UD Composites Research Articles

Interaction of multiple micro-defects on the strength and failure mechanism of UD composites by computational micromechanics

The mechanical properties of unidirectional fiber-reinforced plastic (UD-FRP) are affected by a variety of micro-defects, such as random fiber arrangement, fiber misalignment and micro-voids. This study aims to investigate how these multiple micro-defects interact with each other and how they affect the strength and failure mechanism of UD-FRP by means of computational micromechanics. The composite behavior was simulated by the finite element analysis of a representative volume element of the composite microstructure in which the random distribution of fibers, the micro-voids, and the fiber misalignment are explicitly included. Both matrix and interface failure were considered for the loadings of transverse tension/compression, longitudinal compression, transverse/longitudinal shear, and their combination. It was found that these three micro-defects significantly weakened the compressive strength of UD-FRP along the longitudinal direction. Especially, the fiber misalignment magnified the effect of fiber arrangement, while the micro-voids reduce the effect. Besides, the fiber arrangement and micro-voids significantly weakened the tensile and compressive strength of UD-FRP along the transverse direction, but their interaction effect was not obvious. Moreover, transverse and longitudinal shear strength are significantly affected by micro-voids, but only longitudinal shear is affected by geometric fiber arrangement, and this effect is also weakened by micro-voids. Finally, the damage envelope under the combined longitudinal compression and transverse loads was obtained and compared with the Tsai-Wu failure criterion. The results showed that the Tsai-Wu criteria can provide an effective estimation for the failure locus under this biaxial loading condition.

Modelling of size-dependent plasticity in polymer-based composites based on nano- and macroscale experimental results

A number of experimental evidences indicate that the local response of polymer matrices near fibres is inadequately represented by classical continuum models relying on the bulk polymer behaviour. This results from size-dependency associated to large plastic strain gradients, complex interphase behaviour and/or changes of polymer structure. Classical multiscale models require artificial tuning of the properties to provide realistic macroscale predictions. We demonstrate that an unprecedented modelling approach based on a micromorphic theory is able to capture such size effects in long-fibre composites. The model is identified via nano digital image correlation strain fields and validated by predicting the strengthening found in transverse compression of UD composites, not captured by classical models. Micro-shear bands are properly regularised by the model, thus correctly handling the size-dependent plasticity and softening effects. The improved prediction of the strain localisation pattern in the matrix opens avenues to more accurately model interfacial failure and damage processes.

Study of the thermomechanical behavior of composite based on Elium acrylic resin, carbon nanotubes, and flax fibers: Experimental approach

AbstractThis study evaluated the effect of multi‐walled carbon nanotubes (MWCNT) addition on the thermomechanical, and electrical responses of unidirectional flax fibers/Elium acrylic modified carbon nanotubes‐based composite. The manufacturing process of these laminated composites involved infusing the nanofillers onto the fibers, ensuring good dispersion of MWCNT within the matrix. Regardless of the rate of incorporated nanotubes, this study revealed a significant deterioration of the mechanical properties in tension and viscoelastic properties of the Elium acrylic resin‐based nanocomposite. The electrical conductivity significantly increased for all nanocomposites and laminated composites. In contrast to the nanocomposite, the unidirectional flax fibers/MWCNT/Elium acrylic composite exhibited a considerable improvement in mechanical properties. An increase in Young's modulus and stress at break of about 27% and 23% respectively, was achieved for the material loaded with 2 wt% of MWCNT. The presence of MWCNT reduces the saturation weight by 5% for the unidirectional composite filled with an amount of 1.5 wt% in MWCNT.Highlights Improving the electrical properties of nanocomposites and unidirectional fibers (UD) composites. Improving the mechanical properties of Elium/flax/multi‐walled carbon nanotubes (MWCNT) composites. Influence of MWCNTs on the water absorption of UD composites. Influence of MWCNT on the glass transition temperature of nanocomposites. Inhibiting polymerization of MWCNT with Elium.

Optimising Recycling Processes for Polyimine-Based Vitrimer Carbon Fibre-Reinforced Composites: A Comparative Study on Reinforcement Recovery and Material Properties.

