FIELD OF INVENTION
The present invention relates to a method of fatigue testing thin ductile steel sheets of
less than 2mm thickness under fully reversed stress and strain amplitudes. More
particularly, the invention relates to optimized testing methodology that eliminates
bending and buckling of the sheet specimens during testing under fully reversed loading
condition.
BACKGROUND OF THE INVENTION
Low carbon and ultra low carbon ductile steel sheets of less than 2mm thickness are
increasingly being used for the fabrication of automobile components like roof, door,
front and side panels. These components experience cyclic loading in service and these
are to be adequately designed against fatigue failure. At the design stage, certain basic
fatigue data are required like (a) Stress-life curves and (b) Strain-life curves. Generation
of these data involves two basic types of fatigue tests e.g. (i) load or stress controlled
high cycle fatigue test (covered by ASTM E466) and (ii) strain controlled or low cycle
fatigue test (covered by ASTM E606). ASTM stands for American Standard Test Method
and ASTM is a leading international standard that is widely followed. In case of stress
controlled test, the stress is comparatively low (below the yield stress of the material),
strain is elastic in nature and the number of cycles to failure, N, is high (N ~ 106 to 107
cycles); hence the name high cycle fatigue. During these tests, stress is elastic but
metal undergo localized plastic deformation in regions where inhomogenities and
defects exist. The spread of plastic deformation lead to failure. The strain controlled
fatigue test is conducted with controlled cycles of elastic and plastic strain. Here, strain
is mostly plastic in nature; stress is higher and is above the yield stress. The number of
cycles to failure is low (N < 104 to 105 cycles), hence it is called low cycle fatigue. In
both types of tests, specimens of thin ductile sheets are generally subjected to fully
reversed (equal amount of tensile and compressive) stress and strain amplitudes.
But, imparting fully reversed stress or strain amplitudes to thin, ductile sheets of less
than 2 mm thickness is difficult. This is because of heavy buckling of the sheet
specimens under compressive loading and/or under plastic straining. Information on
this aspect is extremely limited in open literature or in national or international
standards. Few research groups in the world working in this field are not keen to share
the detailed procedures. The testing procedure is, therefore, to be optimized by the
researchers in terms of specimen configuration, surface preparation, loading rate etc.
so as to eliminate various problems encountered like bending and buckling of the
specimens during testing, handling, storage. In India, no research groups are known to
work in this particular field. But, Tata Steel is pioneer in the production of soft and
highly formable steel sheets like Interstitial-free (IF) steel sheet. In Interstitial-free
steels the amount of interstitials (impurities like carbon, nitrogen remain less than 100
parts per million) are extremely low in amount. Tata Steel is in the process of
developing a number of high strength IF steel grades through special alloying addition

which have higher strength combined with good formability. But, assessment of their
fatigue properties is extremely important for this development and to decide upon their
potential use. This invention was necessary to cater to the above need.
Problems of thin sheet fatigue testing
Buckling: The most prevalent problem encountered during fatigue testing is low
buckling resistance of thin sheets. It can be easily visualized how a thin soft sheet will
buckle under compressive loading. Photograph of a specimen buckled during testing is
shown in Fig.3. The buckling resistance of a specimen decreases with reduction in sheet
thickness. Occurrence of buckling causes early fatigue failure and makes the test
invalid. For a given thickness, the buckling resistance can be increased by decreasing
the effective buckling length or free span of the specimen. Free-span is that portion of
the specimen which lies outside the grips of the machine as shown in Fig.4. This is the
most important consideration while deciding the configuration of the sheet specimens
for using in the fatigue tests.
Requirement of high stress: The second most common problem associated with fatigue
tests of thin ductile sheets is requirement of high stress to induce fatigue failure in
sheet specimens. In order to estimate the fatigue life of the sheet specimens,
specimens must fail during the fatigue tests. It was revealed during some of the initial
studies that the specimens of low and ultra low carbon steel sheets exhibit a unique
behaviour under fatigue loading. These specimens do not fail at an applied stress level
below the yield stress. These are clean steel and the normal material
defects/inhomogentities in these steels are not sufficient to cause localized plastic

