Shear and compressive impact of polypropylene bead foam

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