Residual Stress

Topics: Welding, Gas tungsten arc welding, Residual stress Pages: 8 (2570 words) Published: April 17, 2013
Department of Mechanical Engineering Chendhuran college of Engineering and technology Pudhukkottai, Tamilnadu Abstract— This project is aimed to understand the influence of residual stresses on the fatigue failure of butt welded stainless steel pipe joints. In order to carry out this study it requires an experimental fatigue failure data and a computer aided analysis of these results. A finite element scheme will be developed to simulate the residual stress in weld using the experimental data. A Thermo-elasto-plastic analysis will be used to replicate the butt welded pipe joint and the residual stress will be determined.

Residual stress that arises in welded joints by heating and cooling cycles during the welding process is another major factor in fatigue failure of welded structures. Welding residual stresses might lead to a drastic reduction in the fatigue strength of welded elements. In multi cycle fatigue (N > 106 cycles), the effect of residual stresses can be comparable to the effect of stress concentration. The effect of residual stresses on the fatigue life of welded elements are significant as regards relieving harmful tensile residual stresses and introducing beneficial compressive residual stresses in the weld toe zones. The fatigue failure can be classified in to two categories based on the number of cycles taken to fail. A. High cycle fatigue High-cycle fatigue is when the number of cycles to failure is large, typically when the number of cycles to failure, Nf is greater than 103. B. Low cycle fatigue Low- cycle fatigue is when the number of cycles to failure is small, typically when the number of cycles to failure, Nf is less than 103. III. RESIDUAL STRESS ON WELD Residual stresses can be defined as the stresses that remain within a material or body after manufacture and material processing in the absence of external forces or thermal gradients. They can also be produced by service loading, leading to inhomogeneous plastic deformation in the part or specimen. Residual stresses can be defined as either macro or micro stresses and both may be present in a component at any one time. They can be classified as: Macro residual stress that develop in the body of a component on a scale larger than the grain size of the material. Micro residual stresses that vary on the scale of an individual grain.

I. INTRODUCTION Welding has a number of detrimental effects on the structural integrity and in-service performance of the weldments. These detrimental effects are due to imperfections induced by the welding in the weldments, of which the structural shape change behavior, residual stresses and the weld solidification cracks are reported to have very severe degrading effects on the mechanical strengths and possibly can lead to catastrophic failure. Fatigue is a type of fracture that occurs in welds that are subjected to changing or varying stresses over time. Fatigue is mainly caused by the environment in which the welded joints are utilized. Fatigue fracture is a ductile fracture, and therefore occurs by non-uniform plastic deformation. Micro cracks and voids form after a certain number of cycles and grow proportional to the number of cycles eventually grow large enough to overcome recovery mechanisms and move quickly to fracture. The rate of this crack nucleation is proportional to the frequency of the applied stress. II. FACTORS IN FATIGUE FAILURES Fatigue failures of welded joint are initiated by many factors, such as stress concentration, environment, loading and residual stresses. Stress concentration is mainly caused by the geometrical discontinuity and thus initiates fatigue crack at the locations of discontinuity. Stress concentration may also result from weld defects and metallurgical discontinuity. Fatigue strength of weld component may also be significantly affected by the environment,...
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