Predicting the effect of constraint on cleavage and ductile fracture toughness using area contour toughness scaling

The influence of constraint on the fracture toughness properties of materials containing defects has long been recognised. To maintain conservative design and assessment principles, lower bound measurements of fracture properties are commonly used. These are measured using test pieces with a high crack tip constraint. With more accurate design processes and tools becoming more widespread, there is a move toward using more representative properties than these lower bound values. The work presented provides a method that can be used to predict the influence of constraint on the cleavage and ductile fracture toughness of a range of ferritic steels, and hence the associated benefit to the onset upper shelf temperature, defined here as the intersection between the fracture toughness loci associated with 5% cleavage fracture and a 50% ductile initiation probability. The Anderson and Dodds toughness scaling procedure, based on the maximum principal stress, has been used with a range of normalised material tensile properties to generate solutions that allow the constraint benefit to cleavage fracture toughness to be predicted for different ferritic steels. A comparison of predictions with published data shows that this results in conservative predictions, similar to those using the Beremin Weibull stress. For the purposes of predicting constraint benefits to ductile initiation toughness, defined at 0.2 mm stable tearing, the scaling method proposed by Anderson and Dodds was extended to the Rice and Tracey model as a way to provide solutions for the same matrix of tensile properties. This approach was validated against literature data for constraint effects on ductile initiation. Finally, a worked example is provided, showing how the influence of crack tip constraint on the onset upper shelf temperature can be predicted using the solutions provided.