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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 or cyclic crack-tip hysteresis energy, to model crack-growth rate behavior was studied. Crack-growth simulations using the modified Dugdale crack closure model showed a close relation between traditional ΔKeff and the cyclic crack-tip displacement (Δδeff) for an aluminum alloy and a steel over a wide range in stress ratios. Evaluations of the cyclic hysteresis energy demonstrated that the cyclic plastic damage below the crack-opening stress was negligible in the Paris crack-growth regime. Some of the standard and newly proposed remote measurement methods to determine crack-opening stresses and the “effective” crack-tip driving parameter were evaluated from crack-growth simulations made on middle-crack tension specimens. Here, analyses were conducted under both constant-amplitude and single-spike-overload conditions. A potential source of the Kmax effect on crack-growth rates was studied at high stress ratios and at high stress levels for an aluminum alloy. Results showed that the ratio of Kmaxto 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.
fatigue crack growth, fracture mechanics, cracks, stress-intensity factor, crack closure, plasticity, constraint
Senior scientist, NASA Langley Research Center, Hampton, VA