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...

A synchrotron technique, differential aperture X-ray microscopy (DAXM), has been applied to characterize the microstructure and analyze the local mesoscale residual elastic strain fields around graphite nodules embedded in ferrite matrix grains in ductile cast iron. Compressive residual elastic strains are measured with a maximum strain of ~ 6.5-8 × 10-4 near the graphite nodules extending into the matrix about 20 µm, where the elastic strain is near zero. The experimental data are compared with a strain gradient calculated by a finite element model, and good accord has been found but with a significant over prediction of the maximum strain. This is discussed in terms of stress relaxation during cooling or during storage by plastic deformation of the nodule, the matrix or both. Relaxation by plastic deformation of the ferrite is demonstrated by the formation of low energy dislocation cell structure also quantified by the DAXM technique.

Biomass gasification experiments were carried out in a bench scale entrained flow reactor, and the produced solid particles were collected by a cyclone and a metal filter for subsequent characterization. During wood gasification, the major part of the solid material collected in the filter is soot. Scanning electron microscopy (SEM) images coupled with energy dispersive spectroscopy (EDS) show agglomerated nanosize spherical soot particles (<100 nm) that are very rich in carbon. In comparison to wood gasification, the soot content in the filter sample from straw gasification is quite low, while the contents of KCl and K2SO4 in the filter sample are high. SEM images of the straw filter samples show that with steam addition during gasification, where the soot yield is lower, the filter sample becomes richer in KCl and K2SO4 and appears as irregular crystals, and the typical particle size increases from below 100 nm to above 100 nm. During gasification of dried lignin, the filter sample mainly consists of soot and nonvolatilizable inorganic matter. SEM images of the parent wood particles and the derived char samples show that they have similar structure, size, and shape but the derived char particle surface looks smoother indicating some degree of melting. The reactivity of the organic fraction of the samples was determined by thermogravimetry, and it was found that char was more reactive than soot with respect to both oxidation and CO2 gasification. The activation energy for the soot conversion is higher than for the char conversion. These results support the observation from gasification experiments that char is more easily converted than soot. Surprisingly, the soot produced at a higher temperature is more reactive than the soot produced at a lower temperature.

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