Laser induced phased arrays for volumetric imaging

Student thesis: Doctoral Thesis

Abstract

Ultrasonic phased array imaging is an effective tool to detect and characterise defects for NDE relevant applications. In order to achieve ultrasonic focusing in all dimensions for volumetric imaging, two dimensional arrays are required. However, in various applications, currently available transducer-based phased arrays are challenging to apply, such as in extreme environments, in places of restricted access or on objects with complex geometries.Laser ultrasound is an alternative transduction method that utilises lasers for generation and detection of ultrasound. It is able to address various current challenges of NDE, as it is remote, non-contact, couplant-free, has a small footprint and can adapt to complex shapes. In comparison to transducers, the hardware for laser ultrasonic sources and detectors are significantly more costly per element. In consequence, laser ultrasound arrays with multiple lasers for generation and detection is not a viable option. Laser Induced Phased Arrays (LIPAs) overcome this challenge by scanning a single generation and a single detection laser over the desired array aperture, independently of each other, in order to capture the signal from each generation and detection element combination.This data acquisition method is called the Full Matrix Capture. Images are formed in post-processing by synthetically focusing on each pixel of the image, using a delay-and-sum algorithm called the Total Focusing Method. LIPAs require long acquisition time, due to the mechanical scanning of lasers, and the inability to perform parallel data acquisition. These characteristics of LIPAs, coupled with the fact that two dimensional (2D) arrays require significantly higher number of array elements have been limiting their application for three dimensional 3D imaging towards remote volumetric evaluation of defects.In this Thesis, 2D laser induced phased arrays are explored for 3D imaging. Sparse array designs, that can perform imaging without producing high amplitude grating lobes are investigated towards reducing the number of LIPA elements without introducing imaging artefacts. For this purpose, two approaches are implemented: aperiodic designs and designs with decoupled generation and detection layouts. By utilising these sparse array designs, the imaging efficiency of LIPAs is increased, leading to faster acquisition.A novel adaptive data acquisition method is proposed that is able to adjust array parameters, towards optimising the array designs for the need of inspection. This method utilises a selective array building strategy that maximises generation and detection efficiency. The ultrasonic image quality achieved by this method is compared to that of the Full Matrix Capture with respect to signal-to-noise ratio and defect characterisation ability. It is shown that images of comparable signal-to-noise ratio or characterisation ability are achieved with a 10 times faster data acquisition time, utilising the adaptive acquisition method. Finally an infrastructure is developed to synthesise the first 2D Laser Induced Phased Array. This system is utilised to remotely inspect a Wire Arc Additively Manufacture component in 3D. Image combination is performed to provide high quality 3D images of the interior of the test component. High accuracy in defect sizing and defect locating abilities is demonstrated. Furthermore, the improvements in array design and data acquisition method are adapted to 2D LIPAs in order to increase the data throughput and reduce the subsequent acquisition time, towards improving the industrial adaptability of 2D LIPAs.
Date of Award24 Jan 2023
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsEPSRC (Engineering and Physical Sciences Research Council) & University of Strathclyde
SupervisorTheodosia Stratoudaki (Supervisor) & Anthony Gachagan (Supervisor)

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