A 3D microstructural constraint-based remaining ductility model for creep crack growth in metals

Creep crack growth (CCG) is a critical aspect of safety evaluation for high-temperature structures. This study investigates the influence of both geometric constraint levels and microstructure on CCG rates. A novel prediction model called NSW-SC (i.e., stochastic constraint Nikbin-Smith-Webster creep crack growth model) is proposed in which the geometric constraints and stochastic microstructural effects are combined with the well-known remaining ductility-based model (NSW) to predict crack growth in metals. The geometric constraint parameters are derived from a 3D higher-order asymptotic solution of creep crack tip fields. This is presented in a modified NSW-SC model to quantify the microstructural effect on CCG. Random multiaxial ductility influenced by crack tip fields at the sub-grain level is introduced. The model is applicable for predicting steady-state creep crack growth dominated by grain boundary degradation. Findings reveal that higher constraint levels lead to increased CCG rates whilst at the same time reducing peak stress rate fluctuations to induce a more stable CCG behaviour. The NSW-SC model is compared to stochastic finite element calculations for different specimens and validated by CCG results in Inconel 617 to confirm the findings.