| Author: | He, Yi |
| Title: | Optoacoustics-based defect inspection and material characterization for multiscale structures |
| Advisors: | Su, Zhongqing (ME) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Ultrasonic testing Nondestructive testing Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Mechanical Engineering |
| Pages: | xl, 235 pages : color illustrations |
| Language: | English |
| Abstract: | In recognition of the limited detectability of currently prevailing defect inspection and material characterization methods based on ultrasonic waves, there have been impending demands for noncontact and in situ nondestructive testing techniques that are applicable to multiscale materials and structures, to which conventional contact-type transducers have difficulty accessing. Scenarios that reject the contact-type transducers are exemplified by the inspection of cavitation, nonmetallic inclusion, or cracks in turbine disc of the aero-turbofan engines, or in situ material characterization of semiconductor chips. In this connection, optoacoustics-based approaches, involving the optical generation and acquisition of ultrasonic waves triggered by two separated laser beams (referred to as pump and probe beams, respectively), have demonstrated a promising avenue to high-resolution defect detection, thickness gauging and material characterization in a fully noncontact manner. Despite apparent advantages, the optoacoustic inspection techniques today, however, often show compromised performance and lowered deficiency when used for thick structures. The difficulty is brought when the inspection is required to be conducted under heavy background noise and reach subwavelength image quality. And there is a lack of discussion for fully anisotropic photoelasticity that induces orientation-dependent perturbation to the optical polarization in anisotropic media. Moreover, existing ultrafast optoacoustic techniques also show the incapability of registering three-dimensional (3-D) particulate motion when the ultrasonic waves are operated in the gigahertz domain. Here, the ultrasonic waves of GHz frequency are often referred to as GHz phonons. To circumvent the above deficiency of today’s optoacoustics-based defect inspection and material characterization for multiscale engineering structures, this PhD study is dedicated to improving/developing the robustness of optoacoustic subwavelength imaging approaches, the fully anisotropic photoelasticity of monocrystalline semiconductors, and 3-D ultrafast optoacoustic measurement for GHz phonons. A dedicated multiphysics optoacoustic model is first developed to interpret the principles of thermoelastically generating optoacoustic ultrasonic waves (OUWs). With the model, the OUW generation over a duration from few nanoseconds to hundreds of femtoseconds is investigated. The spatial, temporal and frequency characteristics of OUWs, under different conditions of irradiation of the pump beams, in the waveguides with the spatial scale ranging from millimetres to nanometres, are calibrated. Finite element (FE) simulation is performed to verify the multiphysics modelling and examine the accuracy when the model is used to depict the characteristics of OUWs in different media. Facilitated by the model, a two-step optoacoustic framework is established for inspection of defect in multiscale structures. The two phases sequentially refine the noise-polluted OUW-signals and then beamform the refined signals to image the defect. This framework approach is supplemented with i) a denoising algorithm based on the entropy-polarised bilateral filtering (Entropy-P-BF) and ii) a subwavelength defect imaging method based on minimum-variance (MV) beamforming. To start with, a thick structure, with a thickness ten times the wavelength of OUWs and bearing a submillimetre defect in a jet aero-engine turbine disc (~0.7 mm in its characteristic dimension), is inspected using the two-step framework. Experimental results demonstrate the effectiveness and robustness of the proposed framework. The developed framework is not restricted by the scale of the inspected structures. When extended to the mesoscopic material characterization, such as the identification of anisotropy of semiconductors, the developed optoacoustic framework is proven tenable. The framework measures the shear strains of bulk modes of OUWs in mesoscopic materials, outperforming existing optoacoustics-based approaches in measuring OUW-induced shear strains. As a demonstration, the framework is used to characterise the anisotropic features of monocrystalline semiconductors. A multiphysics modelling is conducted to formulate the mutual interaction between the optical polarization and shear strains in anisotropic monocrystalline semiconductors with the diamond-cubic crystal structure. A slicing strategy is introduced to ascertain the solution to polarization of the probe beam induced by the shear strains. With the model, the shear-strain-induced perturbation to the optical polarization, when a monochromatic laser beam interacts with OUWs propagating in the MS, is determined. It is revealed that the optical polarization perturbated by OUWs in Zinc-coated monocrystalline silicon wafers depends on the crystalline orientations, and such orientation-dependence exhibits a potential for the selective measurement of OUW-induced shear strains. The above framework is further advanced with a femtosecond-laser pump-probe technique, to make it applicable to microscopic characterization. The femtosecond laser generates and acquires picosecond phonons, for capturing and visualising the 3-D spin angular momentum (SAM) of GHz evanescent acoustic phonons (EAP) propagating in anisotropic semiconductors. Theory-wise, the propagating behaviours of both the bulk acoustic phonons (BAP) and EAP in silicon wafers are modelled using lattice dynamics, to illustrate the features and difference between SAM of BAP and EAP. Results reveal the SAM of EAP is intrinsic and 3-D. Surface acoustic phonons (SAP), one typical form of EAP, are particularly studied to intuitively depict the nature of SAM of EAP. To validate the theoretical results, a femtosecond-laser pump-probe system is configured, in conjunction with the use of a knife-cutting setup. The system enables sensing the transverse components of atomic velocity of SAP, with which the 3-D SAM maps of GHz SAP are obtained. In summary, this PhD study aspires to gaining an insight into the fundamentals of optoacoustics in generation and acquisition of ultrasonic waves. And based on the optoacoustic fundamentals, an optoacoustic framework for defect inspection and material characterization has been developed. The effectiveness of the framework is not restricted by the scale of the inspected structures. Experimental validation has been implemented on multiscale structures, spanning from macroscopic turbine discs, through mesoscopic monocrystalline semiconductor wafers, to microscopic crystalline structures. |
| Rights: | All rights reserved |
| Access: | open access |
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