Research On Nondestructive Testing Of Composite Plates Using Ultrasonic Guided Wave Technology

With the increasingly widespread use of composite materials, the health condition ofcomposite materials attracts wide attention, which directly affects the overallstructural integrity, safety and service life. Because of wide inspection range, highefficiency, and complete inspection of ultrasonic guided wave technology, it hasbecome one of effective means for composite structure inspection. In this thesis,ultrasonic guided wave imaging method is proposed for health monitoring ofcomposite plate structures. Based on this method, the defect inspection and image incomposite plate structures are achieved by theoretical analysis and experimentalverification.In this thesis, main contents and contributions are given as follows:(1) The dispersion equation of Lamb waves was obtained by analyzing ultrasonicguided wave propagation characteristics in composite plate theoretically. Thisdispersion equation of Lamb waves in composite materials was solved numericallyand the theoretical dispersion curves of the T300/QY8911carbon fiber compositeplate in different ultrasonic guided wave propagation directions were obtained.(2) Based on a variety of time-frequency analysis methods such as short-timeFourier transform, time-frequency rearrangement and the rearrangement methodbased on the nonparametric Snakes, experimental guided wave signals obtained in thecomposite plates were processed. Time-frequency spectra were obtained andcompared with the theoretical dispersion curves. These results proved that time-frequency processing methods can be used for mode identification of ultrasonicguided waves in composite plates. Phase velocity measurement of ultrasonic guidedwaves in the composite plates was implemented experimentally. The results show thatnon-contact air-coupled transducers can be used for accurate phase velocitymeasurement in all directions of composite plates.(3) Because of the presence of the skew effect in anisotropic composites, thegroup velocity of ultrasonic guided waves in all directions of composite plates wascalibrated by group velocity calibration algorithm. By comparing with group velocitywithout calibration, calibrated group velocity is more consistent with the experimentalresults with high accuracy. Guided wave propagation characteristics of carbon fibercomposite plate structures are greatly affected by the ply direction of the fibers and the number of layers. Meanwhile, it also proves that the group velocity calibration isimportant and necessary when ultrasonic guided wave technology is applied to defectdetection in composite plate structures.(4) By the use of sensor arrays distributed in the composite plate in advance, thesimulated defects were inspected in multi propagation paths. Discrete ellipse imagingalgorithm was used for processing guided wave signals and image and localization ofthe defects in composite plates were achieved. Furthermore, data fusion was carriedout by using several defect images to improve defect image quality. Single and doubledefects in the composite plate were perfectly achieved by using this developedmethod.