EVALUATION METHOD OF PLASTIC MATERIAL AND
EVA1,UATION METHOD OF DEFORMATION PROCESSING OF P1,ASTIC
MATERIAL
[Technical Field of the Invention]
[OOOI]
The present invention relates to an evaluation method of a plastic material and
an evaluation nlethod of deformation processing of a plastic material.
Priority is clainled on Japanese Patent Application No. 2014-097227, filed on
May 8,2014, Japanese Patent ApplicationNo. 2014-097228, filed on May 8,2014, and
Japanese Patent Application No. 2014-097229, filed on May 8,2014, the contents of
which are incorporated herein by reference.
[Related Art]
[0002]
In deformation processing of a plastic material, such as a metal material, in
order to predict forming defects, such as cracks, wrrinklcs, springbacks, and a
thickening defect; and fornling conditions, such as a forming load, forming analysis
\vhich uses a finite element method is performed as described in Patent Doculnent 1.
In general, in finite element analysis, stress-strain cut-ve data which is a material
properties parameter which defines a relationship between a plastic strain and stress of
the plastic tnaterial is input to a computer, and the finite element analysis is performed
in the computer. In the related art, the stress-strain cullre data obtained by a uniaxial
tension test is approximated by a \%ark hardening law, such as a Swift equation
described in Nan-Patent Document 1, and the approximated parameter of the work
hardening law is input to the cotnputer as the stress-strait1 curve data.
In the stress-strain curve data obtained by the uniaxial tension test, in a strain
region having uniform elongation until reaching a certain tensile strength, flow stress
increases as the strain increases due to the work hardening. Meanwhile, in a strain
region having local elongation until a test piece fractures exceeding the tensile
strength, plastic instability occurs, necking occurs in the test piece, and the flow stress
deteriorates as the strain increases. In the fornling analysis of press forming using
finite eletnent analysis, from the stress-strain cul-ve data, stress-strain curve data until
reaching the strain region haviug u11ifol.m elongation is mainly used.
In addition, aecording to a hydraulic bulging test described in Non-Patent
Document 2 or Non-Patent Document 3, and further, according to a cylit~drical
upsetting test or a simple shearing test, stabilized stress-strain cunre data in which
uuevenness is relatively small until reaching a strain region in which a uniform
elongation is exceeded from a yield point, is obtained.
[0004]
However, for example, in the press forming of an actual metal sheet, there is a
case where a strain much greater than that of the strain region having uniform
elongation is applied to a part of the metal sheet.
[OOOS]
In addition, in the aulo~nobilef ield, a for~ni~pirgo cessing method which is
called sheet forging is used together with press forming, cold forging, roll forming, or
incremental forming (swaging or the like). Sheet forging is a technique which is a
combination of press forming and cold forging. An example of sheet forging includes
a n~ethodo f sequentially performing a process of press forming with respect to a metal
sheet in a shape of a cup by using a die and a punch, aid a process of upsetting
processing by pushing a tip part of the cup by another punch in a state where a bottonl
surface of the cup is pressed by a pad. In the method, a part \vhich is thinned when
the press forming is perfonned is compressed and thickened by the upsetting
processing. In this mannel; in the sheet forging, since cold forging is added to press
forming, a strain tnuch @eater than that in the press fornling is added to the metal
sheet.
[0006]
Therefore, in a case where forming analysis, such as press forming which uses
the finite element method, sheet forging, and cold forging, is performed, by
extrapolating the stress-strain cunre data of the strain region having unifortn elongation
until reaching the strain region in which the nnifom~e longation is exceeded, the stressstrain
cut-ve data is approximated considering the influence of the work hardening, and
the finite element analysis needs to be performed by using the approximate curve data.
[0007]
As a method for extrapolating the stress-strain cul-ve data until reaching the
strain region in which the unifornl elongation is exceeded, for example, there is a
method for obtaining an average gradient of a stress-strain curve of the strain region
having uniform elongation, and extrapolating a straight line having the average
inclination until reaching the strain region in which the uniform elongation is
exceeded. In addition, as another nlethod, there is a method for obtaining a partial
inclination of the stress-strain curve of the strain region having uniform elongation,
and extrapolating the straight line having the inclination until reaching the strain region
in which the uniform elongation is exceeded. However, even when the stress-stlain
cul-ve data is extrapolated, the stress-strain curve data is approximated, and the forn~ing
analysis of the deformation processing is performed based on the approximate curve
data, and in a case where accuracy of the approxiniate curve data is low, tliere is a
concerli that tlie accuracy of the fortiiing analysis wlien tlie defamation processilig is
performed may deteriorate.
[OOOS]
hi addition, in tlie approximate curve data obtained by extrapolating the
stress-strain curve data, tliere is a trend that at1 error is likely to increase as a strain
amount increases. Therefore, in the forming analysis of the sheet forging or the cold
forging of tlie related art, the approximate curve data of a high-strain region in which
the error is relatively large needs to be used in tlie analysis, and there is a concern that
the accuracy of the foniiing analysis results may be degraded to that in press forming.
[0009]
Furthermore, in a case where the stress-strain curve data until reaching the
strain region in which the uniform elongation is exceeded is obtained by approximating
the data by the Swift equation from the stress-strain curve data of the strain region
having uniform elongation obtained by the uniaxial tension test, it is difficult to obtain
highly accurate stress-strain cu~vcda ta across all of the strain regions. Therefore, it is
necessary to select a strain region in which the relationslip between the stress and the
strain can be approxhiiated in accordance with an object of the finite element method.
[OOl 01
However, even \vhen the strain region is selected in accordance with the
object of the finite element method, the relationship between the stress and the strain is
approximated, and the formirig analysis, such as the press forming, the sheet forging,
and the cold forging, is performed based on the approximate curve data, and in a case
where the approxiiiiatio~ia ccuracy other than tlie approximation accuracy in the
selected strain region is retnarkably Ion; there is a concern that generation of forliiing
defects may not be able to be correctly detected in the press forming, the sheet forging,
and the cold forging.
[OOI 11
In addition, as a \vork hardening law other than the Swift equation, a Voce
equation described in Non-Patent Document 4 is known. Ho\vever, an object of the
Voce equation is to improve the approximation accuracy of the stress-strain curve data
of the strain region fiom the yield point of aluminum or an aluminam alloy until
reaching the urlifortn elongation, and sin~ilatro the Swifi equation, there is a problerli
in the approxitnation accuracy including a strain region in which uniform elongation
has been exceeded.
Furthermore, as work hardening law other than the Swifi equation, a conlplex
hardening law of Lernaitre-Chaboche is also known. However, the complex
hardening law of Lemaitre-Chaboche has an excellent approximation accuracy
regarding a Bauschinger effect which is a phenomenon of deterioration of yield stress
when an inverse load is applied, but the approximation accuracy of the stress-strain
curve data including the strain region in which the unifoml elongation is exceeded is
not excellent.
[Prior Art Document]
[Patent Document]
[0012]
[Patent Document 11 Japanese Examined Patent Application, Second
Publication No. 8-16644
Boll-Patent Docnment]
[0013]
[Non-Patent Document 11 Yoshida and Itou, "Material models in sheet
fortning simulatio~i", Plasticity and Process, Vol. 40, No. 460, pp. 34-39
won-Patent Document 21 Yoshida, Yoshii, Komorida, and Usuda,
"Deformation form dependence (hardening anisotropy X) of hardening stlength, and
application thereof to fortilability evaluation", Plasticity and Process, Val. 11, No. 114,
pp. 513-521
mon-Patent Document 31 Yoshida, Yoshii, Usuda, and Watanabe, "Second
axial tension n and c, and application thereof to fort~~abilietyva luation" Plasticity and
Process, Val. 11, No. 1 16. pp. 670-675
[Non-Patent Document 41 B.K.Choudliary, et.al, "Tensile stress-strain arid
work hardening behaviour of 3 16LN austenitic stainless steel", materials Science and
Technology, February 2001, Vo1.17, pp. 223-23 1
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[00 141
Cot~sideringth e above-described situation, an object of the present invention
is to provide an evaluation method of a plastic material which can obtain highly
accurate stress-strain curve data until reaching a strain region in which uniform
elongation is exceeded, and an evaluation method of deformation processing of a
plastic tilaterial which can perfornl forming analysis of deformation processing with
high accuracy based on the stress-strain curve data obtained by tile evaluation method
of the plastic material.
[Means for Solving the Proble~ii]
[0015]
An outline of the present invention is as follows.
(1) A first embodiment of the present invention is an evaluation method of a
plastic nlaterial including: a first shearing process of perforn~ings inlple shearing
defornlatioti with respect to a first plastic sheet by dividing the first plastic sheet
havine a first strain amount wliicl~in cludes 0 into two regions by a virtual section
perpendicular to a surface thereof, arid by applying a shear stress to the first plastic
sheet so as to nlake relative positions of the two regions shifted along the virtual
section be on the same surface; a second shearing process of performing simple
shearing defonnation with respect to a second plastic sheet by dividing the second
plastic sheet having a second strain amount which is different from the first strain
amount and includes 0 into two regions by a virtual section perpendicular to a surface
thereof, and by applying a shear stress to the second plastic sheet so as to make relative
positions of the twvo regions shifted along the virtual section be on the satlie surface; a
first partial stress-strain cunre data obtaining process of obtaining first partial stressstrain
cunre data from a relationship between the shear stress applied to the first plastic
sheet in the fust shearing process, and a total strain amount which is a sum of a shear
strain amount which is applied to the first plastic sheet in the first shearing process and
the first strain aniount, by measuring the shear stress and the shear strain which are
applied to the first plastic sheet in the first shearing process; a second partial stressstrain
curve data obtaining process of obtaining second partial stress-strain curve data
from a relationship between the shear stress applied to the second plastic sheet in the
second shearing process, and a total strain atnount which is a st in^ of a shear strain
amount which is applied to the second plastic sheet in the second shearing process and
the second strain amount, by measuring the shear stress and the shear strain which are
applied to the second plastic sheet in the second shearing process; and a synthesized
stress-strain curve data obtaining process of obtaining synthesized stress-strain curve
data based on the first partial stress-strain curve data and the second pal-tial stressstrain
curve data.
[0016]
(2) The evaluation method of a plastic material according to the abovedescribed
(1) tnay further include: an outer form renloving process of obtaining the
second plastic sheet by removing an outer for111 past of the first plastic sheet after
unloading the shear stress applied in the first shearing process.
(3) In the evaluation nlethod of a plastic material according to the abovedescribed
(2), in the outer form removing process, the outer form part may be removed
across tile two regions of the first plastic sheet along the surface direction
perpendicularly intersecting the virtual section and a flat surface of the first plastic
sheet.
[0017]
(4) In the evaluation method of a plastic material according to the abovedescribed
(I), the first plastic sheet and the second plastic sheet may be individual
plastic sheets different from each other.
(5) In the evaluation method of a plastic material according to the abovedescribed
(4), the second strain amount is greater than the first strain amount, and is
equal to or less than the strain amount applied to the first plastic sheet in the first
shearing process.
(6) In the evaluation method of a plastic material according to the abovedescribed
(4), in the synthesized stress-strain eunre data obtaining process, the
synthesized stress-strain curve data may be obtained by combining the cuwe data of a
strain region other than a part which receives the influence of a cross-over effect fiom
the first partial stress-strain eunre data and the second partial stress-strain curve data.
(7) The evaluation nlethod of a plastic material according to the abovedescribed
(4) may fin-thcr include: an outer form retnovi~igp rocess of obtaining a third
plastic sheet having a third strain amount different fiom the first strain amount and the
second strain amount by removing an outer form part fornied by the simple shearing
deformation in tlic first plastic sheet; a third shearing process of perfonnirig simple
shearing deformation with respect to the third plastic sheet by dividing the third plastic
sheet into two regions by a virtual section perpendicular to the surface thereof, and by
applying a shear stress to the third plastic sheet so as to make relative positions of the
two regions shifted along the virtual section be on the same surface; and a third partial
stress-strain cul-ve data obtaining process of obtaining third partial stress-strain curve
data from a relationship between the shear stress applied to the third plastic slieet in the
third shearing process, and a total strain amount which is a sum of a shear strain
amount which is applied to the third plastic sheet in the third shearing process and the
third strain amount, by measuring the shear stress and the shear strain which are
applied to the third plastic sheet in the third shearing process, and in the synthesized
stress-strain cut-ve data obtaining process, the synthesized stress-strain curve data may
be obtained based on the first partial stress-strain culx7e data, the second partial stressstrain
curve data, and the third partial stress-strain cut-ve data.
[0018]
(8) In the evaluation method of a plastic material according to the abovedescribed
(I), in the synthesized stress-strain curve data obtaining process, the
synthesized stress-strain cul-ve data is obtained by approximating the first partial stressstrain
cunre data and the second partial stress-strain cul-ve data based on a work
hardening la\\.
(9) In the evaluatioti method of a plastic material according to the abovedescribed
(I), hi the synthesized stress-strain curve data obtaining process, the
synthesized stress-strain curve data is obtained by approximating the first partial stressstrain
curve data and the second partial stress-strain curve data by a relational equation
expressed by the followitig equation (A).
o=K(cP+a)"' ... (A)
m = ti* + l/{b(cP + c)} . . . (B)
here, in equation (A), o is an eq~~ivalesnttr ess, K (MPa) and a are niaterial
factors of the plastic material, cP is an equivalent plastic strain, and m is as illustrated
in the above-described equation (B), and in equation (B), n* is a convergence value of
a work hardening coefficient, b is a paranieter indicating the rate of convergence of the
work hardening coefficient, and c is a parameter indicating tlie rate of development of
the work hardening coefficient.
(10) In the evaluation method of a plastic nlaterial according to the abovedescribed
(I), the application direction of the shear stress in the first shearing process
and the application direction of the shear stress in tlie second shearing process may be
opposite to each other.
(11) In the evaluation method of a plastic material according to the abovedescribed
(I), in the first shearing process, the application direction of the shear stress
may be reversed in the middle.
(12) In the evaluation niethod of a plastic material according to the abovedescribed
(10) or (1 l), in tlie sy~ithesizeds tress-strain curve data obtaining process, the
synthesized stress-strain curve data may be obtained by approximating tlie first partial
stress-strain curve data and the second partial stress-strain curve data based on a
kinematic hardening law.
(13) In tlie evaluation niethod of a plastic ~nateriaal ccording to the abovedescribed
(I), tlie first plastic sheet and the second plastic slieet may have a shape of a
rectangular flat surface.
(14) In the evaluation method of a plastic material according to the abovedescribed
(I), in tlie first shearing process and tlie second shearing process, the largest
amount of change in the sheet thickness of the first plastic sheet and the second plastic
sheet may be equal to or less than 1% of the sheet thickness.
(15) In the evaluation method of a plastic material according to the abovedescribed
(I), the shear strain applied in each of tlie first shearing process and the
second shearing process, may be in a range of 0.4 to 1.2.
(16) In tlie evaluation method of a plastic material according to the abovedescribed
(I), the first plastic sheet and the second plastic sheet may be steel sheets.
[0019]
(17) A second embodiment of the present inve~~tiios ~anl evaluation method
of deformation processing of a plastic material, in which a computer provided with an
analyzer which performs forming analysis of deformation processing of the plastic
material by a finite element method is used, in which the synthesized stress-strain
curve data obtained by the evaluation method of the plastic material according to any
one of the above-described (1) to (16) is input to the analyzer of the computer, and in
wvhich the analyzer is operated by the conlputec
(18) In tlie evaluation method of deformation processing of a plastic
nlaterial accorditig to the above-described (17), the forn~itiga nalysis may obtain at
least one of a strain distribution, tlie maximum strain, and a forming load of the plastic
material in a case where the deformation processing is perfornled with respect to the
i plastic material.
