RTM工艺中单向纤维排列中的缺陷迁移的数值模拟EI

Decreasing the amount of residual voids during the resin infiltration into fibrous porous media is an important aspect in manufacturing high performance composite materials.In order to better understand void transports and flow behaviors in filling process,which affects immediately the final void content,a finite-element scheme for transient simulations of the void migration in a transverse flow through the uniaxial micro-structured fibrous media is developed in this work.A volume-of-fluid (VOF) method has been incorporated in the Eulerian frame to capture the free surface of the resin flow.The implementation of periodic boundary condition to the vertical direction avoids unwanted wall effect.The void migration in a dual-scale fiber tow model was investigated.The voids are observed to be transported through the inter-tow region as well as entrapped into fiber tow.It is that the motion of void lagged behind macro flow front which implies that the adequate resin bleeding after mold filling is crucial to remove the entrapped air.     

RTM工艺中单向纤维排列中的缺陷迁移的数值模拟

Decreasing the amount of residual voids during the resin infiltration into fibrous porous media is an important aspect in manufacturing high performance composite materials.In order to better understand void transports and flow behaviors in filling process,which affects immediately the final void content,a finite-element scheme for transient simulations of the void migration in a transverse flow through the uniaxial micro-structured fibrous media is developed in this work.A volume-of-fluid (VOF) method has been incorporated in the Eulerian frame to capture the free surface of the resin flow.The implementation of periodic boundary condition to the vertical direction avoids unwanted wall effect.The void migration in a dual-scale fiber tow model was investigated.The voids are observed to be transported through the inter-tow region as well as entrapped into fiber tow.It is that the motion of void lagged behind macro flow front which implies that the adequate resin bleeding after mold filling is crucial to remove the entrapped air.     

Correlation of void distribution to VARTM manufacturing techniques

Vacuum assisted resin transfer molding (VARTM) is a liquid composite molding (LCM) technique used to manufacture large scale composite structures. Fiber preforms are placed on a tool surface and covered by a flow enhancement layer and a plastic bag. A vacuum is drawn on the system to infuse the resin. When the resin does not fully saturate the empty regions in between the fibers, voids are created. The fiber tows in woven and stitched preforms have a much lower permeability as compared to the bulk permeability of the fabric. Hence, fiber tows saturate with resin later than the pores between fiber tows and are more prone to voids.This study explores the impact of extended resin bleeding time and additional flow resistance at the vent on the void content within fiber tows both experimentally and by numerical simulation. Samples were machined from each of the manufactured panels and analyzed using image analysis techniques to obtain a relative void content. The experimental results were compared to results obtained by numerical simulation.The experimental void distribution showed that if resin is not allowed to bleed or if no external resistance is attached at the vent, the void content over the length of the part is not uniform. All void levels reduced when resistance was added or bleeding was allowed. The discrepancy between experimental and numerical results was addressed by including deformable distribution media in numerical model to capture the continuation of resin flow after the injection gate is closed.     

An efficient solver of the saturation equation in liquid composite molding processes

A major issue in Liquid Composite Molding Process (LCM) concerns the reduction of voids formed during the resin filling process. Reducing the void content increases the quality of the composite and improves its mechanical properties. Most of modeling efforts on process simulation of mold filling has been focused on the single phase Darcy’s law, with resin as the only phase, ignoring the formation and transport of voids. The resin flow in a partially saturated region can be characterized as two phase flow through a porous medium. The mathematical formulation of saturation in LCM takes into account the interaction between resin and air as it occurs in a two phase flow. This model leads to the introduction of relative permeabilities as a function of saturation. The modified saturation equation is obtained as a result, which is a non-linear advection-diffusion equation with viscous and capillary phenomena. In this work, a flux limiter technique has been used to solve a modified saturation equation for the LCM process. The implemented algorithm allows a numerical optimization of the injected flow rate which minimizes the micro/macroscopic void formation during mold filling. Some preliminary numerical results are presented here in order to validate the proposed mathematical model and the numerical scheme. This formulation opens up new opportunities to improve LCM flow simulations and optimize injection molds.     

Voids and their effect on the strain rate dependent material properties and fatigue behaviour of non-crimp fabric composites materials

The manufacture of composite structures is inevitably linked to the formation of voids. Several non-destructive techniques are potentially able of detecting defects, but just the exact knowledge of the effects of defects on the mechanical properties allows the definition of thresholds for the purpose of quality management. In this paper an experimental program for characterizing the effect of voids on the composite materials behaviour is presented. Therefore glass fibre non-crimp fabric reinforced epoxy composites were produced using vacuum assistant resin transfer moulding. For obtaining various void contents specially modified process parameters were used. Nominally defect free specimens are compared with flawed specimens. Tensile testing at different loading speeds and fatigue tests in tension-compression loading are performed.     

