Measurement of spot welding nugget diameter using power spectral density variation of laser ultrasonic Lamb wave

In applications of spot welding, the nugget determines reliability of the material joining, which is characterized by nugget diameter. However, the conventional methods are complex and rough to measure the diameter of the nugget. This paper aims to propose a novel convenient method for measuring the diameter of spot welding nugget. The characteristic curve of laser ultrasonic Lamb waves in nugget area are obtained by laser ultrasonic detection technology under completely non-contact conditions. The variation characteristics of power spectral density in nugget area are analyzed by Fast Fourier Transform, to determine the geometric relationship between the scanning area and the nugget diameter, and the nugget diameter are obtained by further calculation. Experiments show that the accuracy of the proposed method is within 0.2 mm, and the relative errors of experimental data are within 3%. This paper provides an alternative method for the measurement of spot welding nugget diameter, and has great significance for the application of laser ultrasonic detection in spot welding. •A novel convenient method for measuring the diameter of spot welding nugget by laser ultrasonic detection (LUD) is proposed.•The variation characteristics of power spectral density in nugget area are analyzed, and the nugget diameter are obtained by further calculation.•Experiments show that the accuracy of the proposed method is within 0.2 mm, and the relative errors are within 3%.