Journal of Minerals and Materials Characterization and Engineering, 2013, 1, 331-335
Published Online November 2013 (ht t p:/ / www.scirp.org/journal/jmmce )
http://dx.doi.org/10.4236/jmmce.2013.16050
Open Access JMMCE
Design and Development of Grip for Circular Test Piece
Inuniversal Tensile Testing Machine
Ojo Jeremiah Akinribide, Adetunji Kolawole Ogunkoya, Itopa Monday Momoh,
Olasupo Daniel Ogundare, Ba’aku Emmanuel AttahDaniel, Samuel Olugbenga Oloruntoba Olusunle
Engineering Materials Development Institute, Akure, Nigeria
Received October 5, 2013; revised November 8, 2013; accepted November 20, 2013
Copyright © 2013 Ojo Jeremiah Akinribide et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
The design and development of jaw grip for circular tensile test samples in a universal mechanical tester were under-
taken in this work . In developing econo mies, the cost of acquiring labora tory testing equipment and accessor ies is huge,
thereby depriving most of the supposedly advanced laboratories of most of this necessary research equipment. Where
the equipment is available, they are either non-fun ctional due to inadequate maintenance know-how or non-availab ility
of ne ce s s ar y a cc es s or i es . Th e de v e lo p ed gr ip in t h is wo r k is part of an effort at providing accessories as the ir need arises.
Advanced design and manufacturing too ls were deployed to develop the jaw grip by using austen itic stainless steel. The
developed jaw grip was used on the test equipment to conduct tensile tests on steel samples and the results were found
to conform to international standard. Consequently, replacement of worn-out accessories has been carried out in resent
time, which eventually saves idle time of equipment or otherwise importation for replacement.
Keywords: Mechanical Testing; Tensile Test; Jaw; Grip
1. Introduction
The strength of a material under tension has long been
regarded as one of the most important characteristics
required for design, production, quality control and life
prediction (ASTM E8 -24T, 1924). Man y materials, when
in service, are subjected to both internal and external
forces/loads that could affect the material directly or in-
directly, and examples include the aluminum alloy from
which an airplan e wing is constructed and th e steel in an
automobile axle. In such situations, it is necessary to
know the characteristics of the material and to design the
member from which it is made in order to minimize or
avoid any resulting excessive deformation and sudden
fracture that could occur [1]. The mechanical behavior of
a material reflects the relationship between its response
to an applied load or force(s). Some of the required im-
portant mechanical properties are strength (yield and
ultimate), hardness, ductility, and stiffness [2].
The mechanical properties of a material are related to
its behavior when subjected to continuously increasing elon-
gations up to rupture/fracture [3,4]. A thorough under-
standing of a material’s mechanical properties is required
by engineers if catastrophic failures are to be avoid e d . A c-
cording to Aegerter and associates [5], the tensile test is a
common standard test and is a valuable method of deter-
mining important mechanical properties of engineering
materials. The procedural details of the test vary for differ-
ent material types, but tensile tests are gen erally conduct-
ed at room temperature and relatively slow loading rates.
The mechanical properties of materials are ascertained
by performing carefully designed laboratory experiments
that replicate as nearly as possib le the service conditions.
Factors to be considered include the nature of the applied
load and its duration, as well as the environmental co ndi-
tio n s. I t i s po ss ib l e for the load to b e tensile, co mpressiv e ,
or shear, and its magnitude may be constant with time, or
it may fluctuate continuously [6]. Application time may
be only a fraction of a second, or it may extend over a
period of many years. Service temperature may be an
important factor (depending on the area of application).
Mechanical properties are of concern to a variety of par-
ties (e.g., producers and consumers of materials, research
organizations, and government agencies) that have dif-
fering interests. Consequently, it is imperative that there
is some consistencies in the manner in which tests are
conducted, and in the interpretation of their results. This

O. J. AKINRIBIDE ET AL. 332
consistency is accomplished by using standardized test-
ing techniques established and published by international
professional bodies. In several practical cases, the ulti-
mate ductile fracture strain determined with ten sile test is
accepted as a material plasticity measure [7]. In this case,
the plasticity has to be defined as an ability of a material
to accommodate high permanent strains until fracture
appears where this strain reaches certain value called
ultimate fracture strain. The strain value until fracture
depends not only on the material type, but also on other
several factors, as: strain speed, strain history, material
starting structure, temperature, specimen geometry, etc.
