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Author's accepted manuscript The rain-induced fatigue damage in the wind turbine blade coating has attracted increasing attention owing to significant repair and maintenance costs. The present paper develops an improved computational framework for analyzing the wind turbine blade coating fatigue induced by rain erosion. The paper first presents an extended stochastic rain field simulation model that considers different raindrop shapes (spherical, flat, and spindle), raindrop sizes, impact angles, and impact speeds. The influence of these raindrop characteristics on the impact stress of the blade coating is investigated by a smoothed particle hydrodynamics approach. To address the expensive computational time, a stress interpolation method is proposed to calculate the impact stress of all raindrops in a random rain event. Furthermore, coating fatigue analysis is performed by including the fatigue crack initiation in the incubation period and the fatigue crack propagation in the mass-loss-rate increasing period due to raindrop impact. Finally, the proposed computational framework is verified by comparing the estimated fatigue life with those obtained in literature. The results from the study show that by incorporating the statistics of rainfall data, the proposed framework could be used to calculate the expected fatigue life of the blade coating due to rain erosion.

Rain erosion on the leading edge of wind turbine blades is an intricate engineering challenge for the wind industry. Based on an energetic approach, this work proposes a methodology to characterise the erosion capacity of the raindrop impacts onto the leading edge blades. This methodology can be used with meteorological data from public institutions or from direct measurements at the wind turbine locations. The erosion characterisation is analysed using accumulative and per impact erosive variables, that is, total kinetic energy and kinetic energy per impact. To consider the frequency of impacts, two erosive variables are proposed, namely, total kinetic power and kinetic power per impact. These variables are calculated using the data from the Royal Netherlands Meteorological Institute (Koninklijk Nederlands Meteorologisch Instituut, KNMI) of the last 25 years jointly with the operation specifications of an actual wind turbine model (Suzlon S111). The main contribution to the erosive variables was found to be the wind speed because it controls the rotational velocity of the wind turbine. Also, the intensity of the rainfall and the frequency of meteorological data logging, that is, the temporal resolution of data, play a significant role.

Continuous ultrasonic welding is a promising technique for joining thermoplastic composites structures together. The aim of this study was to gain further insight into what causes higher through-the-thickness heating in continuous ultrasonic welding of thermoplastic composites as compared to the static process. Thermocouples were used to measure temperature evolutions at the welding interface and within the adherends. To understand the mechanisms causing the observed temperature behaviours, the results were compared to temperature measurements from an equivalent static welding process and to the predictions from a simplified heat transfer model. Despite the significantly higher temperatures measured at the welding interface for the continuous process, viscoelastic bulk heat generation and not thermal conduction from the interface was identified as the main cause of higher through-the-thickness heating in the top adherend. Interestingly the top adherend seemed to absorb most of the vibrational energy in the continuous process as opposed to a more balanced energy share between the top and bottom adherend in the static process. Finally, the higher temperatures at the welding interface in continuous ultrasonic welding were attributed to pre-heating of the energy director due to the vibrations being transmitted downstream of the sonotrode, to reduced squeeze-flow of energy director due to the larger adherend size, and to heat flux originating downstream as the welding process continues.

Rain-induced leading-edge erosion of wind turbine blades is associated with high repair and maintenance costs. For efficient operation and maintenance, erosion models are required that provide estimates of blade coating lifetime at a real scale. In this study, a statistical rainfall model is established that describes probabilistic distributions of rain parameters that are critical for site-specific leading-edge erosion assessment. A new droplet size distribution (DSD) is determined based on two years’ onshore rainfall data of an inland site in the Netherlands and the obtained DSD is compared with those from the literature. Joint probability distribution functions of rain intensities and droplet sizes are also established for this site as well as for a coastal site in the Netherlands. Then, the application of the proposed model is presented for a 5 MW wind turbine, where the model is combined with wind statistics along with an analytical surface fatigue model that describes lab-scale coating degradation. The expected lifetime of the blade coating is found three to four times less for the wind turbine operating at the coastal site than for the inland site - primarily due to rainfall at higher wind speeds. Further, the robustness of the proposed model is found consistent with varying data periods used for the analyses.

