A combined modeling–experimental study of the crack opening displacement fracture criterion for characterization of stable crack growth under mixed mode I/II loading in thin sheet materials

Abstract Recent experimental studies have shown that a critical Crack Opening Displacement (COD) has the potential to be a viable parameter for predicting the onset of crack growth in thin sheet 2024-T3 aluminum under combined tension (mode I) and in-plane shear (mode II) loading conditions. To assess the viability of using a critical COD criterion for prediction of crack growth in components experiencing mixed mode loading, the enclosed work presents a complete set of simulation studies and experimental measurements for crack growth under nominally mode I/II conditions. For the first time, finite element crack growth simulations under mixed mode I/II conditions have been performed for the Arcan test specimen using recently measured crack growth path information and the load–crack extension data for the mode I/II experiments. Simulation results indicate that the predicted COD is in excellent agreement with measurements. Specifically, simulation results indicate that (a) the magnitude of COD during simulated crack growth is nearly constant for Δa>5 mm; (b) for Φ≤60°, the predicted COD is primarily perpendicular to the crack path (i.e., mode I in nature); (c) for Φ>60°, the predicted COD is primarily parallel to the crack path (i.e., mode II in nature); (d) near the transitional angle between mode I and mode II fracture, the COD components are more unstable during the crack growth process and (e) simulation predictions for the strain fields are in quantitative agreement with measurement. Taken together, the results from the combined simulation–experimentation program provides strong justification for the use of a COD-based fracture criterion to predict crack growth in thin-sheet materials.