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High-performance polymer fibres offer promising tensile and physical properties but are rarely used in structural composite applications due to their poor compressive performance. To better understand their performance inside a composite, unidirectional thermoset composites made with three types of polymer fibres were tested in shear-loaded compression. The results confirmed the poor compressive performance of the composites despite good fibre alignment, low porosity, and decent interfacial adhesion. Showing that the fibres collapse rather than microbuckle proves that intrinsic fibre behaviour in compression is the key culprit. The developed finite element model predicted that the short gauge lengths required to avoid buckling lead to significant strain concentrations due to the grips. This causes the test to underestimate the compressive modulus and potentially the rest of the compressive stress in the stress–strain diagram.

The deformation and damage development of nano-engineered composites have not yet been investigated in 3D, although it can provide a deeper insight into their damage behavior. To fill this gap, we perform a tensile test on a nano-engineered composite with in-situ X-ray micro-Computed Tomography (micro-CT). The composite is made from woven alumina fibers with grafted carbon nanotubes (CNTs) and epoxy. More diffuse damage seems to exist for the materials with CNTs compared to the baseline material. However, at such resolution where individual fibers are vaguely visible, grayscale thresholding does not accurately characterize the matrix cracks due to their small opening and low contrast with the material itself. Thus, we employ a deep-learning tool, called RootPainter, for segmentation of cracks with small opening in relation to the voxel size, in the 3D images. The results show that RootPainter can reliably identify these small cracks. In addition to the investigation of the mechanical performance of the nano-engineered composite, this study provides a novel and reliable method for the characterization of micro-cracks in in-situ tomograms of these composites.

Although the addition of graphene fillers can transform an electrical insulator to a conductor at a certain threshold of loading ratio, similar transition in thermal conductivity has not been confi...

Longitudinal tensile failure of unidirectional fibre-reinforced composites remains difficult to predict accurately. The key underlying mechanism is the tensile failure of individual fibres. This paper objectively measured the relevant input data and performed a detailed experimental validation of blind predictions of six state-of-the-art models using high-resolution in-situ synchrotron radiation computed tomography (SRCT) measurements on two carbon fibre/epoxy composites. Models without major conservative assumptions regarding stress redistributions around fibre breaks significantly overpredicted failure strains and strengths, but predictions of models with at least one such assumption were in better agreement for those properties. Moreover, all models failed to predict fibre break (and cluster) development accurately, suggesting that it is vital to improve experimental methods to characterise accurately the in-situ strength distribution of fibres within the composites. As a result of detailed measurements of all required input parameters and advanced SRCT experiments, this paper establishes a benchmark for future research on longitudinal tensile failure The research leading to these results has been conducted in the framework of the FiBreMoD project and has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 722626. Compressive tests on the neat epoxy resin were performed at the mechanical lab of the Institute of Mechanics, Materials and Civil Engineering (IMMC), Université Catholique de Louvain (UCLouvain) with the kind support of J. Chevalier and T. Pardoen. M.J. Emerson from QIM, Technical University of Denmark (DTU) is thankfully acknowledged for the handson training sessions and support of the Insegt Fibre segmentation toolbox. Support of C. Schlepütz from TOMCAT beamline at Swiss Light Source for optimising the acquisition settings during the beamtimes under proposal IDs 20161157 and 20171494 is thankfully acknowledged. J.M. Guerrero would like to acknowledge the grant BES-2016-078270 from the ‘Subprograma Estatal de Formación del MICINN’ cofinanced by the European Social Fund, and all authors from the University of Girona acknowledge the funding from the Spanish project RTI2018-097880-B-I00. The work of M. Hajikazemi forms part of the research programme of DPI, project 812T17. S. Pimenta acknowledges the funding from the Royal Academy of Engineering in the scope of her Research Fellowship on “Multiscale discontinuous composites for high-volume and sustainable applications” (2015-2019). R.P. Tavares and P.P. Camanho would like to thank the financial support provided by FCT - Fundação para a Ciência e a Tecnologia through National Funds in the scope of project MITP-TB/PFM/0005/2013

This paper proposes a 0° tensile strength prediction model for unidirectional composites with a low interfacial strength or low matrix yield stress applied to carbon fiber reinforced polypropylene....

In-plane permeability of small area (100 × 50 mm) alumina fiber woven fabrics grafted with aligned carbon nanotubes (CNT) was quantified by placing them in series with a glass mat of known permeability during a flow experiment. The methodology was first validated on a reference woven textile. Permeability values matched those obtained by a direct method within a margin of ±15%. Permeabilities of radial-aligned (short CNT, SCNT) and so-called ‘Mohawk’ (long CNT, LCNT) morphologies of the CNT-grafted samples were then measured and compared to the non-grafted alumina, showing a decrease attributed to a change in local textile structure as assessed in previous studies. Unsaturated permeability decreased by 77% after SCNT- and 88% after LCNT-grafting, while saturated permeability further decreased by 90% and 93%, respectively. The high ratio of unsaturated to saturated permeability (in the range of 1.14 – 2.89) implies that capillary wicking contributes largely to the impregnation of CNT-grafted fabrics.

