Driving forces for localized corrosion-to-fatigue crack transition in Al-Zn-Mg-Cu

TLDR

Research on fatigue crack formation from a corroded 7075‑T651 surface provides insight into the governing mechanical driving forces at microstructure‑scale lengths that are intermediate between safe life and damage‑tolerant feature sizes. Crack surface marker‑bands quantify cycles to form 10–20 µm fatigue cracks from isolated pits and EXCO surfaces, and fatigue crack formation results from elastic stress concentration by 3‑D pit macro‑topography, local micro‑topographic plastic strain, and microstructure, with low‑stress life dominated by these combined elastic–plastic concentrations predicted by finite‑element analysis and low‑cycle fatigue models. Ni decreases as applied stress increases, and the interaction of macro‑ and micro‑scale driving forces causes high variability in cycles to crack, so that a broadly corroded surface can drive life to near zero; at low stresses, crack formation dominates life, as predicted by coupled elastic–plastic concentrations, and the experimental results validate next‑generation crack‑formation models and prognosis methods.

Abstract

Research on fatigue crack formation from a corroded 7075-T651 surface provides insight into the governing mechanical driving forces at microstructure-scale lengths that are intermediate between safe life and damage tolerant feature sizes. Crack surface marker-bands accurately quantify cycles (Ni) to form a 10–20 μm fatigue crack emanating from both an isolated pit perimeter and EXCO corroded surface. The Ni decreases with increasing-applied stress. Fatigue crack formation involves a complex interaction of elastic stress concentration due to three-dimensional pit macro-topography coupled with local micro-topographic plastic strain concentration, further enhanced by microstructure (particularly sub-surface constituents). These driving force interactions lead to high variability in cycles to form a fatigue crack, but from an engineering perspective, a broadly corroded surface should contain an extreme group of features that are likely to drive the portion of life to form a crack to near 0. At low-applied stresses, crack formation can constitute a significant portion of life, which is predicted by coupling macro-pit and micro-feature elastic–plastic stress/strain concentrations from finite element analysis with empirical low-cycle fatigue life models. The presented experimental results provide a foundation to validate next-generation crack formation models and prognosis methods.