Applied Composite Materials (v.25, #4)

Editorial by Xiaogang Chen (707-708).

Simulation and Experimental Validation of Spacer Fabrics Based on their Structure and Yarn’s Properties by Julia Orlik; Kathrin Pietsch; Achim Fassbender; Olena Sivak; Konrad Steiner (709-724).
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.
Keywords: 3D warp-knitted spacer fabrics; Simulation; Modelling; Elastic properties; Permeability; Experimental methods

Spacer fabrics belong to the category of 3D hollow structures, and consist of two separate fabric layers that are connected with pile yarns or fabric layers maintaining hollow space between adjacent connecting yarns or fabric layers. In this study, spacer structures connected with woven cross-links having three different cross-sections of the hollow tunnels: rectangular, trapezoidal and triangular, along the weft direction, were produced using 600 tex E-glass tows. All the sections of the structures were plain 2D fabrics with all constituent layers having the same construction. These fabric structures were then converted to composites, with epoxy resin as matrix, using vacuum assisted resin infusion molding (VARIM) technique. The produced composite samples were characterized for their lateral compressive properties. This study provides an insight into the production of sandwich structures connected with woven cross-links, and their load bearing capabilities. The results indicated that the compressive strength of structure depends mainly on the thickness of the cell walls and its angle with the horizontal layer.
Keywords: Hollow composites; Compression properties; Woven spacers; Energy absorbency

Stabilising and Trimming 3D Woven Fabrics for Composite Preforming Applications by Alice E. Snape; Jody L. Turner; Hassan M. El-Dessouky; Mohamed N. Saleh; Hannah Tew; Richard J. Scaife (735-746).
The work presented here focusses on the developments in the stabilising and trimming of 3D woven preforms. Dry fibre preforms are notoriously difficult to trim; once a fabric is cut, it loses its edge stability and consequently the fabric frays. The result is an unstable fabric which can easily be displaced/ distorted prior to composite manufacturing. In this work, three stabilisation and three trimming techniques were investigated. Of the stabilisation techniques these included powder binder, thermoplastic binder yarn (activated to give fabric stabilization); and polyester stitching. The stabilised fabrics were trimmed to near-net-shape using different trimming techniques. The trimming techniques investigated were laser, clicker press and ultrasonic knife. Each stabilisation method was trialled with each trimming method to assess the most suitable combination. The assessment of quality and suitability was made by observing the level of stabilisation, amount of fraying fibres, quality of the cut, ease of application and repeatability of the process. This paper details the assessments made for each combination alongside practical application conclusions. The key findings were; cutting by means of a laser is capable of sealing the fabric edges, producing high edge quality. Stitching as a method of stabilising is not sufficient in preventing fibres from moving during the cutting process, hence producing an unclean cut.
Keywords: Stabilisation; Trimming; 3D woven fabrics; Carbon fibre

Development & Characterization of Green Composites Using Novel 3D Woven Preforms by Yasir Nawab; Muhammad Kashif; Muhammad Ayub Asghar; Ali Asghar; Muhammad Umair; Khubab Shaker; Muhammad Zeeshan (747-759).
Green composites are the emerging materials made using natural fibers and environmentally degradable matrix such as green epoxy. Natural fiber composites are the motivation of researchers for low to medium impact applications as well as structural applications like automobiles. In this research work, 3D orthogonal layer to layer (LL) and through the thickness (TT) woven structures with different interlocking patterns, used as preforms in composites are presented. The mechanical properties of preform as well as associated composites are studied on equivalent fiber volume fraction. Jute yarn was woven into four layered 3D woven structures. The use of bridgeable and sustainable fiber, with its prospective use with the biodegradable matrix, is the objective of this work. The focus of this study is to improve mechanical performance by changing weave pattern, so that the resulting composite is robust in design.
Keywords: Warp and weft interlocks; Hybrid interlocks; 3D woven composites; Green composites

The Need to Use Generalized Continuum Mechanics to Model 3D Textile Composite Forming by P. Boisse; R. Bai; J. Colmars; N. Hamila; B. Liang; A. Madeo (761-771).
3D textile composite reinforcements can generally be modelled as continuum media. It is shown that the classical continuum mechanics of Cauchy is insufficient to depict the mechanical behavior of textile materials. A Cauchy macroscopic model is not capable of exhibiting very low transverse shear stiffness, given the possibility of sliding between the fibers and simultaneously taking into account the individual stiffness of each fibre. A first solution is presented which consists in adding a bending stiffness to the tridimensional finite elements. Another solution is to supplement the potential of the hyperelastic model by second gradient terms. Another approach consists in implementing a shell approach specific to the fibrous medium. The developed Ahmad elements are based on the quasi-inextensibility of the fibers and the bending stiffness of each fiber.
Keywords: 3D textile reinforcements; Continuum mechanics models; Second gradient models; Finite element curvature