We investigated the recycling process of carbon fibre-reinforced polyimine vitrimer composites and compared composites made from virgin and recycled fibres. The vitrimer matrix consisted of a two-component polyimine-type vitrimer system, and as reinforcing materials, we used nonwoven felt and unidirectional carbon fibre. Various diethylenetriamine (DETA) and xylene solvent ratios were examined to find the optimal dissolution conditions. The 20:80 DETA-xylene ratio provided efficient dissolution, and the elevated temperature (80 °C) significantly accelerated the process. Scaling up to larger composite structures was demonstrated. Scanning electron microscopy (SEM) confirmed effective matrix removal, with minimal residue on carbon fibre surfaces and good adhesion in recycled composites. The recycled nonwoven composite exhibited a decreased glass transition temperature due to the residual solvents in the matrix, while the UD composite showed a slight increase. Dynamic mechanical analysis on the recycled composite showed an increased storage modulus for nonwoven composites at room temperature and greater resistance to deformation at elevated temperatures for the UD composites. Interlaminar shear tests indicated slightly reduced adhesion strength in the reprocessed composites. Overall, this study demonstrates the feasibility of recycling vitrimer composites, emphasising the need for further optimisation to ensure environmental and economic sustainability while mitigating residual solvent and matrix effects.

A Non-linear Mean-Field Debonding Model at Large Strains for the Analysis of Fibre Kinking in UD Composites

A Non-linear Mean-Field Debonding Model at Large Strains for the Analysis of Fibre Kinking in UD Composites

Development of Kevlar®/polypropylene towpregs through optimized commingling nozzle and processing parameters for thermoplastic composites

The current study aims at optimizing nozzle diameter and processing conditions for producing quality Kevlar®/polypropylene commingled towpregs meant for thermoplastic composite applications. ANSYS 3D CFD simulation and various experimental techniques are used for the same purpose. The study also investigates the effect of optimized commingled towpregs on the consolidation quality, void % and flexural properties of the developed UD composites. It is concluded that the nozzle diameter of 7 mm has proven to be ideal for producing quality towpregs as it leads to the formation of high nip frequency at 4.5 bar air pressure. The other nozzle diameters, 3 mm, 5 mm, and 9 mm, performed poorly in forming nips. Among these diameters, the 7 mm nozzle has optimum free space between the internal wall of the nozzle and the feed fibre bundle. This optimum space keeps the filaments within the potential air interaction zone and thus leading to the individual filament displacement and entrapment to form stable nips. The Box-Behnken design expert is used for obtaining ideal commingling conditions. The optimized Kevlar®/PP towpregs are produced using a custom fabricated commingling machine with a 7 mm diameter nozzle at the processing conditions - 4.5 bar air pressure, 2 m/min delivery speed, 3% and 1% overfeed, respectively for Kevlar® and polypropylene (PP). These commingled towpregs are flexible, compact, and regular in structure. These towpregs have the highest nip frequency, nip stability, weavability and homogeneous fibre-matrix mixing quality in yarn cross-section along the length. Using such towpregs makes the textile pre-forming effortless and yields composites with better resin impregnation quality and low void content. The UD Kevlar®/PP composites made through these towpregs offered good consolidation quality with a low void (4.3%). These composites showed flexural strength and modulus of 130 MPa and 28 GPa, respectively.

The Tensile Behaviour of Unaged and Hygrothermally Aged Asymmetric Helicoidally Stacked CFRP Composites

