电子散斑干涉和Video全息干涉

本文综述了有关电子散斑干涉法(ESPI)和Vidco全息干涉技术的一些近期成果.主要包括:有机玻璃模型中主应力和的测量,二维气体温度场和轴对称气体温度场的测量,三维物体表面形状和三维位移场的测量.条纹图均用数字图象处理系统处理.文中介绍了各种测量光路和操作技术,分析了可能出现的误差,提供了部分实验结果.     

云纹干涉法同时测定三维位移场(U_0,U_(45),U_(90),ω)

本文介绍了用云纹干涉法同时获得三维位移场——面内位移(U_0,U_(45),U_(90))和离面位移(w)的两种方法:实时观测法和加载波条纹的差载法.前者可一次加载,分别观测各个位移分量条纹图;后者可用一张底板同时记录全部三维位移信息,通过光学滤波分离出各位移分量条纹图。测试方法和计算都很简便,不仅能测静荷,同时也可测动荷,文中还介绍了实验用的仪器,给出了实验结果。     

虚位移场方法在石墨材料力学参数测量中的应用

光测实验技术在现代力学研究中得到了广泛的应用。对于材料力学参数如杨氏模量和泊松比的测量,可利用典型加载试验如拉伸试验、弯曲试验并结合光测方法(如云纹和数字图像相关技术)得到位移值,利用载荷信息和应变场信息通过计算获得相关的力学参数。本文利用虚位移场方法测量石墨材料的力学参数。结合石墨材料的三点弯曲实验,由数字图像相关法测量得到试件表面的非均匀变形场。通过选择两组不同的虚位移场,可以反算出材料的力学参数:杨氏模量和泊松比。结果表明这种方法可以有效测量石墨材料的弹性参数。该方法可望在材料力学行为检测中得到推广应用。     

Enhanced measurement of strain distributions

A theoretical approach is presented that uses multiple strain gages to accurately measure complicated strain distributions. The technique is based on the method of weighted residuals in conjunction with measured strain data and is applicable for arbitrary in-plane strain distributions. Conventional measurements using strain gages are shown to represent a particular case of the approach presented. The experimental characterization of unidimensional strain fields is discussed in detail. Two approaches are presented; these are based on linear and quadratic approximations of the strain field. The strain distribution for two important practical problems is evaluated assuming ideal conditions to assess the performance of the proposed approach. In both cases, the simulated results demonstrate that measurement error resulting from the finite size of a strain gage may be reduced. That is, a larger strain gage may be used for a given maximum admissible error. The method also allows a minimal error of measured nonlinear strains.     

Mesoscale measurement of strains by analyzing optical images of the surface of loaded solids

An algorithm for estimating the strains in solids at the mesoscale level, which is based on constructing fields of displacement vectors and calculating strain components, is studied and tested. Verification of the method is performed by comparing the analytically calculated strain components with experimental data obtained on the basis of model images and images recorded by a TOMSC optical TV measurement system during tension of polymer samples. The estimates of the strain components obtained by a correlation analysis of images are shown to be in good agreement with the results of analytical calculations for known loading parameters and sizes of the images under analysis. The results of the study confirm that the TOMSC system can be used to estimate local strains by processing images of the surface of loaded samples of materials. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 6, pp. 158–164, November–December, 2006.     

Stiffness and Damping Identification from Full Field Measurements on Vibrating Plates

The paper presents an experimental application of a method leading to the identification of the elastic and damping material properties of isotropic vibrating plates. The theory assumes that the searched parameters can be extracted from curvature and deflection fields measured on the whole surface of the plate at two particular instants of the vibrating motion. The experimental application consists in an original excitation fixture, a particular adaptation of an optical full-field measurement technique, a data preprocessing giving the curvature and deflection fields and finally in the identification process using the Virtual Fields Method (VFM). The principle of the deflectometry technique used for the measurements is presented. First results of identification on an acrylic plate are presented and compared to reference values. Results are discussed and improvements of the method are proposed.     