It is impossible to account for all factors in a single ma-
thematical description, due to a complexity of phenom-
ena and an insufficient state of the art, mainly for pheno-
mena present during a plastic strain. Several experiments
according to Bao and Wierzbicki [8], have demonstrated
that the material fracture process strongly depends on the
hydrostatic stress.
In Tensile Testing, the test specimen is deformed, usu-
ally until complete rupture or fracture occurs, with a
gradually applied increasing tensile load that is applied
uniaxially along the longitudinal axis of the specimen
(metals and non-metals) which could be circular, rectan-
gular with dimensions in accordance to internationally
acceptable standards (ASTM or BS). During testing, de-
formation is confined to the narrow center region which
has a uniform cross section along its length. The test spe-
cimen is clamped together in the machine with the aid of
upper and lower jaws that has grip ability designed to
firmly hold the test specimen. This work is focused at
design and producing alternative jaws grip that could
perform the same function meeting the required interna-
tional standard of material characterization by evaluating
the tensile properties of a material.
2. Materials and Method
2.1. Basic Development
The aim of this work is to design and develop jaw grips
for circular test piece which is an accessory commonly
used in commercially available universal mechanical
testing machine. The equipment accessory designed was
developed at minimum possible cost without compro-
mising the expected efficiency. The designed compo-
nents were modeled and analyzed using parametric 3-D
design software-Pro Engineer and machined using state-
of-the-art advanced manufacturing equipment which in-
clude, power Hacksaw, lathe, Computer Numerical Con-
trol (CNC) vertical Machining Center and surface grind-
ing machine.
Considering the expected uniform distribution in the
ap pli ed forces on the test specimen [8], a professional En-
gineering (Pro-E) software was used to design and model
the jaws (upper and lower) as shown in Figure 1. Each
Figure 1. Model of the two pairs of the jaw grip required
for tensile test operation.
jaw is designed to have two stage stoppers on the speci-
men [9]; this is to avoid the likelihood of stress concentra-
tion at the end/edge of the gauge length of the specimen.
The maximum load of the machine used in this investi-
gation is 50 KN. To this effect, a suitable material (Aus-
tenitic Stainless Steel) that could withstand the load re-
quired-maximum of 30 KN to pull conventionally heat-
treated metallic materials to failure was selected, this is
as a result of its high strength, high resistance to oxida-
tion and its availability. The selected material was sub-
sequently cut with the aid of power Hacksaw to a spe-
cific configuration, drilled using drill bits and bored to
conform to the designed profile so as to ensure firm grip
on any specimen machined to its accepted configuration.
2.2. Results and Discussion
The model of the computer aided design of the jaw grips
are presented in the figures below:
Figure 2 shows the intricate parts of the designed jaw
grip. The jaw was designed and produced to have two
stage stopping points (first and second steps), the second
stopper is to further reduce or eliminate the possible
stress concentration at the end of the gauge length that
could have ensued if it is just one. Stress concentration at
a particular point of a material is a disadvantage that
leads to unexpected premature fracture or deformation in
the specimen. In the type of tensile testing machine tar-
geted for this design, the pins are required to hold the
grips at the designated position firmly thus enabling ri-
gidity during operation, for this reason, the pins were
incorporated in the design and developed as shown in
Figure 1 with their corresponding gro ove through which
they are being held firmly to the machine frame. As
shown in Figures 2 and 3(a), two pairs of grip are n eed-
ed to conduct any tensile test. Each of this pair consists
of two grips forming the upper and the lower jaw grips.
The newly produced jaws grips were subjected to test on
a Tensile Testing Machine [as shown in Figure 3(a)]
following the predetermined mounting setup shown in
Figure 3(b) to conduct several tensile tests on a circular
Open Access JMMCE

O. J. AKINRIBIDE ET AL. 333
Figure 2. Model showing the intricate part of a piece of the
jaw grip.
