Detection of Defects on Reverse Side of Metal Plate Using MHz-Range Air-Coupled Lamb Wave

A novel noncontact air-coupled ultrasonic system using MHz-range Lamb waves was constructed. In both the S0- and A0-modes, dispersion curves agreed well with the k – f image, which was obtained by applying a two-dimensional Fourier transform of the received waveforms using our system. The detection of square, flat-bottomed defects on the reverse side of a metal plate was demonstrated using two types of experimental arrangement. The position and width of the defects were estimated using the arrival time of the Lamb wave reflected from the defects. The profiling of the defect using the change in the amplitude of the Lamb wave confirms the usefulness of this system in the field of noncontact nondestructive evaluation.