Mechanical heterogeneity and validity of J-dominance in welded joints

Fracture criterion of the J-integral finds wide application in the integrit y evaluation of welded components, but there exist some confused problems s uch as the dependence of the fracture toughness on the strength mis-matchin g and specimen geometry which need to be clarified. It is rough and unsuita ble to attribute the variation of J-integral fracture parameter simply to t he effect of mechanical heterogeneity. In the present paper, a two-dimensio nal finite element method is employed to analyze the distribution and varia tion of crack tip field of welded joints with different strength mis-matchi ng in four kinds of specimen geometry, and then the validity of J-dominance in welded joints is investigated. It is found that the crack tip field of mis-matched joint is different from that of either the weld metal or base m etal of which the joint is composed, but it is situated between those of we ld metal and base metal. Under the plane strain, there is obvious differenc e in stress triaxiality for different strength mis-matched joints. The vali dity of J-dominance in welded joint can not be obtained by comparing whethe r the stress triaxiality meets that required by the HRR solution because of the existence of mechanical inhomogeneity. By ascertaining if the stress t riaxiality of welded joint near the crack tip is dependent of specimen geom etry, the conclusion can be arrived at: for plane stress the validity of J- dominance is valid, whilst for plane strain the validity of J-dominance is lost. Based on the above, attempt has been made to point out that the influ ence of mechanical heterogeneity on the fracture toughness of weldment aris es from the variation of constraint intensity-crack tip stress triaxiality. Compared with the effect of mechanical heterogeneity on the stress triaxia lity, the losing of validity of J-dominance in mis-matched joint under plan e strain may play a more critical role in the variation of J-integral fract ure parameter of weldment.