Abstract:When a crack is subjected to shear force, crack branching usually occurs. Theoretical study shows that the crack branching under shear loading is caused by tensile stress, but not caused by shear fracture. The co-plane shear fracture could be obtained if compressive stress with given direction is applied to the specimen, subsequently, calculated shear fracture toughness, K_ⅡC , is larger than K_ⅠC . A prerequisite of possible occurrence of mode Ⅱ fracture was proposed. The study of shear fracture shows that the maximum circumferential stress theory considered its criterion as a parametric equation of a curve in K_Ⅰ,K_Ⅱ plane is incorrect; the predicted ratio K_ⅡC /K_ⅠC=0.866 is incorrect too.

Key words: stress intensity factor; fracture toughness; mode of fracture; mode of loading