[0020]
(19) A third embodiment of the present invention is at1 evaluation method of
a plastic material co~nprisinga: first shearing process ofperfornlitlg simple shearing
deformation with respect to a first plastic sheet by dividing the first plastic sheet
having a first strain amount which includes 0 into two regions by a virtual section
perpendicular to a surface thereof, and by applying shear stress to the first plastic sheet
so as to make relative positions of the t\vo regioris shifted along the virtual section be
on the same surface; a first partial stress-strain curve data obtai~lingp rocess of
obtaining first partial stress-strain curve data from a relationship between the shear
stress applied to tlie first plastic sheet it1 the first shearing process, and a total strain
amount which is a sum of a shear strain amount which is applied to the first plastic
sheet in tlie first shearing proeess and the first strain amount, by measuring the shear
stress and the shear strain which are applied to the first plastic sheet in the first
shearing process; and a synthesized stress-strain curve data obtaining process of
obtaining synthesized stress-strain curve data by approximati~igth e first partial stressstrain
curve data by a relational equation expressed by the following equation (C).
o = K(sP + a)"' . . . (C)
m = n* + ll{b(~"+ c)) .. . (D)
here, in equation (C), o is at1 equivalent stress, K (MPa) and a are material
factors of the plastic material, E* is an equivalent plastic strain, and In is as illustrated
in the above-described equation (D), and in equation (D), n* is a convergence value of
a work hardening coeficient, b is a parameter indicating the rate of convergerlce of the
work hardening coefficient, and c is a parameter indicating the rate of developme~lto f
tlie work hardening coefficient.
[0021]
(20) A fourtli embodiment of the present invention is an evaluation method
of deformatiou processing of a plastic material, in which a computer provided with an
analyzer which perforlns forming analysis of deformation processing of the plastic
material by a finite element method is used, in which the synthesized stress-strain
curve data obtained by the evaluation tnethod of the plastic material according to the
above-described (19) is inpiit to the analyzer of the computer, and in which the
analyzer is operated by the computer.
(21) In the evaluation method of defornlation processing of a plastic
material according to the above-described (20), the forming analysis may obtain at
least one of a strain distribution, the maximum strain, and a forming load of the plastic
material in a case where deformation processing is performed with respect to the
plastic material.
[Effects of the Invention]
[0022]
According to the evaluation method of a plastic material described in the
above (I), by performing the shearing process at least two times with respect to the
first plastic sheet and the second plastic sheet having strain atnounts different from
each other, it is possible to obtain at least two pieces of partial stress-strain curve data
having different strain regions from each other. By obtaining the synthesized stressstrain
curve data based on the partial stress-strain curve data, for example, it is possible
to obtain the relationship between the shear stress and the shear strain until reaching
the strain region in which the uniform elongation is exceeded in a tension test in the
related art.
[0023]
According to the evaluation method of a plastic material described in the
above (2), by performing the outer fonn removing process which is performed with
respect to the first plastic sheet after the first shearing process, it is possible to obtain
the second plastic sheet in which cracks which are starting points of fracture are
removed. Therefore, since it is possible to repeatedly perform the shearing process
with respect to one plastic sheet, it is possible to reduce the number of plastic sheets
prepared in advance. In addition, since the strain regions of the first partial stressstrain
curve data and the second partial stress-strain curve data do not overlap each
other and are not separated from each other, it is possible to obtain synthesized stressstrait1
curve data over a wide range of the strain region by a small number of tests with
high accuracy.
According to the evaluation tnethod of a plastic material described in the
above (3), since crack are removed across two regions of the first plastic sheet along
the surface direction perpendicularly intersecting the vistual section and the flat surface
of the first plastic sheet, it is possible to more reliably remove cracks which are sta~ting
points of fiactnres.
[0024]
According to the evaluation method of a plastic material described in the
above (4), it is possible to obtain plural pieces of partial stress-strain curve data by the
actual measurement by performing the simple shearing deformation with respect to
each of the plurality of plastic sheets having strain amounts different kom each other.
Therefore, it is possible to obtain synthesized stress-strain curve data over a wide range
of the strain region based on the partial stress-strait1 curve data.
According to the evaluatio~mi etiiod of a plastic material described in the
above (5), since the strain region of the first partial stress-strain curve data and the
strain region of the second partial stress-strain curve data overlap each other, it is
possible to avoid the separation of the strain regions of the first pastial stress-strain
curve data and the second pattial stress-strain curve data k o n ~ea ch other. Therefore,
it is possible to obtain tile syntl~esizeds trcss-strain c~uvcd ata over a wide range of the
strain region.
According to the evaluation tilethod of a plastic niaterial described in tl~e
above (6), in the synthesized stress-strain curve data obtaining process, since the curve
data of the strain region other than the pait which receives the influcucc of a cross-over
effect is combined from each piece of partial stress-strain cul-ve data, it is possible to
obtain synthesized stress-strain curve data with a small atnount of error.
According to the evaluation method of a plastic material described in the
above (7), by performing the outer form removing process with respect to the first
plastic sheet after the first shearing process, it is possible to obtain the third plastic
sheet of which cracks which are the starting points of the fiacture are removed.
Therefore, since it is possible to repeatedly perform the shearing proccss with respcct
to one plastic sheet, it is possible to reduce the tlutnber of plastic sheets to be prepared
in advance. In addition, since the strain regions of tlie first partial stress-strain curve
data and the third partial stress-strain cun7e data do not overlap each other and are not
separated from each other, it is possible to obtain synthesized stress-strain curve data
over a wide ratige of the strain region by a small number of tests with high accuracy.
[0025]
According to the evaluation method of a plastic material described in the
above (S), since the first partial stress-strain curve data and the second partial stressstrain
curve data are approximated based on a work hardening law, it is possible to
obtain a wider range of the synthesized stress-strain curve data.
According to the evaluation method of a plastic material described in tlie
above (9), since tlie first partial stress-strain curve data and tlie second partial stressstrain
curve data are approximated based on the above-described equation (A) it is
possible to obtain a wider range of the synthesized stress-strain curve data with high
accuracy.
According to the evaluation method of a plastic material described in the
above (lo), it is possible to obtain tlie synthesized stress-strain curve data when an
inverse load is applied. Therefore, it is possible to evaluate a Bauschinger effect
which is a phenonleno~ol f deterioration of yield stress when the inverse load is
applied.
According to the evaluation method of a plastic material described in the
above (ll), it is possible to obtain the synthesized stress-strait1 curve data when the
inverse load is applied. Therefore, it is possible to evaluate a Bauschinger effect
which is a phenotnenon of deterioration of the yield stress when tlie inverse load is
applied. In particular, since it is possible to reverse the load direction at the time
when a desirable amount of strain amount is loaded, it is possible to obtain synthesized
stress-strain curve data having higher practicability.
According to the evaluation method of a plastic material described in the
above (12), since the partial stress-strain curve data obtained in the above-described
(10) or (11) is approximated based on the kinematic hardening law, it is possible to
obtain a wider range of synthesized stress-strain curve data.
According to the evaluation method of a plastic material described in the
above (13), since each of the plastic sheets have a shape of a rectangular flat surface, it
is possible to suppress generation of cracks which can be the starting points of the
fiacture when the shearing processing is performed.
According to the evaluation method of a plastic material described in the
above (14), sirice an amount of decrease of the sheet thickness of the plastic sheet is
reduced in each shearing process, necking is not generated in the sheet thickness
direction. Therefore, it is also possible to obtain the partial stress-strain curve data in
a wide strain region by a uniaxial tension test. Therefore, the number of shearing
processes does not increase; the nletal sheet does not fracture in the shearing process,
and it is possible to obtain a wide range for the synthesized stress-strain curve data.
According to the evaluation method of a plastic material described in the
above (1 5), since the shear strain per one instance in each shearing process is in a
range of 0.4 to 1.2, the nutnber of shearing processes does not increase, the metal sheet
does not fracture in the shearing process, and it is possible to obtain a wide range for
the synthesized stress-strain curve data.
According to the evaluation nlethod of a plastic material described in the
above (16), since the steel sheet is used as the plastic sheet, it is possible to obtain
synthesized stress-strain cullre data of the steel.
[0026]
According to the evaluation method of deformation processing of a plastic
material described in the above (17), by inputting the synthesized stress-strain curve
data obtained by the evaluation method of the plastic materials described in any one of
the above-described (1) to (16) to the comnputer, it is possible to perform the forming
analysis in a case where the deformation processing is performed with respect to the
plastic material with a liigh strain amount, with liigh accuracy.
According to the evaluation method of deformation processing of a plastic
material described in the above (1 S), it is possible to correctly predict at least one of
the strain distribution, the nlaximu~ns train, and the forming load of the plastic material
when the deformation processing is performed wih respect to the plastic material.
For exariiple, in a case where the press forming is employed as the deformation
processing, it is possible to correctly predict the strain distribution atid the tnaxinlum
strain of the plastic sheet in the press for~iii~iagn,d to correctly detect geueration of
cracks. In addition, in a case \vhere shcct forging or cold forging is employed as the
deformation processing, for example, by obtaining the forming load with respect to the
plastic material due to a form, it is possible to correctly predict the for~niuglo ad
required for the processitlg.
[0027]
According to the evaluatiou method of a plastic material described in the
above (19), since the first partial stress-strain curve data is approximated based on the
above-described equation (C), it is possible to obtain a wide ratige for the spntl~esized
stress-strain cunre data with high accuracy.
[002S]
According to the evaluation method of deformation processing of a plastic
tnaterial described in the above (20), by inputting the synthesized stress-strain cuwe
data obtained by the evaluation method of the plastic material according to abovedescribed
(19) to the computer, it is possible to perform the forming analysis in a case
where the deformation processing is performed with respect to the plastic material with
a high strain atnount, with high accuracy.
Furthermore, according to the evaluation method of deformation processing of
a plastic material described in the above (21), it is possible to correctly predict at least
one of the strain distribution, the ~naximums train, and the forming load when the
deformation processing is performed \vitIi respect to the plastic material. For
example, in a case where the press foniling is e~nplopeda s the deformation processing,
it is possible to predict the strain distribution and the maximum strain of the plastic
sheet in the press fornling, and to correctly detect generation of cracks. In addition, it1
a case where sheet forging or cold forging is employed as the defornlation processing,
for exaniple, by obtaining the forn~inglo ad with respect to the plastic nlaterial due to a
fol-111, it is possible to correctly predict the for~~iinloga d required for the processing.
[Brief Description of the Drawings]
[0029]
FIG. 1 is a graph illustrating an example of stress-strain curve data obtained
by a uniaxial tension test, and extrapolated curve data obtained by extrapolating tile
stress-strain curve data.
FIG. 2 is a plan view illustrating a first steel sheet which is used in an
evaluation method of steel according to a first embodiment of the present invention.
FIG. 3 is a plan view illustrating the first steel sheet to which a shear stress is
applied.
FIG. 4 is a plan view illustrating a second steel sheet obtained by removing an
outer form part of the first steel sheet.
FIG. 5 is a graph illustrating synthesized stress-strain curve data obtained by
the evaluation method of the steel according to the fnst embodiment of the present
invention, and equivalent stress-equivalent plastic strain cul-ve data obtained by a
method of the related art.
FIG. 6 is a graph illustrating tile synthesized stress-strain curve data obtained
by the evaluation niethod of the steel according to the first embodiment of the present
invention, and equivalent stress-equivalent plastic strain curve data obtained by
extrapolating the equivalent stress-equivalent plastic strain curve data obtained by the
method of the related art based on a S\vift equation.
FIG. 7 is a graph illustrating a relationship between an instantaneous n value
and an equivalent plastic strain.
FIG. 8 is a schematic view illustrating a method for applying a preliminary
strain to the steel sheet.
FIG. 9 is a plan view illustrating a first steel sheet (or second stecl shcct)
which is used in an evaluation method of the steel according to a second embodiment
of the present itlve~ltion.
FIG. 10 is a plan view illustratirig the first steel sheet to which the shear stress
is applied.
FIG. 11 is a graph illustrating plural pieces of partial stress-strain cullre data
obtained by the evaluation method of the steel according to the second embodiment of
the present invention.
FIG. 12 is a graph illustrating syrithesized stress-strain curve data obtained by
the evaluation method of the steel according to tlie second embodiment of the present
invention.
FIG. 13 is a graph illustrating an example of the synthesized stress-strain
curve data in the evaluation method of the steel according to the second embodiment of
the present invention.
FIG. 14 is a graph illustrating partial stress-strain curve data of each steel
obtained by the evaluation method of tlie steel of the second embodiment of the present
invention.
FIG. 15 is a graph illustrating a relationship between an instantaneous work
hardening rate obtained from the partial stress-strain curve data illustrated in FIG. 14
and an equivalent plastic strain.
FIG. 16 is a plan view illustrating a first steel sheet which is used in an
evaluatio~mi ethod of steel according to a modification example of the second
erilbodinient of the present invention.
FIG. 17 is a plan view illnstratilig a first steel sheet to which the shear stress is
applied.
FIG. 18 is a plan view illustrating a third steel sheet obtained by re~novingth e
outer fortn part of the first steel sheet.
FIG. 19 is a graph illustrating the partial stress-strain cullre data of each steel
sheet obtained by the evaluation method of the steel according to the second
embodiment of the present invention.
FIG. 20 is a graph illustrating a relationship between an instantaneous work
hardening rate obtained from the partial stress-strain cunre data illustrated in FIG. 19
and an equivalent plastic strain.
FIG. 21 is a graph which compares the stress-strain cnrve data obtained by
various methods.
FIG. 22 is a graph which compares the stress-strain curve data obtained by
various methods.
FIG. 23 is a plan view illustrating a first steel sheet which is used in an
evaluation method of steel according to a third embodiment of the present invention.
FIG. 24 is a plan view illustrating a first steel sheet to which the shear stress is
applied.
FIG. 25 is a graph illustrating partial stress-strain curve data obtained by a
si~nplesh earing deformation test, and equivalent stress-equivalent plastic strain cullre
data approxinrated by the Swift equation.
FIG. 26 is a graph illustrating a relationship between an instantaneous n value
obtained fiorn the partial stress-strain curve data illustrated in FIG. 25 and an
equivalent plastic strain.
FIG. 27 is a graph illustrating synthesized stress-strain curve data obtained by
the evaluation method of the steel according to the third e~nbodimenot f the present
invention.
FIG 28 is a graph illustrating a relationship between the instantaneous n value
obtained frotn the synthesized stress-strain culvc data illustrated in FIG 27 and at1
equivalent plastic strain.
FIG 29 is a flow chart illustrating an evaluation method of defornlation
processing of steel according to a fourth embodiment of the present invention.
FIG. 30 is a partial perspective view illustrating a processing tnethod of a
cylindrical hole expanding processing.
FIG. 31A is an outline view illustratitig a result of performing analysis of the
cylindrical hole expanding processing of the steel sheet based on a material parameter
obtained by the Swift equation of the related art.
FIG. 31B is an outline view illustrating a result of performing analysis of the
cylindrical tole expanding processing of the steel sheet based on the sylithesized
stress-strain curve data obtained by the evaluation method of the steel according to the
first embodiment of the present invention.
FIG. 32 is a graph illustrating a distribution of the maximum strain amount at
an edge of a hole after the cylindrical hole expanding processing of the steel sheet.
FIG. 33A is a first process view illustrating sheet forging of the steel sheet.
FIG. 33B is a second process view illustrating tlie sheet forging of tlie steel
sheet.
FIG. 33C is a third process view illustrating the sheet forging of the steel
sheet.
