A Research Of Ultrasonic Tomography On Composite Panels

Composite materials are widely used in lots of fields because of their excellent performance that their components are complementary for each other. Defect detection of materials has been the necessary part in industry, especially for composite materials. Ultrasonic nondestructive testing has been always an advanced technology of nondestructive testing. It has the advantages of strong penetration value and high sensitivity and so on. Computer tomographic is a technology of detecting internal geometry or physical parameters(such as density, etc.) of objects without any damage of the objects. Ultrasonic tomographic imaging that combines ultrasonic nondestructive testing with computer tomographic has gradually been a hot topic in the field of non-destructive testing in recent years. Research on composite panels defect detection by ultrasonic tomographic imaging is conducted in the thesis. And the contents are given as follows:The characteristics of lamb waves propagating in isotropic and anisotropic materials were studied through consulting lots of reference. Then its dispersion equations and dispersion curves in the two different medias have been given. They provided a theoretical basis for the selections of signal exciting frequency and modes.The relevant theories of computer tomographic have been introduced. The equations of parallel and fan beam filter back projection algorithms based on fourier slice theorem were derived and steps of image reconstruction were given based on the two kinds of algorithm. By comparing the steps, the fan beam filter back projection algorithm has been chosen for image reconstruction in this thesis because it has advantages of simple steps and less calculation.The simulation models with the circular defect in different positions which were made of material T300/QY8911 were built to simulate lamb wave beam scanning process and achieve time of flight(TOF) data of lamb wave by the finite element method. The date has been used for image reconstruction by convolution filter back projection algorithm. The basic information of defects can be identified from the reconstructed images with some artifacts brought by the anisotropy of composite materials. A data revising method based on the data of the undamaged mode has been proposed. Then the revised data has been applied for reconstructing image again. The quality of reconstructed images has been improved by data revising.Finally the experimental platform has been set up for lamb wave beam projection and data extraction of lamb wave travel time. The images were reconstructed by original and revised data, respectively. The location of the defect appears in the images reconstructed by revised data is closer to it’s true position than that in the images reconstructed by original data. Thus it proves that the data revising method can effectively solve the problems that anisotropy of the material degrade reconstruction image quality. And ultrasonic tomography technology can detect the different position of the defects in composite panels, which lays a foundation for application in engineering practice of the technology.