Characterisation of fibre entanglement in nonwoven fabrics based on knot theory

Owing to the structural complexity, the entanglement of fibres within nonwoven fabrics is generally characterised in indirect way by determining fabric mechanical properties using destructive testing and empirical modelling. Using elements of knot theory, a new approach is presented in this paper to characterise degree of entanglement in webs and nonwoven fabrics by topological representation of the fibrous assembly. The identification of fibre crossings followed by calculation of splitting number gives a numerical estimate of the degree of entanglement that can be potentially linked to the mechanical properties of the fibrous assembly.     

In situ monitoring of structural changes in nonwoven mats under tensile loading using X-ray computer tomography

Changes in the internal structure of nonwoven mats during tensile testing were investigated in situ with micro X-ray computer tomography (CT). Fiber orientation and volume fraction, as well as fiber–fiber contact, were quantitatively characterized at several strain levels. These parameters are apt to change under tensile loading and are important in determining the mechanical properties of nonwoven mats. The reorientation of fibers along the tensile direction was restricted at large deformations due to interlocked structures, which formed as a result of inherent entanglements in the nonwoven mats. In addition, contact efficiency, which describes the relative degree of fiber–fiber contact and was shown to be a suitable geometrical parameter for characterizing the microstructure of nonwoven mats, decreased at low strain and then increased with increasing strain until failure.     

Notch effects and crack propagation analysis on kenaf/polypropylene nonwoven composites

This paper has studied the open-hole and pin filled-hole effects on the tensile properties of Kenaf/Polypropylene Nonwoven Composites (KPNCs) in production of automotive interior parts. The influence of specimen width-to-hole diameter (W/D) ratios of 6, 3, and 2 on failure load was studied. Two sample thicknesses of 3 mm and 6 mm were evaluated. Mechanical properties of the KPNCs in terms of uniaxial tensile, open-hole tensile (OHT), and pin filled-hole tensile (FHT) were measured experimentally. A preliminary model by extended finite element method (XFEM) was established to predict the failure load and simulate crack propagation of 3 mm thick open-hole and pin filled-hole specimens. Good agreement was found between experimental and simulation results. By calculating the stress concentration factor K_t for brittle materials, the net section stress factor K_n for ductile materials, and the strength reduction factor K_r, it was found that KPNC was relatively ductile and insensitive to the notch.     

Computation of permeability of a non-crimp carbon textile reinforcement based on X-ray computed tomography images

The paper demonstrates the possibility of a correct (within the experimental scatter) calculation of a textile reinforcement permeability based on X-ray micro-computed tomography registration of the textile internal architecture, introduces the image segmentation procedures to achieve the necessary precision of reconstruction of the geometry and studies variability of the geometry and local permeability. The homogenized permeability of a non-crimp textile reinforcement is computed using computational fluid dynamics with voxel geometrical models. The models are constructed from X-ray computed tomography images using a statistical image segmentation method based on a Gaussian mixture model. The computed permeability shows a significant variability across different unit cells, in the range of (0.5…3.5) × 10⁻⁴ mm², which is strongly correlated with the solid volume fraction in the unit cell.     

Microstructure,flexural properties and durability of coir fibre reinforced concrete beams externally strengthened with flax FRP composites

This study investigated the flexural behaviour of plain concrete (PC) and coir fibre reinforced concrete (CFRC) beams externally strengthened by flax fabric reinforced epoxy polymer (FFRP) composites. PC and CFRC beams without and with FFRP (i.e. 2, 4 and 6 layers) reinforcement were tested under three- and four-point bending. The microstructures of coir fibre, coir/cement matrix, flax/epoxy matrix, and FFRP/concrete interfaces were analysed using scanning electronic microscope (SEM). Test results indicated that the peak load, flexural strength, deflection and fracture energy of both PC and CFRC specimens enhanced proportional to an increase of FFRP layers. Coir further increased load, strength and energy of the specimens remarkably. It was also found that the thickness and coir influenced the failure modes while the test method influenced the load and energy of the specimens remarkably. SEM studies showed effective bond at coir/cement, flax/epoxy and FFRP/concrete interfaces. Therefore, it concluded that natural FFRP composites can be used to repair or retrofit existing concrete structures.     

