Discontinuous Fiber Research Articles

Determination of shear modulus in bamboo fibers composite in torsion tests

Natural Fibers, when employed as reinforced in composites, contribute to the reduction of environmental impacts, moreover, its use decreases costs and reduces the weight of components. Several studies are carried out for the application of these materials in the automobile industry. Some mechanical behaviors such as elasticity moduli have already been experienced in these types of composites. However, for the use of these materials in quality and safety products, more mechanical information is required. The purpose of this study is to determine the shear moduli G13 and G23 of composites with vegetable fibers. Torsion tests were performed on epoxy resin composites with bamboo fibers without chemical treatment: one with 280-mm unidirectional long fibers and another one with 25-mm discontinuous short fibers. The torsion test was based on the variation of the base/height ratio (b/h) of the test specimen and at each base reduction a new torsion test was carried out. All tests featured a linear relationship between the torsional angle and the torque. On the experimental points plotted, the Saint-Venant equation modified to orthotropic material was interpolated, thus allowing the determination of the shear modulus G13 and G23 for the two composites. The shear moduli for long fiber composites presented the following values: G13 = 1.441 GPa, G23 = 1.726 GPa, and for short fibers G13 = 1.889 GPa, G23 = 1.290 GPa. The results showed that the composites are orthotropic for shear modulus due to the compression molding process.

Numerical material testing based on statistically similar representative volume elements for discontinuous fiber composites

AbstractA computational method is proposed for the construction of statistically similar representative volume elements (SSRVEs) for discontinuous fiber composites (DFCs) in order to efficiently calculate material properties based on computational homogenization. The SSRVE is obtained by solving an optimization problem which minimizes differences between the power spectral density of a real target microstructure and the SSRVE. The target microstructure is obtained by means of computed tomography scanning of a DFC plate. As a result of a comparative study, the mechanical behavior of the SSRVE turns out to agree with that of the target microstructure. However, the SSRVE reduces extremely the computational costs of finite element analyses to derive macroscopic material properties of DFCs.

Reclaimed Carbon and Flax Fibre Composites: Manufacturing and Mechanical Properties

The feasibility of using the HiPerDiF (high performance discontinuous fibre) method to manufacture highly aligned discontinuous fibres intermingled hybrid composites with flax and reclaimed carbon fibres (rCF), and the potential benefits of so doing, are investigated in this paper. It is demonstrated that, despite their hydrophilic nature, flax fibres are not affected by this water-based process. Intermingled flax/rCF hybrid composites are characterised in terms of their tensile and vibrational response. It is concluded that natural/rCF fibre hybrid composites can be a viable solution for those applications where a reduction in primary mechanical properties, e.g., stiffness and strength, is an acceptable trade-off for the enhancement of secondary properties, e.g., noise, vibration, and harshness (NVH) mitigation, and the reduction of monetary costs.

A micromechanics model to predict extensional viscosity of aligned long discontinuous fiber suspensions

A micromechanical unit cell model is formulated to investigate the deformation in a unit cell of highly concentrated suspension of long discontinuous aligned fibers. The periodic unit cell is many fiber lengths long to allow for microstructural variations due to random fiber length overlaps and end-to-end gap distances between the fibers. This feature of the unit cell makes it possible to examine the effect of such variabilities on the macroscopic material behavior (effective viscosity) during the forming process. The influence of microstructure variability on viscosity is predicted. The standard deviation of the effective extensional viscosity is significant and increases with material extensional deformation and the variation of the end to end fiber spacing. This could have ramifications on the forming process and the final physical properties of the materials of interest. The predicted dependence of the mean extensional viscosity value on the usual material parameters (fiber aspect ratio, fiber volume fraction) is found to be consistent with the relations predicted by reported models using deterministic periodic unit cells. The behavior of standard deviation of our model follows the same dependence as the mean value. On the other hand, it shows negligible variability of effective longitudinal-transverse shearing viscosity of such materials, regardless of the random perturbations of fiber length overlaps and end to end fiber spacings within the unit cell.

