Quantification of residual stresses induced by prior loading at high temperatures

High temperature components generally undergo cyclic loading conditions. Prior tensile/compressive loading of a fracture specimen can induce compressive/tensile residual stress fields at the crack tip. These residual stresses will influence the subsequent fracture behaviour of the cracked body. This work forms part of a project to examine the influence of creep induced damage at a crack tip on subsequent fatigue crack growth and fracture toughness properties of austenitic type 316H stainless steel. Creep damage is introduced local to the crack tip of a fracture specimen by interrupting a creep crack growth test, performed at 550 °C. Prior to testing, the material was pre-compressed in order to strain harden the material. The compact tension, C(T), specimen geometry has been considered in this work. Since residual stresses are known to influence fatigue and fracture toughness properties of a cracked body, it is important that the residual stress levels at the crack tip are quantified. Neutron diffraction (ND) measurements have therefore been performed to quantify the extent of residual stress in these samples after initial loading, and compared to finite element model predictions. Two specimens have been considered with the crack plane orientated in parallel and perpendicular to the pre-compression direction. Compressive residual stresses of around 100 MPa have been measured directly ahead of the crack tip. Reasonable predictions of the principal residual stress distributions have been obtained by the simplified FE analysis. Though the tensile properties differ significantly in for specimens orientated parallel and perpendicular to the pre-compression direction, no significant differences in the residual stress field are predicted in the C(T) specimens orientated in both directions.