【作者】 何跃娟；

【导师】 朱日宏； 沈中华；

【作者基本信息】 南京理工大学 ， 光学工程， 2006， 博士

【摘要】 本文建立了脉冲激光线源非轴对称作用在圆管状金属材料上热弹激发超声波的理论模型，并建立了该模型的有限元数值计算方法。对圆管中激光产生材料中的温度场、激光产生材料中的应力场以及激光激发材料中超声波的产生机理、波形特征及沿管周向的传播特征等方面进行了理论分析和数值模拟研究。 根据经典热传导方程，分析了材料在激光辐照下的光学及热物理性能的变化情况，得到了脉冲激光线源非轴对称作用在圆管状材料表面时材料内部的瞬态温度场分布。数值结果表明：材料内部的瞬态温度场分布在一个很小的局部区域，该区域存在很大的温度梯度，且分布极不均匀，材料热物理参数随温度的变化对整个瞬态温度场的影响很大。 在热分析基础上，根据经典线弹性动力学方程，应用准静态过程中线弹性有限元数值模拟计算激光热应力的方法，得到了脉冲激光线源非轴对称作用在圆管状材料表面时材料内部的应力场分布。冯．密赛斯应力随周向和法向的变化情况和温度场的变化类似，周向正应力在受热区域附近为压应力，远离激光辐照区域为拉应力，热应力是激光超声的源。 分析了纳秒量级的激光超声产生的热结构耦合的物理过程，研究了网格大小和时间步长参数和有限元求解的稳定性的关系，建立了优化的有限元模型，数值模拟圆管中激光非轴对称热弹激发超声的过程以及超声导波沿周向的传播过程。数值模拟结果表明：薄圆管中的激光超声导波是典型的类似于薄板中类Lamb波，且其中S₀模式是传播速度最快且频散较小的导波；厚圆管表面声波中显著的波为掠面纵波、头波和瑞利波，第一个瑞利波脉冲的极性在传播过程中发生了倒转现象，瑞利波沿圆周向传播时180°范围内瑞利波脉冲正好倒转了一次；忽略衍射效果，只考虑色散效应时，圆管中第一个瑞利波脉冲的极性和试样的尺寸无关，仅和探测点离波源的角度相关。 本文的研究结果可为圆管材料的无损检测及圆管材料中的激光超声的实验研究提供理论依据，并为圆管状复合材料或新型材料中激光超声特性的研究提供基础。更多还原

【Abstract】 In this paper, a theoretical model of thermo-elastic generation of laser-ultrasound in hollow cylindrical metallic materials has been studied and established. By using the finite element method, the transient temperature fields and the thermal stress fields generated by a pulsed line-source laser in hollow cylinders are analyzed. And using the same method, the generation of laser ultrasonic wave and its propagation, waveform characteristics are also studied by theoretical analysis and numerical simulation.In accordance with the classic thermal conduction equation, this paper analyzed the changing of material’s optical and thermal physical properties under laser irradiation. The transient temperature fields are obtained. The numerical results denoted: the transient temperature field inside the material is being distributed in a very small-localized region, where great temperature gradient exists quite unevenly.In accordance with the linear elastic dynamics equation, a method of linear elastic finite element numerical simulation laser thermal stress in a quasi-static process is used. The stress fields are obtained based on the thermal analysis above. The distribution of Von Mises stress is similar to the temperature distribution, there appears circumferential compressive resistance near the laser irradiation range and circumferential tensile stress far away the laser irradiation range. The thermal stress is the origin of the laser-ultrasound.Since the temperature field induces the stress and displacement fields and the effect of the stress and displacement fields on the temperature field is negligibly small, the sequential field coupling is used in the present modeling. An optimized finite element is developed to simulate the laser thermo-elastic generation and propagation of surface acoustic waves in hollow cylinders based on a well understanding of the influences of element size and integration time step on the solution stability. A series of the time domain waveforms of the surface normal displacements are shown according to the angle between the source and the detection point from 5° to 180°. The numerical results denoted: in a cylindrical shell, the typical ultrasonic guided wave is Lamb wave like in a thin plate and the So mode is the fastest transmitting and less dispersion of guided waves; the main features of the surface waves in a thick hollow cylinder are three kinds of waves-- the surface skimming longitudinal wave , head wave and the Rayleigh wave, the first Rayleigh pulse’s polarity changes gradually with the propagating angle increment, and reverses one更多还原