Super-resolution 3D ultrasound tomography for material microstructure characterisation

超声断层扫描是一种强大的非侵入性工具，广泛应用于医学诊断和无损检测。传统的层析成像方法代表了一种基于模型的介质声速重建，它为每个体素分配一个代表局部材料属性的声速。例如，这种类型的信号处理可以使用弹性导波测量来绘制管道和板状结构的壁厚图，或者用于头骨内的人脑成像。然而，超声断层扫描在多光谱、角度相关和区域特定成像方面具有巨大的未开发潜力。非破坏性体积材料的表征是一个重要的问题，在许多领域有着广泛的应用。例如，材料微观结构参数的知识对于准确估计航空航天（喷气发动机和起落架）或能源部门（核电站）的安全关键部件的寿命至关重要。可以使用几种已建立的表征方法，如光学显微镜、x射线、电子背散射衍射（EBSD），对金属成分进行详细的微观结构检查。然而，所有这些方法都局限于表面或近表面检测。此外，一些技术本质上是破坏性的（EBSD），需要复杂的表面处理，并且仅限于相对较小的检测区域。因此，显然需要能够在大样本量中量化材料微观结构的其他方法。同样重要的是，与光学显微镜和EBSD等传统方法相比，这些技术显著减少了时间和表面质量要求。该项目是由使用超声波阵列的缺陷表征的最新进展推动的，这是基于对小亚波长目标的后向散射模式的分析。主要目的是从根本上扩展超声层析成像功能，以创建一种有效的工具来生成对应于每个局部材料区域的金属微观结构参数的定量图。这需要一个完全不同的创新成像概念。测量装置将在机械臂扫描系统中实施，该系统将允许对大范围的入射和散射角度进行超声波发送-接收测量。成像方法利用声波传播的时间反转特性，并基于可逆的正向和反向传播成像操作。

Ultrasonic tomography is a powerful non-invasive tool widely use in medical diagnostics as well as non-destructive testing. Conventional tomographic methods represent a model-based sound velocity reconstruction of a medium, which assigns every voxel a sound velocity representing local material properties. This type of signal processing can be performed, for example, using elastic guided wave measurements for a wall thickness mapping in pipes and plate-like structures, or for the human brain imaging inside the skull. Ultrasonic tomography, however, has substantial untapped potential for multispectral, aspect-dependent, and area-specific imaging. Non-destructive volumetric material characterisation is an important problem with a wide range of applications in many areas. For example, the knowledge of material microstructural parameters is crucial for accurately estimating the lifetime of safety critical components in aerospace (jet engines and landing gears) or energy sectors (nuclear power plants). Detailed microstructural examination of metallic components can be performed using several established characterisation methods, such as optical microscopy, X-ray, Electron Back-Scattered diffraction (EBSD). However, all these methods are restricted to surface or near surface inspections. Moreover, some techniques are essentially destructive (EBSD), require complex surface preparation and are limited to relatively small inspection regions. Therefore, there is a clear need for additional methods capable of quantifying material microstructure in large sample volumes. It is also critical that such techniques significantly reduce both the time and surface quality requirements compared to conventional methods such as optical microscopy and EBSD. This project is driven by the recent advances in defect characterisation using ultrasonic arrays, which are based on the analysis of backscatter patterns of small subwavelength targets. The main aim is to expand ultrasonic tomography functionality fundamentally to create an efficient tool to produce a quantitative map of microstructural parameters of metals corresponding to each local material region. This requires a fundamentally different and innovative imaging concept. The measurement set-up will be implemented in a robotic arm scanning system, which will allow to perform ultrasonic transmitter-receiver measurements for a wide range of incident and scattered angles. The imaging approach exploits time-reversal properties of sound wave propagation and is based on reversible forward- and back-propagation imaging operations.