Analysis of Crack Extension Mechanism in the Near-Threshold Regime in an Aluminum Alloy

Short cracks initiated from pre-existing flaws are known to propagate in an intermittent way below threshold as the crack tip field interacts with the microstructure. If the stress amplitude is very low, similar effects as observed for short cracks may happen to long cracks leading to unexpected crack extension. This phenomenon is studied in this paper using flat dogbone specimens of a commercial aluminum alloy in two heat treatment states. Compression pre-cracked specimens were used to determine the threshold by continuous load increase according to a procedure proposed by Pippan et al. [Pippan et al. (1994)]. On this basis, crack growth experiments with an approximately constant ΔK-value propagation rate were performed at a stress ratio of R = 0.1. Results indicate that primary precipitates act as microstructural barriers causing crack deflection and crack branching. If the stress amplitude is low enough, shear-dominated crack extension has been found to be possible even for long cracks in the near-threshold regime. Both mechanisms keep the crack from extending continuously.