**1. Introduction**

Wind power has become a key technology to provide electricity from renewable and low-emission sources [1]. There is a need to improve existing technologies, by increasing the size of offshore wind turbines to capture more wind energy [2]. Composites use opened up great prospects in the design and manufacture of future wind turbine blades, due to the versatility offered in the material optimization and design. Nevertheless, composites perform poorly under transverse impact (i.e., perpendicular to the reinforcement direction) and are sensitive to environmental factors, such as heat, moisture,

icing, salinity and/or UV. Blade manufacturers employ surface coatings to protect the composite structure from exposure to these factors. When considering the repeated impact of rain droplets, the high required tip speed is a key contributor to surface erosion damage on the leading edges of wind turbine blades.

The leading-edge protection (LEP) coating system analyzed in this work [3] is usually molded, painted or sprayed onto the blade surface during whole blade manufacture or during a repair in-field. Industrial processes state that LEP systems can be outlined as a multi-layered system, where a putty filler layer between the laminate and the surface LEP coating is included to smooth the composite surface. A primer layer may be also integrated under the coating and over the filler layer to guarantee adhesion, circumventing delamination between layers, see Figure 1.

**Figure 1.** Leading Edge Protection (LEP) system application procedures, i.e., (**a**) spray; (**b**) roller; (**c**) trowel. Multilayer configuration.

Analytical and numerical models are commonly applied to relate top coating erosion lifetime prediction [4–6] or alternative accelerated rain erosion testing assessment is also used [7,8]. In order to identify suitable coating and composite substrate combinations based on their potential stress reduction on the surface and interface different studies are related with the droplet impact phenomena [9,10]. Recent studies treat the complexity of the single droplet impact problem with the fatigue analysis under repeated impact [11], and considering material viscoelastic approaches [12–14]. The Springer [4] model is applied and industry validated [5] for wear top-coating rain erosion lifetime assessment. It is used in this research [15] to predict wear fatigue failure analysis and as a computational tool for top-coating LEP design. In this work, its application is discussed, focusing on the required coating and substrate suitable combinations, and on the appropriate speed of sound measurements as input material parameters. The numerical model applied for the analysis of rain erosion lifetime estimation is limited to a linear elastic response of the polymer subjected to drop impact loads [4]. It is important to note that polymeric materials recently applied on the LEP systems are mainly viscoelastic materials with good properties for impact energy attenuation in erosion applications [16], that develop different mechanical response depending on temperature and on stress and strain rates [17–19]. If these parameters are not incorporated in the mechanical modeling, the predicted stresses of the coating behavior under impingement may wrongly consider the material capabilities.

In order to develop an appropriate parametric approach based on the viscoelastic material characterization, it is also necessary to consider a computational tool that allows one to design and validate the proposed modelling. In this research, a previous analysis of candidate materials in the temporal and frequency domain was developed to define applicable strain rate range for the required characterization. The simulated analysis developed in this research in a linked reference [15] limits the frequency for wind turbine rain erosion applications in a range of 0.5–7 MHz. The analysis has been done considering the constant values of material speed of sound and density for the impedance definition, in order to reproduce the Springer modelling assumptions.

The speed of sound of viscoelastic materials is directly related with its modulus of elasticity [20]. The viscoelastic characterization of the LEP materials at the appropriate working frequency range is limited for dynamic tests based on the vibration of rods or beams [21,22] and only possible using ultrasonic waves [23–25]. Moreover, the use of the ultrasound technique in thin film applications has additional issues as coupled thickness layer determination [26–30]. Alternatively, it is well known for viscoelastic materials, that the frequency (strain rate) and temperature dependencies of polymer properties are both related. One may use the time–temperature superposition principle to generate the frequency-dependent curve, but in this case, other testing based on temperature variations are also complex and limited as described in [15]. It is important to point out here that the frequency sensitivity of ultrasound velocities is usually weak, of order tens m/s/decade, as described in [23], but since it depends mainly on the polymers relaxation and Tg, it may be a remarkable source of property variations in the performance analysis developed in this work.

The higher limit of 5 MHz proposed in [15] permits one to consider a conservative method for the suitable measurement of the material impedance, providing an upper bound limit on the stiffness variation of the viscoelastic response of the selected material, as demonstrated in [23,24], and for specific impact erosion applications in [16]. Hence, a procedure for the measurement of acoustic impedance with a time-of-flight technique of a thin viscoelastic layer using a planar ultrasonic transducer for the frequency regime of interest is done in this work, in the next section.

In the current work, impedance measurements at suitable working frequency with Ultrasonic testing are presented and developed as the input material data for the lifetime prediction based on Springer modelling exposed with different application case analysis. An investigation into various LEP coating application cases has been undertaken and related with the rain erosion durability factors. LEP erosion performance at rain erosion accelerated testing technique is used as the key metric in an effort to assess the response of changing material and processing parameters involved and to evaluate the lifetime accuracy analysis.
