On criteria for craze initiation in glassy polymers

Abstract

The previous published data on biaxial critical stresses for craze initiation on the surface of cylindrical specimen of glassy polymers have been reexamined. A new interpretation is presented for the trend of crazing stress near pure shear state in the second quadrant of principal stress space which was observed in air at elevated temperatures. That is, an increase in the tensile stress for crazing with an increase in the magnitude of the compressive stress is interpreted not to be followed by a decrease in the dilatational stress but by development of shear yielding. On the basis of this interpretation, a new empirical criterion for craze initiation is proposed by considering the stress concentration due to surface scratch. The theoretical crazing locus accorded with the previous experimental results, except for the data near the shear yield locus in the second quadrant, which were considered to be affected by shear yielding. It was also indicated in the calculation that the shape and direction of surface scratch exert a considerable influence on the trend of biaxial crazing stress.