This paper presents latest development of a modelling algorithms and simulation results, related to 3D braiding machines with individually driven switches and horn gears. The early developed 3D braiding machines and the current software from the braiding producer covers only rectangular sets of horn gears. Based on complete emulation of the braiding process with the carrier motion, a generalized method for the simulation of any configuration of horn gears with different sizes was developed and reported earlier by the author. In this work, an extension of the algorithms with individually controlled switches is presented. These switches allow production of profiles for textiles reinforced composites with complex cross section, changing during the production. The machine emulation can be coupled with FEM based braiding process simulation and the complete product can be virtually produced and analyzed.
Keywords: 3D braiding; Process simulation; Variable track; Programmable switches

Inserting discrete through-the-thickness reinforcing elements such as fibrous tufts into a composite laminate may increase dramatically its damage tolerance and impact resistance. The three-dimensional fibre architecture generated locally by such elements (sometimes referred to as micro-fasteners) has been demonstrated to slow down or arrest altogether delamination crack growth. Where a closed, matched tool is used for infusion and cure, the insertion of tufts may result in a local increase in fibre volume fraction. On the other hand, when a flexible sealant bag or film is used, the extra fibrous yarn may cause an increase of the laminate thickness, with potential alteration of the component geometry. The latter effect may impact upon the component performance, especially if aerodynamic surfaces are involved, and therefore needs systematic quantification. This paper quantifies the alteration in thickness of a flat laminate made with non-crimp carbon fabric and reinforced with carbon tufts using a range of material and manufacturing variables. A design of experiments approach is applied and a 3D laser scanner used to gather reliable data to identify those parameters most likely to influence the change of geometry (including yarn weight, tuft areal density and tuft angle). A case study is presented to relate the experimental findings to the case of a 900 mm long, double curvature carbon fibre tufted T-stringer made with a 3D woven π-section.
Keywords: Tuft; Tufting; Composites; Through-thickness reinforcement; Geometry; Thickness; Non-crimp fabric; 3D woven

Nonlinear Multi-Scale Modelling, Simulation and Validation of 3D Knitted Textiles by Oliver Weeger; Amir Hosein Sakhaei; Ying Yi Tan; Yu Han Quek; Tat Lin Lee; Sai-Kit Yeung; Sawako Kaijima; Martin L. Dunn (797-810).
Three-dimensionally (3D) knitted technical textiles are spreading into industrial applications, since their geometric, structural and functional performance can be tailored and optimized on fibre-, yarn- and fabric levels by customizing yarn materials, knit patterns and geometric shapes. The ability to simulate their complex mechanical behaviour is thus an essential ingredient in the development of a digital workflow for optimal design and manufacture of 3D knitted textiles. Here, we present a multi-scale modelling and simulation framework for the prediction of the nonlinear orthotropic mechanical behaviour of single jersey knitted textiles and its experimental validation. On the meso-scale, representative volume elements (RVEs) of the fabric are modelled as single, interlocked yarn loops and their mechanical deformation behaviour is homogenized using periodic boundary conditions. Yarns are modelled as nonlinear 3D beam elements and numerically discretized using an isogeometric collocation method, where a frictional contact formulation is used to model inter-yarn interactions. On the macro-scale, fabrics are modelled as membrane elements with nonlinear orthotropic material behaviour, which is parameterized by a response surface constitutive model obtained from the meso-scale homogenization. The input parameters of the yarn-level simulation, i.e., mechanical properties of yarns and geometric dimensions of yarn loops in the fabrics, are determined experimentally and subsequent meso- and macro-scale simulation results are evaluated against reference results and mechanical tests of knitted fabric samples. Good agreement between computational predictions and experimental results is achieved for samples with varying stitch values, thus validating our novel computational approach combining efficient meso-scale simulation using 3D beam modelling of yarns with numerical homogenization and nonlinear orthotropic response surface constitutive modelling on the macro-scale.
Keywords: 3D knitting; Technical textiles; Digital design; Multi-scale modelling; Homogenization

