Fatigue Crack Propagation Under Variable-Amplitude Load: Part I — A Deterministic State-Space Model

Abstract

Abstract The two-part paper addresses modeling of fatigue crack damage in metallic structures for on-line diagnostics and health monitoring of operating machinery. This paper which is the first part presents a dynamical model of fatigue crack propagation in the deterministic state-space setting based on the crack closure concept under cyclic stress excitation of variable-amplitude loading where the model state variables are crack length and crack opening stress. The second part which is a companion paper extends the deterministic model to formulate a stochastic state-space model of fatigue crack propagation under both constant-amplitude and variable-amplitude load. The state-space model is capable of capturing the effects of stress overload and underload on crack retardation and sequence effects, and the model predictions are in fair agreement with experimental data on the 7075-T6 aluminum alloy. Furthermore, the state-space model recursively computes the crack opening stress via a simple functional relationship and does not require a stacked array of peaks and valleys of stress history for its execution. Therefore, savings in both computation time and memory requirement are significant. As such, the state space model of fatigue crack propagation allows real-time execution of decision algorithms for diagnostics, risk analysis, and life prediction on inexpensive platforms such as a Pentium processor.