Low Fiber Volume Fraction Research Articles

Quasi-Static Penetration Properties of 3D-Printed Composite Plates.

This work investigated the impact and piercing load resistance (energy absorption capabilities) of 3D-printed composites plates manufactured by means of the Fused-Filament-Fabrication (FFF) technique. Two sets of reinforced composite plates were produced. The first set of plates was printed with short-carbon-fiber-reinforced polyamide-12, while the second set was reinforced with continuous fibers. The plates were tested with quasi-static indentation tests at various Span-to-Punch ratios and with three different indenter nose shapes (blunt, hemispherical, and conical). The quasi-static measurements were subsequently elaborated to estimate the energy absorption capability of the plates during a ballistic impact. The addition of continuous fibers increased the quasi-static energy absorption capability by 20-185% with respect to the short-fiber-reinforced plates. The quasi-static results showed that by including the continuous reinforcement in the plates, the normalized energy absorbed increased by an order of magnitude. Finally, a comparison with data from the literature concerning continuous-reinforced composite plates manufactured by means of traditional techniques was carried out. The comparison revealed that FFF-printed composite plates can compete with traditional composite ones in terms of both ballistic and quasi-static penetrating load conditions, even if limited by the lower fiber volume fraction. Thus, these findings confirm that this novel Additive Manufacturing technique is promising and worth investigating further.

Reinforcement Effects on Tensile Behavior of Ultra-High-Performance Concrete (UHPC) with Low Steel Fiber Volume Fractions.

Ultra-high-performance concrete (UHPC) with a low steel fiber volume fraction offers lower material costs than UHPC with typical steel fiber volume fractions, and has the potential to mitigate the ductility degradation of rebar-reinforced UHPC (R-UHPC). This study explores the reinforcement effect on the tensile behavior of UHPC with a low fiber volume fraction with the aim of facilitating more cost-efficient UHPC applications. The axial tensile behavior of 30 UHPC specimens with low fiber volume fractions at different reinforcement ratios was tested through direct tensile tests. The findings indicate that adopting UHPC with a low fiber volume fraction can significantly mitigate the ductility deterioration of rebar-reinforced UHPC (R-UHPC), and both increasing the reinforcement ratio and decreasing the fiber volume fraction contribute to the improvement in ductility. The failure modes of R-UHPC are determined by the ratio of reinforcement ratio and fiber volume fraction, rather than a single parameter, which also means that R-UHPC with different parameters may correspond to different methods to predict tensile load-bearing capacity. For UHPC with a fiber volume fraction low to 0.5%, incorporating steel rebars gives superior multi-crack cracking behavior and excellent capacity to restrict the maximum crack width. Increasing the fiber volume fraction from 0.5% to 1.0% at the same reinforcement ratio will yield little benefit other than an increase in tensile load-bearing capacity.

Recent studies on the mechanical properties of Engineered Cementitious Composite

Engineered Cementitious Composite (ECC) is a type of High-Performance Fiber-Reinforced Cementitious Composite (HPFRCC) that has emerged as a promising material for use in construction and engineering. ECC is designed based on micromechanical theory, resulting in a remarkable tensile strain capacity of over 6%, while maintaining a low fiber volume fraction of no more than 2%. The material’s high flexibility enables it to be molded into various shapes, and it exhibits excellent mechanical properties, making it suitable for field applications such as the construction of joint-less deck slabs, link slabs, and retrofitting of critical structures like dams and tunnels. However, the accurate characterization of ECC’s properties requires the use of appropriate testing methods, particularly regarding its mechanical characteristics, including compressive strength, flexural strength, and direct and indirect tensile strength. This review highlights the significance of understanding ECC’s mechanical properties and emphasizes the importance of using appropriate testing techniques to evaluate the material’s properties and structural behavior.

