Dynamics and time-stepping schemes for elastic shells undergoing finite rotations

In this work we study the time-stepping schemes for shell models, which describe the shell-director vector motion by the finite rotations. Different possibilities for choosing director rotations are examined and their relationships are cast in terms of the commutative diagram. The Newmark time-stepping schemes, making use of different rotation parameters, are then developed. The mid-point scheme modified to either conserve or dissipate the total energy is further examined. Several numerical simulations are presented to illustrate the performance of each developed scheme.     

Application of differential quadrature method to the delamination buckling of composite plates

The delamination buckling response of a composite panel containing through-the-width delamination is numerically modeled using a solution that is based on the differential quadrature method (DQM). The composite is modeled as a general one-dimensional beam–plate having a through-the -width delamination that can be at any arbitrary location through its thickness; hence, dividing the domain into four regions. The DQM is applied to each region and with the imposition of appropriate boundary conditions, the problem is transformed into a standard eigenvalue problem. Numerical results are presented, illustrating the stability and validity of the method. The results also demonstrate the efficiency of the method in treating this class of engineering problem.     

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.     

Numerical evaluation of the asymptotic energy behavior of intermediate shells with application to two classical benchmark tests

We consider the class of shell problems which are neither purely bending neither purely membrane dominated. In such cases the asymptotic energy norm behavior (which is useful not only because it represents the structure stiffness, but also for numerical comparison purposes) is not a priori known. In this work we apply a numerical procedure in order to estimate the energy behavior of a general shell problem. In order to test its reliability, the method is applied to various problems for which the theoretical energy behavior is known and the results can be compared. Among the problems tested, we have two classical engineering shell benchmarks which are neither bending neither membrane dominated, and for which an analytical evaluation has been obtained in a recent work. All the energy behavior estimates obtained with the numerical method are in perfect agreement with the theoretical values.     

Design and analysis of stiffened composite panels including post-buckling and collapse

The European aircraft industry demands reduced development and operating costs, by 20% and 50% in the short and long term, respectively. Contributions to this aim are provided by the completed project POSICOSS (5th FP) and the running follow-up project COCOMAT (6th FP), both supported by the European Commission. As an important contribution to cost reduction a decrease in structural weight can be reached by exploiting considerable reserves in primary fibre composite fuselage structures through an accurate and reliable simulation of post-buckling up to collapse. The POSICOSS team developed fast procedures for the post-buckling analysis of stiffened fibre composite panels, created comprehensive experimental data bases and derived suitable design guidelines. COCOMAT builds up on the POSICOSS results and considers in addition the simulation of collapse by taking degradation into account. The results comprise an extended experimental data base, degradation models, and improved certification and design tools as well as extended design guidelines.One major task of POSICOSS and COCOMAT is the development of improved analysis tools that are validated by experiments performed within the framework of the projects. Because the new tools must comprise a wide range of various aspects a considerable number of different structures had to be tested. These structures were designed under different objectives (e.g. large post-buckling region). For the design process, the consortiums applied state-of-the-art simulation tools and brought in their own design experience. This paper deals with the design process as performed within both projects and with the applied analysis procedures. It is focused on the DLR experience in the design and analysis of stringer-stiffened CFRP panels gained within the scope of these two projects.     

Characterization of delamination effect on composite laminates using a new generalized layerwise approach

A dynamic analysis method has been developed to investigate and characterize the effect due to presence of discrete single and multiple embedded delaminations on the dynamic response of composite laminated structures with balanced/unbalanced and arbitrary stacking sequences in terms of number, placement, mode shapes and natural frequencies. A new generalized layerwise finite element model is developed to model the presence of multiple finite delamination in laminated composites. The new theory accurately predicts the interlaminar shear stresses while maintaining computational efficiency.     

A grid-based algorithm for the generation of hexahedral element meshes

An algorithm for the generation of hexahedralelement meshes is presented. The algorithm works in two steps: first the interior of the volume is filled with a regular grid; then the boundary region is meshed by using basically twodimensional operations.The algorithm was designed for use in the fem-simulation of metal forming processes where a remeshing must be done very often. In principle, it can be used for meshing any geometry with hexahedral elements and examples of meshes for geometries arising from various applications are given. The algorithm is checked against the criteria proposed by Sabin [1] (Advances in Engineering Software, 13, 220–225).     

