A negative Poisson’s ratio carbon fibre composite using a negative Poisson’s ratio yarn reinforcement

The manufacture of negative Poisson’s ratio (auxetic) composites, containing inherently auxetic phases is rare and has been confined to relatively low modulus composite systems with stiffnesses several orders of magnitude below those of structural composites. This paper presents the use of an auxetic double helix yarn that is used to produce a unidirectional fibre composite with both relatively high stiffness (4 GPa) and negative Poisson’s ratio (−6.8), at 30% fibre volume fraction, compared to other auxetic composites. This is the first structural auxetic composite to be produced using carbon fibre and importantly it was produced using standard manufacturing techniques and therefore is potentially applicable in a variety of engineering applications.     

The helical auxetic yarn - A novel structure for composites and textiles; geometry, manufacture and mechanical properties

This paper introduces a novel fibre structure known as the helical auxetic yarn (HAY). The geometry of the yarn is defined and the manufacturing process described. A range of HAYs have been manufactured that vary the geometric properties of the structure. A systematic study of the yarns has been completed to evaluate the effect on the auxetic behaviour of the geometry. We also characterise the component fibres and yarns and discuss the influence of geometric and material effects on the observed Poisson’s ratio of the yarns.It can be shown that the starting wrap angle of the yarn has the greatest effect on auxetic behaviour both in terms of magnitude and the strain range over which it may be observed.The maximum negative Poisson’s ratio observed for a yarn manufactured from conventionally available monofilaments with positive Poisson’s ratio is −2.7.     

Quantification of effects of stochastic feature parameters of yarn on elastic properties of plain-weave composite – Part 2: Statistical predictions vs. mechanical experiments

This paper presents a linear discretized theoretical model on the basis of the ideal theoretical model to evaluate elastic constants of plain-weave composite by using the statistics of the feature parameters of yarn measured from Micro CT data. A finite element method is utilized to calculate the elastic constants of the composite using the modified and global mean feature parameters of yarn, respectively. Uniaxial tensile and in-plane shear experiments are then completed to measure in-plane elastic constants of the composite. Finally, comparisons among the predictions of two theoretical models, FEM and experimental results are conducted. The results show that the stochastic fluctuations of yarn feature parameters decrease the in-plane elastic moduli and increase the in-plane shear moduli and Poisson’s ratios of the plain-weave composite. The discretized theoretical model with taking account of real yarn stochastic features can predict more accurate elastic constants of the composite than deterministic models.     

Local strain in a 5-harness satin weave composite under static tension: Part II - Meso-FE analysis

This paper presents the local strain analysis in a thermo-plastic 5-harness satin weave composite under uni-axial static tensile load using meso-FE simulations. In order to predict the local strain profiles as observed in the experiments (Part I) at various locations of the composite, different unit cell stacking models with appropriate boundary conditions are used for the FE analysis. Apart from the calculation of local strain values at different locations (inside/traction free surface) of the composite laminate, the aim of the numerical simulations is to understand the ‘shadowing’ effects of the internal ply shifting on the surface strain behavior of a 5-harness satin weave composite. Comparison of the experimental local strain values (Part I) at various locations of the satin weave composite reveals that the effects of local yarn constraints are negligible on the local longitudinal strain behavior of the composite.However, local stress-strain profiles obtained from unit cell meso-FE simulations indicate that the longitudinal strain and the transverse stress distribution in the weft yarn at the yarn crimp location is sensitive to the unit cell stacking as well as to the applied boundary conditions to the unit cell.     

