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.