A uniaxial tensile test is a useful method for determination of material properties, especially longitudinal tensile strength. To accurately derive the longitudinal tensile strength, it is desired that the specimen fails in in the gauge section defined here as ‘working zone’. Unidirectional (UD) composites require use of end tabs during this tensile testing to avoid damage to the specimen due to grip serrations. The grip pressure, along with sudden geometry change at the edge of end tabs leads to longitudinal stress concentrations. The conventionally used rectangular and tapered end tabs suffer from these longitudinal stress concentrations under the edge of end tabs, causing premature failure of specimen outside of the working zone. In the present paper, a simulation study is performed for comparison of conventional end tabs with hybrid specimen geometry [1] and a novel arrow-shaped end tab geometry to determine the effect of end tab geometry on longitudinal stress concentrations. The study is focused on high modulus carbon fibre HS40/epoxy UD (0°) composite. The numerical model replicates the actual set-up for uniaxial tensile testing, including contact interactions between testing machine components. The simulation results are used to further optimise the geometry and provide recommendations to eliminate or minimise longitudinal stress concentrations.

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Self-reinforced composites (SRCs) offer a high toughness, low cost, good recyclability, formability, and potential for hybridisation. Because matrix and fibres are of the same material, SRCs have a perfectly compatible fibre-matrix bond. Several commercial SRCs are available, but no direct comparison of these materials can be found in the literature. In this study, we therefore compared the thermal, tensile, flexural and impact of four self-reinforced polypropylene (SRPP) materials (known under the brand names Curv, PURE, Torodon and Terralys) and one self-reinforced polyethylene terephthalate material (SRPET from Comfil). PURE had the highest tensile stiffness and strength, Torodon the highest impact resistance and SRPET the highest bending strength. However, none of these materials outperformed the rest on all the tested properties, implying that the best self-reinforced composite depends on the targeted application. This paper can be used as a tool for initial screening of the commercial self-reinforced composites for a given application.

The longitudinal and transverse strength of three unidirectional thermoplastic prepreg systems: carbon fiber/polypropylene (CF/PP), polyamide 6 (CF/PA6), and polyphenylene sulfide (CF/PPS) are stud...

© 2019 Elsevier Ltd Highly aligned discontinuous fiber composites have demonstrated mechanical properties comparable to those of unidirectional continuous fiber composites. However, their ductility is still limited by the intrinsic brittleness of the fibers and stress concentrations at the fiber ends. Hybridization of aligned discontinuous carbon fibers (ADCF) with self-reinforced polypropylene (SRPP) is a promising strategy to achieve a balanced performance in terms of stiffness, provided by the ADCF, and ductility, delivered by SRPP. The current work focuses on interlayer hybridization of these materials and their tensile behavior as a function of different material parameters. Effects of the carbon layer thickness, carbon/SRPP layer thickness ratio, layer dispersion and interface adhesion are investigated. The carbon fiber misalignment is characterized using X-ray computed tomography to predict the modulus of the aligned discontinuous carbon fiber layer. The hybrids exhibit a gradual tensile failure with high pseudo-ductile strain of above 10% facilitated by multiple carbon layer failures (layer fragmentation) and dispersed delaminations. At the microscopic scale, the carbon layer fails mainly through interfacial debonding and fiber pull-out. ispartof: COMPOSITES PART A-APPLIED SCIENCE AND MANUFACTURING vol:123 pages:97-107 status: published

Debonding at the fiber/matrix interface is an important failure mechanism in composite materials. It controls, among others, the intralaminar cracking process. A large amount of experimental data exists indicating that the addition of carbon nanotubes (CNTs) in a composite can hinder onset and development of these cracks. Here we report results of a modelling study aiming to better understand the effect of carbon nanotubes on the fiber/matrix debonding. The study is performed on an example of a carbon fiber/epoxy unidirectional composite with two practically realizable CNT configurations: agglomerated CNTs in the matrix and CNTs grown on fibers. The composites are subjected to transverse tension. The stress distribution prior to debonding is predicted using a two-scale finite element model based on the embedded element technique. The onset and propagation of the debond are then analyzed using a surface-based cohesive zone model with a linear traction-separation law. Our results show that CNTs have a strong effect on the stress distribution in the matrix and the debonding process, although their effect on the latter is less pronounced. CNT agglomerates are found to act as stiff microscopic inclusions leading to magnification of stress concentrations and earlier initiation of the debonding process. The composites with CNTs grown on fibers, on the other hand, show suppression of stress concentrations at the interface and slight delay in the debonding onset compared to the composite without CNTs.

