Crack tip constraint analysis in welded joints with pronounced strength and toughness heterogeneity

Application of standardized fracture testing methods in heterogeneous welded joints might lead to overestimation or underestimation of fracture toughness and consequentially to inaccurate estimation of loading capacity of welded structure. Combining experimental testing and numerical modelling of double mismatched welded joints provided a view on influence of pronounced strength heterogeneity on fracture behaviour under monotonous loading. Investigated double mismatched welded joints had a fatigue crack located in one mismatched weld material region. The fatigue crack plane was perpendicular to the interface of two distinctive mismatched weld material regions, while the stable crack extension was estimated to be from one weld material region to other, crossing the interface. Single edge notched bend (SE(B)) specimens were used to investigate double mismatched welded joint fracture behaviour under high constraint conditions. Crack extension has been evaluated using normalization data reduction technique and crack driving force has been evaluated as specified in ASTM E1820 standard. Stress triaxiality has been implemented as a second fracture parameter, evaluating level of constraint. Obtained results show that combination of strength overmatched and undermatched weld material has a significant effect on the weld load capacity and stress triaxiality ahead of the crack tip. The latter leads to reduced or increased fracture toughness of the weld as the crack tip is closer to the fusion line between overmatched and undermatched weld material. Finally, computational simulations revealed how double mismatched weld configuration alters the stress field near the crack tip and corresponding values of J-integral. This implies that standard fracture test overestimate or underestimate fracture toughness of the double mismatched welded joint.