Acoustic metamaterials for ultrasound manipulation for non-destructive testing applications

Abstract

Non-destructive testing (NDT) is a collection of techniques for evaluating the health of a component without damaging it. Due to this characteristic, it can be widely applied for component manufacturing and the assessment of service life in industrial applications, such as weld joints inspection, and composite inspection. There exist several categories of NDT techniques including ultrasound testing (UT), x-ray testing and eddy current testing. One common configuration in UT is oblique incidence inspection and this is widely used for weld inspection. To achieve an angled incident beam, solid/liquid wedges are used to couple between the transducer and the flat surface. However, these wedges take up space, make the experimental system bulky and result in complex wave propagation in the wedges themselves. This thesis aims to replace such coupling wedges with a low-cost metamaterial structure with an aim of simplifying the experimental setup, enhancing coupling efficiency and reducing manufacturing costs. The metamaterial devices will also enable other acoustic fields, such as focused beams, to be created, making it suitable for a range of defect inspection applications. This thesis starts by exploring the feasibility of ultrasound manipulation in air using an acoustic metamaterial lens composed of cross-shaped unit cells for beam focusing. First, it examines the effective material properties of the cross-shaped unit cells using both the retrieval method and the band structure method, comparing the results from these two approaches. Then, the desired refractive index (or speed of sound) distribution of the lens is derived based on a time delay law, and its performance is validated through finite element analysis. A prototype of the metamaterial lens, targeting 40 kHz, was fabricated using three-dimensional printing, and its acoustic field was experimentally confirmed revealing that good focusing performance can be achieved. The impact of the inevitable manufacturing errors on the performance of the metamaterial lens is then analyzed to select the most appropriate manufacturing method and unit cell structure. Various types of unit cells, such as steel cross-shaped, resin circular void, and resin-silicone layered designs, are considered for lens construction. By accounting for manufacturing errors, the performance of the unit cells is evaluated, and the time delay distribution of the lens is determined using the accumulated delay method. A Huygens’ principle model is then used to calculate the acoustic pressure field, incorporating these manufacturing errors, and the lens performance is assessed. Finally, a set of criteria for choosing the optimal manufacturing method and unit cell structure is proposed. Optimal here means that the metamaterials are relatively insensitive to manufacturing errors leading to a robust device. Following on from the optimal metamaterial work, metamaterial plates are constructed using silicone-resin layered unit cells targeting MHz operation. This frequency is chosen for its relevance for applications in defect inspection in the field of NDT. Here both frequency and time domain methods are used to design the lens, allowing for a comparison of these design methods. Subsequently, thin metamaterial plates with steering and focusing capabilities are designed by using the selected time-domain method and manufactured. The effectiveness of these plates for defect inspection is experimentally validated through tests on hole-type and slot-type defects in an aluminium sample. Overall, this thesis takes a critical step forward towards the concept of metamaterial-based devices for NDT. It does this by first exploring the underlying physics of the wave phenomena involved, and then by practical realization. In these ways the work here paving the way for the broader application of metamaterials in ultrasonic NDT.

Date of Award	30 Sept 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Jie Zhang (Supervisor) & Bruce W Drinkwater (Supervisor)