激光超声检测带过渡圆角平板表面缺陷的数值研究

为了研究激光激发出的超声波在带过渡圆角的金属平板上的传播规律和检测表面缺陷的方法,采用有限元法模拟了该类平板中的激光超声现象,分析了表面波在圆角区域的传播规律和与表面缺陷的作用过程。数值结果表明:激光激发出纵波、横波和Rayleigh波等,其中Rayleigh波主要存在于表面mm量级,并且在过渡圆角处发生模式转换生成了直达波R′和模式转换波R_R等多种表面波;经过过渡区域后的声波在表面缺陷处发生了反射和透射现象,通过B扫图可以检测缺陷的位置。随着缺陷深度的增加,表面波的透射系数不断减小,且透射波R_t和R_st存在0.5 μs左右的到达时间差,该时间差与缺陷深度近似成线性正相关。数值结果为激光超声检测带过渡圆角的平板表面缺陷提供了有价值的参考。     

黏弹性胶粘涂层-基底结构中激光激发类瑞利波的传播特性

研究黏弹性胶粘涂层/基底结构中激光激发类瑞利波的传播特性。考虑涂层及基底的黏弹性,在频域中建立环氧树脂涂层/铝基底、环氧树脂涂层/铜基底、环氧树脂涂层/泡沫基底等结构中激光激发类瑞利波的有限元数值模型,研究类瑞利波的波形及传播特性,并分析环氧树脂涂层的透明性、厚度、黏性与基底的黏性对类瑞利波传播特性的影响,在此基础上,结合理论计算的色散与衰减曲线进行验证。研究结果表明,涂层的黏性引起类瑞利波高频成分的衰减,而对低频成分没有影响;基底的黏性对类瑞利波的高频与低频成分的衰减均有影响,其中对低频成分影响较大,此外,类瑞利波的模式及频散特性与基底材料密切相关。本研究为反演黏弹性胶粘涂层/基底结构的力学参数及评估结构的粘接质量提供理论指导。      

分层半空间表面非线性瑞利波的激发*

针对非线性瑞利波在均匀分层半空间结构中的激发和传播规律进行研究。根据摄动理论和模态分解将分层半空间结构中瑞利波的二次谐波声场表示为二倍频瑞利波模式的线性组合,经由互易关系得到各模式的展开系数表达式。对不同分层半空间结构中瑞利波二次谐波的激发和传播特性进行讨论,结果表明相速度匹配的瑞利波模式其二次谐波分量随传播距离线性增长,非匹配模式的二次谐波分量则沿传播方向周期震荡传播。此外,文中定义非线性参数表征瑞利波模式产生的非线性程度,这有利于选择出具有明显非线性效应的匹配点,为后续检测工作提供理论依据,具有指导意义。     

黏弹性胶粘涂层-基底结构中激光激发类瑞利波的传播特性

研究黏弹性胶粘涂层/基底结构中激光激发类瑞利波的传播特性。考虑涂层及基底的黏弹性,在频域中建立环氧树脂涂层/铝基底、环氧树脂涂层/铜基底、环氧树脂涂层/泡沫基底等结构中激光激发类瑞利波的有限元数值模型,研究类瑞利波的波形及传播特性,并分析环氧树脂涂层的透明性、厚度、黏性与基底的黏性对类瑞利波传播特性的影响,在此基础上,结合理论计算的色散与衰减曲线进行验证。研究结果表明,涂层的黏性引起类瑞利波高频成分的衰减,而对低频成分没有影响;基底的黏性对类瑞利波的高频与低频成分的衰减均有影响,其中对低频成分影响较大,此外,类瑞利波的模式及频散特性与基底材料密切相关。本研究为反演黏弹性胶粘涂层/基底结构的力学参数及评估结构的粘接质量提供理论指导。     

扫描激光线源技术检测金属表面缺陷位置实验研究

利用扫描激光线源(Scaing Laser Line Source,SLLS)技术,提出一种金属表面缺陷位置检测方法,并搭建实验平台,实验研究该检测方法对金属表面缺陷位置的检测效果。方法中利用扫描激光线源技术对样品表面进行扫查,在样品内部激发超声信号,采用双波混合干涉方法实现对激发信号的探测,根据激发的瑞利波和反射回波与缺陷位置的关系,确定激发点和探测点到缺陷的距离,从而确定缺陷的位置。实验研究中分别以线源位置和探测点位置为基点确定缺陷位置,结果表明以探测点位置确定缺陷位置时,定位相对误差为0.23%。     

