Non-proportionality analysis of multiaxial fatigue stress histories in trailing edge adhesive joints of wind turbine rotor blades

Abstract. Wind turbine rotor blades are exposed to complex multiaxial stress states due to the aero-(hydro-)servo-elastic behaviour of the turbine. The dynamic response of rotor blades can result in non-proportional stress histories in the adhesive joints. These are not properly considered in current design guidelines and standards. However, knowledge about the degree of non-proportionality is crucial for choosing an appropriate fatigue analysis framework.

This paper investigates the degree of non-proportionality of three-dimensional stress histories in trailing edge adhesive joints of wind turbine rotor blades. For the quantification of non-proportionality, the concept of so-called non-proportionality factors is utilized. Existing approaches show weaknesses for the application to adhesives. Hence, a novel non-proportionality factor is introduced in this work that combines two formulations from literature. After a concise verification, it is applied to analyze the trailing edge adhesive joints of three different blade designs in the framework of a numerical comparative study.

The results do not reveal any correlation between the degree of non-proportionality and the blade size. General conclusions are hard to draw, as the blade response does not only depend on the turbine size, but also on the blade design philosophy. However, each blade shows significant degrees of non-proportionality that should not be neglected in fatigue damage analyses.