Materials Science

Squeezing Holes

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Science  25 Sep 2009:
Vol. 325, Issue 5948, pp. 1601
DOI: 10.1126/science.325_1601c

Most materials stretched in one dimension will contract in the other two, a property captured in the substrate's positive Poisson's ratio. Auxetic materials, in contrast, expand in a lateral direction when stretched, or correspondingly shrink in a lateral direction when compressed. Bertoldi et al. consider a two-dimensional rubber sheet punctuated with an array of circular holes that shows auxetic behavior only under compression. The sheet initially has a positive Poisson's ratio, which decreases with increasing compressive strain. The holes deform into orthogonal ellipses, as there is insufficient space for all of them to collapse in the same direction. Using finite element simulations, the authors find that a minimum void fraction of 0.34 is needed in order to obtain auxetic behavior. Below this value, the compression leads to a macroscopic instability whereby an entire row of holes collapses to accommodate the strain, leaving the rest of the sample mostly undeformed. The void density also influences the critical strain at which the Poisson's ratio becomes negative; thus, it should be possible through engineering the specific void pattern to make a highly tunable compressively auxetic material.

Adv. Mater. 21, 10.1002/adma.200901956 (2009).

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