Colloquia

Summary

During operation, wind turbine blades are subjected to impact fatigue caused by rain and hail, leading to a phenomenon known as leading edge erosion. This erosion reduces the aerodynamic efficiency and structural integrity of the blades, ultimately increasing the cost of wind-generated energy. The tips of the leading edges, which experience the highest velocities, are particularly susceptible to such damage. To reduce impact erosion and extend blade lifetime, thermoplastic elastomer (TPE) coatings are applied to the leading edges. These coatings enhance durability and performance, leading to reduced maintenance and thus helping to lower the cost of energy. However, these coatings can fail abruptly, which makes the predictability of the performance of these coatings challenging.

This research aims to investigate the fatigue damage behaviour of TPE coatings under repeated high-velocity impacts, with a focus on identifying failure mechanisms and the onset of material degradation.

Impact fatigue tests were conducted using a Single Point Impact Fatigue Test (SPIFT) setup, varying parameters such as impact energy, frequency, and specimen production method. The SPIFT setup was designed during this research and is used to experimentally apply repeated impacts of high-velocity projectiles on the test specimen with controlled velocity and impact frequency.

The results of these tests show that increased impact frequencies, energies, and velocities reduce the coating's fatigue lifetime. Moreover, specimens produced using Vacuum Assisted Resin Transfer Moulding show better fatigue properties compared to those fabricated with the adhesion-gelcoat method. Microscopic analysis of the damaged area further revealed that these parameters influence the dominant failure mechanisms of the coating. Specifically, for low impact energies and impact frequencies, the dominant failure mechanism appears to shift from crack-driven failure to plastic deformation. This change is caused by the combined effects of the load case (strain rate, stress magnitude, etc.) and the material behaviour (relaxation time, visco-elastic behaviour, etc.). Higher impact velocities induce brittle material behaviour due to elevated strain rates. It is known that higher strain rates lead to a stiffer response, but this work shows that also the failure mechanisms are affected. This complex relation is one of the reasons why performance predictions are challenging.

This thesis highlights the importance of considering impact rate, velocity, and production method when assessing tests of coating fatigue and predicting real-world coating life-time with a SPIFT setup.