The success of carbon nanotubes (CNTs) in increasing toughness of polymers and their composites is often attributed to the additional energy consumed by CNT debonding and pull out. In this work we demonstrate that this mechanism alone can only lead to modest improvements in toughness and that the true toughening power of CNTs lies in activation of multiple mechanisms. Our virtual experiments reveal three mechanisms: 1) suppression of stress concentrations leading to delay in damage initiation, 2) damage diffusion/crack branching and 3) CNT debonding and pull-out. The first two mechanisms dominate energy dissipation although they have received less attention in the literature. These mechanisms act concurrently at different scales and have complex dependence on the morphology of the CNT network and properties of the CNT/matrix interface. When CNTs' position, orientation and compatibility with polymer are optimised, the strength and toughness of the nanocomposite can be increased significantly (in the studied case by 90% and 277%, respectively, compared to unfilled polymer). Without morphological optimisation these improvements were only 6% and 14%, respectively. Thus, for strengthening and toughening of nanocomposite structures, it is insufficient to only modify the interface, one also needs to optimise CNTs’ spatial distribution and orientation.

© 2018 Elsevier Ltd The strain-rate sensitivity and stress relaxation of self-reinforced polypropylene (SRPP) and hybrid carbon fibre/SRPP composites were analysed under bending loads. For this purpose, three point bending static tests were performed at room temperature and displacement rates of 200, 20, 2, 0.2 and 0.02 mm/min. Tests of stress relaxation were also carried out, where a fixed strain was applied and the stress was recorded during the loading time. The study concluded that, for all materials, the bending stress and strain at the maximum bending stress were sensitive to the strain rate. The bending modulus, however, was found to be insensitive to the strain rate for hybrid composites, and sensitive (with a linear increase) for SRPP composites. From the stress relaxation tests and for both materials, the stress decreased with time and this decrease was more significant for higher strains. The results were fitted following the Kohlrausch-Williams-Watts model, evidencing good accuracy of the model for the stress relaxation time. ispartof: COMPOSITE STRUCTURES vol:209 pages:802-810 status: published

Digital image correlation is applied to the images of a deforming composite to obtain strain maps at three length scales: micro-scale (ply level, hundreds of micrometers), meso-scale (laminate level, millimeters), and macro-scale (specimen level, tens of millimeters). The images are acquired in-situ with optical cameras and an electron microscope. The strain mapping at the macro- and meso-scales allows semi-automatic detection of matrix cracks and quantification of their density evolution in function of the applied strain. The micro-scale examination provides additional insights into the failure mechanisms. The technique is developed and then applied to characterize transverse cracking in cross-ply carbon fiber/epoxy composites in the absence and presence of manufacturing defects (including voids). Laminates with defects were produced by lowering the autoclave pressure and the cure temperature, intentionally. The strain for cracking onset and the saturation crack density are found to be different in the inner and outer transverse plies of both types of laminates. The change in processing conditions that led to the presence of voids and incomplete matrix cure resulted in a lower strain for cracking onset and up to 3.5 times increase of the crack density in comparison with the reference material without defects.

Many models for prediction of longitudinal tensile failure of unidirectional (UD) composites have been developed in the last decades. These models require significant assumptions and simplifications, but their consequences for the predictions are often not clearly understood. This paper therefore presents a blind benchmark of seven different models applied to two virtual materials. Reliably capturing the localisation of stress concentrations was vital in predicting the effect of matrix stiffness and strength on composite failure strain and strength as well as fibre break and cluster development. Although the models have different assumptions regarding stress redistributions around fibre breaks, the 2-plet (clusters of two fibre breaks) development was similar. Distance-based criteria were shown to be inadequate for monitoring cluster development. The discussions provide detailed insight into how the model assumptions are linked to the differences in the predictions. European Union’s Horizon 2020 research and innovation programme published

We performed in-situ tensile tests on two carbon fibre/epoxy composites with continuous scanning using synchrotron computed tomography (CT). Both composites were cross-ply laminates, and two specimens were tested for each compos- ite. The voxel size was sufficiently small to recognize individ- ual fibres and fibre breaks. For each test, 16-19 volumes were reconstructed, cropped down to the 0° plies and analysed to track fibre break and cluster development. This dataset provides the last CT volume before failure for each of the four specimens as well as the individual fibre break locations in all reconstructed volumes. These data are then plotted against predictions from six state-of-the-art strength models. The target is that these data become a benchmark for the development of new models, inspiring researchers to set up refined experiments and develop improved models. © 2021 Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