Thermomechanical properties of bio-based composites made from a lactic acid thermoset resin and flax and flax/basalt fibre reinforcements

Low viscosity thermoset bio-based resin was synthesised from lactic acid, allyl alcohol and pentaerythritol. The resin was impregnated into cellulosic fibre reinforcement from flax and basalt and then compression moulded at elevated temperature to produce thermoset composites. The mechanical properties of composites were characterised by flexural, tensile and Charpy impact testing whereas the thermal properties were analysed by dynamic mechanical thermal analysis (DMTA) and thermogravimetric analysis (TGA). The results showed a decrease in mechanical properties with increase in fibre load after 40 wt.% for the neat flax composite due to insufficient fibre wetting and an increase in mechanical properties with increase fibre load up to 60 wt.% for the flax/basalt composite. The results of the ageing test showed that the mechanical properties of the composites deteriorate with ageing; however, the flax/basalt composite had better mechanical properties after ageing than the flax composite before ageing.     

In-situ damage sensing of woven composites using carbon nanotube conductive networks

We report an in situ analysis of the microstructure of woven composites using carbon nanotube (CNT)-based conductive networks. Two types of specimens with stacking sequences of (0/90)_s (on-axis) and (22/85/−85/−22) (off-axis) were manufactured using ultra-high-molecular-weight polyethylene fibers and a CNT-dispersed epoxy matrix via vacuum-assisted resin transfer molding. The changes in the electrical resistance of the woven composites in response to uniaxial loading corresponded to the changes in the gradient of the stress–strain curves, which is indicative of the initiation and accumulation of microscopic cracking and delamination. The electrical resistance of the woven composites increased due to both elongation and microscopic damage; interestingly, however, it decreased beyond a certain strain level. In situ X-ray computed tomography and biaxial loading tests reveal that this transition is due to yarn compaction and Poisson’s contraction, which are manifest in textile composites.     

Analysis of a two-way tension-shear coupling in woven fabrics under combined loading tests: Global to local transformation of non-orthogonal normalized forces and displacements

Dependence of shear rigidity of woven fabrics on yarn pre-tension has been reported in the recent literature, however some conflict regarding the trend of this effect is observed. Sources of this conflict are discussed and resolved in the present article using a new characterization framework and a custom-design combined loading fixture. It is shown that in order to correctly characterize the tension-shear coupling behavior in woven fabrics, instead of using global measured data, local normalized forces and displacements should be driven via a non-orthogonal transformation procedure, while considering kinematic force coupling in the setup. In addition, the effect of fabric shear on the tensile behavior of yarns has been investigated, suggesting that the coupling under question is in fact two-way. In particular, results revealed that applying yarn pre-tension increases the shear resistance of the fabric reinforcement, while the tensile behavior of the material becomes more compliant when undergoing shear deformation.     

Structural behaviour of fabric reinforced cementitious matrix (FRCM) strengthened concrete columns under eccentric loading

The structural behaviour of eccentrically loaded reinforced concrete columns with rectangular cross sections strengthened with a cement based composite materials wrapping system, is analysed in the paper, both experimentally and analytically.The main issues focussed in the paper were: i) the effectiveness of the cement based wrapping systems to improve the strength of the reinforced concrete columns, ii) the influence of the load eccentricity and the reinforcement ratio on the structural response of wrapped columns, iii) the prediction, by an analytical procedure, of the structural behaviour of wrapped columns.A total of 8 reinforced concrete columns with end corbels, wrapped with fabric meshes of PBO (short of Polypara-phenylene-benzo-bisthiazole) fibers embedded into a cement based matrix (PBO-FRCM system), were tested varying both the reinforcement ratio, ρ_f, and the eccentricity-to-section height ratio (e/h). The influence of mechanical and geometrical parameters on the structural response of wrapped columns was analysed in terms of failure modes, strength and ductility.To predict the structural response of wrapped columns, a non linear second-order analysis that takes into account the changes in geometry caused by lateral deformations is, also, developed. Theoretical results were compared with experimental ones to validate the effectiveness of the proposed procedure.     

