Study on high accuracy ultrasonic testing method by separating a composite ultrasonic waves

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A flaw existed very near the surface can not be detected by using normal ultrasonic testing method and a ultrasonic microscope. In this study, a method to detecting the flaw existed very near the surface has been developed. The reflected echo from the flaw existed very near the surface is superimposed to that from surface of the measured sample. Therefore, in this study, in order to measure the time difference between the reflected echo from the flaw and that from surface, the composite echo is separated to the flaw echo and the surface echo. As the result, the following conclusions are obtained. (1) The composite echo can be separated by using a simplex method. (2) When the materials of the coated layer and the base material, the composite echo can be easily separated by considering the reflected ratio that is calculated from the acoustic impedance of the both materials. (3) The surface roughness of the base material has an influences on the amplitude of the reflected echo. However, the surface roughness can be measured by measuring the amplitude of the scattering echo. (4) When the flaw inclines to the surface, the flaw can be detected within 10 degree to the surface. (5) It can be seen that the suitable frequency of the ultrasonic transducer is 20MHz. (6) The diameter of the focused ultrasonic waves by the focusing type ultrasonic transducer is 0.2mm.