Sub-wavelength characterisation of defects in inaccessible regions using guided waves

在越来越多的工业中，包括发电、石油化工和核部门，工厂的使用寿命超过了原来的设计寿命，因此需要确定原来预计不需要检查的区域的完整性。声波导波为远程检查提供了必要的范围（几十米），商业上可用的系统通常用于管道工程的快速筛查。申请人最近的研究已经导致用于管道检查的导波合成聚焦技术的发展，其已经显示出在对小缺陷的灵敏度方面给出了数量级的改进。然而，申请人还表明，图像中的定量信息仍然不具有足够的分辨率以使得能够将关键缺陷的尺寸确定为结构完整性评估所需的准确度。无法实现精确尺寸的原因是，与其他成像系统一样，可分辨的细节受到探测波波长的衍射限制。分辨率可以通过在更高的频率下操作来增加，但是这是以由于增加的衰减和散射而减小的范围为代价来实现的。在许多成像领域中，已经研究了所谓的超分辨率技术，其使得能够提取低于衍射极限的细节。虽然超分辨率的好处已经在某些应用中得到了证明，但它的开发是高度特定于具体情况的。这是因为，它必须根据探测波与可能遇到的特征的相互作用的先验知识进行定制。本项目的目的是开发使用亚波长（超分辨率）表征技术，用于使用导波表征安全关键管道中不可接近的缺陷。主要目标是确定裂纹或腐蚀斑的最大穿透深度，以及其方向或形状。在与工业合作者的讨论中，已经商定了以下目标规范，这些规范代表了必须满足的最小值，以便导波尺寸确定实际上是有用的。对于横向范围大于3 T且深度大于0.3T的缺陷，目标是实现+/-0.1T（T =管壁厚度）的深度分辨率。该项目将推进对真实形状缺陷的导波散射的理解，并将开发新的超分辨率技术，以使用换能器阵列收集的散射导波场来表征缺陷。因此，该项目涉及应用于高度相关的工业问题的基础科学，这使得它适合EPSRC资助。

In an increasing number of industries, including those in the power generation, petrochemical and nuclear sectors, plant is extended beyond its original design life and there is a need to ascertain the integrity of areas that were not originally anticipated to require inspection. Guided acoustic waves provide the necessary range (tens of metres) for remote inspection and commercially available systems are routinely used for rapid screening of pipework. Recent research by the applicants has lead to the development of guided wave synthetic focusing techniques for pipe inspection, which have been shown to give an order of magnitude improvement in the sensitivity to small defects. However, the applicants have also shown that the quantitative information in the images is still not of sufficient resolution to enable sizing of critical defects to the accuracy required for structural integrity assessment. The reason why accurate sizing cannot be achieved is because, like other imaging systems, the resolvable detail is diffraction-limited by the wavelength of the probing wave. Resolution can be increased by operating at higher frequency but this is achieved at the expense of reduced range due to the increased attenuation and scattering. In many imaging fields so-called super-resolution techniques have been investigated that enable detail below the diffraction limit to be extracted. Although the benefits of super-resolution have been demonstrated in certain applications, its exploitation is highly case-specific. This is because, it must be tailored according to a priori knowledge of the interaction of the probing wave with the features likely to be encountered.The purpose of this project is to develop the use of sub-wavelength (super-resolution) characterisation techniques for the characterisation of otherwise inaccessible defects in safety-critical pipes using guided waves. The principal goal will be to determine the maximum penetration depth of a crack or corrosion patch, and also its orientation or shape. In discussion with the industrial collaborators the following target specifications have been agreed that represent the minimum that must be satisfied in order for guided wave sizing to be practically useful. The target will be to achieve a depth resolution of +/-0.1T (T = pipe wall thickness) for defects that are larger than 3T in lateral extent and deeper than 0.3T. This project will advance the understanding of guided wave scattering from realistic-shaped defects and will develop new super-resolution techniques to enable defects to be characterised using the scattered guided wave field collected by a transducer array. The project therefore involves basic science applied to a highly relevant industrial problem, which makes it appropriate for EPSRC funding.