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Tunable stiffness using negative Poisson's ratio 333

the elongation of the sample causes the auxetic material to expand to create
jamming at the boundary. We chose 6mm as it was the maximum elonga-
tion before cracks appeared on the surface of the sample. We also wanted to
see the effects with half the maximum possible elongation, so we chose 3mm
as another test case. The test was conducted with a 5mm per minute rate of
vertical loading at 5mm per minute and an end test criterion of 10mm total
vertical flexural elongation. Measurements were done using a 50N load cell.
We hypothesize that with increasing horizontal elongation, the load vs
vertical extension curve should shift upwards. This is because jamming
resulting from the horizontal elongation will increase the stiffness of the sam-
ple causing it to resist bending. Hence, for the same vertical extension, a
higher load will be required. Between 3 and 6mm horizontal elongation,
we expected the load vs extension graph to be higher for the 6mm test case
because a larger horizontal elongation should lead to a larger expansion of
the auxetic material and hence increase jamming and the associated stiffness.
The test results and the associated discussions are presented in the next
section.

6.2 Continuum metastructural tests
6.2.1 Re-entrant honeycomb structure tests
We used the standard hexagonal re-entrant design that is snugly fitted into
the hexagonal PPR outer tube and subjected it to the bending test. As shown
in Fig. 15, among the three materials, only TPU showed a negligible
increase in stiffness after the effects of jamming following the application
of a horizontal elongation. Furthermore, this increase in stiffness was
load-dependent and not consistent over all the load values. The other
two materials did not show such an increase, but rather showed a decrease
in the stiffness, especially at higher levels of load.

6.2.2 Missing rib structure tests
Like in the case of the re-entrant design, the missing rib design only showed
an increase in stiffness upon elongation when using the TPU material. The
increase in stiffness was consistent across different loads. We saw a wide var-
iance when using PVA, suggesting that the material was not suited for such
test applications. We also observe a small increase in stiffness when using
Pythonflex, which is consistent across the loads. However, there was not
much of a difference between the 3 and 6mm elongations, and the differ-
ences in stiffness between the base case and the elongated case were minute
(Fig. 16).