deformation leading to failure. Hence, fatigue testing of thin sheet specimens requires
high stress in the plastic strain range. But a high and/or plastic strain range, risk of
buckling increases.
Scatter in test results: It is evident that there are two major difficulties of working with
thin ductile steel sheets of low and ultra low carbon varieties. First, the sheet specimens
buckle under compressive and/or plastic straining and second, the requirement of very
high stress for these specimens to fail. At high stress, risk of buckling increases further.
Buckling causes premature failure of specimen leading to scatter in the test results.
Furthermore, thin sheets have large surface area and fatigue failure always starts from
surface. Hence, presence of scratch marks, dent marks, improper machining and
environmental degradation can make the specimen fail early and increase scatter in the
test results. Both buckling and presence of surface defects result into considerable
amount of scatter in the generated fatigue data on thin sheets. Sometimes, the
specimen undergoes slippage while fixed in the anti-buckling fixture during strain-
controlled tests. It is a very frequent problem. During slippage, extensometer losses
contact with specimen and hence can not measure strain and specimen fails early
increasing scatter in test results.
Lack of well-documented technique: The risk of buckling can be eliminated by selection
of proper configuration and dimensions of specimens. Scatter in test results can be
eliminated by choosing proper fabrication, testing and handling techniques. But there is
lack of well documented testing methodologies in the open literature and in the national
or international standards to generate reliable test results on sheet specimens using

standard testing machines and instruments. For example the relates ASTM standards
(ASTM E646 and ASTM E606) only mentions a few experimental steps and precautions
to be considered during testing of sheets upto 2.54 mm thickness sheets and does not
provide any guidelines for sheets of thickness lesser than 2.54 mm.
OBJECTS OF THE INVENTION
Therefore it is an object of the invention to propose a method of fatigue testing thin
ductile steel sheets of less than 2 mm thickness under fully reversed stress and strain
amplitudes using specimens of new design which is capable of generating data for
stress-life curves and strain-life curves.
Another object of the invention is to propose a method of fatigue testing thin ductile
steel sheets of less than 2 mm thickness under fully reversed stress and strain
amplitudes which can eliminate problems of bending and buckling of the sheet
specimens under fully revered stress and strain amplitudes during testing with and
without a commercial anti-buckling fixture.
A further object of the invention is to propose a method of fatigue testing thin ductile
steel sheets of less than 2 mm thickness under fully reversed stress and strain
amplitudes which can eliminate overheating of the specimen during testing at high
strain by selecting optimum frequency of loading.

A still further object of the invention is to propose a method of fatigue testing thin
ductile steel sheets of less than 2 mm thickness under fully reversed stress and strain
amplitudes which can reduce scatter in test results by (i) introducing methods of
fabrication, surface-polishing, handing and storage to minimize surface defects (ii)
preventing premature failure at the grips for holding the specimen for testing and (iii)
preventing slippage of the specimen leading to premature failure.
SUMMARY OF THE INVENTION
This invention provides a methodology for carrying out axial stress and strain controlled
fatigue tests using specimen of thin and ductile steel sheets. These sheets are prone to
severe buckling under compressive and/or high plastic strain. The developed
methodology is suitable for conducting tests using commercially available standard
servo-hydraulic testing machines and instruments like dynamic extensometer. In
addition, guidelines for reducing scatter in the test results by proper methodology of
sample fabrication, handling, storage and testing of sheets specimens are provided. The
important objects of the invention are,
1. Configuration of specimen: The inventor relates to the establishment of most
optimum specimen configuration for thin ductile sheets. It is recommended that
for carrying out stress controlled fatigue tests under fully reversed loading hour-

