Accurate prediction of residual stiffness is very useful and important for assessing the remaining life of composite structures, particularly in wind turbine blades. However, traditional approaches rely on destructive testing, which is costly and time-consuming. This study presents a hybrid analytical-experimental framework that integrates modal analysis with progressive fatigue damage modelling to enhance fatigue life prediction in laminated composites. A strong correlation was experimentally established between the first natural frequency and the degradation of tensile elastic modulus in [0/90]7 cross-ply glass/epoxy laminates subjected to tensile-tensile fatigue loading. A previously known phenomenological stiffness degradation model was modified using the observed frequency-stiffness relationship, particularly emphasizing the early stages of fatigue life where conventional models show limited accuracy. The modified model was used to estimate first-mode natural frequencies across multiple fatigue life intervals based on the experimentally derived stiffness–frequency relationship. Comparison with experimental results demonstrated strong agreement, validating the model\\\'s predictive capability. This integrated approach offers a reliable, non-destructive alternative for tracking fatigue damage progression and predicting structural life in composite materials.

Keywords