Investigation of shear response due to variation of fiber volume fraction, transverse steel, and placement method on short span R/UHPC beams

This study investigates the response of small-scale, reinforced ultra high performance concrete beams. Seven specimens were cast and experimentally tested. Steel fiber volume fraction, amount of longitudinal and transverse steel reinforcement, and casting direction varied between specimens. High longitudinal reinforcement ratios of 4.1% and 9.5% were expected to drive relatively large shear demands and low steel fiber volume fractions of 0.5% and 1% were expected to decrease the UHPC’s ability to resist tension, shear, and compression. Specimens were either cast at one end or mid-span. All specimens failed in shear, as expected. R/UHPC beams with only 0.5% fibers and a stirrup spacing equal to half the beam’s depth carried more ultimate load than beams with 1% fibers and twice the stirrup spacing. At a 0.5% fiber volume fraction, UHPC performed well, exhibiting fiber bridging in tension and resisting spalling in compression. As longitudinal reinforcement ratio increased from 4.1% to 9.5%, load carrying capacity generally increased, but not proportional to the increase in steel. Placement method did not influence fiber orientation in the R/UHPC beams when measured in 2D planes transverse or longitudinal to the specimens’ major axis. The longitudinal and transverse reinforcement interrupted fiber flow and prevented significant fiber alignment; combined with the short span of the beams, these factors mitigated the influence of placement method.

Influence of Epoxy Spray Binder on Infusion and Cure in Liquid Composite Molding Processes

Binders are known to influence the different steps in liquid composite molding process chains. Most reseach focuses on powder binder and veils. Spray binders are rarely studied. Therefore, the effect of an epoxy spray binder on the infusion and cure steps of liquid composite molding processes is studied in this paper. Permeability measurements, solubility tests and measurements of glass transition temperature, resin cure time and resin viscosity show a complex interaction between binder and resin depending on the process conditions. The binder mostly increased the permeability of the tested preforms. This effect increases with increasing binder loading. At low fiber volume fractions the binder had a positive effect on preform permeability, most likely by to delaying the closure of macro flow channels or textile inhomogeneity. The binder did not dissolve in the resin until temperatures of around 120 °C, while remaining as a separate phase at 22 °C, 40 and 80 °C. This has to be considered when defining the cure profile of the resin, as the binder is expected to participate in the cure reaction. The glass transition temperature remained unchanged for the binder-resin combination used. An increase in viscosity by a factor of 1.5-2 was observed when the binder was dissolved in the resin. Samples with dissolved binder cured slightly faster than pure resin. The experimental results were theoretically transferred to real infusion processes.

Optimizing the thermal properties of fiber reinforced phthalonitrile composites

AbstractThe development of lightweight composites with desirable thermo‐mechanical properties is progressively increasing. Phthalonitrile (PN) based composites have shown great potential in this regard. However, the basic thermal properties of PN composites required for engineering design are not yet fully understood. In this work, we investigated the thermal stability, thermal expansion behavior and thermal conductivity of PN composites reinforced with carbon fiber (CF) and high silicon fiberglass (HSF) via combined experimental studies and numerical simulations. The results indicated that CF/PN performs better in thermostability than HSF/PN at temperatures below 500°C. Moreover, the incorporation of CF and HSF lowered the coefficient of thermal expansion (CTE) and thermal insulation of PN. At room temperature, the in‐plane CTE of CF/PN and HSF/PN were 1.97E‐6 and 9.24E‐6°C−1, respectively, while the out‐plane thermal conductivities of CF/PN and HSF/PN were 0.65 and 0.34 W/(m K). It is worth noting that an excessively high fiber volume fraction would lead to poor thermal insulation and lightweight properties of the PN composites, while a low fiber volume fraction would result in poor stiffness and thermal dimensional stability. Determined via TOPSIS model, a fiber volume fraction range of 50%–60% was ideal for both composites with comprehensive optimal properties, respectively.

Strain hardening magnesium-silicate-hydrate composites with extremely low fiber dosage of 0.5% by volume

Formulation of strain hardening cementitious composites typically engage 2% or more fiber by volume, resulting in higher cost and difficult processing. This study presents the development of strain hardening magnesium-silicate-hydrate composite with an extremely low fiber volume fraction of 0.5% via micromechanics-guided design approach. The developed composite demonstrated a tensile strain capacity of 7.2% with a tensile strength of 2.24 MPa, and a compressive strength of 86.1 MPa. The fiber/matrix interfacial bond was characterized using single fiber pullout test. The microstructural characterization of fiber surface and fiber tunnel in the matrix was carried out to understand the fiber/matrix interface properties. The micromechanics-based assessment of critical fiber volume fraction required to achieve strain hardening was also conducted. The material sustainability of the developed composite was evaluated and compared with existing Portland cement-based strain hardening cementitious composites, and strategies to further reduce embodied carbon and primary energy were proposed.