(a)
(b)
Figure 3. (a) Tensile circular test piece inside the developed
jaw grips on a mechanical tester during performance
evaluation (b) Models showing the mounting arrangements
of both 1-step and 2-step test pieces on the develope d upper
and lower jaw Grips.
specimen (mild steel) in accordance with ASTM A352/A
352M-03 and the results were found to conform to inter-
national standard (ASTM E8M3) as presented in Table
1. The grip portion of the circular test piece applicable to
the designed and developed jaw grips could be of differ-
ent configurations but with the same gauge length di-
mensions as required by international standard. The ten-
sile test results presented in Table 1 shows that 2-step
test piece sample yielded the best result in conformity
with international standard and this is further confirmed
by its simulated performance presenting the best struc-
tural stability in Figure 4; Its graph conform with theo-
retical expectation (ASTM E8M3), in comparison with
others presented in Figures 5 and 6. The 2-step grip con-
figuration is the test piece that best match the grip cavity
in the developed jaw grip; therefore, the developed jaw
grip exhibits reliable performance and therefore serves as
a good replacement to the defected one.
Figures 4-6 show typical stress-strain curves obtained
in uniaxial tensile tests on mild steel. In these three
graphs, the detail explanation of the stress-strain curve is
as follows: in Figures 4 and 6, there is a straight line or
linear relationship between stress and strain from origin
until it attains a stress level of 250 MPa which is other-
wise known as the proportional limit. Thereafter, the
stress/strain relationship became nonlinear up to a point
300 MPa which is very near to proportional limit. At 300
MPa, the deformations are largely elastic and on unload-
ing, the specimen regains the original dimensions. But
beyond the point 300 MPa, the metal yields and suffers
plastic deformation. This is indicated by a sudden bend
in the curve. Most of the strain after this point is plastic
strain which is not recovered on unloading . The value of
stress at 300 MPa is called upper yield strength. With
further increase in strain, the stress falls a bit to a lower
level at 285 MPA. This is due to formation of Lüder
bands or slip band otherwise known as the bands in the
metal where permanent, plastic deformation begins. With
increase in tension, localized plastic flow takes place in a
narrow band with boundary planes inclined at a certain
angle with the axis of test specimen. However, due to
strain hardening of the material in the band, the load
again increases till another Lüd er band app ears in the test
piece. This goes on till the whole specimen is full of
Lüder bands. Thus between the points 0.07 mm/mm and
0.09 mm/mm of th e tensile strain level the stress is oscil-
lating between two narrow limits. This occurs in alloys
having interstitial solid solution structure. The stress at
285 MPa is called lower yield strength.
However with furth er increase in tensile strain beyond
the point 0.09 mm/mm, when the test piece is full of
Lüder bands, the load or stress again starts increasing.
The distinction between the two yields may disappear
with strain hardening and only a small kink may remain
on the stress strain curve. Some author like Zhongchun
Chen et al., prefer to take stress value at 285 MPa as the
flow stress at the yield point, however, the data given in
material standards generally refer to upper yield point.
After the 0.09 mm/mm the stress-strain curve moves
upwards, however, with further deformation, its slope
gradually decreases at 450 MPa which is the highest
point on the curve. After 450 MPA, the curve goes down.
Before the 450 MPA, it was observed that increase in
strain increases the load on the specimen due to strain
hardening [10]. Even after the point 450 MPA, the strain
hardening is still there but at some point the area of cros s
Open Access JMMCE

O. J. AKINRIBIDE ET AL.
Open Access JMMCE
334
Table 1. Results of tensile test performe d by the developed jaw grip on different test piec e configuration.
Tensile Test Results Yield Strength (Y.S) N/mm2
or Mpa Ultimate Tensile Strength (UTS) N/mm2 or Mpa. Elongation (%)
Obtainable (Inter national Standard) 275 485 - 655 22
Old grip (Conventional) 268 491 21
1-step grip 261 425 18 Obtained
2-step grip 265 485 20
Figures 4. 2-step configuration of grip region, its corresponding simulated behaviour under tensile load and graph of tensile
test conducted on it.