Coupled visco-mechanical and diffusion void growth modelling during composite curing

Most critical processing step during long fiber reinforced epoxy matrix composite laminate manufacturing is the polymerization stage. If not optimized, it gives birth to defects in the bulk material, such as voids. These defects are considered as possible sources of damage in the composite parts. The aim of this work is to model the evolution of void growth in thermoset composite laminates after ply collation (autoclave processes) or resin impregnation (RTM, LCM process). A coupled mechanical and diffusion model is presented to better predict the final void size at the end of polymerization. Amongst the parameter investigated, onset of pressure application and diffusive species concentration where found to have a major effect on void size evolution during curing process.     

Numerical prediction of saturation in dual scale fibrous reinforcements during Liquid Composite Molding

This paper presents a fractional flow model based on two-phase flow, resin and air, through a porous medium to simulate numerically Liquid Composites Molding (LCM) processes. It allows predicting the formation, transport and compression of voids in the modeling of LCM. The equations are derived by combining Darcy’s law and mass conservation for each phase (resin/air). In the model, the relative permeability and capillary pressure depend on saturation. The resin is incompressible and the air slightly compressible. Introducing some simplifications, the fractional flow model consists of a saturation equation coupled with a pressure/velocity equation including the effects of air solubility and compressibility. The introduction of air compressibility in the pressure equation allows for the numerical prediction of the experimental behavior at low constant resin injection flow rate. A good agreement was obtained between the numerical prediction of saturation in a glass fiber reinforcement and the experimental observations during the filling of a test mold by Resin Transfer Molding (RTM).     

Simulation of void formation in liquid composite molding processes

Air entrapment within and between fiber tows during preform permeation in liquid composite molding (LCM) processes leads to undesirable quality in the resulting composite material with defects such as discontinuous material properties, failure zones, and visual flaws. Essential to designing processing conditions for void-free filling is the development of an accurate prediction of local air entrapment locations as the resin permeates the preform. To this end, the study presents a numerical simulation of the infiltrating dual-scale resin flow through the actual architecture of plain weave fibrous preforms accounting for the capillary effects within the fiber bundles. The numerical simulations consider two-dimensional cross sections and full three-dimensional representations of the preform to investigate the relative size and location of entrapped voids for a wide range of flow, preform geometry, and resin material properties. Based on the studies, a generalized paradigm is presented for predicting the void content as a function of the Capillary and Reynolds numbers governing the materials and processing. Optimum conditions for minimizing air entrapment during processing are also presented and discussed.     

Determining the effect of voids in GFRP on the damage behaviour under compression loading using acoustic emission

Defects in composite structures, such as voids, have a major influence on the damage behaviour and mechanical properties. Based on the conducted experiments the influence of voids on the mechanical properties of multi-axial glass fibre non crimp fabric composites under quasi-static compression load was examined. Optical in-situ inspection and acoustic emission measurement were applied to detect the failure behaviour during compression tests, while paying special attention to the early stages of damage appearance where the Young’s modulus is determined. The specimens were produced by resin transfer moulding with different processing parameters to obtain various void contents and morphologies.     

Prediction of optimal flow front velocity to minimize void formation in dual scale fibrous reinforcements

Liquid Composite Molding (LCM) is an increasingly used class of processes to manufacture high performance composites. Engineering fabrics commonly used in LCM generally have a dual scale architecture in terms of porosity: microscopic pores exist between the filaments in the fiber tows, while macroscopic pores appear between the tows. Capillary flows in fiber tows play a major role on the quality of composites made by resin injection through fibrous reinforcements. This paper reports on an investigation on fabric imbibition characterization and subsequent evaluation of the optimal flow front velocity during resin injection through fibrous reinforcements. The goal is to devise more robust LCM processes and improve part quality. In order to evaluate a priori the injection conditions that minimize void formation, an impregnation model is developed based on imbibition characterization. This approach allows predicting the optimal front velocity without having to model complex dual scale flows through fibrous reinforcements and without performing expensive and time-consuming fabrication tests. After a summary of previous imbibition results obtained with a probe fluid, the optimal modified capillary numbers are computed by the new predictive model and the values are compared with results reported in the literature on void formation in LCM processes. Afterwards, capillary rise measurements are carried out with four infiltration fluids in order to evaluate the range of optimal flow front velocity that minimizes void formation. This characterization is implemented with vinyl ester resin, epoxy anhydride resin, styrene and anhydride. Finally, the optimal flow front velocity is evaluated for several fabric configurations.     

