Effect of initiation feature on microstructure-scale fatigue crack propagation in Al–Zn–Mg–Cu

High fidelity measurements of constituent particle or corrosion topography nucleated fatigue crack growth rates (da/dN) are established for 7075-T651 in humid air. Values of microstructure-scale da/dN are determined by microscopy of programmed load-induced crack surface markers, rather than surface-only measurements. Both pristine and corroded specimen da/dN from various applied stress levels are successfully correlated using continuum-elastic stress intensity (ΔK or ΔK and Kmax) or dislocation-based (Bilby–Cottrell–Swindon) crack tip opening displacement (cyclic ϕ and ϕmax), with the former accounting for the gradient of elastic stress concentration due to the initiating feature. Values of da/dN vary by an order of magnitude at each fixed driving force due to microstructural influences that result in a locally irregular crack front. Grain-scale models using stress intensity closure or slip-based crystal plasticity do not capture experimental da/dN variability. Due to an inadequate mechanistic basis, mechanics-inspired models of da/dN do not predict multiple growth regimes that are typical of environment enhanced cracking. An elastic ΔK-based description of long crack da/dN data for a given alloy-environment can be transformed to a continuum elastic–plastic ϕc basis to provide a mean crack growth rate description. Coupling mean rates with a statistical description of microstructure sensitive variability, and dislocation or crystal plasticity-finite element modeling of component ϕc for non-continuum cracking, will enhance prognosis in the MSC regime.

► High fidelity small crack da/dN are established for 7075 via fracture surface SEM.
► K- and ϕ-based approaches capture mean da/dN trends for corroded and pristine data.
► Extrapolation of long crack data reasonably captures mean small crack behavior.
► High variability in da/dN is attributed to local microstructure interactions.
► Variability about da/dN trends is not captured by current mechanism based models.