The importance of small fatigue cracks in advanced materials

Damage-tolerant design and "lifing" methodologies have been practiced for metallic structures for decades; however, their application to ceramic and intermetallic materials poses particular problems. This is because these so-called advanced materials are generally highly flaw-sensitive. Moreover, like metals, they are susceptible to premature failure by cyclic fatigue, which provides a prominent mechanism for subcritical crack growth that further limits life. One specific problem involves the large dependency of growth rates on the applied stress intensity, which necessitates that design is based on the concept of a fatigue threshold, particularly in the presence of small cracks. In this paper, studies on the role of small cracks in influencing thresholds and near-threshold growth rates indicate that behavior in both advanced and traditional materials is remarkably similar. However, microstructures with optimal small-crack growth resistance are rarely similar to those optimized for large crack-growth properties.