1 - Mechanism of fatigue in the absence of defects and inclusions

Abstract

In order to evaluate quantitatively the effects of defects and inclusions we must first understand the basic mechanism of fatigue. Researchers who are mainly interested in the mechanism of fatigue on a microscopic scale may study the behaviour of dislocations during the fatigue process. In fact, active research in this field, including many experiments and theories on persistent slip bands and various dislocation structures, has led to understanding of some aspects of the fatigue phenomenon. However, study from the viewpoint of dislocation structure is somewhat qualitative, and has not so far been developed to a level that permits the quantitative solution of practical engineering problems. In this chapter, discussion of the fatigue mechanism is based on more macroscopic phenomena such as those observed with an optical microscope. The phenomena observed with an optical microscope are those that may be detected within one grain, in commercial materials, ranging in size from a few μm to several tens of μm. Thus, the process of initiation and propagation of so-called small cracks is perhaps the most important phenomenon discussed in this book. Although several theories of small cracks have been proposed, this chapter is restricted to the presentation of experimental evidence during the fatigue of unnotched specimens and to the derivation of practically useful conclusions.