Dynamic fracture of linear medium density polyethylene under impact loading conditions

In the present study three-point-bend impact experiments were conducted using a Modified Split Hopkinson Pressure Bar (MHPB) to better understand the correlation between impact velocity and critical dynamic energy release rate at crack initiation in linear medium density polyethylene (LMDPE). The MHPB experiments were conducted at impact velocities ranging from 1 to 7 m/s. The dynamic energy release rate at crack initiation was determined from the force-displacement history of the load point obtained from measurements of input and reflected strain profiles on the incident bar. Moreover, extensive scanning electron microscopy was used to elucidate the micro- and macro-failure mechanisms operative during the dynamic fracture event. In particular two micro-mechanisms of failure were of primary interest: (a) formation of a craze zone at the notch root, and (b) the development of shear lips at the surface of the specimen following crack initiation. The results of this investigation were compared to those obtained by other investigators on linear low-density polyethylene (LLDPE) and linear high-density polyethylene (LHDPE).