Chapter 12: Fatigue of Metals

number of cycles) to differentiate between the endurance limit found in ferrous metals and the fatigue strength of nonferrous alloys. Significant attention is devoted to the statistical nature of fatigue data and the influence of mean stress on fatigue life, employing analytical models like the Goodman, Soderberg, and Gerber diagrams to predict failure probabilities under combined static and dynamic loading. The discussion advances to cyclic stress-strain behavior, exploring phenomena such as the Bauschinger effect, cyclic hardening, and softening, which serve as the basis for analyzing low-cycle fatigue using the Coffin-Manson relation to correlate plastic strain amplitude with fatigue life. Structural aspects of fatigue are detailed through the stages of damage evolution: crack initiation at persistent slip bands (intrusions and extrusions), Stage I crystallographic shear, and Stage II tensile crack growth characterized by microscopic striations. The chapter integrates linear elastic fracture mechanics to describe crack propagation rates, introducing the Paris law which relates crack growth per cycle to the stress intensity factor range. Practical engineering design factors are thoroughly evaluated, including the impact of stress concentrations (notch sensitivity), size effects, and surface finish, alongside methods to enhance performance such as shot peening and surface rolling to induce beneficial compressive residual stresses. Furthermore, the text covers cumulative damage theories like Miner's rule for varying load histories, the interaction of combined stress states, and the effects of metallurgical variables like grain size and inclusions. Finally, the chapter outlines modern design philosophies—including infinite-life, safe-life, and damage-tolerant approaches—and addresses environmental challenges such as corrosion fatigue, fretting, and thermal fatigue caused by cyclic temperature changes.