ABSTRACT
A member may fail at stress levels substantially below the yield strength of the material if it is subjected to time-varying loads rather than static loading. The phenomenon of progressive fracture due to repeated loading is called fatigue. Its occurrence is a function of the magnitude of stress and a number of repetitions, so it is called fatigue failure. Photographs (Figure 7.1) represent two components failed by fatigue. We observe throughout this chapter that the fatigue strength of a component is signi cantly affected by a variety of factors. A fatigue crack most often is initiated at a point of high stress concentration, such as at the edge of a notch, or by minute ¡ows in the material. Fatigue failure is of a brittle nature even for materials that normally behave in a ductile manner. The usual fracture occurs under tensile stress and with no warning. For combined ¡uctuating loading conditions, it is common practice to modify the static failure theories and material strength for the purposes of design.
