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Hierarchical design down to the nanoscale has become possible in structural composite materials with the discovery of carbon nanomaterials such as carbon nanotubes (CNTs) and graphene. Composites that simultaneously combine microscopic continuous fibers and nanoscale reinforcements are known in the field as hierarchical or nanoengineered composites. The additional reinforcement at the nanoscale promises high-performance composites with unique combinations of mechanical properties and new functionalities. Here, we review advances in fiber-reinforced polymers modified with CNTs. Three routes for integration of CNTs in composites are discussed: deposition on fibers/plies, dispersion in the matrix, and assembly into fibers. We highlight opportunities and challenges focusing on mechanical performance and processing.

In this work, we investigate the effect of the matrix on the mechanical performance of woven carbon fibre composites. More specifically, composites with the same 5-harness satin carbon fabric reinforcement and different thermoplastic matrices, PPS and PEEK, are compared in various mechanical tests (tensile, interlaminar fracture toughness and compression-after-impact tests). The results of tension tests show the influence of the matrix type on the development of cracks in yarns. The cracks in carbon fabric/PEEK composite appear later than in carbon fabric/PPS composite. Their density is also lower. A correlation between cumulative acoustic emission energy and transverse crack appearance in tensile tests is shown. The most evident difference is demonstrated for the Double Cantilever Beam tests and End Notch Flexure tests. The interlaminar fracture toughness for both mode I and mode II is more than 1.5 times higher for carbon fabric/PEEK laminates as compared to carbon fabric/PPS ones. The higher fracture toughness of carbon fabric/PEEK results in its higher residual compressive strength after impact (∼25%). Thus, the study concludes that the performance of textile composites is highly sensitive to the performance of the matrix. Matrices that have higher strength, ductility and fracture toughness lead to structural composites with lower crack densities, better performance in the bias direction, higher resistance to delaminations and higher residual strength after impact.

The authors investigate the interlaminar fracture toughness of unidirectional carbon fiber composites with eight different matrix systems. The matrices are blends of epoxy and polyethersulfone (PES) thermoplastic that are additionally reinforced with carbon nanotubes (CNTs). Based on the content of PES, different multi-phase morphologies (including particulate, co-continuous, and phase inverted) are obtained. The presence of fibers and CNTs is found to affect these morphologies in the composite. The interlaminar fracture toughness increases for high PES contents where co-continuous and inverted morphologies are present. No synergy between CNTs and PES is observed. The improvements achieved in the study are highest (above 100%) for the composites containing 15% of PES (with and without CNTs).

Few studies have been conducted to monitor inter-fiber deformations in fiber-reinforced composites. In the present work, we demonstrate full-field strain measurements in the composites at the micro-scale, using digital image correlation (DIC). The study is performed on a unidirectional glass fiber reinforced composite loaded in transverse three-point bending inside an environmental scanning electron microscope. A nano-scale random speckle pattern of high quality is created. Validity of the measured fields is assessed against results of a finite element (FE) model with boundary conditions retrieved from the experiment. A good agreement is found between the DIC-measured and FE-predicted results. The precise recognition of very small-scale strain concentrations requires enhancement of the correlation process and removal of microscopy imperfections. The investigated methodology shows promise for real-time deformation measurements in composites at the micro-scale.

The failure strain of fibre-reinforced composites can be increased by fibre hybridisation. A recently developed model for unidirectional composites was extended to hybrid composites to analyse this synergetic effect, called the hybrid effect. The model predicts individual fibre breaks and interactions among clusters of fibre breaks. Three key parameters were studied to understand how they can maximise the hybrid effect, namely low elongation fibre strength scatter and hybridisation fibre stiffness and failure strain. Larger strength scatter of the low elongation fibres leads to larger hybrid effects, as the scatter spreads out the cluster development over a larger strain interval. The failure strain ratio of the two fibre types should be above 2 for the properties used here, but a higher ratio did not yield any additional benefits. Increasing the stiffness of the hybridisation fibre reduces the stress concentrations on the low elongation fibre and may also enlarge the hybrid effect. These conclusions provide guidelines for designing optimal hybrid composites. publisher: Elsevier articletitle: Maximising the hybrid effect in unidirectional hybrid composites journaltitle: Materials & Design articlelink: http://dx.doi.org/10.1016/j.matdes.2015.12.137 content_type: article copyright: Copyright © 2015 Elsevier Ltd. All rights reserved. ispartof: Materials & Design vol:93 pages:39-45 status: published

While poor dispersion of carbon nanotubes (CNTs) is often blamed for degradation of properties of CNT reinforced polymers and their composites, the evidence that CNT agglomerates generate stress concentrations is based on common sense and, to a large extent, indirect. The present study investigates the effect of CNT agglomerates on the inter-fiber stresses in a unidirectional fiber reinforced composite using a numerical approach. The two-scale model recently developed by the authors allows simulating realistic CNT agglomeration scenarios. A parametric study is performed focusing on the size and density of CNT agglomerates as well as on the degree of agglomeration from perfectly dispersed to partially and fully agglomerated states. The study concludes that CNT agglomerates of a higher density and bigger size produce higher stress concentrations. They also give rise to higher stresses at the fiber/matrix interface. The least disturbance on the stress fields is introduced by homogeneously dispersed CNTs.

