Inspection of Material Internal Defects Using Double Shadow Method Based on Laser Ultrasonic Reflected Shear Waves

Because the amplitude of diffraction waves from the internal defects is weak and the depth information of the defects cannot be obtained by transmission bulk waves, it is still difficult to inspect the small internal defects of material by using laser ultrasonics (LU) in nondestructive conditions. In order to achieve remote nondestructive testing of internal tiny defects, we propose a double shadow method based on the LU reflected shear waves. By combining the advantages of the ultrasonic transmission method and the reflection echo-wave method, this method utilized the time of flight method to inspect the internal defects based on the twice attenuation effect of the defects on the reflected shear waves. The cross-correlation algorithm is used to calculate the time delay of the surface acoustic wave and the reflected shear wave, respectively. On basis of the accurate measurement of the distance between the pump and probe lasers, the distance between the two shadow positions and the thickness of the sample, the internal defect of 0.8 mm diameter is successfully detected by the double shadow method, and the depth position of the defect is measured. By comparing with the result of X-ray digital radiography imaging and the detection of conventional ultrasound transducers, the experimental results demonstrate that the LU method has the ability to inspect the internal tiny defects and locate the position of the defects accurately under nondestructive conditions.