相干梯度敏感方法的改进及应用

介绍了光测力学方法:相干梯度敏感(CGS-Coherent Gradient Sensing)技术的基本原理,并分析了原始方法的不足,在此基础上提出了改进的相干梯度敏感方法。改进方法利用原始方法的实验光路,仅需要拍摄同一试件在不同载荷下的多幅干涉条纹图(至少三幅),就可以精确获取相干梯度敏感干涉条纹图任意位置点的条纹级数。改进方法不需要额外的实验装置,也不需要进行复杂的图像处理过程。最后采用改进方法对静态Ⅰ型裂纹尖端变形场进行了三点弯曲实验,并提取了应力强度因子。实验结果表明改进方法能够显著减少由于条纹提取带来的误差,从而提高相干梯度敏感方法的测量精度。     

气流场ESPI载频条纹图的消噪处理与相位提取方法

为拓展ESPI方法在流动可视化测量技术中的应用,对气流场ESPI载频条纹图进行了傅立叶变换、数字全低通滤波,和傅立叶逆变换的处理以消除散斑噪声,并直接将其替换为原散斑相关条纹图,进而成为保留了流场信息的高对比度、低噪声的灰度条纹图,便于提取原散斑图中流动相位信息。根据模拟数据确定了由这两种条纹图互换所引起的空间相移量和相应流场测量中的修正量。提出了用灰度扫描法对互换后的条纹图进行一维灰度扫描,由灰度计算得出气流扰动的一维相位差分布和流场一维温度分布;同时也用FTP法对该图进行了二维相位展开和温度分布计算,亦可对多帧条纹图进行时间轴上的逐点相位提取以计算其他流场参数。结果表明,计算数据与实测符合,方法简单易行,为在气流场参数测量中应用动态ESPI方法提供了一种实用有效的方法。     

希尔伯特变换实时全息干涉条纹相位提取

实时全息干涉法可以观察记录整个测试过程中条纹图的动态变化,传统相位提取算法只适合于静态干涉条纹图相位的提取.根据实时全息干涉条纹和希尔伯特变换的特点,提出了利用希尔伯特变换提取实时全息干涉条纹相位值的方法,采用了高通滤波的方法减少背景光强的影响,对铝片受力变形实验中实时全息干涉条纹的相位变化分布进行了提取.实验表明:希尔伯特变换法适合于动态条纹的相位提取,可以自动提取实时全息干涉测量过程中全场各点在任意两个时刻间的相位变化值,且测量结果与实时全息干涉条纹人工分析结果一致.     

干涉图条纹数据的快速自动采集

干涉图条纹的数据采集是干涉图数据处理的前提和基础，提出了一种有效的从数字化干涉图中由计算机自动采集干涉条纹数据的算法，该算法的主要特点是首先用尺度验证技术对灰值干涉图进行干涉条纹细化，然后利用细化后的二值干涉图对干涉条纹进行条纹跟踪和数据采集。     

爆破模型的动态光测力学方法研究综述

动态光测力学方法是爆破模型研究十分重要的工具,各种光测力学方法对爆炸应力、应变、位移场的测量及相应研究有不同的作用.在总结大量国内外文献的基础上,对爆破模型研究中使用的动态光测力学方法进行了回顾和评述,阐述了它们在不同阶段实验技术和理论研究的进步.重点介绍动光弹、激光全息干涉法在爆破研究中的运用及最新进展,简略探讨了动态焦散、云纹、云纹干涉法等其它光测力学方法在动载研究中的使用情况.通过分析上述文献,指出了这些方法的运用前景和发展方向.     

