基于Wigner-Ville分布的焊缝特征导波信号分析方法

超声导波信号的波形具有多模态、波包混叠严重的问题,因此寻找有效的信号分析和模态识别方法成为超声导波焊缝检测技术中的重要研究内容。利用时-频分析方法中的Wigner-Ville分布及其改进方法对实验获得的焊缝特征导波回波信号进行处理,分析各波包对应的导波模态,有效识别缺陷回波信号,提取焊缝的结构和缺陷信息,实现模态分离和缺陷识别。分析结果与焊缝的实际结构和尺寸参数相吻合,该分析方法可为焊缝特征导波信号的分析处理提供现实依据。     

Application of Wigner-Ville distribution to measurements ontransient signals

This paper deals with an application of the Wigner-Ville distribution (WVD) and with usual digital-processing techniques, such as the short-time Fourier transform (STFT), used in dedicated instrumentation for measuring nonstationary signals. The processed real signals are made analytic by means of Hilbert transformations; then suitable implementations of the windowed STFT and of the pseudo Wigner-Ville distribution (PWVD) in the time domain have been performed. Particularly, the fast Hartley transform (FHT) is used to evaluate the PWVD in the real domain. Furthermore, the use of an efficient interpolation algorithm and of a suitable flat-top windowing function is proposed in order to give accurate real-time frequency and amplitude measurements, respectively. With this aim, a dedicated digital system was set up, which uses the LabVIEW software to create virtual instruments (VI), suitable to process the data sequences. Finally, applications of the suggested techniques in analyzing noisy data were also investigated     

基于ITD和WVD的旋转机械故障诊断方法

针对Wigner-Ville分布(WVD)分析多分量旋转机械故障振动信号存在交叉项干扰的问题,提出一种基于本征时间尺度分解(ITD)和WVD的旋转机械故障诊断方法.首先利用ITD将原始振动信号分解为若干个合理旋转(PR)分量,然后运用相关系数原则剔除其中的伪分量,再对每个真实的PR分量进行WVD分析,最后将分析结果重构并提取原信号的时频分布特征.仿真分析结果表明:该方法保留了ITD和WVD的优点,同时能有效抑制WVD的交叉项干扰,分析效果优于平滑伪Wigner-Ville分布(PWVD).同时该文给出转子油膜涡动的故障诊断实例,验证了该方法的工程实用性.     

A fully smoothed XFEM for analysis of axisymmetric problems with weak discontinuities

In this paper, we propose a fully smoothed extended finite element method for axisymmetric problems with weak discontinuities. The salient feature of the proposed approach is that all the terms in the stiffness and mass matrices can be computed by smoothing technique. This is accomplished by combining the Gaussian divergence theorem with the evaluation of indefinite integral based on smoothing technique, which is used to transform the domain integral into boundary integral. The proposed technique completely eliminates the need for isoparametric mapping and the computing of Jacobian matrix even for the mass matrix. When employed over the enriched elements, the proposed technique does not require sub‐triangulation for the purpose of numerical integration. The accuracy and convergence properties of the proposed technique are demonstrated with a few problems in elastostatics and elastodynamics with weak discontinuities. It can be seen that the proposed technique yields stable and accurate solutions and is less sensitive to mesh distortion. Copyright © 2016 John Wiley & Sons, Ltd.     

Adaptive smoothed molecular dynamics for multiscale modeling

In the smoothed molecular dynamics (SMD), the high frequency modes are eliminated from the motion of atoms to enlarge the time step significantly. In some situations, however, rearrangements or atoms disorder may occur. Hence, it is desirable to use MD in localized regions to capture the interesting high frequency motion, while use SMD elsewhere to save the computational cost. In this paper, an adaptive smoothed molecular dynamics (ASMD) is developed. During the simulation process, if the high frequency motions of atoms are dominant in a region, the background grid in the region is refined hierarchically until it is able to capture the high frequency motion of the atoms.     

