Crack tip constraint is known to affect the fracture resistance of materials. The effect of biaxial loading on a center crack in an X100 steel plate has been investigated. The crack driving force and the constraint parameter are estimated based on the two-parameter J-A2 theory in elastic-plastic fracture mechanics with the aid of finite element analysis. The center-cracked plate is subject to various degrees of biaxiality (defined as the ratio of the transverse stress parallel to the crack and the opening stress normal to the crack). Using the constraint parameter (A2) in uniaxial loading condition as a reference value, a Constraint Enhancement Factor is introduced to facilitate the investigation of crack tip constraint under biaxial loading. The analysis carried out in this paper has established a relationship between the Constraint Enhancement Factor and the biaxiality. With the J-A2 fracture model, the critical applied load and the critical crack driving force can be expressed as functions of biaxial loading ratio. The methodology and analysis results can be used in structural integrity assessment of a pressure vessel or piping which contains a crack under biaxial loading.

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