Design and fabrication of high-performance 3D printed continuous flax fibre/PLA composites

The mechanical properties of 3D-printed continuous plant fibre-reinforced poly(lactic) acid (PLA) composites are insufficient due to the low fibre volume fraction (∼30 %). Further increase of fibre volume fraction of composites by reducing the line width may result in a composite with low fibre impregnation degree or the clogging of printer nozzle because of the twisted nature and uneven diameter of plant yarns, which also degrades the mechanical properties. This study proposed a method to obtain 3D printed continuous flax fibre reinforced composites (CFFRCs) with a high fibre volume fraction and excellent mechanical performance by statistical analysis of the dimensions of flax fibre yarns as well as surface modification. The CFFRCs were printed by a fibre prepreg-based 3D printing method, and an in-situ impregnation method was also used to prepare CFFRCs for comparison. The results showed that the CFFRCs by the fibre prepreg method had a fibre volume fraction of 44.1 % and a void content of ∼1.9 % due to the high impregnation degree. Correspondingly, the tensile modulus and strength of the CFFRCs were the highest among the existing reported values of the composites made by 3D printing and even greater than those of the CFFRCs with a fibre volume fraction of ∼51.4 % prepared by the in-situ impregnation method.

Analysis of the transverse compressive behavior of cotton yarns and fabrics

The inherent complexity of textile preforms and a high degree of consistency in dry fabrics and yarns present a number of modeling challenges, including uncertainty in geometrical characteristics under external loads, non-elastic deformations of fibrous media, and multiple deformation modes at the fiber and yarn scales. The prediction of the compaction reaction for different preforms is still possible thanks to the direct measurements of yarn compaction. The data are checked against compression stress-thickness curves that were produced by compacting yarns and preforms. The yarn and fabric compaction model given in this article demonstrates the final characteristics of woven preforms. The yarn compaction curve exhibits asymptotic hardening with a restricted compaction state attained at high compression stress and as the limit deformations are being approached. The yarn count, yarn fiber volume ratio, and the spinning method all affected the compression modulus. The transverse compression behaviour of yarns and fabrics was studied both analytically and experimentally. Mechanisms of fabric compressibility have been found to be reliant on both fabric and yarn specifications of warp and weft Young’s modulus. Investigations were made into the fabric’s fiber volume fraction under compression stress. When producing composite with low fiber volume fraction preforms, it may be more efficient to use more compression, according to research on the link between compressive stress and fabric volume fraction. The relationship between the compressive stress and fabric volume fraction was investigated, as well as the value of maximum compression stress.

Enhancing the mechanical performance of carbon fiber reinforced polymer using carbonized coconut shell particles

Inorganic fillers have been used to improve the out-of-plane mechanical properties of carbon fiber reinforced polymer (CFRP) composite laminates for decades. Nonetheless, its associated high cost and environmental unfriendliness is a concern. Biomaterials are currently being explored as fillers in polymeric materials due to their low cost, wide availability, and biodegradability. However, the use of coconut shell based biofillers together with carbon fibers in epoxy matrix has not been investigated. This research seeks to improve the out-of-plane mechanical properties of CFRP with low fiber volume fraction using carbonized coconut shell particles (CCSP). Five hybrid epoxy biocomposites with varying concentrations of CCSP were used to impregnate four plies of woven carbon fabric, making up a fiber volume fraction of 29%. The tensile, flexural, and impact behavior of the laminated biocomposites were investigated. The mechanical properties of the biocomposite laminates were enhanced compared to a reference CFRP without CCSP. This work provides a cheaper and greener alternative to inorganic CFRP hybrid composite for potential use in automotive and aerospace industries.