水下结构声辐射问题之计算机仿真

该文采用混合有限元／边界元法计算水下结构物声辐射问题。为有限元软件包Nastran开发的Fortran代码用于计算外域流体的附加质量和附加阻尼,DMAP实现：Fortran代码与Nastran的结合,成功实现了应用现有有限元软件计算无界流域种结构辐射噪声的问题。采用Patran前后处理软件实现了对水下结构响应及其辐射声场的仿真。     

Micromechanical analysis of interfacial debonding in unidirectional fiber-reinforced composites

In this paper the behaviour of unidirectional fiber-reinforced composites with imperfect interfacial bonding is investigated by using the finite element method. The matrix and the fibers are considered homogeneous, isotropic and linearly elastic; an interfacial failure model is implemented by connecting the fibers and the matrix at the finite element nodes by normal and tangential brittle-elastic springs. Numerical analyses are carried out on a unit cell extracted from a composite model characterized by a periodic microstructure. The micromechanical simulations provide the first failure loci (FFL), i.e. the envelopes of the composite average strains corresponding to the initiation of the debonding, and the instantaneous overall moduli of the damaged composite corresponding to several interface damage configurations. The results show that the intersections of the FFL with prescribed planes are defined by curves associated with different interface failure modes. Besides, damage induced by fiber–matrix debonding is strongly anisotropic and dependent on the load history.     

Finite Element Simulation of Three-Dimensional Free-Surface Flow Problems

An adaptive finite element algorithm is described for the stable solution of three-dimensional free-surface-flow problems based primarily on the use of node movement. The algorithm also includes a discrete remeshing procedure which enhances its accuracy and robustness. The spatial discretisation allows an isoparametric piecewise-quadratic approximation of the domain geometry for accurate resolution of the curved free surface. The technique is illustrated through an implementation for surface-tension-dominated viscous flows modelled in terms of the Stokes equations with suitable boundary conditions on the deforming free surface. Two three-dimensional test problems are used to demonstrate the performance of the method: a liquid bridge problem and the formation of a fluid droplet.     

运动系统稳定性的计算机仿真研究

运动稳定性对机构工作性能有重大影响。对于复杂的多参数系统，难以通过理论分析来预测系统的运动趋势，尤其对于存在弹簧，阻尼器等非线性零部件的机械系统。该文采用有限元分析对运动过程进行实时仿真（real time simulation,RTS)研究，模拟运动系统的实际运动过程，从而分析研究运动稳定性与结构参数的关系。该文介绍了这种方法的基本原理和建模方法，并分析了一组弹簧连接物体的运动稳定性。分析结果表明，该方法能够有效地在设计阶段对结构的动力学性能进行预测与评价。     

On spurious velocities in incompressible flow problems with interfaces

We show that a non-physical velocity may appear in the finite element calculation of incompressible two-phase flows subjected to an external local force. There are different sources responsible for this phenomenon: approximation of the incompressibility constraint, the interface, and the local external force. Furthermore, we demonstrate that this non-physical velocity does not vanish with the expected order of convergence. We evaluate different concepts and give recommendations for a proper handling of this phenomenon.     

计算机仿真在管道漏磁检测中的应用

磁路的设计、校验和提高设计开发效率是漏磁检测的关键问题。该文介绍了管道漏磁检测技术的基本原理和设计要素。采用有限元仿真的方法对漏磁检测进行分析仿真,实验结果表明这是一种有效的设计校验方法。在有限元仿真、三维cad、产品数据管理等子系统的基础上构建了漏磁检测器的设计开发系统,使产品研发工作得以优化重组,有效提高了开发效率、缩短开发时间、减少开发成本。     

有限单元法在塑料注射充模流动模拟中的应用

对塑料熔体注射成型充模流动过程的流变方程进行了熔体充模过程流变方程参数基于3维有限单元法的数值求解研究,得到基于形函数的压力刚度矩阵、温度刚度矩阵的方程式,进而得出压力场、速度场、温度场数值解的方程式。据此完成了注射熔体流动充模过程由M atlab编程实现的有限元分析本体程序。用具体的注射成型塑料制品作为检验实例,用2种不同的塑料进行注射成型模拟,其结果与国际著名的M old flow商品化软件和文献数据进行比较,表明本研究的设计软件算法正确、程序运行速度快。     

