Plane-strain fracture energy instability and mixed mode crack propagation in polycarbonate at room temperature

A study of crack propagation in double cantilever beam specimens of polycarbonate has revealed a large velocity-dependent instability in the plane-strain fracture energy G_Ic. At a crack velocity of 10^?2 in./min, G_Ic accords with published values obtained from tensile studies of precracked specimens. Crack propagation in doubly grooved double cantilever beam specimens is unstable at higher velocities. The G_Ic's during crack jumping and at crack arrest are estimated to be 0.2 and 2%, respectively, of the low crack speed value, based on the amounts of crazing produced at the various crack speeds. Evidence of plane-strain shear deformation at the low speed crack tip is presented. The G_Ic instability is suggested to arise from differences in the kinetics of shear failure and craze breakdown.