Three-dimensional reconstruction of resin flow using capacitance sensor data assimilation during a liquid composite molding process: A numerical study

Liquid composite molding (LCM) is a method to manufacture fiber-reinforced composites, where dry fabric reinforcement is impregnated with a resin in a molding apparatus. However, the inherent process variability changes resin flow patterns during mold filling, which in turn may cause void formation. We propose a method to reconstruct three-dimensional resin flow in LCM, without embedding sensors into the composite structure. Capacitance measured from pairs of electrodes on molding tools and the stochastic simulation of resin flow during an LCM process are integrated by a sequential data assimilation method based on the ensemble Kalman filter; then, three-dimensional resin flow and permeability distribution are estimated simultaneously. The applicability of this method is investigated by numerical experiments, characterized by different spatial distributions of permeability. We confirmed that changes in resin flow caused by spatial permeability variations could be captured and the spatial distribution of permeability could be estimated by the proposed method.     

纺织复合材料预制件多层机织布内气泡形成机理

以多层机织布为对象,首先基于两个分别对应于两种典型叠加模式的单元结构模型,通过理论分析,建立了多层机织布纵向切面内气泡形成的分析模型;然后采用有限元-控制体积方法对单元结构内树脂的扩散及气泡形成的过程进行了数值模拟。理论分析和数值模拟的结果符合一致,均表明:不同局部区域差异很大的渗透系数是气泡形成的最根本原因,而纬线的轴向渗透系数和经线的横向渗透系数之比对多层机织布纵向切面内气泡的形成及尺度起着决定性的作用。     

热固性树脂中孔隙形成条件的定量测试方法与影响因素

根据热固性树脂基复合材料的热压成型特点和扩散形成孔隙机制 , 建立了一种离线的孔隙形成条件量测试方法 , 可实现孔隙率2树脂压力变化关系的定量测定。采用该方法研究了两类环氧树脂体系固化过程中胶温度、 相对湿度、 树脂压力等条件对孔隙形态和分布的影响 , 并与修正的 Kardos气泡模型的理论计算结果进了对比分析。结果表明: 所建立的测试方法可以模拟热压工艺条件离线测定水汽形成孔隙情况 , 结果重复性好不同条件下树脂压力与孔隙率的关系均呈指数衰减的变化规律 ; 凝胶温度和相对湿度对孔隙率影响显著; 树脂类明显影响孔隙率大小和形态分布。研究结果为热压工艺复合材料孔隙缺陷的控制提供了重要的测试方法和验依据。      

Void growth and coalescence in metals deformed at elevated temperature

For metals deformed at elevated temperatures the growth of voids to coalescence is studied numerically. The voids are assumed to be present from the beginning of deformation, and the rate of deformation considered is so high that void growth is dominated by power law creep of the material, without any noticeable effect of surface diffusion. Axisymmetric unit cell model computations are used to study void growth in a material containing a periodic array of voids, and the onset of the coalescence process is defined as the stage where plastic flow localizes in the ligaments between neighbouring voids. The focus of the study is on various relatively high stress triaxialities. In order to represent the results in terms of a porous ductile material model a set of constitutive relations are used, which have been proposed for void growth in a material undergoing power law creep.     

A non-local void filling model to describe its dynamics during processing thermoplastic composites

A model is developed to describe the void dynamics within thermoplastic composite tape during the tape placement process. The model relates the volatile pressure in voids, the applied compaction load, fiber bed response and the resin pressure due to squeeze-flow of resin from resin-rich regions to fill void regions. This model relies on some geometric simplifications, but incorporates the relevant physical phenomena.This void consolidation model was implemented in a numerical code which predicts the void development during the process. The initial void geometry can be introduced either manually, using a random generation algorithm or from actual processed tape micrographs.The model predicts that the final void content depends on the original void content but also on the initial void distribution. Presented results analyze the influence of void distribution on tape consolidation. Limitations of the consolidation process rate by the resin squeeze flow pressures are clearly demonstrated.     

