Mechanical analysis of bi-component-fibre nonwovens: Finite-element strategy

In thermally bonded bi-component fibre nonwovens, a significant contribution is made by bond points in defining their mechanical behaviour formed as a result of their manufacture. Bond points are composite regions with a sheath material reinforced by a network of fibres’ cores. These composite regions are connected by bi-component fibres — a discontinuous domain of the material. Microstructural and mechanical characterization of this material was carried out with experimental and numerical modelling techniques. Two numerical modelling strategies were implemented: (i) traditional finite element (FE) and (ii) a new parametric discrete phase FE model to elucidate the mechanical behaviour and underlying mechanisms involved in deformation of these materials. In FE models the studied nonwoven material was treated as an assembly of two regions having distinct microstructure and mechanical properties: fibre matrix and bond points. The former is composed of randomly oriented core/sheath fibres acting as load-transfer link between composite bond points. Randomness of material’s microstructure was introduced in terms of orientation distribution function (ODF). The ODF was obtained by analysing the data acquired with scanning electron microscopy (SEM) and X-ray micro computed tomography (CT). Bond points were treated as a deformable two-phase composite. An in-house algorithm was used to calculate anisotropic material properties of composite bond points based on properties of constituent fibres and manufacturing parameters such as the planar density, core/sheath ratio and fibre diameter. Individual fibres connecting the composite bond points were modelled in the discrete phase model directly according to their orientation distribution. The developed models were validated by comparing numerical results with experimental tensile test data, demonstrating that the proposed approach is highly suitable for prediction of complex deformation mechanisms, mechanical performance and structure-properties relationships of composites.     

Modeling of coupled dual-scale flow–deformation processes in composites manufacturing

The present contribution is a part of the work towards a framework for holistic modeling of composites manufacturing. Here we focus our attention onto the particular problem of coupled dual-scale deformation–flow process such as the one arising in RTM, Vacuum Assisted Resin Infusion (VARI) and Vacuum Bag Only (VBO) prepregs. The formulation considers coupling effects between macro-scale preform processes and meso-scale ply processes as well as coupling effects between the solid and fluid phases. The framework comprises a nonlinear compressible fiber network saturated with incompressible fluid phase. Internal variables are introduced in terms of solid compressibility to describe the irreversible mesoscopic infiltration and reversible preform compaction processes. As a main result a coupled displacement–pressure, geometrically nonlinear, finite element simulation tool is developed. The paper is concluded with a numerical example, where a relaxation–compression test of a planar fluid filled VBO preform at globally un-drained and partly drained conditions is considered.     

Numerical simulation of metal matrix composites and polymer matrix composites processing by infiltration: a review

The injection of a liquid metal through a fibrous preform is one of the techniques used to manufacture metal matrix composites (MMCs). The flow of metal through fibrous preform is a problem of fluid mechanics in porous medium. Numerical simulations of this process were developed in particular for non-isothermal infiltrations which take into account the phenomena of phase change. In addition, numerical models were developed to predict the appearance of defects in the end product and to study the evolution of the deformation of the fibrous preform during metal infiltration. After pointing out the analogous numerical studies devoted to the Resin Transfer Moulding (RTM) process, we give a progress report on the models developed to date for MMCs.     

Shape optimization of preform tools in forging of aerofoil using a metamodel-assisted multi-island genetic algorithm

Preform design plays an important role in improving the material flow, mechanical properties and reducing defects for forgings with complex shapes. In this paper, a study on shape optimization of preform tools in forging of an airfoil is carried out based on a multi-island genetic algorithm combined with a metamodel technique. An optimal Latin hypercube sampling technique is employed for sampling with the expected coverage of parameter space. Finite element (FE) simulations of multistep forging processes are implemented to obtain the objective function values for evaluating the forging qualities. For facilitating the optimization process, a radial basis function surrogate model is established to predict the responses of the hot forging process to the variation of the preform tool shape. In consideration of the compromise between different optimal objectives, a set of Pareto-optimal solutions are identified by the suggested genetic algorithm to provide more selections. Finally, according to the proposed fitness function, the best solution of multi-objective optimization on the Pareto front is confirmed and the corresponding preform tool shape proves optimal performances with substantially improved forging qualities via FE validation.     

Flow analysis during compression of partially impregnated fiber preform under controlled force

Compression resin transfer molding (CRTM) is an alternative solution to conventional resin transfer molding processes. It offers the capability to produce net shape composites with fast cycle times making it conducive for high volume production. The resin flow during this process can be separated into three phases: (i) metered amount of resin injection into a partially closed mold containing dry fiber preform, (ii) closure of the mold until it is in contact with the fiber preform displacing all the resin into the preform and (iii) further mold closure to the desired thickness of the part compacting the preform and redistributing the resin. Understanding the flow behavior in every phase is imperative for predictive process modeling that guarantees full preform saturation within a given time and under specified force constraints.     