Leading edge erosion (LEE) of a wind turbine blade (WTB) is a complex phenomenon that contributes to high operation and maintenance costs. The impact between rain droplets and rotating blades exerts cyclic fatigue stresses on the leading edge — causing progressive material loss and reduced aerodynamic performance. One of the most important parameters for erosion modelling and damage prediction is the relative impact velocity between rain droplets and rotating blade and depends upon the environmental conditions. The environmental condition, in general, could vary for onshore and offshore wind turbines (OWTs) — for instance, the presence of wave-induced loads along with less turbulent wind and varying rainfall conditions in the offshore environment. The present paper tries to provide guidelines whether all these parameters need to be included for LEE modelling. Aero-hydro-servo-elastic simulations are carried out for a rotating blade based on the NREL 5 MW turbine by considering realistic environmental conditions for a land-based wind turbine and monopile-supported OWT. Further, the impact velocities and erosion damage rate, evaluated using a surface fatigue model, are analysed and compared for different environmental conditions. It is found that rainfall intensity and turbulence intensity influences the impact velocity minorly, however, has a substantial effect on the overall erosion damage rate. For instance, for the investigated load cases, an 8% increase in the impact velocity is observed when the turbulence intensity increases from 6% to 26%, which indicates an increase of erosion damage rate by more than 40%. Furthermore, no substantial influence is found due to the effects of wave-induced loads on the wind turbine.

The presence of rain-induced leading edge erosion of wind turbine blades (WTBs) necessitates the development of erosion models. One of the essential parameters for erosion modeling is the relative impact velocity between rain droplets and the rotating blade. Based on this parameter, the erosion damage rate of a WTB is calculated to estimate the expected leading edge lifetime. The environmental conditions that govern this parameter have site-specific variations, and thus, rain and wind loading on a turbine differ for onshore and offshore locations. In addition, there are wave loads present in the offshore environment. The present paper tries to provide guidelines for erosion modeling and investigates whether there are differences in erosion of blades due to (1) varying rainfall conditions modeled using different droplet size distributions for onshore and offshore locations in combination with (2) winds of varying turbulence intensities and (3) wave-induced loads. Aero-hydro-servo-elastic simulations are carried out for an onshore wind turbine (WT) and a monopile-supported offshore WT. Furthermore, erosion variables such as the relative impact velocities and the associated erosion damage rate of a blade are analyzed for various blade azimuth angles. The study shows that the rainfall intensity and turbulence intensity minorly influence the impact velocity and pressure but have a substantial effect on the overall erosion damage rate. Additionally, a significantly higher erosion damage rate is found for blades exposed to offshore rainfall conditions than for blades under onshore rainfall conditions. Furthermore, no substantial influence on erosion is found because of wave-induced loads.

Epoxies are inherently brittle materials and to overcome this brittleness, a second microphase (i.e., thermoplastic) is typically added. This modification of epoxy resin using thermoplastics results in reaction-induced phase separating morphologies in the micrometer range. In this study, the influence of the curing history, beyond phase separation, on the interphase formation and final morphology of PEI and the high Tg epoxy system is investigated. Several cure cycles were examined, each with a first dwell temperature ranging from 120 to 180 °C for a given time up to the onset of phase separation (OPS) or up to the 80% degree of cure (80% DOC) and then with a second dwell at 200 °C for 20 min to complete the cure cycle. Hot-stage microscopy experiments were carried out at several first dwell temperatures before final conversion at the second dwell. The morphologies and resulting droplet size distribution at the interphase, after the final cure, were analyzed through scanning electron microscopy. Results showed that the diffusion distance was significantly higher in the case of OPS as compared to the 80% DOC case, particularly at lower first dwell temperatures. This behavior was attributed to the fact that, in the case of OPS, polymeric chains were still in a mobile state and diffused further during the second dwell curing stage, while at 80% DOC, polymeric chains were completely bound but still diffuse due to non-stoichiometric curing. This restricted mobility of polymeric chains after phase separation (80% DOC) resulted in a larger number of smaller droplets as compared to the OPS case.

