CHAPTER 7 - EFFECTS OF RESIDUAL STRESS ON BRITTLE FRACTURE

Abstract

The usual definitions of applied stresses and residual stresses, arising from incompatibilities of strain in a body, are considered to depend on arbitrary definition of the boundaries of the body, from which it is argued that the fracture response of the material must be the same for each. Examples of residual stress systems are described, such as bolted, riveted, and welded joints, shrink fits, lack of fit in assembly, and rolled, peened, and differentially hardened surfaces, illustrating the extent to which residual stresses are susceptible to control and are often beneficial. The mechanisms of residual and reaction stress accumulation in welded joints are described in a semiquantitative manner to show, in particular, the influence of the equalization temperature and plastic yielding. The effects of residual stress on brittle fracture in casualties are illustrated in terms of nucleating defects and spontaneous cracking. The Fawley oil storage tank is identified as a model casualty, susceptible to exact analysis, and providing the key to laboratory study by notched and welded wide plate tests. The results of those summarized also contribute to knowledge of the effects of furnace and mechanical stress relief and of multirun welds in thick plates, and show that local damage at the nucleating flaw from thermal strain activated plastic flow is at least as important as elastic residual stress in producing spontaneous or low stress fractures. The methods of linear fracture mechanics are used to study crack propagation and arrest in low-stress fractures of notched and welded wide plates, such that the applied and residual elastic stress contributions to fracture are quantitatively compared; and the metallurgical nature of the local embrittlement is identified, and its influence on toughness estimated. It is shown that critical fracture conditions depend on balance between the static loading toughness in the damaged region and the propagation resistances, as affected by strain rate in the undamaged material beyond. Optimum conditions for furnace, mechanical, and local thermal stress relief are quantitatively described. It is considered that the topic of welding residual stresses is now of waning research interest, having been overtaken by a demand for investigation of the morphology of defect growth and local thermal damage in low alloy steels of structural quality.