Engineering Design and Mechanical Property Characterisation of 3D Warp Interlock Woven Fabrics by A. C. Corbin; A. Kececi; F. Boussu; M. Ferreira; D. Soulat (811-822).
3D warp interlock fabrics have been used both in composite materials as fibrous reinforcement as well as in protective solutions against impact mainly due to their improved capacity to absorb energy by higher intra-ply resistance to delamination. However, depending on the type of architecture used, the binding warp yarns may provide different types of mechanical behaviour. By the same, the choice of the yarn raw material coupled with the suited 3D warp interlock architecture is still a challenge to solve due to the lack of knowledge on the optimized fabric parameters to be chosen. Thus, to fill this gap, we have designed, produced on same dobby loom and tested different types of 3D warp interlock architectures (O-T 4 3–4 Basket 3–3 and A-T 4 5–4 Twill 6) with different types of raw material (E-glass EC9 900 Tex, para-aramid 336 Tex and flax Tex 500 yarns). Thanks to these tests, it has been highlighted different mechanical behaviours of 3D warp interlock fabrics with the same weave pattern but with different types of yarns (E-glass, flax and para-aramid) both in the warp and weft directions. It has been also revealed that the warp shrinkage of warp yarns inside the woven structure has a major influence on the whole fabric behaviour.
Keywords: 3D warp interlock fabric; Textile composite; Mechanical characterization; Weaving technology

Meso-Scale Finite Element Simulations of 3D Braided Textile Composites: Effects of Force Loading Modes by Chao Zhang; Chunjian Mao; Jose L. Curiel-Sosa; Tinh Quoc Bui (823-841).
Meso-scale finite element method (FEM) is considered as the most effective and economical numerical method to investigate the mechanical behavior of braided textile composites. Applying the periodic boundary conditions on the unit-cell model is a critical step for yielding accurate mechanical response. However, the force loading mode has not been employed in the available meso-scale finite element analysis (FEA) works. In the present work, a meso-scale FEA is conducted to predict the mechanical properties and simulate the progressive damage of 3D braided composites under external loadings. For the same unit-cell model with displacement and force loading modes, the stress distribution, predicted stiffness and strength properties and damage evolution process subjected to typical loading conditions are then analyzed and compared. The obtained numerical results show that the predicted elastic properties are exactly the same, and the strength and damage evolution process are very close under these two loading modes, which validates the feasibility and effectiveness of the force loading mode. This comparison study provides a suitable reference for selecting the loading modes in the unit-cell based mechanical behavior analysis of other textile composites.
Keywords: 3D braided composites; Unit-cell; Periodic boundary conditions; Loading mode; Meso-scale FEA

Surface Modification of Aramid Fibres with Graphene Oxide for Interface Improvement in Composites by Lei Zeng; Xuqing Liu; Xiaogang Chen; Constantinos Soutis (843-852).
A novel method of biomimetic surface modification was used for aramid fibres aiming to enhance the interface properties between epoxy resin and the modified aramid fibre. Inspired by the composition of adhesive proteins in mussels, a thin layer of poly(dopamine) (PDA) was self-polymerized onto the surface of the aramid fibre. The graphene oxide (GO) was then grafted on the surface of PDA-coated aramid fibres. The microstructure and chemical characteristics of the pristine and modified fibres were characterised using Scanning Electron Microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), indicating successful grafting of GO on the PDA-coated aramid fibres. Single fibre tensile test and microbond test were carried out to evaluate the mechanical properties of the modified fibres. It was found that the fibre surface modification improved the interfacial shear strength by 210% and the fibre tensile strength was protected by GO-PDA coating.
Keywords: Surface modification; Graphene oxide; Interface; Aramid fibre composites; Epoxy resin

Surface Modification of Carbon Fibres for Interface Improvement in Textile Composites by Jiawen Qiu; Jiashen Li; Zishun Yuan; Haoxian Zeng; Xiaogang Chen (853-860).
The performance of carbon fibre-reinforced composites is dependent to a great extent on the properties of fibre-matrix interface. In this research, based on the reviewed surface modification technique and inspired by the in situ growth of three-dimensional graphene coatings on nanomaterials, a new method of in situ growth of a graphene-related structure on the surface of carbon fibres is to be applied, for which it is intended to use a mixed solution of Ferrous Sulfate Heptahydrate (FeSO4· 7H2O) and D-Glucose monohydrate (C6H12O6 · H2O) to treat the carbon fibres under specific conditions to in-situ growth of a graphene-related coatings on the surface of carbon fibres. Firstly, the method was carried out by heating the mixed solution under specific temperature on the silicon wafer substrate and followed with characterisation experiments such as Raman and Scanning Electron Microscopy (SEM). Then, the mixed solution was applied on the carbon fibres and treated under the same condition. The characterisation results indicated successfully growth of the porous carbon coatings on the surface of the carbon fibres, which contained with graphene-related structures, while other characterization experiment like Transition Electron Microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS) will need to be used to further characterise the porous carbon structure. The interfacial shear strength between the fibre and the porous carbon coating also need to be characterised by using the micro-bond test.
Keywords: Carbon fibre modification; Graphene; Composites; Characterisation; Interface