We propose to treat propagation of a fatigue-driven delamination in composite laminates as a sequence of incremental delamination onsets and develop a cohesive zone model (CZM) to simulate it. The model employs a fatigue damage evolution law that is based on the reduction of critical strain energy release rate. Unlike existing fatigue CZMs, which assume and use Paris’ law derived from propagation tests as input, the proposed model relies solely on the experimental data from fatigue onset tests (namely the G-N curve, where G is the supplied strain energy release rate and N is the number of cycles to onset). The model is implemented within an implicit finite element code using three-dimensional cohesive elements and is demonstrated to predict fatigue delamination propagation in unidirectional carbon fibre/epoxy composites under Mode I loading. This incremental onset model predicts the Paris’ law, which compares well with experimental data for this case. Composites with toughened inter-laminar interface and under other modes of deformation and their combinations may exhibit a different fatigue behaviour.

Hybrid composites combining brittle and ductile reinforcing fibres can be designed to have pseudo-ductile behaviour by controlling their failure mechanisms. The recently developed carbon-fibre/self-reinforced polypropylene (SRPP) hybrids have demonstrated a unique combination of stiffness (7 GPa), toughness (>6% strain) and density (±1000 kg/m³). Here, the influence of a circular notch on their failure mechanisms is investigated. Discontinuous carbon-fibre/polypropylene composites retained 69% of the gross strength and 83% of the net strength. The strength retention of SRPP was highly influenced by the processing parameters, and a notch-insensitive composite could be produced. Pseudo-ductility was obtained even with the notched hybrids. Hybrids that do not achieve the optimal ductility, still exhibit high gross and net strength retentions and can be classified as notch-insensitive. These results illustrate the significant potential of this new hybrid composite for applications with notches or holes.

The answer to this question is revealed by our dual scale models, which say that the composite is the strongest and the toughest not when nanotubes themselves absorb energy but when they serve a supporting role in facilitating matrix to do the energy absorption instead. Toughening by means of only nanotube debonding/pull-out is not the most optimal strategy. The model concurrently considers five energy absorbing mechanisms acting at different scales. These mechanisms are nanotube debonding from the matrix, nanotube breakage, nanotube detachment from the (microscopic) fiber, fiber debonding and finally microdamage in the matrix. Our virtual experiments show that the benefit of CNT grafting largely depends on the tensile strength of CNTs and their interfacial cohesion with the matrix. A map of failure modes is constructed in the parametric space of these two CNT properties to help establish a window of properties beneficial for the strength and toughness improvement. The maximal enhancement is obtained when matrix is allowed to absorb energy through micro-scale damage and CNT debonding at the nano-scale has only partial contribution to it. Recommendations for the CNT tensile strength and CNT/matrix interfacial strength, necessary for creation of strong and tough hierarchical composites, are formulated.

Thermoplastic composites are emerging in the automotive industry as serious competitors to typical epoxy composites thanks to their higher toughness and reprocessability. Additionally, composites with polymer fibres can yield higher impact resistance than composites with conventional structural fibres like carbon and glass, while still delivering a high stiffness. In this work, woven composites with different fiber types (aramid, poly(p-phenylene-2,6-benzobisoxazole) (PBO), polyarylate (PAR), carbon) and different matrices (epoxy, polypropylene) were investigated under penetrating impact. It was found that aramid-PP composites absorb 2.82 times the energy of aramid-epoxy. PBO-PP absorbed almost twice as much energy as the other polymer fibre composites. PBO-PP and PAR-PP fractured the specimen with an unusual “X” shape while all other composites exhibited a “+“shape, which is typical for woven fabric composites. It is hypothesised that the unusual failure shape and the high energy absorption are linked to the high fibre anisotropy.

© 2018 Elsevier Ltd The present work investigates the influence of the laminate layup on the flexural properties and failure mechanisms of corrugated panels made of self-reinforced polypropylene (SRPP) hybridised with carbon fibre. The influence of the layup on the stiffness and strength of the corrugated hybrid composite panels was much less important than the carbon fibre content. The experimental program was complemented by finite element analysis. The model revealed the importance of SRPP's non-linear behaviour and difference in tensile and compressive behaviour in controlling the flexural properties of corrugated panels. The compressive behaviour of SRPP was characterised to provide reliable input data for the model. ispartof: COMPOSITES PART B-ENGINEERING vol:153 pages:437-444 status: published

A Digital Image Correlation (DIC) based method is proposed to characterize Mode I fatigue delamination onset and propagation in laminated composites. With the help of DIC, the displacement field around a delamination crack is obtained and further processed to determine the position of the crack tip. With this method the delamination length can be measured automatically in each cycle with a precision on the order of few hundreds of micrometers. The fatigue delamination onset life is then determined by detecting the increase of the delamination length, and the fatigue delamination propagation rate is calculated. The proposed method produces more conservative fatigue life measurements in comparison with the compliance increase method in ASTM D6115.

