Svjetlana Stekovic, research coordinator at LiU’s Grants Office, and researcher at the Department of Management and Engineering, is coordinating two projects aimed at improving materials used in aircraft. Materials in aircraft turbine engines are affected by heat and mechanical wear – in particular the rotating turbine discs, which must withstand huge variations in temperature and mechanical stress. At the same time, their strength is crucial to engine safety. Also, the turbine disc is the part that most affects the total efficiency of the engine.
Svjelana Stekovic investigates materials for aircrafts in different ways. Photo credit: Susanna Lönnqvist“The turbine disc rotates very quickly and is subject to extreme loads and temperatures. The disc is thicker at the middle than at the outer edges, and the variations in material thickness mean that different parts of the disc heat up and cool down at different rates. This causes tension in the middle of the disc during the engine’s start-up and shutdown. So the temperatures at the disc’s centre and its edges are different, which causes strain in the material the disc is made of”, explains Svjetlana Stekovic.
The work with the turbine engines has focussed on increasing their temperature resistance and making the blades thinner, in order to achieve lighter engines and aircraft. So how do we know whether the material used in the turbine blades will withstand the stresses and the differences?
The project DevTMF, where TMF stands for thermo-mechanical fatigue, aims to develop methods and models for predicting how materials in turbine discs behave in various conditions. When evaluating a material, one studies different phases of how damage to the material occurs and develops. Today there is no standardised method for the evaluation of damage propagation during thermo-mechanical fatigue, that is, how cracks spread and grow in a material, and how propagation is affected by the material’s properties.
“In the project we’re trying to improve an experimental method for studying thermo-mechanical fatigue, and to develop a standardised way of evaluating thermo-mechanical crack propagation. And then to combine both with modelling and simulations”, says Svjetlana Stekovic.
Ultimately, the work is about being able to predict more accurately the product life of turbine discs. This would enable a longer operating time and product life of turbine components, which in turn would lead to fewer service hours for aircraft, and reduced material waste of machine parts. Optimising the use of materials and engine parts is also part of increasing efficiency:
“Aviation benefits from the development of the most efficient engines possible, with as good materials as possible, since a more optimised engine uses less fuel, which means good environmental effects for EU’s aeronautics industry”, says Svjetlana Stekovic.
A test rig for thermomechanical fatigue. Photo credit: Svjetlana Stekovic.The project also includes developing new materials for Rolls-Royce’s turbine discs, which in this case withstand higher temperatures.
“Over the past 40 years, the development of materials for turbine parts has been impressive: their temperature capability has increased by 250 degrees during that period. Today we are almost at the maximum temperature for operation, but if the engines can manage a small increase, just 25 to 40 degrees more than today, the engines would be more efficient and use up to a half to one per cent less fuel.”
That might seem like a small decrease, but in the long run it can help cut greenhouse gas emissions.
“On their own, optimised turbine discs can’t lead to large reductions in carbon dioxide emissions, but combined with other research and innovative technology we can make a difference.”
3D-printed materials
Using new, optimised materials, it’s also possible to build new components. For aircraft, having every component as light as possible brings multiple benefits. A lighter aircraft uses less fuel and produces less emissions. Also, the complexity of airplane components can be optimised during production. In AddMan, the second project that Svjetlana Stekovic is coordinating, they are investigating the use of additive manufacturing, that is, 3-D printing technology. The project is developing a component for the door of an aircraft together with SAAB.“Complex aircraft parts require production in several steps that involves several suppliers, which leads to longer lead times and requires resources and coordination. We’re developing different additive manufacturing processes from metallic materials, which can be 3-D printed layer by layer, in order to create a door component that weighs less and is easier to produce”, says Svjetlana Stekovic.
With 3-D printing the entire production process is simpler than with traditional methods, and it generates less material waste. The challenge is that the component must have the same mechanical properties and be as damage-resistant as the conventionally produced parts that are currently used in aircraft.
“With 3-D printing, we can optimise and vary the geometry, shape and structure, as well as the material’s mechanical properties, by changing the material’s microstructure. At the moment the challenge is the surface of the material, because strength can be affected by surface irregularities, which can lead to cracking.”
Clean Sky
DevTMF and AddMan are funded through Clean Sky 2, which is an EU-financed programme for the improvement and development of various aircraft-related innovative technologies in Europe. Clean Sky 2 will run until 2024, and its aim is to meet the targets of Flightpath 2050, which is the European Commission’s vision for aviation and the aircraft industry in Europe. The aims are to reduce fuel consumption and accompanying carbon dioxide emissions by 75 per cent, and emissions of nitric oxide by 90 per cent, by 2050. Also, aircraft noise is to be cut by 65 per cent. These aims are based on typical values for new aircraft in the year 2000.
DevTMF
The DevTMF project develops experimental technologies and predictive tools for characterising thermomechanical fatigue behaviour and damage mechanisms.The researchers at LiU are Johan Moverare, Daniel Leidermark, Viktor Norman and Svjetlana Stekovic. The partners are Rolls-Royce, Swansea University and the University of Nottingham.
