All machine and structural designs are problems in fatigue because the forces of Nature are always at work and each object must respond in some fashion.

Carl Osgood, Fatigue Design

This chapter introduces the essential concepts of fatigue and impact. Fatigue is a process wherein a material accumulates damage due to a cyclic load; this damage compromises strength and can lead to brittle fracture, even for ductile materials. In fatigue, flaws initially present in the material grow until one dominates; this crack then propagates through the part with every stress cycle. The chapter begins with an analysis of Mode I fracture with uniaxial stresses, and presents S-N diagrams and associated theories for uniaxial fully alternating stresses. Some materials have an endurance limit; if the stresses remain below the endurance limit, then a fatigue failure is not likely. Many factors that affect the endurance limit are discussed, including stress concentrations, surface finish, temperatures, residual stresses, part size, and desired reliability. For materials without an endurance limit, fault tolerant design approaches are needed to avoid fatigue failures. The chapter also describes failure when there is a significant mean stress, as well as when the stress state is more complicated. Fracture mechanics approaches to fatigue are also introduced. The chapter ends with a discussion of impact stresses that result from dynamic loadings.