The strain dependent indentation resilience of auxetic microporous polyethylene

A series of tests have been conducted on (i) auxetic, (ii) compression moulded and (iii) sintered ultra high molecular weight polyethylene. The auxetic material possesses a negative Poisson's ratio, ν, due to its complex porous microstructure which consists of nodules interconnected by fibrils and the sintered material has a positive ν and is microporous but does not contain fibrils. It was found that the auxetic material was both more difficult to indent than the other materials at low loads (from 10–100 N) and was the least plastic with the most rapid viscoelastic creep recovery of any residual deformation. Indeed, at low loads, where the resistance to local indentation is most elastic, the hardness increased by up to a factor of 8 on changing the Poisson's ratio from ν ≈ 0 to ν ≈ −0.8. A mechanism is proposed based on local densification under the indentor of the nodules and fibrils which explains how the microstructural response of an auxetic polymer can be used to interpret the results.