基于激光超声临界频率的表面缺陷检测与评价

激光超声能够激发宽频带的表面波,可实现金属材料表面微缺陷的定位分析与深度检测。根据表面波在缺陷处的透射/反射阈值现象,提出了测量表面缺陷深度的临界频率法;基于光弹激发原理搭建了激光超声检测平台,得到了铝合金样品的B扫成像,实现了表面缺陷的定位;结合小波变换分析了表面缺陷的透射与反射波形能量分布,测得透射/反射阈值θ_0=1/4,进而采用临界频率法实现了表面微缺陷的深度估计。此外,还分析了激发点与探测点距离、待测样品材料对透射/反射阈值的影响。结果表明,基于激光超声临界频率法可以实现表面微缺陷的定位及深度检测,且透射/反射阈值大小与表面波传播距离、待测样品材质无关。     

用于铝板缺陷无损检测的激光超声有限元模拟研究

激光超声技术能够在材料表面形成超声波，是实现材料缺陷无损检测的重要环节。借助Abaqus有限元分析工具，基于激光超声热弹机制建立了轴对称铝板的表面缺陷模型，模拟了激光激发产生的表面波在材料中的传播特性及其与铝板缺陷的相互作用过程。数值模拟实验表明，铝板表面缺陷的分布深度值越大，反射波越强，并且当缺陷深度达到一定程度时，反射波的幅值趋于稳定；但缺陷的分布宽度对于反射波的影响则十分有限。所得结论为基于激光超声的材料缺陷的定量检测及识别提供有效的理论依据。     

含孔隙分层半空间中瑞利波的传播特性研究

为了研究含孔隙介质分层半空间中瑞利波的传播规律,分析孔隙介质参数对瑞利波频散曲线的影响,本文进行了数值模拟研究。采用传递矩阵算法,计算了含孔隙分层半空间中一定频率范围内瑞利波所有模式的频散及激发强度曲线,并与均匀弹性固体分层半空间情况作了类比分析,在含孔隙覆盖层的两层模型和含低速孔隙夹层的三层模型下,详细研究了孔隙度、渗透率、层厚度等参数对瑞利波各模式的影响,发现孔隙度及层厚度的变化对频散曲线影响较大,而渗透率的变化对频散曲线影响较小。      

研究激光激发的声表面波与材料近表面缺陷的振荡效应

根据激光激发声表面波的热弹运动方程及热传导方程, 采取有限元技术对方程进行求解, 得到声表面波传播波形图. 当声表面波经过近表面缺陷时, 声表面波与近表面缺陷之间产生一种振荡效应, 通过近表面缺陷的振荡波形幅值存在一个逐渐增加后又逐渐减小的过程. 当声表面波经过不同深度的近表面缺陷时, 振荡信号中心频率存在一定的变化规律. 数值仿真结果表明: 当近表面缺陷深度从0.1 mm到0.5 mm变化时, 振荡效应产生的振荡信号中心频率从0.4 MHz到0.76 MHz变化, 振荡信号中心频率与近表面缺陷深度呈近似线性关系, 这为近表面缺陷的定量检测提供了一种理论基础.     

激光超声的多模式合成孔径聚焦成像仿真分析*

针对激光超声体波在检测材料内部缺陷时反射信号弱、信噪比较差的问题,提出了一种基于激光超声的多模式合成孔径成像方法。通过耦合激光激发超声方向性、干涉仪探测超声方向性、超声波与缺陷作用后声场分布方向性得到声场灵敏度图,再根据声场灵敏度图和不同模式波的传播特性预测不同位置分布缺陷的最佳成像模式波,从而使用多模组合的合成孔径成像方法对缺陷成像。使用有限元方法对该成像过程进行数值模拟来验证其可行性,成像结果与理论预测一致。采用这一缺陷成像方法可减小盲区,消除伪像,从而提高图像质量。     

激光激发声表面波在缺陷板材中散射过程的有限元分析

利用有限元法模拟了金属板材中激光激发的声表面波经过缺陷位置时发生散射的瞬态过程,采用线状激光源作为超声导波的激发源.针对三种不同深度的表面缺陷以及三种亚表面缺陷的模型进行了对比计算,结果显示缺陷的深度及位置对声表面波的时域特征存在显著的影响.表面缺陷深度越深将产生较大幅度的表面反射回波,亚表面缺陷的影响将取决于缺陷上顶面距离板材上表面的距离.因此,数值模拟结果表明通过分析激光产生的表面波形可以判定近表面缺陷的尺寸和所处的位置.     

