SAMPE Journal - November/December 2016 - 42

Feature Article

Figure 9. Tensile elastic modulus results.

Compressive Response Results
Compressive testing was carried out on puck-shaped
coupons cut from 0.75 cm thick strips, using a 1.5 cm
diameter punch die. Five coupons were tested for each
material and cure number, and three properties were
measured: a) initial stress-strain slope, b) maximum
stress-strain slope, and c) maximum true (also known
as Hencky or logarithmic) strain. An example of
compressive stress-strain plot (TC 40, sample 1, no
cures) is displayed in Figure 11. The maximum stressstrain slope was measured automatically by the Instron
Bluehill® testing software in the region at the end of the
test. For example, the slope of the coupon whose stressstrain curve is displayed in Figure 11 was calculated in
the region from about 60 to 65% strain.
The three properties were measured at initial and
maximum stress levels, because it was unknown at what
stress level the possible changes in physical-mechanical
properties would occur. One other factor affecting

compression response of rubber is shape factor SF,
equal to coupon's loaded area divided by its bulging
area that is the lateral area. The higher SF the stockier is
the coupon, and the higher the stress required to reach
the same strain at lower SF, and hence the higher the
stress-strain slope. The tested coupons' SF was about
0.5, which is on the low side of rubber testing range
reported in technical literature.
The initial stress-strain slope values (average and
standard deviation) are listed in Table 7, and plotted in
Figure 12. The slope increased and then slightly dropped
for KE1310ST and M4641 to 17% and 6% respectively,
whereas steadily decreased for TC 40 and V-340 to a
final -12% and -15% respectively.
The maximum stress-strain slope values (average
and standard deviation) are presented in Table 8, and
plotted in Figure 13. The slope steadily increased for
KE1310ST and M4641 to 4% and 3% respectively,
whereas oscillated for TC 40 and V-340 to a final +1%.
The maximum true-strain values t (average and standard
deviation) are included in Table 9, and plotted in Figure
14, and were calculated as it follows16:
t = ln(T/T0) = ln(e+1)
with T0 and T the initial and final thickness respectively,
e the engineering strain
e = (T- T0)/T0
t was preferred to e because the former is a truer
indication of deformations above 10% than the latter17.
The strain steadily increased for TC 40 and V-340 to a
final 3% and 7% respectively, indicating more compliant
materials, consistently with the fact that TC 40 and
V-340 featured the lowest increase in maximum stressstrain slope (less than 1%). Instead, the maximum strain
dropped by 10% and stayed unchanged for KE1310ST
and M4641, characterizing them as stiffer
materials, consistently with the fact that
they featured a maximum stress-slope
higher than TC 40 and V-340.
Compressive Modulus and Hardness
In compressive loading, the force is
applied evenly to the entire test coupon
cross-section, whereas in hardness testing
the force is exerted to a concentrated
location through a tip. Since the material is
compressed in both cases, it was expected
that harder material featured a steeper
stress-strain slope, and hence some
similarity existing between the hardness
data set and initial compressive stressstrain slope data set, if both data sets
were valid. Therefore, hardness and initial
compressive stress-strain slope data were

Figure 10. Plots of hardness-Young's
modulus in tested materials.
42

SAMPE Journal, Volume 52, No. 6, November/December 2016

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