Flexural creep behavior of hierarchical bamboo structure using time-temperature-stress superposition principle

The long-term flexural creep properties of graded hierarchical bamboo are important to structural design in civil construction. The present study conducted the short-term flexural creep tests at a series of temperature (30–170 °C) and stress (7–15 MPa) conditions, for the bamboo from both inner and outer parts. The application of time-temperature superposition principle (TTSP) and time-temperature-stress superposition principle (TTSSP) was analyzed for the creep behavior of bamboo with regard to hierarchical structure. The results confirmed the feasibility of TTSP and TTSSP in flexural creep of bamboo by horizontal shifting. A creep model based on Findley power law was successfully employed to predict the flexural creep response of bamboo. Additionally, the sample from inner part of bamboo displayed a more pronounced flexible creep response compared with the sample from outer part, attributing a low fiber volume fraction in inner part due to bamboo growth strategy. The findings in this study broaden the knowledge of bamboo rheology, and is helpful for further work on the bamboo growth strategy effect on mechanical properties.

Microbuckling behavior of unidirectional fiber-reinforced shape memory polymer composite undergoing compressive deformation

The microbuckling mechanics of unidirectional fiber-reinforced shape memory polymer composite (SMPC) with low fiber volume fraction were investigated, and the mechanical models were formulated considering the attenuation of shear strain in the resin matrix near the buckled fibers. We deduced the analytical expression of the attenuation function and the key parameters during the microbuckling process of SMPC, including the critical buckling wavelength and strain. The values determined by finite element analysis verified the accuracy of the above theoretical predictions. The nonlinear stress–strain relationship during the post-buckling process was also investigated. Additionally, the classical elastic–viscoelastic correspondence principle was established to determine the dynamic buckling behaviors of SMPC at different temperatures. The viscoelastic parameters of shape memory polymer (SMP) were obtained from the isothermal stress relaxation experiments, and then the variation of buckling wavelength with time at different temperatures was evaluated.

Resin Film Infusion Process: Numerical Algorithm

The liquid composite molding (LCM) belongs to the composite manufacturing processes. In this family, a fabric preform material is placed into the mold cavity, and then it is impregnated with a thermosetting resin of low viscosity, until the fiber skeleton is entirely filled and finally polymerized to create a polymeric composite product. Due to its advantages, LCM has gained attention and competitiveness against other composite manufacturing processes. The resin film infusion (RFI) belongs to the LCM family, but unlike the other variants, such as resin transfer molding (RTM) and vacuum assisted resin infusion (VARI), in which the liquid resin is injected or infused into the mold cavity, the resin in the RFI process is placed into the mold cavity in the semi-cured state. Then, under pressure and temperature, the resin film will be liquefied and impregnated the fibrous reinforcement in the thickness direction. This particularity permits to RFI to fabricate large complex composite structures and reduce significantly the equipment cost as compared to the conventional resin transfer molding processes. However, as this variant used only a vacuum bag as the upper half-mold, the fabricated part has non-uniformity in the thickness, low dimensional tolerances and low fiber volume fraction. The main objective of this paper is to propose a numerical algorithm allowing to study the influence of part thickness on the RFI’ filling time. Numerical simulation is based on the explicit finite difference method. The results obtained show that the filling time increases parabolically with the part thickness.

Durability evaluation of GFRP rebars in harsh alkaline environment using optimized tree-based random forest model

GFRP bars reinforced in submerged or moist seawater and ocean concrete is subjected to highly alkaline conditions. While investigating the durability of GFRP bars in alkaline environment, the effect of surrounding temperature and conditioning duration on tensile strength retention (TSR) of GFRP bars is well investigated with laboratory aging of GFRP bars. However, the role of variable bar size and volume fraction of fiber have been poorly investigated. Additionally, various structural codes recommend the use of an additional environmental reduction factor to accurately reflect the long-term performance of GFRP bars in harsh environments. This study presents the development of Random Forest (RF) regression model to predict the TSR of laboratory conditioned bars in alkaline environment based on a reliable database comprising 772 tested specimens. RF model was optimized, trained, and validated using variety of statistical checks available in the literature. The developed RF model was used for the sensitivity and parametric analysis. Moreover, the formulated RF model was used for studying the long-term performance of GFRP rebars in the alkaline concrete environment. The sensitivity analysis exhibited that temperature and pH are among the most influential attributes in TSR, followed by volume fraction of fibers, duration of conditioning, and diameter of the bars, respectively. The bars with larger diameter and high-volume fraction of fibers are less susceptible to degradation in contrast to the small diameter bars and relatively low fiber's volume fraction. Also, the long-term performance revealed that the existing recommendations by various codes regarding environmental reduction factors are conservative and therefore needs revision accordingly.

