DIC Strain Measurements at the Micro-Scale in a Semi-Crystalline Polymer

To improve physical motivation of mechanical modeling for semi-crystalline polymers, a better understanding of micro-mechanisms and a quantification of the mechanical response at the microstructure scale are needed. Strain field evolutions during deformation would be useful information but experimental techniques are still lacking for these materials. In this paper, an in-situ Digital Image Correlation (DIC) technique in the Scanning Electron Microscope (SEM) was developed to access strain fields at the spherulitic scale of a polypropylene. The evolution of strain heterogeneities during deformation were analyzed and correlated to the microstructure.     

<Emphasis Type="Italic">In-situ</Emphasis> Full Field Measurements During Inter-Facial Debonding in Single Fiber Composite Under Transverse Load

The fibre/matrix interfacial damage mechanisms of fiber-reinforced composites (FRCs) are investigated for single-fiber composites under transverse load. A stereo microscope setup is used for 3D digital image correlation during in-situ quasi-static tests of single-fiber standard dog-bone specimens. Macro-fibers (0.9 mm diameter) with radically different interfacial bonding with the epoxy matrix are used. Damage appears to initiate with fiber debonding at the free surface along the tensile direction. The crack then propagates around the interface while slightly growing along the fiber until a lateral crack initiates on the debonded free surface, provoking specimen failure. The final failure mechanisms appears to be different for strong and weak fiber/matrix bonding. 3D DIC is used to provide precise measurements of displacements, strains, and out-of-plane displacement during the whole test. Quantitative differences in the displacement fields are measured in the cases of strong and weak bonding between the fiber and matrix. 3D DIC with macro-fibers is shown to be a promising technique to provide a better understanding of the damage mechanisms in a single-fiber composite and to determine interfacial toughness of a specific fibre/matrix couple in order to perform accurate modeling of damage in FRCs. Displacement, strain, and confidence field results for each pixel from each experiment and at each time step are also provided for detailed comparison with simulation results.     

Static Model Describing the Inhomogeneous Deformation of Unidirectional Fibrous Composites under Plane Strain

A mathematical model is proposed to describe the static deformation of a unidirectional fiber-reinforced composite under plane-strain conditions in a plane perpendicular to the fiber direction. The model is based on the stochastic static equations for an elastic homogeneous two-component medium with nonzero body forces. Applying the method of conditional moments and double Fourier transform yields a nonlocal model in the form of integro-differential equations. Expanding the integral kernels into series in expansion coefficients yields differential equations whose order depends on the number of terms in the series. The zero-order approximation leads to the theory of effective moduli, the first-order approximation to the refined theory of effective moduli, and the second-order approximation to fourth-order equilibrium equations for mean displacements (mathematical expectations) and formulas for displacements, strains, and stresses averaged over the composite and its components. All the coefficients in theses formulas are expressed in terms of the elastic constants of the components and the geometric parameters of the structure. The model is valid for heavy stress gradients. In a particular case, the static theory of two-component elastic mixtures follows from the model__________Translated from Prikladnaya Mekhanika, Vol. 41, No. 2, pp. 41–51, February 2005.     

高温下C/SiC复合材料弯曲断裂性能实时测试和微观结构表征分析

采用先驱体浸渍裂解法制备三维编织的C/SiC复合材料,利用自主研发的高温散斑制作技术和改进的三维变形光学测试系统,基于数字图像相关技术测试原理,在高温小尺度下实时表征分析C/SiC复合材料的力学性能。运用X射线衍射和扫描电子显微镜测试分析了材料高温氧化情况、显微组织结构及裂纹扩展情况。研究表明:温度对C/SiC复合材料的性能有很大的影响。随着温度的升高,材料由脆性断裂逐渐转变为韧性断裂;材料的断裂韧性由7.98±0.12MPa·m~(1/2)减小到2.76±0.11MPa·m~(1/2),断裂强度从251.40±2.71MPa减小到86.94±1.82MPa。本文为掌握C/SiC复合材料的高温失效机理提供了一种有效的实验测试技术和方法。     

正交各向异性材料电阻应变测量的横向效应修正

本文讨论了电阻应变片测量正交各向异性材料应变的横向效应修正。将测量各向同性测量的表观弹性模量、表观泊松比推广应用于正交各向异性材料，得出了一组与广义虎克定律形式上相同的计算式，并讨论了横向效应对应力测量精度的影响。     