Mechanical properties and radiopacity of flowable fiber-reinforced composite

The aim was to evaluate the effect of different zirconia discontinuous fiber fractions on radiopacity and other selected properties of glass discontinuous fiber-reinforced flowable composite (Exp-SFRC). Exp-SFRC was prepared by mixing 30 wt% of resin-matrix and 45 wt% of particulate-fillers to 25 wt% of various weight-fractions of E-glass/zirconia discontinuous fiber-fillers (25:0, 20:5, 15:10, 10:15, 0:25 wt%). Flexural strength and fracture toughness were determined for each experimental material. Radiograph of each Exp-SFRC and aluminium step wedge were taken to determine the radiopacity. Degree of conversion and light-transmission were also measured. Scanning electron microscopy was used to evaluate the microstructure of the Exp-SFRC. Analysis of variance (ANOVA) revealed that fractions of E-glass/zirconia discontinuous fiber-fillers had significant effect (p<0.05) on radiopacity and other tested properties of the Exp-SFRCs. Replacing low fraction of E-glass fiber with zirconia fiber-fillers can increase the radiopacity of the fiber-reinforced composite without deteriorating the mechanical properties, although, degree of conversion was decreased.

3D printing of discontinuous and continuous fibre composites using stereolithography

3D printing technology has revolutionized the field of machinery, aerospace, and electronics. To address the shortcomings of previous studies on improving the poor mechanical properties of the resin used in 3D printers, this study presents a technology for fabricating short fibres or a continuous fibre-composite material using stereolithography 3D printing. Glass powder and fibreglass fabric were used as the discontinuous and continuous fibre reinforcement of light-cured resin material. The tensile strength and Young’s modulus showed a marked increase: these were 7.2 and 11.5 times higher than those of the resin specimen, respectively.

CHARACTERIZATION OF THE FRACTURE MECHANICAL BEHAVIOR OF C-SMC MATERIALS

The fracture mechanics of random discontinuous Carbon Fiber Sheet Molding Compound (C-SMC) materials compared to traditional carbon fiber composites are not well understood. An experimental study was carried out to characterize the fracture behavior of such C-SMC materials. Mode I tests, using double cantilever beam specimens, and mode II tests, adopting the four-point bend, end-notched flexure configuration, were performed. Results show high variations in the forcedeflection responses and scatter in the fracture toughness properties GIc and GIIc, due to the complex mesostructure defined by random oriented carbon fiber chips. To investigate the influence of the mesostructure, tensile tests with varying specimen width and thickness were assessed by stochastic measures to find the representative specimen size.

Editorial for the Special Issue on Discontinuous Fiber Composites

The papers published in this special edition of the Journal of Composites Science will give the polymer engineer and scientist insight into what the existing challenges are in the discontinuous fiber composites field, and how these challenges are being addressed by the research community. [...]

Comparison of strength and Young modulus of aligned discontinuous fibre PLA composites obtained experimentally and from theoretical prediction models

The present paper describes work carried out to compare the strength performance of PLA reinforced aligned discontinuous harakeke and hemp fibre mat composites evaluated experimentally and using various simple elastic mathematical prediction models. Fibre mats were produced by means of Dynamic Sheet Former (DSF), and stacked alternately with PLA sheets to produce composites with up to 40 wt% fibre content. The fibre content was increased from 5 wt% to 40 wt% using 5 wt% interval. In this work, both harakeke and hemp composites were manufactured using hot-press. It was found that, better agreement to the experiment was obtained for Young’s modulus compared to those composite strengths. The Modified Rule of Mixtures is found to give accuracy within 41% for strength, while the Cox model showing the best agreement (<10%) for Young’s modulus for fibre contents up to 25 wt%. Better agreement obtained for Young’s modulus is believed to due to less mechanism involved at where Young’s modulus is measured.

