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© 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

© 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

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.

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.

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.

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.

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

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

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

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

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.

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.

The focus of the present study is on hybrid composites with interplied carbon fiber reinforced polypropylene (CFRPP) between self-reinforced polypropylene (SRPP) layers. SRPP is produced by hot compaction of a woven fabric of highly oriented polypropylene and has an intrinsic behavior of shrinkage under high temperatures. The aim of this research is to enhance the tensile properties of the CFRPP/SRPP hybrid composites by using the SRPP shrinkage to introduce a compressive pre-strain in CFRPP. The results from tensile testing show that the failure strain of the hybrid composites is improved in comparison with CFRPP. The modulus and strength are noted to be lower than the ones expected from the rule of mixture. This may be attributed to the introduction of local misalignment (waviness) of carbon fibers caused by the SRPP shrinkage during consolidation.

Natural fibres are studied as alternatives for man-made fibres to reinforce composites while keeping the weight lower. The assessment of the value of some commonly available tropical fibres for the composite industry starts with the determination of the strength, E-modulus and strain to failure through single fibre tensile tests. The mean strength and standard deviation is calculated following the normal and Weibull distribution resulting in the questionable benefit of applying the Weibull distribution. Furthermore, a correction method assesses the real fibre elongation from the measured clamp displacement. This procedure seems to be useful for strong, brittle fibres to produce more reliable results for the E-modulus and strain to failure.

Carbon nanotubes (CNTs) were integrated in glass fibres epoxy composites by either including CNTs in the fibre sizing formulation, in the matrix, or both. The effects of such controlled placement of CNTs on the thermophysical properties (glass transition temperature and coefficient of thermal expansion) and the Mode I interlaminar fracture toughness of the composites were studied. The present method of CNT-sizing of the glass fibres produces an increase of almost +10% in the glass transition temperature and a significant reduction of −31% in the coefficient of thermal expansion of the composites. Additionally, the presence of CNTs in the sizing resulted in an increased resistance of crack initiation fracture toughness by +10%, but a lowered crack propagation toughness of −53%. Similar trends were observed for both instances when CNTs were introduced only in the matrix and in combination of both matrix and sizing.

The continuous damage mechanics (CDM) approach is a popular tool for modelling of damage evolution in textile composites on the meso-level. It is based on the assumption that a material with defects can be replaced by a fictitious material with no defects but with degraded elastic constants. In such way the presence of defects is only reflected in the material elastic properties and damage evolution is recorded through the loss of these properties. The CDM approach incorporated into finite element analysis often predicts unphysically wide damage zones and in some cases failure across yarns – findings that are not supported by experimental data. The current work is geared toward identifying the source of inconsistencies between experiment and modelling by revisiting basic assumptions of CDM. A test problem is proposed to illustrate a break down of the CDM approach where a single crack-like defect in a yarn is modelled as an inhomogeneity with elastic constants reduced according to Murakami–Ohno model. It is shown that CDM in combination with local stress analysis of failure may predict a different direction of damage evolution as well as an incorrect failure mode in comparison with the crack problem. We also investigate whether the Murakami–Ohno model adopted for calculation of properties of a fictitious inhomogeneity contributes to the unphysical results. For this we compare contributions of a crack and an inhomogeneity into material elastic response. A new property degradation procedure is suggested (referred here as an effective elastic response model) where the size of an inhomogeneity and properties of the surrounding material are taken into account.

Experimental results are presented which allow the hybrid effect to be evaluated accurately for thin ply carbon/epoxy-glass/epoxy interlayer hybrid composites. It is shown that there is an enhancement in strain at failure of up to 20% for very thin plies, but no significant effect for thicker plies. Hybrid specimens with thick carbon plies can therefore be used to measure the reference carbon/epoxy failure strain. The latter is significantly higher than the strain from all-carbon specimens in which there is an effect due to stress concentrations at the load introduction. Models are presented which illustrate the mechanisms responsible for the hybrid effect due to the constraint on failure at both the fibre and ply level. These results give a good understanding of how variability in the carbon fibre strengths can translate into hybrid effects in composite laminates. publisher: Elsevier articletitle: Hybrid effects in thin ply carbon/glass unidirectional laminates: Accurate experimental determination and prediction journaltitle: Composites Part A: Applied Science and Manufacturing articlelink: http://dx.doi.org/10.1016/j.compositesa.2016.04.014 content_type: article copyright: © 2016 The Authors. Published by Elsevier Ltd. ispartof: Composites A, Applied Science and Manufacturing vol:88 pages:131-139 status: published

We apply Digital Image Correlation (DIC) to X-ray Computed Tomography (CT) slices for characterization of individual fibers misalignment in continuous fiber composites. This is an atypical application of DIC since the input is slices of a CT tomogram acquired at one time step, whereas the input of the typical deformation analysis application is 2D images captured over consecutive loading steps. The methodology is demonstrated and validated on synchrotron CT images of a glass/epoxy laminate, where individual fibers can be clearly identified. It is found that the average/standard deviation of the DIC displacement field in every CT slice is a good estimation of the average/standard deviation of the actual displacement of all fibers’ cross-sections in that slice. This provides a shortcut to the measurement of the moments of fiber orientation distribution, compared to the algorithms that reconstruct the fiber center lines. Using this shortcut, the fiber misalignment was also characterized in synchrotron CT images of a carbon/epoxy laminate, where individual fibers were not clearly observed and where, fiber segmentation tools failed to characterize the fibers. Furthermore, being applied to synchrotron images acquired during in-situ tension, the methodology is employed to quantify the evolution of misalignment during tension. The investigated 3D volumes are provided in [1] and described in [2]. ispartof: Composites Part A - Applied Science and Manufacturing vol:142 status: published