FIELD OF THE INVENTION
The present invention relates to a tensile sample to undergo tensile deformation
in an in-situ tensile testing stage under a scanning electron microscope (SEM).
More particularly, the invention relates to propose a tensile specimen for in-situ
tensile testing in a scanning electron microscope, which is constructed with a
miniature notch to maintain minimum variation of triaxiality in the centre
corresponding to increase in strain.
BACKGROUND OF THE INVENTION
In the automotive industry there is an increasing demand for lighter vehicles with
reduced fuel consumption. This has necessitated replacement of conventional
low strength steels with higher strength steels so that thinner sections can be
used and reduction in the weight of the auto components is possible without
compromising safety and functional requirements. Advanced high strength steels
(AHSS) offer attractive combinations of high strength and formability at
affordable costs as a result of which AHSS are being considered as the promising
materials for autobody applications.
AHSS are characterized by their multiphase microstructure. The complexity of the
microstructure and vast differences in relative strengths and flow behaviour of
various phase transformations results in inhomogeneous stress and strain
distribution during deformation. Another important factor which significantly
influences the deformation behaviour of AHSS, is the stress triaxiality defined as
the ratio of the hydrostatic stress to the equivalent stress.

The study of the microstructure/deformation relationship involves different
length scales from macro - to microscopic level. For classical materials such as
steels, the typical length of the local heterogeneities is usually governed by the
grain size or size of second phases. The SEM is the appropriate tool for
observations at such length scales. In order to study the evolution of the
microstructure during deformation of steel, in situ-SEM studies are conducted.
This method is suitable for investigations of fracture mechanisms as it allows
observation and documentation of the deformation processes directly leading to
the initiation and development of plastic deformation and fracture can be
described quite reliably.
In-situ tensile testing under the SEM is carried out using miniature tensile
specimens. In order to study the effect of triaxiality, notched tensile specimens
are generally preferred. But depending on the notch tip radius, the triaxiality at
the centre of the specimen changes with the applied equivalent plastic strain.
Therefore, the notch tip radius should be such that the variation in triaxiality with
plastic strain is minimum.
The American Society for Testing and Materials (ASTM) specifies standard
specifications for the shape of tensile specimens. However, the shape of existing
specimen specified by ASTM are higher in length and cannot serve the purpose
of miniature tensile testing. Moreover, the standards for notched tensile test
have been withdrawn in the year 2010.
There are various tensile test specimen designs disclosed in the prior art, for
example patents US 2454850. US 4606230, US 4895750, US 5078843. But all
these patents disclose specimen design that are not suitable for testing the effect
of triaxiality and also cannot be tested inside a SEM.

US patent no. 7150200B1 describes a miniature axisymmetric streamline tensile
specimen for tensile of ceramics. Though the sample is miniature in size it cannot
be used to study the effect of triaxiality in AHSS sheets.
OBJECTS OF THE INVENTION
It is therefore an object of the invention is to propose a tensile specimen for in-
situ tensile testing in a scanning electron microscope, which eliminates the
disadvantages of the prior art.
Another object of the invention is to propose a tensile specimen for in-situ tensile
testing in a scanning electron microscope, which enables to study the effect of
triaxiality on deformation behaviour of the specimen.
A still another object of the invention is to propose a tensile specimen for in-situ
tensile testing in a scanning electron microscope, which is constructed to have a
miniature notch that can be tested inside a SEM chamber.
Yet another object of the invention is to propose a tensile specimen for in-situ
tensile testing in a scanning electron microscope, whose triaxiality at the centre
remains constant with equivalent plastic strain.
A further object of the invention is to propose a tensile specimen for in-situ
tensile testing in a scanning electron microscope, which is configured such that
the maximum deformation is concentrated at the centre of the specimen to allow
conducting SEM analysis at a fixed region during the deformation.

