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Accurate characterization of fiber and void volume fractions of natural fiber composites by pyrolysis in a nitrogen atmosphere

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Abstract

This paper proposes a simple and accurate technique to quantify both fiber and void volume fractions of natural fiber composites. This technique involves the separate pyrolysis of natural fibers, polymer resin, and natural fiber/polymer composite in a pure nitrogen atmosphere. The remained fractional residuals of these materials are used for the determination of fiber and matrix weight fractions. Consequently, the void and fiber volume fractions of the natural fiber composite are calculated by having weight fractions, as well as the density of natural fiber, epoxy resin, and composite sample. To validate this new method, a number of silk/epoxy composites at various levels of void content were manufactured. Specific results are given for the random mat silk/epoxy laminates made by wet lay-up vacuum bagging (WLVB) process, as well as plain weave silk/epoxy laminates fabricated by the vacuum assisted resin transfer molding (VARTM) process. Using the proposed method, the void volume fractions of the random mat and plain weave silk composites are calculated to be 9.5% and 1.7%, respectively. The fiber volume fraction of random mat and plain weave laminates are also determined to be 18.7% and 46.3%, respectively. The representative scanning electron microscopy (SEM) images of those laminates are also presented to visually compare the void and fiber contents of the parts. The results demonstrate that the proposed method enables an accurate determination of the fiber and void contents in silk/epoxy laminates and can be extended to composites containing other types of natural fibers.
AIP Conference Proceedings 2205, 020032 (2020); https://doi.org/10.1063/1.5142947 2205, 020032
© 2020 Author(s).
Accurate characterization of fiber and void
volume fractions of natural fiber composites
by pyrolysis in a nitrogen atmosphere
Cite as: AIP Conference Proceedings 2205, 020032 (2020); https://doi.org/10.1063/1.5142947
Published Online: 10 January 2020
Mehrad Amirkhosravi, Maya Pishvar, Youssef K. Hamidi, and M. Cengiz Altan
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Proceedings of the 35th International Conference of the Polymer Processing Society (PPS-35)
AIP Conf. Proc. 2205, 020032-1–020032-5; https://doi.org/10.1063/1.5142947
Published by AIP Publishing. 978-0-7354-1956-8/$30.00
020032-1
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... The density of the fibres (PALF and WTLF) was determined experimentally using a straightforward immersion technique in accordance with the standards specified in ASTM Standard D 1505-18a. 25,26 Initially, the mass of the fibre was measured using a balance or scale and then the initial volume of the liquid was measured in a container. The fibre was immersed in the liquid, ensuring it was fully submerged, and the change in volume of the liquid was observed after 24 h. ...
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Interest in biocomposites is growing worldwide as companies that manufacture high-performance products seek out more sustainable material options. Although there is significant research on biocomposite material options and processing found in the literature from at least the last two decades, there are few experimentally based case studies published to help guide product designers and engineers when considering these materials. This paper discusses the use of biocomposites in the seat of an electric bus. Although it is clear that biocomposite material options are quite limited, the authors eventually settled on three natural reinforcements (cellulose, hemp, flax), two epoxies (one low and the other high viscosity) with high biobased carbon content, and one flax precoated with bioepoxy for consideration. Laminate plates with a 4mm nominal thickness are manufactured using VARTM (low viscosity epoxy only), hand layup as a surrogate for prepregging (high viscosity epoxy only), compression molding, and an out-of-autoclave process called the Pressure Focusing Layer (PFL) method. Permeability of the three reinforcements infused with the high viscosity epoxy and fiber volume fractions are determined experimentally to provide insight into VARTM processing and mechanical performance. The tensile modulus, maximum tensile stress, flexural modulus, and maximum flexural stress are measured for all combinations of reinforcement, resin, and processing using tension testing and three-point bending based on ASTM standards. Basic conclusions are drawn about the specific application and more generally about the process of using biocomposites in commercial products.