International audience; Self-reinforced polypropylene (SRPP) is a composite in which the reinforcement and the matrix consist of the same polymer, namely polypropylene. The present work focuses on the development and experimental validation of an FE model for thermoforming of SRPP. The constitutive model of SRPP sheet is based on the Boisse-Charmetant hyperelastic law. Material properties required to fit the model were obtained from the tensile and picture frame shear tests performed at forming temperatures and bending test of a non-consolidated fabric at room temperature. The model is validated in comparison with the experimental draping data, with special attention to parameters and patterns of wrinkles. Samples, preheated to a surface temperature of 170-175°C, were deformed up to 1-4 cm heights using a hemispherical mould with a 4.5 cm diameter. Overall, good qualitative correlation between experimental and modelling results is observed, which justifies use of the proposed model with the experimentally identified parameters for prediction of SRPP draping and subsequent analysis of the mechanical response of the consolidated part to loading.

Wind turbine blades are a key growth market for fibre-hybrid composites, as they offer the potential for higher turbine efficiency at a reduced cost. This paper therefore overviews the developments of KU Leuven models for mechanical properties relevant to wind turbine blades, including fibre-hybridisation, size scaling effects, flexure, transverse cracking, stress relaxation and fatigue. The strengths and limitations of the models will be highlighted with a particular focus on relevance for wind turbine blades. While significant steps forward have been made, the various developments still need to be incorporated in one all-encompassing model, and further work is needed on gathering the required input data and detailed experimental validation studies. ispartof: IOP Conference Series: Materials Science and Engineering vol:942 ispartof: 41st Risø International Symposium on Materials Science: "Materials and Design for Next Generation Wind Turbine Blades" location:Roskilde, Denmark date:7 Sep - 10 Sep 2020 status: Published online

This paper presents a comprehensive review of the scientific and engineering contributions of Prof. Stepan V. Lomov in a career spanning over 3 decades. Starting from his early work on the modelling of dry textiles, different aspects of composite materials have benefited from his inspiring contribution. His wide scientific curiosity drove the research on several topics, covering permeability, forming, damage, fatigue, nano-engineered composites, as well as development of analytical and numerical models at different scales. The paper presents a full spectrum of his scientific career aiming to link his achievements and the future research on composite materials.

© The Author(s) 2018. Voids, the most studied type of manufacturing defects, form very often in processing of fiber-reinforced composites. Due to their considerable influence on physical and thermomechanical properties of composites, they have been extensively studied, with the focus on three research tracks: void formation, characteristics, and mechanical effects. Investigation of voids in composites started around half a century ago and is still an active research field in composites community. This is because of remaining unknowns and uncertainties about voids as well as difficulties in their suppression in modern manufacturing techniques like out-of-autoclave curing and parts with high complexity, further complicated by increased viscosity of modified resins. Finally, this is because of the increasing interest in realization of more accurate void rejection limits that would tolerate some voidage. The current study reviews the research on formation, characterization, and mechanical effects of voids, which has been conducted over the past five decades. Investigation and control of void formation, using experimental and modeling approaches, in liquid composite molding as well as in prepreg composite processing are surveyed. Techniques for void characterization with their advantages and disadvantages are described. Finally, the effect of voids on a broad range of mechanical properties, including inter-laminar shear, tensile, compressive, and flexural strength as well as fracture toughness and fatigue life, is appraised. Both experimental and simulation approaches and results, concerning voids' effects, are reviewed. ispartof: JOURNAL OF COMPOSITE MATERIALS vol:53 issue:12 pages:1579-1669 status: published

The spatial distribution and orientation of carbon nanotubes (CNTs) have significant effects on the mechanical properties of nanocomposites. But what is the ideal CNT network morphology to achieve a desired mechanical performance? To help answer this question, we propose a novel computational approach to optimize position and orientation of CNTs in a nanocomposite to reduce stress concentrations. The presence of stress risers (either intrinsic or intentionally introduced) is known to control failure of materials and structures at different scales. The present methodology is based on the combined implementation of genetic algorithm and finite element method. The optimization problem is formulated as minimization of a global measure of stress field, found to be more efficient than minimization of the stress concentration factor directly. For the nanocomposites with 0.15% and 0.30% of CNTs, the nanostructure optimization is responsible for 39.5% and 54.4% of stress-concentration reduction compared to unfilled polymer, and 35.6% and 48.9% compared to the polymer filled with uniformly distributed CNTs, respectively. The predicted nanostructures redistribute stresses in the material and decrease the volume of high-stress region. The proposed optimization approach supports ongoing developments in the field of 3D printing that made the design and production of spatially resolved nanostructures possible. Keywords: Carbon nanotubes, Optimization, Micro-scale morphology, Stress concentration, Genetic algorithm

This work presents a concept for creating different chemistries along thin, micrometer sized fibers to be used as substrates for selective localization of nanoparticles. Glass fibers have been surface modified to introduce hydrophilic and hydrophobic regions along their length. Dynamic contact angle measurements using the Wilhelmy technique shows a marked change in the wetting behavior, confirming the transition in chemical functionality along the fiber. These fibers were then used as substrates to localize polystyrene nanoparticles from an aqueous dispersion at the air-water interface via a dip-coating method. Low contact angles and faster de-wetting allowed the particles to predominantly confine onto the hydrophilic regions of the glass fibers, indicating that both the fiber surface properties as well as the curvature of the fiber influences the profile of the particle carrying fluid meniscus. Our results also indicate a velocity-dependent particle deposition morphology and image quantification analysis revealed a significantly higher density of nanoparticles to be deposited at higher withdrawal velocities.

