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A special issue of Journal of Composites Science (ISSN 2504-477X). This special issue belongs to the section "Fiber Composites".

Deadline for manuscript submissions: closed (30 June 2018) | Viewed by 68942

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Prof. Dr. Tim A. Osswald

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Department of Mechanical Engineering, University of Wisconsin Madison, Madison, WI 53706, USA
Interests: short fiber composites; long fiber composites; fiber orientation, fiber attrition; failure criteria
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Special Issue Information

Dear Colleagues,

Discontinuous fiber-reinforced polymers have gained importance in the transportation industries due to their outstanding material properties, lower manufacturing costs and superior lightweight characteristics. One of the most attractive attributes of discontinuous fiber reinforced composites is the ease with which they can be manufactured in large numbers, using injection and compression molding processes.

Typical processes involving discontinuous fiber reinforced thermoplastic composite materials include injection and compression molding processes as well as extrusion. Furthermore, the automotive and appliance industries also use thermosets reinforced with chopped fibers in the form of sheet molding compound and bulk molding compound, for compression and injection-compression molding processes, respectively.

A big disadvantage of discontinuous fiber composites is that the configuration of the reinforcing fibers is significantly changed throughout production process, reflected in the form of fiber attrition, excessive fiber orientation, fiber jamming and fiber matrix separation. This process-induced variation of the microstructural fiber properties within the molded part introduces heterogeneity and anisotropies to the mechanical properties, which can limit the potential of discontinuous fiber reinforced composites for lightweight applications.

The main aim of this Special Issue is to collect various investigations focused on the processing of discontinuous fiber reinforced composites and the effect processing has on fiber orientation, fiber length and fiber density distributions throughout the final part. Papers presenting investigations on the effect fiber configurations have on the mechanical properties of the final composite products and materials are welcome in the Special Issue. Researchers who are modeling and simulating processes involving discontinuous fiber composites as well as those performing experimental studies involving these composites are welcomed to submit papers. Authors are encouraged to present new models, constitutive laws and measuring and monitoring techniques to provide a complete framework on these groundbreaking materials and facilitate their use in different engineering applications.

Prof. Dr. Tim A. Osswald
Guest Editor

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Keywords

discontinuous fibers
chopped fibers
short fiber reinforced thermoplastics (SFT)
long fiber reinforced thermoplastics (LFT)
sheet molding compound (SMC)
bulk Molding Compound (BMC)
fiber orientation distributions
fiber length distributions
fiber density distributions
fiber attrition
micro computed tomography
compression molding
injection molding
compounding

Published Papers (13 papers)

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Editorial

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3 pages, 168 KiB

Open AccessEditorial

Editorial for the Special Issue on Discontinuous Fiber Composites

by Tim A. Osswald

J. Compos. Sci. 2018, 2(4), 63; https://doi.org/10.3390/jcs2040063 - 23 Oct 2018

Viewed by 2160

Abstract

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 [...] Read more.

(This article belongs to the Special Issue Discontinuous Fiber Composites)

Research

Jump to: Editorial

13 pages, 25739 KiB

Open AccessArticle

Prediction of Young’s Modulus for Injection Molded Long Fiber Reinforced Thermoplastics

by Hongyu Chen and Donald G. Baird

J. Compos. Sci. 2018, 2(3), 47; https://doi.org/10.3390/jcs2030047 - 06 Aug 2018

Cited by 12 | Viewed by 3805

Abstract

In this article, the elastic properties of long-fiber injection-molded thermoplastics (LFTs) are investigated by micro-mechanical approaches including the Halpin-Tsai (HT) model and the Mori-Tanaka model based on Eshelby’s equivalent inclusion (EMT). In the modeling, the elastic properties are calculated by the fiber content, fiber length, and fiber orientation. Several closure approximations for the fourth-order fiber orientation tensor are evaluated by comparing the as-calculated elastic stiffness with that from the original experimental fourth-order tensor. An empirical model was developed to correct the fibers’ aspect ratio in the computation for the actual as-formed LFTs with fiber bundles under high fiber content. After the correction, the analytical predictions had good agreement with the experimental stiffness values from tensile tests on the LFTs. Our analysis shows that it is essential to incorporate the effect of the presence of fiber bundles to accurately predict the composite properties. This work involved the use of experimental values of fiber orientation and serves as the basis for computing part stiffness as a function of mold filling conditions. The work also explains why the modulus tends to level off with fiber concentration. Full article

