激光激发表面波与亚表面缺陷作用的理论研究

为了研究激光激发表面波与亚表面缺陷的作用机理，实现激光超声技术对该类型缺陷的定量检测，论文基于激光超声热弹机制，采用有限元方法模拟了激光激发表面波与亚表面矩形缺陷之间的相互作用，进而探讨缺陷的埋藏深度及其纵向尺寸对表面波的影响。首先模拟了激光激发表面波与矩形缺陷前沿（波与缺陷最先作用的纵向边沿）的相互作用，分析了埋藏深度对波形的影响，提取了缺陷作用的散射回波特征并分析其成因。然后对一定的缺陷埋藏深度，通过改变缺陷纵向尺寸，计算和分析了纵向尺寸对散射回波特征的影响。研究结果表明，散射回波的特征点到达时间与缺陷的埋藏深度和纵向尺寸有关，可能由此实现其反问题缺陷纵向尺寸的估算。

Theoretical Study of the Interaction of Laser Excited Surface Acoustic Waves with Subsurface Defects

Nondestructive testing of subsurface defects plays an important role in ensuring the performance and safety of a component or a structure. However, the subsurface defects are often deeply buried and invisible, and therefore, are difficult to be identified. Laser ultrasonic technology (LUT), which is further developed from the traditional piezoelectric ultrasonic technology, has a series of advantages, such as non-contact, broadband, high sensitivity and high spatial resolution. And LUT has been used in detecting surface defects for a long time. The interaction of laser excited surface acoustic wave (LESAW) with surface defects is well understood, but its interaction with subsurface defects is still unclear, even though these two groups of defects have some similar features. In this paper, the interaction of LESAW with subsurface defects has been studied in detail, and the quantitative detection of such defects by LUT has been discussed. The finite element method and the thermoelastic model of laser ultrasound have been employed to simulate the interaction, and to discuss the influences of burial depth and vertical size of the rectangular defect. Firstly, simulation has been made for the initial interaction of LESAW with the frontier of subsurface defect at the vertical edge. The impact of burial depth on the waveform has been analyzed, and the scattered feature caused by the defect has been extracted and interpreted. It is shown that the arrival time of the reflected surface wave is insensitive to the burial depth when the depth is less than the center wavelength. Then for a fixed burial depth, the influences of vertical size and dimension on scattered echo have been simulated and analyzed. It is found that there exists a specific linear relationship between the arrival time of reflected surface wave and the vertical size of defect. At last, based on the results of the study, an equation for the evaluation of vertical size of defect has been presented. The findings on the interaction between LESAW and subsurface defects may provide possible means for nondestructive testing.