Crack-growth calculations in 7075-T7351 aluminum alloy under various load spectra using an improved crack-closure model

This paper is concerned with the application of an improved crack-closure model, FASTRAN, to calculate crack growth under aircraft spectrum loading. The improvements are related to more accurate development of the plasticity-induced closure mechanism, especially, for high stress ratio spectra, and to using improved crack-growth rate results in the near threshold regime. The model was used to correlate fatigue-crack-growth-rate data under constant-amplitude loading over a wide range in rates and in stress ratios from the NASGRO materials database and the literature. The model was then used to calculate crack growth under three simulated aircraft load spectra studied by Brot and Matias on 7075-T7351 aluminum alloy plate. The paper demonstrates how constraint (plane-stress and plane-strain behavior) plays a leading role in the retardation and acceleration effects that occur under variable-amplitude loading. Comparisons are made between measured and calculated crack-length-against-cycles for the aluminum alloy and two crack configurations under three simulated aircraft load histories. The calculated fatigue-crack-growth lives were within ±20% of the test lives.