Effect of molecular weight on craze shape and fracture toughness in polycarbonate

The shape of the craze at the tip of a crack has been studied using optical microscopy on polycarbonates of various molecular weights at ?30°C. For all molecular weights studied the craze shape was well approximated by the Dugdale plastic zone model and this model was used to calculate the craze stress and the release rate in plane strain. It was found that the craze dimensions, the craze stress and the strain energy release rate in plane strain all increased with increasing molecular weight. Fracture of macroscopic specimens showed a mixed mode fracture in all molecular weights. By studying the effect of thickness the strain energy release rate in plane strain was calculated for various molecular weights. Agreement was found between these values and those determined from the craze shape measurements. The overall strain energy release rate, the strain energy release rate in plane strain and the contributions from the plane stress mode increased with increasing molecular weight.