State-space modeling of fatigue crack growth in ductile alloys

This paper presents a nonlinear dynamical model of fatigue crack growth in ductile alloys under variable-amplitude loading. The model equations are formulated in the state-space setting based on the crack closure concept and capture the effects of stress overload and reverse plastic flow. The state variables of the model are crack length and crack opening stress. The constitutive equation of crack-opening stress in the state-space model is governed by a low-order nonlinear difference equation that does not require storage of a long load history. The state-space model can be restructured as an autoregressive moving average (ARMA) model for real-time applications such as health monitoring and life extending control. The model is validated with fatigue test data for different types of variable-amplitude and spectrum loading including single-cycle overloads, irregular sequences, and random loads in 7075-T6 and 2024-T3 alloys. Predictions of the state-space model are also compared with those of the FASTRAN-II model. (C) 2000 Elsevier Science Ltd. All rights reserved.