Crack propagation in polycarbonate

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Abstract
Crack velocity measurements and fracture toughness tests have been carried out on extruded sheets of bisphenol‐A polycarbonate. Slow bend tests provided quasi‐staticK1cdata while dynamic initiation values were obtained from instrumented Charpy impact tests. In both types of tests high‐speed crack velocity measurements were made using conductive silver grids applied to the specimens. The variation ofK1cwith strain rate and temperature was found to be small and in general agreement with expectations from the relaxation properties of polycarbonate. Notch acuity was found to have little effect in that values ofK1cand crack velocity from specimens having the standard Charpy notch were similar to values obtained from sharp‐cracked specimens. Some anisotropy was present in the material and gave rise to a small variation in K1cvalues with direction of crack propagation. Crack velocity and also the fracture mode transition temperature showed considerable dependence upon orientation. It was thought that dynamic toughnessKd, was influenced more thanK1cby partial orientation of amorphous regions in the material and that the relaxation time for re‐orientation was probably significant here.

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