基于PFC~(3D)的砂土直剪模拟及宏细观分析

为能更好地分析砂土在剪切过程中的体积变化及力学行为,通过PFC~(3D)颗粒流程序,按照室内实际级配建立数值试样,进行了不同垂直压力下的砂土直剪试验模拟,并将模拟结果与室内试验结果进行了对比;研究了砂土在剪切过程中的体积变化,并用两种方法分析了砂土剪切带的演变过程;从细观角度对试样颗粒的速度场及力链网络的发展变化进行了分析研究。结果表明,数值模拟结果与室内试验结果基本一致,砂土的体积变化表现为先剪缩后剪胀,剪胀量与垂直压力成反比;剪切带厚度约为11～12倍d_(50),在剪切带内颗粒的位移和欧拉角变化较大;试样内部强力链的演变较为明显,力链网络对外荷载变化的敏感性很高;剪切过程中上盒颗粒速度场方向的变化能够较好地解释砂土产生的剪胀现象。     

基于离散元模型的土石混合体直剪试验分析

土石混合体是由高强度块石和低强度土体组成的一类特殊工程地质材料,其力学性质可通过直剪试验进行确定。本文针对土石混合体的细观材料特性,分别采用球形颗粒单元和非规则组合颗粒单元模拟土体和块石材料,对其在不同含石量和颗粒粘结强度下的直剪试验过程进行离散元分析。计算结果表明,土石混合体的抗剪强度随着含石量和粘结强度的增加而增加;通过不同法向应力下直剪试验的离散元分析,确定了不同含石量下土石混合体材料的内摩擦角和粘聚力。本文研究结果有助于进一步揭示土石混合体的抗剪强度特性。     

胶结砂砾石层面直剪试验及破坏行为细观研究

本文采用宏观试验和细观模拟相结合的方法研究胶结砂砾石层面在剪切过程中的破坏行为．首先进行了不同法向应力作用下的胶结砂砾石层面直剪试验,获得了不同的剪切面破坏特征：随着法向应力的不断增加,剪切破坏面凹凸起伏程度、骨料脱落现象越发明显．其次为了深入探究层面破坏现象,按照室内试验采用的骨料级配粒径建立细观颗粒数值模型,结合物理试验对模型进行参数标定,并进行数值模型的层面直剪模拟和细观分析．结果表明,数值模型可以再现宏观直剪试验层面破坏特征;法向应力越大,层面区域颗粒发生错动和翻转的数量越多;层面破坏方式为颗粒间的张拉和剪切混合破坏,裂隙均集中在层面位置,随着法向应力的增加,裂隙的集中区域逐渐由“面”向“带”转变．     

砂土统一模型的显式计算及加载速度影响研究

以砂土体渐进破坏问题为背景,采用显式方法进行高、低压下砂土统一模型的二次开发以克服隐式算法在软化计算中的强非线性不收敛问题,实现具有剪胀软化、剪缩硬化特性材料的模拟计算．在此基础上开展三个压力等级、不同位移加载速度条件下的平面应变压缩试验,分析位移加载速度对显式计算结果的影响效应．通过分析得到结论：(1)采用动力学显式算法对常压至高压范围的砂土统一模型进行计算是可行的,受荷响应随加载速度的降低逐渐趋于稳定并最终得到静力学计算结果,能够反映砂土在低压下的剪胀软化以及高压下的剪缩硬化特性;(2)砂土统一模型的显式计算结果受加载速度的影响呈双折线趋势,并存在加载速度界限值,试样在加载速度界限值前后呈现不同的受荷变形破坏形态;(3)采用显式计算方法开展砂土统一模型的准静态模拟过程中,须结合计算结果偏差面积比随加载速度的关系曲线,来确定满足一定偏差面积比并且小于界限值的准静态位移加载速度．     