glass configuration is to be used to minimize buckling. For strain-controlled tests,
dumbbell-configuration has to be used.
2. Dimensions of the specimens: For stress controlled tests, hourglass specimens of
110-120 mm total length (minimum) with radius of curvature as large as feasible
(preferably between 50-100 mm) are suggested. The minimum width in the
middle of the hourglass shaped test section is to be decided based on the
dimensions of anti-buckling fixture used. For standard fixture, width can be kept
at 8 mm. The drawing in Fig.l can be referred. For strain controlled tests,
parallel length of uniform cross section should be 2 to 3.5 mm more than the
gauge length of the extensometer used for its easy travel over the test section
and accurate measurement of strain. The drawing in Fig. 10 can be referred. The
grip for both types of specimens should be minimum 20 mm wide and 30 mm
long.
3. Reduction of scatter in test data: Specimens are to be machined using fresh
tools, undercuts, scratch and dent-marks are to be avoided. Surfaces of the
specimens are to be ground and polished mechanically applying only moderate
pressure. Standard metallographic technique is to be used. Freshly polished
samples are to be preserved in desiccators to minimize any environmental
degradation of the surface. The handling during testing is to be done carefully in
order to avoid any scratching and denting which can bring down the fatigue life.
The fixing of the anti-buckling fixture on the specimen surface prior to each test
is to be done manually. Check-nut in addition to nuts should be used while fixing

the specimen in anti-buckling fixture to avoid slippage of the specimen while
testing. Slippage of specimen makes a test invalid.
4. Conducting fatigue tests without anti-buckling fixtures: When anti-buckling
fixtures are not used, tests can be carried out using specimens of lower gauge-
length by bonding two/three/four specimens together with the help of
commercial adhesive recommended for metallic materials. Typical configuration
of a specimen is shown in Fig.2.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.1(a)- Specimen of hourglass configuration
Fig.1(b) - Specimen of dumbbell configuration
Fig.2 - Specimen (thickness 0.7-1.6 mm) used for fatigue testing under fully
reversed strain amplitudes
Fig.3 - (a) A sheet specimen with anti-buckling fixture over it and (b) a buckled
specimen
Fig.4 - A sheet specimen gripped in the machine
Fig.5 - A typical configuration of sheet specimens used for (a) the stress controlled
and the (b) strain controlled fatigue tests
Fig.6 - schematic diagram of a anti-buckling fixture
Fig.7 - S-N curves of (a) IF and (b) IFHS steel sheets
Fig.8 - Strain-life curves of IF and IFHS steel sheets (a) Basquinn plot and (b)
Coffin-manson plot
Fig.9 - Strain-life curve generated by testing 0.7 mm thick sheet specimens without
the use of commercial anti-buckling fixtures. Specimens were tested by bonding 3-5
specimens together with adhesive
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
This invention is generally related to development of a novel fatigue testing
methodology for thin and ductile steel sheets of less than 2 mm thickness. Fatigue
testing of thin steel sheets requires application of very high tensile and compressive
stress (close to and/or above the yield strength of steel sheet) to the sheet
specimens. Thin sheets buckle under high stress and/or compressive stress and are
difficult to test using standard testing machines and instruments. Information on this
aspect is limited in open literature. In addition, estimated fatigue data of thin sheets
exhibit scatter due to presence of surface-defects. The new fatigue testing
methodology is developed to estimate accurate and reproducible fatigue data on
thin steel sheets by eliminating buckling and other problems.
In this invention, the first aim is to eliminate buckling of specimens during fatigue
tests by optimization of specimen configuration through suitable trials. For trials one

of the softest thin sheets e.g. 1.2 mm Interstitial-free and Interstitial-free high
strength steel sheet was selected. The chemical composition and mechanical
properties of the selected IF and IFHS steel sheets are shown in Table 1 and 2
respectively. Different specimen configurations were tried out using these thin
sheets.

