An Evaluation of the Plasticity-Induced Crack-Closure Concept and Measurement Methods

An assessment of the plasticity-induced crack-closure concept is made, in light of some of the questions that have been raised on the validity of the concept, and the assumptions that have been made concerning crack-tip damage below the crack-opening stress. The impact of using other crack-tip parameters, such as the cyclic crack-tip displacement, to model crack-growth rate behavior was studied. Crack-growth simulations, using a crack-closure model, showed a close relation between traditional ∆Keff and the cyclic crack-tip displacement (∆δeff) for an aluminum alloy and a steel. Evaluations of the cyclic hysteresis energy demonstrated that the cyclic plastic damage below the crackopening stress was negligible in the Paris crack-growth regime. Some of the standard and newly proposed remote measurement methods to determine the “effective” crack-tip driving parameter were evaluated on middle-crack tension specimens. A potential source of the Kmax effect on crack-growth rates was studied on an aluminum alloy. Results showed that the ratio of Kmax to Kc had a strong effect on crack-growth rates at high stress ratios and at low stress ratios for very high stress levels. The crack-closure concept and the traditional crack-growth rate equations were able to correlate and predict crack-growth rates under these extreme conditions. INTRODUCTION In 1968, Elber observed that fatigue-crack surfaces contact each other even during tension-tension cyclic loading and he subsequently developed the crack-closure concept [1]. This observation and the explanation of crack-closure behavior revolutionized the damagetolerance analyses and began to rationally explain many crack-growth characteristics, such as crack-growth retardation and acceleration. Since the discovery of plasticity-induced fatigue-crack closure, several other closure mechanisms have been identified, such as