As discussed in earlier chapters, the total life of a cyclically loaded component is composed of both the crack initiation and crack propagation stages. Modern defect-tolerant design approaches to fatigue are based on the premise that engineering structures are inherently flawed; the useful fatigue life then is the time or the number of cycles to propagate a dominant flaw of an assumed or measured initial size (or the largest undetected crack size estimated from the resolution of the nondestructive inspection method) to a critical dimension (which may be dictated by the fracture toughness, limit load, allowable strain or allowable compliance change). In most metallic materials, catastrophic failure is preceded by a substantial amount of stable crack propagation under cyclic loading conditions. The rates at which these cracks propagate for different combinations of applied stress, crack length and geometrical conditions of the cracked structure, and the mechanisms which influence the crack propagation rates under different combinations of mean stress, test frequency and environment, are topics of considerable scientific and practical interest.

In this chapter, we examine the mechanics and micromechanisms of stable crack propagation in ductile solids subjected to uniaxial and multiaxial cyclic loads of fixed amplitudes. Attention is focused on circumstances for which linear elastic fracture mechanics concepts are expected to be valid. Fatigue crack advance where considerable nonlinear deformation occurs ahead of a crack tip or notch tip is considered in Chapters 15 and 16. Situations involving crack growth under variable amplitude fatigue are examined in Chapter 14.