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. We observe throughout this chapter that the fatigue strength of a component is significantly 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 flows 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 fluctuating loading conditions, it is common practice to modify the static failure theories and material strength for the purposes of design.

The fatigue failure phenomenon was first recognized in the 1800s when railroad axles fractured after only a limited time in service. Until about the middle of the 19th century, repeated and static loadings were treated alike, with the exception of the use of safety factors. Poncelet’s book in 1839 used the term fatigue owing to the fluctuating stress. At the present time, the development of modern high-speed transportation and machinery has increased the importance of the fatigue properties of materials. In spite of periodic inspection of parts for cracks and other flaws, numerous major railroad and aircraft accidents have been caused by fatigue failures. The basic mechanism associated with fatigue failure is now reasonably well understood, although research continues on its many details [1-31]. The complexity of the problem is such that rational design procedures for fatigue are difficult to develop. The great variation in properties makes it necessary to apply statistical methods in the evaluation of the fatigue strength.