Ultrasonic guided wave phased array for isotropic and anisotropic plates.

摘要

This thesis covers both ultrasonic guided wave mechanics and phased array principles and also combines them into ultrasonic guided wave phased array developments for plates. The principal motivation of this thesis is to achieve the ultimate goal of inspecting plates and plate-like structures in aircraft using ultrasonic guided wave phased array techniques to ensure aviation safety. Besides isotropic metallic materials, anisotropic fiber-reinforced composite materials are also included in the thesis as the inspection objects of the phased arrays due to their increasing usage in the aircraft industry.;On the basis of the free wave solutions obtained using a semi-analytical finite element (SAFE) method, a normal mode expansion technique and a Green's function based method are employed to investigate the problems of guided wave excitations by various source loadings for both isotropic plates and anisotropic fiber-reinforced composite plates. The derived closed form solutions for guided wave excitations by point sources can be applied to evaluate the guided wave excitations by various transducers. Finite element (FE) models are used to validate the closed form solutions.;Miscellaneous topics in phased array principles are discussed, including a comparison between beam steering and beam focusing, parametric studies on linear and circular array designs, real-time phased arrays and synthetic phased arrays, a deconvolution method for suppressing phased array image artifacts caused by sidelobes in array directivity profiles, and a back-propagation beamforming technique. To apply the phased array principles to guided wave applications, the influence of unwanted wave modes, guided wave dispersions and the influence of anisotropy in composite plates are identified as the main technical challenges. A concept of a mode selection spectrum is developed and utilized to deal with the influence of unwanted wave modes based on the closed form solutions for guided wave excitations. The guided wave dispersion relations obtained in wave mechanics studies are integrated into the back-propagation beamforming technique. To control the influence of anisotropy, guided wave modes with quasi-isotropic behaviors are selected. Mode 1 in composite plates is found to be suitable for guided wave phased array beam steering at low frequencies. The feasibilities of inspecting plates and plate-like structures using guided wave phased array beam steering are validated by laboratory experiments.;Wave mode selection using phased comb type arrays is another application of the guided wave phased array technique. Based on the guided wave excitation spectrums, the capability of the phased comb type arrays on selecting specific wave modes and frequencies to meet different inspection requirements is explored. A blind discontinuity detection technique is developed for isotropic plates and plate-like structures such that discontinuity detection can still be performed even if the precise isotropic material properties of the objects being inspected are unknown. A novel method of utilizing guided wave skew effects in defect detection for fiber-reinforced composite plates is presented as well. On the basis of skew angle dispersion curves, different wave modes with different skew angles are selected by means of applying different phase delays to the comb array elements to inspect different directions in composite plates. Experimental results are provided as a validation of the proposed techniques.