A novel experimental method to evaluate the fatigue behaviour and damage evolution of bi-directional composites under variable amplitude loading

• A new experimental framework is proposed for assessing the fatigue performance of GFRP composites . • A nonlinear damage law is introduced to characterize cumulative damage progression. • Influence of cycle mix and load sequence is investigated on the fatigue behaviour . • Crack initiation and propagation are qualitatively analysed using IR thermography. This study presents a novel experimental approach to evaluate the fatigue performance of bi-directional glass fiber-reinforced polymer (GFRP) composites under variable amplitude sequential block loading conditions. The proposed method incorporates advanced loading protocols to simulate realistic service conditions, capturing the complex nonlinear stress interactions and damage progression unique to composite materials. The experimental setup ensures precise control over load sequences and stress blocks, facilitating a comprehensive understanding of fatigue performance and failure mechanisms based on fatigue life, stiffness degradation and thermal response. Results demonstrate a significant influence of load sequence, cycle mixing and stress level on fatigue behaviour , providing critical insights into material performance and damage evolution. Key findings highlight the pronounced effect of high-to-low (H-L) compared to low-to-high (L-H) loading regimes on fatigue performance, showing up to 38 % reduced fatigue life. The study also demonstrates the influence of crack initiation and propagation rates on fatigue performance under both loading regimes. A load sequence sensitive damage law has been proposed to capture the nonlinear damage accumulation under variable loading conditions. This work provides a robust framework for improving the design and reliability of GFRP composites subjected to variable amplitude loading in engineering applications.