Continuous ultrasonic welding is a promising high-speed and energy-efficient joining method for thermoplastic composite structures. Our aim was to identify and understand differences between the static and continuous ultrasonic welding process for thermoplastic composites. In particular, melting of the interface, consumed power and energy density, temperature evolution at the weld interface, and optimum welding conditions for both types of processes were investigated. This was done for three combinations of welding force and vibrational amplitude, parameters which are known to have a significant effect in both welding processes. Our results showed that for the continuous process the amount of non-welded area under the sonotrode remains constant, while for the static process the amount of non-welded area gradually decreases to zero. Additionally, the optimum vibration times and welding speeds in both processes are similar.

The paper tackles the multi-objective optimisation of the cure stage of the Vacuum Assisted Resin Transfer Moulding (VARTM) process to manufacture the root insert of wind turbine blades. The aim of the study is to compare the Pareto front obtained from a pure heat transfer analysis, where temperature overshoot is an objective, with the one achieved when a coupled thermo-mechanical analysis is used, in which spring-in deflection is directly addressed. The optimisation methodology links the finite element solution of a heat transfer and a coupled thermo-mechanical problem of the cure process with a genetic algorithm suitable for multi-objective problems. The paper highlights that whilst minimising overshoot temperature is effective in reducing the spring-in, optimal design points might remain hidden. Furthermore, by showing the evolution of transverse residual stresses the paper suggests that designing cure cycles that introduce compressive stresses before vitrification is beneficial to counteract the subsequent tensile stresses generation.

The paper addresses the role played by the cure stage of a vacuum assisted resin transfer molding process in residual stresses generation. The Airstone 780E epoxy resin and Hardener 785H system broadly used in the wind turbine blade industry has been used in this study. The viscous–elastic properties of the resin have been characterized and implemented in a thermo-mechanical FE model. The model has been validated against manufactured [0/90]4 asymmetric laminates. Analysis of residual stresses generation highlighted that compressive stresses generation occurs when the cure is shrinkage dominated and tensile stresses when expansion dominated in the 0° plies. The finding points out that 10% reduction in warpage and 33% reduction in process time can be obtained by selecting cure cycle parameters that allow tensile stresses development during the cure process in the 0° plies.

Continuous ultrasonic welding is a high-speed joining method for thermoplastic composites. Currently, a thin film energy director is used to focus the heat generation at the interface. However, areas of intact energy director remain in the welded seam, which significantly lowers the weld strength, and result in a non-uniformly welded seam. To improve the weld uniformity of continuous ultrasonically welded joints, we changed to a more compliant energy director. A woven polymer mesh energy director was found to give a significant improvement in weld quality. The mesh was flattened in between the composite adherends during the welding process. This flattening promoted a good contact between the energy director and the adherends, fully wetting the adherend surfaces, resulting in a more uniformly welded seam without areas of intact energy director.

The effects of laser-induced deconsolidation on the compaction process of CF/PEEK tapes were investigated. First, tapes with different degrees of deconsolidation were manufactured using a laser heater. This procedure resulted in samples with different waviness, thickness, void content and surface roughness values. Then, as-received and laser-deconsolidated tapes were compacted under two different temperature histories and four different pressure levels. Waviness induced by laser-deconsolidation vanished when the material was heated up to the glass transition temperature even at a very low compaction pressure. Unlike waviness; increased thickness, void content and surface roughness due to laser-deconsolidation remained between the glass transition and melting temperatures. After the melting temperature was exceeded, the effects of laser-deconsolidation were dependent on the applied pressure and initial degree of deconsolidation. The final surface roughness, thickness and degree of effective intimate contact were affected by the degree of laser-deconsolidation when a compaction pressure of less than 300 kPa was applied.

