There is continuing interest in defining a fracture parameter which is capable of characterising the ductile crack growth resistance o~" a material under conditions of large scale plasticity. The aim of this study is to assess the capability of a numberof, elastic-plastic fracture parameters for the case of a C-Mn steel. The fracture parameters considered are: the crack opening displacement at the original crack tip (CTOD); the crack opening angle at the growing crack tip (CTOA); and the crack opening displacement at the growing crack tip (6°). The parameters were assessed by determining experimentally whether they were affected by changes in test specimen size and geometry in cases where there was no change in the crack tip constraint. Changes in the latter were assessed from changes in the CTOD at crack initiation (CTOD i). Only 6~ was capable of characterising the crack growth resistance, as unlike both CTOD~ and 6a, dCTOD/da and CTOA were dependent on specimen geometry.

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One mode by which a structure can fail in an unstable manner is ductile crack propagation from a pre-existing defect.Two approaches have been suggested to predict the conditions for such an instability. These are the T approach of Paris et al. and the I approach of Turner. The Paris T approach is based on the stability of the equilibrium state between the applied value of the J integral and the J versus crack extension material resistance curve. Under linear elastic conditions instability may be interpreted as occurring when the second differential of available energy (with respect to crack extension) exceeds that required for crack extension. In the elastic-plastic regime this interpretation is lost and the tearing modulus approach uses the J integral as a crack tip field characterising parameter. The Turner approach, on the other hand, is based on the balance between the energies available and required for crack extension.Under linear-elastic conditions instability occurs when the first differential of available energy exceeds that required for crack extension. Under elastic-plastic conditions the Turner approach continues to be an energy balance with the driving force being the elastic energy release rate, I , rather than the linear-elastic energy release rate, G. A comparison of the two approaches has been carried out by re-analysing the instability results of Paris et al., obtained using single-edge-notched, three-point bend specimens under conditions where the J integral can be considered as a crack tip field characterising parameter. Under these conditions both approaches, although based on different assumptions, are numerically similar and the predicted conditions for instability are in good agreement with experimental results. In addition, the two approaches are shown to be numerically similar for the case of centre-cracked tension specimens under limit load conditions. This agreement in two geometries of differing constraint gives tentative support for the generality of this result

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