FIG. 34 is a flow view illustrating an e\raluation method of deformation
processing of steel according to a fifth embodiment of the present invention.
FIG. 35A is a view illustrating a result of performing analysis of the sheet
forging of the steel sheet based on the synthesized stress-strain curve data obtained by
the evaluation method of the steel according to the second embodiment of the present
iuve~ltion.
FIG. 35B is a view illustrating a result of performing analysis of the sheet
forgirig of the steel slieet based on the stress-strain curve data obtained by the uuiaxial
tetlsion test.
FIG. 36 is a flow view illustrati~igth e evaluation method of the deformatiot~
processing of the steel according to the fourth emboditnent of the present invention.
FIG. 37 is a partial perspective view illustrating a processing method of the
cylitidrical hole expanding processing.
FIG. 38A is an outline view illustrating a result of performing analysis of the
cylindrical hole expanditlg processing of the steel sheet based on the material
parameter obtained by the Swift equation of the related art.
FIG. 38B is an outline view illustrating a result of perforn~inga nalysis of the
cylindrical hole expanding processing of the steel sheet based on the synthesized
stress-strain curve data obtained by the evaluation method of the steel according to the
third embodimetlt of the present invention.
FIG. 39 is a graph illustrating a distributioti of the maxinlum main strain
a~ilounat t the edge of the liole after the cylindrical hole expanding proeessi~~ogf the
steel sheet.
FIG. 40 is a graph illustrating the sy~~thesizesdtr ess-strain curve data in a case
where the applicatiot~d irection of the shear stress is reversed in the middle of the first
shearing process.
FIG. 41 is a graph illustrating rigidity stress-strain cu~veda ta in a case where
the application direction of the shear strcss is reversed it1 the third shearing process.
[Embodiments of the Itiventioa]
[0030]
In forming analysis of plastic material processing wliich uses finite element
analysis, stress-strain curve data to a strain region having uniform elongation of a
uniaxial tension test is used fioni the stress-strain curve data. However, for example,
in press forming of an actual plastic material, sheet forging and cold forging, a straiti
much greater than that of the strain region having uniform elongation is applied to a
plastic material. Therefore, in the related art, in a case where the forming analysis,
such as tlie press forming ~vllichu ses the finite element method, the sheet forging, and
the cold forging, is performed, by extrapolating the stress-strain curve data of the strain
region having uniform elongation to a strain region in which the aniform elongation is
exceeded, tlie stress-strain curve data is approximated considering tlie influence of the
work hardening, and the finite element a~ialysisis perfornied by using the approximate
curve data.
[0031]
As a method for extrapolating the stress-strain curve data until reaching the
strain region in which the uniform elongation is exceeded, for example, a method for
obtaining average inclination of a stress-strain curve of the strain region having
uniform elongation, and extrapolating a straight line havitig the average inclination
until reaching the strain region in wvluch the uniform elongation is exceeded, may be
employed. In addition, as another method, a method for obtaining a partial
inclination of the stress-strain curve of the strain region having unifornl elongation,
and extrapolating the straight line having the inclination until reaching the strain region
in which the uniforn~e longation is exceeded, may be employed. hi example of the
approximate curve data is illustrated it1 FIG. 1.
LO0321
In FIG. 1, the stress-strain curve data obtained by the uliiaxial tension test and
extrapolated curve data extrapolated frotn the stress-strain curve data of the uniaxial
tension test, are illustrated. As illustrated in FIG. 1, it1 the extrapolated curve data,
unevenness is generated according to a calculation method thereof, and the nneve~mess
increases as the strain amount increase. In particular, in the strain regioti of the sheet
forging, there is a case \vl~eref low stress varies by a width which is equal to or greater
than 10%.
[0033]
The stress-strain curve data decreases after exceeding the maximum tensile
strength. The reason thereof is that plastic illstability occurs and local necking occurs
in a test piece when a strain greater than a strain which corresponds to the maximum
tensile strength is applied. Instead of the uniaxial tension test, a method for obtaining
the stress-strain curve data by a hydraulic bulging test is also employed, but this
method merely obtains the stress-strain curve data of the strain region which is
approximately two times the strain region having uniform elongation. Furthermore, a
method for obtaining the stress-strain curve data by the cylindrical upsetting test is also
employed, but for example, in order to obtain the stress-strain curve data of a steel
sheet by the method, it is necessary to prepare a cylindrical body of which the diameter
is approxinlately the thickness of the steel sheet as the test piece, and much labor and
costs are required for adjusting the test piece.
[0034]
In addition, for example, in a case where the deformation processing is
performed with respect to the steel sheet, there is a case where cracks are generated in
the steel sheet. In particular, cracks are likely to be generated in the press fanning
with respect to a high-strength steel sheet. There is a case where a strain amount
applied to the location at which cracks are generated when the press forming is
performed, reaches approxi~natelyf our times that of the strain region of the stressstrain
curve data obtained by the uniaxial tension test. Therefore, in a case wllere the
forming analysis in a case where the press forming is performed with respect to the
steel sheet is performed by the finite element method or the like, the extrapolated
stress-strain curve data is used, but since the stress-strain curve data is merely
extrapolated stress-strain curve data and is not actually measured data, there is a
concern that an error occurs.
[0035]
Here, the inventors investigated a method for obtaining the stress-strain curve
data in a wide strain region, and found that it is more advantageous to use the simple
shearing test than the uniaxial tension test of the related art. This is because a simple
shearing test applies at1 in-plane shear stress to a plastic sheet, and even when applying
the strain greater than the strain which corresponds to the maximum tensile strength,
necking does not occur in the sheet thickuess direction.
Furthermore, the inventors found that highly accurate synthesized stress-strain
curve data is obtained across the wide strain region without performing the
approximation, such as the extrapolation, by obtaining plural pieces of partial stressstrain
curve data (a) by performing the simple shearing test with respect to the plastic
sheets by preparing the same type of plural plastic sheets having strain amounts
different from each other, or (b) by repeating simple shearing deformation again with
respect to a single plastic sheet by removing an outer form part deformed after the
simple shearing test, and by obtaining one piece of synthesized stress-strain curve data
based on the partial stress-strain curve data.
Furtlienilore, the inventors have found an approximation liaving higher
accuracy than that of the approximation of the related art, sucli as the Sjvift equation,
as a result of arialyzitig highly accurate synthesized stress-strain curve data wliicl~ is
actually obtained by the above-described method.
The present invention is based on the above-described discovery.
[0036]
Hereinaftel; the present invention will be described in detail based on a first
embodiment to a sixth etnbodiment. Furthermore, in any etnbodiment, steel (that is,
the steel sheet \vl~ich is the plastic sheet) \vhich is the plastic material is used, but as the
plastic material, it is possible to use a metal tiiaterial, such as aluminum or titanium, a
glass fiber-reinforcing resin material, such as FRP or FRTP, and ful-thel; a coniposite
material thereof.
[0037]
In addition, in the specification, the "partial stress-strain curve data" means
data which is obtained by performing the shearing process one time, and which
indicates a relationship between the stress and the strain. In addition, the
"synthesized stress-strain curve data" means stress-strain curve data which is combined
by the plural pieces of partial stress-strain curve data, or which has a wide strain region
obtained by employing the approximation to at least one piece of partial stress-strain
curve data.
[0038]
(First Embodiment)
Hereinafter, at1 evaluation metliod of the steel according to the first
embodiment of the present invention \\'ill be described with reference to FIGS. 2 to 7.
Tlie evaluation method of the steel according to the etiibodilnent includes: [lI]
a first shearing process of perfom~ingth e shearing dcforniation with respect to a
first steel sheet 101; [I-21 a partial stress-strain culve data obtaining process of
obtaining first partial st~.css-strainc urve data fro111 a measurement rcsl~lot f the first
shearing process; [l-31 an outer form removing process of removing the outer form
part of first steel sheet 101 after the shearing deforniatio~a~n,d obtairii~iga second steel
sheet 102; [I-41 a second shearing process of performing the shearing deformation
\vitli respect to the second steel sheet 102; [l-51 a partial stress-strain culve data
obtaining process of obtaining second partial stress-strait1 curve data from a
measurement result of the second sliexing process; and [I-6]a synthesized stressstrain
curve data obtaining process of obtaining synthesized stress-strain curve data
based on the first partial stress-strain curve data and the secotid partial stress-strain
curve data. Hereinafter, each process will be described in detail.
[0039]
[I-11 First Shearing Process
As illustrated in FIG. 2, the first steel sheet 101 is a steel sheet having a shape
of a rectangular flat surface. The preliminary strain may be applied to tlie first steel
sheet 101 as necessa~y. In other words, the first steel sheet 101 has a first strain
amount (which may include 0).
[0040]
As illustrated in FIG. 2, between one side lOla and the other side lOlb of tlie
first steel sheet 101, a virtual section lOlc which divides the first steel sheet 101 into
t\vo regions is set. The virtual section lOlc is set perpendicularly to the surface of the
first steel sheet 101. In addition, in a case where the virtual section lOlc is a
boundary, a part including the one side lOla of the first steel sheet 101 and a part
i~lcluditigth e other side .l Olb are respectively bound by a fixer w11ich is schetnatically
illustrated. As the fixer, a cliucking device wl~ichg rabs and fixes the first steel sheet
101 can be illustrated as an example.
[0041]
Next, as illustrated in FIG. 3, the simple shearing deformation is performed
with respect to the first steel sheet 101 by applying the shear stress to make the two
regions of the first steel sheet 101 divided by the virtual section lOlc shifted fro111 each
other it1 the sheet width direction along the virtual section l0lc (that is, to make
relative positions of the two regions shifted be on the same surface).
Specifically, in a state where each of the hvo regious of the first steel sheet
101 divided by the virtual section lOlc is bound by the chucking device, the chuckiug
devices are moved to be shifted from each other in the sheet width direction along the
virtual section 101c. Accordingly, shearing deformation part lOld is formed near the
virtual section 101c. Since the shear stress is applied in the sheet width direction
along the 101c, the sheet thickness at the shearing deformation part lOld does not
change compared to that before the stress is applied. Furthermore, there is also a case
where the sheet thickness can decrease according to the material, but the largest
amount of change in the sheet thickness is at most equal to or less than 1%.
Therefore, the local ueckir~gd oes not occur in the test piece similar to a short axial
tension test.
However, the shape of sides lOle and lOle which connect the one side lOla
and the other side lOlb of the first steel sheet 101 to each other, are largely deformed
by applying the shear stress to the first steel sheet 101.
[0042] :
The shear strain applied to the first steel sheet 101 by performing shearing
process one time is preferably it1 a range of0.4 to 1.2, and is more preferably in a
range of 0.5 to 1.0. Wien tlie applying amount of the shear strait1 performed one time
is set to be equal to or greater that1 0.4, the strain amoutit due to the shearing
defor~llationp erformed one time does riot become extremely small, and the number of
repeats of the shearing process aud the outer for111 removing process does not increase.
In addition, when tlie applying amount of tlie shear strain per one instatice is set to be
equal to or less than 1.2, it is possible to prevent the fracture of the steel sheet in an
early stage.
[0043]
[I-21 First Partial Stress-strain Cul-ve Data Obtaining Process
In the first partial stress-strain curve data obtaining process, the shear stress
and the shear strain wvhich are applied to the first steel sheet 101 in the above-described
first shearing process, are nleasured. In addition, the first partial stress-strain curve
data is obtained from a relationship between (I) the shew stress applied to the first
steel sheet 101 in the first shearing process, and (2) the total strain amoutit which is a
sulll of the shear strain amount which is applied to the first steel sheet 101 in the first
shearing process and the first strain atnoutit.
[0044]
[I -31 Outer Form Removing Process
In the outer form removing process, as illustrated in FIG. 4, by relnoving the
outer form part deformed by applying the shear stress after unloading the shear stress
first, the second steel sheet 102 having a shape of a rectangular flat plane is obtained.
Specifically, by cutting off the part includirig the sides 101e and 101e in the first steel
sheet after the first shearing process, the shape of the first steel sheet 101 in a plan
view becomes a rectangular shape.
Among the sides lOle atid lOle \vhich are deformed by applying the shear
stress in the first shearing process, there is a possibilitj~th at cracks are generated at a
location at which the deformation is particularly large. In a state where cracks
renlaiu, in the second shearing process which will be described later, there is a concern
that the second steel sheet 102 fractures considering cracks as starting points.
However, by perfomling the outer form removing process, since it is possible to obtain
the second steel sheet 102 of which cracks \vl~icha re the starting points of the fracture
are removed, it is possible to solve the above-described problem.
Furthermore, in the outer form removing process, the outer form part may be
removed across tlie two regions of the first steel sheet 101 along the surface direction
perpendicularly intersecting tlie vil-h~als ection lOlc and the flat surface of the first
steel sheet 101. In other words, the outer form part may be removed along two lines
A in FIG. 4. Accordingly, it is possible to more reliably remove cracks which are the
starting points of the fracture.
[0045]
[l-41 Second Shearing Process
In the second shearing process, the shear stress is applied to the second steel
sheet 102 obtained by the outer form removing process by a method similar to the
method described in the above [l-11.
The second steel sheet 102 has the second strain amount wvhich is caused by
the strain applied in the first shearing process. Therefore, in the embodiment, the
second strain amount of the second steel sheet 102 is greater than the first strain
amount of the above-described first steel sheet 10 1.
[0046]
[l-51 Second Partial Stress-strain Curve Data Obtaining Process
In the second partial stress-strain curve data obtaining process, the shear stress
and the shear strain \\~hicha re applied to the second steel sheet 102 in the second
shearing process, are measured. In addition, the second partial stress-strain curve
data is obtained fiotn a relatiot~shipb etween (1) the shear stress applied to the secorld
steel sheet 102 ia the second shearing process, and (2) the total strain a ~ l ~ owuh~ic~h tis
a suu~of tlie shear strain amount which is applied to the second steel sheet 102 in the
second shearing process and the second strain amount.
[0047]
[l-61 Sy~ltl~esizeSdtr ess-strain Curve Data Obtaining Process
In the synthesized stress-strain curve data obtaining process, tlie synthesized
stress-strain curve data is obtained at least based on the first partial stress-strain curve
data and the second partial stress-strain curve data.
[0048]
In the description above, the synthesized stress-strain cul-ve data is obtained
based on the two pieces of partial stress-strain curve data obtained by perfor~niingt he
shearing process two times during the outer form removiug process, but according to
the embodiment, it is possible to repeat the shearing process and the outer form
removing process with respect to one steel sheet plural times. It is not necessary to
pat-ticularly determine an upper litnit of the number of repeats, the processing may
continue until the steel sleet fractures during applying the shear stress.
Therefore, according to the evaluation method of the steel according'to the
en~bodinienti,t is possible to reduce the number of plastic sheets to be prepared in
advance. In addition, since the strain region of the first partial stress-strain curve data
and the strain region of the secotid partial stress-strain curve data do not overlap each
other and are not separated from each othel; it is possible to obtain the synthesized
stress-strain curve data over a wide range of the strain region by s~llalnl u~l~boefr t ests
with higll accuracy.
[0049]
In FIG 5, regarding a case (method of the embodiment) where the shearing
process is repeated three times during the two times of onter form removing process,
and a case (method in the related art) where the simple tension test is performed, the
obtained equivalent stress-equivalent plastic strain curve data is illustrated. I11 the
steel sheet supplied for the test, the tensile strength is 600 MPa, the yield strength is
400 MPa, and the sheet thickness is 1.6 mtn.
[0050]
As illustrated in FIG. 5, in the method of the related art, the stress-strain cunfe
data to an equivalent plastic strain of 0.19 is obtained, but according to the method of
the embodiment, it is possible to obtain the stress-strain curve data to approximately
0.7 which is four times thereof.
