The effects of residual stress on elastic-plastic fracture: two diffraction studies

Brittle fracture in structural materials is strongly influenced by the presence of residual stresses. During more ductile fractures the influence of residual stress is reduced by plastic deformation prior to the initiation of fracture. In structural integrity assessments, it is therefore important to account for the interaction between applied loads and residual stresses. This interaction was studied in two experiments. In the first, residual stresses in aluminium alloy 7075-T6 three-point bend specimens were measured prior to loading using neutron diffraction. In the second, the stress field in compact tension specimens of ferritic pressure vessel steel was mapped using energy-dispersive synchrotron X-ray diffraction as the specimens were loaded. In both cases, finite element modelling of the elastic-plastic fracture was performed using residual stresses either reconstructed from, or validated using, experimental results. This method of analysis allowed us to compare the effect of residual stress on two different fracture mechanisms. The aluminum specimens fractured in brittle manner, and analysis of experimental load-displacement results showed that residual stresses were superimposed almost linearly with externally-applied stresses at the point of fracture initiation. In the steel specimens, fracture initiated by ductile tearing and it was shown using finite element analysis that residual stress had only a very small effect on the crack-driving force at higher levels of applied load. In fact, at the point of tearing initiation prior strain-hardening caused by the indentation process used to introduce residual stresses had a greater effect on the crack-driving force than the residual stress state itself.