内聚力界面单元在胶接接头分层仿真中的应用

分层是胶接接头的主要破坏形式,而分层主要发生在胶层与被粘物的界面层上,因此,对胶接接头界面层的力学行为的分析是很有必要的.由于界面层厚度太小,用常规的有限元法或边界元法很难对其进行仿真,更加无法仿真胶接接头的分层损伤过程.针对上述情况,为提高接头结构承载力,采用零厚度的内聚力界面单元来仿真界面层,克服无法仿真界面层的困难,对胶接接头的分层损伤过程进行仿真,并与试验结果进行对比,验证方法的有效性.通过仿真分析结果与试验结果对比可知,仿真分析结果与试验结果吻合较好.因此,运用内聚界面单元仿真界面层方法是有效的,并为复杂胶接接头承载能力分析提供有力的依据.     

基于内聚力模型的纤维复合材料细观损伤模拟

基于代表性体积单元和内聚力模型，使用内聚力单元和扩展有限元方法分别模拟界面损伤和基体损伤，通过ABAQUS二次开发建立了纤维增强复合材料细观力学模型。以受横向拉伸的碳纤维/树脂代表性体积单元为研究对象，分析对比了不同强度下仅考虑界面损伤时复合材料的力学特性和同时考虑界面、基体损伤时的力学特性。结果表明纤维对基体的增强效果随界面强度提高；扩展有限元方法能在不依赖网格数量的条件下准确描述基体中多条裂纹的萌生及扩展过程，并且捕捉到基体裂纹引起的界面损伤；仅考虑界面损伤时的代表性体积单元平均应力-位移曲线后期呈上升趋势，而同时考虑界面和基体损伤的平均应力-位移曲线后期呈下降趋势。     

火灾后型钢混凝土十字形柱的敏感性分析

为准确分析型钢混凝土柱的抗火灾行为,采用Abaqus软件对型钢混凝土十字形柱进行顺序热力耦合分析,将十字形柱的截面温度分布、温度-时间关系、火灾后的破坏模式以及载荷-位移关系的模拟结果与试验结果进行对比,验证有限元模型的有效性。讨论十字形柱模型的参数敏感性,选择最优混凝土材料属性、建模单元和材料的热膨胀系数,为型钢混凝土组合结构模拟提供更精确的参考依据。     

扩展的多尺度有限元法基本原理

阐述一种适用于非均质材料力学性能分析的扩展的多尺度有限元法（Extended Multiscale Finite Element Method,EMsFEM）的基本原理．该方法的基本思想是利用数值方法构造能反映胞体单元内部材料非均质影响的多尺度基函数,在此基础上求得粗网格层次的等效单元刚度阵,从而在粗网格尺度上对原问题进行求解,很大程度地减少计算量．以该方法进行的具有周期和随机微观结构的材料计算示例,通过与传统有限元法的结果比较,说明这一方法的有效性．EMsFEM的优势在于,能容易地进行降尺度计算,可较准确地求得单元内部的微观应力应变信息,在非均质材料强度和非线性分析中有很大的应用潜力．     

功能梯度双层悬臂梁的有限元建模分析

为考察不同建模形式和界面处理方式对由功能梯度材料构成的双层悬臂梁计算结果的影响,通过有限元计算结果与理论解的对比发现,对于功能梯度悬臂梁,选取八节点二次单元能更好地消除剪力自锁现象,比四节点线性单元的求解结果更加精确;对于双材料理想界面,采取强制位移约束条件比消除重合节点的约束条件更符合真实情况;梁端部附近应力场的有限元解比理论解更加合理.     

基于光滑粒子动力学方法的爆轰波对碰实验数值仿真

为分析爆轰波对碰实验及不同材料飞层在爆轰波对碰作用下的动力学响应,采用能处理断裂和复杂界面的光滑粒子动力学(Smoothed Particle Hydrodynamics,SPH)方法进行数值模拟.采用二维轴对称SPH离散形式,光滑函数采用B-样条函数;利用二维流体动力学程序进行数值模拟.数值模拟结果与实验结果符合较好...     

岩石材料变形破坏过程应用程序开发

为了研究岩石表面变形破坏过程的变化特征,设计了一个可视化应用程序.该应用程序以岩石常规力学性质试验视频作为研究对象,包括静态图像处理界面和视频图像处理界面.静态图像处理界面由图像类型转换、图像边缘检测、图像形态学处理、图像滤波处理4个模块组成.视频处理界面则提供试验视频帧数、历时、帧图像大小和维数等基本信息.通过在可视化界面上进行所需参数设置实现了单帧图像特征纹理参数计算和岩石试样表面位移场的计算.本文还以两个示例说明了使用该应用程序进行岩石材料变形破坏过程分析的方法.本文成果对分析岩石材料变形特点和破坏机制具有一定的参考价值.     