[0051]
Next, the equivalent stress-equivalent plastic strain curve data obtained
according to the evaluation method of the steel according to the embodiment was
investigated in detail. FIG. 6 illustrates the investigation result thereof. In FIG. 6,
the synthesized stress-strain curve data obtained by the evaluation method of the steel
according to the embodiment, and stress-strain curve data obtained by extrapolating
and the stress-strait1 curve data lo an equivale~p~lta stic strain of 0.19 obtained by the
method of the related art, utltil the plastic strain becomes equal to or greater that1 0.7
based on a Swift equation, are illustrated.
[0052]
The synthesized stress-strain curve data obtained by the evaluation method of
the steel according to the emboditne~i~st d ata which plots the equivalent stress and the
equivaletit plastic strail1 \vl~iclia re obtained fro111 tlle shear stress and the shear strain
which are actually measured. 111 addition, the extrapolated stress-strain curve data is
data computed by applying the stress-strain cui-ve data to the plastic strain of 0.1
obtained by tlie method of the related art, to the Swift equation of the followitlg
equation (1).
[0053]
o = a(sP + 0)" ... (1)
[0054]
here, in equation (I), o is an equivalent stress, u and are constants
determined for each steel sheet, sP is an equivalent plastic strain, and n is a work
hardening coefficient.
[0055]
As illustrated in FIG. 6, it is ascertained that the equivalent stress-equivalent
plastic strain curve data extrapolated based on the Swift equation is separated from the
synthesized stress-strain cui-ve data obtained by the method of the embodiment when
appmaching a high-strain region.
[0056]
The reason of the separation of the two pieces of curve data was investigated,
and a relationship between an instantaneous n value and the equivalent plastic shairi
was investigated. The result thereof is illustrated in FIG. 7. Tlle instantaneous ti
value is at1 instantaneous gradient in the curve data which plots the stress-strain curve
data illustrated in FIG. 6 in both logarithmic graphs. As illustrated in FIG. 7, the
stress-strain cuwe data extrapolate based on the Swift equation is converged to a
substantially constant value as the strait1 amount increases. Meanwliile, the
instantaneous n value of tlie stress-strain curve data obtained by the method of the
e~i~bodinietnetn ds to be converged as the strain amount inc~easesa, nd continues to
change without being converged to the constant value. The Swift equation is at1
equation on the assun~ptionth at tlie n value is constant, bnt it is ascertained that the
Swift equation is not enlployed in a region in wvl~icht he strain amount is particularly
large in a case of the actual steel.
[0057]
(Second Embodiment)
The evaluation method of the steel according to the second embodiment of the
present invention is an evaluation method of tlie steel for obtaining the partial stressstrain
curve data by preparing hvo or more steel sheets having the preliminary strain
amounts different from each other, and by performing the simple shearing deformation
\vitli respect to each of the steel sheets, and fi~alierf,o r obtaining the synthesized
stress-strain curve data based on the partial stress-strain curve data.
III addition, as a modification example, in the shearing process, similar to the
first embodiment, the simple shearing deformation tnay be repeatedly performed again
one or more times \vith respect to the steel sheet of which the outer form part of the
steel sheet deformed by the simple shearing deformation is removed.
[OOS8]
In the embodiment, unlike the first embodiment, two or more steel sheets of
which the components and structures are the same, are prepared. One of the prepared
steel sheets may be a steel sheet to wliich the strain amount is not applied. As a
method for applying the preliminary strain to the steel sheet, as illustrated in FIG. 8, the
rolling direction may be, for example, the direction which is not parallel to.tlie sl~earing
direction, and the rolling direction tnay be the direction parallel to tlie shearing
direction.
[0059]
In a case where n steel sheets are prepared, a plurality of steel sheets to \\,hich
p~.eliniinarys trains E", E ~E ~~, ,., . . E~,,a re gradually applied, are prepared. The
~iumbeor f steel sheets to be prepared may be determined in accordance with the size of
a target equivalent strain. hl particular, in a case where the stress-strain curve data to
the high eqoivalent strain region is obtained, the plurality of steel sheets to which the
strains are applied in advance may be prepared.
[0060]
Furthermore, the preliminary strain &"applied to the steel sheet by the cold
rolling is applied in the following equation (2) in a case where the sheet thickness
before the rolling is ho and the sheet thickness after the rolling is h.
[0061]
= (2/d3)1n(lho) . . . (2)
[0062]
Not being limited to the cold rolling, a method for applying the preliminary
strain to the steel sheet is not particularly limited as long as the method is a method
which can control the preliminary strain amount, and processing of applying tensile
stress which uses the uniaxial tension test and the flat surface strain tension test, can be
illustrated as an example.
[0063]
Hereinafter, the evaluation method of the steel according to the second
embodiment of the present invention will be described with reference to FIGS. 9 to 22.
The evaluation of the steel according to the embodiment includes: [2-11 a first
shearing process of performing the shearing deformation with respect to a first steel
sheet 201; [2-21 a pa~tiasl tress-strain curve data obtaining process of obtaining iirst
partial stress-strain CUIT~Cd ata fron~a nleasorement result of the first shearing process;
[2-31 a second shearing process of perfonning the shearing deformation 116th respect to
a sccond steel sheet 202 which is prepared separately from the first steel sheet 201; [2-
41 a partial stress-strain curve data obtaining process of obtaining second partial strcssstrain
curve data fro111 a measurement result of the second shearing process; and [2-51 a
synthesized stress-strain curve data obtaining process of obtaining synthesized stressstrain
curve data based on the first partial stress-strain curve data and the second partial
stress-strain cutve data. Hereinafter, each process will be described in detail.
[0064]
[2-11 First Shearing Process
As illustrated in FIG. 9, the first steel sheet 201 is a steel sheet having a shape
of a rectangular flat surface. The preliminary strain may be applied to the first steel
sheet 201 as necessary. In other words, the first steel sheet 201 has a first strain
amount (which may include 0).
[0065]
As illustrated in FIG. 9, between one side 201a and the other side 201b of the
first steel sheet 201, a virtual section 201c which divides the first steel sheet 201 into
two regions is set. The virtual section 201c is set perpendicnlarly to the surface of the
first steel sheet 201. In addition, in a case where the virtual section 201c is a
boundary, a part including the one side 201a of the first steel sheet 201 and a part
including the other side 201b are respectively bound by a fixer which is schematically
illustrated. As the fixel; a chucking device which grabs and fixes the first steel sheet
201 can be illustrated as an example.
[0066]
Next, as illustrated in FIG. 10, the simple shearing deformation is performed
wit11 respect to tllc first steel sheet 201 by applying the shear stress to make the two
regions of the first steel sheet 201 divided by the virtual section 201c shifted frorn each
other in the sheet width direction along the virtual section 201c (that is, to make
relative positions of the two regions shifted be on the same surface).
Specifically, in a state where each of the two regions of the first steel sheet
201 divided by the virtual section 201c is bound by tlie chucking device, the chucking
devices are moved to be shifted fro111 each other in the sheet width direction along the
virtual section 201c. Accordingly, shearing deformation part 201d is formed near the
virtual section 201c. Since the shear stress is appliedin tlie sheet width direction
along the virtual section 201c, the sheet thickness at the shearing deformation part
201d does not change co~nparedto that before the stress is applied. Furthernlore,
there is also a case wvllere the sheet thickness can decrease according to the material,
but the largest amount of change in the sheet thickness is at most equal to or less than
1%. Therefore, the local strain is not generated in the test piece similar to a s1101-t
axial tension test.
Howevel; the shape of sides 201e and 201e which connect the one side 201a
and the other side 201b of the first steel sheet 201 to each other, are largely deformed
by applying the shear stress to the first steel sheet 201.
[0067]
The shear strain applied to the first steel sheet 201 by performing shearing
process one time is prcfcrably cqoal to or greater than 0.4. When the applying
amount of the shear strain performed one time is set to be equal to or greater than 0.4,
the strain amount due to the shearing deformation performed one time does not
become extremely small, and the number of steel sheets to be used can be reduced. In
the embodiment, the upper limit of the applying amount of the shear strain is not
necessarily partictllarly limitcd, and the shear strcss nlay be applied until tlie first steel
sheet 201 fsactores.
[0068]
[2-21 First Partial Stress-strain Curve Data Obtaining Process
In tlie first partial stress-strain curve data obtaining process, tlie shear stress
and the shear strain wvliich are applied to the first steel sheet 201 in the above-described
first shearing process, are measured. In addition, the first partial stress-strain curve
data is obtained from a relationship between (1) the shear stress applied to the first
steel sheet 201 in the first shearing process, and (2) the total strain amount which is a
sum of the shear strain anloutit which is applied to the first steel sheet 201 in the first
shearing process and the first strain amount.
[0069]
[2-31 Second Shearing Process
In the second shearing process, the shear stress is applied to the second steel
sheet 202 which is prepared separately from the first steel sheet 201 by tlie method
similar to the method described in the above [2-11.
The second steel sheet 202 may have the secolid strain amount different from
the first strain amount of the first steel sheet. Therefore, in the embodiment, the first
strain atiiount may be greater or less than the second strain amount.
However, for example, in a case where the second strain amoutit is greater
than the first strain amount, it is preferable that the second strain amount is equal to or
less than the strain amount applied to the first plastic sheet in the first shearing process.
. Accordingly, it is possible to avoid tlie separation of the strain regions of the first
partial stress-strain curve data and the second partial stress-strain cullre data fiom each
other, and to obtain the synthesized stress-strain curve data over a wide range of the
strain region with high accuracy.
[0070]
[2-41 Second Partial Stress-strain Curve Data Obtaining Process
In the second partial stress-strain cut-ve data obtaining process, the shear stress
and the shear strain which are applied to the second steel sheet 202 in the abovedescribed
second shearing process, are measured. In addition, the second partial
stress-strain curve data is obtained from a relationship between (1) the shear stress
applied to the second steel sheet 202 in the second shearing process, atid (2) the total
strain atnount which is a sun1 of the shear strain amount whicli is applied to the second
steel sheet 202 in the second shearing process and the second strain amount.
[0071]
[2-51 Synthesized Stress-strain Cunle Data Obtaining Process
In the synthesized stress-strain cunTe data obtaining process, the synthesized
stress-strain cu~veda ta is obtained at least based on the first partial stress-strain curve
data and the second partial stress-strain curve data.
Hereinafter, a method for obtaining one piece of synthesized stress-strain
curve data by combining the first pastial stress-strain curve data atid tlie second partial
stress-strain curve data with each other, will be described with a specific example.
[0072]
FIG. 11 illustrates five pieces of partial stress-strain curve data (equivalent
stress-equivalent plastic strain curve data) obtained by perfornling the sliearitig process
with respect to five steel sheets having the preliminary strain aniounts (0, E'I, E'Z, E'~,
EP4) different from each other.
[0073]
As illustrated in FIG. 11, in the partial stsess-strain curve of the steel sheet to
wvliich the preliminary strain is not applied, the curve fro111 the position at \vI~ictlh~e
equivalent strain is 0 rises, tlie equivalent stress contitlues to rise, and after this, tlie
stress returns to 0 as the steel sheet fractures.
hi addition, in the partial stress-strain curve of the steel sheet to wvl~iclit he
prelimi~larys train E"I is applied, the curve rises fio~omt he position at wl~ictlh~e
equivalent strain is E", the equivalent stress continues to rise, and after this, the stress
returns to 0 as the steel sheet fractures.
Similarly, in the stress-strain cullre of the steel sliect to which the prelimina~y
strains ~~z to ~ " a4r e applied, each cunre rises from the positions at which the equivalent
strains are E~~ to cP4, the equivalent stress coritinues to rise, and after this, the stress
returns to 0 as the steel sheet fractures.
[0074]
In this manner, in the embodiment, a starting position of the stress-strain
cuwe of the steel sheet to which the prelitninary strain E~ is applied is the equivalent
strain cP. In a case where the preliminary strain E" is applied to the steel sheet in
advance, the strain applied to the steel sheet becomes a strain obtained by integrating
tlie preliminary strain cP and the shear strain. In addition, by performing the uniaxial
shearing test with respect to the steel sheet to which the preliminary strain E~ is applied,
the stress-strain curve data of the total strain obtained by integrating the prelimina~y
strain and the shear strain, and the equivalent stress wl~iclci orsespot~dsto the total
strain, is obtained. Therefore, as described above, the starting position of the stressstrain
curve of the stecl sheet to which the preliminary strain E~ is applied is the
equivalent strain E ~ .
[0075]
Next, as illustrated in FIG. 12, from each of the partial stress-strain curve data,
by coillbiliing tlie curve data which is equivalent to the range of the unifo1.m elongation
other than thc elastic defonnation, one piece of sytithesized stress-strain curve data is
obtained.
At this time, in a case where each cutve data substantially overlaps each otliel;
the curve data may be one piece of synthesized stress-strain curve data by combining
each curve data while maintaining the overlapping state.
[0076]
In addition, in a case where a shift occurs ~lliilea ch cunre data does not
overlap each other, as illustrated in FIG. 13, curve data bl or b2 of the steel sheet having
a high preliminary strain are shifted in the upward-and-downward direction along an
axis of the equivalent stress so that the cullre data bl or b2 of the steel sheet having a
high preliminary strain overlap cullre data a of tlie steel sheet having a low preliminary
strain. For example, as illustrated by one-dot chain line of FIG. 13, in a case where
the equivalent stress of the curve data bl of the steel sheet having a high preliminary
strain is high with respect to the curve data a of the steel sheet having a low
preliminary strain, the cunle data bl of the steel sheet having a high preliminary strain
is shifted downward and overlaps the curve data of the steel sheet having a low
preliminary strain. In addition, as illustrated by a dotted line of FIG. 13, in a case
where the equivalent stress of the curve data b2 of tlie steel sheet havitig a high
preliminary strain is low with respect to the curve data a of the steel sheet having a low
preliminary strain, the curve data b2 of the steel sleet having a high preliminary strain
is shifted upward and overlaps the curve data a of the steel sheet having a low
prelimitiary strain. Curve data c after the overlapping is curve data obtained as the
original curve data a and the cutve data after the shift overlap each other.
[0077]
As will be described later, tlie reasoti why tlie above-described handling is
possible in thc synthesized stress-strain cuve data obtaining process, is that an
instantaneous work hardening rate obtained from the stress-strain cut-ve data of the
steel sheet to which tlie preliminary strain is applied, and an instantaneous work
hardening rate obtained from the stress-strain curve data of the steel sheet to wlich the
preliminary strain is riot applied, substatitially match each other.
[0078]
In addition, tlie reason why tile cunre data bl or bl of the steel sheet having a
high prelu~~inarsytr ain are shifted fiom the curve data a of the steel sheet having a low
preliminary strain and the curve data bt or b2 of the steel sheet having a high
preliminary strain, is that the other culve data sequentially overlap each other by
considering the stress-strain curve data of the steel sheet having a low preliminaty
strain as a reference. In addition, the reason why the stress-strain cusve data of the
steel sheet having a low preliminary strain is a reference, is that there is a case where
the equivalent stress is influenced by the preliminary strain in the steel sheet to which
the preliminary strain is applied, and the influence is removed as much as possible.
