Papers by Author: N.P. O'Dowd

Abstract: This paper considers the prediction of the effects of tensile and compressive residual stress in fracture mechanics specimens by the application of a mechanical pre-load. This is considered in the context of a ‘C’ shape specimen which is mechanically pre-tensioned or pre-compressed to produce, respectively, a compressive or tensile residual stress in the region where the crack is introduced. Finite-element analysis is performed to simulate the pre-loading and the subsequent fracture loading of the cracked specimens. The finite-element predictions are compared with experimental data including residual stress measurements using neutron diffraction. A discussion is presented on modelling and material issues pertaining to the use of mechanical pre-loading as a means for introducing residual stress.

Abstract: This paper considers the measurement of residual stresses induced by mechanical loading in a weld Type 347 stainless steel. The work is based in part on an ongoing Round Robin collaborative effort by the Versailles Agreement on Materials and Standards, Technical Working Area 31, (VAMAS TWA 31) working on ‘Crack Growth of Components Containing Residual Stresses’. The specific objective of the work at Imperial College London and HMI, Berlin is to examine how residual stresses and prior straining and subsequent relaxation at high temperature contribute to creep crack initiation and growth for steels relevant to power plant applications. Tensile residual stresses have been introduced in the weld by pre-compression and neutron diffraction measurements have been carried out before and after stress relaxation at 650 oC. Significant relaxation of the residual stresses has been observed, in agreement with earlier work on a stainless steel. Preliminary results suggest that the strains local to the crack drop by over 60% after 1000 h relaxation at 650 oC for the weld steel. The results have been compared with finite element studies of elastic-plastic pre-compression and stress relaxation due to creep.