© 2018 Elsevier Ltd Several modelling approaches are available in the literature to predict longitudinal tensile failure of fibre-reinforced polymers. However, a systematic, blind and unbiased comparison between the predictions from the different models and against experimental data has never been performed. This paper presents a benchmarking exercise performed for three different models from the literature: (i) an analytical hierarchical scaling law for composite fibre bundles, (ii) direct numerical simulations of composite fibre bundles, and (iii) a multiscale finite-element simulation method. The results show that there are significant discrepancies between the predictions of the different modelling approaches for fibre-break density evolution, cluster formation and ultimate strength, and that each of the three models presents unique advantages over the others. Blind model predictions are also compared against detailed computed-tomography experiments, showing that our understanding of the micromechanics of longitudinal tensile failure of composites needs to be developed further. ispartof: COMPOSITES PART A-APPLIED SCIENCE AND MANUFACTURING vol:111 pages:138-150 status: published

The high performance fiber-reinforced plastics have hierarchical reinforcement architectures. They exhibit complex failure processes that occur interactively and concurrently on multiple scales and are closely linked to the constituents’ behavior and the reinforcement architecture. The chapter describes different stages of the damage development and their link to structural levels. At the micro-level the onset of damage is influenced by the microscopic morphology inside fiber bundles and the submicroscopic organization of nano-reinforcements. The latter introduce strong inhomogeneity in stress fields and change stress concentrations at the fiber/matrix interface. The transition from the fiber/matrix debonding to the intra- and inter-ply damage is then discussed focusing on differences between laminates with unidirectional and textile plies. The experimentally observed phenomenology of the damage processes is supported by findings from simulation studies.

© 2018 Author(s). This paper investigates the formability characteristics of polypropylene composites reinforced by randomly oriented discontinuous carbon fibres and/or woven polypropylene tapes, which are also known as self-reinforced composites. Hybridised configurations when both reinforcements are used were also studied. Material behaviour during forming is studied by thermostamping domes of different depths ranging from nearly flat to fully hemispherical. It was found that carbon fibre preforms and hybrids deconsolidate and develop wrinkles during forming. These wrinkles collapse during the final consolidation stage and good surface quality of the hemispherical parts is achieved. ispartof: PROCEEDINGS OF 21ST INTERNATIONAL ESAFORM CONFERENCE ON MATERIAL FORMING (ESAFORM 2018) vol:1960 ispartof: 21st International ESAFORM Conference on Material Forming (ESAFORM) location:ITALY, Univ Palermo, Palermo date:23 Apr - 25 Apr 2018 status: published

A promising strategy to increase the tensile failure strain of carbon fibre-reinforced composites is to hybridise carbon fibres with other, higher-elongation fibres. The resulting increase in failure strain is known as the hybrid effect. In the present article, a global load-sharing model for hybrid composites is developed and used to carry out a parametric study for carbon/glass hybrids. Hybrid effects of up to 15% increase in failure strain are predicted, corresponding reasonably well to literature data. Scatter in the carbon fibre strength is shown to be crucial for the hybrid effect, while the scatter in glass fibre strength is much less important. In contrast to reports in earlier literature, the ratio of failure strains of the two fibres has only a small influence on the hybrid effect. The results provide guidelines for designing optimal hybrid composites. publisher: Elsevier articletitle: Global load-sharing model for unidirectional hybrid fibre-reinforced composites journaltitle: Journal of the Mechanics and Physics of Solids articlelink: http://dx.doi.org/10.1016/j.jmps.2015.08.009 content_type: article copyright: Copyright © 2015 Elsevier Ltd. All rights reserved. ispartof: Journal of the Mechanics and Physics of Solids vol:84 pages:380-394 status: published

Carbon fibre composites possess excellent mechanical properties, but suffer from brittleness. Hybridisation with self-reinforced polypropylene (SRPP) is a promising strategy to introduce ductility into carbon fibre-reinforced polypropylene (CFRPP). The present work demonstrates how different damage mechanisms in these hybrid composites change as a function of the carbon fibre volume fraction, the directionality of CFRPP and SRPP and their relative layer thickness. Multiple fractures of the CFRPP layers or “fragmentation” is achieved by optimising these parameters. This leads to a ductile hybrid composite with a gradual failure development. publisher: Elsevier articletitle: Introducing ductility in hybrid carbon fibre/self-reinforced composites through control of the damage mechanisms journaltitle: Composite Structures articlelink: http://dx.doi.org/10.1016/j.compstruct.2015.04.069 content_type: article copyright: Copyright © 2015 Elsevier Ltd. All rights reserved. ispartof: Composite Structures vol:131 pages:259-265 status: published

Three issues are investigated that may influence the accuracy of strength models for unidirectional composites. Firstly, most authors limit themselves to 25–100 single fibre tests to determine the Weibull distribution. This is insufficient to accurately determine this crucial input parameter, leading to significant errors in the predicted composite failure strains. Secondly, random, square and hexagonal fibre packings are shown to lead to a similar predicted failure strain and cluster development. This is the first strong indication that the type of fibre packing has little influence on modelling predictions. Finally, boundary fibres introduced at the model perimeter were shown to prevent preferential cluster formation near the model perimeter. This makes the models less sensitive to the number of fibres. Boundary fibres do not influence the early cluster development, but do increase the critical cluster size. The results provide guidelines for when such boundary fibres should be included. publisher: Elsevier articletitle: Issues in strength models for unidirectional fibre-reinforced composites related to Weibull distributions, fibre packings and boundary effects journaltitle: Composites Science and Technology articlelink: http://dx.doi.org/10.1016/j.compscitech.2015.04.002 content_type: article copyright: Copyright © 2015 Elsevier Ltd. All rights reserved. ispartof: Composites Science and Technology vol:114 pages:42-49 status: published

Deposition of carbon nanotubes (CNT) on fiber surfaces is a promising technique to add functionalities to polymer composites. Upon deformation, these CNTs generate local stress heterogeneities which may affect properties of the interphase and the composite. The stress disturbance depends on the morphology of deposited CNT assemblies also linked to a deposition technique. In the present work, two types of morphologies are investigated: (1) CNTs grown on fibers; and (2) CNTs deposited in fiber coatings. The difference in the two cases is the orientation of CNTs near the fiber interface: radial for grown CNTs and tangent for CNTs in the coatings. Different parameters including CNT length, morphology of the grafting and density of the coating are evaluated. The analysis is performed using a recently developed two-scale model. Results show that CNT grafted and coated fibers produce distinctly different interface stresses.