Recursive linear smoothed Newton predictors for polynomialextrapolation

Newton predictors have considerable gain at the higher frequencies, which reduces their applicability to practical signal processing where the narrowband primary signal is often corrupted by additive wideband noise. Two modifications that can be used to extrapolate low-order polynomials have been proposed. In both approaches, the highest order difference of successive input samples, approximating the constant nonzero derivative, is smoothed before it is added to the lower order differences, reducing the undesired noise gain. The linear smoothed Newton (LSN) predictor is extended in this work by including a recursive term in the basic transfer function and cascading the rest of the successive difference paths with appropriately delayed extrapolation filters of corresponding polynomial orders. This leads to computationally efficient IIR predictors with significantly lowered gain at the higher frequencies. The recursive predictor is analyzed in the time and frequency domains and compared to the other predictors     

光滑有限元的声学研究：时域和频域分析

摘　要：在使用有限元进行声场的数值模拟中,存在着两个主要误差,一个是数值方法中常规的插值误差,另外一个是计算声学中所特有的耗散误差(dispersion error),后者则是影响声学模拟仿真置信度的最重要因素。产生耗散误差的本质原因是由于有限元的数值模型刚度“偏硬”造成的。为了控制耗散误差,最重要的是使数值模型更好的反映真实模型。本文采用了一种基于边光滑的有限元方法(ES-FEM)来对声场的时域和频域进行数值模拟研究。该方法只采用对复杂问题域适应性很强的三角形网格,通过引进基于边的广义梯度光滑技术,能够使得有限元系统得到适当的“软化”。关于时域和频域的算例表明了在使用同样网格的情况下,本方法在声学模拟中的精度都要比有限元模型的高。     

Phase retrieval in digital speckle pattern interferometry by use of a smoothed space-frequency distribution

We evaluate the use of a smoothed space-frequency distribution (SSFD) to retrieve optical phase maps in digital speckle pattern interferometry (DSPI). The performance of this method is tested by use of computer-simulated DSPI fringes. Phase gradients are found along a pixel path from a single DSPI image, and the phase map is finally determined by integration. This technique does not need the application of a phase unwrapping algorithm or the introduction of carrier fringes in the interferometer. It is shown that a Wigner-Ville distribution with a smoothing Gaussian kernel gives more-accurate results than methods based on the continuous wavelet transform. We also discuss the influence of filtering on smoothing of the DSPI fringes and some additional limitations that emerge when this technique is applied. The performance of the SSFD method for processing experimental data is then illustrated.     

基于Wigner-Ville分布交叉项的独塔自锚式悬索桥损伤识别试验研究

为研究以Wigner-Ville分布的交叉项作为指标进行损伤识别效果,探究完整的提取交叉项方法,通过对二重余弦信号的Wigner-Ville分布推导,获得主项与交叉项的位置关系及交叉项震荡特性;据此设计相应的提取方法,将Wigner-Ville分布中各频率段正负能量分别沿时间轴积分,将正负能量之和绝对值作商,通过设定阈值进行筛选获得交叉项能量分布;以工程实例为研究对象,通过对比无、有损状态下对应测点交叉项能量差的突异点识别损伤位置与程度。试验结果表明,交叉项可作为损伤识别指标,且效果显著。     

Power spectrum estimation of complex signals and its application to Wigner-Ville distribution: A group delay approach

In this paper, a method of estimating the power spectrum of a complex signal based on the Group Delay function (GD) is proposed and also applied to the Wigner-Ville Distribution (WVD) to reduce the ripple effect due to the truncation of the autocorrelation sequence. The proposed method is realised by the GD for a complex signal and the modified GD concept. This extends the performance advantages of the modified GD applicable to a real signal, to a complex one. Further, its application to WVD, reduces the truncation/ripple effect without sacrificing any frequency resolution, as nocommon window function is used. Significant improvement in performance, in terms of reduction in variance without any compromise on resolution and higher noise immunity, has been found over those of the periodogram and windowed WVD.     

A boundary elements pseudo heat source method formulation for inverse analysis of solidification problems

 An efficient methodology is presented to solve inverse solidification problems. In the procedure, the latent heat effects are implemented by introducing pseudo heat sources near the moving interface. The material properties can be temperature dependent. To account for the nonlinear part of the governing differential equations, a finite-boundary element formulation is employed. To reduce the oscillations in the solution, a sequential regularization scheme is used. A procedure for proper selection of regularization parameters is presented. To smooth the solutions further, a secondary regularization scheme is introduced and employed. Two complete examples are presented to demonstrate the applicability and the accuracy of the methods. Received: 1 March 2002 / Accepted: 10 February 2003     