In this study, the effect of rapid laser heating, which is typical during laser-assisted fiber placement (LAFP), on the micro- and meso- structure of the thermoplastic tape was investigated. Thermoplastic tapes were heated above the melting temperature with different heated lengths (30 and 80 mm and heating times (0.2 and 0.8 s) in a dedicated experimental setup. In-situ and ex-situ characterization techniques were used to observe the differences between the micro- and meso- structure of the tape before and after heating. The experiments resulted in significant changes in the tape structure, namely increased out-of-plane deformation, waviness, arc-length width, roughness, thickness and volumetric void content. This study shows for the first time that a unique deconsolidation behavior takes place during the heating phase of LAFP: the deconsolidation mechanisms are exacerbated by the non-uniform temperature at the tape surface, which is caused by roughness increase and waviness formation.

Research on ultrasonic welding of composites has focused mostly on studying parameters that are inputs for the process or material-related parameters, but almost no attention has been given on the effect of manufacturing tolerances. In this work, we investigated how an angle between adherends impacts the welding process and the weld quality. By increasing the angle between top and bottom adherends, it was found that the duration of the process increased while the power consumed, the weld uniformity and the weld strength decreased. However, by increasing the clamping distance, which increased the compliance of the adherends and hence their ability to deform under the applied welding force, the effect of the misalignment on both the welding process and weld quality could be substantially reduced.

Although aerospace traditionally has always had a multidisciplinary approach to engineering and design, the increasing complexity of aircraft and spacecraft and the rapid digitization within the aerospace industry has led to a large number of related engineering and scientific disciplines such as electrical engineers, computer scientist to work much more directly within the aerospace domain than before. Next to that there is a shortage of highly trained engineers worldwide to meet demand. As a result, there is a clear need to provide basic knowledge to non-aerospace engineers working in the field and to motivate and attract more people to engineering and aerospace in particular. This paper details how the creation of a Massive Open Online Course (MOOC) at an introductory level in Aerospace Structures and Materials provides an efficient and fit-for-purpose tool to achieve both aims. The paper will discuss the course design, the course set up, the course evaluation and how the course fits within the online learning philosophy of Delft University of Technology. It will use learning analytics to analyze our learners and their needs.

Operations and maintenance of offshore wind turbines (OWTs) play an important role in the development of offshore wind farms. Compared with operations, maintenance is a critical element in the levelized cost of energy, given the practical constraints imposed by offshore operations and the relatively high costs. The effects of maintenance on the life cycle of an offshore wind farm are highly complex and uncertain. The selection of maintenance strategies influences the overall efficiency, profit margin, safety, and sustainability of offshore wind farms. For an offshore wind project, after a maintenance strategy is selected, schedule planning will be considered, which is an optimization problem. Onsite maintenance will involve complex marine operations whose efficiency and safety depend on practical factors. Moreover, negative environmental impacts due to offshore maintenance deserve attention. To address these issues, this paper reviews the state-of-the-art research on OWT maintenance, covering strategy selection, schedule optimization, onsite operations, repair, assessment criteria, recycling, and environmental concerns. Many methods are summarized and compared. Limitations in the research and shortcomings in industrial development of OWT operations and maintenance are described. Finally, promising areas are identified with regard to future studies of maintenance strategies.

The effect of thermal contact resistance (TCR) correlated to the degree of intimate contact (DIC) between the incoming tape and the substrate on the temperature history during laser-assisted fiber placement (LAFP) was investigated. A novel experimental methodology was designed to understand the effect with a non-contact method which did not influence the local consolidation quality. To assess the influence of TCR numerically, a three-dimensional optical-thermal model was developed. Experimental results indicated that, for the same tape temperature near the nip point, an increase in the compaction force resulted in a decrease in the temperature at the roller exit and the following cooling phase, in correlation with an increase in the final DIC. Also, the effect of the laser power on the final DIC was less pronounced than the compaction force. In the thermal model, when TCR at the tape-substrate interface was not considered, the temperature predictions underestimated the experimental measurements.

