High Fiber Volume Fraction Research Articles

Three-point Bending Behavior and Energy Absorption Capacity of Composite Tube Reinforced by Gradient Braided Structure in Radial Direction

This study aims to investigate the radial gradient structural effect on energy absorption and failure behavior of braided composite tubes. Two-layer tubular braided fabrics were fabricated using over-braiding technology with three types of radial gradient configurations in terms of uninform, descending and ascending pattern. The structure of surface layer touching with indenter roll was found to have a significant role on the bending behavior and failure mode. The composite tube possessing the small angle surface layer tends to fail in top-surface mode characterized by compression damage in contact area between indenter roll and tube. With the increasing of the braiding angle in surface layer, the bottom-surface failure mode characterized by penetrating crack in circumferential direction was observed, which accompanies abruptly loading drop and loss of energy absorption capacity. In addition, the tube in descending gradient pattern contributes to higher flexural modulus and peak load due to high fiber volume fraction, while keeps top-surface failure mode because of load spreading ability provided by surface layer with small braiding angle. The results show that a proper selection of stacking sequence and braiding angle in multi-layer braided tube is capable of effectively enhancing the energy absorption of tubal structures under bending load.

IGFEM-based shape sensitivity analysis of the transverse failure of a composite laminate

This manuscript presents a shape sensitivity analysis method based on an Interface-enriched Generalized Finite Element Method (IGFEM) formulation and its application to the sensitivity of the transverse failure of a fiber-reinforced composite laminate with respect to the geometrical parameters that define its microstructure. The analytical sensitivities with respect to individual fiber radius and placement are first derived within the context of a cohesive IGFEM solver specially developed to simulate the fiber/matrix debonding observed in the transverse failure of composite laminates with high fiber volume fraction. The IGFEM solver utilizes $$C^{-1}$$ continuous enrichment functions and a cohesive failure model to capture the transverse cracking associated primarily with fiber/matrix interface debonding. In addition to the sensitivities with respect to individual geometrical parameters such as the radius of individual fibers, the sensitivities of the transverse stress–strain response with respect to the parameters that define the distributions of the geometrical parameters such as the mean and standard deviation of the fiber radius and nearest-neighbor distance distributions are also derived. The sensitivity analysis is performed on realistic microstructures composed of hundreds of fibers to characterize the influence of the geometrical parameters and their distributions on the transverse failure response of the composite laminate.

Effect of fabric architecture, compaction and permeability on through thickness thermoplastic melt impregnation

To reduce the cycle time of structural, automotive thermoplastic composites, we investigated the potential of direct thermoplastic melt impregnation of glass fabrics using an injection moulding process. At the high pressures that occur during the process, the effect of the fabric architecture on the impregnation, compaction, volume fraction and permeability of two unidirectional fabrics was studied. Using impregnation experiments with a low viscosity PA6 melt, we identified a favourable processing window resulting in an impregnation time of 5 min. The impregnation experiments with thermoplastic melts demonstrate that textile architectures promoting dual scale flow during impregnation are favourable for complete filling. Based on our findings, thermoplastic compression resin transfer moulding is an efficient processing route for automated production of composite parts with a high fibre volume fraction, if the fabric architecture is adapted for higher processing pressures and by fully utilising dual scale flow.

Manufacture of carbon-fiber prepreg with thermoplastic/epoxy resin blends and microencapsulated solvent healing agents

Unidirectional carbon-fiber/thermoplastic/epoxy prepreg is manufactured with embedded microcapsules containing a healing agent for a thermoplastic-toughened epoxy matrix. A laboratory-scale prepreg fabrication line was designed to produce a prepreg fabric of which fiber-interstitial spaces accommodate microcapsules of an average diameter of 2.8 µm. Microcapsules containing the healing agent, ethyl phenylacetate (EPA), were coated with polydopamine (PDA) to withstand harsh manufacturing conditions. The prepreg fabrics were laminated and hot-pressed to produce composites of a high fiber volume fraction (ca. 62%), exceptional thermal stability (Tg=170 °C) and fracture toughness. The laminated composite achieved a uniform distribution of intact microcapsules with an overall concentration of 3.1 vol% and a 20 wt% thermoplastic-toughened (poly(bisphenol a-co-epichlorohydrin); PBAE) epoxy matrix developed from cure-induced phase separation. The co-existence of intact microcapsules filled with healing agent and the phase-separated PBAE/epoxy matrix has significant potential to mitigate small scale damage in the composite.

