Fatigue

Abstract

Fatigue fracture and its characteristic appearance. Various types of fluctuating stress cycle and its components. Standard fatigue test and S–N diagram. Fatigue properties, and reason for existence of fatigue limit. Statistical nature of fatigue and estimation of anticipated fatigue life by means of standard statistical procedure. Fatigue crack initiation through persistent slip bands, such as slip-band extrusions and slip-band intrusions. Fatigue crack growth: Stage I and Stage II. Crack growth rate and ‘Paris’ law. Cumulative fatigue damage with definitions of overstressing, understressing and coaxing. Methods of presenting S–N data, where mean stress is not zero. Design criteria for mean stress effects: Gerber relation, Goodman relation or Goodman diagram to prevent fatigue failure, and Soderberg relation to prevent yielding. Effects of stress concentration, specimen size, metallurgical variables, frequency of stress cycling, corrosive environment, low and high temperature on fatigue and thermal fatigue. Effects of specimen surface, such as its roughness, properties and residual stress on fatigue. Surface treatments beneficial to fatigue and metallurgical processes detrimental to fatigue. Cyclic strain-controlled fatigue, describing cyclic strain hardening and cyclic strain softening, and their dependency on material’s stacking fault energy. Low-cycle fatigue and Coffin–Manson relation. Total fatigue strain–life equation, and its plot approaching towards the plastic strain–life curve at large total strain amplitudes and the elastic strain–life curve at low total strain amplitudes. Creep–fatigue interaction, where suggested important design approaches are: cumulative damage rule, modification of Goodman law, frequency-modified fatigue relation and strain-range partitioning method. Increasing amplitude tests, such as step test and Prot test. Problems and solutions.