Damage mechanics models of ductile crack growth in welded specimens

Two-dimensional, plane strain, finite element analyses of strength-mismatch ed welded joints have been performed using the modified boundary layer form ulation. The welds were idealized as two-material joints with the material interface running parallel to the crack, which was embedded in the weld mat erial. The Rousselier ductile damage model was employed within the weld mat erial to simulate crack extension due to the growth and coalescence of micr ovoids. By analysing models with different levels of material mismatching, weld dimensions and applied T-stress levels, it was possible to analyse the effects of crack tip constraint due to both material mismatching and speci men geometry on the fracture resistance of the weld material. The results show that material strength overmatching (where the weld materi al is stronger than the base material) reduces the level of constraint ahea d of the crack, which can increase the resistance to fracture of the weld m aterial. Conversely, material strength undermatching increases crack tip co nstraint, reducing the fracture resistance of the joint. By employing estim ates for the crack tip constraint levels, Q(M), based on the applied load, level of material mismatching and weld region thickness, it has been possib le to 'order' the F-resistance curves of overmatched joints by generating a family of F-Q(M) loci which describe the effects Of constraint on the frac ture resistance of the weld material. However, it is shown that the Q(M)-st ress parameter is not capable of describing the effect of material strength undermatching on the fracture resistance of a joint, which can be much low er than that obtained from a high-constraint homogeneous specimen of weld m aterial.