Modeling of stress ratio effect on Al alloy SAE AMS 7475-T7351: Influence of loading direction

The main goal of this work was to evaluate the effectiveness of Walker's equation in collapsing the fatigue crack propagation data of a SAE AMS 7475-T7351 aluminum alloy loaded either longitudinally (L-T) or transversely (T-L) to the rolling direction. T-L orientation testpieces presented lower ductility and fracture toughness values than L-T orientation. As a consequence, during the fatigue crack propagation tests, T-L testpieces exhibited a stronger influence of monotonic modes of fracture, resulting in higher Paris exponent values, m. Walker's model was able to collapse fatigue crack propagation data of L-T test pieces at different applied stress ratios, R. However, for the T-L orientation, due to the R ratio dependency on in and C, simply averaging of m values for the calculations of Walker's exponent proved to be inefficient. A simple analytical procedure was proposed by the authors to modify Walker's model to take into account such effect. For T-L test pieces, when Walker's model is modified by considering both Paris's exponent as well the coefficient as a function of the R ratio, the fatigue crack growth data collapses within a narrow band, thus allowing predictions to be made satisfactorily. The collapsed band is even narrower if the empirical relation m = a+blogC is used instead of simple polynomial equations due to a better correlation coefficient.