3D Strain measurement from gratings on both surfaces of sheet metal

Summary  The deformation of thick sheet metal is analyzed using 3D coordinates of cross-gratings on both sides of a specimen. The basic equations are presented for evaluating the 3D strain tensor from displacement functions. Provided the grating coordinates are measured in the undeformed and the deformed states, the difference of the coordinates yields displacement vectors. These are used to determine polynomial displacement functions, which approximate the displacement vectors in a least-squares sense. The condition of volume invariance at plastic deformation implies the strain in the direction of the thickness. The whole procedure is tested by a radial symmetric forming of a rectangular sheet metal into a half-tube, because it can be evaluated analytically, too. Finally, a real forming process of a plane circular sheet metal into a cup-like object is analyzed. Received 10 May 1999; accepted for publication 22 September 1999     

金属塑性变形极限判据

研究出金属材料塑性变形极限判据，它为断裂力学和失效分析提供理论依据。利用这判据，裂纹尖端的临界应变和相应的临界应力可以确定。在板料成形中，拉应力下颈缩失稳的临界应变值可以定出。节省大量试验工作。     

Analysis of strain localization during tensile tests by digital image correlation

This paper presents an imaging technique developed to study the strain localization phenomena that occur during the tension of thin, flat steel samples. The data are processed using digital speckle image correlation to derive the two in-plane components of the displacement vectors. The authors observe that the calculation of the intercorrelation function reveals a systematic error and propose a numerical method to limit its influence. Plastic incompressibility and thin-sheet assumptions are used to derive the third displacement component and, hence, the various strain and strain rate components. Numerous checks are presented at each step in processing the data to determine the final accuracy of the strain measurements. It is estimated that this accuracy is quite sufficient to track the inception and the development of localization. Examples of possible application are presented for mild steels whose strain localization mechanisms appear to be precocious and gradual.     

板料三维成形过程的有限元数值模拟

基于有限变形理论建立了三维金属板料成形过程的刚塑性有限元数学模拟,该数学模型采用物质坐标系中的Ｕｐｄａｔｅ－Ｌａｇｒａｎｇｅ描述,等向强化假设,考虑了板料的厚向异性,对于金属板料与模具有摩擦采用近似的库仑摩擦定律以改善计算的收敛性。为简经计算采用薄膜单元,并根据此模型编制程序。     

塑性各向异性板料本构关系及冲压成形的数值模拟

推导了具有一般屈服函数形式的弹塑性速率型本构关系;给出了用于板料成形的Hill塑性各向异性屈服模型下本构关系的具体形式;用有限元动力显式计算程序MSC/DYTRAN模拟了金属板料的冲压成形;通过算例分析,考察了塑性各向异性对凸耳形成和大小以及对成形模拟结果准确性的影响;数值结果和实验结果表明：各向（厚向）异性本构模型比各向同性本构模型更真实地反映了板料的成形性。     

Strain tensor for large three-dimensional surface deformation of sheet metal from an object grating

Grating techniques are used to determine the three-dimensional deformation and the tangential strain of sheet metal. A grating is fixed on the surface and taken by stereo CCD cameras in different deformation states. By suitable line-following software, the grating coordinates in the images are determined with subpixel accuracy. Using photogrammetric methods, the three-dimensional coordinates are calculated from the image coordinates. The strain usually is determined by means of a deformation gradient, which is calculated from every deformed triangle. In this paper, the gradient is determined in the center of four neighboring meshes using a polynomial approximation of the displacement function in a reference position. The influence of the nontangential deformation is considered. By simulation, a flat sheet metal is deformed to a rotational symmetric surface. The difference of the known exact strain is compared with the numerically derived strain with respect to different grating pitches. The proposed method yields good results even in the case of large spatial deformation. It is applied to the deformation of a hatlike test specimen.     

