超声无损探测中光致声波的原理及应用

【作者】 罗敏慧；

【导师】 余学才；

【作者基本信息】 电子科技大学 ， 光学工程， 2007， 硕士

【摘要】 激光超声技术是近年来活跃在无损检测领域的一种新型检测手段。本文详细的介绍了光学无损检测的原理和实验技术。实验系统采用共焦Fabry-Perot干涉仪(CFPI)对激光超声信号进行接收和提取。本文重点针对CFPI系统进行光学检测原理的分析和检测系统的结构设计,文中还对光纤Fabry-Perot(F-P)干涉系统在激光超声信号的接收和提取进行了理论设计。对有源腔干涉系统在激光超声信号检测方面的应用前景做了实验研究。本文详细论述了激光脉冲激励固体表面产生超声脉冲的原理。利用有限元法(FEM)对板状金属铝材料中激光超声导波进行了理论研究。对不同激光光源激发条件下产生的超声信号进行了分析和讨论。从声波方程出发,阐述了声表面波的基本性质和传播特性。激光超声无损检测可解决很多常规超声检测方法无法解决的问题,本文对激光超声在材料特性测定、缺陷探测及新型复合材料评估检测等方面的应用进行了研究。验证了一种检测金属材料弹性系数的方法——通过将激光超声在金属内传播的纵波和表面波相结合,分析运算出金属材料弹性系数的方法。建立了激光线源激发声表面波的平面应变有限元理论模型,分析了激光声表面波与不同深度的表面缺陷的相互作用过程。分析了反射声表面波中两种模式成分的到达时间随表面凹痕深度的变化规律。分析结果表明反射声表面波两峰的时间间隔随表面缺陷的深度增加而增加,而与缺陷宽度无关。本文分析了激励源、检测距离、检测手段以及样品特性等因素对检测效果的影响,说明了线光源激励产生的表面波具有指向性好,幅度强等特点。通过理论分析,得出减小探测激光入射角和检测方向散射角可以增大输出光强。适当减小共焦F-P干涉仪的带宽、增加腔长、增大镜面反射率,可以提高系统的信噪比。更多还原

【Abstract】 Laser ultrasonic technique is a very active new testing method in the non-destructive testing (NDT) field. .In the dissertation, the principle and the advantages of the optical NDT are discussed in detail. Confocal Fabry-Perot interferometer(CFPI) is employed to receive and extract the laser ultrasonic signal .On the basis of discussing the principles of detecting ultrasonic signal with CFPI , two new methods of laser ultrasonic NDT, the fiber-optical Fabry-Perot interferometer and active interferometer are theoretical designed and experimental studied respectively.In the dissertation, the mechanism of ultrasonic generated by laser pulse are discussed systematically. The law of finite element is utilized to carry on the theoretical research to the laser ultrasound in the tabular aluminium. The ultrasonic signal excited by pulse laser which could be either point source or line source was analyzed. According to acoustic wave equation, the property and propagation of surface acoustic wave(SAW) was described. The influences of the characteristics of pulsed laser source, detecting distance, detecting method and the characteristics of the specimen on the results were detailed analyzed. In conclusion, the ultrasound excited by a laser line source has much more advantages than by a laser point source; the output of signal intensity would increase with decrease of the angle between laser propagating direction and the normal of the scattering surface and the one between scattering direction and the normal; the SNR would increase by reducing CFPI’s bandwidth, but increasing cavity’s length and reflectivity.Different kinds of applications on laser ultrasonic NDT were demonstrated, including elastic constants measurement, defect recognizing and composite performance testing. A nonconductive method of metallic elastic constants measurement—to combine longitudinal waves with Rayleigh waves—has been verified through comparing my theoretically results with experimental published results . The ultrasound generated by pulse laser has been modeled by the plane strain finite element method. The interaction process between SAW and surface defects with varied structure has been analyzed.更多还原