The Mechanism of the Recovery of Impacted High Density Polyethylene Foam

The impact and recovery behaviour of three high density polyethylene (HDPE) closed cell foams were investigated. Impact stress strain curves were measured using a falling striker impact rig, then the recovery monitored from 10 s after the impact. The variation of the initial yield stress with foam density and temperature followed the expected models. Cell deformation was observed during compression and recovery in a Scanning Electron Microscope. Recovery occurs by the viscoelastic straightening of the buckled faces but it is incomplete due to some plastic deformation in the structure, so the faces remain slightly buckled. Some of the observed inhomogeneity of the cell deformation is due to the removal of cell faces in the sectioning for examination. There is little difference in the recovery dynamics as a result of changing the foam density.