Shear and Compressive Impact of Polypropylene Bead Foam

Abstract

Polypropylene bead foams were subjected to oblique impacts, in which the material is compressed and sheared. This strain combination can occur when a cycle helmet hits a road surface. The results were compared with simple shear tests at low strain rates, and to uniaxial compressive tests at impact strain rates. The observed shear hardening was greatest when there was no imposed density increase, and practically zero when the angle of impact is less than 15 degrees. The shear hardening appears to be a unique function of the principal tensile extension ratio and is a polymer contribution, whereas the volumetric hardening is due to the isothermal compression of the cell gas. Foam material models for Finite Element Analysis need to be reformulated to consider the physics of the hardening mechanisms, so their predictions are reliable for foam impacts in which shear occurs.

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