Non-contact Detection of Subsurface Defects in Metallic Bars with Small Diameter Using Laser-Generated Rayleigh Waves: from Localization to Quantification

The subsurface defect in metallic bars with small diameter was detected and evaluated using laser generated Rayleigh wave for the first time. First, a finite element model was established to study the interaction between Rayleigh waves and subsurface defects, and the mechanisms of phase variation of Rayleigh wave caused by structure's curvature and subsurface defects were revealed, respectively. Subsequently, the correlations between the location, width and depth of subsurface defects and the parameters of transmitted Rayleigh wave were studied. A time-frequency parameter kr that independent of defect's width was proposed to characterize the depth of subsurface defect. A theorical expression of the amplitude of Rayleigh wave that includes the width and depth of the subsurface defect was proposed to evaluate the defect's width. Finally, the laser ultrasonic experiment on aluminum alloy bars with diameter of 20 mm was conducted to detect subsurface defects. The experimental results proved that the proposed parameter kr and theorical expression can be used to detect and evaluate location, depth and width of subsurface defects in cylindrical structure with small diameter, which further improved the practical application capability of laser ultrasonic technology.