Rapidly cured out-of-autoclave laminates: Understanding and controlling the effect of voids on laminate fracture toughness

Voids are one of the most significant defects found within composites and have been demonstrated to reduce the performance of composite structures. The understanding of the impact of the size and distribution of voids on laminate properties is still limited because voids have proven difficult to deliberately control. This study aims to understand the mechanisms by which voids are generated within out-of-autoclave cured laminates. In this study, a process of prepreg conditioning was developed to control the level of voids within test laminates. Non-conditioned laminates highlighted signs of void growth (1.5%), while conditioned laminates showed consistently low levels of voids (<0.3%). Mass spectrometry indicated higher levels of aqueous and solvent volatiles within the non-conditioned prepreg. Finally, Mode II fracture testing revealed a 21% improvement in toughness for the non-voided laminates. A model on the effect of voids within the Mode II stress state has also been proposed.     

基于三维流场模型的含孔隙复合材料弹性常数有限元预测模型

为预测含孔隙复合材料单向层合板的有效弹性常数, 基于孔隙周边纤维分布和形态与三维Rankine椭圆体绕流流场的相似性, 提出了一种基于三维Rankine椭圆体绕流流场比拟的含孔隙复合材料弹性常数计算模型与方法。建立了含孔隙复合材料的有限元单胞计算模型, 用流场的速度变化比拟单胞内纤维体积分数的变化, 用流线形状比拟孔隙周边纤维的形态。通过对单胞施加周期性边界条件, 结合孔隙形态的概率分布模型和刚度平均法, 计算了含孔隙复合材料单向层合板的弹性常数。计算结果与实验数据有较好的一致性, 数值计算可以有效反映孔隙对复合材料单向层合板弹性常数的影响。      

热残余应力对考虑微观孔隙碳纤维增强环氧树脂复合材料横向拉伸性能的影响

为研究由于材料固化产生的热残余应力对碳纤维增强环氧树脂复合材料横向拉伸性能预测结果的影响,发展了一种基于摄动算法的纤维和孔洞随机分布代表性体积单元（RVE）生成方法,建立更加接近真实材料微观结构的RVE模型。通过施加周期性边界条件,并赋予组分（纤维、基体和界面）材料本构关系,进而实现温度和机械荷载下模型的热残余应力和损伤失效分析。从结果中发现,材料固化过程会在纤维之间产生残余压应力,在模型孔隙周围产生沿加载方向的残余拉应力。所建立不含孔隙RVE模型的失效均是由于界面脱黏引起,材料固化在纤维之间产生的残余压应力会增加模型的预测强度。含有孔隙的RVE模型失效起始于孔隙周围的基体中,而材料固化在模型孔隙周围产生的热残余拉应力对含孔隙RVE模型预测的失效强度有降低作用。对于具有不同孔隙尺寸的RVE模型,模型的失效强度随着孔隙尺寸的增加而不断降低,但是热残余应力减弱了孔隙尺寸对模型预测结果的降低作用。对于具有不同长宽比椭圆形孔隙的RVE模型,热残余应力增加了孔隙长宽比对模型强度的降低作用。      

Interaction between anisotropic plastic deformation and damage evolution in Al 2198 sheet metal

Deformation anisotropy of sheet aluminium alloy 2198 (Al-Cu-Li) has been investigated by means of mechanical testing of notched specimens and Kahn-type fracture specimens, loaded in the rolling direction (L) or in the transverse direction (T). Fracture mechanisms were investigated via scanning electron microscopy. Contributions to failure are identified as growth of initial voids accompanied by a significant nucleation of a second population of cavities and transgranular failure. A model based on the Gurson-Tvergaard-Needleman (GTN) approach of porous metal plasticity incorporating isotropic voids, direction-dependent void growth, void nucleation at a second population of inclusions and triaxiality-dependent void coalescence has been used to predict the mechanical response of test samples. The model parameters have been calibrated by means of 3D unit cell simulations, revealing the interaction between the plastic anisotropy of the matrix material and void growth. The model has been successfully used to describe and predict direction-dependent deformation behaviour, crack propagation and, in particular, toughness anisotropy.     

Key factors in electrical engineering with composite materials

The electrical properties of glass fabric epoxy laminates can be very dependant on the chemical composition of the ‘size’ applied to the glass fibres to protect them during weaving and to enhance their mechanical performance. The electrical losses in the warp and weft direction can be two orders of magnitude greater than in the plane perpendicular to the fabric plies. The mechanism is one of ionic conduction along the size coating the fibre and is aggravated by the moisture generated during the cure of an epoxy resin (0.2%) and the presence of voids acting as capillaries for the diffusion of moisture into or out of the laminate. A size giving good mechanical strength does not necessarily ensure good electrical performance. Examples of successful structures and components are included.