二步法三维编织变截面薄壁壳体RTM 工艺数值模拟与试验研究

采用分段-集合计算方法, 对二步法三维编织变厚度变截面薄壁壳体RTM 充模工艺过程进行了较深入的理论研究。提出了较准确的树脂流动速度、树脂充模时间和树脂流动压力计算方程。数值预测值与充模试验结果具有良好的一致性, 所推导理论方程为合理设计RTM 充模工艺参数提供了理论依据。      

RTM工艺参数对树脂充模过程影响 的模拟与实验研究

通过开发计算机程序与有限元/控制体积分析软件,能够实现对任意复杂三维形状复合材料构件的造型和RTM工艺充模过程的模拟。研究了改变工艺参数时工字孔平板的RTM工艺模拟结果和实验结果,两者基本保持一致;证明了恒压注射情况下,充模时间与注射压力、渗透率成反比,与树脂粘度成正比;也证实了该模拟软件确实可用于预测树脂流动模式以及成型效率,为RTM实际工艺设计与优化提供了有效的技术手段。      

RTM 工艺树脂注射温度优化

在RTM 工艺模拟及RTM 工艺专用树脂黏度特性研究的基础上, 提出充模时间与安全系数的乘积小于树脂低黏度平台时间的树脂注射温度优化标准和充模时间、树脂低黏度平台时间的计算方法, 制定了程序化流程并编制了软件, 使优化得以实施。在此基础上, 选取Q Y8911-Ⅳ双马树脂对所选制件进行模拟充模作为注射温度优化的实例, 确定了树脂低黏度平台时间的变化范围, 得到了不同条件下的优化注射温度。优化实例表明, 优化软件可实现对树脂注射温度的优化。      

RTM 工艺充模过程数值模拟及实验比较

树脂传递模塑(RTM ) 工艺越来越成为一种高效的先进纤维增强复合材料的制造方法, 其中RTM 的充模过程是一个很重要的步骤。本文对增强材料各向异性的二维RTM 充模过程进行数值模拟, 并把数值模拟结果和实验结果相比较。      

基于VOF/PLIC界面追踪方法的RTM工艺充模过程数值模拟

针对基于Darcy定律的树脂传递模塑(RTM)工艺的充模过程数值模拟的局限性,将纤维预制体内的充填流动作为两相流(树脂相和空气相)处理,在动量方程中考虑了惯性项和粘性项,采用有限体积方法(FVM)离散控制方程,并与VOF/PLIC界面追踪方法相结合,发展了求解树脂在纤维预制体内非稳态流动问题的数值模拟方法.在此基础上开发了RTM工艺的充模过程数值模拟程序,其算例的数值模拟结果与解析解或实验结果吻合良好,验证了此数值模拟方法的有效性和可靠性.     

视窗化RTM工艺充模过程模拟仿真技术研究

根据RTM工艺树脂流动充模模型,研究和开发了基于FEM/CV算法的RTM工艺复杂渗流充模过程数值模拟软件平台-BHRTM-2。BHRTM-2在视窗系统下运行,带有FEM网格捕捉器窗口可直观方便地设置注射口、溢料口和工艺参数,操作简单,能够模拟复杂边界制件的树脂流动充模过程、显示充模过程中任意时刻模腔内压力的分布场、流动前峰和预测充模时间及可能的干斑缺陷位置,为RTM工艺设计与优化提供了有效技术手段。文中对BHRTM-2的模拟结果的正确性和可靠性进行了理论与实验验证,并给出了具体算例。      

RTM工艺树脂流动过程数值模拟及实验比较

树脂充模是RTM工艺成型过程中的重要一环。研究了RTM工艺树脂流动过程的特点,建立了树脂渗流控制方程。采用贴体坐标/有限差分法模拟了树脂渗流过程,给出了不同时刻树脂流动前沿曲线及终止时刻压力场分布,计算结果与试验结果吻合良好。     

基于孔隙控制的车身结构树脂传递模塑成型工艺设计

以典型车身结构B柱为研究对象,结合实验与仿真分析研究其树脂传递模塑(RTM)工艺的优化设计方法。研究了通过注射方式的优化控制树脂流动前沿,从而达到降低制件孔隙率和保证制件质量的目的。首先通过自制的变厚度渗透率测试模具获取所选用织物的渗透率,之后通过真空辅助RTM实验与对应模拟仿真进行对比分析来验证所采用仿真方法与渗透率数据的可靠性。最后结合充模周期与孔隙率控制理论对RTM工艺注射口分布及注射方式进行优化设计。结果表明,针对所选定车身结构,优化速率注射方式所获得的制件孔隙率最低,但充模周期较长,而基于双点注射的恒流量注射方式能较好地兼顾充模周期与制件孔隙率的要求。     

