Professor Steve Dixon, Royal Society Industry Fellowship
Unlocking the Potential of Eddy Current Arrays
Jet engines are extremely complex machines, constructed from hundreds of different components, made from metal alloys that are designed to withstand the harshest of environments. Based in the Department of Physics, Professor Steve Dixon and his team are developing new ways of detecting small cracks in the surface with devices called "eddy current sensors."
Like all manufactured components, the various metal components in a jet engine will degrade with use, and we need to ensure that we find the smallest defects at the earliest stage possible to ensure that the engine remains safe to operate throughout its life. We also need to carefully inspect the parts of a jet engine during manufacture. The same type of technology that can test them in service can also look for tiny surface defects whilst the part is being made.
Many components in a jet engine are constructed from metals such as Titanium alloys of nickel-based superalloys, which are used because of their outstanding mechanical strength and fatigue life, and their low thermal expansion and high corrosion resistance.
By flowing cooling air through some of these components, they are even used in environments where the surrounding temperature is actually above the melting point of the metal.
Many jet engine components are constructed from metals such as Titanium alloys. Titanium possesses outstanding mechanical strength, low thermal expansion and high corrosion resistance.
As engines become more fuel-efficient, manufacturers have to use new materials and designs in the engine. The eddy current technology will help ensure the continued safe operation of new more powerful and efficient engines.
How do we test jet engine parts for defects?
There are various methods for inspecting jet engine components like turbine blades, but with varying degrees of accuracy.
Professor Steve Dixon and his team are developing a method called "eddy current testing" to detect and size surface cracks in the metal that are less than one-quarter of a millimetre long and maybe one-tenth of a millimetre deep.
These surface cracks are often very narrow, making them extremely difficult to see by eye. When we pass alternating current through a coil of wire that is close to the surface of a metal, it will induce an eddy current in a very thin surface layer of the sample. As we scan this coil across the surface of a metal, the path of the eddy current in the sample will change slightly if there is a small surface crack present, going under or around the crack.
The eddy current in the sample produces a magnetic field outside the sample, and this can be picked up by another coil. A defect will produce a change in the signal in the detection coil, with larger and deeper cracks producing bigger changes, enabling us to detect a defect.
So what do we do that is different?
One common feature of the alloys used in a jet engine is that they tend to have relatively low electrical conductivities compared with more familiar metals such as steel or aluminium.
Low electrical conductivity makes the eddy current travel to a greater depth in the metal surface, meaning that small surface cracks have less of an influence on the eddy current and are more difficult to detect.
If we increase the frequency of the current that we drive through the coil, we can concentrate more of the eddy current in a thinner surface layer, making it more sensitive to small surface defects. To detect defects as shallow as 0.1 mm (around the thickness of a human hair), we need to generate currents and detect voltages in small coils at frequencies of up to 40,000,000 Hz – around a million times faster than the alternating current in the electrical mains of your house.
To achieve these high frequencies, the team has built new designs of tiny, but complex electrical circuits. These are situated close to the coils used to generate and detect the eddy current. Doing this increases sensitivity, meaning that very small surface defects can be detected. It can enable the detection of much smaller defects than ever before.
As jet engines become more fuel-efficient and last longer, manufacturers have to use new materials and designs in the engine, and the new eddy current technology will help to ensure continued safe operation of new designs of more powerful and efficient jet engines.