Identification of the elastic stiffness of composites using the virtual fields method and digital image correlation

This paper presents an effective methodology for characterizing the mechanical parameters of composites using digital image correlation combined with the virtual fields method.By using a three-point bending test configuration,this method can identify all mechanical parameters of the material with merely a single test.Successful results verified that this method is especially effective for characterizing composite materials.In this study,the method is applied to measure the orthotropic elastic parameters of fiber-reinforced polymer-matrix composites before and after the hygrothermal aging process.The results indicate that the hygrothermal aging environment significantly influences the mechanical property of a composite.The components of the parameters in the direction of the fiber bundle decreased significantly.From the accuracy analysis,we found that the actual measurement accuracy is sensitive to a shift of the horizontal edges and rotation of the vertical edges.     

Characterization of material mechanical properties using strain correlation method combined with virtual fields method

Acta Mechanica Sinica - The isotropic and anisotropic behaviors are considered as the important formats of the constitutive behaviors, and can also be called the global properties. To improve the...     

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     

基于数字图像相关性方法的脆性材料拉氏反分析实验技术

利用分离式Hopkinson压杆作为加载系统,借助超高速数字图像相关性分析方法,发展了长杆试件拉氏反分析实验技术,并用于研究脆性材料在小变形条件下的动态本构特性。通过超高速相机实时拍摄冲击加载下长杆试件变形的散斑图像,再对散斑图像进行数字图像相关性（digital image correlation,DIC）分析,获得长杆试件表面速度场和应变场。随后,以脆性材料PMMA（polymethyl methacrylate）为例,从DIC分析得到的速度场中提取出不同拉格朗日位置上质点速度时程曲线,构建路径线连接整个速度场,再结合零初始条件,数值求解得到了试件中的应力时程曲线,消去时间参数后,获得了脆性材料PMMA的动态应力-应变曲线,并与Hopkinson压杆实验和准静态压缩实验的结果进行了对比,揭示了PMMA材料在小应变条件下的黏弹性本构响应特征。     

Application of three-dimensional digital image correlation to the core-drilling method

We present a non-destructive technique for the determination ofin situ stresses in concrete structures, reterred to as the core-drilling method. The method is similar to the American Society for Testing and Materials (ASTM) hole-drilling strain gage method, except that the core-drilling method is formulated in the current work are performed with traditional photogrammetry, and the more novel (and more accurate) three-dimensional digital image correlation. In this paper we review the background elasticity theory and we discuss the results of verification experiments on steel plates. Calculated normal stresses are within 17% of applied values for photogrammetry, and 7% for three-dimensional digital image correlation.     

Application of the virtual fields method to mechanical characterization of elastomeric materials

This paper deals with the identification of constitutive parameters of a Mooney model suitable for hyperelastic materials. These parameters are retrieved from a multiaxial mechanical test that gives rise to heterogeneous stress/ strain fields. Since no analytical relationship is available between measurements and unknown parameters, a suitable tool, namely the virtual fields method, is developed in case of large deformations and used to identify these unknowns. Several results obtained with numerical simulations and experiments performed on rubber specimens illustrate the approach.     

Characterization of the post-necking strain hardening behavior using the virtual fields method

The present study aims at characterizing the post-necking strain hardening behavior of three sheet metals having different hardening behavior. Standard tensile tests were performed on sheet metal specimens up to fracture and heterogeneous logarithmic strain fields were obtained from a digital image correlation technique. Then, an appropriate elasto-plastic constitutive model was chosen. Von Mises yield criterion under plane stress and isotropic hardening law were considered to retrieve the relationship between stress and strain. The virtual fields method (VFM) was adopted as an inverse method to determine the constitutive parameters by calculating the stress fields from the heterogeneous strain fields. The results show that the choice of a hardening law which can describe the hardening behavior accurately is important to derive the true stress–strain curve. Finally, post-necking hardening behavior was successfully characterized up to the initial stage of localized necking using the VFM with Swift and modified Voce laws.     

Note on the macroscopic elastic constants of materials with a foam-like structure

Summary It is assumed that the materials considered consist of a large number of small elements so that on a macroscopic scale the behaviour approaches that of homogeneous continuous media. Assuming that the small elements follow prescribed strains it is possible to find an expression for the specific energy from which the average elastic constants can be derived.     

The most restrictive bounds on change in the applied elastic constants for anisotropic materials

Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, No. 1, pp. 107–114, January–February, 1992.     

Calculations of elastic constants for noble metals with pseudopotential method

The third order elastic constants and the pressure derivatives of the second order elastic constants for noble metals (Cu, Ag and Au) at OK are calculated using the pseudopotential model and the overlap energy of noble metals proposed by us in Ref [1]. Five undeter mined parameters are determined by experimental data of the total energy, three second order elastic constants and the lattice constant of the crystal at OK and zero pressure. The comparisons between theoretical calculations and experiments show that the present results for gold are closer to the experimental data than those obtained by other authors.     

