Ultrasonic Guided Waves Imaging Testing Based On Mode Conversion

Shell structures, such as aircraft wing, hull, etc. inevitably have inherent defects in the materials and usually are subjected to load of air pressure and corrosion of sea water for a long period of time. Those will cause unrecoverable fatigue damage and failure which will affect the normal use of the whole structures and lifetime. To guarantee the safe operation of the system, detection defects of the shell structures has important significance.The traditional NDT(Nondestructive Testing) technologies, represented by X-ray testing, eddy current testing and magnetic testing etc., have some shortcomings including slow testing speed, long processing time, limited detection area and low sensitivity.However, the technique of guided wave can rapidly detect micro-damage in large area shell structures, which has advantages of fast testing speed, short processing time, high sensitivity.Based on experimental and finite element method analysis, this dissertation presents a imaging approach to detecting defects in the plate by guided wave. Firstly, the significance and the application value of guided wave detecting are introduced. The state and trend of guided wave detecting also discuss in this chapter. Then the constitutive equations of guided wave in three-dimensional free plate are derived. The propagation characteristics and dispersion of symmetric and anti-symmetric mode of the guided wave are analyzed. The theories about mode conversion and ellipse location principle are also studied, which provides theoretical basis for defect detecting and imaging in the experiment and finite element simulation.Experimental platform which used to detect defect in plate by guided wave is setup.The PZT wafers which used to excite and receive guided wave are affixed on the top and bottom surface on the plate. Then the signals of guided wave are processed based on the theory of mode conversion and matching pursuit algorithm. The damage index which extracted from the signals is used to locate defects and obtain imaging results.The 3-D solid models are established to study the defect detection in plate by using finite element simulation software ABAQUS, which are used to study the interaction between defect and guided wave. The influences of the defects on acoustic field and energy distribution of guided wave signal are discussed. The results show uniformity between finite element method and experiment.The data from experiments and simulations were processed by the traditional correlation algorithm. The testing results are compared with MP algorithm. It shows that theMP's result is more precise than that of correlations'. Furthermore, the process method based on MP algorithm does not depend on the healthy signal, and the damage index is completely derived from the damage signal. The CFRP composite plate is also detected by guided wave and the testing result is given.