5. Fatigue and Environment-Assisted Testing

Fatigue is the process of damage and failure of materials and structures due to cycling loading. It was demonstrated that under repeated loading materials fail at stresses well below the ultimate strength, and microscopic damage accumulates until cracks or other forms of macroscopic damage develop. Failure due to fatigue loading is called “fatigue failure”. The number of loading cycles leading to failure of structural or machine components is called fatigue life. The main objective of fatigue analysis is to determine the fatigue life for a repeated fluctuating load of constant or variable amplitude. In this chapter we will consider two major approaches for the study of fatigue failure. The stress-based approach which is based on nominal stresses and does not account on the mechanisms of fatigue failure, and the fracture mechanics approach which considers the micro and macromechanisms of fatigue failure and uses the principles of fracture mechanics for initiation and propagation of cracks. This approach provides a better understanding of fatigue failure by analyzing the initiation, propagation and instability processes of fatigue cracks. For both approaches we present the mechanical tests performed on specimens subjected to cyclic loading. The objective of the tests is to generate fatigue life and crack growth data and demonstrate the safety of a material or structure. The chapter concludes with a study of the environment-assisted fracture.