Low cycle fatigue estimation of welded butt joints and weld treatments subjected to cyclic bending

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In this report a procedure to estimate fatigue life for welded joints under bending load has been presented. The methodology of fatigue life prediction used in this investigation is based on strain-life method. The research carried out within the project develops a method for determination of plastic stress and strain distribution in welded parts by using three-dimensional solid cubic finite elements. The first part of the research deals with fatigue analysis of a welded butt joint in a cantilever type flat plate under periodic bending load. Elasto-plastic finite element analysis has been carried out for this joint using software ANSYS. The local plastic stress and strain obtained from ANSYS were used in Morrow’s modified Manson-Coffin equation and Smith-Watson-Topper damage equation to estimate the number of cycles the weld can tolerate. One of the most important and influential phenomena in fatigue life prediction is residual stress. The presence of residual stresses in welded parts can noticeably affect fatigue behaviour. Tensile Residual stress generated during the welding process is inevitable. Therefore, applying residual stress as an initial condition in finite element analysis is an important part of the modelling. The stress and strain distributions at critical regions caused by combination of residual stress and loading were obtained for chosen specimens and subsequently used to determine the fatigue life. The effect of residual stress on fatigue life and positive improvement of weld durability against cyclic loads by eliminating of residual stress or converting tensile stress to compressive stress in high cycle fatigue cases are well known and clear but these effects in low cycle cases are less clear and known. The finite element modellings undertaken in this study show that although the presence of tensile residual stress has a significant effect on fatigue life in high cycle cases, in low cycle fatigue situations, it is almost unimportant and negligible. This finding reveals that any effort and cost to eliminate the residual stress or improve it, when the low cycle fatigue is the main problem, would be unnecessary and uneconomical. The stress concentration is another phenomenon that effects on fatigue life. Any irregularity, discontinuity or abrupt changes in cross section such as holes, notches or angels could change stress distribution so that the common stress equations no longer work to describe the stress field at these locations. Real stress due to stress concentration is higher than nominal stress calculated by regular methods. Stress concentration factor is directly related to the geometry. The materials protruded outside the base plate at the weld zone produce sharp angles between weld material and base plate material which is the main reason of the stress concentration. By removing the material protruding outside the base plate to eliminate the discontinuity angels, it could be assumed that the stress concentration factor has been eliminated and the stress has been reduced, subsequently, the fatigue life has been improved. Grinding is a very common weld finishing and treating method that removes the stress concentration sources. Comparison between regular weld and grinding finite element models reveals that the stress concentration factor has important effects on both high-cycle and low-cycle fatigue situations so that improving the stress concentration condition by using a grinding process can significantly increase the fatigue life. In order to compare and validate the predicted fatigue lives by numerical modellings with the experimental data, a series of laboratory tests were performed. Therefore, a fatigue testing set-up was prepared. This set-up consists of a cantilever type flat plate set on a frame and the whole assembly is then installed on a shake table. A loading arm installed to a fixed frame next to the shake table, generated constant amplitudes of bending deflection on the specimens. The experimental testing included two parts. Firstly, fatigue tests were performed for five different loading situations applied on the regular weld specimens. Secondly, the same loading situations were used for welded specimens ground by hand disk grinder. To achieve an acceptable statistical accuracy three samples for each loading were tested. The experimental tests show a good agreement with computational models for regular weld specimens but contrarily, in ground specimen cases the opposite results were observed which would be discussed in the body of the thesis.
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