基于混合网格方法的VARTM工艺充模仿真与实验验证

针对VARTM工艺的特点,建立了充模过程树脂流动和预成型体变形行为数学模型。提出了基于混合网格方法的VARTM充模仿真算法,在该算法中,模具型腔几何模型进行二维或三维网格划分,在每个真空袋表面单元上增加一个一维附属单元,用于在仿真过程中实时地吸收或挤出因真空袋变形而产生的局部树脂体积变化,形成混合网格仿真模型;求解过程中,对树脂流动和预成型体变形分别进行求解后,基于上述混合网格模型进行两者耦合操作,实现了仿真精度和速度的统一。搭建了VARTM充模实验平台,进行了一维充模实验,通过仿真结果与实验测量结果对比,验证了本文算法的正确性。最后,通过三维仿真算例,验证了算法对三维复杂结构和顺序浇口策略仿真的可行性。     

A new implementation of the spectral crystal plasticity framework in implicit finite elements

We present a new implementation of a computationally efficient crystal plasticity model in an implicit finite element (FE) framework. In recent publications, we have reported a standalone version of a crystal plasticity model based on fast Fourier transforms (FFTs) and termed it the spectral crystal plasticity (SCP) model. In this approach, iterative solvers for obtaining the mechanical response of a single crystal of any crystallographic orientation subjected to any deformation mode are replaced by a database of FFTs that allows fast retrieval of the solution. The standalone version of the code facilitates simulations of relatively simple monotonic deformation processes under homogeneous boundary conditions. In this paper, we present a new model that enables simulations of complex, non-monotonic deformation process with heterogeneous boundary conditions. For this purpose, we derive a fully analytical Jacobian enabling an efficient coupling of SCP with implicit finite elements. In our implementation, an FE integration point can represent a single crystal or a polycrystalline material point whose meso-scale mechanical response is obtained by the mean-field Taylor-type homogenization scheme. The finite element spectral crystal plasticity (FE-SCP) implementation has been validated for several monotonic loading conditions and successfully applied to rolling and equi-channel angular extrusion deformation processes. Predictions of the FE-SCP simulations compare favorably with experimental measurements. Details of the FE-SCP implementation and predicted results are presented and discussed in this paper.     

TC11钛合金盘模锻过程微观组织的数值模拟

通过热力模拟压缩试验和定量金相组织测试数据,运用BP神经网络方法建立TC11钛合金等轴组织两相区变形的流变应力模型和α相组织演变模型.将所建立的流变应力模型和α相组织演变模型集成到MARC有限元分析软件中,实现TC11钛合金等轴组织两相区变形-传热-微观组织演变的多场耦合模拟技术.采用该技术模拟预测TC11钛合金盘模锻后的α相晶粒尺寸,实验验证结果表明预测误差在10%以内.     

Towards a realistic morphological model for the meso-scale mechanical and transport behavior of cementitious composites

In this work, we investigate the mechanical and transport properties of cementitious composites using a realistic morphological 3D matrix-inclusion-ITZ model for the mesoscale behavior. Assuming a compression damaged plasticity-based behavior for the matrix phase under quasi-static loading, the model is used to determine the effect of inclusion (aggregate) shape, volume fraction and segregation on the mechanical response of the composite specimen under uniaxial loading. We then use the model to evaluate the effect of the previously mentioned variables as well as aggregate intrinsic permeability on the macroscopic permeability of the same composite specimen using a series of non-conforming FE meshes and level set functions to monitor the heterogenities. The model is then applied to a self-compacting concrete (SCC) with a higher volume of cement paste in order to flow more freely and be formed into complex shapes without shaking. However, this elevated fluidity predisposes SCCs to a higher risk of segregation, i.e. separation between the suspending phase and coarse aggregates, which can affect the mechanical behavior of the composite.     

Mechanical Properties of a Novel Zero Poisson's Ratio Honeycomb

In this paper, a novel honeycomb is proposed by embedding a rib into every cell of the existing zero Poisson's ratio (ZPR) configuration, semi re‐entrant honeycomb (SRH). Analytical model is developed to investigate the in‐plane mechanical properties of the new honeycomb, and the resulting theoretical expressions are compared with the experimental tests and numerical results obtained from two different finite element (FE) models (3D beam model and 3D solid model), leading to a good correlation. FE analysis, analytical modeling, and experimental tests of the new honeycomb show that it can achieve ZPR effect in two principal directions. For further studies, parameters analyses are carried out to explore the dependence of the in‐plane mechanical properties versus the geometric parameters. The results show that bending is the dominated deformation model when the new honeycomb is compressed along the x‐ direction, while stretch controlled in the y‐ directional compression. It is remarkable that the new proposed honeycomb features superior specific stiffness and more flexible in mechanical properties tailoring compared to the other ZPR honeycombs in the literature. Given these benefits, the new honeycomb may be promising in some practical applications.

