PURPOSE: To separate a void defect from a normal defect by moving an ultra sonic probe from immediately above a defect to a position with a predetermined angle or more shifted, detecting center frequency of a received echo signal and determining the void defect when a shift amount from the center frequency exceeds a threshold. ;CONSTITUTION: When a probe 16 having received a pulse signal from a pulse 1a applies an ultrasonic beam to an object 18, its center frequency (coinciding with the maximum peak) is high at the center and lower on outer parts. Therefore if a defect echo is detected from a slant direction, it shifts from the center frequency from a case where the echo is detected from immediately above the defect. Such an echo signal is received 1b, A/D-converted 2, stored in a waveform data memory 3 and stored in a RAM 6. At the time of detecting a void, the probe 16 is separated from the defect position by 0.5° or more, the echo waveform is stored in the waveform data memory 3 to make frequency of the maximum peak the center frequency, and when difference between the center frequency and a center frequency of a hammered waveform of the probe 16 is at a certain value or higher, a void defect is determined.;COPYRIGHT: (C)1993,JPO&Japio
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