This paper concerns the effects of hygrothermal ageing on the tensile behaviour of assymetric helicoidally stacked carbon fibre reinforced plastic (CFRP) composites. MR70 12P carbon fibre epoxy prepreg sheets were manufactured into laminated composites comprising constant inter-ply pitch angles ranging from 0° to 30°. The composites were tested in tension (according to BS ISO 527-5:2009) as either dry unaged specimens or following hygrothermal ageing in seawater at the constant temperatures of 40 °C and 60 °C for 2000 h. Both tensile modulus and tensile strength are found to be detrimentally affected by hygrothermal ageing, and the extent to which ageing affects these properties is a function of the inter-ply pitch angle. Higher hygrothermal ageing temperatures are found to decrease the tensile modulus and strength ratios of asymmetric helicoidally stacked composites when compared against UD composites subjected to the same conditions and the strength and stiffness ratios of all composites when compared against unaged equivalents. Significantly, therefore, we show that the degradation of helicoidal composite properties under hygrothermal conditions, in general, occurs more rapidly than it does in UD composites, and thus the long-term use of helicoidal composites in immersed environments should take into account these differences. A second order relationship is observed for the mechanical properties of the composites when plotted against their inter-ply helicoidal pitch angles. As such, a mixtures model was modified to incorporate the observed effects of laminate inter-ply pitch angle and used to predict the tensile modulus of unaged composites. The predictions are within one standard deviation of the experimental arithmetic mean; however, the model can only be used for dry helicoidal composites, as ageing alters the microstructures in an irregular manner between the different sample sets. The development of this mixture model is useful as it provides a justifiably simple route to predicting the properties of dry helicoidal structures, albeit within the bounds of specific interply-pitch angles. Finite element analyses (Hashin failure) elucidate the plies that are most likely responsible for composite failure. The validity of these numerical predictions is evidenced by observing primary fracture paths in the composites. Finally, hygrothermal ageing is found to enable greater in-plane (mode III) twisting of individual laminates under loading, with certain laminate angles being more prone to twisting than others.

Experimental investigation of the effect of weave type on the mechanical properties of woven hemp fabric/epoxy composites

Natural fiber composites have a growing popularity as alternative materials to petroleum-based plastics and glass fiber reinforced composites due to various economic and environmental reasons. Textile preforms such as woven, knitted, braided, nonwoven, and multiaxial fabrics are commonly used in composite industry due to their high mechanical properties, tailorability, ease of handling during production, and versatility in material design. A range of properties can be obtained by varying the weave pattern and fabric architecture in woven fabric composites. In this study, epoxy composites reinforced with hemp woven fabrics with four different weave types were produced. The weave types chosen were quasi-unidirectional (UD), plain, basket 2/2, and twill 2/2 which are commonly used in the composite industry. Fabric properties such as yarn angle, yarn density, and crimp ratio were determined to evaluate their possible effect on composite properties. Tensile and three-point flexural tests were conducted in order to determine the effect of weave type on the mechanical properties of the resulting composites. It was found that UD composites show the highest tensile strength, tensile modulus, flexural strength, and flexural modulus in the 0° (yarn) direction. Plain weave fabric composites showed the second highest tensile and flexural strength and moduli followed by basket 2/2 weave composites. Composites reinforced with twill 2/2 woven fabrics showed the lowest mechanical properties due to high yarn crimp and yarn angle, less balanced structure, and resulting shear forces involved during various loading conditions.

Continuous fibre composite 3D printing with pultruded carbon/PA6 commingled fibres: Processing and mechanical properties

Continuous Fibre composite 3D printing (CF-3DP) technology is a newly developed Additive Manufacturing (AM) process. By employing continuous fibre as a reinforcement for the composite, the mechanical properties of the 3D printed part can be significantly improved. However, owing to the process's early stage of development, the mechanical performance of the printed material is still unable to compete with composites produced using conventional manufacturing processes. This can manly be attributed to lower fibre volume fractions and/or higher void content. The grand objective of the research project is to establish a demonstrator of practical applications, and in this paper, preparatory developments were carried out to pave the ground for the subsequent developments, including feedstock materials development, printing process development, and materials properties evaluation. Efforts has been made to increase fibre volume fracture, to reduce voids contents, to evaluate effective properties systematically. The static mechanical test results showed relatively good longitudinal tensile properties while matrix sensitive properties were low, as to be expected in UD composites with a relatively weak matrix material. The advantage of CF-3DP is to achieve fibre steering, and the steered fibre design can be achieved by the current process to realize novel composite structures in future study.