表面裂纹对掠入射垂直偏振横波散射的动态光弹实验分析

利用动态光弹法研究掠入射垂直偏振横波(SV波)在表面裂纹处的散射声场,给出不同倾斜角度的表面裂纹引起的散射声场分布和简化波前示意图。以垂直表面裂纹为例,有限元仿真和电法测量分别直接地和间接地证明了光弹实验结果的可靠性。表面裂纹引起的各类散射波与裂纹参量密切相关且容易区分,可使用一阶散射声波到达时间反演表面裂纹的长度和倾角,实验测量值与真实值的相对误差在6%以下。      

激光激发黏弹表面波有限元数值模拟

研究黏弹性材料中激光激发的Rayleigh波的传播特征. 考虑到黏弹性材料的黏性特征,在频域内建立黏弹性材料中激光激发Rayleigh波的有限元数值模型. 在验证有限元频域数值模型正确性的基础上,模拟脉冲激光作用在黏弹性材料上激发出Rayleigh波,进而讨论激光激发的黏弹Rayleigh波的传播特征,并比较黏弹性材料与弹性材料中激光激发的Rayleigh波差异,同时分析了材料的黏性劲度参量变化对Rayleigh波特征的影响. 关键词： 表面波 激光超声 有限元方法 黏弹性     

Laser Generation of Surface Waves on Cylinder with a Slow Coating

An analytical model of acoustic field excited by a pulsed-laser line source on a coated cylinder is presented. Surface wave dispersive behaviours for a cylinder with a slow coating are analysed and compared with that of a bare cylinder. Based on this analysis, the laser-generated transient response of the perturbed Rayleigh wave and the higher modes of steel cylinder with a zinc coating are calculated from the model using residue theory and FFT technique. The theoretical result from the superposed waveform of the perturbed Rayleigh wave and higher modes agree well with the waveform obtained in experiment. The results show that the model and numerical method provide a useful technique for quantitatively characterizing coating parameters of coated cylinder by the laser generated surface waves.     

Pulsed Rayleigh wave scattered at a surface crack

This paper investigates Rayleigh wave interaction with machined slots on flat aluminium blocks to simulate surface breaking cracks. Using a finite element method, Rayleigh wave scattering by narrow slots of varied depth ranging from 0.5 mm to 20 mm is calculated. Pulsed wideband Rayleigh waves with a centre frequency of 590 kHz and -6 dB bandwidth of 520 kHz is considered. Reflection and transmission coefficients are calculated and compare well with the published literature. We and other workers have reported enhancement of the measured amplitude or particle velocity of an apparent Rayleigh wave close to a surface defect. In this paper, it is found that the predicted enhancement of in-plane components of particle velocities close to a crack is significantly higher than that of the out-of-plane components of particle velocities which appears to be mainly due to the mode-converted surface skimming longitudinal wave from the crack that has mainly in-plane components near the sample surface. The enhancement of the in-plane particle velocity will be observed regardless of the type of in-plane sensitive ultrasonic detector used. The explanation of the discrepancy of the reflection and transmission coefficients obtained by pulsed and narrow band or pseudo continuous Rayleigh waves is discussed. The later-arriving Rayleigh waves from reverberation along the inside of the crack surface are observed, as has been previously reported by other workers, and this may also be used to gauge slot depth.     

Finite element analysis of Rayleigh wave interaction with finite-size,surface-breaking cracks

The interaction of surface acoustic waves with finite-size, surface-breaking, semi-circular cracks is studied numerically, and experimentally. We focus on the behavior of the reflection coefficient of the Rayleigh wave from such cracks in the far field of the crack, when the depth of the crack is comparable to the wavelength of the interrogating surface wave. The cases in which the depth of the crack is much smaller or much larger compared to the wavelength have been extensively investigated by many authors and are not considered here except for validating the numerical and experimental results in these regimes. The theoretical, finite element, and experimental results presented are in very good agreement over the range were the crack depth is much smaller or much larger compared to the wavelength of the incident Rayleigh wave. In the transition regime, between these two limiting cases, only the finite element and experimental data show good agreement since the theoretical predictions are no longer applicable. In the high crack depth to wavelength ratio (a/lambda(R)) regime, the finite element and experimental results close to the crack approach the limiting value of the reflection coefficient from a 90 degrees corner.     