基于DIC的颗粒间接触力计算及力链分析

首次利用数字图像相关方法(Digital Image Correlation method,DIC),依据牛顿力学理论和颗粒线性动量平衡,分析了集中力作用下的二维钢珠颗粒体系,定量地获得了颗粒间接触力。采用CCD相机进行观测并采集了集中力作用下钢珠颗粒体系的变形图像;利用数字图像相关方法对采集的序列图像进行处理,获得了全场位移及应变,并根据胡克定律求得全场应力。对每个颗粒面积区域内各点应力值求和并取平均,获得各个颗粒的平均应力。对采集的数字图像进行图像分析,得到各个颗粒及接触点的排列结构。以颗粒平均应力和颗粒的排列结构作为初始条件,根据牛顿力学理论和颗粒线性动量平衡,最终获取了颗粒间接触力的大小和方向,实现了真实颗粒体系接触力的定量计算。另外从受力方面对集中力作用下颗粒体系内部力链的发展与演变进行了分析。研究结果表明:力链起源于加载位置,随着外荷载的部分耗散,颗粒间接触力逐渐发生变化,力链向四周扩展而形成力链网络。通过对不同压入深度颗粒间接触力变化和分布的分析,定量地描述了加载过程中颗粒间的滚动或滑动引起的力链断裂和重构的现象。     

黏性土宏细观参数相关性研究

运用颗粒流程序中的bond原理结合Mohr-Coulomb破坏准则对黏性土双轴试验进行了数值模拟,探讨了黏性土宏观强度指标与其对应的细观参数之间的关系.分析结果表明,黏性土内摩擦角与颗粒摩擦角呈线性关系,黏性土黏聚力-颗粒接触强度两者拟合曲线的斜率与颗粒间摩擦系数呈幂函数关系,且两者的相关性都很好.通过该宏细观关系,得出上海第②层褐黄色粉质黏土细观参数,对其进行了双轴试验的颗粒流模拟,计算得到的应力应变曲线与实测结果一致.研究成果对于黏性土细观参数的取值及颗粒流模拟具有指导意义.     

基于细观拓扑结构演化的颗粒材料剪胀性分析

剪胀性是包括岩土材料在内的摩擦性颗粒材料的重要特征之一,其形成机制与颗粒体系内部拓扑结构的演化有关.基于颗粒体系细观数据,可对颗粒体系内部的拓扑结构特征及演化进行分析,进而建立拓扑演化与宏观剪胀变形之间的联系.采用离散单元法,根据密实、中密和松散摩擦性颗粒材料双轴试验的宏微观数据,从拓扑参量演化及接触网络拓扑变化所引起...     

一种颗粒复合材料物理性能计算模型与工程应用

结合傅里叶描述子法与格林函数法提出了任意形状夹杂物的Eshelby张量计算方法,基于分步夹杂法建立了一种具有任意形状颗粒的复合材料物理性能预测理论模型。通过傅里叶描述子法对扫描电镜(SEM)图像中颗粒形貌进行数学表征,构建由多个细观结构参数组合的几何模型;利用格林函数求解夹杂物Eshelby张量,考虑颗粒形态、大小和组分性能,研究细观结构对颗粒复合材料有效性能的影响。在此基础上,设计了一款颗粒复合材料物理性能预测软件。以铜铬合金为研究对象,分析重构的细观模型与预测的结果,并与有限元计算结果进行对比,验证了该计算模型的有效性和可靠性,以及在工程计算应用中的可行性。     

颗粒摩擦对颗粒材料剪切行为影响的试验研究

通过对一种类似于土的颗粒材料--玻璃珠开展一系列室内直剪试验,研究颗粒间摩擦对颗粒材料剪切行为的影响. 试验一共考虑了4种不同的摩擦情况:干燥状态、用水浸润状态、完全淹没在水中和用油浸润状态. 分析试验结果发现,与干燥状态试样相比,用油浸润能明显降低试样的剪胀性和抗剪强度,而用水浸润和淹没在水中的方法没有产生显著的影响. 此外,通过在剪胀关系式中引入可变剪胀系数来考虑颗粒摩擦对颗粒材料剪胀性的影响,并从颗粒滑动与滚动的细观机理上初步解释了颗粒滑动摩擦角对临界状态摩擦角的影响规律.     

