The Effect of Material Strength Mismatching on Constraint at the Limit Load of Welded Three-Point Bend Specimens

This paper describes the results of a series of finite element analyses performed to investigate the suitability of the coefficient of the J-CTOD relationship, dn, as a parameter to quantify constraint. Analyses have been performed which employ the modified boundary layer solution to demonstrate the relationship between the T-stress, Q and dn parameters. Analyses have also been performed to analyse the effects of constraint in strength mismatched welded three-point bend specimens. These results are compared with predictions of constraint made using values of dn derived from slip-line field solutions. Material strength overmatching is shown to cause a significant loss in constraint, whilst undermatching increases constraint. On the whole, predictions of the effects of constraint from slip-line field solutions are shown to agree with the measured constraint levels obtained using the finite element method, although the results from highly undermatched joints are not as accurate as the others examined. This is shown to be due to the effect of the base material outside the weld on the crack tip stress fields. By employing a two-material idealisation of the modified boundary layer formulation, using elastic T-stresses to model the constraint due to the specimen geometry and the normalised load parameter, J/hσYw, to control the size of the plastic zone relative to the thickness of the weld material, it was possible to reproduce the complex stress fields encountered in each of the specimens.