TABLE OF CONTENTS In plain metallic specimens, such as those shown in Figures 3.2 and 3.4, or in the presence of a mild notch, such as that shown schematically in Figure 4.18, fatigue lives are dominated by fatigue crack initiation, rather than by fatigue crack propagation (Frost et al. 1974). If fatigue crack initiation is defined as the development of a small crack, say 0.5 mm deep, then typically 80 per cent of the fatigue life is occupied by fatigue crack initiation.
Very few components are of uniform cross section; most contain some form of change of cross section resulting from a discontinuity such as a fillet, a hole, or an external groove or notch. The bicycle crankshaft shown in Figure 4.19 is an example. For convenience, in metal fatigue, any of these discontinuities is generally referred to as a notch, irrespective of its geometric shape. Failure analysis of components often shows that a fatigue crack had initiated at some point at a notch root. A crack initiates here because the fatigue stresses at, or near, the notch root are higher than the nominal fatigue stresses away from the notch. The car drive line component shown in Figure 1.1 and the marine steam turbine blade shown in Figure 3.15 are examples of fatigue failures originating at a notch. The brass chain link, Figure 1.4, is an example of a fatigue failure in a region of uniform cross section. All three are examples of crack initiation dominated situations, that is
