Impact properties of uniaxially thermoformed auxetic foams

This work describes large strain quasi-static and impact tests of a new class of low-cost, uniaxially thermoformed, transverse isotropic auxetic foams with compression ratio ranging from 20 to 80%. A custom drop tower rig with high-speed cameras suitable for these soft porous foam materials is designed and used to perform impact tests with energy ranging from 0.38 to 1.90 J, corresponding to strain rates from 64/s to 143/s. The thermoformed foams only show auxeticity when loaded along the transverse direction to the uniaxial thermoforming compression, with Poisson's ratio V-21 reaching as low as-1.5 at 20% strain and then close to 0 at large strains. The samples deform with large shear band deformations, also due to the auxeticity. The negative Poisson's ratio foams along that transverse direction also show enhanced impact energy absorption performance, with normalized peak force reduction as high as similar to 40% against the pristine foam; the reduction is however 20% along the thermoforming direction. A constitutive model based on Nagy's approach is applied to describe the enhancement of dynamic stress during the impact tests compared with the quasi-static one, due to strain rate effects.

Automated summary

This study describes large strain quasi-static and impact tests of a new class of low-cost uniaxially thermoformed transverse isotropic auxetic foams. The results show that these foams only exhibit auxetic behavior when loaded along the transverse direction to the uniaxial thermoforming compression. The impact energy absorption performance of the foams is enhanced, and the Poisson's ratio approaches 0 at large strains.