We performed a numerical study to evaluate the influence of carbon nanotubes (CNTs) on the stress build-up in and around a broken fibre in a single carbon fibre/epoxy composite loaded longitudinall...

Carbon fiber layer failure is vital for the tensile behavior of interlayer hybrid carbon fiber (CF)/self-reinforced polypropylene (SRPP) composites. Introducing cuts, termed here as discontinuities, into the carbon layer is a promising way to tailor its failure behavior. Inspired by structural features of biological composites, we designed and produced hybrid composites with hierarchical and polygonal arrangements of discontinuities in the carbon layer. Increasing the number of levels in the hierarchical patterns delayed the onset of carbon layer failure, hence improving the damage resistance of the hybrid composites. A progressive carbon layer failure was achieved with the polygonal patterns by creating a transition from fiber bundle pull-out to fiber bundle fracture. Spreading the polygonal patterns throughout the specimen resulted in a unique diffused delamination distribution that has not been reported in the literature. Pseudo-ductile behavior was achieved by creating dispersed fiber bundle pull-out with the fully dispersed polygonal patterns. The resulting hybrid CF/SRPP composites demonstrated a rare combination of stiffness (10 GPa) and ductility (~16% failure strain) with a pseudo-ductile strain over 14%. This paper delivers and proves the concept of utilizing discontinuities to engineer the tensile behavior of hybrid composites.

© 2018 Hybrid composites combining two fibre types with distinctly different mechanical properties have the potential to surpass the stiffness-toughness dilemma, which is characteristic to standard (single fibre type) composite materials. The current work demonstrates this potential on the example of carbon fibre/self-reinforced polypropylene (SRPP) hybrids. The aim is to understand the transition from brittle to ductile behaviour under tensile and impact loadings and to identify the parameters affecting this transition. It was found that the volume fraction (Vf) of carbon fibres at which the transition occurs can be increased by using a dispersed layup with thinner layers. The use of a high adhesion matrix results in higher modulus and yield strength but lowers the transition Vf. The experimental program is supported by analytical models used to predict modulus, strength and energy absorption. Results indicate that pseudo-ductile carbon fibre/SRPP hybrids are competitive with composites produced from bulk and sheet moulding compounds. ispartof: COMPOSITES PART A-APPLIED SCIENCE AND MANUFACTURING vol:109 pages:20-30 status: published

© 2018, © 2018 Institute of Materials, Minerals and Mining and ASM International Published by Taylor & Francis on behalf of the Institute and ASM International. Fibre-hybrid composites are composed of two or more fibre types in a matrix. Such composites offer more design freedom than non-hybrid composites. The aim is often to alleviate the drawbacks of one of the fibre types while keeping the benefits of the other. The hybridisation can also lead to synergetic effects or to properties that neither of the constituents possess. Even though fibre-hybrid composites are attractive, they also pose more challenges in terms of materials selection than conventional, single fibre type composites. This review analyses the mechanisms for synergetic effects provides guidance on the fibre and matrix selection and describes recent opportunities and trends. It finishes by describing the current applications, and by contrasting how the industrial use is different from the academic research. ispartof: INTERNATIONAL MATERIALS REVIEWS vol:64 issue:4 pages:181-215 status: published

Interlayer hybrid carbon fiber/self-reinforced polypropylene composites possess a rare combination of lightness, stiffness and ductility, but they do suffer from a catastrophic stress drop when the carbon fiber layer fractures. To promote a gradual failure of the hybrids, we introduced discontinuities in the carbon fiber layer by partially cutting it at multiple locations perpendicularly to the fiber direction. By altering characteristics of the discontinuities (their length and number over the specimen width), we were able to influence the failure mechanisms and tensile performance of these hybrid composites. When the cut length was increased to 15 mm, the carbon fiber layer fragmented and delaminated at the cut sites inducing a gradual failure development in the hybrid. The concept of fracture process zone was applied to explain the failure mechanisms in these hybrid composites.

