Damage detection in solid structures by ultrasonic techniques---Analytical and experimental investigation

This dissertation deals with analytical and experimental investigations of a number of distinct problems related to defect detection in solid structures. All these problems have applications in structural health monitoring (SHM) and nondestructive evaluation (NDE). With this broad goal in mind the mode selective excitation and detection scheme for guided Lamb waves in isotropic and anisotropic plates has been investigated. Change in the time of flight technique is applied to the detection of inherent variations in material properties. Symmetric and anti-symmetric modes are resolved with orthogonal phase difference and homodyning. Noncontact Electro-Magnetic Acoustic Transducers (EMATs) are developed for generation and detection of Lamb waves. Its interaction with circular defects having dimensions comparable to the wavelength is investigated. Short time Fourier Transform and Hilbert Transform are applied for feature detection and identification of localization of damages. Distributed point source method (DPSM) is applied to model the scattering of acoustical wave field and to study the stress singularity in penny shaped cracks. Both experimental and numerical investigations are performed in the framework of structural health monitoring and non destructive evaluation for gaining insight in damage initiation and progression in materials.