The influence of constraint on crack tip stress fields in strength mismatched welded joints

This paper describes the results of a series of two-dimensional, plane strain, finite element analyses which were performed using the modified boundary layer formulation to model two material idealisations of welded joints containing cracks. The cracks were assumed to be contained within the weld material and ran parallel to the material interface. The effects of changes in geometrical constraint on the crack tip stress fields were modelled by varying the magnitude of the T-stress term at the crack tip, and the effects of constraint due to strength mismatching were investigated by changing the strength of the base material outside the weld. The results show that constraint due to material mismatching depends on the level of strength mismatching, the applied load, and the width of the weld material. A normalised load parameter, J hσ Yw , scales the size of the plastic zone with the width of the weld material, and can be used to quantify the level of constraint for a given degree of mismatching. Any two specimens loaded to the same value of this parameter will have the same crack tip stress field, regardless of the size of weld material. The effect of geometrical constraint on overmatched welds, applied through the T-stress term was similar to that observed in homogeneous materials. For undermatched specimens the influence of the T-stress term on the stress field depends on the normalised load parameter. A level of constraint can be obtained in undermatched specimens which is higher than that obtainable in homogeneous laboratory specimens. This paper discusses the implications of this on the fracture resistance of undermatched welds.