Recently, interest on fracture toughness under mixed mode loading increases and many questions remain to be answered, especially for ductile fracture. Crack growth resistance curves for mixed mode loading may be interpreted as a result of competition between usual ductile fracture and shear fracture. Analysis is based on previous work in the field of ductile fracture, especially on studies of ductile fracture criteria of the type “loss of stability of homogeneous deformation field”. Phenomena of shear fracture can be successfully predicted by bifurcation analysis of shear band initiation or localized necking. Usual ductile fracture under different stress triaxialities can be analogously predicted by cavitation instability criterion. Usual (cavitation-type) ductile fracture strain depends on strain hardening exponent less than shear fracture strain does and this explains dependence of the KIIc/KIc ratio on strain hardening exponent. These ideas are demonstrated quantitatively using data in the open literature.

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