The Effects of Microstructure, Deformation Mode and Environment on Fatigue

So-called laws for predicting the lives of components at high temperatures have been proposed (e.g. strain range partitioning, frequency modified fatigue life, time and cycle fraction etc.). These “laws” usually work quite well for certain situations but are less successful in predicting life for other conditions and materials that are quite similar. These laws are based on generic notions of damage with several adjustable parameters. Such laws are useful from an engineering perspective when considerable data base is available. However, they are far less successful in predicting behaviour of new materials or existing materials under different external conditions. In essence, most of these models have evolved into very sophisticated curve fits and generally fail to take into account the actual behaviour of the material. For example, interactions between deformation mechanisms in the material and environmental attack and the changing nature of the material are not explicitly considered. In this paper, the physical damage processes that take place in high temperature fatigue and fatigue crack propagation (FCP) of different commercial high temperature alloys is presented. This information is used to develop fatigue physically-based models.