Experimental and numerical investigation on the dynamic characteristics of thick laminated plant fiber-reinforced polymer composite plates

Although considerable attention has been devoted to the experimental research works of plant fiber-reinforced composites, theoretical determination of their structural characteristics is also much needed one in their evolution. But it is still considered as a difficult process due to the several factors such as short and discontinuous fiber form, plant fiber’s intrinsic characteristics, fiber anisotropy and porosity. In this study, vibration characteristics of woven jute/epoxy and woven aloe/epoxy laminated composite plate structures are investigated theoretically and experimentally. The elastic constants of the woven jute/epoxy and woven aloe/epoxy laminated composites are obtained through experimental testing. The governing differential equations of motion for the uniform woven jute/epoxy and woven aloe/epoxy laminated thick composite plates are carried out using the p-version finite element method based on higher-order shear deformation theory. The effectiveness of the developed finite element formulations is demonstrated by comparing the natural frequencies obtained using the present finite element method with those obtained from the experimental measurements. Also a comparative study is carried out between the results of h-version FEM and p-version FEM to check their accuracies and efficiencies. The effects of aspect ratio and angle of fiber orientation under various end conditions on the free vibration responses of the woven jute/epoxy laminated composite plate are also investigated. The forced vibration response of the woven jute/epoxy laminated composite plate under the harmonic force excitation is carried out under various end conditions.

Multi-scale constitutive modeling of natural fiber fabric reinforced composites

Natural fiber reinforced structural composites have a hierarchical structure made from stacked fabrics of interwoven yarns formed by twisting together discontinuous natural fibers. We develop multi-scale constitutive models to investigate the mechanical properties of plain woven composites. The multi-scale constitutive modeling is carried out in two steps: the effective micromechanical properties of a micro-scale representative volume element (RVE) of the twisted yarn are calculated using an orientation averaging method, which are subsequently transferred to a meso-scale RVE of the final composite to compute the elastic constants by homogenization over the RVE. 3D finite element models of the meso-scale RVEs of composites are developed to verify the accuracy of the proposed model. The multi-scale modeling results show that the yarn twist angle has significant effects on the elastic properties of the composites. The predicted results from the multi-scale constitutive model show good agreements with that from finite element analysis and experiment.

Influence of small amount of glass fibers on mechanical properties of discontinuous recycled carbon fiber-reinforced thermoplastics

The effect of residual contamination, such as glass fibers (GFs), on the application of discontinuous recycled carbon fibers (rCFs) is still unclear. In this study, GFs are used to substitute rCF at volume fractions (Vfs) of 0%, 1%, 2%, and 5% in carbon fiber paper-reinforced thermoplastics fabricated through a papermaking process. After an investigation with the three-point bending tests, the flexural modulus will not decrease when the GF content increased. Unlike the flexural modulus, the flexural strength decreased with an increase in the GF volume. In order to illustrate the effect of small amount of GF on flexural modulus, a modified rule of mixture (ROM) is used to derive a coefficient, Factor c1, depicting the effect caused by discontinuous and randomly distributed reinforcement fibers. Furthermore, modified Halpin–Tsai model with Factor c1 illustrated that the experiment results of flexural modulus is not decreased as fast as predicted results. Therefore, small amount of GF will not affect the application of rCF.

Experimental and analytical study of the thermo-mechanical behaviour of textile-reinforced concrete (TRC) at elevated temperatures: Role of discontinuous short glass fibres

During the last few decades, textile-reinforced concrete materials are being increasingly used to repair and/or reinforce civil engineering structures, owing to their many advantages (nontoxicity, availability of raw materials, recyclability, simplicity of implementation, etc.). However, the elevated temperature resistance of these materials has not been studied to a great extent. The present work was aimed at studying the tensile behaviour of two types of such composite materials (3GRI and 3GRI.SF), under combined thermal and mechanical loading. 3GRI was composed of a cement matrix, and alkali-resistant (AR) grid glass textile. 3GRI.SF was also composed of the same matrix and textile reinforcement (as 3GRI), but with discontinuous, short AR glass fibres. Using an original experimental approach, the thermo-mechanical behaviour of these two types of composite materials could be identified at various temperatures (from 20 °C to 600 °C), and the role and efficiency of the addition of discontinuous short fibres were investigated. The experimental study was followed by analytical modelling, which helped calibrate other existing analytical models (Gibson and Bisby). Based on these models, and on the experimental results, the parameters of suitable models for the textile-reinforced concrete composites (3GRI and 3GRI.SF) were identified and commented on. This paper shows that the addition of short glass fibres had a positive effect on the tensile strength, stiffness and resistance to crack. The calibrated analytical models allowed predicting the thermomechanical proprieties of glass TRC with a minimum of experimental tests. The calibrated analytical models can be applied for the prediction of the thermomechanical proprieties of similar TRC.