In the present study, polyoxymethylene (POM), a crystallizable thermoplastic, is investigated for toughening of an epoxy resin and a carbon fiber reinforced composite produced from it. Depending on the blend composition POM forms different morphologies during the curing reaction of epoxy (particulate, co-continuous or phase-inverted). A multi-fold increase in the fracture toughness of the neat resin is obtained for particulate morphology. Fractographic investigation of fracture surfaces reveals a number of energy dissipation mechanisms, including particle crack bridging, matrix shear yielding, and plastic void growth following particle debonding. The presence of fibers is found to severely disturb the phase separation process and to change the resulting morphology in the fiber reinforced composite. With it the interlaminar fracture behavior of the composite is compromised.

Conventional composites are susceptible to impact loading due to their low toughness. Novel annealed steel fibres possess a unique combination of high stiffness and high strain to failure. A range of composite laminates incorporating various steel fibre architectures with thermoplastic and thermoset matrices were produced and the low-velocity penetration impact resistance was characterized by falling weight impact tests, showing the high potential toughness of steel fibre composites. The effect of different parameters was determined. High matrix strain to failure allows a better exploitation of the steel fibre ductility. The higher absorbed energy up to penetration, in case of laminates with lower levels of fibre/matrix adhesion is attributed to fibre debonding, potential for more plastic deformation, and pull-out mechanisms. Thinner fibres are more sensitive to premature failure due to inclusions in the steel fibres. Cross-ply laminates show higher energy absorption, probably due to increased delamination, whereas woven fibre structures lead to more localized damage. A preliminary study shows that layer by layer (‘macro’) hybridization of carbon and steel fabrics leads to penetration impact resistance close to the pure steel fibre composite, in case the steel plies are placed on the outside surfaces, showing a significant positive effect of hybridization.

In this study we investigate the tensile behaviour of unidirectional and cross-ply composites reinforced with ductile stainless steel fibres and modified adhesion to the epoxy matrix. Results show that annealed stainless steel fibres have a potential in designing tough polymer composites for structural applications. The stiffness of the UD composites made from these fibres is 77GPa combined with the strain-to-failure between 15% and 18% depending on the level of adhesion. Silane treatments were used to modify the adhesion. By treating the stainless steel fibres with different silane coupling agents, an increase of 50% in the transverse 3-point-bending strength was realised. Increasing the adhesion by 50% leads to a higher tensile strength and strain-to-failure in both UD and cross-ply laminates and a higher in-situ strength of the 90° plies. It also delays formation of matrix cracks and hinders growth of debonding.

By adding glass fibres to carbon fibre composites, the apparent failure strain of the carbon fibres can be increased. A strength model for unidirectional hybrid composites was developed under very local load sharing assumptions to study this hybrid effect. Firstly, it was shown that adding more glass fibres leads to higher hybrid effects. The hybrid effect was up to 32% for a hybrid composite with a 10/90 ratio of carbon/glass fibres. The development of clusters of broken fibres helped to explain differences in the performance of these hybrid composites. For 50/50 carbon/glass hybrids, a fine bundle-by-bundle dispersion led to a slightly smaller hybrid effect than for randomly dispersed hybrids. The highest hybrid effect for a 50/50 ratio, however, was 16% and was achieved in a composite with alternating single fibre layers. The results demonstrate that thin ply hybrids may have more potential for improved mechanical properties than comingled hybrids. publisher: Elsevier articletitle: The effect of fibre dispersion on initial failure strain and cluster development in unidirectional carbon/glass hybrid composites journaltitle: Composites Part A: Applied Science and Manufacturing articlelink: http://dx.doi.org/10.1016/j.compositesa.2014.12.001 content_type: article copyright: Copyright © 2014 Elsevier Ltd. All rights reserved. ispartof: Composites A, Applied Science and Manufacturing vol:69 pages:279-287 status: published

Despite the crucial significance of failure prediction in composites, such an objective remains challenging, even in unidirectional (UD) systems. A strength model for UD composites was used that has great versatility in handling various matrix and fibre behaviours. This model includes a simplified superposition principle that was found to be reliable in predicting stress concentration factors irrespective of the presence of matrix cracks. The model revealed the negligible influence of matrix cracks on stress concentrations, ineffective length, cluster development and failure strain. The presence of matrix cracks can therefore be safely neglected in models for UD composites. This information is important for experimental validations and for advancing the state of the art in strength models for UD composites. publisher: Elsevier articletitle: Matrix cracks around fibre breaks and their effect on stress redistribution and failure development in unidirectional composites journaltitle: Composites Science and Technology articlelink: http://dx.doi.org/10.1016/j.compscitech.2015.01.002 content_type: article copyright: Copyright © 2015 Elsevier Ltd. All rights reserved. ispartof: Composites Science and Technology vol:108 pages:16-22 status: published

A new micromechanical model that predicts anisotropic elastic properties of cell-wall material in nanoclay-reinforced polymeric foams is proposed. The model accounts for the reorientation of nanoclays during the foaming process, due to the stretching of the foam cell walls. Elastic moduli of bulk nanocomposites were studied first. It was found that given the high aspect ratio of nanoclays, they cannot be randomly placed in a representative volume element without having them intersect each other at high weight fractions. At 1.5 wt% the nanoclays already align and form clusters resulting in a significant effect on the stiffness of the nanocomposite. The number of silicate layers per cluster stack was also found to influence the Young’s modulus of the nanocomposite. When reinforcing polypropylene (PP) with 2 wt% of nanoclays, the modulus of elasticity decreased by 17% when the number of clays in a cluster went from 1 (fully exfoliated system) to 5. The stretching of the nanocomposite was shown to reorient nanoclays and, consequently, to change the stiffness of the cell wall. For the PP reinforced with 1 wt% of nanoclays and a stretching ratio of 6.5, the modulus increased by 15%.

