Fatigue behavior of polystyrene and effect of mean stress

Abstract
The behavior of polystyrene under cyclic loading has been investigated. Tests have been run under completely reversed axial stress and also at various superimposed mean stress values. In one series of tests, the alternating stress component was maintained at a value of 2500 psi, while the mean stress was varied, in steps of 500 psi, from 0 to 2500 psi. Over the stress range investigated, the logarithm of the fatigue lifetime was found to decrease linearly with increase of the mean stress component. In a second test series, the peak stress was maintained constant (2500 psi) while the mean stress component and the alternating stress component were both varied. For this set of tests, the specimen fatigue lifetimes increased steadily with increase of mean stress (to 1500 psi) and decrease of alternating stress (to 1000 psi). For higher mean stress values, and therefore lower alternating stress values, there was a rapid rise toward infinite lifetimes. The fracture surfaces of all specimens were examined by scanning electron microscopy. Introduction of tensile mean stress produced significant changes in some aspects of the fatigue fracture process and these changes are described and illustrated. Design implications of the data are discussed.

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