【作者】 李刚；

【导师】 张国平；

【作者基本信息】 华中师范大学 ， 电路与系统， 2004， 硕士

【摘要】 近年来，随着工业自动化程度的不断提高，人们对无损检测技术，尤其是非接触无损检测技术的需求越来越迫切。激光超声检测技术是利用激光脉冲照射样品表面，激发出超声波，并利用光学方法对超声波进行检测，从而实现对样品进行无损检测的一种方法。目前，激光超声技术已成为无损检测领域中的一种重要技术和手段。 由于激光超声具有非接触，时间和空间分辨率高，对样品的形状和尺寸没有特殊要求等优点，它已被广泛地应用于工业无损检测，材料的表征，以及复合材料和各向异性材料的声传播规律研究等领域。 本文首先阐述了激光超声的热弹激发机理和烧蚀激发机理，并分析和比较了激光超声各种波形的特点。然后，介绍了激光超声的光学检测技术，并重点分析了外差干涉检测技术和共焦Fabry-Perot干涉检测技术的原理和特点。 在此基础上，本文介绍了激光超声检测系统的基本结构及工作原理，并通过实验证实了利用激光超声技术对金属进行无损检测的可行性和有效性。通过实验检测铝板的超声回波，求出了铝的超声波速度，并根据固体力学理论，得出了与理论数值相符合的材料的弹性模量。实验还对铝板的表面人工缺陷进行了检测和分析。 本文进一步分析了激光超声的激励源，检测手段等因素对检测结果的影响。通过对超声振动位移与激光脉冲的光功率密度的关系进行分析，发现二者之间存在正比关系。在分析点光源激励声表面波机理的基础上，求出了线光源激励的超声振动的位移表达式，发现线光源激励的近场区表面波具有波形好，信噪比高，指向性好，幅度强等特点。通过理论推导，得出共焦Fabry-Perot干涉仪的输出光强除了与样品表面的振动速度成正比，并与入射角和散射角有关；适当减小CFPI的带宽，增加腔长，增大镜面反射率，或者减小检测入射角都可以提高系统的信噪比。更多还原

【Abstract】 With the development of industrial automation, there is more and more demand on the non-destructive testing technology. For example, the equipments and productions in manufacturers are wanted to be real-time monitored and detected without any damage on line even in such conditions as high temperature, high pressure, poisonousness. The laser-induced ultrasonic technique, which is based on the generation of ultrasonic by a laser and the detection of stress wave with laser interferometry, and is an ideal combination of laser and ultrasonic for non-destructive testing, has also the advantage of non-contact, and has become very important in the non-destructive testing fields.This technology has been used widely just for its advantages of non-contract operation, non-destructive testing, broad bandwidth , high time resolution, high space resolution, no limit on sample’s shape, one laser pulse exciting many kinds of waves and so on.Firstly, this paper introduces the principles of laser ultrasonic generation, thermoelastic excitation theory and the ablation excitation theory. Besides, it briefly presents the other excitation theories in solids. It also analyses and compares the characteristics of bulk acoustic wave and surface acoustic wave, such as directivities and shapes . And then optical detection methods of laser-induced ultrasonic, especially CFPI detection method, are introduced.Based on the principles of laser-induced ultrasonic generation and detection , the structure of the laser ultrasonic detection system is presented, and its availability is testified by several experiments. Using the velocity of bulk acoustic wave and surface acoustic wave in the aluminium sheet measured by this system, elastic constants of aluminium are deduced. The system also test the surface defects of the aluminium sheet. The experimental results show that laser ultrasonic technique is practical and effective.The influences of the characteristics of pulsed laser source and CFPI on the accuracy of detedtions are analysed in detail. The conclusions are as follows: the ultrasonic intensity isproportion to the laser intensity ; the ultrasonic induced by a laser line source has much more advantages than by a laser point source, such as stronger energy, higher SNR, more directivity and shape; the intensity of CFPI’s output signal is linear with ultrasonic vibration velocity, and is effected by incidence angle and scattered angle; SNR will increase by reducing CFPI’s bandwidth, reducing incidence angle, increasing cavity’s length and reflectivity.更多还原