Abstract
In recent times, a number of studies on Structural Health Monitoring (SHM) and ultrasonic-based Non-destructive Testing (NDT) have been focused on damage detection in materials used for aerospace, railways, nuclear safety and automotive fields, such as stainless steel, aluminium and carbon fibre–reinforced polymer (CFRP) composite.Most of these techniques are based on contact inspections, with coupling medium required to minimise wave attenuation, thus limiting size of the material being inspected. Moreover, smaller-scale damages such as stress corrosion cracks, or barely visible impact damages (BVID) are not ever appreciable with the common ultrasonic-based techniques, and could irreparably affect the integrity of a structure, leading to catastrophic failure and consequences.
Air-Coupled inspection methods have been employed to reduce contact issues in noncontact ultrasonic inspections, thus resulting in poor transmission of ultrasound in air because of the impedance mismatch between solid and air.
Nonlinear ultrasonic approaches have been recently developed to detect small-scale damages, by studying the generation of further harmonics due to damage, which generally occur at multiples of the fundamental frequency. However, amplitude of higher harmonics are typically three orders of magnitude lower than the fundamental frequency amplitude, therefore expensive high-sensitivity transducers and/or complex active filtering systems are required.
Several nonlinear noncontact acousto/ultrasonic approaches are proposed in this thesis to improve detection of micro-damage and damage imaging resolution in nonlinear noncontact inspections. Transmission tests have been performed to validate the analytical/numerical models and assess the effectiveness of the proposed approaches. Experimental investigations show that the developed structures are capable to achieve amplitude amplification and nonlinear filtering of the designed frequencies. Moreover, the novel acoustic prototypes have been employed to perform noncontact investigations on corroded and impacted components, with the devices being moved across the samples by means of a X-Y gantry systems to obtain a 2-D damage mapping. Results have been demonstrated high potential of the proposed methods for improving micro-damage detection and imaging resolution in nonlinear noncontact inspections.
| Date of Award | 29 Mar 2023 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Michele Meo (Supervisor) & Fulvio Pinto (Supervisor) |