A Study Of The Technology Of The Guided Wave Inspection

Ultrasonic guided-wave inspection technology is a new emerging technology in NDT. A rapid large-range inspection of the large metallic structures can be implemented by using guided wave. Furthermore, a nondestructive testing of structures under earth, water or coating comes to be true with the guided wave technology. However, pipes and metallic plates used in industry often are coated by bitumen or insulated layers, and these damped materials will result in an attenuation of the guided-wave and a reducing to the effective testing distance. So it is important to make a study of the characteristics of the propagation and attenuation of the guide waves in coated metallic plates and pipes in order to improve the efficiency of the guided-wave inspection.Characteristics of the propagation and attenuation of the guided waves in bilayer are studied in this thesis. The propagation of the bulk wave in viscoelastic materials is studied first, then the generation of guided-wave in bilayer is discussed. Characteristics of the propagation and attenuation of the SH wave and Lamb wave in bilayer are analyzed and the expressions of the dispersion equation and attenuation of the SH wave and Lamb wave are presented. Then this approach is extended to solve the guided waves in pipes and the distribution of the stress field and displacement field of the T-mode guided wave in pipe is analyzed. Finally, an experiment is done by using a magnetostrictive sensor in order to generate and receive longitudinal guided waves in pipe.The results indicate that the guided wave is a result of the superposition of shear waves and longitudinal waves at the interfaces in the acoustic wave guides. Based on the free traction conditions at the top and bottom surfaces of the structure and the consecutive conditions of the stress and displacement at the interface of the bilayer, a secular equation of the guided wave can be built. The dispersion curve of the guided wave can be drawn by solving the secular equation. Furthermore, the attenuation of the guided wave can be scaled by the energy factors. For the guided waves in elastic materials, the attenuation can be approximated with a Maclaurin's expansion of the zero order;for the low attenuative materials the attenuation of the guidedwaves can be studied by considering the Maclaurin's expansion up to the first order;for large-attenuative materials higher order approximation is required.