In order to optimise the construction of soft body armour panels by hybridization, this study aims to identify materials determination for hybrid panel. Different ballistic characteristics of aramid woven fabrics and Ultra High Molecular Weight Polyethylene uni-directional laminates were investigated through ballistic test and fractorgaphic analysis. With an increasing of total layer numbers in a panel, specific energy absorption of Twaron woven panel shows a decrease trend, and Dyneema UD panel exhibits an increasing trend. Such reverse trend of ballistic performance is due to different failure modes of two materials. According to fractorgraphic analysis, Twaron fabric has large transverse deformation for back layers in a perforated panel. This results in higher energy absorption in back layers. For Dyneema UD, thermal damage is the dominant failure mode, which can result in performance degradation especially for front layers on the strike face. In addition, Dyneema UD exhibits significant advantage of minimize Backface Signature (BFS) and a little higher perforation ratio than that of Twaron woven panels. Based on these findings, an optimized hybrid panel is designed by combing Twaron woven fabric before Dyneema UD. In comparison with other panels with different layer sequences, this hybridization manner exhibited better ballistic performance, including improvement of energy absorption, minimized BFS of the non-perforated panel and reduction of perforation ratio. These findings indicated that material determination for hybrid design should be based on ballistic characteristics of different materials and requirements of different regions in a panel.
Keywords: Hybrid soft armour panel; Materials determination; Aramid fabric; Ultra high molecular weight polyethylene Uni-directional laminate; Ballistic performance

The mode-I interlaminar toughness properties of nanostitched para-aramid/phenolic multiwall carbon nanotube composites were studied. The toughness strength of the stitched and stitched/nano composites demonstrated 40 fold and 38 fold (beam theory) increases compared to the base composites, respectively. It was found that stitching yarn type, especially prepreg para-aramid stitching yarn, was effective. On the other hand, the initiation and propagation of the GIC values for stitched and stitched/nano composites were considerably deviated due to strengthening mechanism of the para-aramid stitch yarn in the transverse direction of the composite. The fracture toughness resistance to arrest crack propagation in the stitched/nano composite was mainly due to through-the-thickness stitching fiber bridging and pull-out, and was also due to warp and weft directional fiber bridging and multiwall carbon nanotubes. The results demonstrated that mainly stitching and some extent the nanotubes arrested the crack growth. Therefore, the stitched/nano and especially stitched para-aramid/phenolic composites showed a better damage resistance performance.
Keywords: Aramid fiber; Carbon nanotubes; Fracture toughness; Nanostitching; Nanoprepreg

Networked fabrics are a type of three-dimensional multilayer fabrics having predetermined interconnections between layers by combining yarns from two adjacent sublayers into one. This paper reports the research on the influence of structural parameters on the ballistic performance of networked fabrics using finite element analysis in parallel with experiment. The widths of separate and combined sections are found to affect the energy absorption (EA) of regular networked fabrics against high-velocity impact. Separate sections of networked fabrics generally outperform combined sections. The optimal width of the separate section is around 9.5 cm for both dense and loose networked fabrics when impacted at the separate section. The optimal width of combined section decreases from 2.38 cm to 1.15 cm with the decrease of weave density in this area. For the studied structural parameters, highest EAs of dense and loose networked fabrics are around 13.3% and 17.1% higher than those of their counterpart layups of dense and loose plain-woven fabrics, respectively. These findings suggest networked fabrics could be engineered to improve the ballistic performance of flexible fabrics.
Keywords: Finite element modelling; 3D networked fabric; Ballistic impact

Finite Element Analysis of Mesh Size Effect of 3D Angle-Interlock Woven Composites Using Voxel-Based Method by Diantang Zhang; Guyu Feng; Mengyao Sun; Song Yu; Yuanhui Gu; Xiaodong Liu; Kun Qian (905-920).
A study is conducted with the aim of developing meso-scale voxel-based model for evaluating the voxel size effect of 3D angle-interlock woven composites subjected to axial tensile loading. Five different mesh size (0.06 mm, 0.08 mm, 0.10 mm, 0.12 mm and 0.16 mm) are chosen for the analysis of the elastic behaviors, global stress-strain curves, local stress distribution and progressive damage process. The results show that the mesh size has little influence on the effective elastic properties of 3D angle-interlock woven composites as long as the fiber volume fraction is reached. Also, the simulated displacement isolines correlate well with the experimental image, regardless the voxel size. Furthermore, the voxel sizes have an important effect on the damage behavior. Based on the statistical distribution, it is found that when the damage variable is higher, the degree of the damage ratio’s dependence on voxel size is reduced.
Keywords: 3D angle woven composites; Voxel-based model; Mechanical properties; Finite element method