Characterization of radial bamboo slivers from different height positions and their effect on the mechanical properties of bamboo-wood composite container flooring

ABSTRACT Bamboo-wood composite container flooring (BWCCF), as a low-cost, sustainable, and eco-friendly product, plays an increasing role in the container flooring industry. In this paper, the structural characteristics of radial bamboo slivers (RBS) from different locations of bamboo culms were studied. Furthermore, the mechanical performance and failure mechanisms of 17-layer BWCCF, made of radial bamboo sliver curtains from the top (top-CF), middle (middle-CF), and bottom (bottom-CF) sections of bamboo culms, were evaluated. It was found that the geometry of the top-RBS was more irregular than the middle and bottom-RBS resulting in more structural defects of the top-CF and higher standard deviation of its mechanical properties. Except for the MOE, the top-CF is 7.0% lower than the middle-CF, there is a small difference between the strengths of the top and middle-CF. Since lower fiber volume fraction and larger bamboo node numbers of bottom-RBS, the density, concentrated load, modulus of elasticity (MOE), modulus of rupture (MOR), and shear strength parallel to grain (fs||) of the middle-CF are 6.3%, 3.3%, 7.3%, 14.7%, and 3.1%, respectively higher than those of bottom-CF. The failure modes of concentrated load for the top, middle, and bottom-CF were different mainly because the number of RBS included in corresponding RBSC was different.

Thermo-elastic properties in short fibre reinforced ultra-high temperature ceramic matrix composites: Characterisation and numerical assessment

The thermo-elastic properties of a novel class of ceramic composites based on an ultra-refractory matrix and containing short carbon fibre to provide failure tolerance and thermal shock resistance were experimentally characterised and numerically simulated. The composites samples were fabricated with fibre volume fractions ranging from 5 to 60 vol% and were featured by homogeneous dispersion of fibre along the basal plane due to the processing route. A random sequential adsorption algorithm was implemented to generate Representative Volume Elements (RVEs), following which finite element (FE) analyses were conducted to determine the elastic modulus and the coefficient of thermal expansion of the composites . Comparison of the experimental and modelling results showed good agreement for the modulus at low fibre volume fractions, and for CTE at higher volume fractions. Discrepancies are attributed to significant evolution of the composite microstructure upon processing at high temperatures, as revealed by microscopic analysis. The observations emphasise the need to account for the effects of microstructural changes on the constituent properties in the modelling of the thermo-mechanical properties in sintered ceramic composites.

3D printing of continuous carbon fiber reinforced thermoset composites using UV curable resin

AbstractAdditive manufacturing is trending toward functional applications beyond the scope of rapid prototyping. This requires the development of high‐performance, lightweight, and complex structures resulting from additive manufacturing. One of the biggest challenges for fulfilling these requirements is the limitation of material properties. The introduction of reinforcement allows for the expansion of current polymer‐based 3D printing technologies to achieve increased material properties while maintaining complex geometries. Commercial 3D printers able to print thermoplastic composites have recently been demonstrated with good repeatability. However, limitations of these thermoplastic composites include low fiber volume fraction and susceptibility to high temperatures. In contrast, thermoset‐based composites offer greater thermal stability and higher mechanical performance due to the cross‐linked structure of the polymer. Moreover, using continuous reinforcement instead of short fiber reinforcement offers higher performance opportunities. Therefore, the challenge of 3D printing with continuous fiber‐reinforced thermoset composite is addressed in this study. A 3D printer capable of printing a UV curable resin system reinforced with continuous carbon fiber was constructed. Variations of printed composites' mechanical properties depending on the fiber tow characteristic were investigated. Two different carbon fiber types and their associated tow twist count and sizing were evaluated in this study. Burn‐off tests were used to determine the printed composites' final fiber volume fraction. Using a rule of mixtures approach, strength and modulus were predicted for each composite and compared against their experimental values. Predicted modulus values were found within 2%–5% of experimental, while strength values were significantly less than predicted.

