基于频散补偿和分数阶微分的多模式兰姆波分离

由于兰姆波的多模和频散特性,实际检测时在同一激发频率下存在多种模式的混合信号,而各模式信号有不同的频散特性,使得在时频混叠的情况下兰姆波的检测变得十分复杂.本文在频散补偿的基础上,通过时延函数建模,依靠不同模式频散趋势的差异性,将时频混叠信号的分离问题转化为部分模式混叠信号的分离问题.基于分数阶微分的理论,用信号幅值谱分数阶微分极大值和对应频率分别与微分阶次拟合多项式实现特征参数的提取并依靠特征参数重建幅值谱.结合相位谱重构时域信号以实现部分混叠信号中频散补偿后的模式的分离.最后恢复频散获得分离后的兰姆波信号.仿真和实验结果表明,本文方法不仅可以实现时频混叠多模式兰姆波信号的分离,更能保证分离精度,有助于复杂多模式频散信号的分离与处理的进一步研究.     

基于赛利斯模型和分数阶微分的兰姆波信号消噪

为降低噪声对超声兰姆波检测信号的影响,提高信噪比和增加特征提取的精度,提出了一种赛利斯模型下分数阶微分方法用于超声兰姆波信号去噪.该方法对含噪声的兰姆波信号幅值谱进行各阶分数微分,用赛利斯分布作为待处幅值谱的模型,提出了幅值谱分数阶微分最大值和过零点与微分阶数的拟合三次关系式,建立了幅值谱特征参数的计算式来提取特征参数和重建原始信号的幅值谱,并结合相位谱重构去噪后的兰姆波信号.仿真结果表明,该方法可以有效地提高兰姆波信号甚至微弱兰姆波信号的信噪比,同时降低均方误差和平滑度.实验结果显示,与小波去噪和集合经验模态去噪方法相比,该方法在没有信号先验知识的情况下,可以更有效地去除兰姆波信号的噪声,同时更好地保留主信号的细节特征.因此,本文提出的方法可以有效地去除兰姆波检测信号中混入的噪声.     

超声导波的频散补偿与模式分离算法研究

超声导波已被广泛应用于无损检测与评价,针对超声导波应用中普遍存在的模式混叠与频散问题,本文在采用频谱相位与信号时延函数建模导波频散信号传递系统的基础上,提出了一种单一模式的频散补偿算法。进一步地,通过将选择性频散补偿技术与导波模式分离相结合,本文算法克服了传统补偿算法所不能解决的多模式导波频散问题。板中的Lamb波A0,A1和S0混合模式仿真分析表明,本算法不仅可用于单一导波模式信号合成与频散补偿,而且可在实现多模式混合导波信号选择性频散补偿基础上,解决多模式导波信号中单一模式提取与分离。本文研究有助于超声导波多模式频散信号的分析与处理.      

超声兰姆波的时频分析

声-超声方法在被测板件厚度较小时,板中传播的超声波模式主要是多模式的兰姆波。由于兰姆波的复杂性,声-超声方法回避了对信号进行进一步深入的分析,而采用所谓“应力波因子”对材料中能量耗散进行相对度量。当多模式与频散同时存在时,信号的识别异常困难,本文采用时-频分析的方法对声-超声信号进行了分离,只需一次测量就可满足分析要求。通过对兰姆波信号的伪Wigner-Ville分布进行时频重排和图像特征提取,能够精确地获得兰姆波的局部群速度频散曲线。     

板状结构Lamb波频散补偿与聚焦接收成像检测

对板状结构中Lamb波的频散和多模现象进行了理论分析,采用有限元仿真方法获取了Lamb波与缺陷作用后的回波信号,对信号进行聚焦接收处理,即在一定距离范围内利用导波频散特性对各个模式信号进行频散补偿,提取补偿后信号的幅值,结果表明当所得信号幅值最大时,所对应的补偿距离等于缺陷与换能器之间的实际距离.利用这一结论,提出了以...     