(This article belongs to the Special Issue Discontinuous Fiber Composites)

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14 pages, 5341 KiB

Open AccessArticle

Process-Induced Fiber Orientation in Fused Filament Fabrication

by Tom Mulholland, Sebastian Goris, Jake Boxleitner, Tim A. Osswald and Natalie Rudolph

J. Compos. Sci. 2018, 2(3), 45; https://doi.org/10.3390/jcs2030045 - 02 Aug 2018

Cited by 39 | Viewed by 4457

Abstract

As the applications for additive manufacturing have continued to grow, so too has the range of available materials, with more functional or better performing materials constantly under development. This work characterizes a copper-filled polyamide 6 (PA6) thermoplastic composite designed to enhance the thermal conductivity of fused filament fabrication (FFF) parts, especially for heat transfer applications. The composite was mixed and extruded into filament using twin screw extrusion. Because the fiber orientation within the material governs the thermal conductivity of the material, the orientation was measured in the filament, through the nozzle, and in printed parts using micro-computed tomography. The thermal conductivity of the material was measured and achieved 4.95, 2.38, and 0.75 W/(m·K) at 70 °C in the inflow, crossflow, and thickness directions, respectively. The implications of this anisotropy are discussed using the example of an air-to-water crossflow heat exchanger. The lower conductivity in the crossflow direction reduces thermal performance due to the orientation in thin-walled parts. Full article

(This article belongs to the Special Issue Discontinuous Fiber Composites)

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12 pages, 4636 KiB

Open AccessArticle

Mechanical Properties and Wear Behavior of a Novel Composite of Acrylonitrile–Butadiene–Styrene Strengthened by Short Basalt Fiber

by Mohammed Y. Abdellah, Hesham I. Fathi, Ayman M. M. Abdelhaleem and Montasser Dewidar

J. Compos. Sci. 2018, 2(2), 34; https://doi.org/10.3390/jcs2020034 - 07 Jun 2018

Cited by 24 | Viewed by 3661

Abstract

Polymer matrix composites (PMC) have a competitive and dominant role in a lot of industries, like aerospace and automobiles. Short basalt fiber (SBF) is used to strengthen acrylonitrile–butadiene–styrene (ABS) polymers as a composite. The composite material is fabricated using injection molding with a new technique to obtain a uniform distribution for the ABS matrix at an elevated temperature range from 140 °C to 240 °C. Four types of specimen were produced according to the mechanically mixed amounts of SBF, which were (5, 10, 15, 20) wt %. The produced material was tested for tension, hardness and impact to measure the enhancement of the mechanical properties of the ABS only and the ABS reinforced by SBF composite. Wear tests were carried out using a pin on disc at a velocity of 57.5 m/s at three normal loads of 5, 10 and 15 kN. Tensile strength increased with up to 5 wt % of SBF, then decreased with an increasing amount of SBF reinforcement, while surface hardness increased with increasing SBF. The impact strength was found to degrade with the whole increment of SBF. Wear resistance increased with the increasing SBF reinforcement amount at all applied normal loads. Full article

(This article belongs to the Special Issue Discontinuous Fiber Composites)

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16 pages, 8398 KiB

Open AccessArticle

A Novel CAE Method for Compression Molding Simulation of Carbon Fiber-Reinforced Thermoplastic Composite Sheet Materials

by Yuyang Song, Umesh Gandhi, Takeshi Sekito, Uday K. Vaidya, Jim Hsu, Anthony Yang and Tim Osswald

J. Compos. Sci. 2018, 2(2), 33; https://doi.org/10.3390/jcs2020033 - 01 Jun 2018

Cited by 11 | Viewed by 5635

Abstract

Its high-specific strength and stiffness with lower cost make discontinuous fiber-reinforced thermoplastic (FRT) materials an ideal choice for lightweight applications in the automotive industry. Compression molding is one of the preferred manufacturing processes for such materials as it offers the opportunity to maintain a longer fiber length and higher volume production. In the past, we have demonstrated that compression molding of FRT in bulk form can be simulated by treating melt flow as a continuum using the conservation of mass and momentum equations. However, the compression molding of such materials in sheet form using a similar approach does not work well. The assumption of melt flow as a continuum does not hold for such deformation processes. To address this challenge, we have developed a novel simulation approach. First, the draping of the sheet was simulated as a structural deformation using the explicit finite element approach. Next, the draped shape was compressed using fluid mechanics equations. The proposed method was verified by building a physical part and comparing the predicted fiber orientation and warpage measurements performed on the physical parts. The developed method and tools are expected to help in expediting the development of FRT parts, which will help achieve lightweight targets in the automotive industry. Full article

