The prediction of constraint-dependent R6 failure assessment lines for a pressure vessel steel via micro-mechanical modelling of fracture

The work demonstrates the ability of micro-mechanical modelling (the local approach) to predict the effects of both temperature and crack tip constraint on the probability of cleavage failure and the anticipated amount of pre-cleavage tearing of a pressure vessel steel. The basic fracture toughness data comprised information from deep-cracked (high constraint) and shallow-cracked (low constraint) specimens tested at temperatures ranging from -45 to 20 deg. C. The micro-mechanical modelling of ductile rupture employed a temperature-independent Gurson model, which was used in combination with a temperature-sensitive cleavage model to analyse the data in the transition regime. The cleavage parameters were tuned using a Monte-Carlo technique, which compared the outcome of the finite element models with a random set of data drawn from the statistical model of the basic experimental data. Application of the modified boundary layer formulation allowed analyses of the effects of both constraint and temperature on the amount of ductile tearing and the probability of cleavage failure to be undertaken. The results of these can easily be seen in the R6 diagram through constraint-modified failure assessment lines as set out in Appendix 14 of R6. In particular, by displaying the constraint corrections available for 0.2 mm ductile crack growth and 5% probability of cleavage failure, this paper demonstrates the enhancement of fracture resistance that can be obtained from low-constraint geometries