Characterizing Crack Growth in Thin Aluminum Panels Under Tension-Torsion Loading Using Three-Dimensional Digital Image Correlation

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Abstract
The enclosed work was performed to determine whether a critical crack opening displacement (COD) criterion can be used to predict the stable crack growth behavior of thin, 2024-T3 aluminum fracture specimens experiencing tension and torsion loading. Due to the complexity of the large deformations that occur near the crack tip in a single edge-cracked specimen under torsion loading, a state of the art three-dimensional computer vision system was developed and employed to make the three-dimensional vector displacement measurements required to determine COD. Results from the experimental program indicate that the three-dimensional surface profile and deformation measurement system was fully capable of making the required measurements, even in the presence of large, out-of-plane displacements and surface strains that occurred during the tension-torsion loading process. Specifically, the measurements show that (a) critical COD for tension-torsion loading is constant during crack growth, (b) COD is approximately 8% larger than observed for in-plane tension-shear and (c) the surface strain fields during crack growth are quite complex due to the coupling of out-of-plane displacements and in-plane surface strains.