The high stiffness (∼193 GPa) and high strain-to-failure (∼20%) of annealed stainless steel fibres present an opportunity to design ductile structural composites. In this research, the effect of the weave architecture on the tensile and impact behaviour of ductile stainless steel fibre/PP composites is investigated. Composites with three different weave architectures are compared: a quasi-unidirectional weave, a basket weave and a satin weave. The tensile test results show that all weave architectures show the same composite strain-to-failure, despite the difference in crimp. The composite with the basket weave (high crimp) has much lower stiffness and yield stress in comparison with the other two composites. This is attributed to significant out-of-plane deformations observed during the tensile test. The penetration impact results show that the high ductility of stainless steel fibre composites in tensile tests is transferred into excellent impact performance.

The intra-laminar damage during quasi-static loading of Carbon Fiber-Reinforced Plastics (CFRPs) leads to degradation of ply mechanical properties. The possible intralaminar damage mechanisms are fiber failure, matrix cracking, and fiber/matrix debonding. In addition to experimental investigation of these mechanisms, modeling methodologies have been developed to predict the degradation of ply properties, accounting for (some of) these mechanisms. One of these methodologies is proposed by Ladeveze and LeDantec [1]. This model requires a set of input parameters, which are obtained through mechanical tests on laminates with prescribed lay-ups. In the current study, we extract these input parameters for an aerospace-grade CFRP, with detailed explanations. Beside the conventional parameter identification procedure, the parameters are linked to the physical progression of damage (in terms of crack densities), which is measured in the in situ images taken during the tests.

Hybridisation of self-reinforced polypropylene (SRPP) with ductile steel fibres is proposed as a way to create composites with a high stiffness, failure strain and toughness. The stiffness and yield stress of SRPP were significantly increased without sacrifice of toughness, even after normalisation to the composite density. The measurements revealed a higher stress level and slope after yielding than the predictions by the linear rule-of-mixtures. This effect was attributed to differences in the Poisson contraction of the two constituents and was supported by modelling predictions. The absolute penetration impact resistance of SRPP was increased, whereas the specific impact resistance remained the same, despite the high density of steel fibres. Placing steel fibre layers on the outside of the laminate maximised the penetration impact resistance, as it allowed the steel fibres to exploit their full plastic potential. These results can guide the optimisation of other hybrid composites with two ductile fibres. ispartof: Composites: Part A - Applied Science and Manufacturing vol:100 pages:48-54 status: published

The state of carbon nanotube (CNT) dispersion in epoxy is likely to change in the process of composite production. In the present work CNT dispersion is characterized at different stages of nanocomposite preparation: in the original masterbatch with high CNT concentration, after masterbatch dilution, in the process of curing and in the final nanocompos-ite. The evaluation techniques included dynamic rheological analysis of the liquid phases, optical, environmental and charge contrast scanning electron microscopy, electrochemical impedance spectroscopy and dynamic mechanical analysis. The evolution of the CNT dispersion was assessed for two CNT/epoxy systems with distinctly different dispersion states induced by different storage time. Strong interactions between CNT clusters were revealed in the masterbatch with a longer storage time. Upon curing CNT clusters in this material formed a network-like structure. This network enhanced the elastic behaviour and specific conductivity of the resulting nanocomposite, leading to a partial electrical percolation after curing. © BME-PT. ispartof: Express Polymer Letters vol:8 issue:8 pages:596-608 status: published

Due to the intrinsic brittleness of high performance fibres, traditional structural fibre-reinforced composites have limited ductility and toughness. In the present work a new class of fibres is explored for the reinforcement of polymers: continuous stainless steel fibres that simultaneously possess a high stiffness and a high strain-to-failure. The fibres are combined with brittle and ductile matrix systems (epoxy and PA-6) to produce unidirectional and cross-ply composites. The composites are investigated in quasi static tensile tests accompanied with acoustic emission registration. The steel fibre composites are found to exhibit a 3–4 times higher strain-to-failure than typical carbon or glass fibre composites. publisher: Elsevier articletitle: Ductile steel fibre composites with brittle and ductile matrices journaltitle: Composites Part A: Applied Science and Manufacturing articlelink: http://dx.doi.org/10.1016/j.compositesa.2014.02.006 content_type: article copyright: Copyright © 2014 Elsevier Ltd. All rights reserved. ispartof: Composites: Part A - Applied Science And Manufacturing vol:61 pages:235-244 status: published

The effect of carbon nanotubes (CNTs) on the impact and after impact performance of woven carbon fiber/epoxy composites is investigated. Three nano-reinforced epoxy systems that differ in the CNT dispersion and functionalization are used as a matrix material. The composites are tested for the resistance to an out-of-plane low velocity impact, the residual compressive strength, the mode II interlaminar fracture toughness and the interlaminar shear strength. The composite with the highest mode II fracture toughness is found to be the best performing system in the compression test. The resin in this composite contains a network-like structure of CNTs. Nanotubes are found to have a dual effect on composite properties: (1) they improve the mode II interlaminar fracture toughness and (with it) damage tolerance of a composite, but (2) they also make composites more susceptible to the onset of matrix cracks leading to a larger delamination area after impact.

