Non-contact evaluation of spot-welded nugget size based on measuring diffracted Lamb waves by focused air-coupled ultrasound

Spot-welding is widely used in various fields of industries. Since the size and quality of a welded nugget are closely related to the bonding strength of the weld, its quantitative evaluation is important to improve the reliability of the welded structure. In this work, a nugget sizing method based on measuring Lamb waves diffracted at the nugget boundary of a spot weld is proposed. In this method, the nugget size is estimated from the amplitude distribution of Lamb wave diffracted at the nugget boundary when a pair of air-coupled ultrasonic transducers is being scanned near a spot weld. A pair of focused transducers of center frequency of 0.3 MHz is employed. An imitation specimen is firstly evaluated to verify to the validity of the proposed method. Based on the result, the method is applied to the nugget sizing of three spot welds fabricated with different electric currents. Although the estimated values almost agree with those obtained by a water-immersion ultrasonic method, there are certain discrepancies between them due to structural disorders at the nugget/base material interface.