Simulation and fabrication of piezoresistive membrane type MEMS strain sensors

Two piezoresistive (n-polysilicon) strain sensors on a thin Si₃N₄/SiO₂ membrane with improved sensitivity were successfully fabricated by using MEMS technology. The primary difference between the two designs was the number of strips of the polysilicon patterns. For each design, a doped n-polysilicon sensing element was patterned over a thin 3 μm Si₃N₄/SiO₂ membrane. A 1000×1000 μm² window in the silicon wafer was etched to free the thin membrane from the silicon wafer. The intent of this design was to fabricate a flexible MEMS strain sensor similar in function to a commercial metal foil strain gage. A finite element model of this geometry indicates that strains in the membrane will be higher than strains in the surrounding silicon. The values of nominal resistance of the single strip sensor and the multi-strip sensor were 4.6 and 8.6 kΩ, respectively. To evaluate thermal stability and sensing characteristics, the temperature coefficient of resistance [TCR=(ΔR/R₀)/ΔT] and the gage factor [GF=(ΔR/R₀)/] for each design were evaluated. The sensors were heated on a hot plate to measure the TCR. The sensors were embedded in a vinyl ester epoxy plate to determine the sensor sensitivity. The TCR was 7.5×10⁻⁴ and 9.5×10⁻⁴/°C for the single strip and the multi-strip pattern sensors. The gage factor was as high as 15 (bending) and 13 (tension) for the single strip sensor, and 4 (bending) and 21 (tension) for the multi-strip sensor. The sensitivity of these MEMS sensors is much higher than the sensitivity of commercial metal foil strain gages and strain gage alloys.     

A Ritz procedure for optimisation of cylindrical shells, formed by a nearly symmetric and balanced angle-ply composite laminate, with fixed minimum frequency

The paper deals with the problem of optimisation of a cylindrical shell profile under a frequency constraint. The minimum value of the thickness has been established a priori. The structure considered is typical of aerospace craft vessels.The same value of the lowest vibration frequency of the reference cylindrical shell with uniform thickness, has been imposed. That is the minimization procedure of the structure weight must not affect its lowest vibration frequency.Instead of the currently applied finite element method (FEM), Ritz series expansions have been utilized in the analytical developments both for the dynamic variables and for the thickness axial distribution over the shell surface. Lagrange multipliers, together with governing equations and objective function, have been utilized to form the Lagrangian functional, as in the classical Euler–Lagrange method. Imposing the stationary conditions with respect to the Lagrangian degrees of freedom gives a non-linear algebraic equations system, whose solution can be found with an appropriate algorithm.A series of repeated optimisation operations have been performed to arrive at the minimized weight profile, but with the pre-established minimum value of the shell thickness.A simplified nearly symmetric and balanced multilayer composite angle-ply laminate of the shell structure is supposed, as in the case of the uniform thickness reference shell, previously considered for the dynamic analysis. Significant results of some computation application cases can be helpful to evaluate the efficiency of the proposed optimisation procedure applied to cylindrical structures.     

基于有限元模型的连铸坯感应加热过程仿真

研究连铸坯感应加热控制过程,提高电热效率和满足连铸坯轧制温度.为了能够直观、准确地获得感应加热过程的温度分布规律,采用计算机数值仿真技术对感应加热过程进行仿真.以某钢铁厂热连轧连铸坯感应加热过程为例,利用有限元分析软件ANSYS建立感应加热模型,再现了连铸坯经过两个感应器的加热过程,并获取了钢坯内部关键点的温度变化数据.结果表明有限元数值仿真与实际相符,可为感应加热装置的优化设计和智能控制提供重要依据.     

Automatic hexahedral mesh generation for multi-domain composite models using a hybrid projective grid-based method

This paper presents an algorithm to generate an all-hexahedral mesh of a multi-domain solid model using a hybrid grid-based approach. This is based on a projective concept during the boundary adaptation of the initial mesh. In general, the algorithm involves the generation of a grid structure, which is superimposed on the solid model. This grid structure forms an initial mesh consisting of hexahedral elements, which intersect fully or partially with the solid model. This initial mesh is then shrunk in an outside-in manner to the faces of the model through a node projection process using the closest position approach. To match the resulting mesh to the edges of the model, a minimal deformation angle method is used. Finally, to match the vertices with the nodes on the mesh, a minimal warp angle method is employed. To create the mesh of a multi-domain solid model, an outside-in and inside-in hybrid of the grid-based method is used. This hybrid method ensures that the meshes of the different domains are conforming at their common boundary. This paper also describes two methods for resolving cases of degenerate elements: a splitting technique and a wedge insertion technique.     

虚拟手术中的针操作仿真研究

详细论述并实现了虚拟手术中针操作仿真技术.首先将材料的各向异件引入基于有限元理论的动态Tensor-Mass生物力学模型,并结合动力学方程以仿真软组织形变;接着根据物理运动规律描述手术针与人体组织间相互作用关系,然后结合结点投影法来论述针力与摩擦力的处理策略,并给出算法具体描述;最后提出一种局部网格细分法与针力等效处理方法相结合策略以提高仿真精度与速度.实验结果证明该方法不仅合理处理针与组织间作用力关系,而且能够避免网格单元数目激增,同时可消除滞后性与网格单元退化等,从而降低时空复杂性,并满足仿真精度与实时性,也提高仿真系统健壮性.实验结果显示针操作能够实时仿真活组织检查、介入治疗与跟骨钻孔等,在医疗中具有实际意义.