三轴试验土样剪切破坏过程的判别

本文在土工三轴试验中应用土样全表面变形数字图像测量系统,通过跟踪包裹土样的橡皮膜表面的标记点,得到加载过程中其全表面的变形和应变分布。根据土样表面的应变分布和实测的应力应变关系曲线,可以得到每一时刻土样表面的应力分布。于是,土样表面剪切破坏开始出现的时刻和剪切破坏带贯穿的时刻可分别依据应变和应力判定。依据应变,将橡皮膜上各点的轴向应变的拐点最晚出现的时刻作为剪切带完全贯穿的时刻;依据应力,将全部点中某一个或几个点最早达到峰值应力的时刻作为剪切带开始出现的时刻;土样各点相继达到应力峰值的过程就是剪切破坏的过程。以剪切破坏的起始点和剪切带贯穿的点为分界,土样剪切破坏过程可以划分成三个阶段:破坏前阶段、破坏阶段和破坏后阶段,在不同阶段土样具有不同的变形特点和变形机理。     

Residual strain evaluation of curved surface by grating-transferring technique and GPA

This paper investigates an advanced grating-transferring technique combined with geometric phase analysis (GPA) for residual strain evaluation of curved surface. A standard holographic grating is first transferred to a pre-produced epoxy resin film and then consolidated to a test region of curved surface. With a rubber mold and silicone rubber the deformed grating is replicated to a sheet metal after hole-drilling for release of residual stress. After that the grating is transferred from the sheet metal to the glass plate, which would be served as an analyzer grating (specimen grating). By GPA the local strain distributions related to the phase difference between the reference grating and analyzer grating for the released stress can be evaluated. A validation test has been conducted on the weld joint of a stainless steel tube and the obtained results demonstrate the ability of the method in measuring the residual strain of curved surface.     

Damage and fracture evaluation of granular composite materials by digital image correlation method

This paper presents the applications of digital image correlation technique to the mesoscopic damage and fracture study of some granular based composite materials including steelfiber reinforced concrete, sandstone and crystal-polymer composite. The deformation fields of the composite materials resulted from stress localization were obtained by the correlation computation of the surface images with loading steps and thus the related damage prediction and fracture parameters were evaluated. The correlation searching could be performed either directly based on the gray levels of the digital images or from the wavelet transform (WT) coefficients of the transform spectrum. The latter was developed by the authors and showed higher resolution and sensitivity to the singularity detection. Because the displacement components came from the rough surfaces of the composite materials without any coats of gratings or fringes of optical interferometry, both surface profiles and the deformation fields of the composites were visualized which was helpful to compare each other to analyze the damage of those heterogeneous materials. The project supported by the National Natural Science Foundation of China (10125211 and 10072002), the Scientific Committee of Yunnan Province for the Program of Steel Fiber Reinforced Concrete, and the Institute of Chemical Materials, CAEP at Mianyang     

应用超声散斑相关法测量水下物内层界面位移

超声散斑相关法可应用于水下物体内层界面的位移测量,在工程建设中具有广泛的应用前景。根据所建立的测量实验系统,对位于水中的有机玻璃试件内层界面的面内位移和离面位移进行了实测,测量误差一般在7%以内。进一步研究结果表明:在刚体位移情况下,一般经粗磨、粗刨和粗铣加工或砂模铸造后的粗糙外层界面对超声散斑相关法内层界面位移测量结果的影响极小;当外层界面发生应变时,其变形前后内层界面上的散斑场的相关性将随应变量的增加而减弱,但外层界面应变小于0.05ε时,内层界面上超声散斑相关系数的减小不超过4%。     

交变电场作用下骨表面温升的实验研究

骨具有力电性质,这一性质可促进骨组织生长。骨既是生理器官又是介电材料,在交变电场作用下,其表现行为可能有生理作用。为此,本文测量了骨悬臂梁试样在交变电场下的表面温升与顶端挠度。发现在电压70V,频率为10~70kHz的交流电场下,最大表面温升达到2~4℃,最大挠度变化为9.0~78.7μm;在频率10kHz,电压为70~175V的交流电场下,最大表面温升达到4~6℃,最大挠度变化为13.0~114.3μm。同时测量了两种经典介电材料有机玻璃和聚乙烯在交变电场下的温升与顶端挠度,最大温升低于0.5℃,但是挠度与骨试样相当,所以相对高温升是骨特有的性质。将交流电压有效值替换成相同幅值的直流电压时,最大温升也低于0.5℃,基本没有挠度变化,所以相对的高温升反映了骨的交流性质。分析后认为,骨在交变电场作用下的温升由胶原的介电损耗引起。实验数据显示,骨在交变电场下,表面温度变化与加载电压的平方成正比,且在实验加载的10~70kHz频段内,骨的介电常数随频率的变化明显。     