Bilinear elastoplastic constitutive model with polyvinyl alcohol content for strain-hardening cementitious composite

Abstract Strain-hardening cementitious composite (SHCC) exhibits good tensile hardening performance and polyvinyl alcohol (PVA) content has a significant effect on its tensile properties. A bilinear elastoplastic constitutive model with the PVA content is proposed for SHCC and validated by comparison with the experimental data available in the open literature. In order to quantitatively evaluate the PVA content effect, the deformation responses of SHCC pipes with various volume fractions of PVA fiber not larger than 2.00% to the three-edge-bearing load were numerically simulated. It is found that both the ultimate load and the maximum plastic energy of SHCC pipe can be remarkably enhanced by adding PVA fibers. Moreover, compared to the fiber content less than 1%, they are improved more obviously when the fiber content is greater than 1%. Accordingly, higher fiber volume fraction has a better modification effect on the structural performance of SHCC pipe. In addition, the conventional steel reinforcement can be replaced with the PVA fiber reinforcement when the volume fraction of PVA fibers is greater than 1.25%.

Impact Behavior of Polyester GFRP for Naval Applications: Influence of the Clamping Device and Fluid–Material Interaction

The low-velocity impact behavior of polyester glass fiber composite laminates has been investigated under different clamping conditions, by wetting the specimens surface with fluids having different densities. Unidirectional composite laminates, based on polymer resin and fibers of naval interest, with different thicknesses have been manufactured by resin transfer molding process in order to have a high control of the final properties and a high fiber volume fraction. Impact tests in air have been, first, performed up to penetration on all manufactured composites having four different thicknesses to get information on the material impact behavior. The effect of the clamping conditions and the interaction of the laminates with two fluids different in density, i.e., water and paraffin oil, have been investigated, then, only on the thickest laminates that showed a lower tendency to impact damage. In particular, the impact tests have been performed by impacting the front surface of the sample and by wetting the back surface of the composite samples under unclamped and clamped conditions, replicating the boundary constraints suggested by ASTM D7136 Standard for the conventional impact tests. Experimental results showed a lower initial rigidity, higher absorbed energy and deflection for specimens exposed to liquids with respect to those exposed to air, confirming the need of taking into account the fluid interaction in the case of marine composite applications.

Fiber motion in highly confined flows of carbon fiber and non-Newtonian polymer

Inks compounded of short carbon fibers suspended in polymer resin can be extruded to produce composite materials during additive manufacturing or 3D printing processes. The flow process induces anisotropic orientation of the fibers which is set into the matrix and significantly affects the mechanical and physical properties of the final product. Therefore, the flow of fiber suspensions needs to be understood in order to predict the orientation distribution of the fibers during such manufacturing processes. There is still a lack of knowledge for extrusion of the complex mixture of a non-Newtonian polymer containing a high-volume fraction of fibers with high fiber aspect ratio, where both inter-particle, fluid-particle and fiber-wall interactions are computationally evaluated. This paper presents predictive numerical simulations and experimental results of such confined flow in a concentrated regime. The code is based on Lagrange multiplier technique and resolves each particle and interaction between the fibers and surrounding fluid and nozzle walls. We investigate numerically how the fiber length impacts the fiber alignment during extrusion. We found that a fiber length above 67% of the nozzle's diameter induces dramatic change in the fiber flow, causing fibers to concentrate at the nozzle boundaries with very low concentration of fibers in the center of the nozzle. It was found that the best fiber alignment is reached for fiber lengths equal to 40–50% of the nozzle diameters. Numerical findings are supported by experimental results. This work improves understanding of fiber orientation during 3D printing and is an important milestone for the prediction of the complex mechanical properties of additively manufactured fiber composites.