砂土地基锚板基础抗拔承载力PFC数值分析

在模型试验的基础上,采用PFC离散单元法对条形锚板基础在中密砂土地基中的抗拔性能进行了数值分析。数值模拟采用簇颗粒单元来模拟砂粒的不规则形状,颗粒级配根据模型试验福建标准砂的级配按照相似级配法生成,细观参数根据数值双轴试验确定。水平锚板数值模拟结果与模型试验结果基本一致。与模型试验结果相比,颗粒流数值模拟能得到颗粒间接触力链的分布及其演化规律,能从细观角度来探明宏观抗拔承载力特性的演化机理。在此基础上对倾斜锚板上拔过程进行了模拟,分析了锚板前后砂粒的运动趋势以及接触力链的演化规律,并与已有承载力结果进行了对比分析。     

Grain-scale processes governing shear band initiation and evolution in sands

A variety of experimental techniques have been used to advance understanding of strain localization phenomena in sands. However, all of these methods have fallen short in characterizing the evolution of the grain-scale processes that necessarily control shear band formation and growth in sands. This paper presents results of application of the non-destructive displacement measurement technique of digital image correlation (DIC) to measure two- and three-dimensional surface displacements on plane strain and axisymmetric sand specimens over short time steps. The abundance of local displacement data, high level of accuracy, and nearly continuous (spatially and temporally) record of displacement evolution afforded by the DIC technique has finally enabled a means to quantify local displacements to particulate-scale intensity. The data have been used to evaluate the local displacement mechanisms leading to the triggering of the formation of persistent shear bands, the timing of shear band formation with regard to the achievement of peak stress, and the character of displacements within fully formed shear bands. Insights are offered regarding the relation between strain localization and global stress-strain behavior, and the ensuing interpretations of shear banding as a hardening or softening phenomenon. Comparison of behavior between plane strain and triaxial tests offer additional perspective on the influences of three-dimensional stresses and boundary conditions on shear banding. The results further shed light on the micro-deformation mechanisms (i.e. buckling columns) responsible for the observed local strain non-uniformities that characterize “steady-state” shear band evolution.     

一种考虑旋转位移分量的数字图像相关法及其精度分析

提出了一种新的考虑旋转位移分量的数字图像相关法[CD2]旋转相关匹配法,其特点是先对图像子区进行预旋转,再进行相关匹配、相关系数、整像素和亚像素位移的计算。在物体产生旋转位移分量时,该方法可有效提高变形前后图像子区间的相关系数和位移计算精度。通过斜拉伸、剪切和悬臂梁弯曲实验,对旋转相关匹配法和直接相关匹配法进行了对比研究,并用灰度梯度法计算旋转后新图像子区和变形后图像子区间的亚像素位移。结果表明,在旋转位移分量小于10°的情况下,相比于直接相关匹配法,旋转相关匹配法可以得到更高的相关系数;在用灰度梯度法计算亚像素位移条件下,位移精度与试件位移不产生旋转时的直接相关匹配相当。     

On the Use of NURBS Functions for Displacement Derivatives Measurement by Digital Image Correlation

In this paper, we propose to investigate the potential improvement of using Non-Uniform Rational B-Spline (NURBS) functions for displacement measurements by digital image correlation (DIC). The aim is at improving the performance of DIC to capture with low uncertainty and low noise levels not only the displacement field but also its derivatives. Indeed, when the displacement field is used to feed constitutive law identification procedures, displacement derivatives are required and thus may be measured with robustness. Two examples illustrate the potential of NURBS for DIC: a compressive test on a wood sample and a bending test on a steel beam. For the latter, beam kinematics are adopted and NURBS are used in order to capture the variation of the curvature (second derivative of the displacement) along the beam axis. For these two examples, an error study based on a decomposition of the error into the correlation error and the interpolation error, is carried out and shows the great potential of NURBS functions for DIC.     