A stiffness volume averaging based approach to model non-crimp fabric reinforced composites

A representative volume element (RVE) based model to evaluate the mechanical performance of non-crimp fabric (NCF) composites has been developed and presented hereafter. By means of the stiffness averaging method, the modelling procedure is able to simulate the NCF’s elastic properties taking into account the effects of process-induced defects and final geometrical configuration. Microscopy analysis has been used to quantitatively evaluate the effects of tow waviness, stitching geometrical parameters and matrix porosity; these features have been included as sub-models into the final model. Numerical predictions have been compared to the results of tensile tests performed on composite coupons. A geometrical parameter characterising the undulation of the tows has been introduced and a sensitivity analysis has been performed on the model with different undulations.     

Effects of fabric parameters on the tensile behaviour of sustainable cementitious composites

The mechanical behaviour of fabric-reinforced composites can be affected by several parameters, such as the properties of fabrics and matrix, the fibre content, the bond interphase and the anchorage ability of fabrics. In this study, the effects of the fibre type, the fabric geometry, the physical and mechanical properties of fabrics and the volume fraction of fibres on the tensile stress–strain response and crack propagation of cementitious composites reinforced with natural fabrics were studied. To further examine the properties of the fibres, mineral fibres (glass) were also used to study the tensile behaviour of glass fabric-reinforced composites and contrast the results with those obtained for the natural fabric-reinforced composites. Composite samples were manufactured by the hand lay-up moulding technique using one, two and three layers of flax and sisal fabric strips and a natural hydraulic lime (NHL) grouting mix. Considering fabric geometry and physical properties such as the mass per unit area and the linear density, the flax fabric provided better anchorage development than the sisal and glass fabrics in the cement-based composites. The fabric geometry and the volume fraction of fibres were the parameters that had the greatest effects on the tensile behaviour of these composite systems.     

Fabrication and mechanical behaviors of cementitious composites reinforced by 3D woven lattice sandwich fabrics

Glass fibers were firstly woven to form three-dimensional (3D) woven lattice sandwich fabrics (WLSFs) which then were applied to reinforce cementitious foams and mortars to fabricate novel ductile cementitious composites. Failure behaviors of WLSF reinforced cementitious composite structures were studied through compression and three-point bending experiments. The WLSF greatly enhances the strength of cementitious foams at a level of four times. For cementitious mortars, compression strength of WLSF reinforced blocks is a little greater for the fraction of the textile is small as well as the compression strength of the textile pillars is not strong. But in flexure, excellent stretching ability of the glass fiber textiles greatly improves the flexural behavior of WLSF reinforced cementitious composite panels. Load capacity and ultimate deflection of these composite panels were greatly enhanced. Flexural capacity of the WLSF reinforced beam is four times greater. Reinforced by WLSF, failure of the cementitious composite is ductile.     

Development of fibrous preforms for FRP pipe connections

This paper describes the experimental work carried on at the University of Minho concerning the design of weft-knitted fleecy fabrics for application in pipe connections produced with composite materials reinforced by 3D weft-knitted fabric preforms. The specifications of a T tube composite connexion have been established according to the information of the composites producer company involved in the project. The weft-knitted fleecy fabric has been optimized to perform a better mechanical performance, i.e., to increase the stiffness due to the use of straight fleece yarns. Special knitting techniques, developed by the authors, have been applied to produce 3D shaped performs to be impregnated by using RTM techniques. A special mould has been produced according to the required geometry. The results of the mechanical tests made on the final produced samples are presented, discussed and compared with those imposed in the initial specifications.     

A non-destructive X-ray microtomography approach for measuring fibre length in short-fibre composites

An improved method based on X-ray microtomography is developed for estimating fibre length distribution of short-fibre composite materials. In particular, a new method is proposed for correcting the biasing effects caused by the finite sample size as defined by the limited field of view of the tomographic devices. The method is first tested for computer generated fibre data and then applied in analyzing the fibre length distribution in three different types of wood fibre reinforced composite materials. The results were compared with those obtained by an independent method based on manual registration of fibres in images from a light microscope. The method can be applied in quality control and in verifying the effects of processing parameters on the fibre length and on the relevant mechanical properties of short fibre composite materials, e.g. stiffness, strength and fracture toughness.     