Numerical modeling of steel fiber reinforced concrete with a discrete and explicit representation of steel fibers

This work presents a novel numerical model based on the use of coupling finite elements to simulate the behavior of steel fiber reinforced concrete (SFRC) with a discrete and explicit representation of steel fibers. The material is described as a composite made up by three phases: concrete, discrete discontinuous fibers and fiber-matrix interface. The steel fibers are modeled using two-node finite elements (truss elements) with a one-dimensional elastoplastic constitutive model. They are positioned using an isotropic uniform random distribution, considering the wall effect of the mold. A non-rigid coupling procedure is proposed for modeling the complex nonlinear behavior of the fiber-matrix interface by adopting an appropriate constitutive damage model to describe the relation between the shear stress (adherence stress) and the relative sliding between the matrix and each fiber individually. An isotropic damage model including two independent scalar damage variables for describing the concrete behavior under tension and compression is considered. To increase the computability and robustness of the continuum damage models used to simulate matrix and interface behavior, an implicit-explicit integration scheme is used. Numerical examples involving a single fiber and a cloud of fibers are performed. Comparisons with experimental results demonstrate that the application of the numerical strategy for modeling the behavior of SFRC is highly promising and may constitute an important tool for better understanding the effects of the different aspects involved in the failure process of this material.

Tension-stiffening effect in steel-reinforced UHPC composites: Constitutive model and effects of steel fibers, loading patterns, and rebar sizes

The tensile performance of steel-reinforced concrete members is closely associated with the bond interaction between concrete and the embedded rebar. Ultra-high performance concrete (UHPC) is a rapidly emerging concrete material that has an ultra-high compressive strength and bond strength. When it is reinforced with short, discontinuous fibers, it features a tensile strain-hardening behavior and a damage pattern of closely spaced narrow cracks. The present study investigated the tensile behavior of steel-reinforced UHPC members. Sixteen samples were tested, with the experimental variables including embedded rebar sizes, loading patterns, and steel fibers. The tensile responses of the steel-reinforced UHPC samples were evaluated using multiple performance measures, including the damage pattern, stiffness, load-deformation relationship, rebar strain, and the tension-stiffening behavior of UHPC. The test results showed that the enhanced bond strength due to the inclusion of steel fibers transformed the failure pattern of the steel-reinforced UHPC from multiple localized cracks into a single localized crack, which intensified the strain concentration in the embedded rebar. Although the addition of fibers substantially strengthened the tension-stiffening response of the UHPC, it also raised critical concerns about premature failure, especially when UHPC members were reinforced with small steel bars and subjected to monotonic loading. In addition to the experimental study, a tetra-linear constitutive model that was able to reasonably represent the tension-stiffening behavior of fiber-reinforced UHPC up to failure was suggested in this study.

Sisal fiber reinforced high density polyethylene pre-preg for potential application in filament winding

A thermoplastic-penetrated natural fiber belt was developed for potential application in filament winding. Discontinuous sisal fiber bundles were impregnated with high density polyethylene and then connected into continuous pre-pregs via hot-pressing. The interfacial bonding was improved by treating the fibers with sodium hydroxide (NaOH) and three types of coupling agents (NH2C3H6Si(OC2H5)3, C2H3Si(OC2H5)3 and C51H112O22P6Ti). Tensile properties of the fiber bundles, pre-pregs, and annular specimens were determined. The fiber surface microstructure and the pre-preg interfacial bonding were evaluated via scanning electron microscopy and Fourier transform infrared spectroscopy. Among the four treatments, 2% C2H3Si(OC2H5)3 yielded the most significant improvement in the tensile properties of the pre-pregs and annular specimens. The fiber bundle was less damaged by the coupling agents treatment comparing to NaOH treatment. When the overlap at the end of the sisal fibers was >11 mm, the presence of the joint had no effect on the tensile strength of the pre-preg. The pre-preg belt containing treated fibers, which was evaluated based on the requirements of filament winding applications, exhibited a high performance.