Step 1: Conducting trials using guidelines of ASTM standard: The first step in the
invention was to optimize the specimen design using selected 1.2 mm thick steel
sheets. ASTM E606 provides some guidelines on dimensions of specimens for sheets of
a particular thickness say 'T'. According to this standard, the gauge length (L), radius of
curvature between grip and test section (R), width of grip (W) and length of grip (A) is
to be calculated as follows:
L = 3T ± T/2
R = 2T ± T/2
W = 4T ± T/2
A = 10T ± 2T
The dimensions of these samples were very small for T = 1.2 mm and could not be
conveniently gripped in standard servo-hydraulic testing machine. Moreover, it was not
feasible to carry out tests with specimens of gauge length as small as 3 mm as the
commercially available and more reliable extensometers generally have gauge length of
10 mm and above. Hence, it was not possible to use ASTM guidelines. It was realized
that a number of factors are to be considered while finalizing a particular specimen
design e.g. capability of the testing machine, convenience of gripping the specimen
within the machine-grips and available gauge length of the dynamic extensometer used.
Step 2: Selection of specimen configuration: The second step was to use longer
specimens that are convenient to grip in standard machines and then to finalize
specimen configuration for two different types of fatigue tests e.g. stress and strain
controlled tests. Longer specimens with total length of 130 mm were fabricated and
stress controlled fatigue tests were carried out using them. Specimens of two different
configurations were machined (a) dumbbell shaped specimens (with tangentially bend
fillets between the uniform test section and the ends) and (b) hour-glass shaped
specimens with continuous radius between ends. The dumbbell shaped specimen was
of gauge length 25 mm. These specimens were tested at various values of cyclic stress
amplitudes starting from 0.5 times the yield strength (YS) to 0.7 times YS. It was found
that specimens with uniform test section (dumbbell shaped) were more prone to
buckling than the hour-glass specimens. Hence, it was inferred that for stress controlled
tests using thin sheet specimen, hour-glass configuration is best as the risk of buckling
is considerably reduced.
For the strain controlled tests, however, it was not feasible to use hour-glass specimens
to reduce buckling. This is because in case of strain controlled tests some parallel
length of uniform cross section at the middle of the specimen is mandatory to serve as
gauge length for the extensometer. The strain is measured and recorded over the
gauge length and is essential for proper running of tests and generation of accurate
data. Hence dumbbell-shaped specimens are recommended for strain-controlled tests.
Step 3: Optimizing dimensions of specimens: Having established the specimen
configurations, exact dimensions of (a) hour-glass shaped specimens for stress-
controlled tests and (b) dumbbell-shaped specimens for strain-controlled tests were
optimized through suitable trials.
Hour-glass specimen for stress-controlled test: Specimens of 50, 100 and 150 mm total
length were fabricated and tested. The specimen length of minimum 100 mm was
found to be essential for proper gripping. Longer the specimen more is the risk of
buckling. Hence total length of 100 to 120 mm is optimum. Next, the grip portion length
was varied between 20, 25 and 30 mm keeping the width of grip as 20 mm. It was
found that longer grip is better for gripping and for avoiding slippage. The minimum
width in the centre of the specimen was varied between 8, 10, 12 mm. It was found
that if the minimum width in the middle of the curved or hour-glass shaped test section
was maintained at 8 mm the anti-buckling fixture could be easily accommodated on the
specimen surface during fatigue tests. A schematic drawing of commercial anti-buckling
fixture is shown in Fig.5. Radius of curvature was varied between 50, 70 and 100 mm.
within this range, no significant effect of radius of curvature was noticed on buckling.
However, to minimize stress gradient, radius of curvature should be maintained as large
as feasible. The final drawing of specimen for stress-controlled test is shown in Fig.1
(a).
Dumbbell-shaped specimens (uniform rectangular cross section at the middle) was
considered for strain-controlled tests. In order to minimize buckling, parallel gauge
length in the middle of the specimen was maintained marginally (15-16)mm more than
the specified gauge length of the extensometer for smooth travel of the extensometer
and for precise recording of displacement. The dynamic extensometer used in this
investigation was having specified gauge length of 12.5 mm. hence, specimens of
parallel gauge length of 13, 15 and 18 mm was fabricated. It was found that parallel
gauge length of minimum 15 mm was to be maintained for proper recording of strain.
The width of the gauge-section was fixed at 10 mm (2 mm more than that for stress-
controlled test) to accommodate both anti-buckling fixture as well as the extensometer
conveniently. The length of the grip portion at the two ends was maintained at 30 mm
so as to enable good gripping of the specimen in the testing machine. The final drawing
of specimen for strain controlled test is shown in Fig.1 (b).
Step 4: Preparation and protection of sheet specimens: Thin sheets have large surface
area and fatigue failure always starts from surface. Hence, presence of scratch marks,

