The Cracked Situation

Abstract

Because of the nature of metal fatigue mechanisms, the fatigue lives of specimens, components and structures are sometimes dominated by fatigue crack initiation, and sometimes by fatigue crack propagation (see Sections 3.4, 6.1 and 6.5). From a practical viewpoint, probably the most significant advance in the understanding of metal fatigue behaviour was the general realisation, some 30 years ago, that many components and structures are crack propagation dominated (Frost 1975). Cracks, or crack like flaws, may be introduced during manufacture, especially if welding or casting is used, or cracks may form early on during service (Pook 2000a, Murakami 2002). As examples of cracked situations, Figure 7.1 shows multiple fatigue cracking from crack like flaws in a 25 mm thick structural steel cruciform welded joint, and Figure 7.2 fatigue cracking from shrinkage in a 30 × 35 mm cast steel bar.