Chapter 11 - Multiaxial loading: Fracture and fatigue under complex stress states

Abstract

This chapter discusses the monotonic fracture of brittle materials and the HCF limits of metals. Current methods used for the assessment of notches and cracks are outlined and the basic theoretical approaches are the same in these two cases, though with some extra subtleties in the case of fatigue. A strategy for applying theory of critical distances (TCD) methods such as the PM and LM is developed and these methods are used to predict experimental data. The findings suggest that the TCD can certainly be used for multiaxial problems in both fracture and fatigue, though a number of decisions need to be made regarding the failure criterion to be used and the direction of the focus path. Further work is needed in this area, with the aim of developing a systematic approach which considers both shear stresses other tensile stresses, which arise due to constraint, specimen thickness, notch orientation, and the T-stress. However, multiaxial fatigue is a much more complex problem than uniaxial, as proved by the large number of competing criteria being used in the current practice. There is no one perfect solution, and different approaches may be optimal for different materials and types of loading. Conclusively, TCD has clear advantages when it comes to assessing notches and other stress concentrators subjected to complex loading states.