Strain-Based Fatigue Analysis—Low Cycle Fatigue

Three regions were identified in Sect.  1.2 and shown in Fig.  1.10 as Region I, II and III concerning the fatigue strength of materials. In Sect.  2.5 Region III pertaining fatigue limit was analyzed. In that region the stress amplitude in not high enough to propagate a micro defect even though it may have been generated. In Chap. 5 we have been discussing of Region II, the region of high cycle fatigue where load levels are low and the main component of stress is elastic, when not totally elastic. In this chapter we will examine as last the first region or Region I of low cycle fatigue where strains and, in particular, plastic strains are the dominant factor. When load levels are low, stresses and strains are linearly related by Hooke’s law. Under such conditions they are fully interchangeable. But when the plastic component cannot be neglected anymore and, indeed, prevails, stresses are no longer uniquely determined and must be put aside. Although most engineering structures and components are design such that nominal loads remain elastic, stress concentrations can easily introduce a non-negligible plastic strain component.