Detecting defects in the main spar of a wind turbine blade

Various defects produced during the manufacture of a wind turbine blade can affect the damage evolution under fatigue loads. In this paper, a calculation model based on energy dissipation was used to predict damage evolution. Based on the structure of the unidirectional laminates in the main spar of a 1.5-MW blade, specimens with voids or delamination defects of different dimensions were fabricated. Fatigue tests of the specimens were monitored with an infrared thermography system. The energy dissipation density was calculated from the surface temperature. A damage parameter D is proposed to represent the degree of damage evolution. By analyzing the energy dissipated during cyclic loading, it is possible to evaluate the critical moment when a defect evolves into initial critical damage. The calculation model agrees well with the observed damage evolution. This innovative method could be used to detect the initial damage in a main spar with defects during a fatigue test.