| Summary: | 碩士 === 國立成功大學 === 系統及船舶機電工程學系碩博士班 === 94 === In this study, the wavelet analysis method is used to detect the fatigue crack on aluminum alloy plate. Under undamaged condition, the shaker generates a vibration wave source, and the strain gage is pasted on the aluminum alloy plate and used as a sensor to measure the wave signals in the plate. The wavelet analysis method is applied to analyze the transformation between time domain and frequency of transient plate wave. Finally the time difference and wave group speed of the dispersive wave and flexural wave generated by the crack are used to calculate the globe minimum value of the target function, and predict the location of the crack on the plate.
In present investigation a rectangular aluminum alloy plate is used as the active detection object, and the boundary conditions of it are fixed on two opposite sides and flexible on the other two opposite sides. Four strain gages are mounted on the surface of the aluminum alloy plate to construct the array for active detection of dispersive wave of the crack. The strain wave signals are obtained after the shaker impact the plate, and wavelet transform is applied to find out the wave path time of flexural wave from the impact point to the strain gage, and to compute the wave group speed of flexural wave in each frequency. Since the flexural wave would create dispersive wave when it contacts with plate cracks during transmission, the plate wave signals of plates with cracks will include direct wave of the flexural wave and dispersive wave caused by cracks. In an ideal situation, the wave signal obtained from the plate with fatigue crack deducts the wave signal obtained from the plate without crack, the dispersive wave signal generated by the cracks can be retained. Using the wave path time difference between the flexural wave and dispersion wave, and the wave group speed of the direct wave and dispersive wave, the contour map of the minimum value (target function) of the entire region can be drawn. The location of the minimum value is the position of the plate crack. The result shown in the contour map is consistent with the actual location of the crack. It is believed that the method explored in this study can achieve non-destructive inspection effect with simple and inexpensive equipment.
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