A Novel Physics-Based Predictive Model for Small Fatigue Crack Growth with Microstructural Sensitivity in Cast Aluminum Alloys

A novel predictive model for microstructurally small fatigue crack growth rates was developed using a three-part methodology. First, a deterministic model was created to predict microstructurally small fatigue crack growth behavior from long fatigue crack growth data using considerations of crack tip plasticity. Subsequently, microstructural barrier characteristic spacing/strength were modeled to introduce characteristic acceleration and deceleration mechanisms of grain- and secondary phase-controlled cracks. Finally, the deterministic model was coupled with a Monte Carlo technique, and used to make predictions of lifetime distributions and S–N curves with material and component specificity. Simple, metallographically measured parameters are used to make predictions, and the model provides insight into their respective roles in controlling fatigue crack growth lifetimes, and enables design practice optimization. The model predicts that, for cast Al-Si-Mg alloy A356, increasing matrix strength, grain size, and secondary dendrite arm spacing enhances overall fatigue crack growth resistance. Comparisons are made to experimental data, and show that successful predictions are made.