Using ultrasound for the non-destructive testing (NDT) of components is a simple yet powerful technique. Ultrasound propagates into the inner structure or surface of the test component and its detection provides information about its properties, thickness and possible defects.
The devices traditionally used for ultrasonic generation and detection are the piezoelectric transducers, and although they have been used successfully in the industry for non-destructive testing, they have several limitations.
Piezoelectric testing is a technique that requires an ultrasonic couplant and the ability to be able to get within close proximity of the component being tested. It also uses wires, which makes it difficult to reach certain parts of the aircraft and engine. In addition, these transducers are of a size and weight that makes them unsuitable for permanent applications or for testing components of unusual geometry. Added to this, they are also expensive, with each costing several hundreds of dollars, which makes it impractical to use them extensively on aircraft.
A new breed
At the University of Nottingham in the UK, a team has been developing a new breed of ultrasonic transducers, the CHOTs (CHeap Optical Transducers), which overcome all of the limitations of the traditional technology.
CHOTs are structures attached to the surface of the test component that are optically excited using a simple laser set-up to either generate or detect ultrasound. The use of lasers allows for remote and wireless operation, and in addition, CHOTs are also very cheap (potentially less than 1p (US$0.015) each) so they can be used in great numbers. They are also extremely small with minimal impact on the inspected component, which allows them to be permanently attached; and robust, meaning they can survive in environments with extreme temperatures, high pressures and vibration.
Due to all of these advantages, CHOTs are ideal for low cost, reliable, in-situ and remote NDT, and the university team is currently developing a portable CHOTs system for endoscopic NDT in partnership with the aerospace industry.
How does it work?
The CHOT is a 2D structure that is deposited, etched, attached or in some way drawn onto the surface of the test component that is optically excited using lasers to either generate or detect ultrasound. There are two types of CHOT: one for ultrasound generation (g-CHOT) and one for ultrasound detection (d-CHOT), and each part can work independently of the other. However, each offers the user full control of the excited/detected wave mode (type of ultrasonic wave), its directivity, and the ultrasonic frequency content. This is achieved through the appropriate design of the CHOT structure.
Both types of CHOT are operated remotely by directing a laser onto the surface of a CHOT. For the generation of ultrasound a pulsed laser is required, while a CW laser is used to detect an ultrasound signal. Aligning these lasers onto their respective CHOT device is a simple task (‘point and shoot’) and, provided the CHOTs are illuminated by part of the laser beam, the ultrasound signal will be generated or detected as appropriate. Thus only simple, low-cost optics are required for this process.
The CHOTs themselves can be produced using a range of methods, for example contact printing, laser etching and photolithography, or attached as printed stickers and left in place. These methods of production mean that the CHOTs themselves are cheap to produce and can be regarded as disposable. This opens up a whole new range of potential markets for ultrasound testing that have yet to be explored.
CHOTs have the potential to result in a significant increase in aircraft and passenger safety, while contributing to substantial cost savings through a decrease in maintenance and operating times.
One of the great advantages of CHOTs over piezoelectric transducers is that they have the potential to be used for in-service, continuous monitoring. The small size and cost of CHOTs systems will mean that multiple CHOTs can be placed on components, enabling multipoint defect detection or scanning. The fact that CHOTs are so small also means that they have minimum impact when installed onto a test component. They can also cope with testing components of unusual shapes and can also produce a range of ultrasonic waves.
For example, it is entirely feasible for CHOTs to be used to test turbine blades while they are in service. This provides a good example of their benefits where other systems wouldn’t work. A CHOT system with a chosen frequency could be placed in the positions where failure of the blade is most likely and could constantly monitor the component throughout its lifetime. Such an application would be particularly appropriate in the example of a turbine blade, which operates in a difficult-to-reach, high-temperature environment, and here CHOT’s robustness comes to the fore and demonstrates its benefits over other transducers that cannot be placed.
The low cost and volume of the CHOTs system could also potentially see its deployment in future as part of an active monitoring system of the airframe. A large number of CHOTs could be placed on the airframe, enabling quick, regular, scheduled testing over a large area in a cost-effective way that would not be possible with other transducers. This could potentially provide an alternative to passive condition monitoring and help to identify local damage in the form of microcracks or delaminations, weakening of adhesive bonds, and thermal and chemical damage to the airframe, as well as overcoming some of the inherent data analysis challenges of condition and health monitoring.
These attributes mean that, as well as in-situ testing, CHOTs can also be used to improve quality and check for manufacturing defects on the production line, enabling aerospace manufacturers to quickly and cheaply assess the quality of components during machining and assembly processes. CHOTs are so versatile that they have the potential to be used in industries as diverse as food and drink or pharmaceuticals, where they could form part of the packaging; or the energy industry, where they could monitor offshore wind turbines.
Dr Theodosia Stratoudaki is a specialist in the department of electrical and electronic engineering at the University of Nottingham in the UK