On the constancy of the specific enthalpy of fracture

The specific enthalpy of fracture due to ductile crack propagation in commercial polycarbonate sheet is calculated as ^* =A _1c/R _1c, whereA _1c is the critical energy release rate associated with the onset of unstable crack propagation andR _1c is the corresponding amount of damage (yielded material) formed per unit crack extension.A _1c andR _1c are determined from fatigue crack propagation experiments conducted at different maximum loads, load ratios and frequencies. The value of ^* obtained from all experiments is found to be 9.8±1.4 cal g^–1 (1cal = 4.184 J) which indicates that ^* is a material constant. This finding substantiates predictions of the crack layer theory.