板壳结构中的非线性兰姆波

鉴于常规超声检测技术对分布式材料细微损伤和接触类结构损伤的检测效果不佳,近年来非线性超声技术逐渐引起广泛关注.超声波在板壳结构中通常以兰姆波的形式进行传播,然而由于兰姆波的频散及多模特性,使得非线性兰姆波的理论和实验研究进展缓慢.本文从经典非线性理论出发,总结了源于材料固有非线性诱发的非线性兰姆波的理论和实验两个方面的研究进展,井综述了兰姆波的二次谐波发生效应在材料损伤评价方面的若干应用;从接触声非线性理论出发,讨论了目前由于接触类结构损伤诱发的非线性兰姆波的研究现状.最后展望了非线性兰姆波的未来研究重点及发展趋势.     

基于时间反转法的Lamb波检测技术的研究进展

Lamb波具有传播距离远、衰减小等特点,已被广泛应用于大型板类结构的损伤检测。时间反转是实现超声波聚焦的有效方法之一。本文回顾并总结了近20年来时间反转方法在Lamb波检测领域中的研究进展。针对Lamb波固有的频散和多模态等传播特性,详细论述了时间反转法在板结构Lamb波检测中的应用。最后对基于时间反转法的免基准信号Lamb波损伤检测方法进行了总结,分析其存在的问题并对其应用前景进行了展望。     

Thermal Fatigue Damage Assessment in an Isotropic Pipe Using Nonlinear Ultrasonic Guided Waves

The use of nonlinear ultrasonic waves has been accepted as a potential technique to characterize the state of material micro-structure in solids. The typical nonlinear phenomenon is generation of second harmonics. Second harmonic generation of ultrasonic waves propagation has been vigorously studied for tracking material micro-damages in unbounded media and plate-like waveguides. However, there are few studies of launching second harmonic guided wave propagation in tube-like structures. Considering that second harmonics could provide useful information sensitive for material degradation condition, this research aims at developing a procedure for detecting second harmonics of ultrasonic guided wave in an isotropic pipe. The second harmonics generation of guided wave propagation in an isotropic and stress-free elastic pipe is investigated. Flexible polyvinylidene fluoride (PVDF) comb transducers are used to measure fundamental wave and second harmonic one. Experimental results show that nonlinear parameters increase monotonically with propagation distance. This work experimentally verifies that the second harmonics of guided waves in pipe have the cumulative effect with propagation distance. The proposed procedure is applied to assessing thermal fatigue damage indicated by nonlinearity in an aluminum pipe. The experimental observation verifies that nonlinear guided waves can be used to assess damage levels in early thermal fatigue state by correlating them with the acoustic nonlinearity.     

材料力学性能退化的超声无损检测与评价

材料性能退化总是伴随着某种形式的材料非线性力学行为, 从而引起超声波传播的非线性, 即高频谐波的产生.基于此,材料和结构的超声无损检测与主人技术发展成起来.首先介绍固体介质内的非线性超声波动方程的基础, 并综述了利用超声波传播的非线性特性对结构材料和粘结面的力学性能退化进行无损检测与评价的研究进展.之后对材料在疲劳、拉伸以及蠕变载荷作用下, 其力学性能退化进行超声无损检测与评价的试验研究进行了介绍;综述了超声波传播非线性的机理研究, 以及利用超声波对粘结面的粘结强度及其力学性能退化评估所开展的研究. 最后指出了今后该领域需要进一步研究的问题.      

Experimental and Numerical Investigation of Second Harmonic Generation by Creep Induced Micro-voids

Background: Identification of damage at an early stage is crucial for critical structural components. Objective: Creep induced micro-voids in heat treated polycrystalline pure copper are experimentally and numerically characterized. Methods: This is accomplished by the use of non-linear ultrasonic waves. Numerically, the study is carried out on an elastic material with randomly located micro-voids. The finite difference in time domain method is used. Results: Experimentally it is found that material damage due to micro-voids that are developed before 40% creep life may effectively be detected with nonlinear ultrasonic waves in the low power regime. Increase in second order nonlinearity parameter with percentage creep life is observed along with the corresponding increase in micro-void concentration. Optical metallography and micro-hardness measurements were used to corroborate the experimental results. For the numerical studies, void dimensions selected for study are much lower than the probing wave length. Concentration of micro-voids is varied from 0.01% to 3%. The second order nonlinearity parameter showed a slight increase in the initial stages and an abrupt increase with higher micro-void concentration in simulations. This agrees with experimental trends. Conclusion: The feasibility of detecting micro-voids in the early stages of creep using nonlinear ultrasonic waves is experimentally demonstrated and a simple model with voids is numerically studied to show the sensitivity of the second order nonlinearity parameter to micro voids.

