Modelling the impact of the cyclic plastic zone on the fatigue crack growth in particle-reinforced MMCs using non-equilibrium thermodynamics

Abstract

Fatigue crack growth (FCG), primarily controlled by localised plastic deformation in cyclic plastic zone (CPZ), develops ahead of the crack tip under cyclic loading conditions. This localised plasticity facilitates crack progression by cyclically weakening the material, thereby influences the overall fatigue behaviour. The present FCG life prediction model for particle-reinforced metal matrix composites (MMCs) highlights the significance of the CPZ in fatigue performance. The model utilises an energy balance methodology to assess FCG rates, with the dissipated energy measured as the area under the cyclic stress-strain curve. Elasto-plastic stress and strain fields at the crack tip are calculated using Rice formulas, while microstructural factors are implemented using modified shear lag theory and enhanced dislocation density. The size of the CPZ is, varies from 250 to 350 µm depending on strain amplitude and particle distribution, identified as a crucial factor affecting crack propagation behaviour, and determined by strain amplitude, microstructural characteristics, and plastic energy dissipation. The results indicate that increasing the particle volume fraction from 20% to 40% enhances fatigue life by approximately 40%, while reducing cyclic strain hardening exponent ( $n^{\mathrm{\prime }}$ ) from 0.05 to 0.3 improves crack growth resistance by 75%. Polar plots are used to analyse crack propagation along the CPZ peripheral under different strain amplitudes (0.00527 to 0.000323), elucidating the interaction of microstructural impacts, initial crack growth length, and final crack growth length. The proposed closed-form analytical model exhibits significant concordance with experimental data for various particle-reinforced MMCs, yielding precise FCG life estimates. This study highlights the significance of CPZ size and energy-based processes in fatigue modelling, providing critical insights for the design and performance enhancement of advanced composites subjected to cyclic loading conditions.