The elastodynamic Boundary Element Method for Ultrasonic Guided Wave Monitoring of poly-crystalline materials at micro-scale

Abstract

This thesis presents the development of the elastodynamic Boundary Element Method (BEM) in the Laplace domain for Ultrasonic Guided Wave Structural Health Monitoring (UGW-SHM) and Non-Destructive Testing (NDT) applications of poly-crystalline materials. Poly-crystalline materials are chosen for this study due to their widespread use in engineering structures, strong mechanical and thermal properties, and susceptibility to micro-scale failure caused by various defects. Understanding microscopic characteristics such as defects, grain size, and orientation is crucial for enhancing material mechanical properties and studying material behavior. As such, this analysis focuses on defect detection and localization. In this thesis, the multi-region elastodynamic boundary element formulation for anisotropic material is developed, for simulating the propagation of UGWs in poly-crystalline materials at micro-scale. The poly-crystalline micro-structures are created employing the Voronoi tessellations, where each grain is considered as a single crystal with random location, morphology, orientation and anisotropic material behaviour within the entire aggregate. The propagation of UGWs in poly-crystalline materials is thoroughly analysed to comprehend the effects of microscopic characteristics and various micro-defects on wave propagation. The study examines intergranular cracks, analysing their position, orientation, length, and distance from a reference point. Both single crack and clusters of cracks are simulated to represent realistic damage scenarios. The phenomenon of porosity is also studied, exploring its influence on material macro properties. Different crystal types are compared with respect to how they are affected by the volume fraction of porosity. Additionally, the detection of voids is explained based on the amplitude and Time of Arrival (ToA) of the back-scattered wave. Furthermore, the propagation of UGWs is investigated in both pristine and damaged scenarios by discretising the internal domain of the micro-structure using internal points. The Probability of Detection (PoD) and damage index analyses are utilised to assess the extent of damage. The numerical BEM platform is thoroughly validated and compared with experimental tests, demonstrating close agreement between numerical and experimental results, thereby confirming the accuracy and effectiveness of the numerical platform. The simulations provide valuable insights into wave propagation, wave-material interaction, and the impact of micro-scale defects. The study illustrates that the guided wave solution is a suitable approach for obtaining information at the microstructural scale. Overall, the BEM and UGW-SHM method offer an effective and efficient approach to understanding the micro-structural features and their influence on the macro-structural properties of poly-crystalline materials.