Development of a 3D finite element model at mesoscale for the crushing of unidirectional composites: Application to plates crushing

This paper presents the development of a new 3D finite element model for the simulation of UD composites materials under crushing, at the mesoscale. It is based on the Discrete Ply Model (DPM), enhanced with a new damage law to represent compression at constant stress due to localised crushing and its propagation from one element to another. The model aims to take into account the various damage types involved in UD laminates during crushing and the associated absorbed energies: delamination, splitting, ply rupture in tension or bending, compression due to localised fragmentation and friction. The model is confronted with experimental results obtained on laminated plates crushing: not only global metrics (force, energy absorbed…) but especially key morphological aspects of the crushing front (delamination lengths, splaying, number of plies under localised crushing, debris wedge…) thanks to high speed camera pictures.

A non-local finite element model to assess the compressive strength of composite structures

Compression failure is a mechanism of non-local ruin that depends on the composite structure (layer thickness, load gradient), which makes it a peculiarity. The mechanism has been described and modeled with particular numerical tools in the literature. Only a few researchers modeled the mechanism at a mesoscopic scale, which is limited to UD composites. A homogenized non-local finite element model is proposed in this article. This is similar to Mindlin's II gradient theory to assess the compressive strength of the carbon/epoxy long fiber composite at the mesoscopic/structural scale. The framework of this non-local modeling is more general than that of Drapier et al. (1999) to assess microbuckling phenomenon in UD and woven composites. The developed non-local numerical model has been implemented in User Element (UEL) subroutine for analysis in ABAQUS®, which permits the simulation of the behavior of 2D and 3D cases. A 2D continuous (C1) type super-parametric non-local element (NL U32) is developed for both linear and non-linear (geometry and material) case. Various tests results are presented to validate the non-local FE model for 2D case by comparing the results of the homogenized non-local element with ABAQUS® iso-parametric elements. The classical (elastic and plastic) and non-local material properties (elastic) of the non-local model are identified by comparison to the responses of a Representative Volume Element (RVE) of full heterogeneous microstructures. A comparison of the compressive behavior of UD composite with the results in the bibliography shows the capabilities of this numerical tool.

The effects of fiber radius and fiber shape deviations and of matrix void content on the strengths and failure mechanisms of UD composites by computational micromechanics

Manufacturing plays an important role in the properties of composites, which may lead to two types of uncertainties: (1) carbon fiber manufacturing deviations, such as the bean-shaped PAN carbon fibers, the lognormal-distributed fiber radius, etc. (2) composites manufacturing deviations, such as the inter-fiber voids and matrix voids. Although several experiments have been conducted to assess their effects on the mechanical properties of composites, a satisfactory result of quantitative characterization of fiber radius and fiber shape deviations and of matrix void content have not been obtained. In this paper, a parametric study based on the computational micromechanics is conducted to reveal the effects of fiber radius and fiber shape deviations and of matrix void content on the strengths and failure mechanisms of UD composites under four loading conditions, i.e. the transverse tension, compression, shear and longitudinal shear. Firstly, the constitutive laws of constituents, i.e. carbon fiber, matrix and interface, are established to characterize their mechanical responses. Then, the UD RVEs with fiber radius and fiber shape deviations and with matrix void are modeled individually based on experimental observation and statistical distribution. After qualitative validation of failure modes and quantitative validation of stress-strain curves, the methodology is applied to predict the stress-strain curves and failure modes of UD RVE with manufacturing uncertainties and the results show that not all manufacturing uncertainties have a detrimental effect on the mechanical properties of UD composites.

Non-linear mean-field modelling of UD composite laminates accounting for average asymmetric plasticity of the matrix, debonding and progressive failure

As an effective and accurate method for modelling composite materials, mean-field homogenization is still not well studied in modelling non-linear and damage behaviours of UD composites. Investigated micro FE-simulations show that the matrix of UD composites exhibits different average plastic behaviour, named as average asymmetric matrix plasticity (AAMP), when the composite behaves different under shear, longitudinal and transverse loadings. In this study, a non-linear mean-field debonding model (NMFDM) combining a mean-field model and a fibre–matrix interface debonding model, is developed to simulate UD composites under consideration of AAMP, fibre–matrix interface damage and progressive failure. AAMP is considered by using so-called stress mode factor, which is expressed in terms of basic invariants of the matrix deviatoric stress tensor and is used as an indicator for detection of differences in the loading mode. The material behaviour of UD composites with imperfect interface is assumed identical as for perfect interface and stiffness reduced fibres. Progressive failure criteria are established with consideration of fibre breakage and matrix crack for different fibre orientations. As a representative example for the NMFDM, a C30/E201 UD composite is studied. To verify the model, experiments are conducted on polymers, carbon fibres and UD CFRPs. Finally, the model is applied to simulate a perforated CFRP laminate, which shows excellent prediction ability on deformation, debonding and progressive failure.