Recycled carbon fibre-reinforced polypropylene thermoplastic composites

Comingled carbon fibre (CF)/polypropylene (PP) yarns were produced from chopped recycled carbon fibres (reCF) (20 mm in length, 7-8 μm diameter) blended with matrix polypropylene staple fibres (60 mm in length, 28 μm diameter) using a modified carding and wrap spinning process. Microscopic analysis showed that more than 90% of the reCF were aligned along the yarn axis. Thermoplastic composite test specimens fabricated from the wrap-spun yarns had 15-27.7% reCF volume content. Similar to the yarn, greater than 90% of the reCF comprising each composite sample made, showed a parallel alignment with the axis of the test specimens. The average values obtained for tensile, and flexural strengths were 160 MPa and 154 MPa, respectively for composite specimens containing 27.7% reCF by volume. It was concluded that with such mechanical properties, thermoplastic composites made from recycled CF could be used as low cost materials for many non-structural applications.     

Elastic constants of 3-, 4- and 6-connected chiral and anti-chiral honeycombs subject to uniaxial in-plane loading

Finite element models are developed for the in-plane linear elastic constants of a family of honeycombs comprising arrays of cylinders connected by ligaments. Honeycombs having cylinders with 3, 4 and 6 ligaments attached to them are considered, with two possible configurations explored for each of the 3- (trichiral and anti-trichiral) and 4- (tetrachiral and anti-tetrachiral) connected systems. Honeycombs for each configuration have been manufactured using rapid prototyping and subsequently characterised for mechanical properties through in-plane uniaxial loading to verify the models. An interesting consequence of the family of ‘chiral’ honeycombs presented here is the ability to produce negative Poisson’s ratio (auxetic) response. The deformation mechanisms responsible for auxetic functionality in such honeycombs are discussed.     

Novel Linear,Piezoresistive, Auxetic Sensors Coated by AAA Battery Active Carbons with Supreme Sensitivity for Human Body Movement Detection

Herein, a novel strategy for creating low-cost, sustainable, piezoresistive auxetic sensors using the active carbon in consumed AAA batteries, promoting a circular economy, is presented. An auxetic structure with a fixed Poisson's ratio during the strain is designed for sensing. The sensor substrate is silicone RTV2, and the sensing element is the active carbon in AAA batteries chopped to microscale particles using an ultrasonic wave. The sensor mold is designed using Solidworks software and produced using a computer numerical control device and EdgeCam2014 software. The coating process is performed by spraying the prepared particles on the molded auxetic structure and putting the coated auxetic structure under ultraviolet ray to prepare the final sensor. Sensitivity tests are performed, and the results show that the proposed sensor has a better sensitivity of about 1000% and 410% than the previous mixed and layered composite auxetic counterparts. The proposed sensor has linear sensitivity during the strain (estimated with a line with a slope of 0.64) while previous ones have a nonlinear performance (estimated at least with two lines). The sustainable sensor is implemented to detect the movements of the human body, including the movements of the wrist, finger, elbow, and forearm.     

Development of novel auxetic structures based on braided composites

Auxetic materials are a class of materials that expand transversely when stretched longitudinally. Recently, auxetic materials are gaining special interest in the technical sectors mainly due to their attractive mechanical behavior. This paper reports, for the first time, the development of auxetic structures from composite materials and the characterization of their auxetic as well as mechanical properties. Five different auxetic structures were developed varying their structural angle using core reinforced braided composite rods, containing glass fibers for axial reinforcement, polyester filaments for braided structure and epoxy resin as the matrix. Auxetic behavior of these structures was studied in a tensile testing machine using an image-based tracking method. Additionally, an analytical model was used to calculate Poisson’s ratio of these structures. According to experimental and analytical results, auxetic behavior and tensile characteristics of these structures were strongly dependant on their initial geometric configuration (i.e. structural angle). These novel auxetic structures exhibited Poisson’s ratio in the range of −0.30 to −5.20.     

