Aircraft manufacturers are under constant pressure to develop designs that use less fuel and therefore cost less to operate. Their response? Turn up the heat. By supporting the development of jet engines capable of performing at higher temperatures for extended periods, manufacturers are setting the stage for a new generation of aircraft that will redefine expectations for fuel efficiency, potentially allowing fully loaded commercial jets to make transatlantic flights carrying up to 15% less fuel. Understanding the properties of the high temperature alloys and ceramic matrix composites that will comprise components of these new engines is a critical aspect of this endeavour.
Some of the most advanced research in this field occurs at the Structures and Materials Performance Laboratory, part of the National Research Council (NRC) of Canada's Institute for Aerospace Research located in Ottawa, Ontario. The NRC is a Canadian government agency that funds more than 20 research institutes in science and engineering and employs more than 4,000 people.
Fifteen of those people are research scientists who specialise in designing, developing and testing new gas turbine materials. This team, called the Materials and Component Technologies Group, pursues in-house research projects and also collaborates with commercial engine manufacturers such as Pratt & Whitney, Rolls-Royce and General Electric, and with aerospace material vendors such as ATI Allvac and Carpenter Technologies.
In both cases, the group works to determine exactly how new materials will perform - and how long they will last - when exposed to increasingly high temperatures. Right now, the group is focusing on a select group of high temperature alloys and ceramic matrix composites. Not surprisingly, much of this work involves a wide array of precision mechanical testing.
“We do anything from tensile, creep and fracture toughness testing to more specialised tests, such as fretting fatigue, thermomechanical fatigue, fatigue crack growth and creep crack growth rate,” says Dr Rick Kearsey, a leading figure in high temperature materials testing who manages the group's High-Temperature Fatigue and Fracture Mechanics Facility. “We are developing the materials, testing them and then developing standard test methods for characterising these materials, which includes figuring out the geometry of the specimen as well as duplicating the service environment of the engine in the lab.”
Fittingly, the group's biggest challenge when it comes to testing is heat. In previous decades, temperatures for testing stainless steel, titanium and aluminium alloys were relatively moderate. Testing today's most advanced materials, however, requires temperatures as much as 25% hotter.
“The temperatures are so high that testing the materials becomes a more complex problem,” states Dr Kearsey. “How do you measure materials at 1,000°C when the standard practice relies on equipment rated for 800°C? How do you add proper instrumentation to measure crack growth rate? It's not impossible, but it's very complicated. These are issues many labs don't have to deal with, and it's a big reason why so few labs are devoted to high temperature materials testing.”
These tests also can be time-consuming and costly to perform, a fact that drives much of the group's contract work. Although the larger aerospace manufacturers have in-house high temperature material testing capabilities, they augment their abilities by leveraging NRC Canada's high temperature testing expertise to expedite test design and make sure tests are optimised to deliver valid, actionable data.
“If we do a specific test for a customer, we eventually transfer the knowledge to them,” explains Dr Kearsey. “For example, we may help them figure out how to do a thermomechanical fatigue test on a specific material. We help them get the test established in their own lab, so they can do it themselves.”
For manufacturers who collaborate with the group to perform these complex and demanding tests, the range of options available is impressive. Today, the Materials and Component Technologies Group deploys 18 systems for high temperature testing, all of which come from MTS Systems Corporation, based in Eden Prairie, Minnesota.
“We expect extreme stability from these systems,” notes Dr Kearsey. “When we have four or five frames running side by side, the lab is shaking. Despite this, we can't have any twisting or torsion that might misalign the specimen. We also have to make absolutely sure that the temperatures and loads are 100% traceable, because our results are being used to make critical engine components.”
Another challenge the group faces is the complex configurations required for the full range of high temperature tests. This typically involves combining and recombining test systems with accessories - including grips and fixtures, extensometers and strain gauges, furnaces and induction heating units - in a dizzying array of configurations.
“The hardware and accessories need to work properly at different test temperatures and conditions,” Dr Kearsey adds. “We also need to be able to take a furnace and stick it on any load frame. Interchangeability is important, especially for repeatability. When we have to do 100 tests on a similar geometry, we don't want to spend time redoing the calibration for every single one.”
Nothing, however, is more important than the reliability of the test hardware and software. Reliability is essential for the certainty of the results, the productivity of the lab and the satisfaction of the aerospace manufacturers who are his external clients.
“It doesn't matter if we're testing an advanced single-crystal alloy or evaluating 50 year-old alloys for life prediction modelling,” he says. “We have to generate the data required to make confident decisions.”
Dr Kearsey reveals that reliability in high temperature materials testing is a holistic challenge, one that applies to the smallest extensometer as well as the software acquiring the test data. Reliability of software becomes especially important during tests that take a long time to complete.
“When you're doing thermomechanical fatigue testing the tests can last three months,” he says. “It's very expensive for aerospace manufacturers to fund these studies. You never want to tell them the software crashed during the test and their results are invalid. As the facility manager, I'm the one who has to explain why it crashed. The software we use has to be extremely dependable.”
Even now, as Dr Kearsey works to overcome the challenges associated with testing at 1,000°C, he expects the upper bound to rise. In fact, he is already considering how to perform these remarkably complex tests at much hotter temperatures.
“As the newer alloys are replaced with ceramic matrix composites, we will need to run tests up to 1,300°C or 1,400°C,” he concludes. “We are already discussing how to make this happen. What size furnace will we need? What will the hot zone be? What extensometry will we need? It's an exciting time for our lab - and for the entire aerospace industry.”