基于DIC技术和无网格法的裂尖应变场分析方法

针对数字图像相关(DIC)技术难以获得精准的裂纹尖端应变场问题,提出一种基于DIC技术和伽辽金无网格法(EFG)的混合方法来分析裂尖应变场。该方法基于DIC技术导出场节点建立无网格模型,采用EFG法结合衍射法表示裂纹不连续性,进而计算全场应变值。以Q235qE钢材制成的紧凑拉伸(CT)试样为例开展了准静态循环拉伸试验,通过比较裂尖应变解析解和试验结果,证明该方法的有效性和稳定性。进一步研究DIC EFG法在裂尖塑性区域和应力强度因子(SIF)计算等方面的应用。研究结果表明：DIC EFG混合方法建模简单,不需要荷载和约束条件,能够获得准确的包括裂尖在内的全平面应变场。相较于传统DIC技术,DIC EFG混合方法解决疲劳裂纹附近结果缺失的问题,结合J积分法计算SIF结果更为精准,可应用于疲劳裂纹的塑性区域和SIF计算等断裂性能研究。     

基于FE/EFG耦合算法的虚拟裂纹闭合法

在裂纹附近区域采用无网格伽辽金(EFG)节点,其余区域采用常规有限单元(FE)节点进行数值离散并求解,获得含裂纹构件的位移场。在裂纹尖端及其附近设置局部辅助有限单元区域,用于求解裂纹尖端处的2个特征参数:裂纹尖端节点力以及靠近裂纹尖端处裂纹面的位移。由这2个参数得到裂纹尖端处的应变能释放率,进而求得相应的应力强度因子,此方法为计算应力强度因子的EFG虚拟裂纹闭合法。数值算例表明,采用EFG虚拟裂纹闭合法能够有效计算裂纹尖端处的应力强度因子。     

Local Kronecker delta property of the MLS approximation and feasibility of directly imposing the essential boundary conditions for the EFG method

The element-free Galerkin (EFG) method is a promising method for solving many engineering problems. Because the shape functions of the EFG method obtained by the moving least-squares (MLS) approximation, generally, do not satisfy the Kronecker delta property, special techniques are required to impose the essential boundary conditions. In this paper, it is proved that the MLS shape functions satisfy the Kronecker delta property when the number of nodes in the support domain is equal to the number of the basis functions. According to this, a local Kronecker delta property, which is satisfying the Kronecker delta property only at boundary nodes, can be obtained in one- and two-dimension. This local Kronecker delta property is an inherent property of the one-dimensional MLS shape functions and can be obtained for the two-dimensional MLS shape functions by reducing the influence domain of each boundary node to weaken the influence between them. The local Kronecker delta property provides the feasibility of directly imposing the essential boundary conditions for the EFG method. Four numerical examples are computed to verify this feasibility. The coincidence of the numerical results obtained by the direct method and Lagrange multiplier method shows the feasibility of the direct method.     

Measurement of residual stress in EFG ribbons using a phase-shifting IR photoelastic method

This paper reports on the measurement of residual stress in EFG silicon ribbons for solar cell applications using the phase-shifting infrared (IR) photoelastic method. The samples analysed were wafers cut from EFG octagons with 100 mm face width and from EFG 125 mm face-width octagon under development. Experimental results show that the distribution of residual stress in both types of samples is similar, within measurement uncertainties. The average residual stress in the samples is about 8 MPa. Maximum stresses of around 30 MPa are associated with twin and grain boundaries. Significant variations of stress along the growth direction, possibly related to buckling, were also measured.     

