【作者】 关建飞；

【导师】 倪晓武； 沈中华；

【作者基本信息】 南京理工大学 ， 光学工程， 2006， 博士

【摘要】 本文采用有限元法研究了激光点源和环源在铝板表面热弹激发超声的物理过程，建立了激光线源激发声表面波的有限元模型，进而研究了线光源激发的声表面波与铝板表面缺陷的相互作用过程。 建立了轴对称的有限元模型，模拟了不同光斑半径及脉宽的点光源激发出的超声导波波形，得到了激光能量的时空分布特征对超声信号波形的影响；模拟了环光源激发的超声导波在样品表面及内部的叠加过程，为利用环光源半径控制各种声波模式的聚焦深度提供了理论依据。 基于平面应变的弹性理论，建立了线光源在铝板中激发声表面波的有限元模型；利用该模型研究了线光源激发的声表面波模式组成及波形特征，分析了具有不同半宽线源激光对声表面波位移信号特征的影响。 采用有限元法模拟了激光线源激发的声表面波与不同深度表面缺陷的相互作用过程，讨论了Rayleigh波中心频率的反射与透射系数随表面凹痕深度的变化规律；分析了反射声表面波中各模式成分的到达时间随表面凹痕深度的变化规律，给出了反射表面波中不同模式成分的产生过程。 采用优化的平面应变有限元模型模拟了激光在靠近表面缺陷的不同空间位置上激发的声表面波；波形信号随激发位置改变而呈现的变化特征，证实了移动激光线源扫查技术(SLLS)在检测表面微小缺陷方面具有的优越性；通过分析表面凹痕产生的反射声表面波的模式成分，研究了规则的表面凹痕导致超声波散射及模式转化的物理过程，解释了SLLS技术中位移信号变化的机理。数值模拟结果还表明：当表面缺陷的深度不大于激光激发的Rayleigh波的中心波长时，反射的Rayleigh波位移信号中含有明显的振荡成分；且振荡周期由表面凹痕的深度决定，与激光线源的半宽无关。 本文的研究结果为激光声表面波的理论与数值模拟研究，以及移动激光光源扫查表面微裂纹的检测技术研究提供了理论依据。更多还原

【Abstract】 Finite element method has been employed to simulate the generation of ultrasonic guided wave generated by the pulsed laser, which has been focused in the point- or ring-pattern. In addition, the finite element model of laser line source induced surface acoustic wave has been build, based on which the interaction of the laser generated SAW and surface notch on aluminates plate has been simulated in detail.The temporal and frequency domain characteristics of guided wave generated by the point laser source with varied radii and pulse durations have been investigated by the axisymmetry finite element model, the results demonstrated that the correlation between the time and frequency domain distribution of the guided wave intensity and the temporal and spatial domain characteristics of the laser energy. In addition, the interference effect of the guided wave induced by the ring-shaped laser source have been studied in detail, the results demonstrated that the focal depth of the ultrasonic modes can be controlled by the inner radius of the ring source.Based on plane strain model, line source irradiation on the aluminum plate has been polarized in two dimensional plane, and the plane strain finite element model have been established to simulate line source generated surface acoustic wave and analyze the influence on the characteristic waveform of half-width of line source.Plane strain finite element model have been used to model the interaction between laser generated surface acoustic wave and surface notch with varied depths, where the surface notch with rectangular shape is employed to represent the fatigue crack for the convenience of modeling, in addition, the reflected and transmitted coefficients for a frequency component have been calculated and employed to evaluate the depth of crack. In addition, the surface notch with a series depths and widths have been etched on the top surface to simulate the scattered process of the laser generated surface wave, the formation of two different components of the reflected Rayleigh wave have been investigate through arrival time of them.A scanning laser line source (SLLS) technique is simulated numerically by the optimized finite element model, and changes in amplitude and frequency content have been observed for ultrasound signals generated by laser scanning over a large aluminum block containing a small surface notch. The experimentally observed SLLS amplitude and更多还原