In this paper, the sensitivity of the structural integrity of wind turbine blades to debonding of the shear web from the spar cap was investigated. In this regard, modal analysis, static and fatigue testing were performed on a 45.7 m blade for three states of the blade: (i) as received blade (ii) when a crack of 200 mm was introduced between the web and the spar cap and (iii) when the crack was extended to 1000 mm. Calibration pull-tests for all three states of the blade were performed to obtain the strain-bending moment relationship of the blade according to the estimated target bending moment (BM) which the blade is expected to experience in its service life. The resultant data was used to apply appropriate load in the fatigue tests. The blade natural frequencies in flapwise and edgewise directions over a range of frequency domain were found by modal testing for all three states of the blade. The blade first natural frequency for each state was used for the flapwise fatigue tests. These were performed in accordance with technical specification IEC TS 61400-23. The fatigue results showed that, for a 200 mm crack between the web and spar cap at 9 m from the blade root, the crack did not propagate at 50% of the target BM up to 62,110 cycles. However, when the load was increased to 70% of target BM, some damages were detected on the pressure side of the blade. When the 200 mm crack was extended to 1000 mm, the crack began to propagate when the applied load exceeded 100% of target BM and the blade experienced delaminations, adhesive joint failure, compression failure and sandwich core failure.

Leading edge protection (LEP) coating systems are applied to protect turbine blade edges from rain erosion. The performance of a LEP system is assessed in an accelerated rain erosion test (RET) as a metric for industrial application, but these tests are expensive. Modelling methods are available to predict erosion, based on fundamental material properties, but there is a lack of validation. The Springer model (1976) is analysed in this work to assess it as a tool for using material fundamental properties to predict the time to failure in a rain erosion test. It has been applied, referenced and industry validated with important partial considerations. The method has been applied successfully for erosion damage by wear performance prediction when combined with prior material data from rain erosion test (RET), instead of obtaining it directly from fundamental properties measured separately as Springer proposed. The method also offers accurate predictions when coupled with modified numerical parameters obtained from experimental RET testing data. This research aims to understand the differences between the experimental data used by Springer and the current industry approach to rain erosion testing, and to determine how it may introduce inaccuracies into lifetime predictions of current LEP systems, since they are very different to those tested in the historic modelling validation. In this work, a review of the modelling is presented, allowing for the understanding of key issues of its computational implementation and the required experimental material characterisation. Modelling results are discussed for different original application issues and industry-related LEP configuration cases, offering the reader to interpret the limits of the performance prediction when considering the variation in material fundamental properties involved.

The relationship between the bulk thermomechanical properties and rain erosion resistance of development polyaspartate-based coatings as candidate leading edge protection (LEP) materials for wind turbine blades is investigated by the combined application of dynamic mechanical analysis (DMA) and rain erosion testing (RET) within a novel test method (DMA+RET). This method introduces the use of DMA+RET to both monitor the change in thermomechanical properties with respect to raindrop impact and subsequently rationalise differences in rain erosion resistance between coating formulations of comparable composition. The application of this combined process has demonstrated the importance of relatively high viscoelastic moduli at increased strain rates and creep recovery after RET as key material properties to be considered for LEP material development, whereas previous research presented in the scientific literature has primarily focussed on the use of routine characterisation procedures by tensile testing or stand-alone DMA to evaluate coating formulations prior to rain erosion testing. This journal article therefore presents a novel method to evaluate key material properties relevant to rain erosion resistance before and after subjection to raindrop impact using standard ASTM G73 RET equipment. The test method is demonstrated on a novel polyaspartate-based coating, PA-U, that exhibits notable rain erosion resistance in comparison to commercial LEP products. PA-U exhibited negligible mass loss after 30 h of rain erosion testing and favourable thermomechanical properties (E″ = 35 MPa at critical strain; equilibrium recoverable compliance of 0.05 MPa⁻¹) in comparison to alternative formulations.

The current wind turbine leading-edge erosion research focuses on the end of the incubation period and breakthrough when analysing the erosion mechanism. This work presented here shows the benefits of splitting and describing leading-edge erosion progression into discrete stages. The five identified stages are: (1) an undamaged, as-new, sample; (2) between the undamaged sample and end of incubation; (3) the end of incubation period; (4) between the end of incubation and breakthrough, and (5) breakthrough. Mass loss, microscopy and X-ray computed tomography were investigated at each of the five stages. From this analysis, it was observed that notable changes were detected at Stages 2 and 4, which are not usually considered separately. The staged approach to rain erosion testing offers a more thorough understanding of how the coating system changes and ultimately fails due to rain droplet impacts. It is observed that during microscopy and X-ray computed tomography, changes unobservable to the naked eye can be tracked using the staged approach.

Blade leading edge erosion (LEE) is a persistent challenge in the wind industry, resulting in reduced aerodynamic efficiency and increased maintenance costs, with an estimated total expense of GBP 1.3M over a 25-year turbine lifetime. To mitigate these effects, leading edge protection (LEP) systems are widely used, but their real-world performance often falls short of predictions based on the standard rain erosion test (RET). This study investigates the limitations of current RET practices, which are designed to accelerate testing but fail to replicate the diverse environmental conditions experienced by wind turbines. Two LEPs with contrasting viscoelastic properties were tested using a novel design of experiments (DoEs) approach. The study explored the droplet impact frequency, combination and sequencing of high or low rainfall intensities, recovery during the inspection period and droplet size effects on erosion behaviour, to uncover significant differences in material performance compared to standard RET conditions. Results, supported by dynamic mechanical analysis (DMA), indicated that the chosen LEPs undergo a transition between elastic and brittle failure modes at a critical impact frequency, influenced by the viscoelastic properties of the material. Importantly, the findings emphasise the need for revised testing protocols across a range of parameters that incorporate realistic environmental conditions to improve the predictability of LEP performance.

Wind turbine blade erosion is typically assessed in situ using visual inspection, which is a rudimentary qualitative assessment of the condition of the blade coating system. On coated test specimens in laboratory test conditions, mass loss can provide a better understanding of the stage of erosion, but cannot be extended to real world applications. In this work, by using analysis of photographs, microscopy images, mass data, and gloss data together, it was found that gloss measurements can effectively quantify changes in coating microstructure as a result of rain erosion. This was achieved by correlating and comparing mass loss measurements over time with surface gloss and verifying the erosion stages with photographs and microscopy images. As such, gloss was shown to represent the erosion stages with greater accuracy than the current industry methods. This novel technique has been shown to identify the incubation period, that is the onset of erosion damage, by detecting microstructure changes which are not visible to the naked eye, nor is determinable by mass loss. The quantitative output from the gloss methodology thus allows wind turbine owners and operators to assess, manage, and plan more efficiently for costly erosion repairs and future inspections. The system is presently being used in a laboratory setting, though it has the potential to be combined with drones or climber robots to be remotely used within the wind farm.