A still further object of the invention is to propose a tensile specimen for in-situ
tensile testing in a scanning electron microscope, which can be manufactured
with low machining costs.
SUMMARY OF THE INVENTION
Accordingly, there is provided a notched miniature tensile specimen, comprising:
at least two grip sections at the two ends; of the specimen; a reduced section
positioned centrally in between the two grip sections, such that the transition
fillet in between the grip sections and the reduced sections has a constant
curvature, characterized by comprising : two U-shaped notches are provided at
the two sides of the reduced section.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 illustrates a miniature notched tensile specimen according to the
invention.
Figure 2 A comparative representation in variation of triaxiality corresponding to
plastic strain for a prior art specimen and a specimen of the invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a miniature notched tensile specimen that can be
tested inside a SEM chamber. The specimen design is such that maximum
deformation occurs at the mid-plane of a gage section (20 of figure 1) which
allows carrying out of SEM analysis at a fixed region during deformation of the
sample under testing. The miniature tensile specimen of the present invention is

notched so that the triaxiality at the centre of the specimen is much larger than
that of an un-notched specimen as shown in figure 2. This enables an accurate
study of the effect of triaxiality on the deformation behaviour. Also the triaxiality
at the centre of the specimen does not change significantly with equivalent
plastic strain.
A preferred embodiment of the invention is shown in figure 1. The specimen has
two grip sections on two ends (9 and 10). Each of these grip sections has 4 holes
(1-8) which are provided for pin loading of the sample in a tensile testing stage
inside the SEM. In between the two grip sections, is a reduced section (11). The
curvature of a plurality of transition fillet (12-15) disposed between the grip
sections (9 and 10) and the reduced section (11) is constant.
At the centre of the reduced section (11) two U-shaped notches are located on
either side (16 and 17). The radii of the notch tips 18 and 19 decide the
triaxiality at the centre of the specimen. Lower is the notch tip radius, higher is
the stress triaxiality at the centre of the specimen and lesser is the variation of
triaxiality with strain. However, manufacturing of samples with very small notch
tip radius is difficult and costly. Hence an optimum notch tip radius has to be
selected.
The width of a region between the notches, i.e. the region marked 20, should be
lower than that between the adjacent holes of the grip section i.e. 1 and 2, or 3
and 4, or 5 and 6, or 7 and 8. This is to ensure that the cross-sectional area at
the centre of the sample is the minimum and the failure process initiates at the
centre of the sample. Another factor that decides the width of the region
between the notches is the strength of the material to be tested. As the strength
of the material to be tested increases, the width of the region 20 has to be

reduced taking into consideration the load cell capacity of the tensile stage to be
used. Still another factor that decides the width of the region 20, is the thickness
of the material to be tested. In contrast, if the thickness of the material is higher,
the width of section 20 has to be reduced taking into consideration the local cell
capacity of the tensile stage to be used.
The exemplary embodiments disclosed above are neither exhaustive nor
intended to limit the scope of the invention. The embodiments were chosen and
described to explain the principles of the present invention so that others skilled
in the art may put this invention into practice. Various modifications can be made
by those skilled in the art while remaining within the scope of the embodiments
described above.

WE CLAIM :
1. A notched miniature tensile specimen for a scanning electron microscope,
comprising:
at least two grip sections at the two ends of the specimen;
a reduced section positioned centrally in between the two grip sections,
such that the transition fillet in between the grip sections and the reduced
sections has a constant curvature, characterized by two U-shaped notches
provided at the two sides of the reduced section.
2. A notched miniature tensile specimen as claimed in claim 1, wherein a
notch tip radius of the two U-shaped notches is approximately 2 times
that of the un-notched specimen.
3. A notched miniature tensile specimen as claimed in claim 1, wherein the
triaxiality at the centre of the specimen remains almost constant with
strain.
4. A notched miniature tensile specimen as claimed in claim 2, wherein the
width of the region between the notches is the minimum in the entire
specimen so that deformation in this region is maximum.
5. A notched miniature tensile specimen as claimed in claim 1, wherein each
of said two grip sections has at least four holes to allow loading of the
specimen for testing.

6. A notched miniature tensile specimen as claimed in claim 1, wherein the
transition fillets are disposed between the grip sections and the reduced
section.
7. A notched miniature tensile specimen as claimed in claim 1 or claim 5,
wherein the width of a region between the notches is lower than that
between the adjacent holes of the grip section.

ABSTRACT

The invention relates to a notched miniature tensile specimen, comprising:
at least two grip sections at the two ends; of the specimen; a reduced
section positioned centrally in between the two grip sections, such that
the transition fillet in between the grip sections and the reduced sections
has a constant curvature, characterized by comprising : two U-shaped
notches are provided at the two sides of the reduced section.