复合材料负泊松比格栅结构设计及力学性能评价

对复合材料负泊松比格栅新结构的设计、制备与评价进行了研究,采用有限元方法模拟了负泊松比结构单元在轴压载荷作用下的力学行为,通过热压罐成型制备复合材料负泊松比格栅结构,并评估其成型质量、蒙皮及筋条的力学性能、结构抗轴压性能。数值模拟结果表明,负泊松比格栅结构与正交格栅结构相比,变形形式从马鞍形变为波纹形,横向膨胀量降低,应力分布均匀性提升,筋条-轴线夹角θ=30°时,负泊松比格栅结构达到最优。采用热压罐成型的MT300/603碳纤维/环氧树脂负泊松比格栅试件成型质量良好,蒙皮及筋条的力学性能优异。力学测试结果表明,筋条-轴线夹角θ=30°时,MT300/603负泊松比格栅结构轴压模量为65.92 GPa,轴压失效载荷为64.65 kN。轴压失效模式为筋条节点处的蒙皮-筋条开裂。筋条-轴线夹角θ=30°的MT300/603负泊松比格栅结构抗压强度高于正交格栅结构,且力学行为呈现明显的负泊松比特征,是一种具备优异综合力学性能的新格栅结构,在航天飞行器蒙皮结构等领域具有潜在的应用价值。      

Development,characterization and analysis of auxetic structures from braided composites and study the influence of material and structural parameters

Auxetic materials are gaining special interest in technical sectors due to their attractive mechanical behaviour. This paper reports a systematic investigation on missing rib design based auxetic structures produced from braided composites for civil engineering applications. The influence of various structural and material parameters on auxetic and mechanical properties was thoroughly investigated. The basic structures were also modified with straight longitudinal rods to enhance their strengthening potential in structural elements. Additionally, a new analytical model was proposed to predict Poisson’s ratio through a semi empirical approach. Auxetic and tensile behaviours were also predicted using finite element analysis. The auxetic and tensile behaviours were observed to be more strongly dependent on their structural parameters than the material parameters. The developed analytical models could well predict the auxetic behaviour of these structures except at very low or high strains. Good agreement was also observed between the experimental results and numerical analysis.     

Characterization of thermoplastic natural fibre composites made from woven hybrid yarn prepregs with different weave pattern

This paper focuses on the effect of weave structure on mechanical behaviour and moisture absorption of the PLA/hemp woven fabric composites made by compression moulding. The unidirectional woven fabric prepregs were made from PLA (warp) and PLA/hemp wrapped-spun hybrid yarn (weft) with two different weave patterns; 8-harness satin and basket. Unidirectional composites with 30 mass% hemp content were fabricated from these prepregs, and compared to winded PLA/hemp hybrid yarn laminates with same composition. The composite from the satin fabric had significantly lowest porosities and best mechanical properties compared to the composite made from the winded hybrid yarn and basket fabric. The tensile, flexural, and impact strength were 88 MPa, 113.64 MPa, and 24.24 kJ/m², respectively. The effect of weave pattern on water absorption is significant. Although the composite from hybrid yarn laminate has larger water absorption than that of the pure PLA, it exhibits lower moisture absorption than both weaves.     

Local strain in a 5-harness satin weave composite under static tension: Part I - Experimental analysis

This paper presents an experimental method for determining the local strain distribution in the plies of a thermoplastic 5-harness satin weave composite under uni-axial static tensile load. In contrast to uni-directional composites, the yarn interlacing pattern in textile composites causes heterogeneous strain fields with large strain gradients around the yarn crimp regions. In addition, depending on the local constraints that are imposed by the surrounding plies, the deformation behavior of the laminate inner layers may vary from that of the surface layers, which are relatively more free to deform, compared to the inner layers. In order to validate the above hypothesis, the local strains on the composite surface were measured using digital image correlation technique (LIMESS). Internal strains in the composite laminate were measured using embedded fibre optic sensors (FOS).Based on the DIC results, the strain profiles at various locations on the composite surface were estimated. Using the FOS results, the maximum and minimum strain values in the laminate inner layers were evaluated. Comparison of the local strain values at different laminate positions provides an estimate of the influence of the adjacent layers on the local longitudinal strain behavior of a satin weave composite. Part II of this paper elucidates the local strain variation computed using the meso-FE simulations. In addition to the comparison of numerical and experimental strain profiles, Part II presents the maximum and minimum strain envelopes for the carbon-PolyPhenelyne Sulphide (PPS) thermoplastic 5-harness satin weave composite.     

