Manufacturing stronger airplane materials


High temperature solid particle erosion (HTSPE) testing on materials used to make airplanes is invaluable to the aviation industry. With the survival of airplanes being dependent on the ability to withstand high velocity and high temperature particles, the most notable benefit of assessing erosion resistance is ensuring aircraft safety, and thus human safety.

The consequences of the 2010 eruption of the Icelandic volcano Eyjafjallajökull contextualize the necessity of HTSPE testing for the aviation industry. A plume of fine glass-rich ash ejected from the volcanic eruption reached a height of over 8km into the atmosphere. With the plume containing silica concentrations of around 58%, the predominant concern was ingestion of these particles into engines, where they would melt and cause engine damage or even failure. Other major fears included reduced visibility for visual navigation due to smoke and ash, and windscreens and navigation lights being sandblasted by the erosive particles.

Following the eruption, concerns over the hazardous impacts on airplane safety caused the largest grounding of aircraft since World War II, with controlled airspace across much of Europe intermittently closing for six days, the cancellation of more than 100,000 flights, and over 10 million people being affected. This shut-down cost £1.1bn (US$1.3bn), quantifying the knock-on impact particulate erosion can have on the aviation industry’s economy, and emphasizing the wider benefits HTSPE testing can have on industry.

Erosion of materials is a complex process, and its rate is influenced by multiple parameters, including temperature, particle velocity, particle shape and composition, the stricken surface’s properties, and impingement angle. Therefore, understanding these parameters’ influence is important for accurately measuring erosion rates, as well as selecting and developing materials. However, for many years, a lack of metrology, well-controlled and instrumented tests, and standardization has hampered the ability to assess erosion resistance of aircraft candidate materials.

Metrology and HTSPE testing

Manufacturing materials depends on research and development, which requires methods for obtaining reliable experimental data and methods for calibrating against standards. As such, metrology is important in fundamental research, both theoretical and experimental in nature. It is an integral driving force of innovation, facilitating the improvement of a process’s quality, the use of new techniques and materials, and the reduction of waste. Demands on metrology have grown in recent years, particularly for material scenarios such as composite materials, coatings, metals, nanomaterials, and more.

With no recognized standard HTSPE testing methods existing before 2014, testing was previously based around a method titled ASTM G76. This test had several key limitations, namely being conducted in conditions that were not adaptable to represent those of real life, including room temperature and limited velocities. Despite the publication of an HTSPE standard test method around 2010 that addressed multiple measurement limitations, it did not provide a more robust test in terms of metrology. As a result, HTSPE testing has been limited to comparative ranking of materials under conditions that replicate service conditions. In addition to methodological limitations, an overarching restriction to the development of improved testing standards has been the existence of only two laboratories with facilities that meet testing requirements in all of Europe.

The Metrosion project

The limitations of previous HTSPE testing have been addressed by researchers at the National Physical Laboratory (NPL), the home of measurement in the UK. This was through the Metrosion (metrology to enable high temperature erosion testing) project, an initiative funded by the European Association of National Metrology Institutes (EURAMET). The project’s primary objective was to develop the metrological framework for instrumenting and monitoring HTSPE testing in real time, in conjunction with modeling of the erosion process, and thereby provide industry with the tools they require to understand and ameliorate the effect of HTSPE.

The project brought together several measurement techniques into one highly efficient HTSPE system designed to operate at 900⁰C and with velocities up to 300ms-1. The techniques include a new gas inlet nozzle for reaching the highest operating velocity, high speed velocity measurements, in situ mass measurement, and in situ volume measurement.

Collating these approaches enables definitive size and shape measurements of erodent particles to be made, and improves the method for measuring particle velocity. These outcomes can subsequently enhance material scientists’ understanding of material performance and mechanistic modelling, driving innovation, improving system performance, and enabling industry to develop better airplanes with higher resistance to HTSPE damage.

Another key advancement for HTSPE testing brought about by this apparatus is the drastic reduction in time required to conduct a test. While a test with a total exposure time of thirty minutes to erosion by particles at high temperature previously took most of a week to conduct, this new method has cut the test time down to being completed within an hour and a half.

This is a result of incorporating the two types of in situ measurement of sample wear, which revolutionized the process of measuring erosion rate by largely replacing the need to interrupt HTSPE tests. Interruptions were previously required for cooling and weighing samples between each eroding interval, before heating them up again. Together with the other advancements, the in situ approaches can help airplane manufacturers design better airplanes in a much shorter time.

Future of Metrosion

Offering the new HTSPE testing method to industry is the long-term plan of Metrosion, in order to aid airplane manufacturers identify degradation modes of materials, and manufacture components from materials that can endure harsh HTSPE conditions. In the short-term, however, the scientists will continue to work on improving the equipment built at NPL, to advance the method to one of high quality that can accurately identify mechanistic effects attributed to velocity, temperature and particle embedding.

The importance of metrology in fundamental research indicates its potential to improve HTSPE testing when integrated into the method. The new metrology-incorporating HTSPE testing system developed at NPL will advance the manufacturing of aircraft materials that are more resilient to destructive particulate erosion. The ultimate outcome of HTSPE-resistant airplanes is invaluable, as this will in turn benefit many facets of the aviation industry, and most importantly of all, human safety.

To find out more about the research, please visit the Metrosion pages on the NPL website.

January 11, 2017

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