Tensile properties of multilayer-connected biaxial weft knitted fabric reinforced composites for carbon fibers

Multilayered-connected biaxial weft knitted (MBWK) fabric reinforced composites have excellent tensile properties. Three kinds of different fabrics reinforced composites are used in this paper, which are three-layer-connected biaxial weft knitted fabric, four-layer-connected biaxial weft knitted fabric and five-layer-connected biaxial weft knitted fabric. The tensile properties of MBWK fabrics reinforced composites are studied with 0° and 90° directional testing with different carbon fiber volume fractions. The results show that the carbon fiber volume fraction has significant effect on tensile strength of MBWK fabrics reinforced composites. The linear correlation between tensile strength and carbon fiber volume fraction is very well in the certain range, and failure analyses are also available by means of sample debris examination to identify the failure modes and the fracture surfaces.     

Concise formula for tensile strength of knitted fabric reinforced composite

Abstract

A simple one-dimensional formula, as concise as Krenchel's model for stiffness calculation, is presented in this paper to calculate the ultimate tensile strength of a knitted fabric reinforced composite in the loading direction. Its deteriorated form can be used to determine the off axial strength of a unidirectional composite. The formula has been developed based on the understanding of internal stresses generated in the constituent fibre and matrix materials. These stresses are explicitly expressed as functions of the overall applied load, and only the stress components in the loading direction are retained. The ultimate strength of the composite is defined as the overall applied stress under which one of the constituent materials fails. The proposed formula has been applied to calculate the off axial strength of a unidirectional composite and the tensile strengths of two plain weft knitted glass fibre fabric reinforced epoxy matrix composites subjected to wale and course direction loads. All the calculated strengths are in reasonable agreement with experimental data.     

不同针织结构经编碳纤维复合材料弯曲性能

通过对3种不同针织方式碳纤维经编织物结构的分析和弯曲性能测试, 研究了织物针织方式对NCFs复合材料力学性能的影响。采用链式缝编的 织物与经平缝编的 织物相比, 束缚效果更好, 经编线引起的纤维变形区的宽度较小, 因此 织物增强的复合材料中的富树脂区和空洞相对较少, 弯曲强度和模量均高于 复合材料。单向经编织物也采用经平缝编, 纤维取向与双轴向织物相比更准确, 由于缝编引起的纤维变形和损伤较少, 复合材料的弯曲性能高于两种双轴向经编材料。      

双轴向衬纱纬编玄武岩纤维复合材料弯曲性能

经纬纱和针织纱分别选用不同线密度的高模高强玄武岩纤维, 以不同衬纱方式编织出机织针织复合(CWK)织物和多层双轴向纬编(MBWK)织物, 并以其作为增强体, 采用真空辅助树脂传递模塑工艺制备了玄武岩纤维/乙烯复合材料。对两种复合材料0°、 90°和45°方向的弯曲性能进行测试, 分析比较了弯曲应力-应变特征曲线及纱线强度。结果表明: 两种复合材料具有较好的弯曲性能, 0°和90°方向的弯曲性能均优于各自45°方向的, 弯曲应力-应变曲线均表现出一定的塑性破坏特征; MBWK织物增强复合材料0°和90°方向的弯曲性能又分别高于CWK织物增强复合材料0°和90°方向的弯曲性能; 复合材料中经纱和纬纱的屈曲程度不同, 致使MBWK织物增强复合材料的比模量和纱线强度均高于CWK织物增强复合材料, 两种复合材料的弯曲性能受不同衬纱方式的影响, 而两种复合材料试样的弯曲破坏形态相近。研究结果为双轴向衬纱纬编玄武岩纤维复合材料的应用提供了参考。     

不同孔隙率CFRP层合板静态力学性能研究

为了研究孔隙率对织物碳纤维/环氧树脂复合材料层合板静态力学性能的影响规律,分别测量了孔隙率为0.33%至1.50%的CFRP层合板的弯曲强度和层间剪切强度,并进行有限元模拟.在适用于复合材料单向板的改进Hashin失效准则基础上,建立了适用于织物纤维增强复合材料静态力学强度的失效准则.通过引入复合材料基本强度参数预测不同孔隙率CFRP层合板的力学性能,结合刚度突然退化模型,采用ABAQUS软件建立了有限元模型.试验结果表明,随着孔隙率的增加,复合材料层合板的弯曲强度和层间剪切强度均呈下降趋势.有限元模型较为准确地预测了不同孔隙率织物碳纤维/环氧树脂复合材料层合板的弯曲强度和层间剪切强度.     

