Study of the interaction of residual stress and applied loading on fracture

Abstract

Residual stresses are often present in engineering components and structures, usually due to manufacturing processes and assembly. Tensile residual stress, especially in the presence of a flaw, reduces the failure load of a system subject to external loading. It is common practice in evaluating the integrity of a structure to use failure assessment codes to assess a structure’s fitness-for-purpose. These codes may attempt to decompose the stresses present in the structure under analysis as either primary stresses (fixed-load and contribute to plastic collapse) or secondary stresses (fixed-displacement and do not contribute to plastic collapse). Traditionally residual stresses are considered as secondary stresses unless they are seen as sufficiently long range and do not self-equilibrate over the cracked section where they may be considered primary. In reality all residual stresses will be somewhere between the extremes of fixed-load and fixed-displacement loading conditions. When considering the damage evolution of a defective region within a structure, be it due to creep, plasticity or crack growth, elastic follow-up determines how the residual load is reduced, between the extremes of fixed-load and fixed-displacement. This paper considers a whole structure approach for the analysis of long-range residual stress. To examine elastic follow-up (EFU) effects with the aim of providing engineers better guidance on the classification of long-range residual stresses as either primary or secondary stresses, two test rigs are designed and manufactured. The experimental results obtained shows that the test rig with low EFU represents global relaxation while high EFU leads to local relaxation. It is concluded from experimental results that the longer crack initiation times are a consequence of the relaxation and redistribution of the residual loads in the test rigs.