     

金属板结构疲劳损伤二倍频兰姆波检测方法研究

针对重大基础设施安全运行需要,本文进行了金属板结构疲劳损伤非线性兰姆波检测方法研究。基于兰姆波二次谐波产生条件,确定了产生二次谐波积累增长效应的两种兰姆波模态及对应的激励频率。通过有限元仿真,研究了材料性能改变对兰姆波非线性效应的影响,证明了二倍频兰姆波非线性系数对材料性能退化表征的有效性。在此基础上,开展了金属板结构疲劳损伤非线性兰姆波检测实验研究。将极性反转方法应用于疲劳试件检测实验中,有效提高了检测信号中二倍频兰姆波的幅值和信噪比。实验结果表明,两种兰姆波模态对的二次谐波非线性系数均随疲劳损伤增加呈线性增长趋势,但基频S(0,2)模态和二倍频S(0,4)模态对对疲劳损伤检测的灵敏度更高,更适合金属板结构疲劳损伤检测。     

INTERNAL SOLITARY WAVE LOADS EXPERIMENTS AND ITS THEORETICAL MODEL FOR A CYLINDRICAL STRUCTURE

在大型重力式密度分层水槽中, 对内孤立波与圆柱型结构的相互作用特性开展了系列实验. 基于两层流体中 内孤立波的KdV,eKdV和MCC理论, 建立了圆柱型结构内孤立波载荷的理论预报模型, 给出了该载荷理论预报模型中3类内孤立波理论的适用性条件.研究表明, 圆柱型结构内孤立波水平载荷包括水平Froude-Krylov力、附加质量力和拖曳力3个部分, 可以由Morison公式计算, 而内孤立波垂向载荷主要为垂向Froude-Krylov力, 可以由内孤立波诱导动压力计算.系列实验结果表明, 附加质量系数可以取为常数1.0, 拖曳力系数与内孤立波诱导速度场的雷诺数之间为指数函数关系, 而且基于理论预报模型的数值结果与系列实验结果吻合.     

结构中微裂纹与超声波的混频非线性作用数值仿真研究

针对结构中微裂纹检测难题,本文对结构中微裂纹与超声波的混频非线性作用进行了数值仿真研究。基于经典非线性理论,得到了两列超声纵波相互作用产生混频效应的理论条件。通过有限元仿真,研究了两列纵波与微裂纹相互作用产生混频的条件,并分析了界面处静应力、摩擦系数和裂纹方向对混频效应的影响。研究发现,超声波与微裂纹相互作用产生混频非线性效应的发生条件仍符合经典非线性理论下的混频产生条件。裂纹界面处施加的静应力对差频横波幅值有明显影响;当施加静应力与无裂纹模型得到的最大应力值接近时,混频非线性效应最强;裂纹界面的摩擦系数对超声波的混频非线性效应影响较小;透射差频横波传播方向与经典非线性理论预测的理论差频分量方向基本一致,且几乎不受裂纹方向变化的影响,而反射差频横波的传播方向随裂纹方向的改变而有所不同。本文研究工作为微裂纹检出及方向识别做了有益探索。     

Modeling nonlinear Rayleigh wave fields generated by angle beam wedge transducers—A theoretical study

Nonlinear Rayleigh wave fields generated by an angle beam wedge transducer are modeled in this study. The calculated area sound sources underneath the wedge are used to model the fundamental Rayleigh sound fields on the specimen surface, which are more accurate than the previously used line sources with uniform or Gaussian amplitude distributions. A general two-dimensional nonlinear Rayleigh wave equation without parabolic approximation is introduced and the solutions are obtained using the quasilinear theory. The second harmonic Rayleigh wave due to material nonlinearity is given in an integral expression with these fundamental Rayleigh waves radiated by the wedge transmitter acting as a forcing function. Multi-Gaussian beam (MGB) models are employed to simplify these integral solutions and to extract the diffraction and attenuation correction terms explicitly. The effect of nonlinearity of generating sources on the second harmonic Rayleigh wave fields is taken into consideration; simulation results show that it will affect the magnitude and diffraction correction of the second harmonic waves in the region close to the Rayleigh wave sound sources. This research provides a theoretical improvement to alleviate the experimental restriction on analyzing the effects of diffraction, attenuation and source nonlinearity when using angle beam wedge transducers as transmitters.     

Assessment of Heat Treated Inconel X-750 Alloy by Nonlinear Ultrasonics

The nonlinear ultrasonic technique is known as a promising tool for monitoring material states related with micro-structural changes, with improved sensitivity compared to conventional nondestructive testing techniques. It is well known that degradation of material properties is generally accompanied by the increase of material nonlinearity. However, the trend has been rarely investigated in the opposite way for improved material properties. In this paper, nonlinear ultrasonic waves are used to assess the material condition of heat treated Inconel X-750 alloy based on the nonlinear acoustic parameters. Material property testing is conducted to compare the influence of heat treatment for comparison with the nonlinear parameter based prediction. The material properties of specimens are improved by applying heat treatment, with significant decreases in the acoustic nonlinearity. The better the mechanical property achieves via heat treatment, the smaller the acoustic nonlinearity becomes. It can be concluded that the nonlinear acoustic technique can be used to evaluate the effect of heat treatment nondestructively, and to optimize the process, thus providing another indication of the feasibility of using the nonlinear ultrasonic technique for material characterization.     