基于MSC Nastran的某电机支架强度优化分析

采用有限元法对一款电机支架进行强度仿真和优化,将刚性单元和集中质量单元组合模拟负载质量连接,具有较高的计算精度和计算效率.强度分析结果表明,电机支架的端部支撑部件有应力集中现象,存在强度破坏风险.针对风险位置进行结构优化设计,使电机支架破坏的风险有效降低.     

巴西圆盘劈裂破坏的近场动力学建模与分析

基于非局部近场动力学（Peridynamics,PD）理论,对含预置裂纹的混凝土巴西圆盘劈裂破坏问题进行建模分析.将结构离散为包含混凝土材料信息的粒子,引入动态松弛、分级加载和失衡力守恒等粒子系统数值算法,构建可以自然模拟脆性裂纹扩展的PD算法体系.对含不同角度单预置中心裂纹巴西圆盘的裂纹扩展过程进行数值模拟,所得结果与试验结果吻合较好,验证所提出的模型和算法正确.进一步采用该方法模拟双预置裂纹巴西圆盘劈裂破坏过程中的裂纹扩展、交汇、贯通过程,通过将所得模拟结果与试验结果进行比较分析,探究该方法处理多裂纹扩展问题的可行性.     

对接梁Benchmark模型的建模与动态特性分析

采用相对描述方式建立了Sandia国家实验室结构连接件与界面研究中提出的对称连接梁Benchmark问题的对称和反对称动力学模型,解析分析了系统的模态特性.解析模型中连接段采用挠性根部法模拟其变形势能,根部挠性参数则根据试验结果识别.并采用虚拟材料法模拟连接段进行了有限元分析.两种方法均获得了该Benchmark模型与其模态试验结果一致的动态特性.     

An open source program to generate zero-thickness cohesive interface elements

An open source program to generate zero-thickness cohesive interface elements in existing finite element discretizations is presented. This contribution fills the gap in the literature that, to the best of the author’s knowledge, there is no such program exists. The program is useful in numerical modeling of material/structure failure using cohesive interface elements. The program is able to generate one/two dimensional, linear/quadratic cohesive elements (i) at all inter-element boundaries, (ii) at material interfaces and (iii) at grain boundaries in polycrystalline materials. Algorithms and utilization of the program is discussed. Several two dimensional and three dimensional fracture mechanics problems are given including debonding process of material interfaces, multiple delamination of composite structures, crack propagation in polycrystalline structures.     

An examination of stability in cohesive zone modeling

Stable crack propagation hinges on the driving force and the resistance. In the context of a cohesive approach to fracture, properly resolving the cohesive zone ensures that the resistance is in equilibrium with the driving force. Additional restrictions on the mesh size can stem from crack stability. For high strength, low toughness (brittle) materials, the requirements for stability can exceed those for cohesive zone resolution. Examples in 1-D and 2-D reveal that decrements in the mesh size transition the system from indefinite to positive definite. Moreover, small decreases in the mesh size beyond the transition provide substantial reductions in the condition number. We contrast a physical instability resulting from material properties, geometry, or boundary conditions from a numerical instability resulting from the mesh size and propose that the selected discretization should ensure that the cohesive zone is both resolved and stabilized.     

A three-dimensional self-adaptive cohesive zone model for interfacial delamination

Discrete crack models with cohesive binding forces in the fracture process zone have been widely used to address failure in quasi-brittle materials and interfaces. However, the numerical concerns and limitations stemming from the application of interface cohesive zone models in a quasi-static finite element framework increase considerably as the relative size of the process zone decreases. An excessively fine mesh is required in the process zone to accurately resolve the distribution of tractions in a relatively small moving zone. With a moderate mesh size, inefficient path-following techniques have to be employed to trace the local discretization-induced snap-backs. In order to increase the applicability of cohesive zone models by reducing their numerical deficiencies, a self-adaptive finite element framework is proposed, based on a hierarchical enrichment of the standard elements. With this approach, the planar mixed-mode crack growth in a general three-dimensional continuum, discretized by a coarse mesh, can be modeled while the set of equations of the non-linear system is solved by a standard Newton–Raphson iterative procedure. This hierarchical scheme was found to be most effective in reducing the oscillatory behavior of the global response.     

