Justification for meso-scale modelling in quantifying constraint during creep crack growth

Meso-scale models over a range of 50–500 μm are employed for predicting crack growth in the creep range. They make use of fracture mechanics, creep damage mechanics under multi-axial loading and limit analysis techniques. A study is made of the several levels of approximations and validations in the modelling process that are needed in order to make the method an acceptable tool for use in a safe defect assessment procedure. The NSW meso-scale model developed previously has been used to compare crack growth rate in a 316H stainless steel tested at 550 °C using compact tension (CT) and centre-cracked panel (CCP) specimens. In this way the steady crack growth region can be explained by an upper/lower bound cracking rate based on the level of multiaxial constraint. However, the model only quantifies the upper/lower plane stress–strain bounds and does not quantify the intermediate levels of constraint. Therefore, a two-parameter concept to predict constraint and the various steps is employed to improve the predictions for defect assessment in components. Preliminary results using 2D modelling of a CCP specimen under plane strain conditions suggest that the method could be used to predict conservative estimates for crack growth rates when compared to experimental results. Further numerical analysis using 3D models of actual geometric sizes are necessary to confirm the usefulness of the two parameter method in defect assessment procedures.