Fatigue crack growth characteristics in cast Al–Si–Mg alloysPart II. Life predictions using fatigue crack growth data

Abstract

Abstract Fatigue crack growth data are critical inputs for damage tolerant designs, which acknowledge initial flaws in materials subject to cyclic loading. Fatigue crack growth data are used for life predictions, and in addition they provide understanding of the microstructural effects on crack propagation at different stages during service life. Linear elastic fracture mechanics (LEFM) methods have been applied to demonstrate how microstructure and heat treatment can be selected and optimized for given design requirements. Life predictions based on LEFM calculations were performed using AFGROW software. AFGROW is a life predictive tool developed by the Wright–Patterson Air Force Research Laboratory and intensely used by NASA's crack growth life prediction program. This software is used here to integrate the fatigue crack growth curves of several cast Al–Si–Mg alloys in different heat-treat conditions. Several examples of life predictions are presented through case studies representing real life applications such as automotive suspension components and cylinder blocks. The theoretical foundation of microstructure and heat-treat effects on the fatigue crack growth behavior of Al–Si–Mg alloys, were mechanistically laid out in Part I of this paper. Part II is a companion to the theoretical work (Part I), and brings to the reader's attention a practical perspective to engineering analysis and design through a simple and effective method of utilizing fatigue crack growth data. Using AFGROW software an engineering problem can be approached in two ways. First, the software can be used (and it was used in this paper) to rank and select an alloy for a given application based on experimental fatigue crack growth data on various existent materials. Second, the software can be used as an alloy/microstructure/heat treatment optimization tool for future alloy development and applications using fatigue crack growth resistance as an alloy design goal. In this case, the desired operating conditions and the expected life are imposed, and the alloy, microstructure, and crack growth characteristics required to meet them are determined. The methodology offered here can be further tailored for both direct and reverse engineering problems for any material and application.