Solid particle erosion and viscoelastic properties of thermoplastic polyurethanes

Top Coating Anti-Erosion Performance Analysis in Wind Turbine Blades Depending on Relative Acoustic Impedance. Part 2: Material Characterization and Rain Erosion Testing Evaluation

Under droplet impingement, surface leading edge protection (LEP) coating materials for wind turbine blades develop high-rate transient pressure build-up and a subsequent relaxation in a range of strain rates. The stress-strain coating LEP behavior at a working frequency range depends on the specific LEP and on the material and operational conditions, as described in this research in a previous work. Wear fatigue failure analysis, based on the Springer model, requires coating and substrate speed of sound measurements as constant input material parameters. It considers a linear elastic response of the polymer subjected to drop impact loads, but does not account for the frequency dependent viscoelastic effects for the materials involved. The model has been widely used and validated in the literature for different liquid impact erosion problems. In this work, it is shown the appropriate definition of the viscoelastic materials properties with ultrasonic techniques. It is broadly used for developing precise measurements of the speed of sound in thin coatings and laminates. It also allows accurately evaluating elastic moduli and assessing mechanical properties at the high frequencies of interest. In the current work, an investigation into various LEP coating application cases have been undertaken and related with the rain erosion durability factors due to suitable material impedance definition. The proposed numerical procedures to predict wear surface erosion have been evaluated in comparison with the rain erosion testing, in order to identify suitable coating and composite substrate combinations. LEP erosion performance at rain erosion testing (RET) technique is used widely in the wind industry as the key metric, in an effort to assess the response of the varying material and operational parameters involved.

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

Top coating are usually moulded, painted or sprayed onto the wind blade Leading-Edge surface to prevent rain erosion due to transverse repeated droplet impacts. Wear fatigue failure analysis based on Springer model has been widely referenced and validated to quantitatively predict damage initiation. The model requires liquid, coating and substrate speed of sound measurements as constant input parameters to define analytically the shockwave progression due to their relative vibro-acoustic properties. The modelling assumes a pure elastic material behavior during the impact event. Recent coating technologies applied to prevent erosion are based on viscoelastic materials and develop high-rate transient pressure build-up and a subsequent relaxation in a range of strain rates. In order to analyze the erosion performance by using Springer model, appropriate impedance characterization for such viscoelastic materials is then required and represents the main objective of this work to avoid lack of accuracy. In the first part of this research, it is proposed a modelling methodology that allows one to evaluate the frequency dependent strain-stress behavior of the multilayer coating system under single droplet impingement. The computational tool ponders the operational conditions (impact velocity, droplet size, layer thickness, etc.) with the appropriate variable working frequency range for the speed of sound measurements. The second part of this research defines in a complementary paper, the ultrasonic testing characterization of different viscoelastic coatings and the methodology validation. The modelling framework is then used to identify suitable coating and substrate combinations due to their acoustic matching optimization and to analyze the anti-erosion performance of the coating protection system.

Erosion characteristics of Teflon under different operating conditions

The present study investigated the solid particle erosion characteristics of Teflon under different impingement angles (15–90°), impact velocities (30–50 m/s), erodent sizes (300–600 μm) and stand-off distances (15–25 mm) at ambient temperature. The Teflon showed ductile erosion behavior exhibited a peak erosion rate at a 30° impact angle. The design of experiments approach utilizing Taguchi’s orthogonal arrays was applied to test the specimens on a compressed air jet type erosion test rig. Erosion efficiency (η) values were 0.29–16.7%, which indicates micro-plugging, plastic deformation and micro-cutting action as dominating erosion mechanisms. The experimental results are closer to the theoretical model. An optimal parameter combination was determined, which leads to minimization of erosion rate. Analysis of variance (ANOVA) was performed on the measured data and signal-to-noise (S/N) ratios. A mathematical correlation, consistent with the experimental observations, is proposed as a predictive equation for estimation of erosion rate of tested material. The morphology of erodent surfaces was examined by using scanning electron microscopy (SEM). Possible erosion mechanisms are discussed.