Based on continuum damage mechanics (CDM), a multi-scale progressive damage model (PDM) is developed to analyze the uniaxial compression failure mechanisms of 2D triaxially braided composite (2DTBC). The multi-scale PDM starts from the micro-scale analysis which obtains the stiffness and strength properties of fiber tows by a representative unit cell (RUC) model. Meso-scale progressive damage analysis is conducted subsequently to predict the compression failure behaviors of the composite using the results of micro-scale analysis as inputs. To research the free-edge effect on the local failure mechanisms, meso-scale models of different widths are also established. The stress-strain curves obtained by numerical analysis are verified with the experimental data. Results show that fiber and matrix compression failure inside the fiber tows are the major failure modes of the composite under axial compression. For transverse compression, the dominated failure modes are recorded for matrix compression failure inside the fiber tows. It is also presented that the free-edge effect plays an important role in the transverse mechanical response of the composite, and the failure behaviors of the internal fiber tows are strongly influenced as well.
Keywords: 2D triaxially braided composite; Multi-scale model; Failure criterion; Progressive damage; Finite element analysis

Mechanical Properties of Thermoplastic and Thermoset Composites Reinforced with 3D Biaxial Warp-knitted Fabrics by Ali Al-darkazali; Pınar Çolak; Kemal Kadıoğlu; Erdinç Günaydın; Ibrahim Inanç; Özgür Demircan (939-951).
In this study, two types of thermoplastic matrices (low melting point polyethylene terephthalate (LPET) fiber and polypropylene (PP) fiber) and glass fiber/epoxy resin/multi-walled carbon nanotubes (MWCNTs) were used to fabricate the thermoplastic and thermoset composite materials with 3D biaxial warp-knitted fabrics. Thermoplastic and thermoset composites were fabricated using hot-press and resin transfer molding (RTM) methods. The fabricated samples were tested with tensile and three-point flexural tests. In thermoplastic composites, samples in the 90° direction and LPET matrix showed the best tensile and flexural properties with an improvement of 39 and 21% tensile modulus and strength, 16 and 8% flexural modulus and strength compared to the PP samples in the same direction. In thermoset composites, samples in the 90° direction and MWCNTs showed the best improvement of the flexural modulus and strength with 97 and 58% compared to the samples without MWCNTs. This improvement can most likely be attributed to an increase in interfacial adhesion due to the presence of the carbon nanotubes.
Keywords: Non-crimp fabric; Thermoplastics composites; Thermosets composites; Carbon nanotubes; Mechanical properties

In this study, a finite element failure model was created using ABAQUS to determine the location where delamination is initiated and its subsequent propagation. The effect of fibre-reinforced structures on delamination behaviour was studied. The composite T-joints were made of glass fabric infused with epoxy resin using a vacuum assisted resin transfer moulding technique. The veil layer and 3D weave techniques were employed to improve the properties in the through-thickness direction that can delay or prevent delamination when in service. All the pull-out tensile tests were conducted in an Instron testing machine using a specially designed test fixture. The 3D weave T-joints were found to have improved performance under both static and fatigue loading. Increasing the static properties increases fatigue life performance. The location for the through-thickness reinforcement plays an important role in extending fatigue life of the T-joints.
Keywords: Composite; T-joint; Finite element analysis; Fatigue; Delamination; 3D fabrics

Carbon fibre-reinforced polymer (CFRP) composites have been increasingly used by aerospace and other industries for their high specific stiffness and strength properties. When in service, non-destructive testing (NDT) methods are required to monitor and evaluate the structural integrity. Microwave-based detection techniques offer the advantages of non-contact, no need for a coupling medium or sensors bonded to the object surface and relatively easy setup. This paper is intended to provide a comprehensive overview of the currently available microwave techniques appropriate for carbon fibre/polymer composites. The electromagnetic properties of carbon fibre composites associated with microwave testing are discussed first. Then, the microwave methods are categorised into self-sensing methods, near-field induction methods, near-field resonance methods, far-field sensing methods and the methods with combination of other NDT (e.g., microwave-based thermography). Principles and applications of each kind are demonstrated in detail. Discussions of the advantages and limitations in addition to research trends of microwave testing methods are presented.
Keywords: Carbon fibre; Polymer composites; Non-destructive testing; Microwave testing