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The authors have developed a defect imaging technique for plate-like structures such as pipes and bridges. The imaging technique uses a characteristic of a low frequency flexural mode of guided waves wherein the amplitude of waveforms varies with thickness in the vicinity of the laser spot when the flexural mode is generated with a pulsed laser and detected by receiving probes fixed on the object. Scanning the laser source with a galvano mirror scanner enabled us to measure waveforms at many points and obtain amplitude distributions corresponding to defect images. ;This study discusses topics related to developing a fast non-contact defect imaging system (elastic wave camera). Although replacing a receiving probe on a laser Doppler vibrometer yields full non-contact measurements in generating and receiving elastic waves, one encounters difficulties in measurements in large structures such as pipes and bridges due to the small signal to noise ratio. To overcome such difficulties, we generated burst waves by modulating high-repetition pulses emitted from a fiber laser. Taking frequency spectra of the burst signals improved the signal to noise ratio and gave clear defect images even with signals with small amplitude. Using the non-contact imaging system, the defect images in an aluminum alloy plate could be obtained within a few seconds in the experiments with a small number of laser emission points.

Many experimental and theoretical studies have shown the great potential of guided waves for rapid non destructive evaluation of large pipes and bar-like structures. But guided wave inspection is still not an easy technique like conventional pulse echo testing with bulk waves, because it cannot be explained by ray tracing. Authors have developed a calculation technique for guided wave propagation over a long range of 100 or 1000 times wavelength. The calculation technique using a semi-analytical finite element method (SAFEM) can carry out a simulation of guided wave propagation in a plate, a pipe and any kind of bar-like structures with very short time and small computational memory compared with the ordinary finite element methods. Animation methods are useful for studying and gaining insight into the propagation of guided waves and an understanding of potential applications. This study shows animations of the results obtained by the SAFEM for many kinds of guided wave applications such as dispersive and non-dispersive waves, wave focusing in a pipe, mode conversion at a delamination and a crack in a plate, mode conversion at an elbow of a pipe, three dimensional scattering at a defect in a pipe, and wave propagation in a rail with many resonance modes.