SAMPE Journal - November/December 2016 - 45
Feature Article
Figure 16. Plots of initial compressive stress-strain slope
and Young's modulus.
* Maximum compressive true strain:
-10 to -7%
* Color change: 10 to 14%
* Mark-off: none on all materials.
Shrinkage and hardness featured a change
rate higher in the first three cures than in final
three cures. Hence, preliminary heating carried
out off production can help "stabilize" the
elastomeric tooling before its use in production
with the penalty of some service life wasted.
Hardness increase was consistent with data
collected at Boeing in previous studies, and
some technical literature10 on ETD in general.
Two types of material response were detected:
* KE1310ST, M4641 shrunk more than the
others, and became harder and stiffer in tension
and compression. One possible reason is that
the samples were not fully cured, although the
supplier's recommendations were followed for
their fabrication, and at high temperatures they
continued to crosslink19.
Six readings/parameter/sample/cure were taken. For
comprehensive evaluation, dL*, da*, db*, dE*, dEc were
plotted against the cure cycles, and featured some
gradual change, whose rate progressively decreased.
For brevity, it is included only Figure 19 illustrating the
variation of dE*, from 0 to 6 cure cycles. Namely, dE*
was 14%, 13%, 10% and 3% for M4641, KE1310ST,
TC 40, and V-340 respectively. However, V-340's values
cannot be compared to the other materials' values since
V-340 was blue before thermal cycling. dE* also levelled
out towards 6 cure cycles.
Precision
Precision is a figure of merit in test methods in general18,
and typically corresponding to standard deviation (SD)
values. However, in this study, in order to compare
different properties and values across the tested RTVs,
precision was expressed as the coefficient of variation
(CV) equal to 100*SD/100. The CV values in Table 10
are the maximum values across the tested RTVs. Since
dE* is calculated from da*, db*, dL*, and hence has no
associated SD, the CV value for dE* in Table 10 is the
largest CV values associated with da*, db*, dL*.
Conclusions
The testing program described in the present study
enabled measurement of the following changes in
properties of RTVs exposed to thermal cycling:
* Shrinkage : 0.3 to 1.3%
* Hardness: -2.5 % to +8.8%
* Young's modulus: -11 to +3.6%
* Initial compressive stress-strain slope:
+0.7 to +3.7%
* Maximum compressive stress-strain slope:
-10 to +7%
SAMPE Journal, Volume 52, No. 6, November/December 2016
Figure 17. Locations on rubber strips of 62 mil thick caul
plates after 6 cure cycles.
* TC40, V-340 shrunk less than the others, and
became harder and less stiff in tension
and compression.
Expected trends between couples of properties were
measured, and supported accuracy and validity of the
results within the limitations arising from measurements
taken on only one lot of each material and small sample
size. A portable spectrophotometer was evaluated as
an instrument to detect material changes inexpensively
and objectively, but it suffered from high scattering. The
measurement technique with optimal combination of
cost, ease of use, and precision was hardness. Finally,
measuring material properties values in actual service
conditions enables to design ET that can better meet
performance requirements, potentially reducing part
defects caused by inadequate tooling performance.
45
Table of Contents for the Digital Edition of SAMPE Journal - November/December 2016
Contents
SAMPE Journal - November/December 2016 - Cover1
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SAMPE Journal - November/December 2016 - Contents
SAMPE Journal - November/December 2016 - 2
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