In order to find an alternative core material to balsa wood in composite sandwich structures, it is important to understand balsa’s elastic properties in relation to its complex microstructural organisation. In the present work, experimental data on the elastic constants and microstructural features of balsa wood were collected for different porosities (densities) and processed into structure–property relations. An inverse problem was solved to predict variation of the cell wall properties with density, such that the collected experimental structure–property relations were satisfied. The Young’s modulus of the cell wall material in the longitudinal direction was found to increase with balsa’s density, which is consistent with the knowledge that the cell wall material stiffens during tree maturation. The value reported in the literature falls in the middle of the predicted range. The proposed micromechanical model also accurately calculated elastic properties of balsa wood at the mesolevel including longitudinal, radial, and tangential directions. The model took into account the presence of ray cells. It was shown that the addition of 15 % of rays increased the radial Young’s modulus up to 4 times with only slight decrease in the longitudinal modulus.

Growth of nanowires, particularly aligned carbon nanotubes (CNTs) on fibre surfaces, is an additive approach to micro/nanoscale reinforcement for realising hierarchical (or nanoengineered) fibre reinforced composites (FRCs). While eliminating dispersion issues and allowing morphology control, the aligned growth could pose challenges when adapting traditional composite manufacturing methods. To quantify the effect of CNT morphology on manufacturing, the compressibility of CNT-grafted alumina fibres (woven fabric and yarn) is studied experimentally. Radially-aligned CNTs are grown on fibres to lengths of 20 μm, with CNT weight fractions up to 10%. To understand compression as a key aspect of composite processing, pressure vs. thickness measurements are made for tows, single plies, and stacked plies through consecutive compression cycles. The results show substantial decreases in compressibility of the fibrous assemblies in the presence of CNT grafting, consistent with earlier observations of random CNT forests morphologies on carbon fibres. The pressure needed to reach a typical-use fibre volume fraction of 40% increases from 176 kPa for non-grafted fabric to 1390 kPa for a fabric grafted with 5 wt.% aligned CNTs, and 2794 kPa for 10 wt.% of aligned CNTs. Compared to the random CNT morphology, the aligned CNT forest-grafted textiles are less compressible, offering higher compression resistance and higher resilience for the same weight or volume fraction of CNTs: at 0.1 MPa pressure, the fabric thickness is increased (or fibre volume fraction is decreased) by a factor of 1.7…2.5 for 5…10 wt.% of the aligned CNT grafting (against a factor of 1.3…1.5 for the same amount of random grafting). The aligned-CNT morphology exhibits significant compression hysteresis that was not observed for the case of random CNT forests. Analysis of the compression curves reveals that pressures needed to achieve typical laminate microfibre packing fractions are at the limit of standard composite processing pressures.

To improve the toughness of carbon fibre composites, unidirectional carbon fibre prepregs were hybridised with highly oriented polypropylene (PP) tapes. The latter tapes are used in self-reinforced PP composites (SRPPs) with high toughness, but relatively low stiffness. The tensile behaviour of intralayer hybrids of oriented PP tapes and CFPP prepreg tapes was investigated by changing the layup and weave pattern, and by adding interleaved PP films. While stiffness and strength of these hybrids was decreased compared to CFPP, their ductility significantly increased by adding oriented PP tapes. The parallel behaviour of the two constituent materials was caused by delaminations, which developed after the CFPP failure and spreads over the entire sample. This behaviour was modelled and proven to be independent of layup and weave pattern. Interleaved films, which are often necessary for thermoformability, limited the delamination development by increasing the interlayer bonding and yielded a lower ultimate failure strain.

We employed X-ray micro-computed tomography to generate an extensive dataset of void characteristics in carbon/epoxy laminates with different stacking sequences. The voids were deliberately induced by modifying the material’s cure cycle. The obtained three-dimensional images demonstrating voids were segmented and quantified using an in-house software package, VoxTex. The void characteristics, i.e. shape, size parameters, orientation, and spatial distribution, were obtained for analysis. It was found that voids are typically elongated features, aligned with the neighboring fibers. The majority of voids had slightly flattened cross-sections. The voids preferred to localize in the inner plies of the laminate, rather than in the outer ones. Near the laminate mid-thickness, they became more elongated. The stacking sequence was found to affect the size and shape of voids, with the void content being higher for a thicker laminate. The planar location of voids is not random and tends to create a pattern, corresponding to the ply orientations in the laminate. The data gathered during this study has been reported in detail and can be used, for example, as input for studies modeling similar composites with voids. The entire micro-CT data is presented as a Data in Brief article in [1]. ispartof: Composites Part A - Applied Science and Manufacturing vol:125 status: published

In recent years the embedded element (EE) technique, which is based on the superposition of meshes, has been introduced into finite element (FE) modelling of composite materials. When compared to conventional approaches employing a single continuous mesh this technique has a number of advantages, among which are simplicity in the model creation and improved mesh quality. However, due to the absence of physical interfaces, simulation of interfacial damage with this technique presents a real challenge. In this work we propose an approach in which the EE technique is combined with the cohesive zone model with the purpose to model interfacial damage in fibrous composites. The new hybrid method is validated against conventional FE models in four case studies. Among others the methodology is applied to predict mechanical properties of nanocomposites reinforced by carbon nanotubes showing good agreement with experimental data.

