Effect of plate thickness on the environmental fatigue crack growth behavior of AA7085-T7451

• Plate thickness dependent fatigue behavior of 7085-T7451 is quantified.
• Crack propagation likely follows precipitate free zones formed in the thicker plate.
• Growth rates are slower in thick plate materials for various loading environments.
• At low R, crack closure governs changes in growth rates.
• At high R, crack path tortuosity and branching governs changes in growth rates.

Fatigue crack growth rates (L–T) for 102 and 178 mm plates of 7085-T7451 are established at R = 0.1 and 0.5 for a wide-range of ΔK and environments. At R of 0.1, the 178 mm plate exhibits slower crack growth rates independent of environment. The crack growth trends correlate with changes in the fracture surface roughness and compliance-based closure indications, suggesting a dominant role of roughness induced closure. At R of 0.5 compliance-based metrics indicate that minimal closure exists, but plate thickness dependent crack growth behavior persists; grain-scale crack path tortuosity and crack branching govern this behavior. Increased crack path roughness, tortuosity, and branching in the 178 mm plate is attributed to strain localization/cracking along precipitate free zones formed in the thicker plate due to a slower quench rate.