The first part of this doctoral thesis explores the application of Magnetic Barkhausen Noise and Non-Linear Eddy Current techniques for the early-stage detection of fatigue in ferromagnetic materials, with a specific focus on Martensitic Stainless-steel samples. Due to its exceptional mechanical properties at elevated temperatures, stainless steel finds extensive use in various applications. However, material fatigue poses a significant challenge in steel structures, leading to potential catastrophic damage and substantial economic consequences. While conventional nondestructive evaluation techniques excel at detecting macro defects, they often fall short in identifying material degradation at the microstructure level, particularly arising from fatigue. The Magnetic Barkhausen Noise technique involves capturing signals generated by the movement of domain walls after applying a time-varying magnetic field. Different fatigue stages yield unique Magnetic Barkhausen Noise signatures, facilitating effective classification. In the Non-Linear Eddy Current technique, a robust external magnetic field induces non-linear behavior in the material's magnetization characteristic. The harmonics extracted from the Non-Linear Eddy Current signal provide insights into the material's microstructure, aiding in the classification of samples at various fatigue stages. The research work systematically investigates the feasibility of Magnetic Barkhausen Noise and Non-Linear Eddy Current techniques by employing customized sensor assemblies to capture and analyze signals in both time and frequency domains. Extracted features are further processed using k-medoids clustering algorithm, and Genetic algorithm for robust classification into distinct fatigue stages. The comparative performance of the two magnetic non-destructive evaluation techniques is thoroughly examined.The research findings indicate that both Magnetic Barkhausen Noise and Non-Linear Eddy Current techniques present promising capabilities for detecting early-stage fatigue in Martensitic Stainless-steel samples and contributes to advancing the fatigue detection in ferromagnetic structures using magnetic non-destructive evaluation techniques.In the second part of this doctoral thesis, the focus is on addressing critical challenges of monitoring the structural health of engineering structures, which are susceptible to damage from both stress and environmental factors. Traditional ultrasonic nondestructive evaluation techniques typically involve contact-based procedures that necessitate the use of a couplant. However, this thesis explores the use of Electromagnetic Acoustic Transducers, which offer a compelling non-contact alternative. Electromagnetic Acoustic Transducers utilize the Lorentz force, acting on induced currents, to excite elastic waves in a sample, eliminating the need for direct contact. The drawback of conventional Electromagnetic Acoustic Transducers being limited to conductive or ferromagnetic samples is addressed through the introduction of a novel Electromagnetic Acoustic Transducer, specifically designed for non-conductive samples.This novel Electromagnetic Acoustic Transducer presents two distinct configurations: (a) Direct excitation and (b) Non-contact induced excitation, both utilizing the Lorentz force transduction mechanism. A thorough investigation into the metal patch geometry employed in both configurations is detailed, providing valuable design insights. The numerical model of these Electromagnetic Acoustic Transducer configurations is developed using COMSOL, and simulation results robustly affirm the feasibility of the proposed approach. By successfully extending the applicability of Electromagnetic Acoustic Transducers to non-conductive samples and introducing the innovative embedded Electromagnetic Acoustic Transducer, this research significantly contributes to advancing the field of structural health monitoring and presents a viable nondestructive evaluation approach for the effective detection of damage in engineering structures.
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