Abstract: In this paper, in- and out-of-plane constraint are considered in the process of fracture toughness determination. A three parameter approach-J IC , Q and T z is utilized. The plane strain fracture toughness J IC must be determined experimentally according to valid standards. It is the only quantity measured experimentally except for the classical uniaxial stress-strain curve determined for the material tested. Two additional parameters should be computed numerically using 2D and 3D FE analysis for the structural member which is actually analyzed. The T z function is the generalized Poisson's ratio for elastic-plastic materials. The general model proposed allows for fracture toughness determination in two cases-when one fracture mechanism dominates the fracture process or two or more mechanisms are active simultaneously. To apply the model the fracture toughness for the particular fracture mechanisms should be known. In the paper simple models to determine these quantities are proposed both for cleavage and ductile fracture. In the latter case a distinction is made between fracture mechanisms along the shear lips and in the central part of the specimen.The model proposed has been validated by an experimental programme on single-edge-notched specimens in bending and associated numerical computations.For ductile fracture both in- and out-of-plane constraints play an important role in determination of the fracture toughness of a structural element. The procedure proposed to compute this quantity, although based on simple 'physical' models, provides good qualitative and quantitative estimation of fracture toughness of a structural element at least for the materials tested.
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