Computational Mechanics Based Validation of Crack Growth Approaches for Fracture Specimen Predictions

The last several decades have seen growth in elastic-plastic fracture mechanics and the modeling of the behavior of structural steels employed in the nuclear, oil and gas, and other construction industries. Among these are a particular class of problems that provide challenges in modeling the physical behavior of structural steels using finite element modeling (FEM) approach that are based on microstructural damage and using parameters that depict the strain and stress states in the material region ahead of an existing crack. In this work, a recently experimented and investigated pipeline steel X80 material was modeled through two different fracture specimen geometries, namely single-edge-notch-tension, SEN(T) and compact-tension, C(T) to compare and contrast the predictions from two material damage models (microstructure and continuum based). The predictions from both these damage models that predict the ductile crack growth have been compared to the experimental findings of the crack growth (obtained using a d-c Electric Potential measurement technique), the corresponding load levels, and crack opening displacements (CODs). The points of similarity between the experimental measurements and the fracture surface observations of crack growth and the predictions from the FEM approach have been discussed. The same X80 material properties and damage model parameters were employed to predict the ductile crack growth in the two different fracture specimen geometries, SEN(T) and C(T) with a subtle change of one of the parameter values. This sheds light on the predictability of the crack initiation event and the subsequent ductile crack growth until failure using these damage models. The findings provide credence to the applicability of either model (after they are carefully tuned to arrive at optimized parameters) for piping materials while providing a framework for flaw evaluation methodologies. The investigation also opens the doors for regions where mesh regularization methods and modeling approaches along with mathematical relations can be developed to form a more efficient framework for modeling specimens with diverse constraints efficiently and develop material fracture resistance curves.