Non-contact laser ultrasonics for detecting subsurface lateral defects

To evaluate subsurface lateral defects, non-contact and non-destructive laser ultrasonics was employed. Surface acoustic waves (SAW) were generated by a Q-switched YAG laser and monitored by a heterodyne laser interferometer. Both the source and monitoring lasers were scanned over the samples to monitor the location of defects and bond quality. Based on the fundamental works of the thin metallic foil and model specimen with an artificial subsurface slit, bond quality of the stainless foil brazed on the carbon steel was evaluated by the quantitative analysis of velocity dispersion of the Rayleigh and lowest order Lamb waves. Velocity dispersion of SAW over the well bond area well agreed with theoretical dispersion of the Rayleigh wave for multi-layer laminate, while that over the unbond area agreed with the lowest order anti-symmetric Lamb wave. Velocity dispersion of the SAW over the weak bond was calculated by inserting a slip layer which allowed displacement discontinuity across the interface. Analysis of the generalized Lamb wave was demonstrated to be promising in evaluating the bond quality as well as identifying the location of defects.