Effective elastic, piezoelectric and dielectric properties of braided fabric composites

Composites based upon 3D textile preforms have found broad structural application. This paper presents an analytical methodology for functional composites using piezoceramic fibers in a 3D braided preform. The effective elastic, piezoelectric and dielectric properties of 2-step braided composites with a polymeric matrix have been investigated. In the analytical approach, the effective properties of the braider and axial yarns of the unit cells are determined first using a 3D connectivity model. Then, the effective properties of the 2-step braided composite are predicted using an averaging technique. Results of a numerical example illustrating the variation of elastic, piezoelectric and dielectric constants with the braider yarn angle are provided. Textile preforming technique in general offers the potential of near net shape forming and 3D fiber placement. The present work provides the analytical basis for 3D piezoceramic textile composites.     

Influence of translational disorder on the mechanical properties of hexachiral honeycomb systems

Chiral honeycombs are one of the most important and oft studied classes of auxetic systems due to their vast number of potential applications which range from stent geometries to composites, sensors and satellite components. Despite numerous works on these systems, however, relatively few studies have investigated the effect of structural disorder on these structures. In view of this, in this study, the effect of translational disorder on hexachiral honeycombs were investigated through a Finite Element approach. It was found that this type of disorder has minimal effect on the Poisson's ratios of these systems provided that the ligament length to thickness ratio remains sufficiently large and the overall length to width ratio of the disordered system does not differ considerably from that of its ordered counterpart. This makes it ideal for use in various applications and products such as sandwich composites with an auxetic core.     

A facile method for preparing CNT-grafted carbon fibers and improved tensile strength of their composites

Carbon nanotube (CNT)-grafted carbon fibers (CFs) have emerged as new reinforcements for improving the mechanical properties of CF-reinforced composites but such enhancement in macroscale composites has not been realized. This paper reports a facile method for preparing CNT-grafted CFs and improving the tensile strength of their composites. A CNT/polyacrylonitrile solution was sprayed onto the surface of the CF woven fabrics, and the CNTs were grafted by a thermal treatment at 300 °C. CNT-grafted CF composites were fabricated using the CNT-grafted CF woven fabrics using a vacuum-assisted resin transfer molding process with epoxy resin. The CNT-grafted CF composite exhibited 22% enhancement in the tensile strength compared to that of the pristine CF composite. Fracture surfaces of the CNT-grafted CF composites showed that the grafted CNTs obstructed the propagation of micro-cracks and micro-delamination around the CFs and also yarn boundaries, resulting in improved tensile strength of CNT-grafted CF composites.     

Predicting dynamic in-plane compressive properties of multi-axial multi-layer warp-knitted composites with a meso-model

Proper prediction of material microstructure from known processing conditions and constituent material properties is a critical step to determine the bulk properties of the composite. This paper reports a meso-structure model of multi-axial multi-layer warp-knitted (MMWK) composites from an elastic–plastic material model considering the strain rate effect for the components of the MMWK composite. The representative unit cell (RUC) of fiber tow is created to obtain the elastic–plastic parameters of the fiber tow. The 3D meso-structure model of the MMWK composite is based on an idealized geometrical model according to the preform structure of the MMWK fabric. The model is used to investigate the effect of the volume fraction of the knitting yarn on the dynamic in-plane compressive properties. The results show that the fiber tow failure at large extent is mainly caused by the micro cracking of the matrix, and the effects of the knitting yarn on the mechanical properties of MMWK composite are very limited. Particularly, MMWK composites could be considered as laminates when the volume fraction of the knitting yarn is low, such as below 1.5%. Experiments were also conducted to validate the results from the simplified meso-structure model of the MMWK composite. The material is found to be strain rate sensitive, and the experimental and predicted results agree well with respect to the compressive strength and modulus of the composite. This confirms that the meso-structure MMWK composite model proposed is capable of capturing the essential features for the response of the composite under different strain rate conditions at the meso-level.     

