Variable Amplitude and Multiaxial Fatigue

Abstract

Early fatigue tests on structures, components and test specimens were all carried out using constant amplitude fatigue loading. However, many structures and components are subjected in service to variable amplitude fatigue loading (or variable amplitude loading), with variations following either a regular or a random pattern (Frost et al. 1974). Variable amplitude fatigue loadings can divided into two broad classes (Pook 1979): those in which individual load cycles can be distinguished, such as narrow band random loading (Figure 4.1), and those in which individual load cycles cannot be distinguished, such as broad band random loading (Figure 4.2). Modern fatigue testing equipment makes it possible to apply virtually any desired load history, and narrow and broad band random loading have been used extensively for some time, especially in structural testing (Pook 1983a, Marsh 1988). The use of broad band random loading can make the analysis of test results difficult (Smith 1965). Concepts taken from random process theory (Papoulis 1965, Bendat and Piersol 1971, 2000) are used in the characterisation of random load histories (see Section B.2.1). Conventions used in the fatigue testing literature sometimes differ from those usual in random process theory. Although referred to as random, load histories used in tests are usually pseudo random in that they repeat exactly after a return period.