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Materials Science in Additive Manufacturing Energy absorption of Moore’s thin-walled structures

performance . It is important to measure and evaluate the With the increase in strain, all the corners experienced very
[50]
stress distribution to ensure that the structure will maintain high stress except for the two corners in the middle due to
its integrity under a given load. Stress concentration, the snap-ins from the adjacent structures, which attempted
which indicates that the localized stress of a segment is to expand the two middle corners and eventually led to
significantly higher than the surrounding region, could curvature reduction. During this process, redistribution
cause structural failure even if the effective stress is smaller and reduction of stress occurred at these two locations. At
than the material yield strength. For a compliant structure, a strain of 45%, there was accumulation of high stress at
instability during compression is often accompanied by the bottom four corners (Figure 10) that were in contact
stress redistribution in a stress concentration region. Thus, with the compression plates, indicating densification.
it is important to study the stress distribution and observe
the stress concentration regions in this study. 3.4. SEA and energy dissipation
To better understand the behavior of structures Based on the force-displacement curves directly obtained
captured in the experiment, simulations were performed from Instron’s machine, the SEAs for all nine designs were
with respect to various designs to observe the stress compared (Figure 11) with respect to different loading
distribution within the structures. Figure 10 shows the directions. SEA was calculated from 0% to 40% strains
(before densification) for all structures. For both loading
deformation and equivalent stress distribution (von Mises directions, the increase in fractal hierarchy led to less
stress) in the 2 order structure (relative density of 20%)
nd
at strains of 25%, 35%, and 45%. In general, the stress was energy absorption for the relative density of 20%. With the
unevenly distributed at the cross-section of the fractal increase in relative density, the energy absorption capacity
significantly improved except for the 3 order structure.
rd
structure. While the concave segments (purple arrow in In particular, the 2 order structure appeared to be the
nd
stress distribution at 25% strain in Figure 10) at the top most sensitive to relative density regardless of the loading
and bottom were still experiencing very low stress, the direction. Specifically, the SEA for the 2 order structure
nd
four corners (yellow arrow in stress distribution at 25% increased to more than double the amount with every
strain in Figure 10) had already yielded. It is interesting 10% rise of relative density. It is interesting to note that for
to note that the fractal structures were inclined to act in structures with 20% relative density, the 1 order exhibited
st
an auxetic way . With increasing compression load, the the best energy absorption capacity among all three
[51]
structures shrunk in a perpendicular direction rather than hierarchies. However, with increasing relative density,
expanding. Similar responses were observed for all the the efficiency of 2 order structures in absorbing energy
nd
fractal-inspired thin-walled structures.
seemed to increase. When comparing LD1 with LD2, it
Since the fractal design contains concaves and could be observed that the energy absorption capacity
convexness, the corners with sudden change of curvature was higher when subjected to LD2 than when subjected
tend to induce more stress concentration than other regions. to LD1.

Figure 10. Deformation and stress distribution within the 2 order structure (20% relative density) under compressive load from direction 2. The purple
nd
and yellow arrows refer to the typical concave and convex in the structural design. The configurations from the simulation are rendered with actual
thickness.

Volume 2 Issue 1 (2023) 10 https://doi.org/10.36922/msam.53