Effect of tailored plies on the energy absorption capability of square CFRP tubes with discontinuous fibers

Research on carbon-fiber-reinforced plastic (CFRP) composite laminates shows that advanced pseudo-ductility can be obtained using a cut-ply approach. This study investigates the energy absorption capability of CFRP square tubes with overlapping discontinuous plies. The cutting-angle, α, which can be used as a design variable, is considered and controlled to cut both spacing and density. Quasi-static axial compression tests were conducted to investigate the failure process, crashworthiness and the corresponding energy-absorption mechanism. The results indicate that the cutting angle has a significant effect on the energy absorption. Increasing the cutting angle increases the specific energy absorption from 50.8 J/g to 74.1 J/g. Compared to one sample (56.2 J/g), the specific energy absorption increases by 32%. This study also indicates that the energy absorption of composite tubes can be tailored by adjusting the position of the cuts.

Behavior of macro synthetic fiber concrete beams strengthened with different CFRP composite configurations

Thirty-two normal-strength macro discontinuous structural synthetic fiber (DSSF) concrete beams were constructed and tested. In order to determine the optimum dosage of DSSF, eight of the beams were prepared with various dosages of DSSF, while the rest beams were prepared with an optimum DSSF dosage of 0.55% and with various carbon fiber reinforced polymer (CFRP) sheet lengths and widths. The tested parameters include mode of failure, load versus mid-span deflection response, ultimate deflection, ultimate load capacity, energy absorption, stiffness, performance factor, energy absorption ductility index, displacement ductility index, and concrete and CFRP sheet strains. The addition of DSSF and CFRP sheet enhanced most of the investigated parameters by providing internal and external crack arresting mechanisms, respectively, and enhancing the post-cracking ductility after reaching the extreme load capacity.

Behavior of plain concrete beams with DSSF strengthened in flexure with anchored CFRP sheets—Effects of DSSF content on the bonding length of CFRP sheets

In this study, the behavior of the anchored carbon fiber reinforced polymer (CFRP) plain concrete beams with various dosages of macro discontinuous structural synthetic fiber (DSSF) were investigated. Seventy two plain concrete beams were casted and tested. The investigated parameters in this study are the dosages of DSSF, length of CFRP sheets, and with and without CFRP anchorage sheets. Therefore, the feasibility of using macro synthetic fibers and anchored CFRP sheets for enhancing the behavior of plain concrete beams is of enormous interest. The behavior characteristic of each tested beam was evaluated in terms of failure mode, load versus mid-span deflection curve, ultimate load capacity, ultimate deflection, stiffness, energy absorption, performance factor, displacement ductility index, energy absorption ductility index, and CFRP and concrete strain. The tested results reveal a considerable enhancement on the majority of the investigated parameters and show the efficiency of using a combination macro synthetic fibers and anchored CFRP sheets in terms of the behavior characteristic of tested beams. The enhancements were recognized primarily to the efficiency of the used anchored CFRP and macro synthetic fibers in providing external and internal, respectively, cracks arresting mechanism after cracking and even after reaching the ultimate load capacity.

Study of fiber morphology characteristics of discontinuous carbon-fiber-reinforced indium tin oxide transparent conductive film by image analysis method

In discontinuous fiber-reinforced composites, short fibers act as reinforcing materials, and their morphological characteristics (length and orientation) can greatly affect the properties of the composites, such as tensile, electrical, or thermal conductivity. There is a very important issue in describing the morphological characteristics of reinforcing fibers, but the disorder of short fibers increases the difficulty in resolving issue. In this study, an image processing method was used to extract the morphological characteristics of discontinuous fibers in composites, and the conductive properties of short-carbon-fiber-reinforced indium tin oxide (ITO) films were analyzed. Among them, the effective length of the fibers consistently correlated with the surface resistance of the composites. At the same time, in this study, we analyzed the relationship between the area ratio of different carbon materials (carbon fiber, carbon nanotubes, and graphene) and the surface resistance of their composites, and also extended the application of the image processing method.