A PFA methodology to investigate UD composites in fatigue comprising a KTF-based model for matrix damage and stochastic fiber failure prediction

A physics-based, multiscale, progressive fatigue damage analysis methodology for continuous fiber-reinforced polymer composites was further developed and applied to the problem of fatigue damage prediction in open hole tension (OHT) coupons comprised of multidirectional laminates undergoing tension–tension loading. Based off lamina level calibration data, cumulative damage analysis of matrix and fiber constituents under cyclic loading condition was implemented with the objectives to: (i) predict the onset and the progression of subcritical matrix failures as well as the ultimate failure due to fiber fracture, and (ii) observe the effects of damage mode interactions on the progressive failure process. A kinetic theory of fracture (KTF) based durability prediction methodology was utilized to keep track of the matrix damage transpiring in the intra- and inter-laminar layers of the laminate. Tensile strengths were randomly assigned to the fibers using two-parameter Weibull statistics, while the maximum stress theory in tandem with the linear damage accumulation rule was applied to capture the tensile rupture of the reinforcements. Progressive fatigue damage prediction of OHT coupons consisting of the unidirectional composite IM7/977-3 and layup schedules of [0/45/90/−45]2S and [60/0/−60]3S were carried out and benchmarked against available experimental data. It was observed that the KTF-based fatigue model captured the damage state at individual layers, overall loss of mechanical performance, and stochastic nature of the ultimate failure well. The effects of interactions among the damage modes, namely matrix cracking, delamination, and fiber rupture were also investigated and it was recognized that such interactions are a critical feature of the progressive failure process in composite structures.

Interlaminar fracture toughness of a quasi 3D braided composite

Fiber reinforced composite materials are a heavily sought after material for next generation vehicles for light-weighting components due to their high specific strength and stiffness. However, these materials have relatively weak interlaminar strength and are prone to delamination. This is especially the case when a delamination crack already exists. Quasi-3D (Q3D) braided composites seek to solve this issue by weaving the bias tows into the adjacent (above and below) plies. The plies are physically connected through fiber tows as opposed to being bonded simply by the epoxy, and the composite will achieve a higher interlaminar strength due to fiber failure being required for crack propagation as opposed to simply matrix failure. The [Formula: see text] UD and Q3D carbon composites are investigated in this study for their better in-plane isotropy. Mode I and Mode II interlaminar fracture toughness tests were conducted on UD and Q3D samples. In Mode I experiments, the samples were continuously loaded to full beam split using the double cantilever beam method to obtain the fracture toughness throughout the sample. 4ENF is used to measure the Mode II fracture toughness to create a full R-curve for the architectures.

The fiber‐matrix interface in Ioncell cellulose fiber composites and its implications for the mechanical performance

AbstractFiber‐reinforced composites based on natural fibers are promising alternatives for materials made of metal or synthetic polymers. However, the inherent inhomogeneity of natural fibers limits the quality of the respective composites. Man‐made cellulose fibers (MMCFs) prepared from cellulose solutions via wet or dry‐jet wet spinning processes can overcome these limitations. Herein, MMCFs are used to prepare single fiber epoxy composites and UD composites with 20, 30, 40, and 60 wt% fiber loads. The mechanical properties increase gradually with fiber loading. Young's modulus is improved three times while tensile strength doubles at a loading of 60 wt%. Raman spectroscopy is employed to follow conformational changes of the cellulose chains within the fibers upon mechanical deformation of the composites. The shift of the characteristic Raman band under strain indicates the deformation mechanisms in the fiber. Provided stress transfer occurs through the interface, it is a direct measure of the fiber‐matrix interaction, which is investigated herein. The shift rate of the 1095 cm−1 band decreases in single fiber composites compared to the neat fibers and continues to decrease as the fiber loading increased.