[0079]
Next, in a case vrliere the sinnple shearing test is performed with respect to the
steel sheet to which the preliminary strain is applied, there is apossibility that a crossover
effect is generated as tlie preliminary strain and the shear strain are applied. In
FIG. 14, the partial stress-strain curve data in wluch the cross-over effect is generated
is illustrated, and tlie stress-strain cuwe data illustrated in FIG. 14 is the partial stressstrain
culve data of the steel sheet to which 0.1 to 1.0 of preliminary strain is applied,
but immediately after tlie yielding of each cut-ve data, since the curve data indicates a
high equivalent stress once, there is a location at which the equivalent stress gradually
decreases. The disorder of the cunrc data is called a cross-over effect, and there is a
case where the cross-over effect is generated due to a difference of a path through ,
which the strain is applied. In a case where the cross-over effect is generated, the
synthesized stress-strain clu-ve data may be obtained other than the part at which the
cross-over effect is generated. The excluded range may be approxitllately 0.5 times to
1.5 times the preliminary strain sP applied to each steel sheet. In addition, in FIG. 14,
the steel sheet to be used is JSC270 material (Japanese Iron and Steel Federation
Standard).
[OOSO]
Next, the stress-strain curve data of FIG. 14 was investigated in detail. The
investigation result thereof is illustrated in FIG. 15. In FIG. 15, regarding the steel
sheet to which the preliminary strains of 0.1,0.3,0.5, and 1.0 are respectively applied,
the relationship between the instantaneous work hardening rate (doldc) and the
equivalent plastic strain is plotted. The instantaneous work hardening rate is the
i~istat~taneognrsa dient of the four partial stress-strain curves illustrated in FIG. 14.
As illustrated in FIG. 15, the instantaneous work hardening rate gradually deteriorates
as the equivalent strain increase, but it is ascertained that the curve data of the
instantaneous work hardening rate of each steel sheet continues to partially overlap
each othe~ In addition, in FIG. 15, the circled part corresponds to the ccurve data
which receives the influence of the cross-over effect illustrated in FIG. 14, and other
than the part, it is ascel-tained that the curve data of the instantaneous work hardening
rate of each steel sheet substantially overlap each other. Furthermore, although not
illustrated iu FIG. 15, the stress-strain curve data of the steel sheet to which the
preliminary strains are applied excellently match the stress-strain curve data of the
steel sheet to which thc preliminary strain was not applied. From the results, it is
ascertained that it is possible to reduce the range of the errors to the miniti~utne ven
when each picce of partial stress-strait1 curve data of the plurality of steel sheets to
which the preli~uinarys trains are gradually applied are integrated with each other in
the synthesized stress-strain culve data obtaining process.
[0081]
As the tnodification example of the embodiment, after the first shearing
process (andlor the second shearing process), a third steel sheet tnay be obtained by
performing the outer form removing process as described in tlie first embodiment, and
third partial stress-strain cul-ve data may be obtained by performing the sitnple shearing
deformation with respect to the third steel sheet.
[0082]
In the evaluation method of the steel according to the modification exatnple,
two steel sheets (the first steel sheet 201 and the second steel sheet 202) having the
preliminary strains different from each other are prepared.
[0083]
Next, as illustrated in FIG. 16, beheen the one side 201a and the other side
201b of the first steel sheet 201, the virtual section 201c which divides the first steel
sheet 201 into two regions is set, and the part including tlie one side 201a of the first
steel sheet 201 at~da part including the other side 201 b are respectively bound by a
fixer which is schematically illi~strated.
[0084]
Next, as illustrated in FIG. 17, the sinlple shearing defor~nationis perfornled
with respect to the first steel sheet 201 by applying the shear stress to make the two
regions of the first steel sheet 201 divided by the virtual section 201c shifted from each
other in the sheet width direction along the virtual section 201c (that is, to make
relative positio~iso ft he two regions shifted be on the same surface).
Specifically, in a state where each of tlie two regions of the first steel sheet
201 divided by thc virtual section 201 cis bound by the chucking device, the chucking
devices are moved to be shifted frolorn each other in the sheet width direction along the
virhtal section 201c. Accordingly, shearitlg defom~ationpa rt 201d is formed near the
virtual section 201c. Since the shear stress is applied in tlie sheet width direction
along the virtual section 201c, the sheet thickness at the shearing defornlation part
201d does not change compared to that before the stress is applied. Furtherlilore,
there is also a case ~vliereth e sheet thickness can decrease according to the material,
but the largest amount of change in the sheet thickness is at least equal to or greater
than 1%. Therefore, the local necking does not occur in the test piece similar to a
short axial tension test.
However, the shape of sides 201e and 201e which connect the one side 201a
and the other side 201b of the first steel sheet 201 to each other, are largely deformed
by applying the shear stress to the first steel sheet 201.
[OOS5]
In the modification example, the shear strain applied to the steel sheet by
perfor~nings hearing process one time is preferably in a range of0 .4 to 1.2, and is more
preferably in a range of 0.5 to 1 .O. When the applying amount of the shear strain per
one instance is set to be equal to or greater than 0.4, the strain amount due to the
shearing deformation performed one time does not become extremely small, and the
number of repeats of thc shearing process and the outer fonn rernoving process does
not increase. In addition, when the applying amount of the shear strain per one
instance is set to be equal to or less than 1.2, it is possible to prevent the fsachtre of the
steel sheet in an early stage.
[0086]
In tlie outer form retnoving process, as illustrated in FIG. 18, by removing the
outer fomi pait deformed by applyitig the shear stress aftel. ualoading the shear stress
first, a third steel sheet 203 having a shape of a rectangular flat plane is obtained.
Specifically, by cutting off the part itlcluding the sides 201e and 201e in tlie first steel
sheet after the first shearing process, the shape of the first steel sheet 201 in a plan
vie\v becomes a rectangular shape.
On the sides 201e and 201e which are defornied by applying the shear stress
in the first shearing process, tliere is a possibility tliat cracks are generated at a location
at which the deformation is particularly large. In a state where cracks remain, in the
third shearing process which will be described later, there is a concern that the third
steel sheet 203 fractures considerilig cracks as starting points. However, by
performing the outer form removing process, since it is possible to obtain tlie third
steel sheet 203 of which cracks which are the startirig point of the fracture are
removed, it is possible to solve the above-described problem.
Furthermore, in the example illustrated in FIG. 18, among the hvo regions
divided by the viitual section 201c, only orie region is respectively removed.
Howevel; in the outer form removing process, the outer form part may be removed
across the two regions of the first steel sheet 201 along the surface direction
perpendicularly intersecliag the virtual section 201c and the flat surface of the first
steel sheet 201. In other words, the outer forni part may be removed along the two
lines A in FIG.18. Accordingly, it is possible to Inore reliably remove cracks which
are the stastitig points of the fracture.
[0087]
The sitnple shcaring is perfonned by a method similar to tliat of tlie abovedescribed
first shearing piocess with respect to the third steel sheet 203 obtained by the
outer fo1.111 removing process (third shcaring pmcess). In other words, according to
the modification examl~lei,t is possible to repeat the shearing process plural times
during the outer form removing process with respect to one steel sheet. The nurnber
of repeats of the process nlay be one or Inore. It is not necessaly to particularly
deternline an upper limit of the nunlber of repeats, and the processing may continue
until the steel sheet is fsactured while applying the shear stress.
Futfhernlore, since the third steel sheet is obtained after the first shearing
process with respect to the first steel sheet, the third steel sheet has the third strain
amount 3 greater than that of the first steel sheet. In addition, regarding the third
partial stress-strain curve data which will be described later, since the synthesized
stress-strain curve data is obtained by synthesizing the first pai-tial stress-strain curve
data and the second partial stress-strain curve data, it is preferable that the first strain
amount, the second strain amount, and the third strain atnount are different from each
other.
[OOSS]
In addition, by measuring the shear stress and the shear strain which are
applied to the third steel sheet 203 in the third shearing process, the thisd partial stressstrain
curve data is obtained from a relationship between (1) the shear stress applied to
the third plastic sheet in the third shearing process, and (2) the total strain amount
which is a sum of the shear strain anlount which is applied to the third plastic sheet in
the third shearing process arid the third stsain aniotmt.
[0089]
Next, as the synthesized stress-strain curve data obtaining process, one piece
of synthesized stress-strain curve data is obtained based 011 the first partial stress-strain
curve data, the second partial stress-strain cullre data, and the third partial stress-strain
curve data.
The synthesized stress-strain curve data obtaining process at this time, inay be
perforn~ed similar to the synthesized stress-strain curve data obtaining process
described above.
[0090]
In FIG. 19, the partial stress-strain culve data obtained from the plurality of
steel sheets is illustrated. In addition, in FIG. 20, a relationship between the
instantaneous work hardening rate (dofd~)a nd the equivalent plastic strain is
illustrated.
[0091]
The partial stress-strain cnl-ve data illustrated in FIG, 19 is in a state before
performing the synthesized stress-strain cui-ve data obtaining process, but it is
ascertained that each curve data s~lbstantiallyo verlap each other. In addition, as
illustrated in FIG. 19, for example, the cui-ve data of the steel sheet to which 1.0 of the
preliminary strain is applied is segmented by a plurality of lines, but each of the
plurality of lines coi~espondsto the result of repeating the shearing process with
respect to one metal sheet as illustrated in the above-described modification example.
[0092]
In addition, as illustrated in FIG. 20, the curve data indicating the relationship
bet\veen the instaiitaneous work I~ardeiiingra te (dold~a)n d the equivalent strain of the
steel sheet to which the preliminary strain is applied, and the curve data indicating the
relationship between the instantaneous work hardening rate (dolde) and the equivalent
strain of the steel sheet to which the preliminary strain is not applied, excellei~tlym atch
each othes. As a result, even in a ease wl~ereth e shear stress is repeatedly applied, it
is ascertained that the error between the stress-strain curve data of each stecl sheet can
be reduced to the nlinitnum.
[0093]
In FIG. 21, various types of stress-strain curve data are illustrated. In FIG.
21, (a) the stress-strain cullre data obtained by tlie l~ydraulicb ulging test, (b) the stressstrain
curve data obtained by apl~lyingth e tensile stress to the steel sheet to which the
preliminary strain is applied, and (c) the plural pieces of partial stress-strain cuwe data
and synthesized stress-strain curve data which are obtained by the evaluation tnethod
of the steel according to the embodiment, are illustrated. It is ascertained that the
synthesized stress-strain curve data obtained by the evaluation method of the steel
according to the emboditnent extends until reaching the strain region wider than that of
the stress-strain curve data obtained by the hydraulic bulging test, by performing the
measurement by repeating the shearing process. In addition, in a case where the
tensile stress is applied to the steel sheet to which the preliminary strain is applied by
the rolling, the steel sheet fractures at the time when the tensile stress which
co~~espontdos t he yield point is applied. It is predicted that the steel sheet is in a
state where the equivalent stress is applied to the steel sheet in advance as the
preliminary strain is applied, the local necking immediately occurs in the sheet
thickness direction as a tensile load is applied in this state, and the fracture occurs. In
a case wl~ereth e simple shearing test is performed similar to the etnbodimnent, since tlie
local necking does not occur in the sheet direction, the fracture does not inunediately
occur even in a case of the steel sheet to which the preliminary strain is applied, and
each piece of partial stress-strain c u m data is obtained.
lo0941
In addition, in FIG. 22, (d) the approximate culve data obtained by
extrapolating the stress-strain curve data obtained by the uniaxial tension test until
reaching the high-strain region, and (e) the plurality of partial stress-strain curve data
and synthesized stress-strain culve data which are obtained by the evaluation method
of the steel according to the embodiment, are illustrated. It is ascertained that the
synthesized stress-strain cunre data obtained by the evaluation method of the steel
according to the embodiment has a difference which is approximately 10% when the
equivalent strain is in the vicinity of 2.0 with respect to the approximate culvc data.
[0095]
As described above, according to the evaluation method of the steel according
to the embodiment, since one piece of synthesized stress-strain cunre data is obtained
based on the partial stress-strain curve data of each steel sheet by respectively
performing the simple shearing deformation with respect to the plurality of steel sheets
having the strain amounts different from each other, and by obtaining the partial stressstrain
curve data for each steel sheet, it is possible to obtain the stress-strain curve data
until reaching the high-strain region by the actual measurement.
In addition, in the synthesized stress-strain cunre data obtaining process, from
the stress-strain cunfe data other than the elastic deformation of each steel sheet, by
combining the curve data of the strain region other than the part which receives the
influence of the cross-over effect, it is possible to obtain the synthesized stress-strain
curve data with a small amount of error.
Furthermore, siniilar to the above-described modification example, in a case
where the shearing process is perforn~edp lural times with respect to one steel sheet
during the outer fonn removing process, the plural pieces of stress-strain curve data
having the strain regions different fiom each other are obtained &on1 one steel sheet.
As a resolt, since the strain region which can be covered by one steel sheet is enlarged,
for example, by two or three steel sheets, it is possible to obtain the synthesized stressstrain
curve data which covers a wide range of the strait1 region.
[0096]
(Third Embodiment)
In the evaluation rnethod of the steel according to the embodiment, by
employing a new approximation instead of the approximation similar to the Swift
equation of the related art with respect to one piece of partial stress-strain cuwe data
obtained by the shearing process described in the f ~ setn lbodinlent and the second
embodiment, highly accurate synthesized stress-strain curve data is obtained.
Furthernlore, highly accurate synthesized stress-strain curve data may be
obtained by e~nployingn ew approximation to the plural pieces of partial stress-strain
cut-ve data obtained by the shearing process described in the first embodinlent and the
second embodiment.
[0097]
Hereinafter, the evaluation method of the steel according to the third
enlbodi~nento f the present invention will be described with reference to FIGS. 23 to
28.
The evaluation method of the steel according to the embodiment includes: [3-
11 a first shearing process of performing the shearing deformation with respect to a
first steel sheet 301; [3-21 a parlial stress-strain curve data obtainirig process of
obtaining first partial stress-strain curve data from a measurement result of the first
shearing process; and [3-31 a stress-strain curve data obtaining process of obtaining
stress-strain curve data based on the first pa~fiasl tress-strain curve data. Hereinafter,
each process will be described in detail.
[0098]
[3-I] First Shearing Proccss
As illustrated in FIG. 23, the first steel sheet 301 is a steel sheet having a
shape of a rectangular flat surface. The preliminary strain may be applied to the first
steel sheet 301 as necessary. In other words, the first steel sheet 301 has a first strain
amount (which may i~lclude0 ).
[0099]
As illustrated in FIG. 23, between one side 301a and the other side 301b of the
first steel sheet 301, a virh~asl ection 301c which divides tlie first steel sheet 301 into
two regions is set. The virtual section 301c is set perpendicularly to the surface of the
first steel sheet 301. In addition, in a case where the virtual section 301c is a
boundary, a part including the one side 301a of the first steel sheet 301 arid a part
including the otlier side 301b are respectively bound by a fixer which is schematically
illustrated. As the fixer, a chucking device which grabs and fixes the fxst steel sheet
301 can be illustrated as at1 example.
[Ol 001
Next, as illustratcd in FIG. 24, the simple shearing deformation is performed
with respect to the first steel sheet 301 by applying the shear stress to make the two
regions of the fxst steel sheet 301 divided by the virtual section 301c shifted fiom each
other in the sheet width direction along the virtual section 301c (that is, to make
relative positions of the two regions shifted be on tlie same surface).
Specifically, in a state where each of the trvo regions of the first steel sheet
301 divided by the virtual section 301c is hound by the chucking device, the clmcking
devices are moved totbe shifted fiom each other in the sheet xvidth direction along the
virtual section 301c. Accordingly, shearing deformation part 301d is formed near the
virtual section 301c. Since the shear stress is applied in the sheet width direction
along tlie virtual section 301c, tlie sheet thicktless at the sliearing dcformatioll part
301d does not change compared to that before the stress is applied. Furthermore,
there is also a case where the sheet thickness call decrease according to the material,
but the largest amount of cliatige in the sheet thickness is at least equal to or greater
than 1%. Therefore, the local necking does not occur in the test piece similar to a
short axial tension test.