Effect of Composition on the Strength and Electrical Conductivity of Cu-Ti Binary Alloy Wires Fabricated by Aging and Intense Drawing

The strength and electrical conductivity of Cu-Ti alloy wires fabricated by over-aging and intense drawing were investigated as a function of Ti content (2.7 to 4.3 at. pct). The microstructure of all over-aged Cu-Ti alloys before drawing showed mainly coarse cellular components laminating the plates of a terminal Cu solid solution and a β-Cu4Ti intermetallic compound precipitated discontinuously by grain boundary reactions. The volume fraction of β-Cu4Ti plates increased with Ti content, although the compositions of the two phases remained unchanged. When the over-aged alloys were drawn to the same deformation strain, the hardness and tensile strength of the wires increased monotonically with Ti content due to strain-induced strengthening; a high volume fraction of hard β-Cu4Ti fibers formed from laminating plates during drawing promoted a high dislocation density in the matrix. The electrical conductivity of the wires decreased gradually with Ti content due to the higher volume fraction of β-Cu4Ti fibers and due decomposition of the fibers during drawing. The overall performance of the Cu-Ti alloy wires improved as the Ti content increased and was superior to that of conventional Cu-Ti alloy wires fabricated by peak-aging and drawing, and to that of commercial Cu-Be alloy wires.

Experimental investigation of through-thickness resistivity of unidirectional carbon fiber tows

In this study, the influence of type of carbon fiber, sizing amount on the fiber surface and the degree of compaction on the through-thickness electrical resistivity of dry unidirectional carbon fiber tows is investigated to validate the conduction pathways and mechanisms proposed by our previously reported micromechanics electrical resistivity model. An automated experimental setup has been developed and implemented, which measures the electrical resistivity and fiber volume fraction of carbon fiber tows under compression in real time. An extensive experimental study is conducted with five types of commercial PAN-based carbon fibers which vary in fiber diameter, number of fibers in a tow including two unsized fibers and three sized fibers with sizing amount of 0.25% and 1.0% by weight. The fiber volume fraction was increased by compacting the fiber tows using a mechanical testing system (Instron, Norwood, MA). The results show that the fiber sizing and fiber volume fraction impact the through-thickness electrical resistivity of carbon fiber tows. Sized fibers demonstrate 1–2 orders of magnitude higher electrical resistivity than the unsized fibers at lower fiber volume fractions (below 45%), while at higher fiber volume fraction (60%–70%), the electrical resistivity of the two fiber systems tends to be of similar magnitude. Fibers with more sizing (1 wt.%) demonstrated 10 times larger through-thickness resistivity than those with less sizing (0.25 wt.%), indicating the significant impact of fiber sizing on electrical resistivity. The results show good agreement with our micromechanics electrical resistivity model.

Fatigue behavior of short carbon fiber reinforced epoxy composites

This paper intends to contribute to a better understanding of the failure mechanisms on short fibers composites with high fiber volume fractions, for which relatively low values of stiffness and resistance efficiency are reached. For the aforementioned purpose, the current work used composite plates that were manufactured by compression molding and incorporating short carbon fibers reinforcements (0.5 mm in length with 5–20% volume fiber fraction) into an epoxy resin. For low volume fractions, the composites exhibit static mechanical properties nearly proportional with fiber content. However, for high volume fraction lower efficiency was observed for both tensile stiffness and strength. As expected, the strain at failure decreases with the increasing of the fiber content. Fatigue strength follows a similar behavior of static strength, increasing until the fiber content is of 17.5% in volume, but afterwards remains nearly constant. Fatigue life is mainly influenced by fiber dispersion and porosity.

