Understanding elastic wave propagation effects in complex polycrystalline materials
File(s)
Author(s)
Yeoh, Wei Yi
Type
Thesis
Abstract
The presence of macrozones, also referred to as micro-textured regions, in Ti–64 has been identified as a potential contributor to the onset of cold dwell fatigue, leading to a significant reduction in fatigue life. Previous studies have demonstrated the potential of ultrasonic testing for macrozone characterisation through variations in attenuation, backscatter, and wave velocity. However, due to the hierarchical complexity of the microstructure, several observed wave–microstructure interactions remain poorly understood.
In this work, a finite element polycrystalline modelling framework is developed to systematically investigate ultrasonic wave–macrozone interactions. Using two-dimensional idealised models, key correlations between macrozone characteristics, including size, shape, and crystallographic texture, and ultrasonic responses such as attenuation, backscatter, and wave velocity are identified through controlled parametric studies. These trends are subsequently validated experimentally, demonstrating how such systematic relationships may support the ultrasonic characterisation of macrozones in Ti–64 samples.
A complementary methodology for generating realistic synthetic macrozone models is then introduced, incorporating hierarchical microstructural information obtained from Electron Backscatter Diffraction scans. This approach enables the construction of representative microstructures that capture the statistical and morphological features observed in real materials, providing a physically faithful numerical environment for studying wave–macrozone interactions. Grain statistical information extracted from two-dimensional micrographs is used to synthesise representative grains and macrozones, which are validated using finite element wave propagation simulations and two-point correlation measurements.
Finally, the framework is extended into the geometric regime to investigate wave–microstructure interactions at higher frequencies using finite element models, with reference to analytical descriptions. This regime is relevant to macrozone characterisation as operation at higher frequencies offers the potential for improved spatial resolution and sensitivity to finer microstructural features. Overall, this study establishes a comprehensive framework for advancing the fundamental understanding and ultrasonic characterisation of macrozones in Ti–64 through controlled investigation of wave–microstructure interactions.
In this work, a finite element polycrystalline modelling framework is developed to systematically investigate ultrasonic wave–macrozone interactions. Using two-dimensional idealised models, key correlations between macrozone characteristics, including size, shape, and crystallographic texture, and ultrasonic responses such as attenuation, backscatter, and wave velocity are identified through controlled parametric studies. These trends are subsequently validated experimentally, demonstrating how such systematic relationships may support the ultrasonic characterisation of macrozones in Ti–64 samples.
A complementary methodology for generating realistic synthetic macrozone models is then introduced, incorporating hierarchical microstructural information obtained from Electron Backscatter Diffraction scans. This approach enables the construction of representative microstructures that capture the statistical and morphological features observed in real materials, providing a physically faithful numerical environment for studying wave–macrozone interactions. Grain statistical information extracted from two-dimensional micrographs is used to synthesise representative grains and macrozones, which are validated using finite element wave propagation simulations and two-point correlation measurements.
Finally, the framework is extended into the geometric regime to investigate wave–microstructure interactions at higher frequencies using finite element models, with reference to analytical descriptions. This regime is relevant to macrozone characterisation as operation at higher frequencies offers the potential for improved spatial resolution and sensitivity to finer microstructural features. Overall, this study establishes a comprehensive framework for advancing the fundamental understanding and ultrasonic characterisation of macrozones in Ti–64 through controlled investigation of wave–microstructure interactions.
Version
Open Access
Date Issued
2025-09-28
Date Awarded
2026-03-01
Copyright Statement
Attribution-NonCommercial 4.0 International Licence (CC BY-NC)
License URL
Advisor
Lowe, Michael J S
Lan, Bo
Sponsor
Singapore. Agency for Science, Technology and Research
Publisher Department
Department of Mechanical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Engineering Doctorate (EngD)