复杂形状三维薄壁构件RTM制造工艺注模过程模拟

采用有限单元/控制体积方法,编写了RTM工艺注模过程模拟程序SHELLCV。SHELLCV程序包含三角形壳元和矩形壳元两种单元,可以模拟复杂形状三维薄壁构件的RTM制造工艺注模过程,得到任意时刻的树脂流动前峰曲线、压力场分布。与解析解比较验证了SHELLCV程序的稳定性和可靠性。算例研究表明SHELLCV程序可为工艺设计:注射口和排气口的布置,合模压力提供有效的参考依据。      

有限元网格对RTM充模模拟的影响

本工作基于有限元控制体积法编写了模拟程序,研究了节点数量、网格密度对模拟过程的影响.以带圆孔的长方形为例,模拟比较了不同网格密度和注入口位置对充模时间、流动前沿以及干点形成区域的影响.结果表明,节点数量增加到一定程度时,只延长计算时间,对充模时间影响很小;网格疏密分布影响流动前沿的位置,根据流动前沿的形状可以确定排气口的位置和数量.     

Applications of a self-adaptive algorithm to non-linear finite element analysis

A self-adaptive algorithm has been developed and implemented for the implicit time integration of non-linear finite element analysis. In this algorithm, a proper time increment for the next time step is estimated based on the deformation pattern at the preceding step. The iteration process for the equalibrium employs expeditious methods such as quasi-Newton updates and line searches as well as an adaptive stiffness matrix update strategy for efficiency. Convergence difficulties induced from inadequate prediction of step size or the change in non-linearities are tackled by the bisection method. These procedures were also successfully applied to static problems by ignoring the damping and the inertia forces. The objective of this paper is to demonstrate the applicability and the effectiveness of the adaptive algorithm in a wide spectrum of non-linear problems. Six example problems are illustrated, some of which are rather novel. As demonstrated in this paper, the self-adaptive algorithm implemented in MSC/NASTRAN is proving to be versatile, accurate and efficient.     

Applying ant colony optimization for load balancing on grid

Grid computing is comprised of many distributed nodes in order to compute and analyze a set of distributed data. To improve the processing performance, an appropriate load-balancing algorithm is required to equally distribute loads among the grid’s nodes. In this article, an algorithm based on ant colony optimization is proposed to deal with load-balancing problems. In this approach, when an ant reaches a node, the ant’s table and the node’s table exchange their information and update each other. In order to move to the most appropriate node, the ant selects the next node from the current node’s table according to the nodes’ loads and their CPU rates. This process is continued until the ant passes the predefined steps. The experimental results show that while implementing the proposed algorithm to the grid environment, increasing the number of jobs and their length has insignificant impact on the system response time.     

Optimization of injection flow rate to minimize micro/macro-voids formation in resin transfer molded composites

Resin transfer molding (RTM) has become one of the most widely used processes to manufacture medium size reinforced composite parts. To further enhance the process yield while ensuring the best possible quality of the produced parts, physically based optimization procedures have to be devised. The filling of the mold remains the limiting step of the whole process, and the reduction of the filling time has an important impact on the overall cost reduction. On the other hand, the injection cycle has to be appropriately carried out to ensure a proper fiber impregnation. Indeed, a partial fiber impregnation leads to the creation of micro-scopic and macro-scopic voids.In the present work, based on a double scale flow model and the capillary number Ca, an optimization algorithm is proposed to minimize the micro/macro-voids in RTM composite parts. The optimized injection flow rate ensures an optimum Ca at the flow front during part filling. The implemented algorithm allows the use of various constraints such as maximum capabilities of the injection equipment (i.e., maximum pressure or flow rate at the injection gates) or maximum velocity to avoid fiber washing. Bounded by these constraints, the optimization procedure is devised to handle any injection configuration (i.e., injection gates or vents locations) for two or three-dimensional parts. The numerical model is based on a mixed (FE/CV) formulation that uses non-conforming elements to ensure mass conservation. The proposed algorithm is tested for two and three-dimensional parts while emphasizing the important void reduction that results from the optimized injection cycle.     

PCG solver and its computational complexity for implicit control-volume finite-element method of RTM mold filling simulation

For isoparametric element meshes, the control-volume finite-element method for resin transfer molding (RTM) mold filling generates an asymmetric matrix, and the performance of the pre-conditioner conjugate gradient (PCG) solver decreases by almost one order of magnitude, even for meshes with very few trivial asymmetric data points. In this paper, the asymmetric parts of the linear equations were transferred to the right-hand sides, and then the linear equations were transformed into an equivalent set of symmetric equations. The right-hand sides of the system of equations were updated only when the set of filled nodes changed. The time steps were controlled by the rule of “one time step, one element-size distance.” Based on the PCG solver and the time-step strategy, the computational complexity of the implicit control-volume method was analyzed and presented. Both analytical and case studies showed that the computational complexity of the PCG solver was of order N squared (where N is the number of nodes) for both 2.5D and 3D meshes. The proposed approach was very suitable for a 3D mesh and had the capability of simulating a mesh with 50,000 nodes in under one hour using a 2.0 GHz CPU, 512M RAM computer.     

