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In a common interpretation of crack closure, the crack is visualized as “peeling” open as stress is applied. This interpretation 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). The current opening load method of closure measurement in accordance with ASTM Standard Test Method for Measurement of Fatigue Crack Growth Rates, Appendix X2 (E 647) is based on this concept. However, based on a comparison with “closure free” data, evidence of significant crack-tip cyclic strain below K-opening will be presented to show that the exclusion of this additional driving force can give misleading values of ΔKeff. This is so particularly 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 both compact tension C(T) and middle crack tension M(T) specimen geometries. The results of this study indicate that ΔKeff obtained by the ACR method is relatively independent of measurement location, crack length and specimen geometry. Comparison of ΔKeff data obtained by both methods with “closure free” crack growth data obtained at high stress ratios indicates that the ACR method is superior to the ASTM opening load method in the near-threshold regime for estimating the “true” driving force for crack propagation.

A B S T R A C T To properly interpret the results of standard fatigue crack growth tests it is often necessary to incorporate corrective techniques to the K applied data. This is especially true in the near-threshold regime where long crack data need to be closure corrected to predict small crack behaviour. It is also an issue in the presence of residual stress. A methodology to separate the influence of sample size, geometry, crack length and residual stress from the standard crack growth test data to obtain a true material response is presented. Stress ratio and residual stress contributions from known combinations of assumed crack size, applied stress and residual stress are also addressed and incorporated in the fatigue crack growth behaviour. a = crack length a i = notch length before crack initiation a n , a n+1 = current crack lengths measured at two successive steps n and n+1 da = change in crack length ACR = adjusted compliance ratio C o = δ app / P, compliance above the opening load C s = δ eff / P, secant compliance C i = δ i / P, compliance of the notch before crack initiation E = modulus of elasticity n = K max sensitivity exponent P max = maximum load P min = minimum load P op = opening load P 0 = zero load R = ratio of minimum to maximum load or stress Z (a) = influence function dδ = change in displacement due to change in crack length dδ max = change in displacement at maximum load due to change in crack length dδ res = change in displacement due to residual stress and change in crack length δ max = change in displacement at maximum load due crack advance from notch δ app = closure free displacement range δ eff = actual measured displacement range δ i = measured displacement range before crack initiation δ cl = δ app − δ eff , displacement difference due to closure