EMATs for non-contact NDE of austenitic steel

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Power generation and petrochemical plant and civil structures require regular inspection and monitoring to ensure continued safe and reliable operation. The various ways that metal structures are routinely tested includes visual inspection, electromagnetic and radiographic methods and ultrasonic inspection, each technique having its own strengths and weaknesses, often being used in a complementary approach. Of all these methods, ultrasonic inspection is most prolific as it is inherently safe, portable and can be used to detect a wide range of defects down to sub-millimetre sizes. In recent years there has been significant and sustained progress in the fundamental scientific research of guided wave non-destructive evaluation (NDE). The majority of existing guided wave technology uses contacting transducers that must be clamped around the circumference of a pipe in the form of a ring of transducers. Typically a particular mode at a particular frequency is selected with suitable properties for being able to propagate over tens of metres, whilst having sensitivity to defects of interest. Target defect sizes are usually around 25% wall loss or more, which is perfectly acceptable for many applications. Guided waves can be used over shorter distances, and in general there is a trade-off between propagation distance and sensitivity.There is a need to maintain the current power generation plant, particularly within the nuclear industry and with an increase in our reliance on nuclear power anticipated, we need to ensure that we have suitable methods for inspecting critical components. As such, this project focuses on the ultrasonic inspection of stainless steel using ultrasonic transducers called EMATs that can generate or detect ultrasonic waves in metals without being in good mechanical contact with the sample. The advantages of using non-contact methods are that the automation of scanning is easier to implement as contact is not required and the EMATs have a unique set of characteristics that enable them to generate a wide range of wavemodes over a wide range of frequencies, unlike contacting piezoelectric transducers that are usually used at a particular fixed frequency. Note though that EMAT inspection does have some limitations, most principally because they are fairly inefficient when compared to piezoelectric transducers, and so the methods developed in this project are designed to complement the existing technology, providing new inspection capability through fundamental research of the transduction process and the wave propagation in the target sample.To realise fully the potential of EMAT based inspection we need to be able to model the problem scientifically from the bottom up, starting with the shape of the component and the target defect. Target components may be pipes, but will often be components with more complex geometries or problematic material properties as is often the case with stainless steel welds. Modelling how ultrasound propagates through such components can now be reasonably tackled on a high specification desktop PC using methods such as finite element (FE) analysis. Computation time obviously depends on the complexity and size of the model, and the range of frequencies being modelled, but typically one would expect models to take several hours to run on representative components. This needs to be complemented by modelling the behaviour of the transducers, again using FE modelling of the electromagnetic behaviour of sample and transducer. In some cases it is appropriate to combine these FE models with analytical models to improve computation time. Rather than simply providing solutions to a limited number of inspection issues, we will develop a scientific methodology for designing techniques to inspect components of any geometry, equipping both researchers and industrial users with an approach for making the right tool for a specific job rather than providing a limited range of tools.