A water droplet: a greater force than you think!


Leading edge erosion, or rain erosion, is a significant issue for wind turbine blades. Pitting is created on the surface of these large blades by rain droplets, which can develop into more substantial damage and ultimately reduce the efficiency of the overall system. The reduced power output is a major concern for windfarm operators, who require a cost-effective, long-term solution – and fast! To combat this issue, researchers have been looking at alternative measures to mitigate rain erosion and enhance the properties, durability and performance of coating materials.

Among the protective measures taken to limit rain erosion, polymeric coating materials have been widely used due to their affordability. The current coatings are generally made from epoxy or polyester, which are used for structural components of the turbine blade, but while these materials offer advantages in structural stiffness and strength, they are not appropriate for leading edge protection. For many years, making the surface harder was seen as the means of improving rain erosion resistance, and while this may hold true for metals, it has the opposite effect for polymers. Softer polymers, like polyurethane, can effectively absorb and dissipate the impact energy from the droplet, before returning to their original shape in advance of the next droplet impact. This viscoelastic behavior, among other material properties, is currently not entirely understood, and in order to develop the next generation of leading edge protection systems there is a need to better understand the relationship between various material properties and the erosion resistance.

Mr. Anthony O’Carroll, Dr. Mark Hardiman, and Prof. Trevor Young at the University of Limerick, Ireland in collaboration with Dr. Edmond Tobin at the Institute of Technology Carlow, have investigated the rain erosion resistance of several polymeric materials with different mechanical properties. They subjected the materials to rain erosion conditions in a custom designed chamber to rank their erosion resistance. In conjunction, a nanoindenter was used to assess key mechanical and surface properties. The study’s main objectives were to investigate what material or surface properties, if any, have a relationship with the resistance to rain erosion for the selected polymers. The research is published in journal, Wear.

Based on the experimental results, the authors observed that there were several correlations between the erosion resistance and specific mechanical properties of the polymer materials. For instance, a reduction in indentation hardness and elastic modulus indicated a clear correlation with improved rain erosion resistance. Consequently, a lower elastic modulus leads to a lower acoustic impedance, which plays a key role in the damage mechanics of rain erosion. Through nanoindentation, it was found that materials which responded quickly to an applied load performed better than materials that had a delayed response. The creep and relaxation behavior of the polymers were extensively investigated using analytical and numerical methods, showing much promise for future studies.

The roughness of the materials was also assessed to elucidate how the repeated impacting of rain droplets affects the surface of the target material. It was found that the materials were roughened up to a critical point, at which time pieces of the target material would be ripped from the surface. One interesting observation was that the rate at which the surface was initially roughened correlated to the rate of mass loss of the target material, which is in a different segment of the rain erosion cycle.

According to the authors, the study provides an empirical insight into the damage mechanisms produced by repeated droplet impacts on polymeric materials, of which there is limited information in the public domain. It is suggested that this will benefit the wind industry by enabling blade manufacturers to fine-tune their coating materials based on mechanical and surface properties, which will reduce maintenance costs and failures of these materials, thus enhancing their performance and lifespan.

A water droplet a greater force Advances in Engineering

A water droplet: a greater force than you think! - Advances in Engineering A water droplet: a greater force than you think! - Advances in Engineering

About the author

Anthony O’Carroll holds a Bachelor’s degree in Aeronautical Engineering and a Masters by Research from the University of Limerick. Once Anthony graduated from the B.Eng. programme, he took up a research position within the Irish Composites Centre, focusing on manufacture, damage detection and mechanical testing of composite components. This work exposed him to the use of ultrasonic inspection and laser shearography systems, which were central to a work package which focused on “kissing-bonds”, a significant issue for large-scale multi-component composite assemblies.

During this time the opportunity to work in the area of Rain Erosion arose, which became the focus of his masters studies. The title of his thesis was “Correlation of Mechanical Properties to Rain Erosion Resistance of Polymeric Materials”. Anthony is currently the Technical Officer for Composite Manufacturing at the University of Limerick. He has now been working in the area of Rain Erosion for over 4 years, and is keen to continue working towards a solution for this ever increasing issue.

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About the author

Dr. Trevor Young is an Associate Professor at the University of Limerick, Ireland. He holds degrees in aeronautical engineering from Cranfield University, UK (PhD and MSc) and the University of the Witwatersrand, Johannesburg (BSc Eng). He was instrumental in the development of the first Bachelor of Engineering, Aeronautical Engineering degree programme in the Republic of Ireland. He has participated in, managed or coordinated more than 40 research grants and contracts (with funding from the European Commission, the Irish state and several private companies). He act as an Expert Evaluator of research proposals for the European Commission. Since 2015, he served as a member of the Brussels-based Clean Sky Scientific Committee . In this capacity, he acts as an Expert Reviewer of collaborative, multi-national aeronautical engineering research projects.

His research interests include the behaviour of composite material structures (for aeronautics and wind energy applications) and aircraft design technologies for reduced environmental impact. Over the past 12 years, he has developed experimental techniques to study water droplet erosion of aircraft or wind turbine leading edges.

He has published, as author or co-author, over 100 technical papers (incl. 55 peer-reviewed journal articles) and 10 book chapters. He is a co-editor of Innovation in Aeronautics (Woodhead, 2012) and author of Performance of the Jet Transport Airplane: Analysis Methods, Flight Operations, and Regulations (Wiley, 2017). His publications have been cited ca 1500 times.

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About the author

Dr. Mark Hardiman is a Lecturer at the University of Limerick, Ireland. He holds a first class honours degree in Mechanical Engineering (BEng) and a completed a PhD in the area of composite materials characterisation using nanoindentation in 2016. He is a member of the Irish Composites Centre (IComp) and the Bernal Institute with expertise in the design, manufacturing and characterization of fibrous composite structures. His main research interests include the development of nanoindentation protocols for polymer and composite materials, experimental and computational composite micromechanics, joining of dissimilar materials for automotive and aerospace applications and advanced characterisation methods for rain erosion coating systems for composite materials.

He is currently project manager of the SFI-funded FALCOM project related to the development of an Advanced Hybrid Composite-Metal Joining Technology. He has published, as author or co-author, 9 peer-reviewed journal articles in the areas of computational mechanics and composite materials and has participated in over €1M of successful funding proposals.

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About the author

Dr. Edmond Tobin is a Lecturer in Aerospace Engineering at Institute of Technology Carlow, Ireland. He completed his BEng in Aeronautical Engineering at the University of Limerick (2005). He was awarded a PhD by the same institution on the topic of rain erosion of aircraft leading edge materials. Dr. Tobin has conducted research in the areas of rain erosion, wind turbine blade monitoring, blade erosion protection and weathering and in development and design of test rigs and test standardisation. He has been involved in projects at national and international levels through IComp (Irish Composites Centre) and EU Framework Programmes such as FP7 and Clean Sky. He is also an expert evaluator for Clean Sky 2 project proposals. Dr. Tobin is involved in standards development with ISO committee TC 35/SC 9/WG 32 and ASTM standards committee G02 on Wear and Erosion. He joined the Department of Aerospace, Mechanical and Electronic Engineering at IT Carlow in 2015 and is a member of the EngCORE research unit.

He has published, as author or co-author, 11 technical papers (incl. 6 peer-reviewed journal articles) and 2 book chapters. His current research interests involve rain erosion of materials, cavitation erosion, erosion resistant coatings and self-cleaning materials.

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O’Carroll, A., Hardiman, M., Tobin, E., & Young, T. (2018). Correlation of the rain erosion performance of polymers to mechanical and surface properties measured using nanoindentation. Wear, 412-413, 38-48.

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