| Summary: | Structural Health Monitoring (SHM) denotes a system with the ability to detect and interpret adverse changes in a structure in order to improve reliability and reduce life-cycle costs. SHM essentially involves the embedding of a Non-Destructive Evaluation (NDE) system into a structure to allow continuous remote monitoring for damage. Limitations of bulk wave and vibration methods have turned Guided Ultrasonic Waves into a highly efficient method for the NDE and SHM of structures with finite cross-sectional dimensions. The increasing use of composites in structures brought SHM and NDE methods to a new level. Thus, modeling guided waves in composite structures has become challenging, because composite materials usually have anisotropic properties and exhibit directional mechanical behaviors. The need for an efficient and accurate technique for solving guided wave propagation problem that can deal with the challenges provoked by the composite structures has become obvious. The aim of the current doctoral dissertation is to develop a quick and robust solution for the guided wave propagation problem in laminated composite plate structures. This solution is capable of exploiting the great sensitivity of guided waves to smaller defects, due to smaller wavelength involved, as well as their ability to monitor larger ranges across the structure. Therefore, the main original contribution of this dissertation is to offer an efficient way to solve the guided wave propagation problem in potentially complex strips and plates.
The proposed semi-analytical method couples the through-the-thickness discretization of the cross-section of the structure, based on the hypothesis of the layerwise theory, with the assumed analytic harmonic motion along the wave propagation direction. Fourier transforms are used in order to convert the differential equations of motion into simpler systems in the frequency-wavenumber domain. 2D and 3D formulations are developed for the strip (plain strain) and the plate cases. At this point, the dispersive characteristics of the structures are obtained in terms of group velocity, phase velocity, wavenumber along with the cross-sectional mode shapes. Afterwards, the modal expansion technique and matrix transformation methods are used in order to form a suitable eigenvalue problem for the forced solution of the problem. The propagation of the guided waves is then calculated for various loading cases and the forced response of the structures is predicted. Experimental verification is provided in order to strengthen the proposed semi-analytical solution. During these experiments composite beams with attached piezoelectric actuators and sensors are excited with an ultrasonic signal. Then, the response of the beam in a certain point is correlated with the predicted results from the current solution.
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