Fracture analysis of adhesive joints in wind turbine blades

Abstract

Modern wind turbine rotor blades are usually made from fibre-reinforced composite subcomponents. In the final assembly stage, these subcomponents are bonded together by several adhesive joints. One important adhesive joint is situated at the trailing edge, which refers to the downstream edge where the air-flow rejoins and leaves the blade. Maintenance inspections of wind turbine rotor blades show that among other forms of damage, local debonding of the shells along the trailing edge is a frequent failure type. The cause of trailing edge failure in wind turbine blades is complex, and detailed information is scarce. This paper is concerned with the fracture analysis of adhesive joints in general, with a particular focus on trailing edges. For that, the energy release rates in prescribed cracks present in the bond line of a generic trailing edge joint are investigated. In connection with this examination, the paper elucidates the influence of geometrical non-linearity in form of local buckling on both the increase of the energy release rate and the change of mode mixity. First, experimental results on adhesively bonded small-scale subcomponents are presented. Thereafter, a practical approach is presented, which links the experimental results conducted on a small scale to the numerical failure prediction of large-scale models. The proposed method is based on the virtual crack closure technique and defines the mode mixity at bimaterial interfaces unambiguously. The method is consequently applied to a wind turbine blade submodel in order to predict crack growth in the trailing edge. Thereby, the influence of different crack lengths on crack initiation and propagation is considered. The paper concludes with general thoughts on adhesively bonded trailing edge joints regarding the prevention of local debonding.

Initiation of trailing edge failure in full-scale wind turbine blade test

“…About 2% of wind 23 turbines during their first 10 years of operation require blade replacements. 24 Rotor blades see increased failure rates or reduced reliability as the concept 25 grows from simple designs with small rotor diameters towards more advanced 26 technologies with a bigger rotor span [1].…”

“…where G tot is the total energy, G I , G II and G III are the strain energy (λ= 2 for brittle resins and λ= 3 for ductile resins) [23]. In this study the 528 exponent was set to λ=2.284.…”

Effects of geometric non-linearity on energy release rates in a realistic wind turbine blade cross section

“…For the sake of simplicity the experimentally obtained values reported in [20] were used. That is, the Mode-I and Mode-II initiation ERRs for Type-B detail in [20, were used under the assumption that the investigated cracks are small prior to the occurrence of fibre bridging.…”

Prospective challenges in the experimentation of the rain erosion on the leading edge of wind turbine blades

“…The damage should be understood as changes to the constitutive material and/or the geometric properties, including alterations to the boundary conditions and structural connectivity, which can adversely affect structural and power performance . Thus, several issues have been studied, eg, manufacturing process, fracture, fatigue, and adhesive joints . Although diverse turbine designs exist, currently the most common design is built in an upwind horizontal axis with three blades.…”