Fatigue Crack Growth in Aluminum Alloys: Microstructural Mechanisms of Long and Small Cracks and Numerical Modeling

Fatigue crack growth (FCG) studies at various stress ratios (R=0.1, 0.5, 0.7) were performed on solution-strengthened (cast A535) and precipitation-strengthened (cast A356, 319, A390 and wrought 6061) aluminum alloys.  Microstructures were altered through processing, chemistry, and heat treatment (T4, T6, T7) in order to systematically investigate the individual and combined effects of various intrinsic material characteristic features on FCG (e.g. Si amount/type/morphology, grain size, secondary dendrite arm spacing, precipitate type/size).  In this context, mechanisms of long and small fatigue crack growth at the microstructural scale of the studied alloys were identified, and loading-microstructure-damage mechanisms design maps were created.  Differences in the FCG responses between long, physically-small, and microstructurally-small cracks were evaluated, and an original fracture-mechanics/materials-science combined model that predicts microstructurally-small crack behavior was developed, having both material and crack size dependency.  The model has been validated on several alloy systems, and further used in an integrated methodology for design and fatigue life predictions.  Techniques that merge the fundamental microstructural knowledge with FCG data and models into an original computational modeling platform will also be discussed for different crack growth regimes.  These tools are important both in design for enhanced FCG resistance and optimization of the alloys and processes for fatigue-critical structural aerospace applications.