Micromechanisms of leading edge erosion of wind turbine blades are studied with the use of X-ray tomography and computational micromechanics simulations. Computational unit cell micromechanical models of the coatings taking into account their microscale and nanoscale structures have been developed and compared with microscopy studies. It was observed that the heterogeneities, particles, and voids in the protective coatings have critical effect on the crack initiation in the coatings under multiple liquid impact. The damage criterion for the formation of initial defects in the top coating is determined, and it is maximum principal stress criterion. Porosity or stiff particles in the coatings change the damage initiation sites, moving it from the contact surface to the pores or particles closest to the surface. Increasing the thickness of the polymer coatings allows reducing the stress amplitude, thus delaying the damage.
KEYWORDScoatings, leading edge erosion, modeling, wind energy
| INTRODUCTIONOne of the most common reasons of reduction of power generation by wind turbine is the leading edge erosion (LEE) of wind turbine blades. 1 The LEE is responsible for more than 5% reduction of annual energy production for wind turbines. 2 The impact of erosion on the rotor performance includes also an increase in drag coefficient 3 by 80% to 200% and a decrease in lift coefficient for higher angles of attack. 4The LEE of wind turbine blades depends on the loading conditions of the wind turbines blades (rain density, rain droplet size distribution, dust, and flow velocity) as well as on the properties of coating/gelcoat protection system (strength, stiffness, viscosity, and damping). 5-7 The removal and roughening of the leading edge surface take place by the material fatigue and damage accumulation because of multiple liquid impacts by the raindrops. 5 Each rain droplet hits the surfaces, creating pressure on the surface and wave propagation in the protective layers. This leads to the deformation and damage initiation, fatigue cracking in coating and composites, debonding, cracks in composite, material loss, and roughening of surface.Other effects, which likely influence the LEE, are abrasion/cutting of coating surface (at low impact angle by raindrops), brittle fracture, and plastic deformation of the surface (at high and medium speed of raindrops, respectively). 8,9 In this paper, X-ray tomography analysis and computational studies are carried out to understand the microscale mechanisms of the LEE.Typical nanoscale structures of various coatings are analyzed and identified using X-ray tomography analysis and scanning electron microscopy (SEM). Coated laminate samples, corresponding to a typical blade coatings, were tested to failure with the rain erosion tester (RET) from R&D Test Systems A/S. The distribution of defects, microcracks, and their locations was characterized by electron microscopy and X-ray tomography, again. Computational unit cell micromechanical models of the typical coating structures have been designed...