The eXtended Finite Element Method (XFEM) allows modelling of the onset and propagation of multiple cracks in a material without prior assumption on their positions. In the current work, the method is examined for its capability to predict matrix cracking in cross-ply composite laminates under quasi-static tensile loading – a phenomenon that is well understood in the field. The study is performed using commercially available ABAQUS tools and focuses on the effect of numerical and physical parameters. The propagation of transverse cracks and delaminations is described by cohesive laws. XFEM predicts physically sound evolution of crack densities in function of the applied load if input data, such as the transverse strength (including statistical parameters of its distribution) and critical energy release rate characteristics in the cohesive laws, are correctly prescribed. These input parameters are difficult to determine experimentally, and their choice may be controversial. The paper investigates the influence of these parameters on the crack development history and discusses issues in their choice.

This paper proposes a methodology for measurement of fibre misalignment in composite laminates based on the structure tensor of fibre orientations extracted from X-ray micro-computed tomography (micro-CT) images. The methodology is applied to carbon fibre/epoxy composites manufactured with two technologies: (1) by prepregging and consolidation in an autoclave and (2) by vacuum assisted resin transfer moulding. The results show that the in-plane inter-ply misalignment of fibres in the prepreg laminate and in the infusion-produced laminate is within 2–4° and 1–2°, respectively. The misalignment results allow better understanding of the manufacturing typical fibre orientation precision level for the low pressure (vacuum assisted resin transfer moulding) and high pressure (autoclave) manufacturing.

The viscoelastic behaviour of self-reinforced polypropylene (SRPP) was investigated under bending loads. For this purpose, an overfed Twill 2/2 weave pattern was used, which in the hot compacted form is known under the trade name of Curv®. The areal density of the SRPP fabric is 130 g/cm3 and the density of this grade polypropylene is 0.92 g/cm3. Three point bending (3PB) static tests were performed with a span length of 20 mm, according to the EN ISO 178:2003 recommendations. The strain rate effects on the flexural properties were obtained by the 3PB static tests carried out at room temperature with a displacement rate of 200, 20, 2, 0.2 and 0.02 mm/min. Finally, tests of stress relaxation were also performed, where a fixed strain was applied and the stress was recorded during the loading time. It was possible to conclude that higher strain rates promote higher maximum bending stresses and bending modulus. For both cases, a linear model fits successfully the data and the strain-rate effect on the bending strain at a maximum bending stress showed that SRPP composites are strain rate sensitive. The stress relaxation tests evidence a decrease of the stress with time, and this tendency further persists with the increase of the strain values. Maxwell and Kohlrausch-Williams-Watts (KWW) equations were used to fit the data obtained from the stress relaxation tests and, while the Maxwell model was not good enough to predict the stress relaxation time, the KWW model could fit the data with good accuracy.

Continuous steel fiber polymer composites are susceptible to high stress concentrations under transverse loading. This is due to the high mismatch in elastic properties of steel fiber and polymeric matrix. In the present work, the effect of a nano-reinforced interphase on the stress concentrations is evaluated. Three types of nano-reinforcements are considered: 1) carbon nanotubes (CNTs) grown on fibers and 2) graphene nano-plates deposited on the fiber surface and 3) graphene nano-plates randomly distributed in the matrix. The study is carried out using finite element (FE) analysis with an intermediate homogenization step to obtain elastic properties of the nano-reinforced interphase. These properties are calculated using an inclusion based approach. Results indicate that the presence of the nano-reinforced interphase increases radial stress concentrations and those in the direction of the load. The suppression is only observed for shear stress concentrations in the case of the CNT interphase around the fibers.

In this work, we have established a multiscale model to accurately calculate the effective thermal conductivity of the composite of graphene and polyamide-6 (PA-6) and use this model to search for the optimal orientation distribution of the graphene flakes to maximize the composite thermal conductivity. Compared with the direct results of large-scale molecular dynamics simulations on the validation case, our model shows 1 % relative error for the effective thermal conductance of the standalone graphene network, and 4 % for the overall composite thermal conductivity. Counterintuitively, our model predicts that, for the percolation-dominated composite structure, randomly entangled graphene network produces superior thermal conductivity, compared to the composite structure with certain graphene alignment. Our results show that, without increasing graphene loading, the composite thermal conductivity can be maximized by simply producing the optimal orientation distribution for the graphene flakes.

Recently, in-situ X-ray Computed Tomography (CT) has shown its potential for 3D damage analysis in composite materials. However, the characterization of damage in X-ray tomograms is not always straightforward, and it requires post-processing of the 3D images. In this study, we explore the potential of Digital Volume Correlation (DVC) for the detection and characterization of damage in fiber-reinforced composites, where fibers provide the required 3D speckle pattern. Preliminary analysis via “digital deformation” of 3D images is performed to verify the applicability of DVC for quantification of deformation and damage in a carbon/epoxy laminate and to estimate the measurement errors. Then, real-deformation images, acquired with synchrotron CT during in-situ tensile loading of the laminate, are analyzed with DVC to detect different damage mechanisms. A rough analysis is performed at the mesoscale using subset-based DVC, followed by a more detailed investigation at the microscale via finite-element-based DVC. Damage appears in the DVC strain fields as local strain magnification. Crack opening displacement can be estimated reliably via the jumps in displacement fields. DVC proves to be a promising tool for damage characterization in X-ray tomograms of fiber-reinforced composites, especially when simple methods such as grayscale thresholding are not adequate.

