A MULTIAXIAL ENDURANCE CRITERION FOR LCF / HCF COMPLEX CYCLES

Abstract. A multiaxial fatigue criterion that takes explicitly into account the defect size was developed in a previous study [1]. The material is assumed to contain defects modeled as cracks. The criterion is developed within the LEFM frameworks. Non-singular terms in the asymptotic developments of stresses at crack tip were considered. A critical elastic distortional energy is used as a yield criterion for the crack tip region. The elastic domain of the crack tip region is viewed as an endurance domain. With this approach, the defect size effect in the Kitagawa and Takahashi diagram [2] and the hydrostatic stress dependency in the Dang Van [3] criterion are both successfully reproduced. However, this model neither accounts for mean stresses effects nor for history effects. Therefore the centre of the elastic domain is introduced as an internal variable that represents history effects induced by plasticity. The FE method is used to analyse plastic deformation in the crack tip region and to build the evolution law of the centre in a (T, KI) diagram. An incremental formulation is used to make easier the simulation of the effect of variable loads on the fatigue resistance. This approach is applied to the Ti-6Al-4V alloy.