基于激光超声的晶粒尺寸动态检测稳定性研究

Dynamic detection stability of grain size based on laser ultrasonics

基于光折变晶体搭建一套用于检测板带晶粒尺寸的激光超声实验系统,并采用带有千分尺的移动平台和旋转平台模拟板带的面外抖动和小角度偏转,采用衰减法对TC4钛合金板材的平均晶粒尺寸进行表征。结合实验数据分析动态样品的不稳定因素对超声波信号的影响,并进一步评估其对激光超声检测晶粒尺寸精度的影响,进而对激光超声技术的工程应用提出量化的稳定性要求。研究结果表明:板带的面外抖动使光学系统产生离焦量,离焦量越大,返回系统中的散斑强度越低、信噪比越低;小角度偏转改变了反射光的法线方向,引起光折变晶体内部干涉条纹的变化,对干涉的稳定性造成影响;小角度偏转也会降低系统接收到的散斑强度,对系统的信噪比造成影响;动态样品的不稳定因素会引起平均晶粒尺寸的表征偏差,因此,工业用激光超声检测系统在设计时要充分考虑环境干扰,以保证检测精度。

A laser ultrasonic experimental system for detecting the grain size of the strip grains was built based on photorefractive crystals.The out-of-plane jitter and small-angle deflection of the strip were simulated on the translation platform and the rotating platform associated with a micrometer,and the average grain size of the TC4 titanium alloy sheet was characterized by the attenuation method.Combined with the experimental data,the influence of the unstable factors of the dynamic sample on the ultrasonic signal was analyzed,and the influence on the precision of the laser ultrasonic detection of the grain size was further evaluated,and then the quantitative stability requirements for the application of the laser ultrasonic technology were put forward.The results show that the out-of-plane jitter of the strip causes the optical system to defocus.The greater the defocus,the lower the intensity of the return speckle in the system and the worse the signal-to-noise ratio;the small angle deflection changes the normal direction of the reflected laser,causes the change of the interference fringes in the photorefractive crystal,and affects the stability of the interference.In addition,the small angle deflection also reduces the speckle intensity received by the system,and affects the signal-to-noise ratio of the system.Meanwhile,the unstable factors of the dynamic sample will cause the deviation of the average grain size.Therefore,the industrial laser ultrasonic inspection system must fully consider the environmental interference in designing to ensure the inspection accuracy.