基于自适应迭代搜索的三维质心定位算法

在加权质心定位算法和接收信号强度指示测距模型分析的基础上,提出了一种基于自适应迭代搜索的三维质心定位算法。首先计算未知节点的初始搜索步长,然后通过多次迭代、更新搜索点和搜索步长,提高节点的定位精度。该算法能自动调节搜索步长,减少迭代次数,具有一定的自适应能力。仿真分析了不同比例的信标节点、通信半径对定位误差的影响。结果表明,该算法的定位精度和稳定性良好,适用于无线传感器网络的三维定位。     

Adaptive mesh generation for mould filling problems in resin transfer moulding

In injection moulding processes such as Resin Transfer Moulding (RTM) for example, numerical simulations are usually performed on a fixed mesh, on which the numerical algorithm predict the displacement of the flow front. Error estimations can be used in the numerical algorithm to optimise the mesh for the finite element analysis. The mesh can be also adapted during mould filling to follow the shape of the moving boundary. However, in order to minimize computer time, it is preferable to optimise the mesh before carrying out the filling calculation. In this paper, these ideas are adapted to 3D shells, which represent the most common type of composite parts manufactured by RTM. An error estimator generally used in planar or solid geometries is extended for curved 3D surfaces in the specific case of RTM calculations. The extension consists of a projection of the solution field in the tangent plane to avoid problems related to the curvature of the part. Some other issues specific to shell geometries are pointed out and the results of a filling simulation made on a real part are presented. Non-isothermal filling simulations are also carried out in a rectangular mould to illustrate the stability conditions that arise from the convective heat transfer problem. Finally, an analytical study of radial injections is carried out to illustrate issues related to four types of different mesh refinement procedures: (1) a constant time step, (2) constant radial density (to allow a regular progression of the flow front at each time step), (3) a constant Courant number (to ensure stable thermal simulations); and (4) finally, a constant interpolation error.     

Adaptive update interval tracking based on adaptive grid interacting multiple model

The update interval is adjustable in the phased array radar system. An adaptive update interval algorithm based on the adaptive grid interacting multiple model is proposed. The moving step size of mid-model's parameter is utilised to adapt the update interval to the target's behaviour. Furthermore, a controllable parameter is introduced to balance the tracking precision and the system load. The effectiveness of the algorithm is verified through simulation. The simulation results also demonstrate that the proposed algorithm can save much system resource while achieving the same tracking quality as the fixed update interval algorithm.     

A semi‐implicit integration scheme for a combined viscoelastic‐damage model of plastic bonded explosives

This paper presents a new implementation of a constitutive model commonly used to represent plastic bonded explosives in finite element simulations of thermomechanical response. The constitutive model, viscoSCRAM, combines linear viscoelasticity with isotropic damage evolution. The original implementation was focused on short duration transient events; thus, an explicit update scheme was used. For longer duration simulations that employ significantly larger time step sizes, the explicit update scheme is inadequate. This work presents a new semi‐implicit update scheme suitable for simulations using relatively large time steps. The algorithm solves a nonlinear system of equations to ensure that the stress, damaged state, and internal stresses are in agreement with implicit update equations at the end of each increment. The crack growth is advanced in time using a sub‐incremental explicit scheme; thus, the entire implementation is semi‐implicit. The theory is briefly discussed along with previous explicit integration schemes. The new integration algorithm and its implementation into the finite element code, Abaqus, are detailed. Finally, the new and old algorithms are compared via simulations of uniaxial compression and beam bending. The semi‐implicit scheme has been demonstrated to provide higher accuracy for a given allocated computational time for the quasistatic cases considered here. Published 2014. This article is a US Government work and is in the public domain in the USA.     

A three-step renumbering procedure for high-order finite element analysis

An efficient renumbering method for high-order finite element models is presented. The method can be used to reduce the profile and wavefront of a coefficient matrix arising in high-order finite element computation. The method indirectly performs node renumbering and involves three main steps. In the first step, nodes at corners of the elements are numbered using an existing renumbering algorithm. In the second step, elements are numbered in an ascending order of their least new corner node numbers. Finally, based on the new element numbers, both corner and non-corner nodes are renumbered using an algorithm that simulates the node elimination procedure in a frontal solution method. The method is compared to the algorithms that directly perform node renumbering. The numerical results indicate that the three-step algorithm presented here is an order of magnitude faster and the resulting renumbering produces excellent profile and wavefront characteristics of the coefficient matrix. © 1998 John Wiley & Sons, Ltd.