Impact of Site-Specific Thermal Residual Stress on the Fatigue of Wind-Turbine Blades

Abstract

Thermal residual stresses can have a substantial impact on the bond-line fatigue of wind-turbine blades and the initiation of tunneling cracks in the adhesive layer early in their operational life. Thermal residual stresses develop during the manufacture of the blade as a consequence of the temperature difference between the curing conditions and the operating environmental conditions. Polymers are used as adhesive or matrix material for fiber-reinforced materials. Their physical properties (for example, the coefficient of thermal expansion, Young’s modulus, and the tensile strength) depend on the curing process (that is, the degree of cure and the temperature). Moreover, the ambient temperature for the blade during its operational life underlies a statistical distribution, which is site specific. Traditionally, the design evaluation is conducted with material properties obtained at a room temperature of 23°C under fully cured conditions. Taking into account the variability of temperature at a specific site (for example, from to 25°C) will increase the fidelity of the design evaluation. This research investigates the impact of thermal residual stress on the fatigue stress exposure along the trailing-edge bond line during operational blade life. A simplified approach using temperature-independent material properties for different temperature sites at northern, central, and southern European wind farm sites is compared to an advanced approach using temperature-dependent and degree-of-cure-dependent properties. It was found that the advanced approach yields a more critical design.