【作者】 王纪俊；

【导师】 倪晓武； 沈中华；

【作者基本信息】 南京理工大学 ， 光学工程， 2008， 博士

【摘要】 本文采用有限元方法建立了用于研究脉冲激光在不同性质固体材料中热弹激发超声波的数值模型,进而对不同性质固体材料中的激光超声体力源和超声波的产生机理、传播特征以及力源和波形之间关系进行了数值模拟研究。在同时考虑脉冲激光的时间分布和空间分布、介质的热传导效应和激光在材料中的光穿透效应以及材料性质随温度变化的条件下,分别研究了金属材料和非金属材料中激光热弹超声力源和超声波的产生和传播过程,分析了金属和非金属材料中激光超声波的特点。结合对心波形,得到了不同力源模型下的应力场和对应超声波波形间的物理关系。利用有限元方法研究了脉冲激光在透明薄膜/基底系统中激发超声力源和超声波的产生和传播过程。分析了该系统中激光激励产生埋藏体力源的特点以及和超声波波形之间的关系,研究了透明薄膜厚度对超声力源和超声波波形的影响及规律。结果表明,透明薄膜厚度的增加对系统温度场和应力场的影响效果并不相同;透明薄膜的约束使热弹应力场的轴向应力增大,但透明薄膜厚度超过一定值后,继续增加厚度对激光超声力源的形成不产生新的影响。研究了脉冲激光在具有横观各向同性的单向纤维加强复合材料平行薄板中激发超声力源的产生和传播过程,利用应力分布和薄板变形的时间演变过程分析了激光超声Lamb波不同模态的力学产生和传播过程。采用上述方法研究了单向纤维加强复合材料厚板中超声体波的产生和传播过程,分析了横观各向同性材料不同传播平面内的应力场和超声波的产生和传播特征。结果显示,激光超声波在各向异性材料中的传播特征与超声波的传播方向有密切关系。本文所建立的不同性质固体材料中的激光超声波和材料参数、激光参数间的定量关系,可用于不同性质材料中的激光超声产生和传播规律的研究,为金属、非金属、薄膜/基底系统和复合材料的激光超声无损检测和评估及参数优化提供理论依据。更多还原

【Abstract】 A numerical model dealing with laser-generated ultrasound in solid materials with different properties is presented by using the finite element method (FEM). By means of this model, the features of laser buried force and consequent ultrasound generated by pulsed laser, as well as the relationship between the force source and the ultrasonic waveforms are studied numerically.Taking into account of the finite width and duration of the laser source, the effects of thermal diffusion and optical penetration, as well as the temperature dependence of material properties, the generation of laser ultrasonic force source and propagation of laser-generated ultrasound in metallic material and non-metallic material are simulated, respectively, and waveform characteristics of ultrasound waveforms in metallic materials and non-metallic materials are analyzed. The relationship between the different force models and ultrasound waveforms are presented based on epicenter waveforms.The generation of buried force and propagation of consequent ultrasound in transparent coating/substrate system are studied using FEM. The relationship between the buried force source and the ultrasound waveforms is analyzed, and the influence of transparent coating thickness on force source and ultrasound waveforms is presented. The numerical results indicate that the increase of the coating thickness has different effects on temperature field and stress field in the transparent coating/substrate system. Due to the constraint of transparent coating, the axial stress increases with the increase of the coating thickness, however, when the transparent coating thickness reaches a certain value, the laser-generated ultrasonic buried force source does not change any longer.The generation of laser ultrasonic force source and propagation of laser-induced Lamb waves in thin unidirectional fiber-reinforced composite plate, transversely isotropic, is simulated. The mechanical generation process of Lamb wave is presented intuitively by analyzing the stress field propagation and the deformations of plate in detail. By the same method, the generation and propagation of laser-generated ultrasonic bulk waves in thick composite plate are also studied. The propagation features of the laser ultrasonic stress filed and ultrasound waves in principal planes are analyzed. The numerical results indicate that the features of laser-generated ultrasound waveforms have closely relation with the propagating direction in anisotropic specimen.The method in this paper provides insight into the generation and propagation of the laser-generated ultrasound wave in different property solid materials and establishes quantitatively relationship between the ultrasound waveform and the laser input as well as the specimen parameters. It presents theoretical basics to optimize ultrasonic signal generation in nondestructive measurement and evaluation by using laser-induced ultrasonic in metallic material, non-metallic material, coating/substrate system and composite materials.更多还原