Numerical and experimental analysis of a triangular auxetic core made of CFR-PEEK using the Directionally Reinforced Integrated Single-yarn (DIRIS) architecture

The Directionally Reinforced Integrated Single-yarn (DIRIS) architecture is a novel, patented concept of creating directionally reinforced high-strength cores for sandwich panels developed at the Institute of Mechanics of Materials and Geostructures S.A. This technology using glass or carbon fibre-reinforced PEEK has been so far implemented on high-strength honeycomb cores of triangular isogrid geometry with impressive results in all constitutive parameters both in-plane and out-of-plane. In this paper the application of the DIRIS technology on creating high-strength auxetic triangular cores is presented. The mechanical behaviour of the resulting core and panel was numerically investigated using FEA and testing on manufactured prototypes confirmed the high strength of the proposed design. In fact the shear modulus of the DIRIS auxetic cores was found superior to that of existing mass-produced honeycomb cores despite the inherent complexity of the geometrical configuration and the non-standardized manufacturing method.     

Mechanics of textile composites: Micro-geometry

Textile fabric geometry determines textile composite properties. Textile process mechanics determines fabric geometry. In previous papers, the authors proposed a digital element model to generate textile composite geometry by simulating the textile process. The greatest difficulty encountered with its employment in engineering practice is efficiency. A full scale fiber-based digital element analysis would consume huge computational resources. Two advances are developed in this paper to overcome the problem of efficiency. An improved contact-element formulation is developed first. The new formulation improves accuracy. As such, it permits a coarse digital element mesh. Then, a static relaxation algorithm to determine fabric micro-geometry is established to replace step-by-step textile process simulation. Employing the modified contact element formulation in the static relaxation approach, the required computer resource is only 1–2% of the resource required by the original process. Two critical issues with regards to the digital element mesh are also examined: yarn discretization and initial yarn cross-section shape. Fabric geometries derived from digital element analysis are compared to experimental results.     

Elastic properties of an orthotropic binary fiber-reinforced composite with auxetic and conventional constituents

Closed-form expressions for the nine effective elastic constants of a binary fiber-reinforced composite with transversely isotropic constituents with positive (conventional) and negative (auxetic) Poisson’s ratio are considered. Such formulae were obtained by means of the asymptotic homogenization method and were verified numerically with an independent finite element model. The overall properties display explicit dependence on (i) the properties of the constituents, (ii) the volume fraction or radius of inclusion and (iii) the array periodicity. They are finally obtained by solving a normal infinite symmetric linear system of algebraic equations by truncation to a relatively small order term. This allows a fast solution and low computation cost. The overall orthotropy of the elastic properties is obtained by varying the distance between the fibers in two of the principal directions leading to different spacial aspect ratio for fiber distribution. In addition to this, an analytical relation between the effective properties based on the symmetry of the stiffness tensor is introduced. With the previous elements, we present reliable predictions for auxetic and conventional composites of this kind wherein a significant enhancement in Young’s modulus is found in a composite with an auxetic matrix reinforced by conventional fibres. Finally, we compute auxeticity windows (i.e., intervals of volume fraction where the composite is auxetic) when the fibres are auxetic. It is reported that spacial fiber aspect ratio plays a key role in the composite auxetic behavior.     

Textile composite double dome stamping simulation using a non-orthogonal constitutive model

Stamping is one of the most effective ways to form textile composites in industry for providing high-strength, low-weight and cost-effective products. This paper presents a fully continuum mechanics-based approach for stamping simulation of textile fiber reinforced composites by using finite element (FE) method. A previously developed non-orthogonal constitutive model is used to represent the anisotropic mechanical behavior of textile composites under large deformation during stamping. Simulation are performed on a balanced plain weave composite with 0°/90° and ±45° as initial yarn orientation over a benchmark double dome device. Simulation results show good agreement with experimental output in terms of a number of parameters selected for comparison. The effects of meshing and shear moduli obtained from bias extension test and picture frame test on forming simulation results are also investigated.