We are all acquainted with the various ways that materials break when they are loaded past their limit: the brittle snap of a plastic part or the tearing of a pop top tab are the familiar results of a large overload. A more insidious type of gradual failure can occur when a component is subjected to a cyclic load. In this case, cracks can develop and grow over time even when the peak stresses from the load are much lower than the material’s tensile strength. This type of progressive failure due to multiple applications of load is known as “fatigue.”
While direct cyclic loading of components can be used to evaluate fatigue resistance, testing of small standardized samples of a material is more often performed. The types of test used for this are broadly separated into those that use smooth samples and those which contain a crack. The former combines the entire life cycle of fatigue from the initiation of a crack from a small, sometimes microscopic flaw, through crack growth to failure of the sample. The latter type of test uses a sample that contains a sharp crack. It addresses only the growth stage of the fatigue process. Such fatigue crack growth rate (FCGR) testing provides a relationship between a crack growth parameter, described below, and the associated rate of crack growth. This information can then be used to predict the life of structures containing cracks of known size in locations where stresses can be determined.