Multi-scale Optical Analyses of Dynamic Gas Saturation During Air Sparging into Glass Beads

A multi-scale optical imaging technique was developed allowing for the 2D observation of two phase flow in porous media at two different scales simultaneously: Using two coupled cameras, a 2D flow cell (0.5 × 0.5m²) is recorded entirely at the bench scale and at the pore scale with a spatial resolution of 0.5 and 0.01 mm, respectively. The technique is applied to study channelized gas flow in saturated glass beads. We analyze the phase distribution at the pore scale and derive a pixel-based method for the measurement of saturation at the larger scale. This method assumes linearity between the mean reflected light intensity and the local gas saturation if averaging is performed over representative areas (REV). The REV depends on the irregularity of the local pore structure and has a lower limit at the correlation length of the porous medium (somewhat above the size of the glass beads) and an upper limit which correlates with the width of gas channels. These limits could be quantified through optical analysis. The optical approach to estimate phase saturations was validated by gravimetric analysis where a characteristic ratio between the optically observed flow cell wall and the saturation within the bulk material was identified, which corresponds to the expectation based on geometrical considerations of the glass bead packing. Considering a transient flow experiment the optical method is demonstrated to be able to quantify the temporal evolution of the residual and the convective gas phase. We conclude that the new technique provides a valuable tool to improve our quantitative understanding of multiphase phenomena across different scales.     

Measuring displacement fields of curved surface by moire interferometry of partial coherent light

This paper presents a method to measure the in-plane displacement fields of curved surface by moire interferometry of partial coherent light. The method has the following advantages: simple optical system, no requirement on vibration isolation, high sensitivity, large measuring range, high contrast of interference fringes and availability to in situ structural testing. the present paper also gives theoretical analysis of the method and the formulas of light intensity and displacement field, and introduces a replication technique to form a high frequency reflectance grating on the curved surface. The expriments achieved the measurement of the surface displacement field of a cylindrical shell— the simultaneous circumferential, axial and 45° displacement fields. The torsional test data for surface displacement of a circular bar agree well with the theoretical result. The project supported by National Natural Science Foundation of China.     

A combination optical method of lateral shearing interferometry and caustics

A novel experimental technique is presented that combines the optical method of caustics and the full-field lateral shearing interferometer of coherent gradient sensing (CGS) in one setup. Collimated light transmitted through a deformed specimen is separated into two legs. One is processed by the lateral shearing interferometer and is recombined with the other on a single image plane. By adjusting the optical path length of each leg, a combination lateral shearing interferogram/caustic image is generated. When this technique is applied to a fracture specimen, information about the stress intensity factor can be obtained from the caustic and the CGS interference fringes for exactly the same deformation field. This allows for a direct comparison between the two methods when investigating phenomena such asK-dominance, three-dimensional deformation and so on. In this paper, the technique is presented in detail, and its feasibility is tested by application to mode I and mixed-mode crack tip deformation fields.     

数字图像相关分析法增量位移场测试技术

利用位移场的连续性,对亚像素位移场的算法进行了一些改进,设计了一套分步计算位移场、应变场的测量计算方法,较好地解决了数字图像相关分析法计算精度和效率.采用增量位移场叠加的方法计算大应变位移场,采用局部平面拟合的方法计算应变场.通过对高分子材料拉伸试验位移场的测量和结果标定,说明该方法具有较强的实用性和计算精度.同时,由于避免了对亚像素点的搜索,大大提高了计算效率.     

静止裂纹尖端实验的HRR奇异场

用近代光学试验方法(面内云纹和投影云纹),测量了不同应变硬化指数材料(n=3.350～9.180)、平面应力Ⅰ型双边裂纹试件、裂纹尖端附近位移场和应变场。由试验结果分析了裂纹尖端位移奇异性,得到J主导区和围绕裂纹尖端附近HRR场分布。分析了HRR分布随载荷、材料不同的变化规律。     

直接增强自然单元法计算应力强度因子

自然单元法是一种新兴的无网格数值计算方法,但应用于裂纹问题计算时,其近似函数并不能准确反映裂纹尖端渐进应力场的奇异性,为获得足够的计算精度,需要在缝尖附近增大结点的布置密度。针对裂纹问题提出一种增强的自然单元法,将缝尖渐近位移场函数嵌入到自然单元法近似函数中,给出了增强试函数的构造方法,推导了总体刚度矩阵和荷载列阵的相关列式。应力强度因子可以作为附加未知量直接算得,也可用J积分或相互作用能量积分方法进行计算,对增强区域的选择和影响进行了分析。算例结果表明,基于增强自然单元法采用围线积分方法计算应力强度因子具有很高的精度,但直接以附加结点自由度形式计算则精度有所降低。     

