THE CRAZE INITIATION RESPONSE OF A POLYSTYRENE AND A STYRENE-ACRYLONITRILE COPOLYMER DURING PHYSICAL AGING

The influence of structural recovery or physical aging on the craze in itiation of amorphous polystyrene and a styrene-acrylonitrile copolyme r has been examined in two states of stress/deformation and at differe nt temperatures. Studies of craze initiation in equibiaxial creep were performed using a ''bubble'' inflation test geometry while uniaxial e longation in stress relaxation conditions was studied by bending the s pecimen over a variable radius of a curvature test jig. All tests were performed by aging the specimen and testing it at the same temperatur e subsequent to a quench from above the glass transition to below it. The results are compared with expectations from three craze models: St ernstein and Ongchin (1), Argon (2), and Kambour (3). The results are most consistent with the model of Kambour, but discrepancies arise bec ause, e.g., the impact of aging on the stress for crazing in the equib iaxial tests is different for the polystyrene than for the styrene-acr ylonitrile copolymer. The results also suggest that there are two regi mes of behavior for craze initiation under equibiaxial stresses, a beh avior not manifested in the uniaxidal stress relaxation experiments.