State-space model of fatigue crack growth for real-time structural health management

This paper presents formulation and validation of a nonlinear dynamical model of fatigue crack growth in ductile alloys under variable-amplitude loading including single-cycle overloads, irregular sequences, and random loads. The model is formulated in the state-space setting based on the crack closure concept and captures the effects of stress overload and reverse plastic flow. The state variables of the model are crack length and crack opening stress. The state-space model can be restructured in the autoregressive moving average (ARMA) setting for real-time health monitoring, non-destructive evaluation, and life extending control. The model prediction is validated with fatigue test data and is compared with those of standard codes such as AFGROW and FASTRAN under different types of variable-amplitude and spectrum loading. On the average, the state-space model executes about ten times faster than other fatigue crack growth models and its predictions are comparable or of better accuracy.