dent marks, improper machining, and environmental degradation can increase scatter in
the test results. It was found that apart from choosing suitable configuration and
dimension of sheet specimens, special care should be taken during fabrication, surface
preparation, handling and testing of the specimen to adhere to desired dimensions, to
eliminate environmental degradation, introduction of scratch marks and dents on the
surface to reduce scatter in the generated test-results. During fabrication of the
specimens from the selected steel sheets, care should be taken to adhere to the desired
dimensions; undercut, scratch and dent-marks are to be avoided. The contour of the
specimens is to be milled using fresh tools. It was found that electro-polishing is not
suitable for sheet specimens as it creates undulations on the surface which can cause
early failure during fatigue-tests. Slow speed mechanical polishing is found to be best
for good polishing. The surfaces of the fabricated specimens were ground with 400 grit
emery paper followed by polishing successively with 6, 3, 1 and finally 0.25 micron
diamond suspension. During grinding and polishing, care was taken to apply moderate
pressure at all stages to avoid bending/buckling of the sheet specimens. Freshly
polished samples were carefully wrapped in paper towels and were preserved in well-
sealed desiccators to minimize any environmental deterioration of the surface.
Desiccators are glass containers with tight leads where samples are kept in the
presence of moisture absorbing substances like calcium-chloride. Freshly polished
samples were used for testing. The handling of the specimens even during
measurement of dimensions was done carefully in order to avoid any scratching and
denting which can bring down the fatigue life. The fixing of the anti-buckling fixture on

the specimen surface prior to each test is to be done manually avoiding any excessive
pressure while tightening. Because, the anti-buckling fixtures are known to induce
increased resistance to axial load. It is recommended that while fixing the specimen in
the anti-buckling fixture, check-nuts should be used in addition to nuts so that
specimen slippage while testing or during application of load is avoided. The slippage of
specimen while testing breaks the contact between extensometer and the specimen and
test is either stopped or error is introduced in the results. It is a very critical problem to
be eliminated.
Step 5: Fatigue testing and reporting of test results: The specimens for stress and
strain controlled tests were fabricated from the selected IF and IFHS steel sheet of 1.2
mm thickness following the design as described above. The longitudinal axis of the
specimen was parallel to the rolling direction of the sheet. Freshly polished specimens
were used in the tests. All the fatigue tests were carried out using standard servo-
hydraulic fatigue testing machine (Instron model: 8801). The machine is equipped with
special hydraulic wedge-type grips for testing thin sheets of 0.2 to 2 mm thickness. The
guide-line provided by the machine supplier was followed for deciding the gripping load
of the specimen of particular thickness used. The alignment of the machine is checked
at regular frequency during calibration using standard specimen fitted with strain-
gauges at various locations to detect any bending strain. It was possible to conduct
both stress and strain controlled tests using the specimens of developed design. The
results generated are reported in graphical form. The applied stress or strain amplitude
is plotted against the fatigue-life of the specimen (number of cycles to failure at a

particular applied stress or strain). Thus stress-life and strain-life curve are generated.
The stress-life and strain-life curves of two types of selected steel sheets were
generated and are shown in Fig.7 and in Fig.8 respectively.
Step 6: Conducting fatigue tests without anti-buckling fixtures: When commercial anti-
buckling fixtures are not used, tests can be carried out by bonding two/three/four
specimens together with the help of commercial adhesive recommended for metallic
materials. This method of (adhesive) bonding of multiple sheet specimens yield similar
results as using single specimen in conjunction with an anti-buckling fixture. The
configuration of the specimens were finalized in such a way that (a) parallel gauge
length (GL) is minimum to reduce the tendency of buckling during testing and (b) the
radius of the connecting portion between GL and grip is as large as feasible (60-100
mm) to minimize stress gradient. Typical configuration of a specimen is shown in Fig.2.
Some of the generated results are shown in Fig.9.

