Virtual source aperture ultrasonic imaging for non destructive testing

Hoyle, Ewan Charles (2020) Virtual source aperture ultrasonic imaging for non destructive testing. Doctoral thesis, University of Wales Trinity Saint David.

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Ultrasonic testing is of huge importance when it comes to assessing the integrity of a huge variety of structures and components. Virtual Source Aperture (VSA) is an new advanced method of testing and is considered to be a hybrid between traditional phased array (PA) techniques and full matrix capture (FMC). VSA overcomes the shortcoming of PA by using the Delay and Sum (DAS) algorithm to synthesise a PA focussing point at every single point in the image. This allows for easier data interpretation than PA and also allows for several extra views, not applicable to PA such as the self tandem and multimode views. VSA overcomes the slow imaging rate of FMC-TFM imaging by using phased array like delay laws on transmission to generate a divergent wavefront that behaves as if it had originated from a singular point behind the aperture. Each element recieves individually as with FMC. However the higher signal to noise ratios at the A-scan level obtained by the virtual source allows a comparable image to FMC to be generated in fewer frings. Therefore an increased acquisition speed can be achieved. Therefore, VSA is of signifcant interest in non destructive testing (NDT). This work takes forward four main areas of investigation in ultrasound NDT: • Evaluation of the strengths and shortcomings of FMC, Plane Wave Imaging (PWI) and VSA in a comparative study. • Investigation of the VSA frequency domain algorithm and establish whether it is a viable alternative solution. • Development of VSA ‘multi-mode‘ and self-tandem algorithms to improve the probability of defect detection by use of the different propagation modes. 2 • Creation of a surface mapping algorithm for VSA inspections of components with nonplanar surfaces (complex geometry). The FMC algorithm provides the highest quality images overall. However, for a relatively small loss (a few dB) in image quality, much higher frame rates can be achieved using either VSA or PWI. VSA offered slightly higher SNR than PWI. For a typical setup, FMC achieved 18fps while VSA and PWI could deliver up to 250fps. The PWI performed better than VSA for the Array Performance Indicator Measurements (API), indicating that in general the PWI transmissions were more tightly focussed. However the measured values for SNR and API varied massively across the imaging region for PWI, correlating to how many transmissions had covered each particular point in space. Where wave fronts overlapped, the images were higher intensity and more tightly focussed. To avoid this problem the number of transmissions must be higher to offer a more uniform distribution of energy. The energy values for VSA were overall more uniform, and resembled those of FMC. Therefore, for high speed inspections, VSA is likely to be the preferred option if a wider angle range must be covered. The VSA frequency domain algorithm gave a narrower range of angular coverage compared to the equivalent time domain algorithm. Time domain was easier to implement through a refractive boundary, a common issue in NDT. Accordingly, the time domain VSA algorithm was used in the main testing. The VSA self tandem and multimode algorithms beneft from the ability to reprocess the data acquired to exploit a wider range of sound paths, including mode conversions, for imaging. The probability of detection and classifcation for certain defect categories can be improved, including vertically aligned faws. Examples were developed illustrating 3 ability of the self-tandem and multimode algorithms to image vertically aligned faws using high amplitude specular responses rather than the low amplitude tip diffracted signals used by the half skip and full-skip algorithms. A VSA surface mapping method was created, the method being compared to the dipping refector and plane wave surface mapping algorithms. For less complex surface geometry the algorithm was comparable to the established algorithms. The VSA data can be used to extract surface information such that the surface mapping and imaging algorithm can be performed with a single transmission sequence.

Item Type: Thesis (Doctoral)
Uncontrolled Keywords: ultrasonic testing aperture
Subjects: T Technology > TA Engineering (General). Civil engineering (General)
Divisions: Theses and Dissertations > Doctoral Theses
Depositing User: Sandra Stedman
Date Deposited: 17 Feb 2021 11:11
Last Modified: 17 Feb 2021 11:12

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