Mechanisms of fatigue failures

Fatigue failures account for 90% of strength failures in engineering components. In order to analyse such failures and learn to prevent them in future, it is necessary to understand how and why fatigue failures occur. In this paper, an attempt has been made to contribute to such understand¬ing. The concept of the weakest link in a structure initiating failure is discussed. It is shown that the weakest link principle can be extended to explain multiple origin of fatigue cracks. The micromechanisms that are responsible for the initiation of a fatigue crack and those that come into play during the growth process are highlighted. The formation of typical features of fatigue fractures, like striations and beachmarks, are dis-cussed in relation to the mechanisms operative during fatigue crack growth. For the choice of materials for providing service under fatigue loading conditions, and to assess the integrity of components and structures already in service under such conditions, the fatigue resistance of material has to be quantified. The conventional approach to such quanti-fication and the more recent fracture mechanics based differential approach to representing the fatigue resistance of materials are discussed.