Evolution of modal parameters of composite wind turbine blades under short- and long-term forced vibration tests

Modal properties of dynamically tested wind turbine blades (WTBs) of a utility-scale wind turbine are identified. A comprehensive experimental program including free vibration and short- and long-term forced vibrations representing resonance and simplified fatigue conditions was carried out to investigate vibration-based features for damage diagnosis and prognosis. A set of 12 undamaged WTBs were tested to study the variability of the identified modal parameters. Results indicate that the variability of the natural frequencies was rather low, while the obtained damping ratios exhibited significant differences. Forced vibration tests were then conducted. To reach the failure of the blades, approximately 1.9 × 10⁴ and 4.2 × 10⁷ cycles were induced in the short- and long-term tests, respectively. Modal properties identified during testing protocols suggest that natural frequencies correlate well with damage. A linear finite element model was also developed, and its modal properties are compared to the identified modal parameters of the undamaged blades.