激光超声无损检测技术研究

【作者】 程琳；

【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】 在工业领域,常常需要在不破坏或不损伤被检对象的前提下,判定和评价物质内部或外表的物理和机械性能,包括各种缺陷及其它特征参数,这对于控制和改进生产过程中的产品质量,保证材料、零部件和产品的可靠性起着关键作用,因此产生了一门综合检测技术——无损探伤技术。激光超声技术是利用激光来激发和检测超声的一门新兴技术,它广泛应用于无损检测领域。本文从理论和实验上研究了激光超声的生成机理、超声脉冲的特性、光外差无损探伤系统以及速度型干涉探测系统的光学设计。对课题的研究背景、应用情况、国内外发展情况以及论文的主要工作进行了介绍。运用点源模型讨论了热弹和烧蚀两种机制下激光激发超声的机理,探讨了热弹机制下激光激发超声脉冲的特性,并给出实验验证。同时,分析了超声波在固体中的衰减,并由实验得到了钢件和铝件中的超声信号回波,通过比较确定钢件为实验材料。为进一步研究非接触式接收方法打下基础。对光外差探伤系统的原理、光学设计进行了详尽地分析,并给出了实验装置以及实验结果处理。重点系统地分析了速度型干涉探伤系统的工作原理,给出了其中关键元件共焦球面Fabry-Perot干涉仪的参数设计以及光电探测器的参数选择。首先讨论了共焦Fabry-Perot干涉仪用于探测激光超声信号的原理,通过计算给出其关键参数:腔镜反射率R和腔长L,由此确定了干涉仪的四个重要参数;接着计算了超声信号对入射光的调制,并分析了干涉仪对超声信号的解调原理,得出结论如下:当激光超声频率与干涉仪带宽接近时,干涉仪有最佳接收灵敏度,同时也证明了我们所选的仪器带宽的正确性;最后依据上述光学系统的带宽和信号要求确定了光电探测器的参数。对本文所述的两个系统(光外差探测系统和速度型干涉探测系统)做了详细地分析比较,提出了进一步研究工作的方向。更多还原

【Abstract】 Detection of the physical and mechanical property of materials without damage is often needed in industrial field, which has been expected to recognize kinds of defects and other characteristic parameters. So a comprehensive detection technology called nondestructive detection has been appearing, which plays a key role on product quality control, and ensures the reliability of the product components. Laser ultrasonic technology using laser to generate and detect ultrasound is an emerging technology, which is widely used in nondestructive field.In this dissertation, the mechanisms of ultrasonic generation by laser pulse, ultrasonic pulse characteristic, optical heterodyning detection system and the optical design of the velocity interferometry system are discussed theoretically and experimentally.Firstly, the research background, the application and the development at home and abroad of the project are introduced. The main work of this dissertation is also described.Secondly, thermoelastic excitation theory and ablation excitation theory are analyzed by point-source model, and laser ultrasonic pulse characteristic of thermoelastic mechanism is discussed. The experiments to confirm these two theories have been done and the results are given. Besides, the attenuation of ultrasonic in solid is analyzed. And ultrasonic signal echo in steel and aluminum is presented through experiments. Steel is chosen as the probed material by comparison. These results build a foundation for the further research on non-invasion measurement.Thirdly, the principle and optical design of optical heterodyning detection system are analyzed in detail. The experiment device is given, and the results are handled.Fourthly, the operating principle of the velocity interferometry system is analyzed systematically. The parameter design of the key device called confocal Fabry-Perot interferometer and the preferences of the photodetector are given. The reflectivity of the mirror R and the cavity length L are calculated. The modulation on the incident lights by ultrasonic signal and the demodulation on the ultrasonic signal by the interferometer are calculated. The conclusion that the interferometer has the best sensitivity while the frequency of the laser ultrasound is approaching the bandwidth of the interferometer is obtained. Parameters of the photodetector are determined by the bandwidth of the optical system and the signal.Finally, optical heterodyning detection system and the optical design of the velocity interferometry system are compared. And further research direction is presented.更多还原