利用数字散斑照相术测量面内位移

采用数字图像处理技术直接对双曝光散斑图进行全场分析,获取了杨氏条纹图．通过对条纹图的滤波增强、二值化、细化等操作,提取了条纹周期,实现了物体面内微小位移的测量．实验结果表明,该方法可获得高对比度的散斑干涉条纹,经过数字图像处理,条纹细化无断点和毛刺,有利于条纹判读,提高测量精度,其微位移测量误差小于4％．由于散斑图及条纹图像均是在计算机内存储、处理,实现了散斑照相术的数字化,具有一定的应用价值．     

散斑图样条纹自动分析的一种方法

本文论述散斑图样条纹的一种新的图象处理方法密度滤波和特征提取。介绍了散斑计量原理和基本的图象处理方法,以及逐点分析散斑图的自动系统。用该系统测得的面内位移和离面空间位移,其测量误差分别小于1.6%和2.3%。     

利用激光干涉测金属线胀系数

文中分析了洛埃镜干涉实验中 ,由金属受热膨胀引起的洛埃镜位置变化对干涉条纹间距的影响 ,导出金属的线胀系数与干涉条纹密度变化之间的关系。介绍了用CCD图象处理技术与计算机技术相结合 ,测量干涉条纹间距的方法     

两种莫尔条纹信号对光栅系统测量精度的影响

对两种莫尔条纹（光闸条纹和横向条纹）信号的谐波含量以及它们对光栅系统测量精度的影响进行了分析，并用实测数据加以比较，指出在光栅系统中，取横向莫尔条纹信号的谐波量小，正弦性好，细分误差小，在其它参数相同的情况下，可提高系统的测量精度2倍左右。     

散斑干涉条纹图的总变分去噪方法

去除散斑条纹图中的噪声是电子散斑干涉测量技术的关键问题.提出将总变分图像去噪方法应用于电子散斑干涉条纹图滤波过程中,并对保真系数进行了改进.用总变分模型定义图像的能量函数,利用变分法求得满足能量函数的最优解,将图像去噪过程转化为求解偏微分方程的过程.分别对计算机模拟的条纹图和实验获得的条纹图进行了测试,定性和定量分析的结果表明该技术能够在显著滤波的同时保持条纹的对比度.     

用密栅云纹法研究高温新材料的断裂力学行为

介绍了高温云纹技术，并对密栅云法用于高温新材料的断裂力学行为测试进行了讨论，包括高温材料的零厚度光栅制作技术，Ｕ、Ｖ场分离装置，以及高温断裂的理论依据及图像的后处理，最后作了国内外情况对比分析和应用介绍，阐述了密栅云纹法研究高温新材料力学行为的重要性。     

Instantaneous frequency and its application to strain extraction in moire interferometry

Moire interferometry is an effective experimental technique for measurement of in-plane deformation. However, it is information on the derivatives of the deformation, i.e., strains, that is usually desired in experimental mechanics. It is shown that the desired strains are the instantaneous frequencies of the fringe pattern and that either an energy operator or wavelet ridges can be used to extract the instantaneous frequencies from a single fringe pattern. The energy operator is a pixelwise processor; thus the strain extraction can be done on the fly, but it is sensitive to noise. The wavelet ridges extract the local features in the fringe pattern. The strain extraction is thus insensitive to noise, and good results are obtainable at the cost of longer computation time. The two methods can thus be chosen for different needs in strain analysis. The properties of the two methods as well as their applications to a real fringe pattern are given. The effectiveness of the proposed methods is illustrated by their comparison with traditional methods.     

Boundary element analysis of moiré fields in anisotropic materials

A procedure for analyzing moiré fringe patterns using boundary elements is presented. The kernels of the boundary integrals are based on anisotropic elastic Green's functions developed for bimaterial problems. The interfacial boundary conditions are incorporated in the Green's functions so the interface does not require discretization. The bimaterial kernels are also appropriate for homogeneous problems as well as degenerate isotropic problems. The moiré fringe data provide full-field displacement information and are analyzed in a least-squares sense. The numerical procedure is shown to be a logical extension of the local collocation method developed for linear elastic fracture mechanics. An example is given to investigate convergence of the method, predictions of stress, and to investigate factors influencing the analysis. It is shown that moiré fields associated with both displacement components are needed for an accurate analysis.     

