Use of simple load-independent constraint parameter for prediction of brittle fracture

Abstract

In this paper, a load-independent constraint parameter based on T-stress is suggested for prediction of brittle fracture in the ductile-to-brittle transition region under different constraint conditions. This parameter is applied to the results of EU project VOCALIST and in particular to fracture toughness data for pre-cracked Charpy specimens with deep and shallow cracks (steel A533B specially heat treated to achieve an elevated yield stress). These were obtained from the VOCALIST project in NRI Řež. In contrast to the usual procedure using a non-dimensional ratio of T-stress and yield stress, T/σy, for constraint evaluation, a slightly different parameter is proposed. This is based on the load-independent ratio of T-stress and the stress intensity factor KI, T/KI, for prediction of constraint-dependent brittle fracture. A simple 'universal' dependence of constraint correction factor f = Kmatc/Kmat on constraint parameter T/KI seems to accurately describe the behaviour of ferritic steels, provided that ductile crack growth preceding brittle fracture is non-existent or negligibly small. In the paper, both calibration of the dependence and its verification through independent data for two other ferritic steels are performed. In addition, a modification of the factor f = 1 + α(-T/ σy)m used in the R6 procedure is proposed and leads to a 'universal' dependence of fracture toughness ratio Kmatc/Kmat on constraint. Finally, results of the application of the constraint corrected toughness concept within the Failure Assessment Diagram of the R6 procedure when applied to VOCALIST data are presented. © 2006 Journal compilation. The Authors © 2006 Blackwell Publishing Ltd.