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Materials Science in Additive Manufacturing 3D-printed composite auxetic structures

A B

C

Figure 13. Test results of rhombus structures. (A) Deformation of test specimens. (B) Stress-strain curves of three specimens under tensile loads. (C) Plots
of Poisson’s ratio versus strain of three specimens.

Equation II). Rotating rigid FR3 was infilled with lateral as a consequence of the weakened mechanism-type
fibers that had little influence on the auxetic behavior, and deformation. There were some overlaps in the fiber paths
thus, the Poisson’s ratio of rotating rigid FR3 was roughly of re-entrant FR2 (Figure 3E). The overlapped fibers
the same as that of rotating rigid. The rotating rigid FR1 and excessive binding materials (CFC PA) increased the
was reinforced by both axial and lateral continuous fibers. rigidity of joints, thus weakening the mechanism-type
The fiber composites (highest percentage) increased the deformation of the re-entrant structure. For re-entrant
rigidity of joints in rotating rigid FR1, whose rotating FR1, the infilled reinforcing fibers along the axial
deformation at the initial stage was less pronounced than direction had little effect on lateral deformation but had
that of other specimens. This illustrates the reason why a constraining impact on axial deformation. This led to a
rotating rigid FR1 had the highest Poisson’s ratio under lower Poisson’s ratio in re-entrant FR1 as compared with
low strain. specimens without fibers.

When rotating rigid FR2 was stretched and horizontally A structure named rhombus, having a positive Poisson’
expanded, warpage (out of plane) deformation occurred ratio, was also evaluated as a supplement to other test
due to the squeezing of the squares (Figure 14B). In specimens through the tensile test. As shown in Figure 13C,
contrast, fracture occurred in the test specimens without the Poison’s ratio of rhombus FR is similar to rhombus
infilled continuous fiber (Figure 14A) before stretching to without fiber. The existence of axial reinforcing fibers in
warpage. Both of the above-mentioned failures can lead to rhombus FR limited the axial deformation, which gives
degradation of the auxetic behavior. the structure a greater inclination toward rotation. On the
other side, the intersections of fiber paths increased the
As shown in Figure 12C, the re-entrant FR2 infilled rigidity of joints, thus restricting the rotation-dominated
with lateral continuous reinforcing fibers (Figure 3E) deformation. Since these two factors mutually negate each
exhibited less auxetic effect than the other two specimens other, rhombus FR and rhombus ended up with similar

Volume 2 Issue 4 (2023) 9 https://doi.org/10.36922/msam.2159