Dynamic fracture of linear medium density polyethylene

The objective of the present study is to better understand the correlation between impact velocity and dynamic energy release rate at fracture initiation in linear medium density polyethylene (LMDPE). A series of experiments is conducted by impacting three-point bend specimens using an instrumented split Hopkinson pressure bar. The impact velocities in the tests range from I to 7 m/s. In order to determine the dynamic energy release rate at fracture initiation the load-point force versus load-point displacement history are obtained from measurements of input and reflected strain profiles on the incident bar. Moreover, extensive optical and scanning electron microscopy is conducted to elucidate the micro- and macro-failure mechanisms operative during the dynamic fracture event. In particular, two mechanisms are of primary interest. Firstly, the formation of a craze zone at the notch root which is understood to be responsible for enhanced energy absorption at crack initiation. Secondly, the development of shear lips at the surface of the specimen following crack initiation. The shear lips provide avenues for additional energy absorption during the crack propagation phase of the dynamic failure event. The results of this investigation are compared to those obtained by other investigators on LLDPE and LHDPE.