Explicit calculation of smoothed sensitivity coefficients for linear problems

A technique of explicit calculation of sensitivity coefficients based on the approximation of the retrieved function by a linear combination of trial functions of compact support is presented. The method is applicable to steady state and transient linear inverse problems where unknown distributions of boundary fluxes, temperatures, initial conditions or source terms are retrieved. The sensitivity coefficients are obtained by solving a sequence of boundary value problems with boundary conditions and source term being homogeneous except for one term. This inhomogeneous term is taken as subsequent trial functions. Depending on the type of the retrieved function, it may appear on boundary conditions (Dirichlet or Neumann), initial conditions or the source term. Commercial software and analytic techniques can be used to solve this sequence of boundary value problems producing the required sensitivity coefficients. The choice of the approximating functions guarantees a filtration of the high frequency errors. Several numerical examples are included where the sensitivity coefficients are used to retrieve the unknown values of boundary fluxes in transient state and volumetric sources. Analytic, boundary‐element and finite‐element techniques are employed in the study. Copyright © 2003 John Wiley & Sons, Ltd.     

A corrective smoothed particle method for transient elastoplastic dynamics

 An incremental approach is presented to model transient, elastoplastic dynamic problems using the corrective smoothed particle method. It uses the corrective first- and second-derivative approximations to solve the nonlinear momentum equations, which is described in terms of displacement increments entirely. This algorithm not only satisfies the nodal completeness condition but also exhibits no integrablity problem. Several 2D examples, including forced vibration, stress wave propagation, and rigid wall impact, are investigated to demonstrate the numerical stability and accuracy of the proposed method. Received 2 May 2000     

An improvement for tensile instability in smoothed particle hydrodynamics

A corrective Smoothed-Particle Method (CSPM) is proposed to address the tensile instability and, boundary deficiency problems that have hampered full exploitation of standard smoothed particle hydrodynamics (SPH). The results from applying this algorithm to the 1-D bar and 2-D plane stress problems are promising. In addition to the advantage of being a gridless Lagrangian approach, improving the above two major obstacles in standard SPH makes it attractive for applications in computational mechanics.     

Completeness of corrective smoothed particle method for linear elastodynamics

 Two different solution algorithms of the corrective smoothed particle method (CSPM) are developed and examined with linear elastodynamic problems. One is to use the corrective first derivative approximations to solve the stress-based momentum equations, with stresses evaluated from the strains. This is an approach that has widely been adopted in smoothed particle hydrodynamics (SPH) methods. The other is new, in which the corrective second derivative approximations are used to directly solve the displacement-based Navier equations. The former satisfies the nodal completeness condition but lacks integrability; on the contrary, the latter is truly complete. Numerical tests show that the latter outperforms the former as well as other existing SPH methods, as expected. Received 1 April 1999     

An edge-based smoothed XFEM for fracture in composite materials

In this work, an edge-based smoothed extended finite element method (ES-XFEM) is extended to fracture analysis in composite materials. This method, in which the edge-based smoothing technique is married with enrichment in XFEM, shows advantages of both the extended finite element method (XFEM) and the edge-based smoothed finite element method (ES-FEM). The crack tip enrichment functions are specially derived to represent the characteristic of the displacement field around the crack tip in composite materials. Due to the strain smoothing, the necessity of integrating the singular derivatives of the crack tip enrichment functions is eliminated by transforming area integration into path integration, which is an obvious advantage compared with XFEM. Two examples are presented to testify the accuracy and convergence rate of the ES-XFEM.     

Consistent multiscale analysis of heterogeneous thin plates with smoothed quadratic Hermite triangular elements

A consistent multiscale formulation is presented for the bending analysis of heterogeneous thin plate structures containing three dimensional reinforcements with in-plane periodicity. A multiscale asymptotic expansion of the displacement field is proposed to represent the in-plane periodicity, in which the microscopic and macroscopic thickness coordinates are set to be identical. This multiscale displacement expansion yields a local three dimensional unit cell problem and a global homogenized thin plate problem. The local unit cell problem is discretized with the tri-linear hexahedral elements to extract the homogenized material properties. The characteristic macroscopic deformation modes corresponding to the in-plane membrane deformations and out of plane bending deformations are discussed in detail. Thereafter the homogenized material properties are employed for the analysis of global homogenized thin plate with a smoothed quadratic Hermite triangular element formulation. The quadratic Hermite triangular element provides a complete C¹ approximation that is very desirable for thin plate modeling. Meanwhile, it corresponds to the constant strain triangle element and is able to reproduce a simple piecewise constant curvature field. Thus a unified numerical implementation for thin plate analysis can be conveniently realized using the triangular elements with discretization flexibility. The curvature smoothing operation is further introduced to improve the accuracy of the quadratic Hermite triangular element. The effectiveness of the proposed methodology is demonstrated through numerical examples.