Progressive fatigue damage modelling of bi-directional composites under block loading: Influence of stress ratio and load sequence

Abstract

This study investigates the fatigue response of bi-directional composites under block loading conditions. A new progressive damage model is employed for the fatigue life evaluation of the bi-directional composites. The proposed model addresses two key aspects essential for accurate fatigue life prediction of composites: (a) stress-ratio-dependent stiffness degradation, and (b) the influence of load sequence and cycle mixing on the cumulative damage evolution. Two separate damage evolution laws are given for tensile and compressive stiffness degradation. This enables a precise representation of damage progression under tension–tension and tension–compression fatigue loading. The model’s predictive capability is evaluated across multiple loading scenarios, encompassing a range of load levels and stress ratios. Special emphasis is placed on assessing the role of load sequence and the nonlinear effects arising from prior compressive damage on subsequent tensile behaviour and accelerated damage induced by load reversals. The numerical predictions demonstrate an excellent agreement with the in-house experimental fatigue test results conducted on bi-directional GFRP to accurately capturing critical phenomena such as sequence-induced life reduction and stress-interaction effects. Overall, the study establishes a robust and simplified framework for the modelling of fatigue damage in composites under variable amplitude loading, offering significant advancements over conventional life prediction methods.