三维电子散斑干涉法在检测残余应力中的应用

本文提出一种新的大错位三维电子散斑干涉法,并将其和钻 法检测残余应力技术结合起来,其特点一是利用三个激光器,从不同的方向照射被测物体,二是在被测物附近放置一参考物,利用大错位棱镜,使物体表面的信息和参考物上的信息都进及ＣＣＤ中,可以得到与三个位移场有关的电子散斑干涉条纹图,并可以运算和分离获得独立的ｕ,ｖ和ｗ场信息,本文介绍了大错位三维电子散斑干涉法的原理、方法和系统,并将其应用于钻孔法释放残余应     

三维位移场(U,V,W)实时观测的云纹干涉法

本文将一特殊布置的云纹干涉系统和一垂直入射试件栅的麦克尔逊干涉系统组和起来,利用云纹干涉系统测量两个面内位移分量U、V,垂直入射的麦克尔逊干涉系统测量离面位移分量 W。调节云纹干涉系统中入射光的入射角,使正、负一级衍涉波的传播方向偏离开试件栅法向一微小角度,从而使一级衍涉波的频谱点与麦克尔逊干涉系统中的零级衍涉波的频谱点在频谱面上分开。利用小反射镜的反射来改变一级衍射波的传播方向,从而使我们在三个彼此分开的成像面上同步实时观测到三维位移场的条纹图。文中给出了严格的理论推导和实验验证。     

Parallel algorithms for panel methods

A parallel algorithm for the solution of potential flow problems using the panel method of Hess and Smith and conjugate and bi-conjugate gradient techniques is presented. Analysis of the parallelism for the matrix. solvers shows the algorithms to have scalable properties as the problem size grows indefinitely large. Speed-up and efficiency values are presented along with experimental and theoretical values for the optimum number of processors for maximum speed-up. It is envisaged that the parallel techniques presented here have applications using other boundary integral methods for solving engineering problems of a more complex nature.     

Atomistic and continuum studies of stress and strain fields near a rapidly propagating crack in a harmonic lattice

Large-scale atomistic simulations of a mode I crack propagating in a harmonic lattice are presented. The objective of this work is to study the stress and strain fields near a rapidly propagating mode I crack. The asymptotic continuum mechanics solutions of the elastic fields are compared quantitatively with molecular-dynamics simulation results for different crack velocities. It is observed that both atomistic stress and atomistic strain can be successfully related to the corresponding continuum quantities. The study reveals that the atomistic simulation results agree well with the continuum theory predictions, which suggests that the continuum theory can be applied for nano-scale dynamic problems.     

Experimental investigation of GH singularity field of the growing crack-tip for different ductile metals

The displacementsu _x ,u _y inx, y directions at growing crack tip of the specimens with double edge cracks for four different alumium alloys and two coppers are measured by using moire method and optical spatial filtering technique. From experimental displacement fields, the displacement singularity fields are obtained and compared with GH theoretical field. Unknown constantsA and _yo in theoretical solution are determined from experimental data. The theoretical singularity field thus compared is given for plane-stress, mode-I, strain-hardening materials. The error in both the experimental and theoretical evaluations is within ± 10%. The experiments show that there exists dominant singularity region around a growing crack-tip. In the experiments, the strain hardening indexn amounts from 3.158 to 14. The shape of this dominant region ranges from butterfly wing to oblate or circular shape. The size and shape of GH dominant region depend on the material property, the specimen geometry and loading type. Inside GH-field, there is a 3-D deformed damage zone, where no GH singularity exists. Very near to the crack-tip, there is a fracture process zone.The project is supported by the National Science Foundation of China.     

基于锁相循环投影的物体形貌检测

本文提出将相位锁定循环技术应用于栅线投影方法,将规则栅线投影到物体表面得到变形栅线,用相位锁定循环法解调含有物体表面高度信息的位相。用该法不需传统的相位去包裹,即可完成对二维栅线图扫描,从而确定其相位。通过对典型试件的检测,得到了比较满意的结果。     

Evaluation of the variable order of singularities by pseudocaustics

An experimental technique based on the optical method of pseudocaustics was developed for the evaluation of the order of singularity for singular elastic fields. According to this method, the mapping of an infinitesimal circle close to the singularity into an infinitesimal ellipse by means of the pseudocaustics may be used for evaluating directly the stress singularity. An experimental procedure and a numerical scheme were used for accurately defining the order of singularity. Results with variable values of stress singularities indicate the potentialities of the method.