Materials being developed and tested for use in aerospace engines are pushing not only material developers, but also those that have to test them. While the general wish is to make aircraft engines run hotter, it makes the wide temperature ranges experienced during operation difficult to recreate in the lab. The temperature difference can be huge from ambient air temperature of a cruising airliner dropping as low as -150°C to the combustion chamber in a turbine engine rising to well over 1000°C.
There are a number of key properties engineers need to investigate, from how effective a thermal barrier coating is to characterising how an advanced metal alloy behaves in terms of heat resistance, re-crystallisation temperatures and thermo-elastic behaviour.
To achieve the ultimate goal of improving engine performance on future aircraft, researchers at US space agency NASA are investigating promising advances in high-temperature materials that can be used to make turbine engine components. These include ceramic-matrix composites (CMCs) that are lighter, stronger and can withstand the demanding forces of the extremely high temperatures generated in the core of engine. Many feel CMCs are in a perfect position to replace the nickel-based super alloys used in today.
In general, the hotter an engine runs, the better the fuel efficiency. Over the years, engines have become hotter as metal parts are treated with thermal barrier coatings. But there is a limit to what the coatings can tolerate. CMCs, on the other hand, have been shown to tolerate temperatures of 1500°C or more with the help of specially designed ceramic coatings called environmental barrier coatings.
“We want to understand how CMCs and protective coatings can not only withstand high heat, but also environmental particle hazards such as dust, sand and volcanic ash,” says NASA materials engineer, Valerie Wiesner. “This is important because as aircraft engine temperatures increase to promote fuel efficiency, sand for example, when it’s ingested into an engine, can actually melt into glass and potentially cause power loss or failure.”
Moving next generation aircraft toward greater operational efficiency will depend, in large part, on advances in engine technology and materials manufacturing capabilities. Elsewhere at Nasa, similar work carried out by test engineer Michael Cuy, pictured, subjects a coated silicon carbide sample to the high temperature and high-velocity environment of an operating aircraft engine at NASA’s Fuel Burner Rig.
The facility’s eight test cells are equipped with jet fuel combustors and can subject coated samples to high temperatures up to 2,700 degrees Fahrenheit, and airflow and particulate velocities up to Mach 0.3.
This unique facility is used to test advanced coatings developed by researchers to protect aircraft engine parts from the punishing combustion environment encountered during flight. Durable coatings protect and minimise damage from hazardous exposure, but these still have limitations.
Elsewhere, Rolls-Royce has opened its University Technology Centre (UTC) at Karlsruhe University in Germany to carry out similar research and testing around the cooling within gas turbine combustors and turbines.
While running the engine hotter in the goal for Nasa, Roll-Royce wants to make sure its engines do not surpass operational temperature limits and so are looking to further develop efficient cooling systems to prevent combustion and turbine components from melting.
The UTC does aim to improve engine efficiency though, and will take a holistic approach by using less air for cooling and more in the combustion process to help further reduce engine emissions. The University has proven academic capability in combustion cooling in very hot environments, film cooling and two-phase-flow, plus a range of rigs and sophisticated hardware, innovative methods and measurement techniques to test theories and develop materials technologies.
It is expected that Karlsruhe University will work closely with a number of existing Rolls-Royce UTCs in the UK – notably those based in Surrey, Loughborough and Nottingham universities, and the Osney Laboratory in Oxford – that collectively focus on research into heat transfer, combustion, computational fluid dynamics (CFD), aerothermal techniques and component interactions, and provide highly specialised modelling, validation and testing capabilities.
How to test?
To enable many of the high temperature tests being increasingly demanded within global aerospace engine development, heating test pieces has become standard practice. However, though this is well established the process is far from straightforward to get right. Traditionally, metals have been tested up to 1000°C. However, recent developments in hybrid materials that include CMCs as well as high strength steels have resulted in engineers looking to push temperatures well beyond the standard limit for specimen tests.
This has been the case with the Aerospace Research Institute in China, again working to develop materials for use within turbine engines. To get the test capabilities it needs, it approached test equipment manufacturer Zwick Roell to see if it is possible to do non-contact tensile tests on material samples that are heated in excess of 1200°C.
“Engineers are looking for greater energy efficiencies, and one of the drivers behind that is temperature,” explains David Phillips, vice president of corporate marketing at Zwick Roell. “If a jet engine can be driven at higher temperatures, they are more fuel efficient. That puts pressure on us to be able to test materials at higher temperature than we have up until now.”
The request has seen the development of the Allround Line Materials Testing Machine 250kN, for the Institute. The machine, like most designed by Zwick, has been engineered with modularity in mind. Indeed, the nuances at Zwick include highly effective use of basic engineering principles, such as using as many common components on machines as possible, minimal parts counts and modular manufacturing bays – all of which is designed and manufactured in-house at its headquarters in the southern Germany city, Ulm.
The AllroundLine testing machine uses AC drive and is said to be zero-maintenance through life. It can attain maximum forces up to 250 kN in tensile and compression as well as offering constant velocity characteristics at very low speeds.
To enable the tests required, the modular Allround Line machine is equipped with both a temperature chamber and a high-temperature furnace. This combination allows measurements in a temperature range between -150 and 1200°C.The temperature chamber is designed for temperatures between -80 and 300°C, but can be used down to -150°C with liquid nitrogen.
For the high-temperature furnace there is a choice between one and three-zone versions, and it can test in air or within a vacuum environment. The minimum temperature for the furnace is 200°C, while the maximum temperature is strain measurement
The range corresponds to the typical low temperatures seen by a cruising commercial airliner as well as the high temperatures occurring in and around the turbines core and critically the combustion chamber.
“Traditionally you would have to fasten a cage-like device to the specimen which had moving parts and sensors mounted underneath in order to track elongation, which is pretty tough to do when you get up to these high temperatures,” says Phillips. “This device has to be attached to every single specimen and the sensors reset each time, so it takes a long time to set up.
“While this is still state-of-the-art technology, we’ve tried to see if we can do the same test without touching the specimen, so taking a non-contact approach.”
Strain measurement on the Allround Line machine is handled by Zwick’s non-contact laserXtens extensometer. These consist of two measuring heads with digital cameras and laser light sources mounted on motorised carriages.
This laser-optical method eliminates the need for specimen marking in tensile tests and instead uses virtual gauge marks that are generated through illumination of the specimen with coherent light, allowing the movement of a specimen during deformation tests to be accurately tracked in real time.
The laserXtens uses a green laser to project the speckled pattern and form the unique pattern on the surface. The laser-optical method remains accurate at almost any temperature range, from sub-zero to glowing red hot.
The work carried out by Zwick answers many of the industry challenges of how to elevate testing temperatures while being able to accurately measure sample behaviours and properties using non-contact methods.
The work on the rigs could well find application in other industries and will no doubt help turbine engine developers improve fuel efficiency by allowing the testing of new materials at hotter temperatures. Work across the industry continues at pace.
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