Figures 5. Old-step configuration of grip region, its corresponding simulated behaviour under tensile load and graph of ten-
sile test conducted on it.
Figures 6. 1-step configuration of grip region, its corresponding simulated behaviour under tensile load and graph of tensile
test conducted on it.

O. J. AKINRIBIDE ET AL. 335
section of the test piece starts decreasing much faster and
a neck formation starts, with the result, the force that the
test piece can bear decreases continuously with further
deformation resulting in an unstable condition. After
some elongation in the neck, the specimen fractures at
the point 350 MPa at the stress level. Since we have de-
fined stress as force divided by original area of cross
section the stress value thus calculated also decreases
after 450 MPa, however, if we take true stress, as ex-
plained below, it will be much higher. The stress at the
450 MPa is known as ultimate tensile strength. At 450
MPa the actual area of cross section is smaller than the
original area of cross-section.
3. Conclusion
In this work, the design and development of jaw grip for
circular tensile test were undertaken. Professional Engi-
neering (Pro-E) software was used in the design and
analysis, in the course of doing this, several laws were
considered in relation to stress d istribution in a specimen
under tensile test. Suitable material (austenitic stainless
steel) was thereafter selected and machined to a desired
and designed configuration. The product was subse-
quently used to conduct a tensile test on a steel sample
and the results were found to conform to an internationa l
standard. Due to the availability and accessibility of the
raw material used and needed ingenuity, the product thus
serves as an excellent alternative to the jaw grips that
come with the imported machine. Thus far, the work has
brought to focus on a new design and manufactured step-
wise grip for circular uniaxial tensile test to serve as an
alternative to the usual multi-jaws grip designed for the
test.
REFERENCES
[1] ASTM E8M3 Standard Test Method for Tension Testing
of Metallic Materials, 2009.
[2] D. Mohr and S. Henn, “Calibration of Stress-Triaxiality
Dependent Crack Formation Criteria: A New Hybrid Ex-
perimental-Numerical Method,” Experimental Mechanics,
Vol. 47, No. 6, 2007, pp. 805-820.
http://dx.doi.org/10.1007/s11340-007-9039-7
[3] Engineer® Wildfire™ 5.0, “Help Center,” Parametric
Technology Corporation, Needham, 2009.
[4] H. Czichos, T. Saito and L. Smith, “Springer Handbook
of Materials Measurement Methods,” Springer-Verlag,
New York, 2006, pp. 302-307.
http://dx.doi.org/10.1007/978-3-540-30300-8
[5] Granta (Materials Intelligence) CES 2007 EduPack, “Get-
ting Started with CSE EduPack,” Granta Design Limited,
Cambridge, 2007.
[6] J. Aegerter and H. Bloching, “Tensile Test on Materials
According to EN 10002-1 (Issue December 2001),” 11th
Trade Fair for Testing Technology, ZwickHausmesse,
October 2002, pp. 167-188.
[7] J. Aegerter and H. Bloching, “Probenformen- und Her-
stellungfür die Prüfungmetallischer Werkstoffe-Schwer-
punkt Zugversuch (Specimen Geometries and Specimen
Preparation for Testing Metallic Materials, Especially
Tensile Test),” 12th Trade Fair for Testing Technology,
ZwickHausmesse, October 2003, pp. 50-62.
[8] W. Eyassu and T. Henry, “Mechanical Testing (Tensile
Testing),” Louisiana State University, 2009.
http://imechanica.org/node/1070StandardE8M.TensileTes
tingofMetallicMaterials.ASTM
[9] Y. Bao and T. Wierzbicki, “On Fracture Locus in the
Equivalent Strain and Stress Triaxiality Space,” Interna-
tional Journal of Mechanical Sciences, Vol. 46, No. 1,
2004, pp. 81-98.
http://dx.doi.org/10.1016/j.ijmecsci.2004.02.006
[10] Z. C. Chen, et al., “Bauschinger Effect and Multi-Axial
Yield Behavior of StressReversed Mild Steel,” Metallur-
gical and Materials Transactions A, Vol. 30A, 1999, pp.
3069-3078.
Open Access JMMCE