Fatigue and fracture surface morphology of polystyrene

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Abstract
The morphology of fatigue fracture surfaces of polystyrene was studied by light microscopy. The character of the fracture surfaces is affected by the method of preparation of the test specimens. In specimens manufactured by injection-molding, there exists a compressive internal stress in the thin layer under the surface which affects the duration of crack initiation and first propagation periods. The dependence of the area of the resulting mirror surface on the number of cycles to fracture shows an exponential course and, in the region of higher stress levels, also shows an exponential dependence on the stress. The propagation rate of the fatigue crack follows the relation dl/dN = kl n, in which the constant n is independent of the stress for the stress conditions and temperature used. The constant k shows an exponential dependence on the stress. The occurrence of rib markings, which can be observed in the last phases of fatigue-induced crack propagation, is discussed.

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