Chapter 16 - Mechanism of Fatigue and Fracture

Abstract

This chapter discusses the mechanism of fatigue and fracture. Fatigue is the cumulative material damage caused by cyclic loading. Many structural members must withstand numerous stress reversals during their service life. Generally, the load amplitude of each cycle is not large enough to cause the structural failure by itself; but failure could occur when the accumulated damage experienced by the structure reaches a critical level. The fatigue life of a structural detail is directly linked to the fatigue process. The methods for fatigue analysis are: simplified fatigue analysis, spectral fatigue analysis, and time domain fatigue analysis. In order to study the fatigue and fracture damage mechanism, numerous experiments have been conducted to investigate the material characteristics. These experiments can be divided into two categories: stress-controlled fatigue and strain-controlled fatigue. Crack growth can be predicted using Paris equation. Integration of this equation involves numerical methods unless F is independent of crack length. The fatigue can be classified as high-cycle fatigue and low-cycle fatigue. These observations indicate that the fatigue is a three-stage process involving initiation, propagation, and a final failure stage. The fatigue of a specimen subjected to strain-controlled loading is generally related to low-cycle high-stress fatigue. The stress associated with low-cycle fatigue will usually be high enough to cause a considerable amount of plastic deformation in the region of the stress concentration. Thus, the relation between stress and strain will no longer be linear.