Leading edge erosion (LEE) repairs of wind turbine blades (WTBs) involve infield application of leading edge protection (LEP) solutions. The industry is currently aiming to use factory based LEP coatings that can applied to the WTBs before they are shipped out for installation. However, one of the main challenges related to these solutions is the choice of a minimum LEP application length to be applied in the spanwise direction of the WTBs. Generally, coating suppliers apply 10–20 m of LEP onto the blades starting from the tip of the blade using the “rule of thumb”, and no studies in the literature exist that stipulate how these LEP lengths can be calculated. In this study, we extend the scope of a recently developed long-term probabilistic framework to determine the minimum LEP application length required for WTBs to combat rain-induced erosion. A parametric study is performed where different wind turbines with varying power ratings of 2.1 MW to 15 MW at different Dutch sites ranging from inland to coastal are considered. The results of the study show that the LEP application length is sensitive to the choice of the site, as well as the turbine attributes. Further, LEP lengths for WTBs are found to be the highest for turbines installed at coastal sites and turbines with higher power ratings. A detailed investigation is further performed to check the sensitivity of the LEP application length with the wind turbine parameters. The results of the study are expected to provide guidelines to the industry for efficient repair strategies for WTBs.

The issue of leading edge erosion (LEE) of wind turbine blades (WTBs) is a complex problem that reduces the aerodynamic efficiency of blades, and affects the overall cost of energy. Several research efforts are being made at the moment to counter erosion of WTBs such as-testing of advanced coating materials together with development of high-fidelity computational models. However, the majority of these studies assume the coated surfaces as flat, while the surface curvature and the shape of the aerofoil at the blade’s leading-edge exposed to such rain fields is neglected. The present study questions the assumption of a flat surface, in the context of LEE of WTBs, and provides guidelines for erosion modelling. The critical parameters associated with rain droplet impingement kinematics on leading edge are compared for blade impact with (a) flat surface assumptions together with (b) the effects of the blade’s surface curvature. A parametric study is performed which includes WTBs of varying sizes and power ratings ranging from 750 KW to 10 MW, different positions along the blade length, and different rain droplet radii ranging from 0.1 mm to 5 mm for a land based wind turbine operating at rated wind speed. It is found in the study that droplet impingement kinematics are influenced by the surface curvature at the leading edge, the effect of which is significant for representing erosion at the blade tip for smaller blades, and for exposure to rainfall intensity with larger rain droplet size. A master curve describing the threshold level along the blade length is established for various WTBs and rainfall conditions, where flat surface approximation of the surface yields noticeable error and violates the impingement process. The results of the study are expected to aid the modeller in developing advanced numerical models for LEE for WTBs.

Intimate contact development under LAFP-specific thermal and mechanical boundary conditions/interactions and the effect of process parameters are investigated. One-layer, unidirectional strips of CF/PEKK material were placed with different process parameters on a flat tool surface to create different intimate contact conditions. The concept of effective intimate contact, which is based on the resin content at the surface, is introduced and a methodology to measure it from surface micrographs is provided. Degree of effective intimate contact measured from the samples was compared with the existing intimate contact models. The temperature history in the compaction zone was estimated with a finite element model and pressure sensitive films were used to determine the compaction pressure. It is shown that in addition to the squeeze flow mechanism, which is the base for the current intimate contact models, through-thickness percolation flow of the resin needs to be considered to explain the effective intimate contact development.

The paper deals with the influence of the convection coefficient and laminate thickness on multi-objective optimisation of the vacuum assisted resin transfer moulding cure stage for the manufacturing of wind turbine components. An epoxy resin system widely used in the wind turbine industry has been chemically characterised and the correspondent finite element implementation validated. The optimisation methodology developed links the finite element solution with a genetic algorithm and identifies a set of optimal cure cycles for a range of thicknesses (10–100 mm) able to minimise cure time (t cure ) and the maximum degree of cure gradient developed through thickness (Δα max ) during the cure stage as a measure of quality of the product. The results highlight that, by adding convection coefficient as design parameter of the process, significant benefits could be obtained when insulation is applied at the vacuum bag side for all thicknesses.

