EMATs for non-contact NDE of austenitic steel

发电厂、石化厂和土木结构需要定期检查和监测，以确保持续安全可靠的运行。金属结构的各种常规测试方法包括目视检查，电磁和射线照相方法以及超声波检查，每种技术都有自己的优点和缺点，通常用于补充方法。在所有这些方法中，超声波检测是最多产的，因为它本质上是安全的，便携式的，可以用来检测各种各样的缺陷，小到亚毫米的尺寸。近年来，导波无损评价的基础科学研究取得了长足的进步。大多数现有导波技术使用接触换能器，其必须以换能器环的形式围绕管道的圆周夹紧。典型地，选择具有合适特性的特定频率下的特定模式，以便能够传播数十米，同时对感兴趣的缺陷具有灵敏度。目标缺陷尺寸通常为约25%壁损失或更多，这对于许多应用来说是完全可接受的。导波可以在较短的距离内使用，通常在传播距离和灵敏度之间存在权衡。有必要维护现有的发电厂，特别是在核工业中，随着我们对核电的依赖预计会增加，我们需要确保我们有合适的方法来检查关键部件。因此，本项目的重点是使用称为EMAT的超声波换能器对不锈钢进行超声波检测，该换能器可以在不与样品良好机械接触的情况下产生或检测金属中的超声波。使用非接触式方法的优点在于，扫描的自动化更容易实现，因为不需要接触，并且EMAT具有一组独特的特性，这些特性使它们能够在宽范围的频率上生成宽范围的波模，这与通常在特定固定频率下使用的接触式压电换能器不同。请注意，尽管EMAT检测确实有一些局限性，最主要的是因为与压电换能器相比，它们效率相当低，因此本项目中开发的方法旨在补充现有技术，通过对目标样品中的传导过程和波传播的基础研究，提供新的检测能力。为了充分实现EMAT检测的潜力，我们需要能够从零部件的形状和目标缺陷开始，自下而上对问题进行科学建模。目标部件可以是管道，但通常是具有更复杂几何形状或有问题的材料特性的部件，如不锈钢焊缝的情况。现在可以使用有限元（FE）分析等方法在高规格台式PC上合理地处理超声波如何通过这些组件传播的建模。计算时间显然取决于模型的复杂性和大小，以及建模的频率范围，但通常情况下，模型需要几个小时才能在代表性组件上运行。这需要通过对传感器的行为进行建模来补充，再次使用样品和传感器的电磁行为的FE建模。在某些情况下，联合收割机这些有限元模型与分析模型相结合，以缩短计算时间。我们将开发一种科学的方法来设计检测任何几何形状部件的技术，而不是简单地为有限数量的检测问题提供解决方案，为研究人员和工业用户提供一种为特定工作制造正确工具的方法，而不是提供有限范围的工具。

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.