An experimental study of crack tip opening displacement (CTOD) for Mode I/Mode II stable crack growth in thin sheet 2024-T3 aluminum has been conducted. To perform the experiments, an Arcan specimen and fixture was modified so that (1) slippage in the grips was eliminated, (2) large rotations of the fixture components were achievable and (3) bending stresses were minimized. Defining Θ to be the angle between the Mode I loading direction (perpendicular to the crack line) and the direction of applied loading, experimental results indicate that (a) for large amounts of crack extension, a ≥ 10 mm, the value of CTOD at 1 mm behind the crack tip appears to approach a constant value of 0.1 mm for all modes of loading, (b) the direction of crack extension varied with applied mixed mode loading, (c) Mode I crack extension is predominant for 0° ≤ Θ ≤ 60°, (d) Mode II crack extension is predominant for 75° ≤ θ ≤ 90° and (e) a transition zone exists for angles θ near 75°.

The effects of material grain orientation and Mixed Mode I/II loading on crack initiation and stable tearing in 2.3-mm-thick, unclad 2024-T3 aluminum is experimentally investigated. Mode I experiments were performed on center-cracked specimens with the crack being oriented at various angles relative to the rolling direction. Defining θ to be the angle between the normal to the initial crack plane and the loading direction, Mode I/II experiments were performed using an Arcan test fixture for 0° ≤ θ ≤ 90° [corresponding to 90° ≥ β ≥ 0°, where β = atan (KII/KI)] with the crack oriented either along the rolling direction (T-L) or perpendicular to the rolling direction (L-T). Results indicate that: 1. The Mode I crack tip opening displacement (CTOD) is a strong function of the orientation of the crack relative to the rolling direction; CTOD for a T-L specimen is 0.84 mm, increasing linearly with orientation angle to 1.05 mm for an L-T case. 2. The Mode I/II CTOD increases rapidly during initial increments of crack growth and then decreases towards a constant value as crack growth continues. 3. For θ < 68° (β > 29°), all cracks kinked and the Mode I/II plastic zones are similar to rotated Mode I plastic zones throughout the crack growth process. 4. JII = 0 reasonably predicts the direction of tension-dominated crack growth, but does not predict the transition to shear crack growth which occurs for θ ≥ 75°. 5. KII ≥ KI for θ ≈ 58° (β = 45°) does not quantitatively predict the transition to shear crack growth for θ ≥ 75° (β ≤ 22°), but does provide an indication of changing conditions in the crack tip region.