一种定征复合板材粘接层性质的非线性超声兰姆波方法

借助于兰姆波频散曲线及导波激发的模式展开分析方法,对基频兰姆波时域信号及二次谐波时域信号的发生过程进行了直观的论述。结合Ritec-SNAP系统的测量功能,详细分析了二次谐波时域脉冲包络积分表达式的物理意义;该积分表达式可表征基频兰姆波时域脉冲传播过程中的二次谐波发生效率,以及基频与二倍频兰姆波模式之间的频散程度。在基频与二倍频兰姆波相速度相等(或近似相等)的频率附近,实验观察到显著的且无模式混叠的二次谐波信号,显示出在兰姆波的传播过程中的确可存在强烈的非线性效应。对于三种不同粘接情形的复合板材,实验结果表明,采用本文引入的非线性兰姆波应力波因子,结合二次谐波幅频曲线峰值所对应的频率值,可有效地对板材粘接层性质进行表征。     

分数阶van der Pol振子的超谐共振

以含分数阶微分项的van der Pol振子为对象,研究其超谐共振时的动力学特性.首先,通过平均法得到了系统的一阶近似解,提出了超谐共振时等效线性阻尼和等效线性刚度的概念,研究了分数阶微分项的系数和阶次以等效线性阻尼和等效线性刚度的形式对系统动力学特性的影响.随后,建立了超谐共振时定常解的幅频曲线的解析表达式,得到了超谐共振周期响应的稳定性判断准则并提出等效非线性阻尼和非线性稳定性条件参数的概念.最后,通过数值仿真比较了分数阶与整数阶系统的幅频曲线,分析了分数阶微分项的系数和阶次对响应幅值、幅频曲线以及系统稳定性的影响.     

基于短时分数阶傅里叶变换域滤波的多项式相位信号时频检测

针对低信噪比下多个多项式相位信号提出了一种基于短时分数阶滤波的时频检测方法。分数阶傅里叶变换作为一种线性变换,能够实现线性调频信号检测与分离。而多项式相位信号在短时间内可以由线性调频信号提供良好的近似,故可以采用短时分数阶傅里叶变换实现多线性调频分量的检测与分离。对每个短时信号的时频分析进行叠加组合,即得到多个多项式相位信号的时频分析检测。计算机模拟仿真证明了此方法的有效性。     

兰姆波非线性效应的实验观察(Ⅱ)

基于Ritec-SNAP系统对固体板中传播的兰姆波的非线性效应进行了实验观察。根据导波的模式展开分析方法和兰姆波的频散曲线,简述了兰姆波的积累二次谐波发生条件。采用一定倾角的斜劈换能器在固体板表面激发和接收兰姆波的基波和二次谐波时域信号,阐述了兰姆波的基波、二次谐波时域脉冲包络的积分振幅的物理意义,在固体板表面分别测量了不同传播距离的兰姆波的基波、二次谐波的幅频曲线。在兰姆波具有非线性效应的频率值附近,分析了兰姆波的二次谐波振幅随传播距离的变化关系。实验结果进一步证明了兰姆波在一定条件下具有强烈的非线性效应,其二次谐波表现出随传播距离积累增长的性质。     

一种锐化误差小波EDXRF光谱解析方法研究

在轻元素自身和实测元素间的特征X射线相互影响之下，受仪器能量分辨率的制约，实测X射线荧光光谱会产生严重重叠。以色谱分离度Rs判定谱峰重叠程度，针对Rs低于0.3的重叠峰，提出一种解析EDXRF光谱的新方法，并对模拟X射线荧光光谱进行了新方法的验证。首先，详细介绍基于四阶导的峰锐化法和提出误差小波变换。通过仿真结果发现：当Rs=0.27时，两种方法皆不能单独实现重叠谱峰的解析与识别；然而，原始信号在四阶峰锐化法处理后保留了峰位特征的同时，还出现了Rs明显增大的有利现象。因此，只需要通过调节四阶峰锐化法的权重完成对低分离度重叠峰的初步锐化处理，再对锐化后的信号进行误差小波变换，结果实现了对模拟重叠峰的分解，证明了结合后的新方法(锐化误差小波变换)针对极低分离度的重叠谱峰具有强大的分解能力。对两组重叠谱峰采用叠加的高斯函数进行模拟，分别是Mn的K_β能量峰与Fe的K_α能量峰的重叠光谱(Rs=0.19)以及Al的K_α能量峰与其K_β能量峰的重叠光谱(Rs=0.11)。用新方法对谱线进行处理，实现了重叠峰分解，结...     

