SAMPE Journal - July/August 2024 - 42

FEATURE / HONEYCOMB STRUCTURES
to smc = 50 MPa. This results in approximately a
3 MPa difference, which is similar to that which
was reported from the 0.50BI samples. These key
stress values are listed in Table 1 for each group of
samples.
It should be noted that the weight of the 0BI and
0.50BI samples averaged 42 g whereas the 0.75BI
and 1.0BI samples averaged 35 g, so that the former
two groups of samples were nominally 20% heavier
than the latter two groups, so that the volume of
metal coating was not consistent between the two
groups of plated samples (Table 1). However, these
results do support the feasibility of electroplating
the SLA preform to increase energy absorption
capacity because stress is supported over a much
wider strain range of crush.
To better understand what occurs as the
samples are crushed, images were taken of the
quasi-static crush tests as a function of strain. The
top rows of Figure 4 depict a representative crush of
a 0BI plated HC sample. The 0BI plated HC sample
initiated crush nearest to the bottom platen and
slowly collapsed upward as the compressive strain
increased. This resulted in a consistent folding,
regardless of which individual hexagon is being
observed. The bottom rows of Figure 4 depict a
representative crush of a 0.50BI plated HC sample.
The peak stress decreases because, at 10% strain,
about half of the hexagonal cells are collapsing at
the BI locations, while the other half are collapsing
at the upper platen. This continues as the strain
increases. By 40% strain, the left-side tubes are
collapsing or folding top to BI, however the rightside
tubes are collapsing or folding in closer
proximity to the BI location. This collapse process
is not uniform and so reduced the mean crush
stress during the test.
From the stress vs. strain data, the strain
dependent crush efficiency was determined.
Figure 5 shows the crush efficiency as a function of
strain for the unplated HC samples (Figure 5a) and
plated HC samples (Figure 5b). The crush efficiency
data exhibit a rapid increase as the stress climbs
to its peak stress value between 5 and 10% strain,
and maximum crush efficiency is reached. At this
point, the crush efficiency decreases until the stress
reaches the plateau stress, which is maintained
until densification begins. Densification is the
strain at which most of the crush deflection has
occurred, or when the peak stress is reached for the
second time during testing, and above this strain
level the crush efficiency will rapidly decay.
Figure 5a, shows the crush efficiency for the
unplated HC samples. For the 0BI and 0.50BI
samples, the unplated HC samples exhibited
primarily brittle failure, and failed at 10% strain
and 8% strain, respectively. In contrast, the sample
with BIs at the top of the HC was able to be crushed
to a relatively high strain range, so that the crush
efficiency is measurable up to 80% strain. However,
Figure 4. Progression of crush as a function of strain: (Top Rows) Plated HC
sample without BIs; (Bottom rows) Plated HC sample with BI at 50% of the
sample height (0.50BI).
because the mean crush stress is very low relative
to its peak stress, this results in low crush efficiency.
The crush efficiency of the plated HC samples
is indicative of energy absorbing materials (Figure
5b). The initial increase in stress occurred between
0 and 8% strain. At this point, the stress in all four
samples settled down to their respective plateau
stresses, as the folding process got underway during
crush. Once the peak crush efficiency was achieved,
there is a slight decrease in crush efficiency, about
10%, as the samples continued folding, until
samples reached their densification strain at
nominally 65% strain. Above this strain level, there
was a rapid decline in crush efficiency from 65%
to 10% over the remaining 20% of measured strain
range. Peak values of crush efficiency are listed in
Table 1 for both unplated and plated samples.
The energy absorbed efficiency assesses how
well a material is able to absorb the energy across
its full strain range. As previously discussed, the
unplated HC samples exhibit a low strain range of
only 10%, with the exception of the 1.0BI sample
(Figure 6a). None of the unplated HC samples
exceeds an energy absorbed efficiency of greater
than 15% over its tested strain range. The plated HC
samples could be tested to a higher strain level of
nominally 80-85% strain, so that more energy can
be absorbed by these samples. The densification
of these plated HC samples occurred between 65
42 | SAMPE JOURNAL | JULY AUGUST 2024
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SAMPE Journal - July/August 2024

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