受剪复合材料加筋平板筋条剥离分析

为研究筋条-蒙皮界面强度对复合材料加筋平板剪切承载能力的影响,采用基于接触属性定义的粘聚区模型分析蒙皮-筋条界面损伤的萌生和扩展,采用基于二维Hashin准则的渐进损伤模型计算复合材料层板失效.仿真结果与试验吻合较好,验证基于接触属性定义的粘聚区模型用于整体复合材料加筋板筋条剥离分析的工程实用性.计算结果表明:为充分发挥复合材料加筋板的承载潜力,筋条-蒙皮界面应具有较高的强度.     

Multiscale Modeling of Concrete

In this paper, a mesoscale model of concrete is presented, which considers particles, matrix material and the interfacial transition zone (ITZ) as separate constituents. Particles are represented as ellipsoides, generated according to a prescribed grading curve and placed randomly into the specimen. Algorithms are proposed to generate realistic particle configurations efficiently. The nonlinear behavior is simulated with a cohesive interface model for the ITZ. For the matrix material, different damage/plasticity models are investigated. The simulation of localization requires to regularize the solution, which is performed by using integral type nonlocal models with strain or displacement averaging. Due to the complexity of a mesoscale model for a realistic structure, a multiscale method to couple the homogeneous macroscale with the heterogeneous mesoscale model in a concurrent embedded approach is proposed. This allows an adaptive transition from a full macroscale model to a multiscale model, where only the relevant parts are resolved on a finer scale. Special emphasis is placed on the investigation of different coupling schemes between the different scales, such as the mortar method and the arlequin method, and a discussion of their advantages and disadvantages within the current context. The applicability of the proposed methodology is illustrated for a variety of examples in tension and compression.     

A one field full discontinuous Galerkin method for Kirchhoff–Love shells applied to fracture mechanics

In order to model fracture, the cohesive zone method can be coupled in a very efficient way with the finite element method. Nevertheless, there are some drawbacks with the classical insertion of cohesive elements. It is well known that, on one the hand, if these elements are present before fracture there is a modification of the structure stiffness, and that, on the other hand, their insertion during the simulation requires very complex implementation, especially with parallel codes. These drawbacks can be avoided by combining the cohesive method with the use of a discontinuous Galerkin formulation. In such a formulation, all the elements are discontinuous and the continuity is weakly ensured in a stable and consistent way by inserting extra terms on the boundary of elements. The recourse to interface elements allows to substitute them by cohesive elements at the onset of fracture.The purpose of this paper is to develop this formulation for Kirchhoff–Love plates and shells. It is achieved by the establishment of a full DG formulation of shell combined with a cohesive model, which is adapted to the special thickness discretization of the shell formulation. In fact, this cohesive model is applied on resulting reduced stresses which are the basis of thin structures formulations. Finally, numerical examples demonstrate the efficiency of the method.     

An Efficient Adaptive Procedure for Three-Dimensional Fragmentation Simulations

We present a simple set of data structures, and a collection of methods for constructing and updating the structures, designed to support the use of cohesive elements in simulations of fracture and fragmentation. Initially, all interior faces in the triangulation are perfectly coherent, i.e. conforming in the usual finite element sense. Cohesive elements are inserted adaptively at interior faces when the effective traction acting on those faces reaches the cohesive strength of the material. The insertion of cohesive elements changes the geometry of the boundary and, frequently, the topology of the model as well. The data structures and methods presented here are straightforward to implement, and enable the efficient tracking of complex fracture and fragmentation processes. The efficiency and versatility of the approach is demonstrated with the aid of two examples of application to dynamic fracture.     

Numerical analyses of peel demolding for UV embossing of high aspect ratio micro-patterning

Ultraviolet (UV) embossing, involving molding against micro-structured molds, is a quick and efficient method to mass produce high aspect ratio micro-features. A crucial challenge to the repeatability and large-scale application of this technique is successful demolding, which escalates in difficulty with increasing aspect ratio, due to increased polymer-mold mechanical interlocking. Some of the key factors affecting UV embossing include the crosslinked polymer shrinkage and material properties, interfacial strength between polymer to mold and the demolding method. This paper presents a new method to simulate the demolding of UV cured polymer from a nickel mold. Hyperelastic material model and rate-independent cohesive zone modeling were employed in the numerical simulation; linear elastic polymer response, although relatively easy to apply, was not adequate. Progressive shrinkage was implemented, leading to delamination of the polymer-mold interface. The subsequent peeling of the cured polymer from the mold was modeled with increasing prescribed displacement. The optimal shrinkage degree was found to increase from 0.92 to 1.9% with increasing mold aspect ratio (aspect ratio is defined as height to width ratio) from 5 to 10.