Experimental investigation of the performance of concrete columns strengthened with fiber reinforced concrete jacket

This paper presents an experimental study on the axial compressive behavior of concrete columns retrofitted and confined by fiber-reinforced concrete (FRC) jackets. Three different engineered cementitious composite (ECC) mixes containing high-strength polyethylene fibers at three fiber volume fractions of 0%, 1%, and 2%, and one ultra-high performance cementitious composite (UHP) mix at fiber volume fraction of 4%, were prepared to produce FRC jackets around the core concrete. Horizontal stirrups were used in the FRC jackets with three different horizontal reinforcement ratios to investigate the effect of the level of confinement and the interaction between the conventional reinforcement and fibers. The results show that specimens having no fiber or stirrup in their retrofitting jacket exhibit a brittle behavior and only those containing both stirrup and fiber provide a ductile behavior. It was also observed that the lateral strain of the jacket layer at a given axial strain increased with an increase in the fiber content, indicating an increased rate of dilation for the jacket concrete at a higher fiber volume fraction. It was also found that the best solution to enhance both ductility and strength of columns using FRC jacket is to include a minimum of both fiber content and reinforcement in the jacketing layer. The observations of this study reveal the great potential of the use of FRC jackets in retrofitting of structural members, especially columns in seismic regions.

Dynamic pressure control in VARTM: Rapid fabrication of laminates with high fiber volume fraction and improved dimensional uniformity

The compaction pressure on the fibrous preform is one of the most critical parameters in vacuum assisted resin transfer molding (VARTM), which significantly affects the preform permeability, mold filling time, and final thickness of the fabricated composite. In this study, the compaction pressure on the vacuum bag was controlled during and after the mold filling to achieve rapid impregnation and improve the fiber volume fraction of the laminate. It was shown that the dynamic pressure control (1) enabled the manipulation of the fabric permeability and faster distribution of the resin to decrease the mold filling time, (2) improved the dimensional uniformity of the laminate by reducing the thickness variation, and (3) increased the fiber volume fraction by further consolidating the preform and removing the excess resin. One of the most essential and prominent features of the process was shown to be the resin removal from the inlet by applying external pressure, which reduced the thickness variation in laminates from 15 to 1%. The mold filling time was reduced by 48% compared with conventional VARTM, while achieving a high fiber volume fraction up to 64% and a low void content of below 1%. POLYM. COMPOS., 40:2482–2494, 2019. © 2018 Society of Plastics Engineers

A micromechanics method for transverse creep behavior induced by interface diffusion in unidirectional fiber-reinforced metal matrix composites

In this study, we present a new micromechanics method to predict the transverse creep rate induced by interface diffusion in unidirectional fiber-reinforced composites and evolution of overall creep strain under constant applied stress. An analytical solution for creep rate induced by interfacial diffusion, depending on the applied stress, fiber volume fraction and radius of the fiber, as well as the modulus ratio between the fiber and the matrix, is obtained. A micromechanics model is proposed to estimate the stress field in the fiber, which is related to the driving force for the interface diffusion, the normal stress on the interface. Comparison with finite element analysis shows that the present micromechanics model is of good accuracy, especially for high fiber volume fraction and large elastic modulus ratio between fiber and matrix. With the proposed micromechanics method based on the average field theory, the variation of overall creep strains and stresses with time in fibers under constant external load are analyzed by the incremental creep analysis procedures.

Severely Damaged Reinforced Concrete Circular Columns Repaired by Turned Steel Rebar and High-Performance Concrete Jacketing with Steel or Polymer Fibers

A new strategy that repairs severely damaged reinforced concrete (RC) columns after an earthquake is proposed as a simpler and quicker solution with respect to the strategies currently available in the literature. The external concrete parts are removed from the column surface along the whole plastic hinge region to uncover the steel reinforcement. The transverse steel is cut away, and each longitudinal rebar is locally substituted by steel rebar segments connected by welding connections to the original undamaged rebar pieces outside the intervention zone. The new rebar segments have a reduced diameter achieved by turning to ensure plastic deformation only in the plastic hinge, protecting the original rebar and the welding connections. The connection is specifically designed to be effective and simple, and is directly realized on column reinforcement. Finally, the removed concrete is restored by a jacket built with high-performance concrete with steel or polymer fibers. The use of concrete with high volume fraction of polymer fibers to repair the column is investigated for the first time in this paper. This concrete was characterized by compression and flexural tests in the laboratory and its mechanical characteristics were compared with those of the concrete with steel fibers, which are being increasingly used in construction. The repair strategy was applied to two RC columns (1:6 scaled bridge piers), tested by asymmetric cyclic tests. The results show that the column strength, stiffness, and ductility were restored, and the energy dissipation capacity improved. The experimental evidence was investigated by fiber models in OpenSees.