WE CLAIM
1. A method of fatigue testing thin ductile steel sheets of less than 2 mm thickness
under fully reversed stress and strain amplitudes comprising:
machining the specimen (S) to produce hour-glass (A) configuration of the
specimen with established dimensions to eliminate buckling of specimen during
stress controlled fatigue tests;
machining the specimen to produce dumbbell configuration (B) with established
dimensions to eliminate buckling of specimen (S) during strain controlled fatigue
tests;
arranging longer specimen to facilitate gripping and to avoid slippage;
providing a dynamic extensometer to record the displacement;
polishing specimen mechanically at slow speed applying moderate pressure to
avoid buckling of the specimen;
grinding the specimens with emery paper applying moderate pressure to avoid
buckling of the specimen;
polishing the specimen further with diamond suspension applying moderate
pressure to avoid buckling of the specimen;
wrapping up the polished specimen in paper towels;
preserving the specimens in well-sealed desiccators to minimize any
environmental deterioration of the surface;

characterized in that a anti-buckling fixture (F) is fixed on the surface of the
specimen (S) and the specimen is fixed under the grip of the machine (M)
wherein checknuts are tightened over nuts of the buckling fixture (F) to avoid
slippage of the specimen.
2. A method of fatigue testing as claimed in claim 1, wherein specimen (S) for
testing is of 1.2 mm thickness.
3. A method of fatigue testing as claimed in claim 1 and 2, wherein the fixing of the
anti-buckling fixture on the specimen surface prior to each test is done manually.
4. A method of fatigue testing as claimed in claim 1 to 3, wherein in the dumbbell
shaped configuration, parallel gauge length in the middle of the specimen is
maintained at 15 mm.
5. A method of fatigue testing as claimed in claim 1 to 4, wherein the dynamic
extensometer is having specified gauge length of 12.5 mm.
6. A method of fatigue testing as claimed in claim 1 to 5, wherein the width of the
gauge section is fixed at 10 mm.

7. A method of fatigue testing as claimed in claim 1 to 6, wherein the surfaces of
the specimen are ground with 400 grit emery paper followed by polishing with 6,
3, 1 and finally 0.25 micron diamond suspension.
8. A method of fatigue testing as claimed in claim 1 to 7, wherein freshly polished
samples are carefully wrapped in paper towels and preserved in well-sealed
desiccators to minimize any environmental deterioration of the surface.
9. A method of fatigue testing as claimed in claim 1 to 8, wherein fatigue testing
are carried out with freshly polished samples.

The invention relates to a method of fatigue testing thin ductile steel sheets of less than
2 mm thickness under fully reversed stress and strain amplitudes comprises machining
of the specimen (S) to produce hour-glass (A) during stress controlled or dumbbell
configuration (B) of the specimen (S) during strain controlled with established
dimensions to eliminate buckling of specimen, arranging longer specimen to facilitate
gripping and to avoid slippage and then polishing the specimen mechanically at slow
speed, grinding the specimens with emery paper and polishing them with diamond
suspension applying moderate pressure to avoid buckling of the specimen. The polished
specimen are wrapped up in paper towels and preserved in well-sealed desiccators to
minimize any environmental deterioration of the surface. A dynamic extensometer is
provided to record the displacement. An anti-buckling fixture is fixed on the surface of
the specimen and the specimen is fixed under the grip of the machine. For fixing the
specimen in the anti-buckling fixture, check nuts are provided in addition to nuts to
avoid slippage while testing.