Numerical simulation of bubble and droplet deformation by a level set approach with surface tension in three dimensions

In this paper we present a three‐dimensional Navier–Stokes solver for incompressible two‐phase flow problems with surface tension and apply the proposed scheme to the simulation of bubble and droplet deformation. One of the main concerns of this study is the impact of surface tension and its discretization on the overall convergence behavior and conservation properties. Our approach employs a standard finite difference/finite volume discretization on uniform Cartesian staggered grids and uses Chorin's projection approach. The free surface between the two fluid phases is tracked with a level set (LS) technique. Here, the interface conditions are implicitly incorporated into the momentum equations by the continuum surface force method. Surface tension is evaluated using a smoothed delta function and a third‐order interpolation. The problem of mass conservation for the two phases is treated by a reinitialization of the LS function employing a regularized signum function and a global fixed point iteration. All convective terms are discretized by a WENO scheme of fifth order. Altogether, our approach exhibits a second‐order convergence away from the free surface. The discretization of surface tension requires a smoothing scheme near the free surface, which leads to a first‐order convergence in the smoothing region. We discuss the details of the proposed numerical scheme and present the results of several numerical experiments concerning mass conservation, convergence of curvature, and the application of our solver to the simulation of two rising bubble problems, one with small and one with large jumps in material parameters, and the simulation of a droplet deformation due to a shear flow in three space dimensions. Furthermore, we compare our three‐dimensional results with those of quasi‐two‐dimensional and two‐dimensional simulations. This comparison clearly shows the need for full three‐dimensional simulations of droplet and bubble deformation to capture the correct physical behavior. Copyright © 2009 John Wiley & Sons, Ltd.     

小尺寸低碳钢试件吕德斯效应的三维数字图像相关测量

吕德斯效应是多种金属和合金材料由于屈服阶段的不均匀变形而在材料表面产生条带状褶皱的现象,它会使冲压件表面质量降低.为了防止它的出现,对吕德斯效应进行研究变得非常重要.采用小视场(15 mm×15 mm)下三维数字图像相关方法对小尺寸低碳钢试件在单轴拉伸载荷作用下的变形场进行测量,实际观测了小尺寸试件的吕德斯效应,结合理论模型解释了其形成机理,并分析了吕德斯带传播过程中应变及应变率的变化规律.实验研究表明,运用三维数字图像相关方法测量试件表面变形场,实现了对小尺寸低碳钢试件的吕德斯带演化过程以及颈缩、断裂等细观力学行为的观测,该方法是研究材料变形细观机理的一种有效测量手段.     

三维非稳定渗流自由面边界积分项的精确数值计算

利用固定网格法分析三维非稳定渗流问题时,将要面对两项积分难题:以自由面及单元表面为边界的空间积分及以自由面为边界的曲面积分。针对常用的任意8结点6平面三维普通单元,提出采用坐标变换及等参变换技术求取空间积分项的精确数值解;至于曲面积分项,建议改用单元非饱和区部分表面作为积分边界,经过坐标变换及等参变换处理积分边界后,利用高斯数值积分可求出曲面积分项的精确数值解。通过一个普通单元及一项均质半无限边界堤坝的实例分析,表明此方法的精确性和稳定性良好。     

MEASUREMENT OF LÜDERS BAND IN SMALL SIZE LOW CARBON STEEL SPECIMEN BY 3D DIGITAL IMAGE CORRELATION METHOD

吕德斯效应是多种金属和合金材料由于屈服阶段的不均匀变形而在材料表面产生条带状褶皱的现象,它会使冲压件表面质量降低. 为了防止它的出现,对吕德斯效应进行研究变得非常重要. 采用小视场(15mm×15 mm)下三维数字图像相关方法对小尺寸低碳钢试件在单轴拉伸载荷作用下的变形场进行测量,实际观测了小尺寸试件的吕德斯效应,结合理论模型解释了其形成机理,并分析了吕德斯带传播过程中应变及应变率的变化规律.实验研究表明,运用三维数字图像相关方法测量试件表面变形场,实现了对小尺寸低碳钢试件的吕德斯带演化过程以及颈缩、断裂等细观力学行为的观测,该方法是研究材料变形细观机理的一种有效测量手段.