However, the shape of sides 301e and 301e which connect the one side 301a
and the other side 301 b of the first steel sheet 301 to each other, are largely deformed
by applpiug the shear stress to the first steel sheet 301. Tlie shearing process may
contitlue utltil the steel sheet is fractured in the middle of applying the shear stress.
[OlOl]
[3-21 First Partial Stress-strain Curve Data Obtaining Process
In the first partial stress-strait] cut-ve data obtaining process, the shear stress
and the shear strain which are applied to the first steel sheet 301 in the above-described
first sllearing process, are measured. In addition, the first partial stress-strain curve
data is obtained from a relationship between (1) tlie shear stress applied to the first
steel sheet 301 in the first shearing process, and (2) the total strain ainoutlt which is a
sum of the shear strain amount wl~iclils applied to the first steel sheet 301 in the first
shearing process and the first strain amount.
[O 1021
[3-31 Stress-strain Curve Data Obtaining Process
In the stress-strait1 curve data obtaining process, the stress-strain curve data is
obtained based on at least the first pat-tial stress-strain curve data and the secoud partial
stress-strain cut-ve data.
Specifically, the stress-strain curve data is obtained by approximating the first
partial stress-strain curve data by the relational equation expressed by the follo\ving
equation (3).
o = K(G+~ a )"' .. . (3)
n1 = n* + l/{b(&" c)) . . . (4)
here, in equation (3), o is an equivalent stress, K (MPa) and a are material
factors of the plastic material, zP is an equivalent plastic strain, and m is as illustrated
in the above-described equation (4), and in equation (4), n* is a convergence value of a
work hardening coefficient, b is a parameter indicating the rate of convergence of the
work hardening coefficient, and c is a parameter indicating the rate of development of
the work hardening coefficient.
(0 1031
As an example, in FIG. 25, the relationship behveen the equivalent stress
(shear stress) and the equivalent plastic strain (shear strain) which are obtained from
the result of the simple shearing test of the embodiment is illustrated by white circled
marks. In this example, the preliminary strain amount of the first steel sheet 301 is 0.
[0 1041
In addition, in FIG. 25, the eqnivalent stress-equivalent plastic strain curve
data approximated by the Swift equation is also illustrated. From the cluve data
illustrated in FIG. 25, curve data 1 is obtained by approximating the equivalent stressequivalent
plastic strain curve daia by the Swift equation illustrated in the following
equation (5), from the relationship behveen the equivalent stress and the equivalent
plastic strain which are obtained by the uniaxial tension test of the related alt. In
addition, curve data 2 illustrated in FIG 25 is obtained by approximating the equivalent
stress-equivalent plastic strain cuwe data by the Swift equation illustrated in the
following equation (5) from the relationship between the equivalent stress and the
equivalctlt plastic strain which are obtaiucd by the simple shearing test method of the
embodimciit. 111 addition, it1 the uniaxial tension test of the related art, measurement
is possible from the yielding until reaching the uniform elongation, and in the simple
shearing test method, tneasurcnlcnt is possible to the strain region of a range which
exceeds the uniform elongation of the stress-strain curve data in the uniaxial tension
test of the related art. The steel supplied for the test is a steel sheet in which the
tensile strength is 1051 MPa, the yield strength is 750 MPa, and the sheet thickness is
1.6 mm.
[OI 051
D = a(cP + f3)" ... (5)
[O 1061
Here, in equation (5), IS is an equivalent stress, a and P are constants
determined for each steel sheet, cp is a plastic strain, and 11 is a work hardening
coefficient.
[0 1071
As illustrated in FIG. 25, it is ascertained that approximation accuracy of the
curve data 1 becomes lower compared to that of the curve data 2.
[0108]
The reason of the separation of the two pieces of curve data was investigated,
and a relationship between the instautaneous n value aud the plastic strain was
investigated. The result thereof is illustrated in FIG. 26. The instantaneous 11 value
is at1 instantaneous gradient in the stress-strain curve data (the cuive data 1 and the
curve data 2) illustrated in FIG. 25. Specifically, the stress-strain ci1il.e data is plotted
by both logarithmic graphs, aud is collinear approxitl~atedin each section of 0.025 of
increment of the equivalent strain, and the inclination thereof is the instat~taneousn
value. The result thereof is illustrated in FIG. 26. As illustrated in FIG. 26, in tlie
actual steel shcct, the instantaneous n value decreases until the strain amount beconles
approxiinately 0.1, and after this, is converged to approxin~ately0 .07 to 0.08.
[0 1091
In addition, the instantaneous n value obtained base on the curve data 1
decreases until the strain anlount becomes approximately 0.05, and the instantaneous n
value is converged to approximately 0.12 in the strain region in which the strain
amount exceeds 0.05. In this manner, ill the cuwe data 1, the behavior of decrease in
the instantaneous n value in the low-strain region causes substantial separation from an
actually measured value, and the instantaneous n value itself in the high-strain region is
substantially separated from the actually measured value.
[OlIO]
Meanwhile, the itistantat~eousn value obtained based on the curve data 2
decreases until the strain arnount becomes approximately 0.025, and is converged to a
constant value of 0.07 to 0.08 when the strain amount exceeds 0.025. In the curve
data 2, in the high-strain region, the separation between the instantaneous 11 value and
the actually measured value is small, but the behavior of decrease in tlie instantaneous
n value in the low-strain region causes substantial separation from the actually
measured value similar to the curve data 1.
[Olll]
As illustrated in the curve data 1 atid curve data 2 of FIGS. 25 and 26, since
ths Swift equatioii is an equation on the assumption that an n value is converged to tlie
I constant value in the lo\v-strain iegion, it is not possible to reproduce a change in the n
value of the actual steel sheet. Since then value indicating the work hardenability of
the inaterial becomes at1 itnportant factor not only in tlie strain distribution or the stress
distribution in the fortiling analysis, but also in the fortiling defect prediction of
wvrinkles or cracks, the approxitilation accuracy thereof substantially influences the
analysis result.
[0112]
Here, after close investigation, the inventors fotnid that tlie following equation
(6) excellently matches the actual stress-strain curve data. The equation is an
approximate which is obtained by analyzitlg highly accurate synthesized stress-strain
cul-ve data obtained by the evaluation method of the steel according to the abovedescribed
first embodinlent and the second embodiment.
[0113]
o = K(cP + a)"' . . . (6)
tn=n* + ll{b(c%c)} ... (7)
[0114]
Here, in equation (6), o is an equivalent stress, K(MPa) and a are material
factors of each steel sheet, E~ is an equivalent plastic strain, and m is as illustrated in
tlie above-described equation (7), and in the equation (7), n* is a convergence value of
a work hardening coefficient, b is a parameter indicating the rate of convergence of the
work hardening coefficient, and c is a parameter indicating the rate of development of
the work hardening coeff~cient. b is in a range of -5000 to 5000, and c is in a range of
0 to less than 1.
[0115]
The above-described equation (6) is obtained by investigating that the
instantaneous n value changes together with the development of the strain in the lowstrain
region and a new function of the work hardening similar to the convergence to
the constant value is created in the high-strain region.
[Oil61
First, as a form of the function, an index type from an experimental factor
which is called convergence to the constant valoe in the high-strain region, may be
etnployed, and the Swift equation which is widely used currently is used as a base.
[0117]
Next, regarding the instantaneous n value in the low-strain region, it1 order to
indicate a high value immediately after the yielding by the test result, and to indicate a
tendency of gradually or rapidly decreasitig and becoming coti\lerged together with the
developn~enot f the strain, an index part of the Swift equation is a sum of a constant
tern1 and a strain dependence tenn (a term having the equivalent plastic strain as a
denominator) which decreases together with the developmerlt of the strain.
[0118]
Coefticients of the strain dependence term are two including b indicating the
rate of convergence of the instantaneous n value, and c indicating the rate of the
developtnent of the instantaneous n value. The coefficient b is in a range of -5000 to
5000, and more preferably in a range of -1000 to 4000. A reference numeral of the
coefficient b can reproduce the behavior of the instantaneous n valoe which is often
found in a nlaterial which is excelle~litn extension in a case of being positive. In
addition, the reference numeral of the coefticient b can reproduce the behavior of the
instantaneous n value which is often found in a material which is excellent in hole
expanding properties in a case of being negative. In addition, the coefficient c is in a
range of 0 < c < 1, and more preferably in a range of 0.01 I c < 0.05. By inserting the
coefficient c thereto, a role of preventing the m value from becoming infinite in a case
wvhere the equivalent plastic strain is 0, and the calculation in the value analysis fiom
being impossible, is also achieved.
[0119]
Based 011 the above-described idea, the equation (6) measures tlie stress-strain
curve data by tlie evaluation method of the steel of the e~~~bodiniwcintht respect to
actually 40 or more types of steel sheets, and is derived fiom the measurement result,
and in a range where the plastic strain amount is fiom 0 to 1.0 or more, excellently
niatches the result with the actually measured value of tlie stress-strain curve data. In
FIG. 27, curve data which is drawn based on the equation (6) is illustrated, and in FIG.
28, cunre data indicating the relationship between the instantaneous n value and the
strain which is drawn based on the equation (6) is illustrated. It is ascertained that
both of tlie curve data excellently match the result obtained by the simple shearing test.
In addition, the test result of FIGS. 27 and 28 are the same as the measurement result
of the steel sheet in FIGS. 25 and 26.
[O 1201
Furthermore, in the embodiment, the stress-strain curve data is obtained by
employing the approximation to one piece of partial stress-strain curve data.
Ho~vevert,h e synthesized stress-strain curve data may be obtained by employing the
approximation to the plural pieces of partial stress-strain curve data obtained by the
method described in the above-described first embodiment and the second
eniboditnent.
[0121]
Next, a method of using thc synthesized strcss-stmiti curve data obtained by
the evaluation method of the steel described in tlie first embodiment to the third
embodiment of the present invention in the analysis of tlie deformation processing, will
be described based on the fourtli cnlbodi~nentto the sixth embodiment.
[0122]
(Font-th E~nbodirnent)
In the analysis of the deformation processing of the steel, a finite ele~nent
method is used. In tlie finite element method of the related art, there are manj1 cases
where a paranieter of a work hardening law, such as the Swift equation, is used.
However, the inventors made it apparent that the S\vift equation is not enlployed in the
actually measured value particularly in the high-strain region. Here, in the
embodiment, the synthesized stress-strain cuve data obtained by the above-described
evaluation method of tile steel is utilized in the finite element method.
[0123]
Specifically, for example, a coniputer provided with an analyzer which
obtains the strain distribution and the maximum strain in a case where the press
forming is performed with respect to the steel sheet by a finite element method may be
used, tlie measured value of the synthesized stress-strain curve data obtained by the
above-described evaluation method of the steel may be input to the analyzer of the
compute!; and the analyzer may be operated by the computer. The analyzer provided
in the computer is realized as each function of a central processing unit (CPU) of the
cotnputer for performing each of steps 101 to 106 illustrated in FIG. 29
[O 1241
Hereinafter, as an analyzing method of the steel deformation processing of the
embodiment, an order of analyzing tlie strain distribution when the hole expanding
processing which is one type of tlie press forming is performed with respect to the steel
sheet, by the finite element method, will be described.
[0125]
First, a process of the hole expanding processing will be described with
reference to FIG. 30. FIG. 30 is a perspective view after cutting out 114 of the entire
steel sheet, the punch, and the die. As illustrated in FIG. 30, a blank 111 (steel sheet)
provided with a hole 11 la is prepared, a punch 112 for expanding a hole is disposed
below the steel sheet 11 1, and a die 1 13 is disposed on the steel sheet I 11. In
addition, by raising the punch 112, the hole expanding processing which widens the
hole 11 1 a provided in the steel sheet 11 1 is performed.
[0 1261
A case where cracks are not generated at an edge part of the hole of the steel
sheet after the hole expanding processing, is not considered as a forming defect, and a
case where cracks are generated, is considered as a forming defect. When excessive
stress is partially concentrated with respect to the edge part of the hole, a possibility of
generation of cracks increases. Here, by the forming analysis which uses the finite
element method, the distribution of the stress of the edge part of the hole of the steel
sheet after the hole expanding processing is predicted.
[0 1271
As illustrated in FIG. 29, first, in step 101, shape data of the steel sheet which
is the analysis target is created.
[0128]
Next, in step 102, a mesh is generated by dividing the shape data created in
step 101 by the finite element. The generation of the mesh can use, for example, a
mesh generation prograni included in an analysis package or the like of a comtnercially
available finite element method. A shape of the finite element may be any of a
triangle and a quadrangle. The size of the finite element may be appropriately set in
accordance with the size, the shape, the thickness, and the boundary condition of the
steel sheet which is the analysis target.
[0 1291
Next, in step 103, the material properties and tllc boundary conditioll of the
steel sheet are set. In addition, in the step 103, the syt~tliesizeds trcss-strain curve data
obtained hy the sitnple shearing test is input.
[0130]
The material properties are the sheet thicktless or elastic ~~~oduolfu thse steel
sheet I1 1 which is the blank, and may use physical properties of the steel sheet 11 1 as
it is. For example, in a case of the steel sheet in which the tetisile strength is 600
MPa, the yield strength is 400 MPa, and the sheet thickness is 1.6 tnrn is used, each of
the physical properties is input.
[0131]
In addition, the boundary conditions are a binding position, a load position,
and a load weight of the steel sheet 11 1 when the cyli~idricahl ole expanding
processing is performed with respect to tlie steel sheet 11 1. When tlie cylindrical hole
expanding processing is performed, the binding position may be a positon at which the
steel sheet 11 1 is bound by the punch 112 and the die 113. In addition, in a case
where tlie load is applied to the steel sheet 111 by tlie cylitidrical hole expatiding
processing, the load position may be a position at which the load is transmitted to the
steel sheet 11 I.
[0132]
111 addition, the sy~ltliesizeds tress-strain curve data input in step 103 is used it1
the finite element analysis instead of the parameter of the work hardening lam: such as
the Swift equation of the related art. The synthesized stress-strain cuwe is obtained
by sequentially performing step 103-1 of performing the simple shearing test, and step
103-2 of creating the sy~ithesizeds tress-strain curve data based 0x1 tlie result of the
simple shearing test. In each of steps 103-1 and 103-2, the above-described
evaluation method of the steel tnay be perforined.
101331
Next, in step 104, the finite element analysis is performed based on the mesh,
the material properties, atid the boundary condition. For example, as analysis
software, NASTRAN manufactured by MSC Software Corporation, or
ABAQUSISTANDARD manufactured by Dassault Systemes S. A,, which is a multipurpose
purpose st~~~ctaunraley sis finite element method code of a static implicit method, cat1
be used.
101341
Next, in step 105, the obtained result it1 the finite element analysis is
extracted. In addition, in step 106, the analysis result is output as a screen to an
output device of the computer. In FIGS. 3 1A and 3 IB, an exatnple of the analysis
result is illustrated.
[0135]
In FIGS. 31A and 3 lB, a cylindrical hole expanding analysis result of the
steel sheet is illustrated. FIG. 31A is an outline view illustrating a result of
performing the analysis of the cylindrical hole expanding processitlg of the steel sheet
based on a material parameter obtained by the Swifi equatiori of the related art. In
addition, FIG. 3 1B is an outline view illustrating a result of performing the analysis of
the cyliiidrical hole expanding processing of the steel sheet based on the synthesized
stress-strain curve data obtained by tlie evaluation method of the steel accordiilg to the
first embodiment of the present invention. FIGS. 31A and 31B illustrate the strain
distribution ~vl~reenac hing a stroke amount by which cracks are generated by actually
performing tlie hole expanding processing. 111 addition, in FIG. 32, the distribution of
the maxitilutn strain amount at the edge of the hole aiter the cyliridrical hole expanding
processiug of the steel sheet, is illustrated.