(This article belongs to the Special Issue Discontinuous Fiber Composites)

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18 pages, 1247 KiB

Open AccessArticle

Prediction of the Fiber Orientation State and the Resulting Structural and Thermal Properties of Fiber Reinforced Additive Manufactured Composites Fabricated Using the Big Area Additive Manufacturing Process

by Timothy Russell, Blake Heller, David A. Jack and Douglas E. Smith

J. Compos. Sci. 2018, 2(2), 26; https://doi.org/10.3390/jcs2020026 - 10 Apr 2018

Cited by 44 | Viewed by 6498

Abstract

Recent advances in Fused Filament Fabrication (FFF) include large material deposition rates and the addition of chopped carbon fibers to the filament feedstock. During processing, the flow field within the polymer melt orients the fiber suspension, which is important to quantify as the underlying fiber orientation influences the mechanical and thermal properties. This paper investigates the correlation between processing conditions and the resulting locally varying thermal-structural properties that dictate both the final part performance and part dimensionality. The flow domain includes both the confined and unconfined flow indicative of the extruder nozzle within the FFF deposition process. The resulting orientation is obtained through two different isotropic rotary diffusion models, the model by Folgar and Tucker and that of Wang et al., and a comparison is made to demonstrate the sensitivity of the deposited bead’s spatially varying orientation as well as the final processed part’s thermal-structural performance. The results indicate the sensitivity of the final part behavior is quite sensitive to the choice of the slowness parameter in the Wang et al. model. Results also show the need, albeit less than that of the choice of fiber interaction model, to include the extrudate swell and deposition within the flow domain. Full article

(This article belongs to the Special Issue Discontinuous Fiber Composites)

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19 pages, 27015 KiB

Open AccessArticle

Multi-Objective Patch Optimization with Integrated Kinematic Draping Simulation for Continuous–Discontinuous Fiber-Reinforced Composite Structures

by Benedikt Fengler, Luise Kärger, Frank Henning and Andrew Hrymak

J. Compos. Sci. 2018, 2(2), 22; https://doi.org/10.3390/jcs2020022 - 30 Mar 2018

Cited by 24 | Viewed by 4589

Abstract

Discontinuous fiber-reinforced polymers (DiCoFRP) in combination with local continuous fiber reinforced polymers (CoFRP) provide both a high design freedom and high weight-specific mechanical properties. For the optimization of CoFRP patches on complexly shaped DiCoFRP structures, an optimization strategy is needed which considers manufacturing constraints during the optimization procedure. Therefore, a genetic algorithm is combined with a kinematic draping simulation. To determine the optimal patch position with regard to structural performance and overall material consumption, a multi-objective optimization strategy is used. The resulting Pareto front and a corresponding heat-map of the patch position are useful tools for the design engineer to choose the right amount of reinforcement. The proposed patch optimization procedure is applied to two example structures and the effect of different optimization setups is demonstrated. Full article

(This article belongs to the Special Issue Discontinuous Fiber Composites)

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17 pages, 17147 KiB

Open AccessArticle

Fibre Length Reduction in Natural Fibre-Reinforced Polymers during Compounding and Injection Moulding—Experiments Versus Numerical Prediction of Fibre Breakage

by Katharina Albrecht, Tim Osswald, Erwin Baur, Thomas Meier, Sandro Wartzack and Jörg Müssig

J. Compos. Sci. 2018, 2(2), 20; https://doi.org/10.3390/jcs2020020 - 28 Mar 2018

Cited by 12 | Viewed by 4875

Abstract

To establish injection-moulded, natural fibre-reinforced polymers in the automotive industry, numerical simulations are important. To include the breakage behaviour of natural fibres in simulations, a profound understanding is necessary. In this study, the length and width reduction of flax and sisal fibre bundles were analysed experimentally during compounding and injection moulding. Further an optical analysis of the fibre breakage behaviour was performed via scanning electron microscopy and during fibre tensile testing with an ultra-high-speed camera. The fibre breakage of flax and sisal during injection moulding was modelled using a micromechanical model. The experimental and simulative results consistently show that during injection moulding the fibre length is not reduced further; the fibre length was already significantly reduced during compounding. For the mechanical properties of a fibre-reinforced composite it is important to overachieve the critical fibre length in the injection moulded component. The micromechanical model could be used to predict the necessary fibre length in the granules. Full article

