Sub-Surface Crack Detection by Using Laser Induced Transient Stress Wave Propagation

Abstract

In this study we attempt to investigate the possibility of detecting sub-surface crack and the understanding of the propagating phenomena of transient stress waves due to impact in thick aluminum plate by the simultaneous measurement of longitudinal and shear creeping and Rayleigh wave resulted from the mode conversion of laser induced transient stress wave impact. The propagation of the transient stress wave generated by laser irradiation is affected by the sub-surface crack and the result is analyzed. It was observed that the longitudinal and shear creeping wave velocities are varied depending on the depth of sub-surface crack. In addition, the variation of amplitude ratio generated by propagating the stress wave is investigated. The longitudinal creeping wave velocity in the presence of the sub-surface crack is somewhat faster than in case of non-crack. And the shear creeping wave velocities represent large variations which are shown nearly 2nd order quadratic curve shape as the sub-surface crack depth increase under the same experimental condition. The results of this study are very useful for the nondestructive evaluation of the surface layer in thick structures by non-contact method and the opposing and the structures difficult to access.