Recently we showed that continuous stainless steel fibres deliver structural composites with high stiffness and high failure strain at the same time. In the current research we further investigate these new materials focusing on the effect of the microstructure on their tensile facture behaviour. The steel fibre composites are produced from quasi-unidirectional woven (Q-UD) and purely unidirectional (P-UD) fibre architectures in combination with epoxy resin. The main difference in their microstructures is in the fibre distribution. In the Q-UD composite, steel fibres are tightly packed in bundles with 85% local fibre volume fraction and these bundles are separated by resin rich areas. In P-UD composite, steel fibres are homogeneously distributed through the composite with an average volume fraction of 50%. With the highly packed fibres, Q-UD composites achieve a higher strain to failure. In-situ characterization of damage development using acoustic emission registration and crack observations on the specimen surface were used to propose a hypothesis to explain the difference in fracture behaviour, caused by the different microstructures.

Ply fragmentation is a dual-scale phenomenon comprised of fibre breaks at the micro-level and ply fractures at the meso-level. A dual-scale stochastic model is proposed to predict the longitudinal tensile behaviour of unidirectional interply fibre-hybrid composites, including ply fragmentation in low elongation (LE) plies. The predictions of the stress-strain diagram are validated for carbon/glass hybrid composites and agree well with experimental results. A parametric study was performed where the Weibull scale parameter of the high elongation (HE) fibres and shape parameter of the LE fibres were varied. The Weibull scale parameter of the HE fibres strongly influences the failure mode of the hybrid composite. An LE fibre type with a low Weibull modulus should be used if a composite with a smooth stress-strain diagram is desirable. The model presented can be used for any layup and material configuration, contributing to a more intelligent and efficient design of hybrid composites. ispartof: Composites Science and Technology vol:181 status: Published online

In the current data article, we present detailed characteristics of voids in carbon/epoxy composite laminates, along with the original image stacks obtained via X-ray micro-Computed Tomography (micro-CT)1. Five different lay-ups are produced with altering the recommended cure cycle in order to intentionally induce voids in the material. For each lay-up, an image stack (consisting of tomographic slices) and a dataset are provided. The image slices are in 8-bit TIF format. The datasets (spreadsheets) include the volume, size parameters, shape parameters, orientation, and location of the ellipsoids that are fitted to the detected voids in the specimen. The segmentation of the images and quantification of voids are performed in VoxTex, an in-house software for processing of micro-CT results. The processing and interpretation of the data is reported in [1]. The data is hosted in the Mendeley Data repository at [2]. ispartof: Data in Brief vol:27 ispartof: location:Netherlands status: published

Self-reinforce polypropylene (SRPP) is a self-reinforced polymeric composite in which the reinforcement and the matrix consist of the same polymer, namely polypropylene. The present work focuses on the development and validation of an FE model for thermoforming of SRPP. The constitutive model of SRPP sheet is based on the hyperelastic law of Boisse – Charmetant. Material properties required to fit the model were obtained from the tensile and picture frame shear tests performed at forming temperatures and bending test of a non-consolidated fabric at room temperature. To understand how wrinkles developed during forming, intermediate forming trials were conducted. Samples were deformed to 1-4 cm heights using a hemispherical mould with a 4.5 cm diameter. Overall, good qualitative correlation between experimental and modelling results is observed.Self-reinforce polypropylene (SRPP) is a self-reinforced polymeric composite in which the reinforcement and the matrix consist of the same polymer, namely polypropylene. The present work focuses on the development and validation of an FE model for thermoforming of SRPP. The constitutive model of SRPP sheet is based on the hyperelastic law of Boisse – Charmetant. Material properties required to fit the model were obtained from the tensile and picture frame shear tests performed at forming temperatures and bending test of a non-consolidated fabric at room temperature. To understand how wrinkles developed during forming, intermediate forming trials were conducted. Samples were deformed to 1-4 cm heights using a hemispherical mould with a 4.5 cm diameter. Overall, good qualitative correlation between experimental and modelling results is observed.

