Fatigue Crack Propagation Behavior and Damage Accumulation Relationships in an Aluminum Alloy

Abstract

Damage accumulation was studied in strain-cycled aluminum Alloy 6066-T6 cylindrical fatigue specimens. The damage was assessed by measuring the ratio of cracked to cross-sectioned areas and monitoring the apparent modulus changes after different life fractions. Both these methods were found to correlate well. Stage II fatigue crack propagation was established when the surface crack length had attained a critical value of approximately 700 μm. A linear relationship with crack depth ensued. Below this value, the crack depth was small by comparison and damage levels were below 1%. Over 60% of the fatigue life was spent in Stage I (or surface-dominated) crack growth where the cracked area was insignificant. From a practical point of view, damage was considered to begin after this life fraction and a damage mechanics model was successfully applied to the experimental results. Damage accumulation rates were related directly to crack growth rates as a result of the relationship between damage and the cracked area. Hence, damage accumulation may be described by a Paris type of equation. Tests at low strains involving a large amount of elasticity (elastic-to-plastic strain ratios of the order of 600) were found to maintain low damage levels over longer life fractions compared with higher strain cycled tests (with ratios of 25 and less). Damage at a given life fraction in tests with elastic-to-plastic strain ratios below approximately 25 were found to be similar. Under these conditions Miner's rule may be applied.