Abstract
The propagation of fatigue cracks through carburized cases has been studied in two high alloy steels at a constant stress intensity range (δK) and at a constant cyclic peak load. The crack propagation rates (da/dN) were significantly decreased in residual compressive stress fields and increased in tensile stress fields. A superposition model was used to interpret these phenomena. We define an internal stress intensity factor, Ki = δidi1/2, where δi is the residual stress at a given point and di is a distance characteristic of the internal stress distribution, and the effective stress intensity becomes, Ke = Ka + Ki, where Ka is the applied stress intensity. Values of Ke are interpreted to be equivalent to δKe, the effective cyclic stress intensity factor, and used to predict the crack propagation rates from experimental da/dN vs δK curves for the unstressed material. A reasonably good fit was obtained using measured values of δi and a value of di = 11 mm (.43).
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