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

Abstract

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...

Toolbox for optimizing anti‐erosion protective coatings of wind turbine blades: Overview of mechanisms and technical solutions

“…Figure 4 presents the micrographs of the coating section, showing manufacturing defects (bubble, a), the spalled part (cracking/spalling started from the bubble) (b) and the finite element model of coating with bubbles. 30 The maximum principal stress distribution obtained from the finite element model shows the highest stress concentration near the bubbles/manufacturing defects.…”

“…Vecchiato 71 proposed multilayered anti-impact coating for automotive applications, with 1st layer: polyvinyl resin (5%-20%), phenolic resin (1%-10%), epoxy (7%-20%), polyisocyanate resin (15%-50%), a corrosion inhibitor in a percentage between 15% and 50%, and an acid or amine (1%-15%) and second paint layer from epoxy resin or hydroxyl functional polyol (50%-75%_, hollow spheres (0.5% to 10%), reinforcing glass fibres (1%-7%), and amine and/or polyisocyanate hardener (23%-55%). upper soft/lower stiff; both medium stiff layers) and comparison of maximum Mises stresses for these three cases with reinforcing aligned platelike particles, as simulated in Mishnaevsky Jr. et al 30 It can be seen that the bilayer protection system with upper soft layer ensures the lowest stresses, and, thus, supposedly best damage protection. 30…”

“…The damage mechanisms in various real coatings on wind turbine blades were analysed using X-ray scanning microscopy and computational simulations. 30 It was observed that if manufacturing defects and heterogeneities are available in the top coating, the defects in the coatings start from pores, bubbles, or hard particles (not from the contact surface under the impact area, as in homogeneous coatings) and develop toward the surface. Figure 4 presents the micrographs of the coating section, showing manufacturing defects (bubble, a), the spalled part (cracking/spalling started from the bubble) (b) and the finite element model of coating with bubbles.…”

“…Computational model of multilayer coating with various stiffnesses of upper, medium, and lower layers (a) and comparison of maximum Mises stresses for these three cases (reprinted from Mishnaevsky Jr. et al30 ). [Colour figure can be viewed at wileyonlinelibrary.com] REINFORCED AND NANOREINFORCED COATINGS Multiple particle scattering Dissipation of waves in composite materials can be caused by viscoelastic nature of resin and by scattering on inclusions.…”

Top Coating Anti-Erosion Performance Analysis in Wind Turbine Blades Depending on Relative Acoustic Impedance. Part 1: Modelling Approach

“…The analysis (or design) of Leading-Edge Protection systems depends on the material properties in the configuration and the operational load to which it is designed during its realistic life, that is, it must be able to withstand accelerated loads and also fatigue field regimes [6,9]. To make a selection or design of a specific coating protection system, appropriate modelling requires to be defined [10][11][12]. Numerical or analytical models can be constructed with their own capabilities and limitations, [13][14][15].…”