Temperature and enviromental effects on the fatigue fracture in polystyrene

Abstract

Fatigue tests have been conducted on polystyrene in air at different mean stress conditions and also over the temperature range−60 to 60°C using notched specimens. In addition, some tests were performed with the specimens immersed in a detergent and in a corn oil. The data were first represented on a conventional log ΔK-log da/dN plot to determine the Paris law parameters and how these varied with mean stress, temperature and environment. Considerable variations were noted but no useful pattern could be discerned. The data were then analysed using a recently proposed two-stage line plastic zone model so that a crack tip zone stress and a fatigue damage factor could be found. These stresses were found to be very high in air and to increase with increasing mean stress, and this is attributed to a high degree of constraint at the crack tip. There was an abrupt change in σc at about 20° C which, it is suggested, is caused by the onset of crazing due to the presence of a loss peak. Associated changes in Kc were also noted. In environments, the stresses were much smaller and agreed with those obtained in static fracture, suggesting that crazes are formed in environments prior to, and not during, fatigue crack growth. The stress reduction factor, α, remained constant at about 0.2 with both temperature and environment.