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Voids, the most studied type of manufacturing defects, are inevitable in processing of fiber-reinforced composites. Due to their considerable influence on physical and thermomechanical properties of composites, they have been extensively studied, with the focus on three research tracks: void formation, characteristics, and mechanical effects. Investigation of voids in composites started around half a century ago, and is still an active research field in composites community. This is because of remaining unknowns and uncertainties about voids as well as difficulties in their suppression in modern manufacturing techniques like out-of-autoclave curing and parts with high complexity, further complicated by increased viscosity of modified resins. Finally, this is because of the increasing interest in realization of more accurate void rejection limits that would tolerate some voidage. The current study reviews the research on formation, characterization, and mechanical effects of voids, which has been conducted over the past five decades. Investigation and control of void formation, using experimental and modeling approaches, in liquid composite molding as well as in prepreg composite processing are surveyed. Techniques for void characterization with their advantages and disadvantages are described. Finally, the effect of voids on a broad range of mechanical properties, including inter-laminar shear, tensile, compressive, and flexural strength as well as fracture toughness and fatigue life, is appraised. Both experimental and simulation approaches and results, concerning void effects, are reviewed.
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With growing environmental awareness, natural fibers have recently received significant interest as reinforcement in polymer composites. Among natural fibers, silk can potentially be a natural alternative to glass fibers, as it possesses comparable specific mechanical properties. In order to investigate the processability and properties of silk reinforced composites, vacuum assisted resin transfer molding (VARTM) was used to manufacture composite laminates reinforced with woven silk preforms. Specific mechanical properties of silk/epoxy laminates were found to be anisotropic and comparable to those of glass/epoxy. Silk composites even exhibited a 23% improvement of specific flexural strength along the principal weave direction over the glass/epoxy laminate. Applying 300 kPa external pressure after resin infusion was found to improve the silk/epoxy interface, leading to a discernible increase in breaking energy and interlaminar shear strength. Moreover, the effect of fabric moisture on the laminate properties was investigated. Unlike glass mats, silk fabric was found to be prone to moisture absorption from the environment. Moisture presence in silk fabric prior to laminate fabrication yielded slower fill times and reduced mechanical properties. On average, 10% fabric moisture induced a 25% and 20% reduction in specific flexural strength and modulus, respectively.
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Voids are the most common process-induced defects in composite laminates fabricated by vacuum assisted resin transfer molding (VARTM). Reduction or total elimination of these defects is essential for the improved performance and long-term durability of the structural composites. This study introduces a novel method that reduces the void content in VARTM laminates to below 1% by compacting the fibrous mat before infusion. The compaction is achieved by applying magnetic pressure on the vacuum bag by either stationary or moving magnets which are removed before the resin infusion. To assess the effectiveness of the proposed method, 6-, 12-, and 18-ply random mat glass/epoxy laminates are fabricated by VARTM using compacted and uncompacted mats and their properties are compared. In addition, different sets of magnets are used to investigate the effect of compaction levels on the resin flow and the quality of the final part. The placement of stationary magnets on the entire vacuum bag surface is practical for fabrication of small parts. For medium to large parts, however, magnets with a smaller footprint can be moved to apply the compaction pressure over a larger vacuum bag surface. The results show that by applying compaction pressure of 0.2 MPa or higher either by stationary or moving magnets on the dry preforms, the void volume fraction was decreased by 65%–95% to 0.1%–0.8% in all laminates.