     

Numerical simulation of injection/compression liquid composite molding. Part 2: preform compression

In the injection/compression liquid composite molding process (I/C-LCM), a liquid polymer resin is injected into a partially open mold, which contains a preform of reinforcing fibers. After some or all of the resin has been injected, the mold is closed, compressing the preform and causing additional resin flow. This paper addresses compression of the preform, with particular emphasis on modeling three-dimensional mold geometries and multi-layer preforms in which the layers have different mechanical responses. First, a new constitutive relation is developed to model the mechanical response of fiber mats during compression. We introduce a new form of nonlinear elasticity for transversely isotropic materials. A special case of this form is chosen that includes the compressive stress generated by changes in mat thickness, but suppresses all other responses. This avoids the need to model slip of the preform along the mold surface. Second, a finite element method, based on the principle of virtual displacement, is developed to solve for the deformation of the preform at any stage of mold closing. The formulation includes both geometric and material nonlinearities, and uses a full Newton–Raphson iteration in the solution. An open gap above the preform can be incorporated by treating the gap as a distinct material layer with a very small stiffness. Examples show that this approach successfully predicts compression in dry preforms for three-dimensional I/C-LCM molds.     

预成型体渗透率预测及其受压缩变形的影响

建立了织物预成型体单胞内纱线间细观流动和纱线内部微观流动的统一的数学模型。基于最小势能原理建立了织物松弛状态下的单胞几何模型,同时对在模具压缩下的单胞变形进行了分析,并建立了不同压缩状态下的单胞几何模型。通过对单胞内树脂流动数学模型的数值求解,获得了流动速度场及压力场,进而预测了预成型体的渗透率。预测1组不同压缩状态下的单胞渗透率,研究了预成型体压缩变形对渗透率的影响。结果显示:随着压缩量的增加,其渗透率逐渐降低。通过实验测量及数据分析,验证了建模和预测方法的正确性。     

Anisotropic creep modeling of coated textile membrane using finite element analysis

Coated textile membranes (CTMs) form a class of flexible textile composites undergoing viscoelastic deformation because they consist of a polymeric reinforcement and matrix and are tensioned in service. In most CTMs, woven fabrics are frequently used as a reinforcement structure, causing anisotropic mechanical behavior including time dependent viscoelastic deformation. To describe such anisotropic and nonlinear time dependent deformation, the creep potential with three orthotropic parameters was introduced and incorporated into finite element software through a user material subroutine. The three parameters included in the creep potential were determined by carrying out off-axis coupon creep tests and using various mathematical formulae for the effective creep compliance. To validate the current creep modeling and its implementation in finite element software, off-axis coupon creep tests were re-simulated and compared with the experiments, showing that the present modeling can describe the anisotropic and nonlinear creep deformation of CTM with acceptable accuracy.     

Mechanical characteristics and strengthening effectiveness of random-chopped FRP composites containing air voids

In random-chopped fiber-reinforced polymer (FRP) composites used as a retrofit material, a high volume fraction of voids is inevitable due to the manufacturing characteristics. In this paper, the mechanical characteristics and strengthening effectiveness of random-chopped FRP composites containing air porosity are investigated through experiments and numerical analysis. Coupon-shaped specimens with various material compositions were manufactured to examine the uniaxial tensile performance, and the air voids in the composites were measured by a microscope camera. In order to predict the overall performance of the composites, a micromechanical formulation that accounts for porosity was newly developed. The derived model was incorporated into a finite element (FE) code, and the model parameters were estimated by comparing uniaxial tensile test results for various systems of random-chopped FRP composites. In addition, concrete beams strengthened with the composites were produced to evaluate their load-carrying capacity. The FE predictions of the composite structures were then compared with experimental data to verify the predictive capability of the proposed numerical framework.     

Optimum groove pressing die design to achieve desirable severely plastic deformed sheets

In this paper, considering the problems of common finite element (FE) codes that consider simple constitutive equations, a developed FE code that considers a new constitutive model is used to simulate the behavior of copper sheets under severe plastic deformation (SPD). The new proposed constitutive model, that considers dislocation densities in cell interiors and cell walls of material as true internal state variables, can investigate all stages of flow stress evolution of material during large plastic deformations and also can explain the effects of strain rate magnitude on the mechanical response of material, during room temperature SPD. The proposed FE analysis is used to investigate the effects of die design on the property of SPDed sheets by groove pressing (GP) processes. To do so, the GP processes through existent designations of dies are simulated and a good agreement between the modeling results and experimental data is obtained. In addition, a new die design is proposed that can eliminate the problems of the existent designations of dies and can produce the sheets with higher strength and more uniform hardness.     