Separation of multimode Lamb waves overlapped in time and frequency domains by using fractional differential

To separate multimode Lamb wave overlapped both in time and frequency domains,a multimode Lamb wave signal separation method by using fractional differential is proposed.The Tsallis function is used as the model of the amplitude spectrum.The amplitude spectra of Lamb wave signals are divided into fractional order differential.The characteristic parameters of each mode are extracted by using the polynomial between the maximum amplitude and the differential order as well as the polynomial between the peak frequency and the differential order.The corresponding phase spectrum of each mode is derived from the dispersion characteristics.The signal of each mode is constructed through the amplitude spectrum and phase spectrum.The simulated results of S1 and A1 modes in a 1 mm-thick steel plate show that the relative errors of amplitude and bandwidth of both modes are less than 6%,and that the relative errors of central frequency of both modes are less than 0.25%.Therefore,this method could be used to separate the time-frequency overlapped multimode Lamb wave signals.It is helpful for the analysis and recognition of multimode Lamb wave signals.     

Wideband dispersion removal and mode separation of Lamb waves based on two-component laser interferometer measurement

Ultrasonic Lamb waves are considered as a sensitive and effective tool for nondestructive testing and evaluation of plate-like or pipe-like structures. The nature of multimode and dispersion causes the wave packets to spread, and the modes overlap in both time and frequency domains as they propagate through the structures. By using a two-component laser interferometer technique, in combination with a priori knowledge of the dispersion characteristics and wave structure information of Lamb wave modes, a two-component signal processing technique is presented for implementing dispersion removal and mode separation simultaneously for two modes mixture signals of Lamb waves. The proposed algorithm is first processed and verified using synthetic Lamb wave signals. Then, the two-component displacements test experiment is conducted using different aluminum plate samples. Moreover, we confirm the effectiveness and robustness of this method.     

Model-based analysis of dispersion curves using chirplets

Time-frequency representations, like the spectrogram or the scalogram, are widely used to characterize dispersive waves. The resulting energy distributions, however, suffer from the uncertainty principle, which complicates the allocation of energy to individual propagation modes (especially when the dispersion curves of these modes are close to each other in the time-frequency domain). This research applies the chirplet as a tool to analyze dispersive wave signals based on a dispersion model. The chirplet transform, a generalization of both the wavelet and the short-time Fourier transform, enables the extraction of components of a signal with a particular instantaneous frequency and group delay. An adaptive algorithm identifies frequency regions for which quantitative statements can be made about an individual mode's energy, and employs chirplets (locally adapted to a dispersion curve model) to extract the (proportional) energy distribution of that single mode from a multimode dispersive wave signal. The effectiveness of this algorithm is demonstrated on a multimode synthetic Lamb wave signal for which the ground-truth energy distribution is known for each mode. Finally, the robustness of this algorithm is demonstrated on real, experimentally measured Lamb wave signals by an adaption of a correlation technique developed in previous research.     

金属板疲劳损伤非线性兰姆波混频检测

针对金属板结构安全运行需要,开展了金属板结构疲劳损伤非线性兰姆波混频检测方法研究.通过数值仿真,研究了两列A0兰姆波与材料损伤间的非线性相互作用.结果 表明,两列共线A0兰姆波在结构材料损伤处产生单向传播的和频S0波,且和频波幅值随传播距离具有积累增长效应.对不同疲劳程度金属板试件进行了共线混频兰姆波检测实验,结果表明...     

An oscillator using high order Lamb wave modes

An oscillator using high order Lamb wave modes is presented. Due to the multimode property of Lamb waves, high frequency oscillators using high order mode Lamb waves can be fabricated. To select the operating mode of the Lamb wave oscillator, a high order mode selector is inserted into the feedback loop. An oscillator using the 13th antisymmetric mode (a₁₃) in Lamb wave is achieved in experiments and the oscillating frequency is 5.30 times higher than that of the a₀ mode excited by the interdigital transducer.     

Combined spectral estimator for phase velocities of multimode Lamb waves in multilayer plates

A novel combined spectral estimate (CSE) method for differentiation and estimation the phase velocities of multimode Lamb waves whose wave numbers are much close or overlap one another in multiplayer plates is presented in this paper, which based on auto-regressive (AR) model and 2-D FFT. Simulated signals in brass plate were processed by 2-D FFT and CSE. And experiments are performed by using two conventional angle probes as emitter and receiver on the same surface of three-layered aluminum/xpoxy/aluminum plates, which include symmetrical and unsymmetrical plates. The multimode Lamb waves are excited in these laminates, and the received signal is processed by 2-D FFT and CSE, respectively. The results showed that the phase velocities of multimode signals whose wave numbers are much closed cannot be differentiated by 2-D FFT, but CSE has strong spatial resolution. Compared the measured phase velocities with the theoretical values, the error is smaller than 2% on the whole. It promises to be a useful method in experimental signals processing of multimode Lamb waves.