Non-linear analytical model of composites based on basalt textile reinforced mortar under uniaxial tension

The recent development of inorganic based composites as low-cost materials in reinforced concrete structural strengthening and precast thin-walled components, requires the creation of models that predict the mechanical behaviour of these materials.Textile Reinforced Mortar (TRM) shows complex stress–strain behaviour in tension derived from the heterogeneity of its constituent materials. This complexity is mainly caused by the formation of several cracks in the inorganic matrix. The multiple cracking leads to a decrease in structural stiffness. Due to the severe conditions of the serviceability limit state in structural elements, the prediction of the stress–strain curve is essential for design and calculation purposes. After checking other models, an empirical nonlinear approach, which is based on the crack control expression included in the Eurocode 2, is proposed in this paper.Following this scope, this paper presents an experimental campaign focused on 31 TRM specimens reinforced with four different reinforcing ratios. The results are analysed and satisfactorily contrasted with the presented non-linear approach.     

Rate-dependent phenomenological model for self-reinforced polymers

The visco-elastoplastic nature of self-reinforced polymers (SRPs) implies that their mechanical behaviour depends on strain rate. Such dependence, when significant, must be taken into account in order to predict the impact response of these materials. In this paper, the strain rate dependence of the mechanical behaviour of a self-reinforced polypropylene (SRPP) and a self-reinforced poly(ethylene terephthalate) (SRPET) is determined and constitutively modelled. To do this, stress–strain curves corresponding to constant strain rates are deduced for each material by using a characterization method presented and validated in previous works. The strain rate dependence of the stress–strain response is quantified based on the ‘strain rate sensitivity coefficient’, defined by G’Sell and Jonas for their material model for semi-crystalline polymers. Such dependence is found to be higher in the SRPET than in the SRPP and, moreover, in both materials it depends on strain. Finally, a modified phenomenological constitutive model based on the G’Sell–Jonas one is proposed. The results show that the modified model improves the prediction of the original model reproducing accurately the rate-dependent behaviour of both SRPs.     

Fatigue and Izod impact performance of carbon plain weave textile reinforced epoxy modified with cellulose microfibrils and rubber nanoparticles

This work details an experimental investigation on understanding the effects of hybrid epoxy resins, filled with micro-fibrillated cellulose (MFC) and carboxylated nitrile-butadiene rubber nanoparticles (XNBR), on the tensile–tensile fatigue performance of carbon plain weave textile reinforced composites. Twelve combinations of MFC and XNBR weight contents in the epoxy resin (from 0% to 0.5% MFC and from 0% to 3% XNBR) were considered for preliminary quasi-static tests and five of them were selected to study the fatigue behaviour considering different loading levels. Moreover, the effect of the twelve fillers contents was observed on the Izod impact strength. The investigation finds that the best fatigue performance, for the considered weight contents of fillers, is of the composite enhanced with the maximum content of MFC. The SEM observations of the fracture surfaces indicate the extensive “plastic” deformation of the matrix and the improved fibre and matrix adhesion.     

Load-sharing and Monte Carlo models of defects in a bundle of fibres

Two methods for modelling and random simulation of progressive deformation and breaks in a bundle of parallel fibres are proposed. First, a stochastic load-sharing reliability model of a parallel system and of its resistance against a stress is utilized. The method of its statistical analysis is presented, too. In order to improve certain limitations of such model, a complementary method based on the Monte Carlo simulation is introduced. The bundle of fibres is modelled as a grid consisting of a set of nodes and connecting arcs. The deformation and breaks are caused by an external load stretching the grid. The first objective is to find an optimal, stabilized, states of the grid corresponding to each load level. Optimal configuration is found with the help of Markov Chain Monte Carlo (MCMC) procedures. In order to model the breaking process, the load is increased sequentially. It is shown that the model is applicable also to other structures, namely to the plain weave fabric and its defects simulation. The results with bundle of fibres are compared with real stress–strain curves. The parameters for simulation were selected in such a way that obtained stress–strain curve corresponds to a real experiment with carbon fibres.     

Influence of low velocity impact on fatigue behaviour of woven hemp fibre reinforced epoxy composites

The purpose of this work is to study the resistance to low velocity impact of woven hemp/epoxy matrix composites and the influence of impact damage on their residual quasi-static tensile and cyclic fatigue strengths. Impact characteristic parameters were evaluated and critically compared to those found in the literature for other similar composites. Damage mechanisms were analysed by using AE monitoring and microscopic observations. An analytical model is used to predict the fatigue lifetime of impacted specimens. Moreover a damage scenario is proposed, reduced to two phases in post-impacted fatigue behaviour, instead of three phases for non impacted specimens.