Super-resolution 3D ultrasound tomography for material microstructure characterisation

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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.