Effects of Mixed Mode I/II Loading and Grain Orientation on Crack Initiation and Stable Tearing in 2024-T3 Aluminum

Abstract

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.