Mechanisms of high temperature fatigue and creep-fatigue failure in engineering materials

In this chapter detailed consideration is given to the failure mechanisms occurring during elevated temperature low cycle fatigue and creep-fatigue cycling conditions. It is shown that in addition to the operation of the basic fatigue mechanism, which often initiates a dominant surface crack, other time-dependent processes operate to a varying degree. These include creep deformation and fracture processes, metallurgical changes and environmental effects. In failures controlled by a dominant crack these processes can markedly influence both crack initiation and growth rates. It is shown that effects on crack growth rate are usually small, the important factor being their effect on crack-tip displacement. Serious acceleration of failure can occur when internal fracture damage resulting from creep processes directly contributes to crack advance. By studying the development of failure under laboratory test conditions it is possible to isolate the parameters governing such failures and the factors controlling their rate of development. (author)