On the Material Characterisation of Wind Turbine Blade Coatings: The Effect of Interphase Coating–Laminate Adhesion on Rain Erosion Performance

Abstract

Rain erosion damage, caused by repeated droplet impact on wind turbine blades, is a major cause for concern, even more so at offshore locations with larger blades and higher tip speeds. Due to the negative economic influence of blade erosion, all wind turbine Original Equipment Manufacturers (OEMs) are actively seeking solutions. In most cases, since the surface coating plays a decisive role in the blade manufacture and overall performance, it has been identified as an area where a solution may be obtained. In this research, two main coating technologies have been considered: In-mould coatings (Gel coating) applied during moulding on the entire blade surface and the post-mould coatings specifically developed for Leading Edge Protection (LEP). The coating adhesion and erosion is affected by the shock waves created by the collapsing water droplets on impact. The stress waves are reflected and transmitted to the laminate substrate, so microstructural discontinuities in coating layers and interfaces play a key role on its degradation and may accelerate erosion by delamination. Analytical and numerical models are commonly used to relate lifetime prediction and to identify suitable coating and composite substrate combinations based on their potential stress reduction on the interface. Nevertheless, in order to use them, it is necessary to measure the contact adhesion resistance of the multi-layered system interfaces. The rain erosion performance is assessed using an accelerated testing technique, whereby the test material is repeatedly impacted at high speed with water droplets in a Whirling Arm Rain Erosion Rig (WARER). The materials, specifically the coating–laminate interphase region and acoustic properties, are further characterised by several laboratory tests, including Differential Scanning Calorimetry (DSC), pull-off testing, peeling–adhesion testing and nanoindentation testing. This body of work includes a number of case studies. The first case study compares two of the main coating technologies used in industry (i.e., gel coating and LEP); the second case investigates the effects of the in-mould gel coating curing; and the third considers the inclusion of a primer layer on a LEP configuration system. Following these case studies, the LEP is found to be a far superior coating due to its appropriate mechanical and acoustic properties and the interface between the coating and the substrate is highlighted as a key aspect, as poor adhesion can lead to delamination and, ultimately, premature failure of the coating.

Micromechanisms of leading edge erosion of wind turbine blades: X‐ray tomography analysis and computational studies

“…Estimation of Young modulus of “heterogeneous” gel coating: for the rough estimation of the Young modulus of gel coating, the rule of mixture was applied (Reuss rule for the averaging of polyurethane/Al 2 O 3 mixture and then Voight rule for adding the SiO 2 effect). The resulting Young modulus is 4.8 GPa (assuming the volume content of Al 2 O 3 0.21^1.6 = 9.6% for 3‐D case) to which the values given in Cortés et al are relatively close. For the filler, we will use the Young modulus value 8.76 GPa, as listed in Cortés et al…”

“…The resulting Young modulus is 4.8 GPa (assuming the volume content of Al 2 O 3 0.21^1.6 = 9.6% for 3-D case) to which the values given in Cortés et al 17 are relativelyclose. For the filler, we will use the Young modulus value 8.76 GPa, as listed in Cortés et al17…”

Radar-derived precipitation climatology for wind turbine blade leading edge erosion

“…Initially there is an incubation period during which impacts occur but no visible damage is observed, although microstructural changes in the materials generate nucleation sites for material removal, which commences when a threshold is reached (i.e., when some level of accumulated impacts is reached). Once the time to damage has been exceeded, additional damage occurs as stress waves propagate from the impact sites into the composite and cause existing pits and cracks to grow, and there is a steady increase of material loss with each additional impact (Cortés et al, 2017;Eisenberg et al, 2018;Traphan et al, 2018). The number of impacts required to reach the threshold for surface fatigue failure is a function of the droplet diameter and phase, the closing velocity, the strength of the material, and the pressure of the impact.…”

Extending the life of wind turbine blade leading edges by reducing the tip speed during extreme precipitation events

“…The reflection of stress waves between the coating surface and the coating/substrate interface may also play a significant role for fatigue of the coated laminate (Springer 1974). Body waves may also cause delamination inside the laminate (Prayago 2011) and debonding of the coating (Cortés et al 2017). 20…”