Abstract
Damage-tolerant design and life-prediction methodologies have been practiced for metallic structures for decades, although their application to brittle materials, such as ceramics and intermetallic alloys, still poses particular problems, primarily because of their extreme flaw-sensitivity. 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 are described for both brittle and ductile materials. Examples are given from the military engine “High-Cycle Fatigue” initiative, which represents an important problem where information on the behavior of small fatigue cracks is critical.