MEASUREMENT AND ANALYSIS OF LASER GENERATED RAYLEIGH AND LAMB WAVES CONSIDERING ITS PULSE DURATION

In this article, laser generated Rayleigh and Lamb waves are studied by taking into account its pulse duration. The physical model and theoretical solution are presented to predict the corresponding waveforms for aluminum samples under the ablation generation regime.The waveforms of the excited Rayleigh and Lamb waves by laser with selected pulse duration were measured by laser interferometer and analyzed theoretically, and the agreement between measurement and analysis is demonstrated for the validation of the theoretical model and solution.The broadening of the Rayleigh wave and the disappearing of high order Lamb wave modes can be found with the increase of the pulse duration by the laser ultrasonic technique.     

基于频率-波数域偏移的损伤被动成像识别研究

提出了一种应用Lamb波对板结构中多部位损伤源进行被动成像识别的方法.基于Mindlin板理论,推导了板结构中弥散性Lamb波频率-波数域的快速偏移方法,结合爆炸成像原理,对损伤源发出的Lamb波信号进行回传成像.由于损伤源的发生时刻未知,将使用不同假设发生时刻(即不同长度)的Lamb波信号生成一系列图像,通过最小熵原理从中确定最优图像,识别出损伤源的位置和发生时刻.进行了数值仿真研究来表明所提出方法的有效性.     

Klein-Gordon波动方程多波传播的非线性特性

基于双波初值问题，讨论非线性对多波传播的影响。通过选取合适的多重尺度，对Klein-Gordon波动方程进行变形，得到方程的解的多尺度展式首项近似和三波传播时速度相互影响的定量关系，揭示了多波传播的非线性特性；最后，应用Mathematica对波动方程进行数值仿真。研究结果表明，另外多个波的存在会使波的传播速度（相速）超过独自传播时的速度（相速）。     

板背部缺陷的单侧非接触式超声无损检测方法

随着板状结构在石化、航空航天和电力等工业领域中的广泛应用,急需发展相应的无损检测技术,对其结构完整性进行定期评估,以保证结构的安全运行。基于空气耦合换能器的Lamb波技术,可以非接触快速地对板状结构进行扫描,在结构安全检测领域有广阔的应用前景。本文采用基于势函数法的空气耦合板状结构声传播模型,通过理论求解得到其Lamb波临界角随频厚积的变化规律。实验中采取空气耦合换能器激发和接收Lamb波,采用傅里叶变换和信号滤波技术识别S0和A0模式,并通过与理论结果比较进行确认。最后,分别采用S0和A0模式对板背部半通孔缺陷进行定位,结果显示该单侧非接触式超声无损检测方法可用于板状结构背部缺陷的检测。     

Characterization of surface damage of a solid plate under tensile loading using nonlinear Rayleigh waves

This letter reports experimental observation of a direct correlation between the acoustic nonlinearity parameter (NP) measured with nonlinear Rayleigh waves and the accumulation of plasticity damage in an AZ31 magnesium alloy plate specimen. Rayleigh waves are generated and detected with wedge transducers, and the NPs are measured at different stress levels. The results show that there is a significant increase in the NPs with monotonic tensile loads surpassing the material's yielding stress. The research suggests an effective nondestructive evaluation method to track the surface damage in metals.     

Lamb波理论及层合板冲击损伤的实验研究

从理论上分析了板中Lamb波信号的传播特性,并给出Lamb波在板中传播的频散方程。理论分析及实验均表明,Lamb波的频散特性与复合材料结构损伤有着直接的联系,而且最低阶的对称和反对称Lamb波模态对层合板的损伤比较敏感,但应用Lamb波的频散效应监测结构的损伤在检测技术上还难以实现。根据板中导波形成Lamb波的共振原理,板中应力波的幅频特性很大程度上反映了Lamb波的谐振特征。因此,利用压电元件的压电阻抗谱分析应力波的各阶模态频率及振幅对结构损伤的变化,能够反映材料内部损伤与Lamb波的频散特性。文中针对表面粘贴压电元件的层合板智能结构,建立了包含Lamb波谐振模式的压电阻抗计算模型。冲击损伤试件的实验表明,由于结构损伤的出现压电阻抗谱中的模态频率及其阻抗幅值等特征信息将发生变化。因此,引入应力波损伤因子可以对结构冲击损伤的存在和程度进行初步评价。该方法基于结构的机-电动态阻抗特性,不受结构的几何形状限制,测试用的压电元件成本低,方法简单可行,有望在智能结构的健康诊断方面获得应用。     