Modelling of the onset and propagation of matrix cracks in fibre-reinforced composites on the micro scale requires adequate representation of the material microstructure. In the present work we compare simulated and real fibre arrangements found in unidirectional composites, using statistical descriptors. The comparison is done for geometrical and mechanical parameters such as distributions of the fibre positions and the stress fields. The real fibre arrangements are extracted from microscopy images of a 3D non-crimp woven carbon/epoxy composite with fibre volume fractions of 58–68%. The modelled fibre arrangements are generated using a heuristic random microstructure generation algorithm. The stress fields are compared for the case of transverse tension. A good correlation between statistical parameters of the real and simulated fibre arrangements is obtained. publisher: Elsevier articletitle: Statistical analysis of real and simulated fibre arrangements in unidirectional composites journaltitle: Composites Science and Technology articlelink: http://dx.doi.org/10.1016/j.compscitech.2013.07.030 content_type: article copyright: Copyright © 2013 Published by Elsevier Ltd. ispartof: Composites Science and Technology vol:87 pages:126-134 status: published

The weave architecture is vital for hot compaction and the mechanical properties of self-reinforced polypropylene. Low compaction quality resulted in early damage initiation and reduced tensile strength. Interleaved films and decreased crimp in the weave architecture increased the compaction quality. The best compaction quality and tensile properties were obtained by standard fed weaves with interleaved films. The penetration impact resistance and peel strength was independent of the weave architecture. Interleaved films increased the peel strength drastically, but the impact resistance only slightly decreased. These conclusions help to select the correct weave architecture and facilitate the hot compaction process. publisher: Elsevier articletitle: The influence of weave architecture on the mechanical properties of self-reinforced polypropylene journaltitle: Composites Part A: Applied Science and Manufacturing articlelink: http://dx.doi.org/10.1016/j.compositesa.2013.06.015 content_type: article copyright: Copyright © 2013 Elsevier Ltd. All rights reserved. ispartof: Composites Part A: Applied Science and Manufacturing vol:53 pages:129-136 status: published

Due to their exceptional mechanical properties, carbon nanomaterials such as carbon nanotubes (CNTs) have been intensively studied as additional reinforcements in structural composites. They have created opportunities to develop advanced composites with improved mechanical performance and new functionalities. CNTs are introduced in fibre-reinforced polymers via various routes. They can be dispersed in the matrix, deposited in fibre sizing, directly grown on fibres or assembled into fibres. Composites, which simultaneously combine nanoscale and micro-scale reinforcements, are frequently referred to as hierarchical or nano-engineered composites. In the present chapter, we highlight challenges and benefits for the use of CNTs in structural fibre-reinforced polymers. The focus is on the mechanical performance of composites with nano-modified matrices, interfaces and fibres.

For the first time, micro-scale digital image correlation (μDIC) is investigated for measurement of strain fields in hierarchical fiber-reinforced composites. The methodology is developed on an exemplary alumina fiber/epoxy composite laminate with aligned carbon nanotubes (A-CNTs) grown on fibers. Utilizing environmental scanning electron microscopy and nano-scale random speckle patterns, sufficient precision is achieved to detect the influence of the A-CNTs on the deformation field around the fibers. Debonded regions at the fiber/matrix interface with openings as small as 35 nm could be detected. μDIC could identify the propagation of the debonded region based on the non-linear increase of the opening. The image correlation uncertainty in the displacement analysis is estimated to be below 5 nm. The experimental results are validated by computational analysis performed on the region of interest. For this, an advanced model with two scales of reinforcement (microscopic fibers and nanotubes) and boundary conditions taken from the experiment is used. As verified by the model, A-CNTs are found to constrain matrix deformation in their longitudinal direction. Keywords: Digital image correlation (DIC); Scanning electron microscopy (SEM); Carbon nanotubes; Mechanical properties; Finite element analysis (FEA)

Fibre-reinforced composites are rapidly increasing their market share in structural applications. Nevertheless, this increase would be much stronger if reliable failure predictions were available. These predictions are not only insufficiently reliable for complex loading of multidirectional composites, but even for longitudinal tensile failure of unidirectional (UD) composites. Since composite failure usually coincides with longitudinal failure of a 0° ply, the reliability often hinges on longitudinal failure predictions of UD composites. Despite great progress in the state-of-the-art models, significant obstacles remain in collecting the necessary input data and understanding the influence of the modelling assumptions. This review therefore surveys the mechanics, chemistry and physics involved in tensile failure of UD composites and highlights potential areas for improvement. Specific proposals are made to advance the state-of-the-art strength models, which could catalyse the use of composites in structural applications. publisher: Elsevier articletitle: A review of input data and modelling assumptions in longitudinal strength models for unidirectional fibre-reinforced composites journaltitle: Composite Structures articlelink: http://dx.doi.org/10.1016/j.compstruct.2016.05.002 content_type: article copyright: © 2016 Elsevier Ltd. All rights reserved. ispartof: Composite Structures vol:150 pages:153-172 status: published

This chapter introduces typical features of damage development in textile composites loaded in quasi-static tensile tests. It highlights similarities and differences in the damage process between textile and unidirectional cross-ply composites. Textile composites possess some distinct damage patterns that are not found in cross-ply laminates. These are attributed to the special geometrical features of the textile reinforcement. The chapter begins with a general description of the damage process in composites and continues with examples of damage accumulation in 2D and 3D woven, braided, and noncrimp fabric composites, reinforced with carbon, glass, and flax fibers.

