Damage Diagnosis using Time-reversal Tomography Technique in Structural Health Monitoring

It is extremely important to ensure that the small damages can be detected at the early age in fields of aerospace, oil, chemical or nuclear industry. Motivated by the property of time-reversal and existing problems of traditional UT and guide wave, the goal of the thesis work is the development of a finite size damage detection method for plate-like structures such as aircraft fuselage and marine hulls using time-reversal tomography technique, which can operate as an early detection of damage in such structures. The problem of detecting damages by generating plane wave in the medium and analyzing the data from the detected waves is considered and the image is also reconstructed to display such damages with little interfering of the structure in inspection.;An analytical solution is derived for the transient Lamb waves of an infinite plate subject to a point loading based on Mindlin plate theory. An explicit scattered wave solution is also discussed by using Born approximation, which is suitable for weak damage of plate damages. Finally, a MacCormack finite difference algorithm is then implemented and used to simulate the scattered waves which will be used to reconstruct the damage image.;Then two imaging reconstruction algorithms---back propagation and MUSIC are introduced and implemented to point-like damage of plate by processing scattered wave data. However, for extended scatters, each single singular vector of transfer matrix does not correspond to an illumination vector which can be refocused back on the scatter because an extended scatter cannot be viewed as a collection of point scatter. Based on traditional diffraction tomography image reconstruction method, the reconstruction for diffraction tomography has been shown to be essentially represented by a time-reversal process. Based on this relationship, a novel two-step time-reversal tomography image reconstruction procedure is proposed to recover the finite size damages.;In numerical studies, a finite difference simulation is used to generate synthetic scattered wavefield data which provide input to the imaging reconstruction program. Both image reconstruction procedure for point-like and finite size damages are implemented and investigated their performance. All the results show that the time-reversal tomography technique is robust to reconstruct a quantitative image with the location and geometry information of finite size damages of the plate structure.