压电超声电机的非线性动力学研究进展

压电超声电机 (ultrasonic motor, USM) 是一个具有闭环控制的机电耦合强非线性系统,在大功率情况下,会表现出明显的非线性现象．其非线性因素主要表现为: (1) 定子与动子之间的接触非线性; (2) 定子材料非线性; (3) 温度、磨损等因素引起的缓变因素; (4) 驱动电网络中的非线性因素．由于诸多非线性因素的存在,使得 USM 系统具有跳跃、滞后、共振频率漂移、死区和饱和等非线性现象,同时,给 USM 的非线性动力学建模带来了困难．而 USM 闭环控制要求有一个合理的动力学模型,因此,研究 USM 的非线性动力学建模是 USM 研究中的一个重要内容．针对超声电机系统中普遍存在的非线性因素和非线性动力学建模方法进行了归纳,对已有文献中有关超声电机非线性动力学的研究进行了总结,指出了有关该方面的若干研究方向．      

Developments of numerical methods for linear and nonlinear fluid-solid interaction dynamics with applications

本文综述了线性与非线性流固耦合问题数值方法的进展及工程应用. 讨论了四种数值分析方法: (1) 混合有限元-子结构-子区域数值模型, 以求解有限域线性流固耦合问题, 如流体晃动, 声腔-结构耦合, 流体中的压力波, 化工容器的地震响应,坝水耦合等; (2) 混合有限元-边界元数值模型, 以求解涉及无限域的线性流固耦合问题, 如大型浮体承受飞机降落冲击, 船舰的炮击回应等; (3) 混合有限元-有限差分(体积) 数值模型, 以求解不涉及破浪和两相分离的非线性流固耦合问题; (4) 混合有限元-光滑粒子数值模型, 以求解涉及破浪和两相分离的非线性流固耦合问题. 文中推荐分区迭代求解过程, 以便应用现有的固体及流体求解器, 于毎一时间步长分别求解固体及流体的方程, 通过耦合迭代收敛, 向前推进以达问题求解. 文中选用的工程应用例子包含气-液-壳三相耦合, 液化天然气船水晃动, 人体步行冲击引起的声腔-建筑结构耦合, 大型浮体承受飞机降落冲击的瞬态动力回应, 涉及破浪和两相分离的气-翼耦合及结构于水上降落的冲击. 数值分析结果与可用的实验或计算结果作了比较, 以说明所述方法的精度及工程应用价值. 文中列出了基于流固耦合的波能采积装置模型, 以应用线性系统的共振及非线性系统的周期解原理, 有效地采积波能. 本文列出了231 篇参考文献, 以便读者进一步研讨所感兴趣方法.     

Quadratically Nonlinear Cylindrical Hyperelastic Waves: Derivation of Wave Equations for Plane-Strain State

A method is proposed for deriving nonlinear wave equations that describe the propagation and interaction of hyperelastic cylindrical waves. The method is based on a rigorous approach of nonlinear continuum mechanics. Nonlinearity is introduced by means of metric coefficients, Cauchy-Green strain tensor, and Murnaghan potential and corresponds to the quadratic nonlinearity of all basic relationships. For a configuration (state) dependent on the radial and angle coordinates and independent of the axial coordinate, quadratically nonlinear wave equations for stresses are derived and stress-strain relationships are established. Four ways of introducing physical and geometrical nonlinearities to the wave equations are analyzed. For one of the ways, the nonlinear wave equations are written explicitly__________Translated from Prikladnaya Mekhanika,Vol. 41, No. 5, pp. 40–51, May 2005.     

Diffraction of cnoidal waves by vertical cylinders in shallow water

Diffraction of nonlinear waves by single or multiple in-line vertical cylinders in shallow water is studied by use of different nonlinear, shallow-water wave theories. The fixed, in-line, vertical circular cylinders extend from the free surface to the seafloor and are located in a row parallel to the incident wave direction. The wave–structure interaction problem is studied by use of the nonlinear generalized Boussinesq equations, the Green–Naghdi shallow-water wave equations, and the linearized version of the shallow-water wave equations. The wave-induced force and moment of the Green–Naghdi and the Boussinesq equations are presented when the incoming waves are cnoidal, and the forces are compared with the experimental data when available. Results of the linearized equations are compared with the nonlinear results. It is observed that nonlinearity is very important in the calculation of the wave loads on circular cylinders in shallow water. The variation of wave loads with wave height, wavelength and the spacing between cylinders is studied. Effect of the neighboring cylinders, and the shielding effect of upwave cylinders on the wave-induced loads on downwave cylinders are discussed.