Bondings on rotor blades – the probably most
heavily stressed bonded components at all
Rotor blades of wind turbines “harvest” the energy of the wind and transfer it to the generator, which generates the electricity from it. They thus have a significant influence on the annual energy production (AEP) of a wind turbine. Naturally, the length of the rotor blade – the size of the area covered – and the wind speeds prevailing at the site of the turbine are determining factors for the amount of electricity generated.
But the aerodynamic properties of the rotor blade are also of great importance. Not only aerodynamic-improving and thus noise-reducing components are attached to the rotor blade, but also those that are necessary, for example, to guarantee lightning protection and to prevent erosion.
Here the bonding technology represents the ideal joining technology. Not only the integrity of the parts to be bonded is maintained, but also surface disturbances that adversely affect the aerodynamics are avoided.
However, rotor blades are among the most heavily stressed components of all. Especially in the case of high-wind and offshore turbines, the bonded joints have to withstand extremely adverse conditions for many years – a service life of at least 25 years is expected. They are not only exposed to ongoing intermittent mechanical stress, but also to changing temperatures in interaction with rain, ice and UV radiation. At circumferential speeds of 300 km/h and more, which are quite realistic for modern offshore turbines with rotor blade lengths of 80 m and more, even impacting raindrops have a not negligible abrasive effect. In addition, both the materials of the components to be attached to the rotor blade and the protective surface coating of the rotor blade have a special composition and are therefore often difficult to bond.
Any premature failure, e.g. of the bonds that improve the aerodynamics of the rotor blade (see figure on the right), is usually only detected in the course of regular inspections and has already led to reduced energy production. The repair can only be carried out under acceptable weather conditions for the execution of bonds and causes an additional loss of produced energy due to the required downtime.
The verification that the bonds meet the requirements over the entire expected service life of the wind power plant is therefore of great importance. The loads occurring during operation are simulated by various time-lapse tests on standardized test specimens (climate change tests, fatigue tests, etc. and combinations thereof) and the validity of the test methods is continuously correlated with the results from the monitoring of real plants. If necessary, the test methods are adapted.
In order to guarantee the expected service life of the bonded components, it is important not only to ensure the properties of the adhesive but also to ensure that the bonds are manufactured free of defects. Due to the relatively low volume production and the large rotor blade dimensions, automated processes are only possible to a very limited extent. Since the adhesive as a process material only gains its final properties in the course of the manufacturing process and the quality of the bond cannot be checked completely non-destructively, errors must be avoided. This requires a dedicated quality management system that describes, controls and documents the bonding process in detail, taking upstream and downstream process steps into account.