Fast, Robust and Accurate Digital Image Correlation Calculation Without Redundant Computations

High-efficiency and high-accuracy deformation analysis using digital image correlation (DIC) has become increasingly important in recent years, considering the ongoing trend of using higher resolution digital cameras and common requirement of processing a large sequence of images recorded in a dynamic testing. In this work, to eliminate the redundant computations involved in conventional DIC method using forward additive matching strategy and classic Newton–Raphson (FA-NR) algorithm without sacrificing its sub-pixel registration accuracy, we proposed an equivalent but more efficient DIC method by combining inverse compositional matching strategy and Gauss-Newton (IC-GN) algorithm for fast, robust and accurate full-field displacement measurement. To this purpose, first, an efficient IC-GN algorithm, without the need of re-evaluating and inverting Hessian matrix in each iteration, is introduced to optimize the robust zero-mean normalized sum of squared difference (ZNSSD) criterion to determine the desired deformation parameters of each interrogated subset. Then, an improved reliability-guided displacement tracking strategy is employed to achieve further speed advantage by automatically providing accurate and complete initial guess of deformation for the IC-GN algorithm implemented on each calculation point. Finally, an easy-to-implement interpolation coefficient look-up table approach is employed to avoid the repeated calculation of bicubic interpolation at sub-pixel locations. With the above improvements, redundant calculations involved in various procedures (i.e. initial guess of deformation, sub-pixel displacement registration and sub-pixel intensity interpolation) of conventional DIC method are entirely eliminated. The registration accuracy and computational efficiency of the proposed DIC method are carefully tested using numerical experiments and real experimental images. Experimental results verify that the proposed DIC method using IC-GN algorithm and the existing DIC method using classic FA-NR algorithm generate similar results, but the former is about three to five times faster. The proposed reliability-guided IC-GN algorithm is expected to be a new standard full-field displacement tracking algorithm in DIC.     

非连续数字图像相关方法在裂纹重构中的应用

数字图像相关方法作为一种新的非接触式位移测量方法,在力学工程中有广泛的应用前景,然而受限于标准方法对图像变形的连续性要求,这种高效的测量方法在断裂力学领域的推广受到了限制. 为解决这一问题,提出采用引入子区分离数学模型,代替标准方法的连续模型,来对非连续区域进行精确识别和匹配的非连续数字图像相关方法. 研究子区被裂纹等非连续分割后原始像素点的位移情况,并引入裂纹张开向量用以表征被分割子区的主区和副区的位移关系;从而建立子区分离模型的数学表达式,并且为所提出的模型设计相应的图像相关算法;然后将所提出的非连续数字图像相关方法应用于重构平板拉伸试验开裂过程中图像的位移. 研究结果表明,相比于标准的数字图像相关方法,所提出的非连续数字图像相关方法解决了图像相关法在非连续区域失效的问题,提高了数字图像相关方法对位移测量的正确率,特别是能够准确重构裂纹面及附近的位移场,其测量精度能够达到亚像素级别.      

基于数字图像体相关的物体内部三维位移场分析

在二维数字图像相关算法的基础上,推导了三维数字图像体相关算法,并应用于物体内部的三维位移场分析。用计算机模拟方法对SR-CT重建的物体内部三维图像施加已知变形,对变形前后三维数字图像进行体相关运算,获得三维位移场。在计算亚像素位移时,本文提出了一种三维的基于灰度梯度的算法,给出了梯度函数,阐述了数字模拟三维体相关的计算过程,通过数字模拟实验证明了算法的正确性和可靠性。     

