Modeling of stress ratio effect on Al alloy SAE AMS 7475T7351: Influence of loading direction
Abstract
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 7475T7351 aluminum alloy loaded either longitudinally (LT) or transversely (TL) to the rolling direction. TL orientation testpieces presented lower ductility and fracture toughness values than LT orientation. As a consequence, during the fatigue crack propagation tests, TL 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 LT test pieces at different applied stress ratios, R. However, for the TL orientation, due to the R ratio dependency on m 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 TL 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+ blog C is used instead of simple polynomial equations due to a better correlation coefficient.
 Publication:

Journal of Materials Engineering and Performance
 Pub Date:
 October 2006
 DOI:
 10.1361/105994906X136160
 Bibcode:
 2006JMEP...15..608D