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Thick three-dimensional (3-D) finite element models of centre cracked plates are used to study the variation in the biaxiality factor ,L3 with the crack aspect ratio a/w. The use of two widely accepted methods to evaluate the T-stress in two-dimensions, namely the boundary layer method and the displacement field method, to calculate the T-stress in three-dimensions is studied. It is shown that the boundary layer method gives results that compare rather well with the two-dimensional plane strain values (maximum difference of 6 percent), while the displacement field method results are about 15 percent lower. Two parameters are shown to affect the threedimensional evaluation of the biaxiality factor, namely the material's Poisson's ratio v and the specimen's thickness t. The biaxiality factor is directly proportional to u and inversely proportional to t. Three-dimensional analysis is required to assess correctly the effect of u and t on ,B. Nomenclature Abbreviations: 2-D = two-dimensional 3-D = three-dimensional b.1. = boundary layer f.e. = finite element f.m. = full model LEFT = linear elastic fracture mechanics SSY = small scale yielding Latin symbols: a = half crack length a/w = crack aspect ratio E = Young's modulus h = half of cracked plate height J = J-integral Irl = stress intensity factor for mode I K/Kc = normalised stess intensity factor r, 0 = cylindrical co-ordinates centred at the crack tip T = T-stress t = model thickness 36 B.S. Henry and A.R. Luxmoore u, v = displacement in the x-direction and y-direction respectively w = half of cracked plate width Greek symbols: /3 = biaxiality factor u = Poisson's ratio p = notch diameter 0"ij = stress tensor 0"us = yield stess of material