Application of narrowband laser ultrasonics to the nondestructive evaluation of thin bonding layers

In this paper, a modified laser induced grating technique (LIG) has been proposed to generate narrowband surface waves in a thin epoxy-bonded copper-aluminum layered structure. A high performance Doppler interferometer system was utilized to detect the laser-generated waves. The dispersion of SAW in layered specimen was measured and compared with the theoretical solution. An inverse algorithm based on the simplex method was then introduced to determine the bonding thickness as well as the elastic properties of the bonding layer. The inversion results demonstrated that the thickness or the elastic properties of the bonding layer could be successfully determined.