扫查式激光超声技术及其在表面缺陷检测中的应用研究

发布时间：2018-09-08 20:28

【摘要】：随着现代工业技术的快速发展,金属材料在工业生产中长期满负荷工作,其结构难免受到不同程度的损伤。金属材料结构的变化将给我们的工业生产造成严重的损失。为了实现对这些金属材料进行快速检测,出现了一种新的检测方法-激光超声技术。激光超声技术以其远距离、非接触、高灵敏度、频带宽等优点,得到了人们的广泛关注和研究。本文主要采用有限元数值计算和实验相结合的方法研究了激光激发的超声波在材料缺陷中的传播特性,同时采用扫查式激光超声检测技术对奥氏体不锈钢及铝合金材料中存在的缺陷进行可视化检测。本文的研究内容及创新点主要包括以下几个方面:根据激光产生超声波的热弹运动方程及热传导方程,采用有限元技术对方程进行求解,得到超声波传播图像。当声表面波经过近表面缺陷时,声表面波与近表面缺陷之间产生一种振荡效应,通过近表面缺陷的振荡波幅值存在一个先增大后减小的过程。当声表面波经过不同深度的近表面缺陷时,振荡信号的中心频率存在一定的变化规律。数值仿真结果表明:当近表面缺陷深度从0.1mm变化到0.5mm时,振荡效应产生的振荡信号中心频率从0.4MHz变化到0.76MHz,振荡信号中心频率与近表面缺陷深度呈近似线性关系,这为近表面缺陷的深度检测提供了条件。采用有限元方法模拟了扫查激光源对含有表面缺陷材料进行扫查,由于材料缺陷的存在,通过缺陷位置和无缺陷位置处的纵波最大峰值时间不同。根据纵波最大峰值时间不同,得到纵波B扫图及传播图像,可以实现对材料表面缺陷进行检测。同时搭建一套扫查式激光纵波检测系统,对不同宽度、不同深度、不同形状的表面缺陷进行一维扫查和二维扫查,得到了激光源在材料中产生的纵波B扫图及传播图像。实验结果表明:这种扫查式激光纵波检测技术能对不同宽度、不同深度的表面缺陷定量检测。采用有限元方法模拟了扫查激光源在表面缺陷中激发超声波的理论模型,分析了声表面波在缺陷位置处的波形变化,经过缺陷位置处的声表面波最大幅值存在一个先增加后减小的过程。同时搭建扫查式激光声表面波检测系统,根据超声波传播的互易性原理,得到激光在材料中激发的声表面波传播图像,实现对材料缺陷的可视化检测。针对铝合金材料和奥氏体不锈钢材料中的缺陷位置、大小无法检测这一问题,提出了一种基于Wigner-Ville分析的算法,并与现有的其他算法进行比较。实验结果表明:这种算法不仅可以实现对材料缺陷的位置检测,还可以直观高效的对缺陷大小、形状进行初步检测。同时针对奥氏体不锈钢焊缝中缺陷无法定量检测这一问题,提出了一种快速离散正弦变换的算法,并与传统快速傅里叶变换的算法进行比较分析得出,这种快速离散正弦的算法可以实现对材料缺陷的大小检测。最后搭建了基于非接触的扫查式激光声表面波检测系统,通过还原激光源在材料中产生的声表面波振动图像,初步实现了对材料缺陷的位置及大小进行检测。本文的研究成果将为激光超声技术在材料缺陷检测中的研究提供理论和实验依据,同时也助于激光超声波成像技术在无损检测领域的进一步发展和应用。
[Abstract]:With the rapid development of modern industrial technology, metal materials work at full load for a long time in industrial production, and their structures are unavoidably damaged to varying degrees. Laser ultrasonic technology. Laser ultrasonic technology has attracted extensive attention and research because of its advantages of long distance, non-contact, high sensitivity and wide frequency bandwidth. In this paper, the propagation characteristics of laser-excited ultrasonic wave in material defects are studied by means of finite element numerical calculation and experiment, and scanning laser ultrasonic technology is also used. The research contents and innovations of this paper mainly include the following aspects: According to the thermoelastic motion equation and heat conduction equation of laser-generated ultrasonic wave, the finite element method is used to solve the equation and get the ultrasonic propagation image. When SAW passes through near-surface defects, there is an oscillatory effect between SAW and near-surface defects, and the amplitude of oscillatory waves increases first and then decreases. When SAW passes through near-surface defects with different depths, the central frequency of oscillatory signals changes regularly. The results show that the center frequency of oscillation signal changes from 0.4 MHz to 0.76 MHz when the depth of near-surface defect changes from 0.1 mm to 0.5 mm. The relationship between the center frequency of oscillation signal and the depth of near-surface defect is approximately linear, which provides a condition for the depth detection of near-surface defect. Because of the existence of material defect, the maximum peak time of longitudinal wave at the defect location and the defect-free location is different. According to the different peak time of longitudinal wave, the B-scan and propagation images of longitudinal wave can be obtained, which can be used to detect material surface defect. In this system, one-dimensional scanning and two-dimensional scanning of surface defects with different widths, depths and shapes are carried out, and P-wave B-scan and propagation images produced by laser source in materials are obtained. Finite element method is used to simulate the theoretical model of ultrasonic wave excited by scanning laser source in the surface defect.The waveform change of surface acoustic wave at the defect location is analyzed. Based on the principle of reciprocity of propagation, the propagation image of SAW excited by laser in material is obtained, and the visual inspection of material defects is realized. The experimental results show that the algorithm can not only detect the position of the material defects, but also detect the size and shape of the defects directly and efficiently. The fast discrete sinusoidal algorithm can detect the size of material defects. Finally, a laser surface acoustic wave detection system based on non-contact scanning is built, and the material defects are primarily realized by restoring the surface acoustic wave vibration image produced by laser source. The research results of this paper will provide theoretical and experimental basis for the research of laser ultrasonic technology in material defect detection, and also contribute to the further development and application of laser ultrasonic imaging technology in the field of non-destructive testing.
【学位授予单位】：南京航空航天大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG115.28

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