Effect of steel fibers on the flexural behavior of RC beams with very low reinforcement ratios

This study aims to investigate the implications of hooked-end steel fibers on the flexural performance of reinforced concrete (RC) beams with very low reinforcement ratios. For this, four different fiber volume fractions, vf, of 0.25%, 0.50%, 0.75%, and 1.00%, were incorporated into the concrete mixture and plain concrete without fibers was considered as a control specimen. Four reinforcement ratios of 0.178%, 0.267%, 0.317%, and 0.406%, which are 44%, 66%, 78%, and 100% of the minimum reinforcement ratio, ρmin, were also adopted to evaluate the steel fiber effect on the flexural behavior of RC beams with various very low reinforcement ratios. The test results indicated that the overall flexural performance of RC beams, in terms of flexural strength, deflection capacity, post-cracking flexural stiffness, and cracking behavior, was improved by increasing the reinforcement ratio up to ρmin. Higher initial cracking and yield loads, post-cracking stiffness, and better cracking performance of RC beams were also obtained by including steel fibers. However, the enhancement of ultimate load carrying capacity by steel fibers was relatively minor, and the ductility index and flexural strength margin, used to guarantee a ductile failure mode, deteriorated with the inclusion of steel fibers. The lower reinforcement ratios and higher fiber volume fractions clearly led to lower ductility indices. Therefore, it was concluded that longitudinal steel rebar could not be replaced with discontinuous steel fibers at moderate volume fractions, vf ≤ 1.0%, in terms of ultimate load carrying capacity, ductility, and flexural strength margin. Lastly, analytical results considering material models for steel fiber-reinforced concrete (SFRC), given by the RILEM recommendation, generally overestimated the flexural capacities of reinforced SFRC beams, and the inaccuracy increased with increasing fiber contents.

Thermal bonded cocoon laminates via simple hot-press treatment

The thin silkworm cocoon is potential for producing lightweight and tough engineering applications. In this paper, cocoon laminates were prepared by a simple hot pressing method, which are thermally bonded by its own sericin. After being hot pressed at 130 °C for 30 s, cocoon pieces get tightly compressed with a high fiber volume fraction of 64.4%, leading to tensile strength of the cocoon pieces enhancing from 32 to 111 MPa. Besides, mechanical properties of the cocoon laminate are improved for strong sericin bonding between cocoon pieces by increasing hot pressing time from 30 to 90 s. The resulting cocoon laminate exhibits the maximum stress of 70 MPa at 11.3% strain. The failure ascribes to the debonding rather than the fracture of cocoon overlaps. Importantly, the tensile toughness is better than that of the high-volume-fraction silk fabric-reinforced epoxy composite. Our study may show a bright prospect in developing lightweight and tough cocoon laminates for natural precursor materials.

The mechanical and durability properties of concrete containing hybrid synthetic fibers