Developments in the wind industry reveal intricate engineering challenges, one of them being the erosion on the leading edge of the wind turbine blades. In this review work, the main issues for the wind industry in the experimentation with respect to erosion are examined. After a historical and general overview of erosion, this review focuses on the rain erosion on the leading edge of the wind turbine blades giving prominence to (1) the rain simulations, (2) experimental erosion facilities, and (3) variables to characterise erosion. These three factors have to be improved to establish a research field enabling the prediction of erosion behaviour and providing useful information about how the rainfall affects the leading edge of the wind turbine blades. Moreover, these improvements in the experimentation of the erosion would be a first step to understand and predict the erosion damage of the wind turbine blades. Finally, this review work also will help to cope with experimental investigations and results in the rain erosion on the leading edge with a deeper critical thinking for future researchers.

Semi-adiabatic temperature measurements are recorded and used to define semi-empirical equations for the simulation and prediction of the anionic polyamide-6 (APA-6) reaction kinetics. The resin mixture used has a long infusion window before the reaction starts. The prediction of the induction time and its corresponding initial temperature of reaction is explored. By means of this semi-empirical approach and an optimised fitting procedure, the reaction kinetics of APA-6 can successfully be described. The adiabatic polymerisation can be predicted on the basis of an autocatalytic Kamal-Sourour model for thermoset resins, and the crystallisation can be described using the isothermal crystallisation model.

Research on postprocessing of glass fiber–reinforced anionic polyamide-6 (APA-6) composites was performed in order to optimize the processing time. The two main points of investigation were the cure time and the annealing cycle. It was shown that the optimal mechanical properties (short-beam strength) coincided with the maximum degree of crystallinity for a cure time of 90 min at a cure temperature of 180°C. Annealing close to the crystallization temperature of APA-6 showed the largest increase in degree of crystallinity. For enhanced mechanical properties, however, the processing history is of importance. It was seen that for short cure times, sufficient annealing time at elevated temperatures is beneficial for the polymer chains to branch, cross-link, or adhere to the fibers, hence, significantly improving the mechanical properties.

In order to manufacture thicker, larger and more integrated thermoplastic composite parts than currently can be achieved by melt processing, a vacuum infusion process is currently being developed at the Delft University of Technology using a reactive thermoplastic polymer called anionic polyamide-6 (APA-6). In previous studies it was demonstrated that the anionic polyamide-6 (APA-6) resin that is used has excellent mechanical properties. The present study assesses infused thermoplastic composites and focuses on fiber-matrix interactions. Part I of this study focuses on the thermal effects, causes for deactivation of the initiator and the restriction caused by the low in-plane permeability of the fiber textiles on various transport phenomena. It will be shown that addition of pre-heated fibers not only shortens the infusion window, but also influences the matrix properties by reducing the exothermic heat production. In addition, the low in-plane permeability of the fiber textiles influences the infusion time and causes the entrapment of voids. Finally, reactions between the matrix and the fiber surface can lead to deactivation of the initiator and bond formation with the activator. Interfacial bonding, however, is discussed in more detail in Part II of this study, whereas the effect of adding a nucleating agent is discussed in Part III.

[Abstract] During development of glass fibre reinforced anionic polyamide-6 composites, it was shown that the current high temperature vacuum infusion process induced a gradient in flow direction, both thermally and chemically. A change in production process was tested to eliminate these problems: both the resin and the dry fibre pre-form are at the same temperature during infusion, after which it is heated to the required curing temperature. The effect of the infusion temperature was investigated and it was shown that at all temperatures a gradient was still present. A trade-off had to be made between the reduction in gradient due to the low infusion temperature and the heating time to the required curing temperature.