Finite Element Assisted Modelling of the Microscopic Impregnation Process in Thermoplastic Preforms

Fibre reinforced composite materials incorporating thermoplastic matrices are gaining increasing popularity in many industrial applications. One of the potential preforms for the manufacture of technical components is commingled yarn composed of reinforcement and matrix in fibre form. These are often employed in the pultrusion process. Another innovative preform consists of polymer powder preimpregnated sheath surrounding fibre bundles. To achieve adequate mechanical properties of the final product it is essential, when producing laminates by a process such as pultrusion with both types of preform, that sufficient matrix impregnation is achieved. The prevention of voids and dry-spots in the laminate requires a theoretical understanding of the mechanisms involved. On a microscopic scale, several finite element (FE) models can be used to simulate the progress of the matrix flow into the interstitial spaces between the single reinforcement fibres. In the present simulations, a hexagonal and a square arrangement account for two of the various fibre packings occurring in a laminate. It permits an estimation of the impregnation performance of commingled and powder impregnated yarns. For each preform the shear rate, to which the polymer matrix is subjected during the impregnation and consolidation process, can be predicted.     

Potential-based constitutive models for cohesive interfaces: Theory,implementation and examples

Cohesive interfaces appear in many materials or structures, e.g. composites or adhesive bonds. Originally introduced to model crack tips in fracture mechanics, cohesive zone models are used to describe the constitutive behavior of cohesive interfaces since the early 1990s. In the present contribution, the concept of generalized standard materials (GSM) is transferred from the modeling of bulk behavior to cohesive zones. The potential-based framework obtained hereby is referred to as standard dissipative cohesive zones (SD-CZ). Within this framework, an irreversible interface model is developed for the one-dimensional case and subsequently extended to three-dimensional transverse isotropy. While the potential structure of the constitutive law may be required for certain applications, it also brings benefits with regard to the numerical implementation. To the best knowledge of the authors, this is the first approach to interface modeling in a GSM-like framework, where dissipative effects like damage and softening are considered as well as normal-tangential coupling for mixed-mode decohesion. Comparisons to experimental data and existing cohesive zone models outline the capabilities of the approach.     

Analysis of non-isothermal mold filling process in resin transfer molding (RTM) and structural reaction injection molding (SRIM)

In this paper, we present a modeling and numerical simulation of a mold filling process in resin transfer molding/structural reaction injection molding utilizing the homogenization method. Conventionally, most of the mold filling analyses have been based on a macroscopic flow model utilizing Darcy's law. While Darcy's law is successful in describing the averaged flow field within the mold cavity packed with a porous fiber preform, it requires experiments to obtain the permeability tensor and is limited to the case of porous fiber preform-it can not be used to model the resin flow through a double porous fiber preform. In the current approach, the actual flow field is considered, to which the homogenization method is applied to obtain the averaged flow model. The advantages of the current approach are: parameters such as the permeability and effective heat conductivity of the impregnanted fiber preform can be calculated; the actual flow field as well as averaged flow field can be obtained; and the resin flow through a double porous fiber preform can be modelled. In the presentation, we first derive the averaged flow model for the resin flow through a porous fiber preform and compare it with that of other methods. Next, we extend the result to the case of double porous fiber preform. An averaged flow model for the resin flow through a double porous fiber preform is derived, and a simulation program is developed which is capable of predicting the flow pattern and temperature distribution in the mold filling process. Finally, an example of a three dimensional part is provided.     

Numerical implementation of an elastic-viscoplastic constitutive model to simulate the mechanical behaviour of amorphous polymers

Due to their high deformation capabilities, polymeric materials are widely used in several industries. However, polymers exhibit a complex behaviour with strain rate, temperature and pressure dependencies. Numerous constitutive models were developed in order to take into account their specific behaviour. Among these models, the ones proposed by Richeton et al Polymer 46:6035–6043 (2005a), Polymer 46:8194–8201 (2005b) seem to be particularly suitable. They proposed expressions for the Young modulus and the yield stress with strain rate and temperature dependence. Moreover, these models were also implemented in a finite elastic-viscoplastic deformation approach using a flow rule based on thermally activated process. The increase of computational capabilities allowed simulating polymer forming processes using finite element (FE) codes. The aim of the study is to implement the proposed constitutive model in a commercial FE code via a user material subroutine. The implementation of the model was verified using compressive tests over a wide range of strain rates. Next, FE simulations of an impact test and of a plane strain forging process were carried out. The FE predictions are in good agreement with the experimental results taken from the literature.