Fatigue life prediction of nanoparticle/fibrous polymeric composites based on the micromechanical and normalized stiffness degradation approaches

In this paper, a new model based on the micromechanical and normalized stiffness degradation approaches is established. It has been assumed that during the fatigue condition, only material properties of composites (fiber and matrix) were degraded and nanofillers remain intact under different states of stress. A normalized stiffness degradation model was proposed for laminated fibrous composites reinforced with nanoparticles to derive a novel model to predict the stiffness reduction. The developed model is capable of predicting the fatigue life of nanoparticle-filled fibrous composites based on the experimental data of fibrous composites without nanofillers. The new fatigue model is verified by applying it to different experimental data provided by different researchers. The obtained results by the new fatigue model are in very good agreement with the experimental data of nano-silica glass/epoxy composites under constant cyclic stress amplitude fatigue and also for silica/epoxy nanocomposites in various states of stress with negligible error.     

A damage model for knitted fabric composites

A progressive damage model is presented for the prediction of the overall non-linear tensile behaviour of knitted fabric composites. The model is an extension of a recently developed inclusion method for textile composites, taking into account the major damage modes of knitted fabric composites simultaneously. Matrix non-linearities, playing an important role in the behaviour of knitted fabric composites, are implemented using the secant stiffness method. Whereas, non-linearities can be attributed to different sources, depending upon the resin toughness, only some limited yielding was currently investigated. Yarn/matrix debonding, predominantly responsible for the knee behaviour of knitted fabric composites, is investigated using a simple interfacial failure criterion. A selective degradation scheme based upon a finite set of interfacial damage state variables is employed in the reduction of the inclusion stiffness. Finally, a Hoffman criterion is used for yarn failure. The model was used to simulate the tensile behaviour of knitted fabric E-glass/epoxy composites, showing the ability to predict the material response with reasonable accuracy in the region before ultimate failure.     

Effect of fabric types on the impact behavior of cement based composites in flexure

Two different fabric types were used to investigate the effect of the fabric types on the static and impact behavior of fabric reinforced cement based composites by using three point bending tests for various drop heights of hammer and position of the specimens on the supports. For each fabric type, 18 specimens with dimensions of 50 mm × 150 mm × 12 mm were produced with the pultrusion process. The vertical specimens have more stiffness, less ultimate deflection and higher load carrying capacity than the horizontal specimens for same drop heights. However, the horizontal specimens subjected to impact loads have higher stresses than the vertical specimens due to the section properties. The tests showed that polyvinyl alcohol (PVA) fabric reinforced cement based composites carried higher impact loads, were stiffer and had less deflection than other composites. At the drop heights over 100 mm, the impact strength of the horizontal specimens sharply decreased, while that of the vertical specimens was remained same.     

芳纶捆绑对纬编双轴向多层衬纱织物增强环氧树脂复合材料层间性能的影响

将芳纶作为捆绑纱制备纬编双轴向多层衬纱(MBWK)织物增强环氧树脂复合材料，研究了MBWK织物增强环氧树脂复合材料层间性能及芳纶捆绑纱对其层间性能的影响。通过三点弯曲和短梁剪切测试，得到MBWK织物增强环氧树脂复合材料的弯曲性能和层间剪切性能，并通过Aramis V6三维场应变测量系统观察实验过程中层间应变变化。与传统涤纶低弹丝捆绑的MBWK织物增强环氧树脂复合材料相比，芳纶捆绑MBWK织物增强环氧树脂复合材料的弯曲性能和层间剪切性能明显提升，弯曲强度和层间剪切强度分别提高了14.21%和12.70%；弯曲模量提高了25.49%。芳纶捆绑MBWK织物增强环氧树脂复合材料在受到面外载荷时，纵向应变(Epsilon X)和层间剪切应变(Epsilon XZ)在中性面区域内较大，且在受到面外载荷时，芳纶捆绑纱起到有效抑制复合材料分层的作用。      

Simulation and Experimental Validation of Spacer Fabrics Based on their Structure and Yarn’s Properties

Warp-knitted spacer fabrics are considered, which are plates or shells composed of two knitted plane layers connected by vertical beams. Our aim is to compute the effective stiffness and permeability of such spacer fabrics on the basis of their structure and properties of yarns and the monofil. In order to reduce the computational effort and simplify the computational model, homogenization and dimension reduction techniques are applied. They replace the fabric by an equivalent two-dimensional plate or shell with effective elastic properties. To compute the effective permeability, the fluid simulation is done on the fully resolved micro-structure. The paper demonstrates the algorithm on application examples. We compute the elastic properties of a spacer fabric and its effective permeability for different outer-plane compression stages. Numerical examples were performed by applying the multi-scale simulation tools, developed at Fraunhofer ITWM and by comparing with the corresponding experimental results, based on measurements performed at the TU Dresden. The developed algorithms and simulation tools enable a full virtualisation of the material design adapted to exposure scenarios in various technical application cases, i.e. infiltration processes with polymers in the field of fiber reinforced composites, which enables new discoveries for the designing and manufacturing process of 3D warp-knitted spacer fabrics.     