基于EFG方法的PBX炸药圆弧巴西实验的数值模拟

为了对准静态圆弧巴西实验过程中PBX试样的裂纹形成和扩展过程进行模拟,基于EFG方法和内聚力模型,利用LS-DYNA软件的隐式分析模拟计算了实验过程。结果表明,计算结果和文献报道的实验结果吻合较好,证明了该方法的可行性;采用线性内聚力模型可较好地描述PBX炸药的断裂过程,PBX试样的起裂发生在压缩载荷达到峰值后,随着压头的进一步压缩,试样中心的裂纹沿直径向压头方向扩展。     

Application of Particle Methods to Static Fracture of Reinforced Concrete Structures

Particle methods for modeling reinforced concrete are described. The reinforcements are modeled by finite elements and are coupled to the particle method by Lagrange multipliers. The method is applicable to nonlinear problems, problems with moderate to severe cracking and deformable interfaces. Applications to the static response of reinforced concrete structures where the concrete is discretized with particles and the reinforcement with elements are described. The method is also tested for several static problems where no relative displacements between the concrete and the reinforcement are allowed.     

二维接触问题的无网格伽辽金-有限元耦合方法

研究了二维接触问题的无网格伽辽金-有限元耦合方法。给出了接触问题的数学模型，对其中的线性规划法进行深入分析，并将它与无网格伽辽金-有限元耦合方法结合求解接触问题。通过编程对光滑表面圆柱体与刚性平面的弹性接触问题以及粗糙表面与刚性平面的弹性-理想塑性接触进行求解，在对无网格区域相关参数研究的基础上，指出了用无网格伽辽金-有限元方法求解接触问题时的合理参数范围。     

Evaluation of mechanical properties using spherical ball indentation and coupled finite element–element-free galerkin approach

In the present work, the mechanical properties of pressure tube material (Zr-Nb2.5) are evaluated using the coupled finite element–element-free Galerkin approach. Penalty approach is used to impose contact constraints and non-penetration condition at the interface. An efficient node-to-segment algorithm is employed to model the contact behavior. An updated Lagrangian approach is used to model the large deformation. Loading and unloading response of the indentation process is analyzed using von-Mises and Gurson-Tvergaard-Needleman (GTN) plasticity models. In multiple indentations, the indentation depth is progressively increased up to a maximum specified limit with partial unloading. Load-indentation depth curves are used to extract the flow properties of the material.     

Solid state 27Al NMR and X-ray diffraction study of alumina–carbon composites

The structural and chemical transformations occurring in alumina–carbon composites upon heat treatment were investigated by using a combination of X-ray diffraction (XRD) and solid-state ²⁷Al nuclear magnetic resonance (NMR) spectroscopy. Two different carbon precursors were employed: a commercial activated carbon and a char obtained by carbonization of the endocarp of babassu coconut at 700 °C. The alumina–carbon composites were prepared by aqueous impregnation of the carbon supports with aluminum nitrate and, after filtering and drying, the as-synthesized powders were heat-treated under argon flow at temperatures up to 1000 °C. The Al compounds present in the as-synthesized samples were identified by XRD and solid-state ²⁷Al NMR as nanocrystalline aluminum oxyhydroxides or hydroxides, depending on the synthesis conditions. All Al-containing phases were XRD-amorphous in the char-derived nanocomposites, with the presence of a distribution of AlO₆ (octahedral Al site), AlO₅ (pentacoordinated Al) and AlO₄ (tetrahedral Al site) units revealed by solid-state ²⁷Al NMR spectroscopy. The heat treatments caused the formation of transition aluminas dispersed over the carbon supports, with the occurrence of different amounts of AlO₆, AlO₅ and AlO₄ units depending on the heat treatment temperature and on the type of carbon precursor used for the preparation of the composites.     

Fracture of concrete structure using simplified meshless method

Numerical analysis of fracture in concrete is studied with a simplified meshless method. In this meshless method, the crack is represented by a set of disconnected cohesive crack segments that are required to pass through meshless nodes. The crack method therefore falls in the category of discrete crack methods. However, in contrast to most other methods where the crack is represented as continuous line/surface, no representation of the crack surface is needed. Hence, there is no need to track the crack path. Branching cracks and coalescing cracks are a natural outcome of the analysis and no specific algorithms need to be incorporated to model such complicated events. Besides the simplicity of the method, the accuracy is maintained. This is demonstrated through several examples.