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The aim of this work is to study the effect of kenaf volume content and fiber orientation on tensile and flexural properties of kenaf/Kevlar hybrid composites. Hybrid composites were prepared by laminating aramid fabric (Kevlar 29) with kenaf in three orientations (woven, 0o/90o cross ply uni-directional (UD), and non-woven mat) with different kenaf fiber loadings from 15 to 20% and total fiber loading (Kenaf and Kevlar) of 27–49%. The void content varies between 11.5–37.7% to laminate with UD and non-woven mat, respectively. The void content in a woven kenaf structure is 16.2%. Tensile and flexural properties of kenaf/Kevlar hybrid composites were evaluated. Results indicate that UD kenaf fibers reinforced composites display better tensile and flexural properties as compared to woven and non-woven mat reinforced hybrid composites. It is also noticed that increasing volume fraction of kenaf fiber in hybrid composites reduces tensile and flexural properties. Tensile fracture of hybrid composites was morphologically analysed by scanning electron microscopy (SEM). SEM micrographs of Kevlar composite failed in two major modes; fiber fracture by the typical splitting process along with, extensive longitudinal matrix and interfacial shear fracture. UD kenaf structure observed a good interlayer bonding and low matrix cracking/debonding. Damage in composite with woven kenaf shows weak kenaf-matrix bonding. Composite with kenaf mat contains the high void in laminates and poor interfacial bonding. These results motivate us to further study the potential of using kenaf in woven and UD structure in hybrid composites to improve the ballistic application, for example, vehicle spall-liner. POLYM. COMPOS., 2014. © 2014 Society of Plastics Engineers
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In this paper, the vibration behaviour and damping performances of Flax Fiber Reinforced Epoxy (FFRE) are investigated. Static tests using FFRE composite beams are carried out leading to the identification of elastic properties of each layer. Then, three FFRE composite plates are elaborated and used in experimental vibration tests to identify their eigenfrequencies and their modal damping. In numerical part, a constant complex representation of the stiffness is assumed and the loss factors are supposed constants and identified from the first experimental vibration mode. The homogenization technique and the finite element method are applied to describe their damped dynamic behaviour. The resolution of the resulting non-linear problem is carried out using the Asymptotic Numerical Method (ANM). Experimental results show that the modal damping is greater when the flax fibers are oriented at 90°. Comparing numerical and experimental results, the proposed finite element modelling enables to estimate the damped eigenfrequencies with high accuracy. However, the predicted modal loss factors are over-estimated compared to experimental ones. It is concluded that the present modelling should be improved considering the frequency dependence of damping.
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Achieving a complete curing of biocomposites without damaging fibers is very challenging. This study assesses the impact of processing upon the mechanical properties of a unidirectional flax reinforced epoxy composite and identifies which component (resin, reinforcement or interphase) is the most sensitive to post-curing at high temperature (100, 120 or 150 °C). Post-curing temperature had a limited impact on the composite mechanical behavior excepted at 150 °C where ultimate stress and strain decreased drastically while the stabilized modulus slightly increases. Post curing is responsible of a slight decrease of the matrix tensile properties attributed to the polymer oxidation but cannot explain on its own the evolution of the composite behavior. Interfacial adhesion played a minor role in the composite behavior probably due to its intrinsic weakness. Finally, the flax fabric was highlighted to be the component most sensitive to thermal treatment thus governing the drop in the composite mechanical properties.
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Detailed analyses of shape, size, and spatial variations of void morphology are presented for a disk-shaped, resin transfer molded (RTM), E-glass/epoxy composite. The disk is molded at constant injection rate and contains 17.5% E-glass random fiber mats. Voids throughout the composite are evaluated by microscopic image analysis of through-the-thickness and planar surfaces obtained from adjacent radial samples. The void content of 2.15% is obtained from the analysis of through-the-thickness images and believed to be representative of the actual void content in the studied part. Relatively large cylindrical voids are observed in cigar shapes in the planar surfaces, whereas these voids only appear as small irregular or elliptical voids on through-the-thickness surfaces. Along the radial direction, combined effects of void formation by mechanical entrapment and void mobility are shown to yield a complex radial void distribution. It is shown that fewer voids are trapped mechanically with increasing distance from the inlet and most of the medium and small voids that are mobile migrate towards the exit during resin injection. These findings are believed to be applicable not only to RTM, but generally to liquid composite molding processes with varying fluid front velocities.
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This paper describes the moisture absorption behavior and its effect on mechanical properties of Lantana camara fiber reinforced epoxy composite. Composite samples reinforced with different wt% of fibers were prepared by hand lay-up technique. Increase in tensile and flexural strength was observed with increase in fiber content up to 30%. Moisture absorption tests were carried out in three different environmental conditions (steam, saline water and sub-zero temperature). The moisture diffusivity constant and equilibrium moisture uptake were calculated. Moisture absorption of the studied composites was proved to follow the kinetics of a Fickian diffusion process. Tensile and flexural strength of composite were found to decrease with moisture absorption. After the mechanical tests, fracture characteristics analysis of the tested specimens was carried out to reveal a reasonable interaction between the reinforcement and matrix.