Predicting the fatigue life of wind turbine rotor blades is a challenging and crucial engineering task. This study investigates the correlation between modal parameters and degradation of residual strength and tensile modulus in the cross-ply glass epoxy laminate [0/90]7, used in wind turbine blades, under fatigue loading. The tensile and vibration characteristics were assessed, followed by constant amplitude fatigue tests at 35%, 43%, and 55% of the Ultimate Tensile Strength (UTS), with R=0.1 and a frequency of 8 Hz. The modal analysis was performed on the cycled specimens at life fractions from 0.05 to 0.70 and residual modulus and strength were obtained. The results establish a well-defined correlation between these residual mechanical properties and the natural frequency. Normalized residual strength, tensile modulus, and natural frequency demonstrated similar behaviors during the fatigue life. An initial rapid decrease in the first tenth of the life fraction was observed, followed by minimal changes up to a life fraction of 0.7. The strong correlation between the first mode natural frequency and both the residual strength and the tensile E-modulus provides a promising basis for developing accurate fatigue life prediction models for fiber-reinforced composite structures.

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