Validation of a finite element method for simulation of components produced by continuous carbon fiber reinforced additive manufacturing

In this research, a method is examined by which the behavior of continuous carbon fiber reinforced additive manufacturing may be simulated using Finite Element Analysis. This technique is used in a simulated tensile test experiment in which the findings are compared to results determined from theoretical calculations according to the Rule of Mixtures method and from existing mechanical testing results. Four different fiber reinforcement configurations are examined with fiber volume fractions ranging from 4% to 32%. It was found that for fiber volume fractions of 11%, the simulation results closely match those predicted theoretically by the Rule of Mixtures as well as the mechanical testing results published in existing research. Lower fiber volume fractions near 4% yield less accurate results, with a 20% error due to the fact that the anisotropic behavior of the polymer matrix is the dominant material trait. Simulation of higher volume fractions near 32% closely approximate theoretical predictions, however neither the theoretical results nor the simulation results accurately reflect real world mechanical testing, indicating that nonideal condition factors such as the effect of micro-voids between the start and end of the fiber reinforcements play a significant role in the overall strength of the material. Thus, for fiber volume fractions near 11%, this simulation method can accurately be used to predict the behavior of end-use components, but more study must be done to increase simulation accuracy in low and high fiber volume fractions.

In situ impregnation of continuous thermoplastic composite prepreg for additive manufacturing and automated fiber placement

Components made from thermoplastic composites (TPC) by additive manufacturing (AM) and automated fiber placement (AFP) are gaining popularity because of the material system’s inherent advantages over thermosets, but high material costs and limited production flexibility are hindering more widespread use. This paper discusses prototyping, fluid flow modeling, and testing of a novel in situ preimpregnator (prepregger) for lower-cost TPC feedstock used in AM and AFP. Prior attempts by researchers to address this technical need have generally resulted in low-quality prepreg with low fiber volume fraction (Vf) and high void content, which can be explained by Darcy’s law applied to the highly viscous polymer melts. Uniformly spreading the fiber tow thin before impregnation is the key to rapid and complete impregnation. Based on initial experiments with a flexible experimental setup to quickly test concepts, a prototype prepregger consisting of dry fiber payout reel with slight torsional resistance, fixed pin fiber spreader, double slot die coater fed with polymer melt from a microextruder, consolidator with pultrusion exit die, and prepreg downstream drive mechanism was designed and built. Predictions from a steady-state flow model of the coating die/microextruder system assuming creeping flow and Newtonian fluid behavior agreed well with experimental flow measurements for a variety of polymer melts and operating conditions. Once ideal input parameters were determined, prepreg tape specimens exhibited complete impregnation but low Vf and excessive encapsulation due to insufficient consolidation and removal of excess resin. Experiments where pultrusion dies were used for filament manufacturing showed significantly improved Vf and prepreg quality. Future work is suggested to facilitate commercialization of this promising process technology.

Elastic/viscoplastic characterization of additively manufactured composite based on continuous carbon fibers

Additive manufacturing (AM) of continuous carbon fiber thermoplastic matrix composites (CFRPs) allows for producing bowtie geometries that are more appropriate to probe the mechanical behavior of these composites. The quasistatic thermomechanical behaviors of different 3D printed composites laminae and laminate were investigated at temperatures below the glass transition of the thermoplastic matrix. The influence of the fiber orientation revealed weak performance of off-axis samples compared to the on-axis lamina and multidirectional laminate. Fractography attributed this degradation to the low fiber volume fraction of carbon fibers; 20-21%, and the inherent defects of the fused filament fabrication (FFF) process. The degradation was most significant along the direction transverse to the loading direction where the stiffness and strength dropped by 44% and 35%, respectively at 90°C compared to the room temperature values. The viscoplastic behavior of the 3D printed composites were probed via series of load relaxation and creep tests at different thermal environments. The results of the experimental tests were utilized to both construct and validate a phenomenological viscoplastic constitutive model of the 3D printed FRPs. The experiments and the model predictions established that 3D printed composites with off-axis fiber orientation exhibit significant stress relaxation and creep deformation. The results also emphasized that the prediction of the viscoplastic model is less accurate for the off-axis AM composite configuration.