为了实现材料内部微小缺陷的非接触无损检测,解决激光超声检测内部缺陷时衍射回波信号弱、透射体波检测无法获得缺陷深度信息等问题,提出了一种激光超声反射横波双阴影检测方法。该方法结合超声透射法和反射法的优点,依据缺陷对反射横波的两次衰减作用,利用时间飞行法对样品进行扫描检测,通过波形互相关算法计算波形时延,精确测量了激光激发点与探测点距离和横波双阴影间距,结合样品厚度实现了对直径为0.8 mm内部缺陷的检出和深度定位。与X射线数字射线照相、传统超声换能器检测的结果进行对比后可知,激光超声方法能够实现材料内部微小缺陷的非接触无损检测和精确定位。

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.