剪切弯曲下短深梁位移数值计算精度的研究

分析了短深梁位移的基本解，评述了有限元解法中关于高次分布力简化为节点力的问题。使用虚功原理和位移线性插值推导了满足二次分布力作用的精确节点力分配公式。通过矩形截面悬臂梁自由终端受集中载荷的算例，阐明了提高短深梁位移计算精度的途径。     

Reducing the Order of the Boundary Condition in a Theory of Plane Strain Elasticity with Boundary Reinforcement

We consider an elastic solid whose boundary is partly reinforced in such a way that the coated part of the boundary can be represented by the union of a finite number of open curves. We show how the (nonstandard) boundary condition on the reinforcement can be integrated resulting in an equivalent (lower-order) boundary condition which, unlike its higher-order counterpart, permits analysis of the corresponding boundary value problems using the boundary integral equation method. Mathematics Subject Classifications (2000) 74B05, 45E05, 35J25     

可压缩材料平面应变滑移线场的数值解和边界条件

简要介绍了可压缩材料平面应变滑移线理论。导出了滑移线场应力和速度数值解。处理了应力边界条件，阐明了应力和速度间断规律。举例说明了边界条件处理和数值解步骤。     

Full-Field Strain Measurement and Identification of Composites Moduli at High Strain Rate with the Virtual Fields Method

The present paper deals with full-field strain measurement on glass/epoxy composite tensile specimens submitted to high strain rate loading through a split Hopkinson pressure bar (SHPB) device and with the identification of their mechanical properties. First, the adopted methodology is presented: the device, including an Ultra-High Speed camera, and the experimental procedure to obtain relevant displacement maps are described. The different full-field results including displacement, strain and acceleration maps for two mechanical tests are then addressed. The last part of the paper deals with an original procedure to identify stiffnesses on this dynamic case only using the actual strain and acceleration maps (without the applied force) by using the Virtual Fields Method. The results provide very promising values of Young’s modulus and Poisson’s ratio on a quasi-isotropic glass-epoxy laminate. The load reconstructed from the moduli and strains compares favourably with that from the readings.     

On the Accuracy of Elastic Strain Field Measurements by Laue Microdiffraction and High-Resolution EBSD: a Cross-Validation Experiment

Determining the accuracy of elastic strain measurements in plastically deformed alloys is an experimental challenge. To develop a novel cross-validation procedure, a controlled elasto-plastic strain gradient was created in a stainless steel single crystal by four point bending deformation. The corresponding elastic strain field was probed, with an intragranular spatial resolution, in-situ by Laue microdiffraction and ex-situ by High Resolution EBSD. Good agreement is found for the two independent measurements and the predictions of a mechanical model, at plastic strains below 0.5 %. The accuracy of the measurements is estimated at 3.2 × 10^{? 4}.     

Analytical Solution of the Problem of Displacement of a Gas Dissolved in a Melt by Plane and Spherical Crystallization Fronts

Selfsimilar solutions of the problem of displacement of a gas dissolved in a melt by plane and spherical crystallization fronts are found for the case where the crystal growth rate is inversely related to the square root of time. A criterion of the absence of gas displacement due to segregation is found. The problem for a plane crystallization front moving with a constant velocity is analytically solved by means of the Laplace transform method.     

PIV Measurements in the Near and Intermediate Field Regions of Jets Issuing from Eight Different Nozzle Geometries

An experimental study was conducted to investigate the effect of nozzle geometry on the mixing characteristics and turbulent transport phenomena in turbulent jets. The nozzle geometry examined were round, square, cross, eight-corner star, six-lobe daisy, equilateral triangle as well as ellipse and rectangle each with aspect ratio of 2. The jets were produced from sharp linear contoured nozzles which may be considered intermediate to the more widely studied smooth contraction and orifice nozzles. A high resolution particle image velocimetry was used to conduct detailed velocity measurements in the near and intermediate regions. It was observed that the lengths of the potential cores and the growth rates of turbulence intensities on the jet centerline are comparable with those of the orifice jets. The results indicate that the decay and spreading rates are lower than reported for orifice jets but higher than results for smooth contoured jets. The jets issuing from the elliptic and rectangular nozzles have the best mixing performance while the least effective mixing was observed in the star jet. The distributions of the Reynolds stresses and turbulent diffusion clearly showed that turbulent transport phenomena are quite sensitive to nozzle geometry. Due to the specific shape of triangular and daisy jets, the profiles of mean velocity and turbulent quantities are close to each other in their minor and major planes while in the elliptic and rectangular jets are considerably different. They also exhibit more isotropic behavior compared to the elliptic and rectangular jets. In spite of significant effects of nozzle geometry on mean velocity and turbulent quantities, the integral length scales are independent of changes in nozzle geometry.     