Temporal fringe pattern analysis with parallel computing

Temporal fringe pattern analysis is invaluable in transient phenomena studies but necessitates long processing times. Here we describe a parallel computing strategy based on the single-program multiple-data model and hyperthreading processor technology to reduce the execution time. In a two-node cluster workstation configuration we found that execution periods were reduced by 1.6 times when four virtual processors were used. To allow even lower execution times with an increasing number of processors, the time allocated for data transfer, data read, and waiting should be minimized. Parallel computing is found here to present a feasible approach to reduce execution times in temporal fringe pattern analysis.     

Optically enhanced tiling (OET) in digital fringe pattern analysis

In digital fringe pattern analysis information is usually lost in high fringe density zones due to digitisation and quantisation errors. A generic approach namely optically enhanced tiling (OET) is presented to construct a composite picture from multiple optically zoomed images. The procedure is explained with the help of the problem of data acquisition in digital photoelasticity using phase shifting technique.     

Isochromatics demodulation from a single image using the regularized phase tracking technique

Abstract

In this work we propose a robust fringe demodulation technique applied to the analysis of a single isochromatics fringe pattern produced in photoelasticity. The method used is a regularized phase tracking algorithm with a new sequential scanning technique specifically adapted for the characteristics of the isochromatic fringe patterns: possible apparition of closed fringes, large dynamic range in its spatial frequency content and low noise. The performance of the method is discussed and experimental results are presented.     

Deformation-Space Method for the Design of Biplanar Transverse Gradient Coils in Open MRI Systems

We propose an efficient real-space algorithm for the design of biplanar transverse gradient coils for use in open magnetic resonance imaging (MRI) systems. In our method, each wire arc is represented by a closed contour (LimaÇon). Using parametric equations, we deform/reshape an ensemble of closed contours in a simple manner, controllable by just a few parameters. These parameters are used to define system rearrangements in the design procedure. We use an iterative optimization procedure to adjust the control parameters in order to minimize cost functions such as gradient homogeneity and inductance. Here, we comapare the coil pattern designed by our deformation-space method with a pattern designed by the conventional stream function approach, and we discuss the merit of the new method.      

Real-time color holographic interferometry

A new optical technique based on real-time color holographic interferometry has been developed for analyzing unsteady aerodynamic wakes in fluid mechanics or for measuring displacements and deformations in solid mechanics. The technique's feasibility is demonstrated here. It uses three coherent wavelengths produced simultaneously by a cw laser (mixed argon and krypton). Holograms are recorded on single-layer panchromatic silver halide (Slavich PFG 03C) plates. Results show the optical setup can be adjusted to obtain a uniform background color. The interference fringe pattern visualized is large and colored and exhibits a single central white fringe, which makes the zero order of the interferogram easy to identify. An application in a subsonic wind tunnel is presented, in which the unsteady wake past a cylinder is recorded at high rate.     

Improvement of the regularized phase tracking technique for the processing of nonnormalized fringe patterns

One of the powerful approaches to demodulate a single fringe pattern is the regularized phase tracking (RPT) technique. Here, a new improvement in the RPT technique is presented. This new improvement consists in the addition of one term that models the fringe-pattern modulation. With this new term, the RPT technique can be used for the demodulation of nonnormalized fringe patterns. The performance of the improved RPT technique is shown on examples of various fringe patterns.     

Numerical analysis of fringe patterns recorded in holographic interferometry using high-order ambiguity function

This letter introduces a new approach for the demodulation of fringe patterns recorded in holographic interferometry using high-order ambiguity function (HAF). The proposed approach is capable of retrieving the phase from a single fringe pattern. The main advantage of this approach is that it directly provides an estimation of the continuous phase distribution and thereby avoids the necessity of using a cumbersome 2D phase unwrapping procedure. This method first computes the discrete-time analytic signal of the recorded fringe pattern. Then, by modelling this analytic signal as a polynomial phase signal embedded in additive complex white Gaussian noise, a parametric estimation procedure based on HAF is employed to directly estimate the unwrapped phase distribution. Numerical simulations and experimental results demonstrate the potential of the proposed approach.