Spatial DIC Errors due to Pattern-Induced Bias and Grey Level Discretization

Digital image correlation (DIC) is an optical metrology method widely used in experimental mechanics for full-field shape, displacement and strain measurements. The required strain resolution for engineering applications of interest mandates DIC to have a high image displacement matching accuracy, on the order of 1/100th of a pixel, which necessitates an understanding of DIC errors. In this paper, we examine two spatial bias terms that have been almost completely overlooked. They cause a persistent offset in the matching of image intensities and thus corrupt DIC results. We name them pattern-induced bias (PIB), and intensity discretization bias (IDB). We show that the PIB error occurs in the presence of an undermatched shape function and is primarily dictated by the underlying intensity pattern for a fixed displacement field and DIC settings. The IDB error is due to the quantization of the gray level intensity values in the digital camera. In this paper we demonstrate these errors and quantify their magnitudes both experimentally and with synthetic images.

     

数字图像相关方法若干关键问题研究进展

本文报道了作者及其所在课题组近期在数字图像相关(DIC)测量方法上取得的重要研究进展。主要包括:(1)通过对DIC方法中反向组合高斯-牛顿算法的理论误差分析,提出了新的理论误差公式,进一步证明了反向组合高斯-牛顿算法在提高计算速度和测量精度方面的综合优势;(2)采用提出的理论误差公式,发展了数字散斑场的优化及制作方法,保证了测量结果的一致性和正确性;(3)基于相机阵列和图像拼接技术,发展了超分辨率数字图像相关方法,大大提高了DIC测量方法的应变测量分辨率;(4)提出了大视场条件下的三维系统标定方法,实现了三维测量系统的外参实时标定和多相机测量系统中相机位姿的自动矫正;(5)研制了便携式原位三维测量仪和多尺度DIC测量系统,实现了三维实时数字图像相关测量,进一步满足了DIC方法在工业在线检测和医学领域中的应用需求。     

Subset-based local vs.finite element-based global digital image correlation:A comparison study

Being the two primary approaches for full-field kinematics measurements,both subset-based local digital image correlation(DIC) and finite element-based global DIC have been extensively studied.Nowadays,most commercial DIC systems employ local DIC algorithm because of its advantages of straight forward principle and higher efficiency.However,several researchers argue that global DIC can provide better displacement results due to the displacement continuity constraint among adjacent elements.As such,thoroughly examining the performance of these two different DIC methods seems to be highly necessary.Here,the random errors associated with local DIC and two global DIC methods are theoretically analyzed at first.Subsequently,based on the same algorithmic details and parameters during analyses of numerical and real experiments,the performance of the different DIC approaches is fairly compared.Theoretical and experimental results reveal that local DIC outperforms its global counterpart in terms of both displacement results and computational efficiency when element(subset) size is no less than 11 pixels.     

Experimental and numerical investigation of fracture in a cast aluminium alloy

This paper describes an experimental and numerical investigation on the fracture behaviour of a cast AlSi9MgMn aluminium alloy. In the experiments, a modified Arcan test set-up was used to study mixed-mode fracture. During testing, the tension load and the displacement of the actuator of the test machine were recorded, simultaneously as a high-resolution digital camera was used to record a speckle-patterned surface of the specimen. The recorded images were post-processed using an in-house digital image correlation (DIC) software to obtain information of the displacement and strain fields in the specimen during the test. In addition, some newly implemented features in the DIC software allowed us to detect and follow the crack propagation in the material. The numerical calculations were carried out with a user-defined material model implemented in an explicit finite element code. In the model, the material behaviour is described by the classical J₂ flow theory, while fracture was modelled by the Cockcroft–Latham criterion, assuming the fracture parameter to follow a modified weakest-link Weibull distribution. With the proposed probabilistic fracture modelling approach, the fracture parameter can be introduced as a random variable in the finite element simulations. Crack propagation was modelled by element erosion, and a non-local damage formulation was used to reduce mesh-size sensitivity. To reveal the effect of mesh density and meshing technique on the force–displacement curves and the crack propagation, several different meshes were used in the numerical simulations of the modified Arcan tests. The numerical results were finally compared to the experimental data and the agreement between the measured and predicted response was evaluated.