An improved longitudinal failure criterion for UD composites based on kinking model

A longitudinal failure criterion for UD composites based on a kinking model was re-constructed guided by the principles of analytic geometry. The proposed method for determining the fiber initial misalignment angle considers the shear nonlinear response of the matrix. An expression accounting for the fiber matrix coupling failure was used to simulate shear-driven fiber compression failure. Different effects of the matrix shear on the fiber tensile and compressive failure was considered. Compared with the kinking model, the proposed criterion better predicted the compression failure mode, gave better agreement with the experimental results, and exhibited better adaptability for different fiber failure modes.

Growth of interface cracks on consecutive fibers: On the same or on the opposite sides?

The growth of fiber/matrix interface cracks (debonds) located on consecutive fibers along the through-the-thickness (vertical) direction is studied in glass fiber-epoxy UD composites. Debonds could appear, along the vertical direction, on the same or on opposite sides of their respective fibers. Determining which configuration is the most energetically favorable to debond growth is the objective of this paper. To this end, two different families of Representative Volume Elements (RVEs) are developed: the first implements the classic condition of coupling of the vertical displacements to model a unit cell repeating symmetrically along the vertical direction; the second uses a novel set of boundary conditions, proposed here by the authors, to represent a unit cell repeating anti-symmetrically along the vertical direction. The model is analyzed in the context of Linear Elastic Fracture Mechanics (LEFM) and the Mode I and Mode II Energy Release Rate are evaluated to investigate crack growth. The calculation is performed using the Virtual Crack Closure Technique (VCCT) in the framework of the Finite Element Method (FEM). It is found that Mode I dominated propagation is favored when debonds are located on the same sides of their respective fibers; while for larger (Mode II-dominated) debonds, Mode II ERR is higher when they lie on the opposite sides. No interaction effect is present when at least two fully bonded fibers are located between the partially debonded ones.

Improving delamination resistance through tailored defects

Delamination is a well-known damage mode exhibited by laminated composites which has been extensively studied in unidirectional laminates (UD). In this work, a novel strategy consisting of the introduction of a small interlaminar defect to enhance the fracture resistance to delamination of UD composites is presented. This toughening concept is based on the simultaneous propagation of multiple interlaminar cracks along different interfaces. A preliminary numerical study showed that the minimum size required by the initial defect to trigger dual propagation was 4 times the ply thickness for intermediate and thin plies for any applied mixed mode, except for pure mode II. An experimental campaign to prove the toughening concept with 1 and 3 defects at adjacent interfaces was carried out and compared against a reference configuration under three different applied mode mixities (ϕglob=0.4,0.2 and 0). The single defect configuration reached an improvement in the fracture resistance of +300% under pure applied mode I, and +80% and +100% for applied mixed modes of 0.2 and 0.4, respectively. The configuration with 3 defects reached even higher fracture toughness values: +430% in pure applied mode I, and +115% and +100% for applied mixed modes of 0.2 and 0.4, respectively. A finite element model employing cohesive zones successfully predicted both, the phenomenon of multiple crack propagation, and the effective fracture resistance of the process. Interestingly, among the dissipation mechanisms reported experimentally, the extensive promotion of fiber bridging under applied pure mode I was observed, which was not reported in the baseline configuration.

Energy release rate of the fiber/matrix interface crack in UD composites under transverse loading: Effect of the fiber volume fraction and of the distance to the free surface and to non-adjacent debonds

The effects of crack shielding, finite thickness of the composite and fiber content on fiber/matrix debond growth in thin unidirectional composites are investigated analyzing Representative Volume Elements (RVEs) of different ordered microstructures. Debond growth is characterized by estimation of the Energy Release Rates (ERRs) in Mode I and Mode II using the Virtual Crack Closure Technique (VCCT) and the J-integral. It is found that increasing fiber content, a larger distance between debonds in the loading direction and the presence of a free surface close to the debond have all a strong enhancing effect on the ERR. The presence of fully bonded fibers in the composite thickness direction has instead a constraining effect, and it is shown to be very localized. An explanation of these observations is proposed based on mechanical considerations.