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In a common interpretation of crack closure, the crack is visualized as “peeling” open as stress is applied. This interpretation, while useful, has led to the assumption that the driving force for crack propagation, ΔKeff, exists only when the crack tip is fully open (above K-opening). However, evidence of significant crack-tip cyclic strain below the K-opening will be presented. The exclusion of this additional driving force can yield misleading values of ΔKeff. This is particularly so in the near-threshold regime where opening loads are typically high. The new analysis technique for estimating ΔKeff is referred to as the adjusted compliance ratio (ACR) method and is based on an interpretation of crack closure as a stress redistribution (or load transfer) on a relatively compliant crack wake. The ACR method is evaluated using the results of fatigue crack growth tests on 6013-T651, 2324-T39 and 7055-T7751 aluminum alloys using the center crack tension M(T) specimen geometry and stress ratios ranging from -1.0 to above 0.96. The experimental results of this study indicate that the fatigue crack growth rate is not determined solely by ΔKeff but also depends on Kmax. It was observed that this Kmax dependence takes the form of a power law with the magnitude of the exponent being a measure of Kmax sensitivity. As a result, ΔKeff curves from all test conditions could be collapsed to a unique intrinsic FCGR curve using a simple modification to the Paris Law. It is expected that continued research in this area will lead to improvements in fatigue life prediction methodology.
fatigue, fatigue threshold, crack propagation, effective stress intensity, crack closure, stress ratio
Staff engineer, Alcoa Technical Center, Alcoa Center, PA
Director, Fracture Technology Associates, Bethlehem, PA