Numerical analyses of strength mis-match effect on local stresses for ideally plastic materials

The effect of strength mis-matched welded joints on the local stresses in the neighbourhood of a crack has been investigated, in the context of ideally plastic materials, using the finite element method and slip line field analysis. Several key parameters affecting the local stresses have been identified: degree of strength mis-match, M, slenderness of the weld, (W-a)/H, and crack location within the weld. Loading geometry effects have been considered by investigating tension and bending specimens. The analyses were performed for plane strain and plane stress conditions.It is found that, for the case when the crack locates within the weld metal, the crack tip stress triaxiality depends not only on M but also on (W-a)/H. The effect of (W-a)/H is worth noting, as its effect on the crack tip stress triaxiality is restricted to certain ranges of (W-a)/H, which in turn depend on M, geometry and loading. For an interface crack, the stress triaxiality at the softer side is higher than that for a crack in a specimen wholly made of the softer material; the reverse is true for the harder side. This explains the ductile failure behaviour of cracks observed in experiments: the crack path tends to deviate into the softer material in spite of its higher toughness. Another notable point is that, under sufficiently high under-matching conditions, a second peak stress ahead of and remote from the crack tip may occur, the magnitude of which depends on M and (W-a)/H. Present findings are believed to be vital for design and defect assessment of welded joints with conventional welding as well as advanced welding techniques.