NDT in Aerospace Industry

Non destructive testing in the aerospace industry

The aersopace industry is leading the way in the application of non-destructive testing techniques

The simplest way to find out about a component’s structural or material properties is to quite literally push it to breaking point.
But while destructive testing can be an effective and economical solution for high-volume, low-cost components, it’s clearly undesirable for larger, more expensive systems. If you want to test the limits of a multi-million-pound jet engine, destroying it is a pretty drastic way to advance your knowledge.
Fortunately, there is an alternative. And a range of so-called non-destructive testing (NDT) techniques – which can be used to probe structures and materials either before they enter use or as part of a maintenance programme – are now widely used across a range of engineering sectors.
Such techniques are particularly useful for monitoring and testing the kind of high-value, safety-critical components used in the aerospace industry. Indeed, according to Peter Milligan, compliance manager with the British Institute of Non Destructive Testing (BINDT), NDT is now compulsory for many aerospace firms.’Most, if not all, aerospace manufactures have to have some form of NDT carried out on their products [in particular, rotating parts] as it is a safety-critical part of the production process,’ he told The Engineer. ’NDT is a vital part of the production process from a quality-assurance point of view, as it gives confidence that the parts being tested won’t have defects in them that could cause problems in future life cycles.’
The variety of techniques available can be roughly broken down into two areas: surface techniques, which are used to identify surface defects such as cracks and surface porosity, and sub-surface techniques – such as ultrasonic testing or radiography – that can be used to detect defects that lie under the material’s surface.
One of the most widely used NDT methods is penetrant testing, a technique in which a visible dye solution is used to reveal surface defects.
During this process, test objects are coated with a dye solution, excess dye is removed from the surface and any dye that has penetrated cracks in the surface is then revealed under ultraviolet light. GKN, which specialises in the development of composite components for the aerospace industry, is a major UK user of this technique.
Meanwhile, the most commonly used sub-surface inspection technique is perhaps ultrasonic inspection – a method that uses beams of high-frequency sound waves to detect sub-surface flaws. The sound waves travel through the material and are reflected by cracks or flaws. The reflected beam is then analysed to define the presence and location of flaws or discontinuities.
Ultrasonic testing is used in the aerospace industry to locate voids, cracks and laminations, as well as inspect welds and carry out thickness measurements. One major UK supplier of ultrasonic inspection equipment is Midas NDT, which supplies automated systems for companies including Rolls-Royce and GKN.
Another popular sub-surface technique, which like ultrasonics has its origins in the medical field, is radiography – the use of X-rays or gamma radiation to spot imperfections. Using this technique, radiation is directed through a part and the resulting image indicates imperfections in much the same manner as an X-ray shows broken bones. Used across a range of engineering sectors, industrial radiography is particularly useful for testing and grading welds.
But perhaps one of the most versatile NDT techniques is eddy current testing, which uses induced electrical currents to detect defects. Essentially, the technique uses an alternating current in a test coil to induce an alternating magnetic field in the component to be tested. This causes eddy currents to flow in the components – the flow of which is influenced by the presence of flaws or defects.
Though, in general, the technique is used to inspect relatively small areas and is therefore better suited for inspecting areas where damage is already suspected, it nevertheless has a variety of applications, from measuring material thickness to detecting corrosion damage. As eddy currents are affected by the electrical conductivity of materials, they can also be used to sort materials and determine, for instance, whether a structure has been exposed to high temperatures. The technique is used to make corrosion measurements on aircraft skins and in the walls of tubing for assemblies such as heat exchangers. Eddy current testing is also used to measure the thickness of paints and coatings.
Magnetic particle testing (MT) – another key tool in the non-destructive tester’s armoury – is specifically used for detecting flaws in ferromagnetic materials. During MT, a magnetic field is applied to the specimen and the behaviour of fine magnetic particles applied to the surface of the specimen is used to monitor the magnetic flux. If the material is damaged, flux ’leaks’ from the specimen’s surface, close to the flaw, attracting the magnetic particles to the area.
MT, like ultrasonics, radiography and eddy currents, is a fairly well-established technique. However, a range of other methods promise to provide the world of NDT with an even wider pallet of tools.
For instance, one method showing increasing promise in the aerospace industry is pulsed thermography, in which infrared cameras can be used to detect sub-surface damage. According to Dr Nick McCormick, a materials specialist at the UK’s National Physical Laboratory (NPL), the technique holds particular promise for detecting flaws in sections of composite materials. McCormick explained that a group at NPL has been investigating the use of thermography to probe barely visible impact damage (BVID) – small surface scars that could point to problems deeper beneath a material’s surface. ’You can have something that looks like a dimple on the surface but has actually delaminated some of the layers beneath and has reduced the strength markedly,’ he said.
However, by firing a rapid pulse of heat at the surface of the composite material and using an infrared camera to measure the temperature change over time, it’s theoretically possible to gauge the sub-surface state of the material. ’If there’s a delamination below an area, there’s an air gap,’ explained McCormick. ’If there’s an air gap it’s more insulating and the temperature won’t drop as quickly. You can use that to measure what might be going on below the surface.’
Elsewhere, Peter Milligan’s team at BINDT is working to create a certification scheme for an emerging technique called guided-wave testing – a form of long-range ultrasonic inspection.
Mostly used on long stretches of pipes that could be used to carry liquids or gases, the main purpose for this method is to detect internal defects such as corrosion and liner defects. A row of transducers is wrapped around the pipe, inducing a range of ultrasonic beams into the pipe and causing it to twist, which, in turn, allows the beams to travel the full circumference of the internal wall of the pipe. If there are any defects within the pipe, the ultrasonic wave will strike the defect and send a reflection back to the transducer ring, allowing engineers to calculate the location of the defect. This method also allows tests to be carried in areas that are often inaccessible.
Meanwhile, a non-destructive testing technique based on resonant frequency response is showing great promise in the aerospace industry.
Originally commercialised by US firm Vibrant, but refined for use in the aerospace industry through a partnership at Sheffield’s Advanced Manufacturing Research Centre (AMRC), process compensated resonance testing (PCRT) is said to detect hidden flaws more effectively than other NDT processes.
Talking to The Engineer’s sister title, MWP, Lem Hunter, chief executive officer for Vibrant, said that the technology works by subjecting a part to a range of resonant frequencies and recording its response. By comparing these results with known standard patterns, it’s possible to identify defective parts.
One of the most compelling uses of the technique is in turbine blade inspection. In the US, following Federal Aviation Administration (FAA) approval, the process has been adopted by Delta Airlines as a replacement for destructive sample-based tests. Delta is also using PCRT to probe aircraft wheels, fasteners and engine components.