Fatigue is a consequence of a metal undergoing repeated cyclic stress. In a commercial context this stress results from a large number of cycles of loading and unloading. Sudden fracture can result. Fatigue damage and the compromise of stability and integrity of the structural member present the constant potential for structural failure.

It is presently not possible, under any generally acceptable theory of fatigue phenomena, to predict by analysis alone when the fatigue/stress limit is reached and when a fracture may occur. Further, in normal usage, damage occurs cumulatively, at microscopic levels, and can only be detected in its early stages by examining the microscopic structure.

This difficulty has caused designers of structures subject to fatigue to avoid this problem by "over-designing" structures to limit the stresses in critical areas to a level well below the known endurance limits of the material employed. This results in extreme expense through overbuilding. In spite of this, catastrophic fatigue failures still occur. Thus, there is a need to measure the microscopic level of fatigue status, since other available levels of analysis do not address this level of necessary detail. There is also an obvious need to inspect the subsurface areas and components of a particular structure or item of equipment, beyond the boundaries of surface visual inspection.