FRESH LOOK AT FATIGUE.

No one is sure exactly what causes fatigue failures, although metallurgists have devised a reasonable theory based on known behavior of metals. Basically, the theory describes fatigue failure in two steps. First is the ″crack nucleation″ step where submicroscopic, intragranular faults or cracks develop into a larger, microscopic crack. Second, in the ″crack propagation″ step, the microscopic crack grows visible and rapidly spreads until the part fails. Fatigue is usually initiated at points of maximum stress concentration such as holes, fillets, keyways, as well as microscopic flaws in the material - inclusions, for example. Significantly, the direction of the applied load, as dictated by part design, may determine where fatigue failure takes place. This is the first of four articles on the mechanism of fatigue.