Academic Insight: Ultrasonic arrays improve non-destructive testing of engineered structures

Professor Bruce Drinkwater, Department of Mechanical Engineering, University of Bristol

Non-destructive testing (NDT) is the unsung hero of engineering. Aircraft would not fly, and power stations would not produce power, without being continually tested and found to be safe. Modern society demands such testing for these and other engineering structures. The process involves finding tiny defects in very large and complex structures. As engineering structures advance, so must NDT research. 

Ultrasonic array technology was first introduced to engineering for NDT from the medical sector in the 1980s. Using this medical technology to form an image inside the very fabric of an engineering structure was a significant step forwards for plant and machine safety. Before this, NDT measurements were made at a single point, and operators viewed the results on an oscilloscope screen. 

However, engineering objects are very different from the human body, and these early array systems were limited in both image resolution and the size of defects that could be seen.

This limitation recently became an urgent issue for one of Canada’s largest electricity providers, Ontario Power Generation, which needed to map wall thickness around welds in several hundred feedwater pipes in its Canada Deuterium Uranium reactors. These components had complex 3D geometries. With traditional NDT technology, the inspection process was becoming time-consuming and costly, and required high levels of human intervention.

The sector needed more advanced ultrasonic technology that could improve image quality. It also needed techniques that could adapt to the diversity of materials now present in structures. But at the heart of this challenge is the drive to find smaller and smaller defects. We believed that ultrasonic arrays would drive these improvements forward.

Our work in Bristol focused on a single advance in array technology: full matrix capture (FMC). This simplified the process for firing array elements, thereby allowing superior imaging and the potential to permanently store the ultrasonic data. Furthermore, the imaging technology used by FMC, total focusing method (TFM), can now dramatically correct distortions due to anisotropy and transmission across material boundaries, simplifying the interpretation of the data – a technique we call ‘array autofocus’.

Ontario Power Generation can now map the thickness of its feedwater pipes efficiently and accurately. Inspections are quicker, and the company collects and evaluates data regularly to build a true picture of any deterioration. 

Rolls-Royce Aerospace has used FMC/TFM to decrease the number of blade failures in its aircraft engines. Before using arrays for its inspections, the company suffered about three engine failures a year, each costing £5 million. Since introducing the technology, there have been no such failures.

One of our next projects at the University of Bristol will be to improve defect characterisation by compiling a huge, searchable database of ultrasonic signals from possible defects for good matches. The ultimate aim is to move away from traditional images and to a world where computer algorithms interpret the data and report back the nature and extent of any problems.