A model for compression-weakening materials and the elastic fields due to contractile cells

We construct a homogeneous, nonlinear elastic constitutive law that models aspects of the mechanical behavior of inhomogeneous fibrin networks. Fibers in such networks buckle when in compression. We model this as a loss of stiffness in compression in the stress–strain relations of the homogeneous constitutive model. Problems that model a contracting biological cell in a finite matrix are solved. It is found that matrix displacements and stresses induced by cell contraction decay slower (with distance from the cell) in a compression weakening material than linear elasticity would predict. This points toward a mechanism for long-range cell mechanosensing. In contrast, an expanding cell would induce displacements that decay faster than in a linear elastic matrix.     

Effect of the mesh size of the vector displacement field on the strain estimate in the digital image correlation method

The influence of the mesh size of the displacement vector field on the strain estimate in the digital image correlation method is considered. The reasons for the emergence and the magnitude of the strain estimate error in processing optical images of material surfaces with different textures are analyzed. The dependence of the mesh size for strain estimation on the magnitude of displacements and also on the presence and size of the discontinuity region in the strain field is studied. An adaptive algorithm for choosing the mesh size is proposed, which allows the strain calculation error to be reduced by a factor of 1.9 with simultaneous reduction of computational expenses by a factor of 2.1.     

The degeneration of image singularities from anisotropic materials to isotropic materials for an elastic half-plane

The degeneration of image singularities from an anisotropic material to an isotropic material for a half-plane is discussed in this study. The Green’s functions for anisotropic and isotropic half-planes with traction free boundary subjected to concentrated forces and dislocations have been obtained by many authors. It was commonly accepted that the solution of isotropic problem cannot be derived from anisotropic solutions. However, we believe that this possibility exists as we will demonstrate in this paper. Anisotropic materials include only image singularities of order O(1/r) (i.e., forces and dislocations) existing on image points. There are many image points for anisotropic materials and the locations of these image points depend on the material constants. However, isotropic materials have only one image point with higher order image singularities (O(1/r²), O(1/r³)). From the analysis provided in this study, it is found that the higher order image singularities for an isotropic half-plane are generated by combining the concentrated forces and dislocations when an anisotropic material degenerates to an isotropic material. The solutions of higher order image singularities for isotropic material are dependent. Therefore, these image singularities can be combined to form only three or four simpler image singularities acting on an image point of the isotropic material.     

Quasi-static tensile deformation and fracture behavior of a highly particle-filled composite using digital image correlation method

Polymer bonded explosives (PBXs) are highly particle-filled composite materials. This paper experimentally studies the tensile deformation and fracture behavior of a PBX simulation by using the semi-circular bending (SCB) test. The deformation and fracture process of a pre-notched SCB sample with a random speckle pattern is recorded by a charge coupled device camera. The displacement and strain fields on the observed surface during the loading process are obtained by using the digital image correlation method. The crack opening displacement is calculated from the displacement fields, the initiation and propagation of the crack are analyzed. In addition, the damage evolution and fracture mechanisms of the SCB sample are analyzed according to the strain fields and the correlation coefficient fields at different loading steps.     

Evaluating the mechanical behavior of arterial tissue using digital image correlation

In this study, digital image correlation (DIC) was adopted to examine the mechanical behavior of arterial tissue from bovine aorta. Rectangular sections comprised of the intimal and medial layers were excised from the descending aorta and loaded in displacement control uniaxial tension up to 40 percent elongation. Specimens of silicon rubber sheet were also prepared and served as a benchmark material in the application of DIC for the evaluation of large strains; the elastomer was loaded to 50 percent elongation. The arterial specimens exhibited a non-linear hyperelastic stress-strain response and the stiffness increased with percent elongation. Using a bilinear model to describe the uniaxial behavior, the average minor and major elastic modulii were 192±8 KPa and 912±40 KPa, respectively. Poisson's ratio of the arterial sections increased with the magnitude of axial strain; the average Poisson's ratio was 0.17±0.02. Although the correlation coefficient obtained from image correlation decreased with the percent elongation, a correlation coefficient greater than 0.8 was achieved for the tissue experiments and exceeded that obtained in the evaluation of the elastomer. Based on results from this study, DIC may serve as a valuable method for the determination of mechanical properties of arteries and other soft tissues.     

Determination of Mooney material constants for highly nonlinear isotropic incompressible materials under large elastic deformations

Experimental Techniques -     

基于混凝土裂纹数字图像的有限元网格生成

通过结合数字图像处理技术、边界矢量化技术与有限元网格自动生成技术,提出了基于数字图像的混凝土构件裂纹有限元分析方法。本文以混凝土裂纹为研究对象,先采用Canny算子提取裂纹边缘,对中断的边缘和伪边缘利用顺序边缘连接算法进行连接和剔除;然后,通过矢量化技术将裂纹边界拟合成小线段;最后在矢量化边界的基础上,利用自动生成技术生成裂纹结构的有限元网格,实现对裂纹区域受力情况的分析。实验结果表明:本文提出的方法可实现混凝土结构裂纹的图像监测与力学分析的一体化,能够大幅提高现役结构的评估速度和效率。