[OI 361
111 FIG. 3 1 A, the maximum strain amount at a part of the edge of the hole
indicates approximately 0.88, and tlie strain amount distribution is also relatively
uoiforni. Meanwhile, in FIG. 3IB, the maximum strain amount at the part of the edge
of the hole indicates approxirnately 0.90, arid the strain locally increases.
[0137]
According to FIG. 3 IA, since the strain amount is distributed to be relatively
stnall and uniform, it is determined that a risk of generation of cracks is low.
According to FIG. 3 IB, it is determined that there is a risk of generation of cracks at a
location at which the strain is locally high. Here, the cylindrical hole expanding
processing is actnally performed by using the above-described steel sheet, and then,
cracks are generated in the vicinity of the edge of the hole, and are as illustrated in FIG.
3 1B. Therefore, it is ascertained that the evaluation method of the deformation
processing of the steel according to the embodiment can be performed as the forming
analysis with higher accuracy compared to the related art.
[0138]
In this mannel; according to the evaluation method of the steel deforn~atio~~
processing according to the embodiment, since the synthesized stress-strain culve data
is obtained k o ~tnh e shear stress and shear strait1 which are obtained by the abovedescribed
evaluatioti method of the steel, and the maxitnum strain distribution in a case
where the cylindrical hole expanding processing is performed by inputting the obtained
synthesized stress-strain cunre data to the computer and by perfomling the press
fornling with respect to tlie steel sheet, is obtained, it is possible to correctly detect the
generation of cracks of the steel sheet in the press fonning. In addition, the
evaluation mcthod of the deformation processing of the steel according to the
embodiment can also be employed in the sheet forging or the cold forging as the
deformation processing, and for example; it is possible to correctly predict the forming
load required for the processing.
[0139]
(Fifth Embodiment)
In the forming analysis of the deformation processing, the finite element
method is used. In the finite element method of the related art, there are many cases
where the parameter of the work hardening law, such as the Swift equation, is used, or
where the curve data which is approximated by extrapolating the stress-strain curve
data obtained by the simple tension test until reaching the high-strain region.
However, it is ascertained that the approxin~atec urve data or the extrapolated culve
data which are obtained by the Swift equation have a large error particularly in the
high-strain region. Here, in the embodiment, the synthesized stress-strain curve data
obtained by the above-described evaluation method of the steel is used in the finite
element method.
[0140]
Specifically, for example, the computer provided with the analyzer which
perfor~nsth e formit~ga nalysis of the steel deformation processing by the finite element
method may be used, the synthesized siress-strain cunre data obtained by the abovedescribed
evaluation method of the steel may be illput to the analyzer of the computer,
and the analyzer tnay be operated by the computer. The analyzer provided in the
i co~nputcirs realized as each function of the central processing unit (CPU) of the
computer for performing each of steps 201 to 206 illustrated in FIGS. 3 3 4 33B, and
33C. In addition,. as the fornling analysis performed by the computer, at least one of
the strain distribution, the ~naximumst rain, and the forming load oft he steel in a case
where the defor~llatiopnr ocessing is perrormed with respect to the steel, can be
obtained.
[0141]
Hereinafter, as an application example of an analyzing method of the steel
deformation processing of the embodiment, a forming analyzing method when creating
a cub-like member from a steel sheet which is circular ~vhenvi ewed in a plan view by
the sheet forging, will be described.
[0142]
First, a process of the sheet forging will be described with reference to FIGS.
33A, 33B, and 33C. One-dot chain lines of FIGS. 33A, 33B, and 33C are symmetric
axes. First, as illustrated in FIG. 33A, a steel sheet 21 1 which is punched in a circular
shape is prepared, and the steel sheet 21 1 is nipped and fixed between a cylindrical
punch 212 and a cushion 213. In addition, on the periphery of cushion 213, a die 214
having an opening into whicli the punch 212 can infiltrate is disposed.
In addition, while maintaining a state where the die 214 is fixed, the press
forming is performed by lowering the punch 212 toward the opening of the die 214,
and the steel sheet 21 1 is formed into the cup 221. In the steel sheet 21 1, the bending
processing is partially performed by the punch 212 and the die 214. The sheet
ti~icknesso ft ile bending-processed part decreases fsom the sheet thickness of'lhe
original steel sl~eet.
[0 1431
Next, as illustrated in FIG. 33B, the putlch 212 is replaced with a pad 215, the
cushion 21 3 is replaced wit11 another die 216, and fi~~tliethre, die 214 is replaced \vith
another cushion 217. In addition, the pad 215 is inserted into a cup 221, the die 216
is pressed fiotn a lowwler side of the cup 221, and fi~l-tllert,h e cushion 217 is disposed on
an outer circumferential surface of the cup 221. In addition, another punch 21 8 is
pressed against an end part 221a of the cup 221, and upsetting processing is perfomled.
[0144]
In FIG. 33C, a state after the forming is conlpleted is illustrated. A part
which is thinned by the initial press forming, is thickened by the upsetting processi~ig.
In this manner, by the sheet forging, the cup of which a bent part is thickened is
obtained.
[0145]
I-Iere, in obtaining the cup which does not have a forming defect, a balance of
the forming load of the cushion 217 and the punch 218 is important. When the
forming load of the cushion 217 is insufficient, cushion 217 is pushed to a lower part
of the die 216, and as a result, the shape of the cup 221 is collapsed. In addition,
when the fortiling load of the punch 218 is insufficient, the upsetting processing is not
performed sufficiently, and the thickness does not sufficiently increase. Here, in
order to obtain an appropriate balance of the forming load of the cushion 217 and the
punch 218, the forming analysis is performed by the finite element method.
[0 1461
In the forming analysis, as illustrated in FIG. 34, first, in step 201, the shape
data of the steel sheet which is circular when viewed in a plan view that is an analysis
target, is created.
[0 1471
Next, in step 202, a mesh is generated by dividing the shape data created in
step 201 by the finite element. The generation of the mesh can use, for example, a
tnesh generation program included in an analysis package or the like of a conunercially
available finite element nietliod. A shape of tlie finite element may be any of a
triangle and a quadrangle. The size of the finite eleliicnt may be appropriately set in
accordance with the size, the shape, the slieet thickness, and the boundary condition of
the steel sheet which is the analysis target.
[0148]
Next, in step 203, the material properties and the boundary condition of the
steel sheet are set. In addition, in the step 203, the synthesized stress-strain cunre data
obtained by the simple shearing test is input.
[0149]
The material properties are the sheet thickness or elastic modulus of the steel
sheet, and may use the physical properties of the steel sheet as it is.
[OlSO]
In addition, the boundary conditions are a binding position, a load position,
and a load weight of the steel slieet when the sheet forging is performed with respect to
the steel sheet. When the sheet forging is performed, the binding position may be a
positon at which the steel sheet is hound by tlie pad 215, the die 216, the cushion 217,
and the punch 218. In addition, in a case where the load is applied to the steel sheet
from the cushion 217 and the puncli 218, the load position may be a position at which
the load is transmitted to the steel sheet.
[OlSl]
In addition, the sy~ithesizeds tress-strain curve data input in step 203 is used in
tlie finite element analysis. The synthesized stress-strain cul-ve data is obtained by
sequentially performing the shearing process (step 203-1) of performing the si~nple
sheasing test, and the synthesized stress-strain curve data obtaining process (step 203-
2) of creating the synthesized stress-strain curve data based on the result of the sinlple
shearing test. In each of steps 203-1 and 203-2, the abovc-described c\~aluation
method of the steel may be perfor~l~ed.
[0152]
Next, in step 204, the finite element analysis is performed based on the mesh,
the material properties, and the boundary condition. For exa~llplea, s analysis
software, NASTRAN which is a multi-purpose st~~ictuarnea lysis finite eletnent
method code of a static implicit method, can be used.
[0153]
Next, in step 205, the obtained result it1 the finite element analysis is
extracted. In addition, in step 206, the analysis result is output as a screen to the
output device of the computer. In FIGS. 35A and 35B, an exat~ipleo f the analysis
result is illustrated.
[0154]
In FIGS. 35A and 35B, the analysis result in a case where the cup 221 is
formed by the sheet forging is illustrated.
FIG. 35A illustrates a result of performing the forming analysis based on the
synthesized stress-strain curve data illustrated in FIG. 22 obtained by tlie evaluation
method of the steel according to the second embodiment of tlie present invention, and
illustrates a cushion load whet1 the upsetting processing is performed, a punch load,
and an equivaiei~pt lastic strain distribution of the cup section.
Jn addition, FIG. 35B is a view illustrating a result of performing the folming
analysis based on the approximate curve data illustrated in FIG. 22 created based on
the stress-strain cunre data obtained by the uniaxial tension test, and similar to FIG.
35A, illustrates the cushio~llo ad when the upsetting processing is performed, the punch
load, and a stress distribution of the cup section.
[0155]
In FIG. 35A, it is ascertained that the punch load is 104 tons and the cushion
load is 17 tons. Mean\vhile, in FTG. 35B, the punch load is 115 tons and the cushion
load is 22 tons, and any one of the puuch load and the cushio~llo ad is a higher value
corr~paredt o that of FIG. 35A. In addition, when focusing on the strain distribution of
the cup section, it is ascertained that the region in which the equivalent plastic strain is
equal or greater than 1 is smaller in FIG. 35A than that of FIG. 350. When
perforn~ingth e steel forging based on the analysis result of FIG. 350, since the cushion
load and the punch load are excessive, there is a possibility of causing any forming
defect.
[0156]
In this manner, according to the evaluation method of the deformation
processing of the steel according to the embodiment, by inputting the measured data of
the synthesized stress-strain cullre data obtained by the above-described evaluation
method of the steel to the computer, it is possible to enhance the accuracy of the
forming analysis in a case where the deformatio~p~ro cessing is performed with respect
to the steel sheet with a high strain amount. For example, in a case where the press
forming is employed as the deformation processing, it is possible to predict the strain
distribution and the maximum strain of the steel sheet in the press forming, and to
correctly detect the generation of cracks. In addition, in a case where the sheet
forging or the cold forging is employed as the deformation processing, by obtaining the
forming load amount to the steel due to the forming tool, it is possible to predict the
forming load required for the processing.
[0157]
(Sixth Embodiment)
In the analysis of the defornlation processing, the iinite element method is
used. In the finite elenlent method of the related art, there are many cases where the
paratneter of the work hardening law, such as the Swift equation is used. However,
the ir~ventorsm ade it apparent that the stress-strain curve data until reaching the strain
region in which the uniform elongation is exceeded fiom the yield point and the
instantaneous n value cannot be reproduced by the Swift equation. Here, in the
embodiment, the forming analysis of the steel deformation processing is perfornled by
utilizing the equation (6) described in the third embodilnetit in the finite element
method.
[0158]
Specifically, for example, the computer provided with the analyzer which
obtains the n~axi~nusltnr ain distribution in a case where the press for~ningis perfornled
with respect to the steel sheet by the finite element method may be used, K(MPa), a, b,
c, and n* which are material parameters in the above-described equation (6) may be
input to the analyzer of the computer, and the analyzer may be operated by the
computcr. The analyzer provided in the computer is realized as each function of the
central processing uuit (CPU) of the computer for performing each of steps 301 to 306
illustrated in FIG. 36. In addition, in the camputel; a computer prosam which
executes a step of inputting the relational equation expressed by the equation (6) as a
relatio~~aaplp roximalion between the equivalent plastic stress and the plastic strain,
and a step of obtaining the strain distribution and the maximum strain in a case where
the defornlation processing is performed with respect to the steel sheet by the finite
element method based on the relational equation (6), in the computer, is provided.
Each step is realized by the analyzer.
[0159]
Hereinafter, as an analyzing method of the steel deforniation processing of tlie
embodiment, an order of analyzing tlic strain distribution when the liole expanding
processing which is one type of the press forming is performed with respect to the steel
sheet, by the finite element method, will be described.
[0 1601
First, a process of the liole expanding processing \\ill be described with
reference to FIG. 37. FIG. 37 is a perspective view after cutting out 114 of the entire
steel sheet, the punch, and tlie die. As illustrated in FIG. 37, a blank 3 11 (steel sheet)
provided with a hole 3 1 la is prepared, a puncli 3 12 for expatiding a hole is disposed
below the blank 3 11, and a die 3 13 is disposed on the blank 3 11. In addition, by
raising the punch 312, the hole expanding processing wlicli widens tlie hole 3 1 la
provided in the blank 3 11 is performed.
[0161]
A case wliere cracks are riot generated at an edge part of the hole of the blank
3 11 after the hole expanding processing, is not considered as a forming defect, and a
case where cracks are generated, is considered as a forming defect. When excessive
stress is pattially concentrated with respect to the edge part of the hole, a possibility of
generation of cracks increases. Here, by the forming analysis which uses the finite
element method, the distribution of the stress of the edge part of the hole of the blank
3 11 after the hole expanding processing is predicted.
[0 1621
As illustrated in FIG. 36, first, in step 301, shape data of the steel sheet (blank)
which is the analysis target is created. t
[0163]
Next, in step 302, a mesh is generated by dividing tlie shape data creatcd in
step 301 by the finite element. The generation of the mesh can use, for example, a
~ncshg eneration progratii included in an analysis package or the like of a cotinnel~cially
available finite eletnent method. A shape of the finite elenlent may be any of a
triangle and a quadrangle. The size of the finite element may be appropriately set in
accordance with the size, the shape, the thickness, arid the boundary condition of the
steel sheet which is the analysis target.
[O 1641
Next, in step 303, the nlaterial properties and the boundary condition of the
steel sheet are set. In addition, in the step 303, K(MPa), a, b, c, and n* which are
material parameters in the above-described equation (6) are input to the analyzer of the
computer. Specific parameters vary in each type of steel sheet, but for example, is
illustrated in the following table 1.
[0 1651
[Table 11
[0166]
The material properties are the sheet thickness or elastic inodulus of the steel
sheet which is the blank 31 1, and may use the physical properties of the steel sheet as it
is. For example, in a case of the steel sheet in \vhich the tensile strength is 1050 MPa,
the yield strength is 730 MPa, and the sheet thickness is 1.6 lnm is used, each of the
physical properties is input.
[0167]
h~ addition, the boundary conditions are a binding position, a load position;
and a load weight of the blank 3 11 when the cylindrical hole expanding processing is
performed with respect to the blank 3 11. When the cylindrical hole expanding
processing is performed, the binding position may be a positon at which the blank 3 11
is bound by the punch 312 and the die 3 13. In addition, in a case where the load is
applied to the blank 3 11 by the cylindrical hole expanding processing, the load position
may be a position at which the load is transmitted to the blank 31 1.
[0168]
In addition, the material parameters, such as K(MPa), a, b, c, and ti*, which
are input in step 303, are used in the finite element analysis instead of the parameters
of tie work hardening law, such as the Swift equation of the related art. Each
material parameter may be obtained in advance for each type of steel may be used. In
addition, each material parameter n~ayb e obtained from the synthesized stress-strain
cume data obtained by sequentially performing step 303-1 of performing the simple
shearing test and step 303-2 of creating the synthesized stress-strain curve data based
on the result of the simple shearing test.
[01 691
Next, in step 304, the finite element analysis is performed based on the tnesh,
the nlaterial properties, and the boundary condition. For example, as analysis
software, NASTRAN or ABAQUS which is a multi-purpose structure analysis finite
element method code of a static implicit tnethod, or LS-Dyna wllic11 is nonlinear
d~rnan~ainca lysis by an explicit method, can be used.
