Impact fatigue caused by rain droplets, also called rain erosion, is a severe problem for wind turbine blades and aircraft. In this work, an assessment of impact fatigue on a glass fibre reinforced polymer laminate with a gelcoat is presented and the damage mechanisms are investigated. A single point impact fatigue tester is developed to generate impact fatigue damage and SN data. Rubber balls are repeatedly impacted on a single location of the coated laminate. Each impact induces transient stresses in the coated laminate. After repeated impacts, these stresses generate cracks, leading to the removal of the coating and damage to the laminate. High-resolution digital imaging is used to determine the incubation time until the onset of coating damage, and generate an SN curve. An acoustic emission sensor placed at the back of the laminate monitors changes in acoustic response as damage develops in the coated laminate. The subsurface cracks are studied and mapped by 3D X-ray computed tomography. A finite element method model of the impact shows the impact stresses in the coating and the laminate. The stresses seen in the model are compared to cracks found by 3D tomography. The damage is also evaluated by ultrasonic scanning.

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Maintenance and repair of wind turbines contribute to the higher costs of wind energy. In this paper, various technologies of structural repair of damaged and broken wind turbine blades are compared. The composite plates, mimicking damaged blade parts, were damaged and repaired, using various available curing and bonding technologies. Technologies of repair with hand layup lamination, vacuum repair with hand layup and infusion, ultraviolet repair and high temperature thermal curing were compared. The repaired samples were tested under tensile static and fatigue tests, and subject to microscopic X-ray investigations. It was observed that both the strength of the repaired structures and the porosity depend on the repair technology used. Vacuum-based technologies lead to relatively stiff and lower-strength repaired plates, while ultraviolet-curing technologies lead to average stiffness and high strength. High-temperature vacuum curing leads to the highest maximum stress. Hand layup (both vacuum and without vacuum) leads to high post-repair porosity in the adhesive and scarf, while vacuum infusion leads to low porosity. Fatigue lifetime generally follows the trend of porosity. There exist risks of micro-damaging the parent laminate and the formation of residual stresses in the repaired structure.

Repair and maintenance operations of wind turbines constitute a significant part of costs of wind energy. In this paper, technologies of structural repair of damaged wind turbine blades are reviewed. Costs of repair, and technological contribution to the costs are discussed. Technologies of repair are compared, including hand layup lamination, vacuum repair with hand layup and infusion, ultraviolet curing and high temperature thermal curing systems. Computational models of repaired blades, and curing as kinetic process are presented. Void formation during repair and curing, and the void influence on the post-repair reliability of blades is discussed.