In this study, the fresh and hardened properties of concrete including hybridization of polyolefin (PO) and polypropylene (PP) fibers were investigated. The twelve mixtures consisting of different dosages of PO and PP fibers were considered. Mixtures consisted of PO fibers at the volume fractions of 0.5, 1 and 1.5% that some part of those replaced with PP fibers at volume fractions of 0.1% and 0.2%. The properties of mixtures were evaluated using slump and Inverted slump cone tests in fresh state, and using compressive, splitting, and flexural tests in hardened state. In addition, the durability properties were investigated using water absorption, electrical resistivity and chloride penetration tests. The reinforced concrete beams including fibers were made and tested under accelerated corrosion. The influence of fibers on structural performance of sound and corroded beams were assessed by four-point loading test.The results indicated that hybridization of PO with PP fibers can improve the compressive and splitting tensile strength of mixtures up to 7.5% and 23% compared to those of control mixture, respectively. However, the hybridization of these fibers did not show a positive effect on post-cracking behavior at the flexural test. From durability viewpoint, replacement of some part of PO fibers with PP fibers can control the negative effect of using PO macro fibers. However, at the high volume fraction of fibers, the hybridization of PO with PP fibers showed the negative effect. Based on an overall view of results, hybridization of PO and PP fibers at the volume fraction of 0.9% and 0.1%, respectively, can be recommended for practical usage. This combination of fibers decreased the corrosion of steel in RC beams and improved the key parameters of structural behavior such as yield and ultimate loading, and stiffness of beams.

Generation of random fiber distributions for unidirectional fiber‐reinforced composites based on particle swarm optimizer

The stochastic nature of fiber distribution over transversal cross‐section in unidirectional fiber‐reinforced composites has a significant effect on the microscopical distribution of stresses and strains. Towards simulation‐based analysis and design of fiber‐reinforced composite, this paper concentrates on the issue of random fiber distributions and a new approach is developed to effectively generate microstructures of composite material with high fiber volume fraction. The method, named random distribution particle swarm optimization (PSO) algorithm, is proposed based on the particle movement regulation and position updating principle of PSO. The method creates microstructures with different fiber volume fractions conveniently by distributing each single fiber with inter‐fiber distance arrangement. The performance of algorithm is analyzed statistically and the results demonstrate that it is capable of catching the typical distribution of these materials for both low and high fiber volume fractions. Finite‐element analysis is conducted to predict effective properties of the generated microstructures and the simulation results exhibit reasonable agreement with experimental results. The developed algorithm is easily realized and highly efficient without introducing heuristic steps to achieve high volume fraction. The approach provides a valuable alternative to generate random numerical models used in micromechanical analysis of unidirectional fiber‐reinforced composites. POLYM. COMPOS., 40:1643–1653, 2019. © 2018 Society of Plastics Engineers

Experimental and numerical study of the influence of integrated load transmission elements on filling behavior in resin transfer molding

Manufacturing parts with the resin transfer molding (RTM) process enables the inline integration of load transmission elements (e.g. inserts). During the preforming process, the inserts are installed in the dry fabric and, consequently, influence the following mold filling stage of the RTM process because fiber structure in the vicinity of the insert may considerably vary. Particularly, a high fiber volume fraction next to the insert raises the risk of a formation of dry spots close to the embedded insert. In this study, the influence of an insert on the RTM mold filling is analyzed experimentally and numerically. Two different insert geometries are analyzed: a standard insert and an optimized insert. For a detailed analysis, the influence of the local fiber structure around the insert is included in the simulation. This is realized by analyzing CT images of manufactured parts and transferring the fiber structure information to the simulation model.

Comparison of two different methods for producing hollow macrospheres with carbon fiber reinforced composite skin used in syntactic foams

The present study focuses on the preparation and characterization of carbon fiber reinforced hollow epoxy macrospheres (CFR‐HEMs) produced by “Rolling Ball” method and “Electrostatic Fiber Flocking” method when the expanded polystyrene (EPS) beads were served as sacrificial templates. Three‐phase epoxy syntactic foam (ESF) filled with the CFR‐HEMs and hollow glass microspheres (HGMs) were also studied. The CFR‐HEMs made by “Electrostatic Fiber Flocking” method have higher fiber volume fraction, which is 70.1% and the fibers are regularly arranged. By contrast, the CFR‐HEMs made by the “rolling ball” method have a lower average fiber volume fraction, which is 55.5% and the fibers are randomly arranged. The test results show that the ESF filled with HEMs prepared by “Electrostatic Fiber Flocking” method has higher density and compressive property than the ESF filled with HEMs prepared by “Rolling Ball” method POLYM. COMPOS., 40:E1075–E1083, 2019. © 2018 Society of Plastics Engineers