Convective-Region Top Front Propagation in a Uniform Medium under the Action of Point, Linear, and Plane Heat and Momentum Sources

A relation between the height of a convective front rising in an unstratified medium and the momentum and heat fluxes released on the substrate surface is proposed for point, linear, and uniform plane sources arbitrarily dependent on time. This relation makes it possible to determine the integral power of a plume on the basis of optical observations of the height of the propagating convective front. As particular solutions, three classes of self-similar regimes related with the heat and momentum sources, whose rate is a step-shaped, power-law, or exponential function of time, are obtained. A one-dimensional integral model of a rising convective jet is constructed. The classes of self-similar jets corresponding to power or exponential heat and momentum sources are described. It is shown that all self-similar jets corresponding to heat and momentum sources governed by a power law with a fairly large exponent are characterized by the same temperature and velocity profiles.     

基于数字图像的分离式霍普金森压杆实验中试件应变及两端应力的同步测量法

本文提出一种基于高速摄像和数字图像相关方法(DIC)的分离式Hopkinson压杆(SHPB)测量技术,从而实现试件应变和两端应力的同步测量。即在与试件接触的输入输出杆两端制作散斑,通过高速摄像获取SHPB实验过程中的散斑变形图像,由DIC测得各时刻试件的应变、输入输出杆端的应变(可直接换算为试件两端的应力)。由于试件和杆端的应变都是从同一张高速摄影的图像上分析得到的,因此它们是同步的。应用该方法对钢纤维混凝土试件的SHPB试验进行了测量,测量结果与传统应变片测量结果吻合,验证了该方法的可行性。该技术不仅实现了SHPB实验中试件应变和应力的同步测量,还将有助于直接检验各材料在SHPB实验中试件两端的力在实验过程中是否平衡。     

On the Cover of this Issue: In-situ Strain Field Measurement and Mechano-Electro-Chemical Analysis of Graphite Electrodes via Fluorescence Digital Image Correlation by H. Xie,W. Yang,Y. Kang,Q. Zhang,B. Han,and W. Qiu

Experimental Mechanics -     

速度和载荷对脂润滑2Cr13钢离子渗氮层摩擦学行为的影响

利用销-盘式摩擦磨损试验机对不同速度和载荷条件下 PFPE脂润滑2Cr13钢离子渗氮层进行了系统的真空滑动摩擦磨损试验.采用扫描电子显微镜(SEM)、X射线光电子能谱仪(XPS)和傅里叶变换红外光谱(FTIR)分别对磨损表面的形貌、化学状态及润滑脂的成分进行分析.结果表明:随着载荷和滑动速度的增加,脂润滑渗氮层的磨损机制由轻微磨损向轻微复合磨损机制转变.在真空摩擦磨损过程中2Cr13钢离子渗氮层与PFPE润滑脂发生化学反应,并有FeF3生成,促进PFPE润滑脂发生降解,形成酸性的氟化物.增加载荷和速度会加速渗氮层与润滑脂之间的反应,进一步促进润滑脂的降解.     

Formation of Regions with High Energy and Pressure Gradients at the Free Surface of Liquid Dielectric in a Tangential Electric Field

The nonlinear dynamics of the free surface of an ideal incompressible non-conducting fluid with a high dielectric constant subjected to a strong horizontal electric field is simulated using the method of conformal transformations. It is shown that in the initial stage of interaction of counter-propagating periodic waves of significant amplitude, there is a direct energy cascade leading to energy transfer to small scales. This results in the formation of regions with a steep wave front at the fluid surface, in which the dynamic pressure and the pressure exerted by the electric field undergo a discontinuity. It has been demonstrated that the formation of regions with high gradients of the electric field and fluid velocity is accompanied by breaking of surface waves; the boundary inclination angle tends to 90?, and the surface curvature increases without bound.