The J-Q two-parameter fracture theory proposed recently by O'Dowd and Shih [1][2][3] has shown that the stress fields in the forward sector ahead a crack tip under very general conditions of loading in finite bodies can be described by two parameters J and Q. The J-integral sets the scale of deformation at the crack tip (i.e. the CTOD) while the hydrostatic stress parameter, Q, quantifies the level of stress triaxiality ahead of the crack tip. The J-Q theory has been successfully applied to brittle cleavage fracture, ductile fracture and brittle-ductile fracture transition.O'Dowd and Shih [3] gave Q solutions for centre-cracked tension, double-edge cracked tension and three-point bend specimens with various crack lengths. Their solutions are based on the small scale yielding solution as the reference stress. In this report Q solutions for ASTM standard compact tension (C(T)) and singleedge cracked tension (SE(T)) specimens are presented and the dependence of the Q-parameter on the material properties is studied. We use the HRR stress as the reference and the definition of Q is [3] Q = ao0--(O'oo)HRR at 0 = 0 , r = 2 J / a o (1) (70 where ao is the yield stress of the material and aoo is the hoop stress and (~rOO)HRR is the HRR stress. Q values for these two geometries have been obtained by plane strain finite element analyses based on small strain deformation plasticity by use of ABAQUS Version 4-9 (HIBBITT, KARLSSON & SORENSEN, INC.). Eight-noded biquadratic displacement, linear pressure elements with reduced intergration are employed. Because of symmetry only one half of the specimen is analysed. 36 wedge-shaped collapsed eight-noded isoparametric elements are used to surround the upper half Int Journ of Fracture 68 R98of the crack tip. 73 nodes sharing initially the same location at the crack tip are allowed to displace independently. A typical mesh has about 1000 elements. The materials modelled obey the Ramberg-Osgood relationQ values for C(T) specimen with crack length-width ratio a / W = 0.3, 0.5, 0.6 and 0.7 are given in Fig.1. The modelled material is material A given in Table 1 but with various hardening exponent n = 3, 5, 10 and 20. In the figure, J is normalised by the crack length a for a/W = 0.3 and by the remaining ligament b = (W -a) for other crack lengths. For these crack lengths high stress triaxiality is maintained at lower deformation level of J/(aao) or J/(bao) less than about 0.01. At the higher deformation level, the global bending stress field causes a rapid decrease of the stress triaxiality. These results are similar to that obtained by O'Dowd and Shih [3] for three-point bend specimen. However, at the medium deformation level Q value decreases with increasing crack length a/W. This behaviour is different to other geometries and may be caused by the global bending stress having a stronger effect on large crack lengths. Since the HRR-stress is used as the reference stress, a significant effect of strain hardening on Q value is observed in that Q increases with increasing n as shown in F...