Electrical and impact properties of the hybrid knitted inlaid fabric reinforced polypropylene composites

A range of conductive knitted fabric reinforced polypropylene composites have been developed and their electromagnetic shielding effectiveness (EMSE), electrostatic discharge (ESD) and impact properties have been investigated. Carbon and aramid fibers are used as the reinforcement phase in the composites, while copper and stainless steel wires are incorporated as conductive fillers to provide the ESD and EMSE properties of the composite materials. The hollow spindle spinning system has been used to make SS/PP, Cu/PP, SS/C/PP, Cu/C/PP and Cu/K/PP uncommingled yarns. The double plain knitted fabric and its inlaid fabrics were fabricated from the yarns using a 5G traverse knitted machine. Changing the yarn composition, fabric knit structure, and stitch density varies the amount of copper and stainless steel conductive fillers in the composites. 4 layer cross-ply laminates were laid-up by hand, then formed into 3-mm thick conductive thermoplastic composites using a compression molding. It was observed that the EMSE and ESD of the composites increase with increasing the incident frequency, especially at higher frequency range. The effects of inlaid ends, materials and yarn constitutions on the EMSE of the conductive thermoplastic composites were investigated. The results indicate that the composites can be used for the purpose of electromagnetic shielding and ESD attenuation, as well as for some microwave applications.     

Modelling of composite sheet forming: a review

The use of composite materials in sheet forming applications is gaining popularity with the rise of consumer demands and specific mechanical properties. In addition to unidirectional (UD) fibres, the use of textile reinforcements such as woven fabric and knitted fabric has been shown to be feasible in recent years. This paper gives a survey on the modelling of composite sheet forming for both UD fibre and textile composites. Two broad approaches are reviewed here—the mapping approach and the mechanics approach. Mapping approaches for UD fibre composites, woven fabric composites and knitted fabric composites are elucidated on the basis of their fibre geometry. For the mechanics approach both the viscous fluid models and elastic solid models, as a means of describing the constitutive properties, are reviewed. Various updating methods for modelling large deformation found in sheet forming are then described. Finally, a guideline for the choice of modelling techniques for various types of fibre/fabric reinforcements and suggestions for future work are given.     

连续石墨纤维增强铝基复合材料准静态拉伸损伤演化与断裂力学行为

针对连续石墨纤维增强铝基(CF/Al)复合材料,采用三种纤维排布方式的代表体积单元(RVE)建立了其细观力学有限元模型,采用准静态拉伸试验与数值模拟结合的方法,研究了其在轴向拉伸载荷下的渐进损伤与断裂力学行为。结果表明,采用基体合金和纤维原位力学性能建立的细观力学有限元模型,对轴向拉伸弹性模量和极限强度的计算结果与实验结果吻合良好,而断裂应变计算值较实验结果偏低。轴向拉伸变形中首先出现界面和基体合金损伤现象,随应变增加界面发生失效并诱发基体合金的局部失效,最后复合材料因纤维发生失效而破坏,从而出现界面脱粘后纤维拔出与基体合金撕裂共存的微观形貌。细观力学有限元分析结果表明,在复合材料制备后纤维性能衰减而强度较低条件下,改变界面强度和刚度对复合材料轴向拉伸弹塑性力学行为的影响较小,复合材料中纤维强度水平是决定该复合材料轴向拉伸力学性能的主要因素。     

Strength prediction of multi-layer plain weave textile composites using the direct micromechanics method

Previously developed micromechanical methods for stiffness and strength prediction are adapted for analysis of multi-layer plain weave textile composites. Utilizing the direct micromechanics method (DMM) via finite element modeling, three methods are presented: (a) direct simulation of a multi-layer plain weave textile composite; (b) micromechanical analysis of a single layer of interest from the force and moment resultants acting on that layer; and (c) application of the previously developed quadratic stress-gradient failure theory to the layer of interest. In comparison to direct modeling, the other two techniques show only 5% difference over a number of random test cases. Several practical design examples of strength prediction are included to illustrate the importance and accuracy of method implementation.     

Effect of Hybridization on Stiffness Properties of Woven Textile Composites

The present study focuses on stiffness properties of woven textile reinforced polymeric composites with respect to hybridization, and geometry of reinforcement. The analyzed composites represent combinations of different fibre materials (E-glass, Kevlar 49, carbon HM) in a predetermined fabric geometry (a plane weave embedded in thermosetting polymeric resin) serving controlled properties and required performance. The effects of hybridization on the stiffness properties of woven textile composites have been studied with respect to the fibres materials, the unbalancing degree of fabrics, and the variation of compactness and undulation of yarns. Some undesirable effects in fabric geometry can be overcome by the combined effects of hybridization and compactness.     

