Fatigue crack growth in metals and alloys: mechanisms and micromechanics

Fatigue crack growth has been studied using several new experimental tools in the past ten years. The observation of fatigue cracks during growth under high resolution conditions has shown that crack advance is an intermittent process. These results, when combined with measurements of crack opening, displacements, crack closure, crack tip strains, fractography, and other information, leads to a reasonable understanding of many intrinsic aspects of fatigue crack growth at ambient temperature in a number of metallic alloys. Models of fatigue crack growth are reviewed from the perspective of this understanding. No model has achieved the capability of predicting fatigue crack growth from a description of microstructural and mechanical properties. The factors limiting a deeper understanding of fatigue crack growth are also more clearly defined, which gives some direction for future research. This paper is a critical review of crack growth mechanisms, mainly for large fatigue cracks subject to constant amplitude loading, but some mention is also made of small cracks and variable amplitude loading. Since the mechanisms of crack growth are the focus of the review, the effects of environment and temperature are largely excluded. Microstructural effects are included to the degree possible.