The aim of this study is to investigate the effect of carbon nanotubes (CNTs) on the initiation and development of damage in a woven carbon fiber/epoxy composite under quasi-static tensile loading. The composite is produced using resin transfer moulding and contains 0.25 wt.% of CNTs in the matrix. The results in the fiber direction report no improvement of the Young’s modulus and a slight improvement of the strength and strain-to-failure. The most important result of the study is a notion that CNTs have a hindering effect on the formation of transverse cracks. The conclusion is drawn from a combined analysis of the acoustic emission measurements (reporting a pronounced shift of all damage development thresholds towards higher strains by more than 30%) and X-ray/SEM observations (revealing a lower crack density in the CNT modified composite). The same analysis also indicates that the mechanism of energy dissipation through transverse microcracking is partially replaced by another mechanism that promotes (distributed) damage through fiber debonding.

The study investigates the effect of carbon nanotubes (CNTs) on the damage development in a woven carbon fiber/epoxy composite under quasi-static tension in the bias direction. The composite is produced by the resin transfer molding and contains 0.25 wt.% of CNTs in the matrix. The tensile tests are carried out till different strain levels and are accompanied with acoustic emission (AE) registration. The nano-modified composite possesses a higher stiffness and strain-to-failure. It also exhibits a significantly increased AE activity, both in terms of the number of events and the energy level, but reveals a lower crack density. The combined analysis of the AE data and X-ray images indicates that in the nano-modified composite cracks progress through the material in smaller jumps than in the virgin composite. The crack faces in the composite with CNTs also display a fine web of secondary fractures, which is not detected in the virgin composite.

Growth of carbon nanotubes (CNT) or carbon nano-fibres (CNF) on fibrous substrates is a way to increase the fracture toughness of fibre reinforced composites (FRC), with encouraging results reported in the recent years. The issues for these materials related to manufacturing of these composites are, however, less investigated. Following the study of compressibility of woven carbon fibre preforms with CNT/CNFs grown on the fibres using the CVD method [Compos Sci Technol 2011; 71(3): 315–325], this paper describes compression tests on the carbon tows used in these fabrics. The results of the measurements include pressure vs. thickness diagrams in consecutive compression cycles and hysteresis of the compression. The results confirm a drastic change of compressibility of fibrous assemblies in the presence of CNT/CNF grafting.

A model for calculation of the compression resistance of a random assembly of carbon nanotubes (CNTs) is proposed. The model is based on the theory of geometry and compression of fibrous materials with randomly oriented fibres. It accounts for the orientation distribution and volume fraction of CNTs in the assembly, the bending rigidity of a CNT and its decrease after onset of buckling and for friction and slippage between CNTs. The model also allows for calculation the number of contacts between CNTs in the assembly and of the percolation threshold. The predicted compression resistance is compared with experimental data on compression of “buckydisks” made of CNTs.

In carbon fiber reinforced polymer composites the onset of damage occurs at the fiber/matrix interface, where stress concentrations are the highest due to the property mismatch of the two materials. This article reports results of a modelling study indicating that carbon nanotubes (CNTs) grown on fibers are effective in suppressing stress concentrations at the fiber/matrix interface. In the case of high density CNT forests, they can even fundamentally change a profile of the interfacial stress. The study is performed using a novel two-scale finite element model of a nano-engineered composite based on the embedded regions technique.

Manufacturing defects such as voids can influence matrix cracking, which is one of the main forms of damage in fiber-reinforced laminates. Characterization and investigation of matrix cracks is not straightforward, neither experimentally nor computationally. In the current study, we develop a methodology for in-situ analysis of matrix cracking evolution in the absence and presence of voids. The cracks are detected using digital image correlation at three different scales: macro, meso, and micro. Additionally, a combined two-scale computational approach is established for prediction of the effect of voids on matrix cracking. Local properties of the material are calculated using computational micromechanics and then serve as input for the meso-scale model based on extended finite element method. The computational approach predicts the crack density evolution in function of the applied deformation. The results of both experimental and computational studies reveal that voids initiate matrix cracks, causing earlier damage in off-axis plies. They also result in higher crack density, for a given applied strain.

VoxTex software for quantification of micro-CT images provides useful tools for analysis of voids in fibre reinforced composites. These tools are applied in the current work to (1) carbon fibre laminate (-45/0/45/-20/0/20/90/0)so, made with automated dry fibre placement and vacuum assisted resin transfer moulding and to (2) carbon fibre laminates [0/90]ns, made by prepreg technology and cured in autoclave witha regime proliferating void formation. The materials contain very different porosity levels (below 1% for (1) and few percent in (2)). The segmentation and image processing algorithms allow identification of the overall void content and quantification of the void dimensions and their orientation in the form of distribution functions. These distributions are presented and their characteristic features are discussed. A typical peculiarity of segmented micro-CT images is presence of small (few pixels in size) voids, which persist even after the spurious “void pixels” are eliminated. Whether these micro-voidfeatures represent real microscopic voids in the material is discussed based on the comparison with SEM images. ispartof: IOP Conference Series: Materials Science and Engineering vol:406 issue:1 ispartof: TEXCOMP-13, 13th International Conference on Textile Composites location:Milan, Italy date:17 Sep - 19 Sep 2018 status: published