玄武岩纤维-碳纤维混杂平纹织物增强环氧树脂基复合材料的制备与力学性能

对以平纹织物为增强体的混杂纤维复合材料（HFRP）的刚度和强度进行研究。设计热压工艺并制备7组具有不同混杂比的玄武岩纤维-碳纤维（玄-碳）混杂增强环氧树脂基复合材料试样进行拉伸试验。基于平纹织物的结构特征,对传统混合定律加以修正,提出以平纹织物为增强体的HFRP刚度估算模型。基于HFRP层合板的破坏机制,提出材料仅发生一次破坏的临界混杂比,并分成三个混杂比范围给出强度估算模型。最终以体现分散度的混杂效应系数对估算结果加以修正。结果表明：计算值与试验值近似,预估模型计算所得临界混杂比与试样拉伸试验时的应力-应变曲线分析结果相符,模型可为今后的实际应用提供理论依据。本文提出的预估方法可以反应混杂比和分散度对平纹织物为增强体的HFRP强度和刚度的影响,扩展了混合定律的应用范围。     

Influence of Fabric Parameters on Microstructure,Mechanical Properties and Failure Mechanisms in Carbon-Fibre Reinforced Composites

The effects of fibre/matrix bonding, fabric density, fibre volume fraction and bundle size on microstructure, mechanical properties and failure mechanisms in carbon fibre reinforced composites （plastic and carbon matrix） have been investigated. The microstructure of unloaded and cracked samples was studied by optical microscopy and scanning electron microscopy （SEM）, respectively whereas the mechanical behaviour was examined by 3- point bending experiments. Exclusively one type of experimental resole type phenolic resin was applied. A strong fibre/matrix bonding, which is needed for high strength of carbon fibre reinforced plastic （CFRP） materials leads to severe composite damages during the pyrolysis resulting in low strength, brittle failure and a very low utilisation of the fibres strain to failure in C/C composites. Inherent fabric parameters such as an increasing fabric density or bundle size or a reduced fibre volume fraction introduce inhomogenities to the CFRP＇s microstructure. Results are lower strength and stiffness whereas the strain to failure increases or remains unchanged. Toughness is almost not affected. In C/C composites inhomogenities due to a reduced bundle size reduce strain to failure, strength, stiffness and toughness. Vice versa a declining fibre volume fraction leads to exactly the opposite behaviour. Increasing the fabric density （weight per unit area） causes similar effects as in CFRPs.     

Geometrical and mechanical aspects of fabric bonding and pullout in cement composites

Fabric reinforced cement composites are a new class of cementitious materials with enhanced tensile strength and ductility. The reinforcing mechanisms of 2-D fabric structures in cement matrix are studied using a fabric pullout model based on nonlinear finite difference method. Three main aspects of the composite are evaluated: nonlinear bond slip characteristic at interface; slack in longitudinal warp yarns, and mechanical anchorage provided by cross yarn junctions. Parametric studies of these key parameters indicate that an increase in the interfacial bond strength directly increases the pullout strength. Grid structures offering mechanical anchorage at cross yarn junctions can substantially increase the pullout resistance. Presence of slack in the yarn geometry causes an apparently weaker and more compliant pullout response. The model was calibrated using a variety of test data on the experimental pullout response of AR-Glass specimens, manufactured by different techniques to investigate the relative force contribution from bond at interface and from cross yarn junctions of alkaline resistant glass fabric reinforced cement composites.     

Nanostructured interfaces for enhancing mechanical properties of composites: Computational micromechanical studies

Computational micromechanical studies of the effect of nanostructuring and nanoengineering of interfaces, phase and grain boundaries of materials on the mechanical properties and strength of materials and the potential of interface nanostructuring to enhance the materials properties are reviewed. Several groups of materials (composites, nanocomposites, nanocrystalline metals, wood) are considered with view on the effect of nanostructured interfaces on their properties. The structures of various nanostructured interfaces (protein structures and mineral bridges in biopolymers in nacre and microfibrils in wood; pores, interphases and nanoparticles in fiber/matrix interfaces of polymer fiber reinforced composites and nanocomposites; dislocations and precipitates in grain boundaries of nanocrystalline metals) and the methods of their modeling are discussed. It is concluded that nanostructuring of interfaces and phase boundaries is a powerful tool for controlling the material deformation and strength behavior, and allows to enhance the mechanical properties and strength of the materials. Heterogeneous interfaces, with low stiffness leading to the localization of deformation, and nanoreinforcements oriented normally to the main reinforcing elements can ensure the highest damage resistance of materials.