[0 1701
Next, in step 305, the result obtained in the finite element analysis is
extracted. In addition, in step 306, the analysis result is output as a screen to the
output device of the computer. Irl FIGS. 38A, 38B, and 39, an example of the
analysis result is illustrated.
[0171]
In FIGS. 38Aand 38B, a cylindrical hole expanding analysis result of the
steel sheet is illustrated. FIG. 38A is an outline view illustrating a result of
performing the analysis of the cylindrical hole expanding processing of the steel sheet
based on a material parameter obtained by the Swift equation of the related art. 111
addition, FIG. 38B is an outline view illustrating a result of performing the analysis of
the cylindrical hole expanding processing of the steel sheet based on the synthesized
stress-strain curve data obtained by the evaluation method of the steel according to the
third embodiment of the present invention. FIGS. 38A and 38B illustrate the strain
distribution when reaching a stroke amount by which cracks are generated by actually
perfornling the hole expanding processing. In addition, in FIG. 39, the distribution of
the maximum nlain strain amount at the edge of the hole after the cylindrical hole
expanding processing of the steel sheet, is illustrated. A developed method of FIG. 39
is the result of the forming analysis of the embodiment ~vhichc orresponds to that of
FIG 38B. and a conventional method of FIG. 39 is the result of the forming analysis of
the related art which corresponds to that to FIG. 38A. In addition, the test result of
FIG. 39 is the result obtained by actually pcrforn~ingth e hole expanding processing.
[O 1721
As illustrated in FIG. 38A and 39, in the conventional method, the tnaximu~n
strain amount at the part of the edge of the Iole is approximately 0.39, and regardless
of the angle from the rolling direction of the edge of the hole, the distribution of the
strain amount is also relatively unifonn. Therefore, in the conventional method, it is
determined that the risk of generation of cracks are low. Meanwhile, as illustrated it1
FIGS. 38B and 39, in the evaluation method of the etnbodirnent, the maxitnum strain
amount at the part of the edge of the hole is approxitnately 0.42, and the strain is
locally high. This tendency excellently matches the result obtained by actually
performing the cylindrical hole expanding processing with respect to the steel sheet,
Therefore, it is ascertained that the evaluatior~m ethod of the defor~nationp rocessing of
the steel according to the embodiment can be performed as the forming analysis with
higher accuracy compared to the related art.
[0173]
According to the evaluation method of the deformation processing of tile steel
according to the embodiment, since the maximum strain distribution in a case where
the deformation processing is performed with respect to steel is obtained by the finite
element method by using the relational equation expressed by the above-described
equatiol~(6 ) as the relational approxi~nationb etween the equivalent plastic stress and
the plastic strain, it is possible to correctly obtain the maxirnum strain distribution in a
case \~fherteh e deformation processing is performed with respect to the streel material.
For example, in a case where the press fonning of the steel sheet is employed as the
deformation processing, it is possible to correctly detect the generation of cracks of the
steel sheet in thepress fornling.
[O 1 741
Above, the present invention is described in detail based on the first
embodiment to tlie sixth embodiment, but any of the above-described embodiments is
merely a specificd example for realizing the present invention, and the technical range
or the present invention is not restrictively interpreted by the ernbodirnents. For
example, it is possible to appropriately employ the contents described in each
embodiment to other embodiments.
[0 1751
As an analyzer which uses the synthesized stress-strain curve data, an analysis
program of a comtnercially available finite element method map be used, an analyzer
in a fracture prediction method in Japanese Unexamined Patent Application, First
Publication No. 2007-232715 may be employed, an analyzer in a fracture prediction
method in Japanese Unexamined Patent Application, First Pnblicatiot~N o. 2007-
285832 may be employed, and an analyzer in a bending fixture prediction method of
a material in Japanese Unexanlined Patent Application, First Publication No. 2012-
33039 may be employed
[0 1761
In the description above, the steel (that is, the steel sheet which is the plastic
sheet) w11ich is tlie plastic material is used, but as tlie plastic material, it is possible to
use a rnetal material, such as aluminutn or titanium, a glass fiber-reinforcing resin
material, suc11 as FRP or FRTP, and further, a composite material thereof.
[0 1771
In the description above, the plural pieces of partial stress-strain curve data
are obtained wvhen the direction in which the shear stress is loaded is considered as a
constant direction in the shearing process, but as illustrated in FIG. 40, for example, tlie
synthesized stress-strain curve data tnay be obtained by reversing the application
tlirection of the shear stress ill the middle of the first shearing process. In this case,
since it is possible to obtain the synthesized stress-strain curve data when tlie inverse
load is applied, it is possible to evaluate a Bauschinger effect which is a phetiomenon
of deterioration of the yield stress.
111 addition, as illustrated in FIG. 41, for exalnple, the synthesized stress-strain
culve data may be obtained by reversing tlie applicatiorl direction of tlie shear stress
after the third shearing process. In this case, since it is also possible to obtain the
synthesized stress-strain curve data when the inverse load is applied, it is possible to
evaluate a Bauschinger effect which is a phenomenoti of deterioration of the yield
stress. In particular, since it is possible to reverse the load direction at the time when
a desirable strain amount is loaded, it is possible to obtain the syntllesized stress-strain
curve data having higher practicability.
Furthernlore, as illustrated in FIG. 41 and 42, in a case where the plural pieces
of partial stress-strain curve data are obtained by reversing the application direction of
the shear stress, by approximating the partial stress-strain curve data based on the
kinematic hardening law, such as a Lemaitre-Chaboche model or a Yoshida-Uemori
model, it is possible to obtain a wider range of synthesized stress-strain curve data.
[Industrial Applicability]
[0178]
According to the present invention, it is possible to provide an evaluatioli
method of a plastic material which can obtain highly accurate stress-strain curve data
ut~tirle aching a strait1 region which exceeds uniform elongation, and an evaluation :
method of defor~natiorpi rocessing of a plastic material \\~Iiichc ar1 perform forming
analysis of deformation processilig with high accuracy based on the stress-strain cut-ve
data obtained by the evaluation method of the plastic material.
[Brief Description of the Reference Sy~nbols]
[0179]
101,201,301 FIRST STEEL SHEET (FIRST PLASTIC SHEET)
101a,201a,301a ONESIDE
101 b, 201 b, 301 b THE OTI-IER SIDE
101c, 201c, 301c VIRTUAL SECTION
IOld, 201d, 301d SHEARING DEFORMATION PART
IOle, 201e, 301e SIDE (OUTER FORM PART)
102,202 SECOND STEEL SI-IEET (SECOND PLASTIC

C1AIMS
1. An evaluation method of a plastic material comprising:
a first shearing process of performing simple shearing deforrnatio~wl ith
respect to a first plastic sheet by dividing the first plastic sheet having a first strain
amount which includes 0 into two regions by a virh~asle ction perpendicular to a
surface thereof, and by applying a shear stress to the first plastic sheet so as to make
relative positions of the two regions shifted along the virtual section be on the same
surface;
a secoud shearing process of perfornling simple shearing deformation with
respect to a second plastic sheet by dividing the second plastic sheet having a second
strain amount which is different from the first strain amount and includes 0 into two
regions by a virtual sectiot~p erpendicular to a surface thereof, and by applying a shear
stress to the second plastic sheet so as to make relative positions of the two regions
shifted along the virtual section be on the same surface;
a first partial stress-strain cuwe data obtaining process of obtaining first
partial stress-strain curve data from a relationship between the shear stress applied to
the first plastic sheet in the first shearing process, and a total strain amount which is a
sutn of a shear strain amount which is applied to the first plastic sheet in the first
shearing process and the first strain amount, by measuring the shear stress and the
shear strain which are applied to the first plastic sheet in the first shearing process;
a second partial stress-strain curve data obtaining process of obtaining second
partial stress-strain curve data from a relationsl~ipb etween the shear stress applied to
the second plastic sheet in the second shearing process, and a total strain amount which
is a sutn of a shear strain amount which is applied to the second plastic sheet in the
second shearing process and the second strain amount, by measuring the shear strcss
and the shear strain which are applied to the second plastic sheet in the second shearing
process; and
a synthesized stress-strain curve data obtaining process of obtaining
synthesized stress-strain curve data based on the first partial stress-strain curve data
and the second partial stress-strain curve data.
2. The evaloation method of a plastic material according to Claim 1, further
comprising:
an outer form removing process of obtaining the second plastic slieet by
removing an outer form part of the first plastic sheet after unloading the shear stress
applied in the first shearing process.
3. The evaluation method of a plastic material according to Claim 2,
wherein, in the outer form removing process, the outer form part is removed
across the two regions of the first plastic slieet along the surface direction
perpendicularly intersecting the virtual section and a flat surface of the first plastic
sheet.
4. The evaluation method of a plastic material according to Claim 1,
wherein the first plastic sheet aud the second plastic sheet are individual
plastic sl~eetsd ifferent from each other.
5. : The evaluation method of a plastic material according to Claim 4,
wherein the second strain anlount is greater than the first strain amount, and is
equal to or less than the strain amount applied to the first plastic material in the first
shearing process.
6. The evaluation method of a plastic material accorditig to Claitil4,
wherein, in tlie synthesized stress-strain culve data obtaining process, the
sy~ithesizeds tress-strain curve data is obtained by combining the curve data of a strain
region other than a pal? which receives the influence of a cross-over effect from the
first partial stress-strain cu~vdea ta and the second partial stress-strain curve data.
7. The evaluation method of a plastic material accordirig to Claim 4, further
comprising:
an outer form removing process of obtaining a third plastic sheet having a
tliird strain amount different from the first strain amount and the second strain amount
by removing an outer form part formed by the simple shearing deforniation in the first
plastic sheet;
a third shearing process of performi~igs imple shearing deformation with
respect to the third plastic sheet by dividing the third plastic sheet into two regions by a
virtual section perpendicular to the surface thereof, and by applying a shear stress to
tlie third plastic sheet so as to make relative positions of the two regions shifted along
the virh~asl ection be on the same surface; and
a tliird partial stress-strain culve data obtaining process of obtaining third
partial stress-strain cunre data from a relationship between the shear stress applied to
the tliird plastic sheet in the tliird shearing process, and a total strain amount which is a
sum of a shear strain amount which is applied to the third plastic sheet in the third
shearing process and the third strain amouut, by measuring the shear stress and the
shear strain which are applied to the third plastic sheet in the third shearing process,
wlierein, in the synthesized stress-strain curve data obtaining process, the
synthesized stress-strain curve data is obtained based on the first partial stress-strain
curve data, the second partial stress-strain curve data, and the third partial stress-strain
cul-ve data.
8. The evaluation method of a plastic material according to Claim 1,
wherein, in the synthesized stress-strain curve data obtaining process, the
synthesized stress-strait1 cul-ve data is obtained by approximating the first partial stressstrain
curve data and the second partial stress-strain curve data based on a \vork
hardening la\v.
9. The evaluation method of a plastic material according to Claitn 1,
wherein, in the synthesized stress-strain cullre data obtaining process, the
synthesized stress-strain curve data is obtained by approximating the first partial stressstrain
curve data and the second partial stress-strain curve data by a relational equation
expressed by the following equation (1).
o=K(cP+a)"' ... (1)
In = n* + l/{b(cP + c)) . . . (2)
here, in equation (I), (3. is an equivalent stress, K (MPa) and a are material
factors of the plastic material, &!'is an equivalent plastic strain, and m is as illustrated
in the above-described equation (2), and in equatiot~(2 ), n* is a convergence value of a
work hardening coefficient, b is a parameter indicating the rate of convergence of the
work hardening coefficient, and c is a parameter indicating the rate of development of
the work hardening coefficient.
10. Thc evaluation method of a plastic material according to'clai~n1 ,
wherein the application direction of the shear stress in the first shearing
process and the application direction of the shear stress in the second shearing process
are opposite to each other.
11. The evaluation method of a plastic material according to Claim 1,
wl~ereini,n the first shearing process, the application direction of the sheat
stress is reversed in the middle.
12. The evaluation method of a plastic material according to Claitn 10 or 11,
wherein, in the synthesized stress-strain curve data obtaining process, the
synthesized stress-strain curve data is obtained by approximating the first partial stressstrain
curve data and the second partial stress-strain cunle data based on a kinematic
hardening law.
13. The evaluation method of a plastic material according to Claim 1,
wherein the first plastic sheet and the second plastic sheet have a shape of a
rectangular flat surface.
14. The e\,aluation method of a plastic material according to Claim 1,
\vherein, in the first shearing process and the second shearing process, the
largest amount of change in the sheet thickness of the first plastic sheet and the second
1 plastic sheet is equal to or less than 1% of the sheet thickness.
15. The evaluation method of a plastic material according to Claim 1,
wherein the shear strain applied in each of the first shearing process and the
second shearing process, is in a range of 0.4 to 1.2.
16. The evaluation tnetliod of a plastic material according to Claim 1,
wherein the first plastic sheet and the second plastic sheet are steel sheets,
17. An evaluation method of deformation processing of a plastic material,
wherein a conlputer provided with an analyzer which performs forming
analysis of deformation processing of the plastic material by a finite element method is
used,
wherein the synthesized stress-strain curve data obtained by the evaluation
method of the plastic material according to any one of Claims 1 to 16 is input to the
analyzer of the computer, and
wherein the analyzer is operated by the computer.
18. The evaluation method of deforniation processing of a plastic material
according to Claim 17,
wherein the forming analysis obtains at least one of a strain distribution, the
nlaximum strain, and a fortning load of the plastic material in a case where the
deformatioli processing is performed with respect to the plastic material.
19. An evaluation method of a plastic material comprising:
a first shearing process of perforniing simple shearing deformation with
respect to a first plastic sheet by dividing the first plastic sheet having a first strain
alnount which includes 0 into two regions by a virtual section perpendicular to a
surface thereof, and by applying shear strcss to the first plastic sheet so as to make
relative positions of the two regions shifted along the virtual section be on the san~e
surface;
a first partial stress-strain curve data obtaining process of obtaini~igfi rst
partial stress-strain cnrve data fro111 a rclatio~ishipb etween the shear stress applied to
the first plastic sheet in the first shearing process, and a total strain amount \vhicti is a
suin of a shear strain amount wliich is applied to the first plastic sheet in the first
shearing process and the first strain amount, by ~neasuringth e shear stress and the
shear strain which are applied to the first plastic sheet in the first shearing process; and
a synthesized stress-strain curve data obtaining process of obtaining
synthesized stress-strain curve data by approximating the first partial stress-strain
curve data by a relational equation expressed by the following equation (3).
o = K(eP + a)"' . . . (3)
m = n* + l/{b(~' + c)} .. . (4)
here, in equation (3), o is an equivalent stress, K ( m a ) and a are material
factors of the plastic material, E~ is at1 equivalent plastic strain, and m is as illustrated
in the above-described equation (4), and in equation (4), n* is a convergence value of a
work hardening coefficient, b is a parameter indicating the ratc of convergence of the
work hardening coefficient, and c is a parameter indicating the ratc of development of
the work harclening coefficient.
20. An evaluation method of deformation processing of a plastic material,
wherein a computer provided with an analyzer which performs forming
analysis of deformation processing of the plastic nlaterial hy a finite element method is
used,
wherein the synthesized stress-strain curve data obtained by the evaluation
method of the plastic material according to Claitn 19 is input to the atlalyzer of the
cornp~lter,a nd
wherein the analyzer is operated by the computer.
21. The evaluation method of defortnation processitlg of a plastic material
according to Claitn 20,
wvl~ereinth e forming analysis obtains at least one of a strain distribution, the
maxitnutn strain, and a fomiing load of the plastic material in a case where the
dcforinatiol~p rocessing is pwfortned with respect to the plastic material.