An accurate knowledge of the dependence of the fatigue crack growth rate (da/dN) on the stress intensity factor range (ΔK) is necessary to perform a safety analysis of any structure. Fatigue crack growth tests are normally performed on simple, two-dimensional finite thickness specimens to determine this dependence. Certain anomalies in this dependence have been observed when specimen thickness and mean stress have been varied. The thickness effect and the mean stress effect on the fatigue crack growth rate are related to the variation in crack closure and the local stress intensity factor along the crack front. A simple model incorporating both of these two effects is proposed. The model is applied to fatigue crack growth data for a nickel-base super alloy (IN-100) with very good success.

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