Ultrasonic thickness measurement for aluminum alloy irregular surface parts based on spectral analysis

Ultrasonic methods of measuring the thin thickness in a non-parallel irregular surface sample require quite a high time resolution of the detection device. Ultrasonic resonance method based on spectral analysis was used for accurate thickness measurement of this kind irregular aluminum alloy sample. Aiming at performing successive measurement of the thicknesses of different positions of one sample, non-contact detection mode was selected by using 20 MHz water immersion ultrasonic focused transducer and ultrasonic PAC testing system with 3D location device. Multiple resonant thickness algorithm was set up, so that the thickness value can be calculated by multiple resonant frequencies. In order to obtain the precise multiple resonant frequencies, in spectral analysis, surface wave in time domain signal needs to be removed and the appropriate window of time domain signal was selected. A high measurement accuracy of ±15 μm can be achieved using this method. In addition, the effect of the transducer location error was investigated and the allowed transducer offset ±500 μm was determined.