© 2019 Elsevier Ltd In this work, we investigate the toughness of an inter-layer Self-Reinforced Polypropylene/Carbon Fibre Polypropylene (SRPP/CFPP) cross-ply hybrid composite and devise strategies to improve two aspects of its damage tolerance: (i) increasing the energy dissipation capability and (ii) enhancing the impact damage tolerance. To this end, we introduced discontinuities in the form of laser-cuts across the fibres of the CFPP plies, tailoring two patterns of laser-cuts to meet each specific damage tolerance requirement. We conducted penetration impact and Double Edge Notched Tensile (DEN-T) tests. The DEN-T tests, analysed via the Essential Work of Fracture method, show that engineering the microstructure successfully diffused damage. This resulted in a great increase in energy dissipation capability — 90% higher than a reference non-engineered structure. Engineering the microstructure of impact samples has led to enhanced impact damage tolerance with increased energy dissipation at a sub-critical level and delayed critical failure. ispartof: COMPOSITES PART A-APPLIED SCIENCE AND MANUFACTURING vol:121 pages:341-352 status: published

© 2018 Elsevier Ltd Voids are an important type of manufacturing defects in fiber-reinforced composites. Matrix cracking is sensitive to the presence of voids. Although this cracking occurs at the ply scale, its dynamics is strongly affected by ply's microstructure, in particular, fiber distribution, fiber content, and the presence of voids. In the current study, a computational approach to simulate the influence of intra-laminar voids on cracking in composite laminates is developed. The approach combines finite element models of two scales: a micro-scale model, where the fibers and voids are modeled explicitly, and a meso-scale model, where the cracking phenomenon is captured on the ply scale. The micro-scale model, incorporating plasticity and damage in the matrix, provides input for the meso-scale model, which simulates the progressive cracking by means of the extended finite element method. The methodology is applied to investigate the effect of voids on the density of transverse cracks in cross-ply laminates in function of the quasi-static tensile load. Different sizes and contents of voids, which are chosen based on experimental micro-computed tomography data, are simulated. The numerical experiments show that the presence of voids leads to earlier start of the cracking, with the crack density evolution less sensitive to voids. ispartof: COMPOSITES PART A-APPLIED SCIENCE AND MANUFACTURING vol:117 pages:180-192 status: published

A digital image correlation (DIC) based method to characterize the Mode I fatigue delamination onset andpropagation in composite laminates is proposed. The characterization is performed using the double cantileverbeam test (DCB) under fatigue loading. The DIC is used to analyse the displacementfield on the edge of a DCBspecimen, from which the crack tip position is determined as a place where opening displacement converges tozero. The method was successfully applied to both the crack onset detection (fatigue onset life) and propaga-tion monitoring under fatigue cycling. The DIC‐based method, which relies on the actual crack tip position,gives conservative values for the fatigue onset life in comparison with the compliance based method recom-mended in the ASTM standard. The DIC assisted method reduces operator and structure dependent errorsand shows promise for real‐time delamination detection under fatigue loadings.1. IntroductionDelamination is one of the primary failure modes in laminatedcomposites. Characterization of material’s resistance to delaminationunder quasi‐static and fatigue loads is essential to develop no‐defect‐growth design and damage tolerance guidelines for composite struc-tures. The double cantilever beam (DCB) test is widely adopted tomeasure Mode I inter‐laminar toughness[1,2]and Mode I fatiguebehaviour[3–8], and standard methods are established in ASTMD5528[9]for quasi‐static and ASTM D6115[10]for fatigue loading.For characterization of the fatigue behaviour two types of the fati-gue tests are normally conducted: fatigue delamination onset andpropagation. In the former case, the growth onset of an existinginter‐laminar crack is characterized by a G‐N curve, which is analo-gous to the S‐N curve, but replacing the stress,S, with the suppliedstrain energy release rate (SERR),G.Nis the number of loading cyclesrequired for the delamination to onset. The characterization methodfor Mode I is described in ASTM D6115[10]. In this standard, threedifferent ways are introduced to determine the delamination onset.They are:(1)visual observation of the crack onset;(2)detection of1% increase in system’s compliance;(3)detection of 5% increase insystem’s compliance. Because of the difficulties in visual mon

Radially aligned carbon nanotubes (A-CNTs) grown on micron-scale fibers promise structural composites with high mechanical performance and multi-functional properties. Changes in the internal structure of woven composite laminates after A-CNT growth are studied here utilizing micro-computed tomography. The laminates are produced by vacuum impregnation in a closed mold with and without clamping pressure. Two A-CNT lengths are investigated: 4–6 μm and 17–19 μm. A-CNTs were found to increase the distance between fibers, scaled by the A-CNT length. This “swelling” resulted in an increased cross-sectional area, crimp and in-plane misalignment of the yarns. The laminate thickness doubled for laminates with long A-CNTs compared to shorter ones. The laminate with long A-CNTs produced under pressure showed a remarkable alteration of the internal structure. Fibers migrated within the fabric plane, filling almost completely the resin rich pockets. Further research is needed to understand the effect of these changes on the composite mechanical performance.