Abstract: Acousto-ultrasonic technique is introduced for rapid testing on unweld and pull out of spot weld of thin steel sheet. The method of waveform and modal analysis is used to study the testing mechanism, and then the characteristic value which can be used for evaluating the integrity of the spot welded construction is obtained. In the research, normal transducers are used to excite and receive acousto-ultrasonic wave, and the main component of the testing wave is A0 modal Lamb wave according to the comparison between the testing signal’s time-frequency distribution and theoretical frequency dispersion curves. As the wave propagates through the weld spot, the testing signals are received as discontinuous wave packets ranged in time domain which correspond to the location of the weld spots. Results reveal that the energy of wave packets distributed in time and frequency domain can be used to discovery unweld and pull-out of weld spot, and the location of problem weld spot also can be decided. Besides, the study proposes the characteristic value which can be used to evaluate the integrity of spot weld construction, and is meaningful for developing testing efficiency and decreasing testing cost.

Key words: Acousito-ultrasonic, Lamb wave, Spot weld, Time-frequency analysis