表面垂直裂痕诱发瑞利波散射的数值分析

激光激发的声表面波为材料表面缺陷的检测提供了有力的工具.针对含缺陷材料在模型边界上的复杂性,建立了基于平面应变的有限元模型并选取了相同厚度但含有不同深度的表面裂痕的单层铝板进行了对比计算,得到了声表面波经过不同深度的表面裂痕时产生的反射及透射信号波形的时域特征.进而引入了基于Wigner-Ville分布理论的时-频分析方法计算裂痕前、后散射的瞬态表面波的能量在时间-频率平面内分布的情形.结果显示：声表面波接近中心频率的某一频率成分在经过深度小于其中心波长的表面缺陷时,随着裂痕深度的增加,对应于该频率的反射系数呈现单调递增的趋势;而透射系数呈现递减的特征,这一结果可以为激光超声检测表面缺陷提供一种定量的表征手段.     

Short range scattering of the fundamental shear horizontal guided wave mode normally incident at a through-thickness crack in an isotropic plate

Interaction of the fundamental shear horizontal mode with through-thickness cracks in an isotropic plate is studied in the context of low frequency array imaging for ultrasonic guided wave nondestructive evaluation with improved resolution. Circular wave fronts are used and the symmetric case where a line from the wave source bisects the crack face normally is considered. Finite element simulations are employed to obtain trends subject to analytical and experimental validation. The influence of the crack length and of the location of source and measurement positions on the specular reflection from the crack face is first examined. These studies show that low frequency short range scattering is strongly affected by diffraction phenomena, leading to focusing of energy by the crack in the backscatter direction. Study of the diffraction from the crack edges reveals contributions due to a direct diffraction at the edges and multiple reverberations across the crack length. A simple diffraction model is shown to adequately represent cracks up to moderate lengths, providing an easy means of estimating the far field of the waves. The presence of multiple diffraction components is quantitatively established and surface waves on the crack face are identified as equivalent to low frequency symmetric modes of rectangular ridge waveguides.     

Nonlinear acoustic response through minute surface cracks: FEM simulation and experimentation

The second harmonic of a Rayleigh wave passing through a minute surface crack has been numerically analyzed by semi-explicit FEM including special elements which account for a nonlinear stress-strain relation at crack surfaces. Minute cracks perpendicular to a free, flat surface close under compressive stress when width of the crack opening is less than the longitudinal amplitude of the Rayleigh wave. Thereafter, compressive and shear stresses are partially transmitted through the closed cracks, whereas tensile and shear stresses are not transmitted through cracks that remain open. This leads to marked nonlinear ultrasonic response. Calculation was performed for an aluminum block having a surface crack. The transverse component of the Rayleigh wave propagating through the cracks shows distorted waveforms, making the second harmonic amplitude clearly noticeable. In an experiment, the second harmonic component of the leaky Rayleigh wave was detected for a simple crack model consisting of two aluminum blocks, by use of a PVDF line-focused transducer. The results of the experiment show that the second harmonic amplitude is a second-order function of the fundamental wave amplitude, and is more pronounced for low compressive stress applied to close the crack surfaces.     

Numerical simulation of evaluation of surface breaking cracks by array-lasers generated narrow-band SAW

Most of the factors limiting the extensive application of laser-based ultrasonic for nondestructive evaluation of surface breaking crack are its poor sensitivity, low efficiency relative to conventional contact ultrasonic methods and limit on the dimension of the cracks. For this reason, a new technique that multiplepulse narrow-band ultrasound generated by laser arrays has been proposed. It is found that crack detection dependent on spectrum of narrow-band ultrasound generated by laser arrays can be operated with low amplitude requirements. In this paper, the narrow-band ultrasound generated by pulse laser arrays interacting with surface breaking cracks has been simulated in detail by the finite element method (FEM) according to the thermoelastic theory. The pulsed array lasers were assumed to be transient heat source, and the surface acoustic wave (SAW) which propagating on the top of the plate was computed based on thermoelastic theory. Then the frequency spectrums of both reflected waves by crack and transmission ones through crack were compared with the direct waves. Results demonstrate that multiple-frequency components of the narrow-band ultrasound were varied with change of the depth of surface breaking cracks significantly, which provides the possibility for precise evaluation of surface breaking cracks.