A Multiaxial Fatigue Theory Including Mean Stress and Ratcheting Effects

Abstract

Most engineering components are subjected to cyclically varying loads with mean stress (or mean strain). Often the loads may result in a multiaxial stress state. It is well known that the fatigue process is sensitive to superposed tensile mean stress in both the high-cycle and low-cycle fatigue regimes. In an elastoplastic regime, a material which experiences a stress-controlled cyclic loading with a non-zero mean stress, will exhibit an accumulated ratcheting strain (cyclic creep strain). This ratcheting strain can cause an additional damage in the material. Thus, a complete fatigue theory should be capable to evaluate the effects of mean stress and ratcheting strain. A large number of multiaxial fatigue theories have been proposed in the past and are reviewed in the literature, for example, see Refs [1–2]. These theories may be divided into four categories, viz: equivalent-stress and -strain, energy-based criteria and the critical plane approaches. The first two approaches usually have difficulty to include the mean stress effect and to account for path dependency of the cyclic plastic deformation. This essential interaction between stress path and strain path is inherently included in the energy approach. In the present paper an energy-based fatigue theory proposed by Ellyin and his collaborators [3] is extended to include the mean stress and ratcheting strain effects on the fatigue life of materials.