Analysis of Force and Energy Measurements in Impact Testing

Complete velocity-time (v-t) relationships for a pendulum striker during impact testing of sharp-notched high-density polyethylene have been recorded by means of laser-Doppler velocimetry. The velocity data have been processed using a microcomputer to give force-deflection curves and hence absorbed energy. Toughness is expressed in terms of the critical strain energy release rate, G_c, and the critical stress intensity, K_c. The plane stress G_c from pendulum energy was 11.3 kJ/m², and that from areas under force-deflection curves was 9.9 kJ/m². The authors observed that an initiation G_c of the order 2.3 kJ/m² might be expected for this material below its glass transition. By defining crack initiation in these room temperature tests as the point of maximum recorded force, an initiation G_c of 2.0 kJ/m² is obtained. The critical stress intensity factor, K_c, is also defined from the maximum force, and a value of 2.1 MN/m^3/2 is calculated. The elastic modulus is computed from compliance measurements to be 1.84 GN/m², whereas from $E=Kc2∕G$ the plane-stress modulus is 1.93 GN/m², and the introduction of Poisson's ratio gives a plane-strain modulus of 1.69 GN/m².