(This article belongs to the Special Issue Discontinuous Fiber Composites)

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21 pages, 22861 KiB

Open AccessArticle

Cruciform Specimen Design for Biaxial Tensile Testing of SMC

by Malte Schemmann, Juliane Lang, Anton Helfrich, Thomas Seelig and Thomas Böhlke

J. Compos. Sci. 2018, 2(1), 12; https://doi.org/10.3390/jcs2010012 - 01 Mar 2018

Cited by 16 | Viewed by 7437

Abstract

This paper presents an investigation of different cruciform specimen designs for the characterization of sheet molding compound (SMC) under biaxial loading. The considered material is a discontinuous glass fiber reinforced thermoset. We define various (material-specific) requirements for an optimal specimen design. One key challenge represents the achievement of a high strain level in the center region of the cruciform specimen in order to observe damage, at the same time prevention of premature failure in the clamped specimen arms. Starting from the ISO norm for sheet metals, we introduce design variations, including two concepts to reinforce the specimens’ arms. An experimental evaluation includes two different loading scenarios, uniaxial tension and equi-biaxial tension. The best fit in terms of the defined optimality criteria, is a specimen manufactured in a layup with unidirectional reinforcing outer layers where a gentle milling process exposed the pure SMC in the center region of the specimen. This specimen performed superior for all considered loading conditions, for instance, in the uniaxial loading scenario, the average strain in the center region reached

87 %

of the failure strain in a uniaxial tensile bone specimen. Full article

(This article belongs to the Special Issue Discontinuous Fiber Composites)

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18 pages, 4478 KiB

Open AccessArticle

Rheology Effects on Predicted Fiber Orientation and Elastic Properties in Large Scale Polymer Composite Additive Manufacturing

by Zhaogui Wang and Douglas E. Smith

J. Compos. Sci. 2018, 2(1), 10; https://doi.org/10.3390/jcs2010010 - 16 Feb 2018

Cited by 49 | Viewed by 8664

Abstract

Short fiber-reinforced polymers have recently been introduced to large-scale additive manufacturing to improve the mechanical performances of printed-parts. As the short fiber polymer composite is extruded and deposited on a moving platform, velocity gradients within the melt orientate the suspended fibers, and the final orientation directly affects material properties in the solidified extrudate. This paper numerically evaluates melt rheology effects on predicted fiber orientation and elastic properties of printed-composites in three steps. First, the steady-state isothermal axisymmetric nozzle melt flow is computed, which includes the prediction of die swell just outside the nozzle exit. Simulations are performed with ANSYS-Polyflow, where we consider the effect of various rheology models on the computed outcomes. Here, we include Newtonian, generalized Newtonian, and viscoelastic rheology models to represent the melt flow. Fiber orientation is computed using Advani–Tucker fiber orientation tensors. Finally, elastic properties in the extrudate are evaluated based from predicted fiber orientation distributions. Calculations show that the Phan–Thien–Tanner (PTT) model yields the lowest fiber principal alignment among considered rheology models. Furthermore, the cross section averaged elastic properties indicate a strong transversely isotropic behavior in these composites, where generalized Newtonian models yield higher principal Young’s modulus, while the viscoelastic fluid models result in higher shear moduli. Full article

(This article belongs to the Special Issue Discontinuous Fiber Composites)

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16 pages, 3323 KiB

Open AccessArticle

Simulation of Reinforced Reactive Injection Molding with the Finite Volume Method

by Florian Wittemann, Robert Maertens, Alexander Bernath, Martin Hohberg, Luise Kärger and Frank Henning

J. Compos. Sci. 2018, 2(1), 5; https://doi.org/10.3390/jcs2010005 - 31 Jan 2018

Cited by 15 | Viewed by 5969

Abstract

The reactive process of reinforced thermoset injection molding significantly influences the mechanical properties of the final composite structure. Therefore, reliable process simulation is crucial to predict the process behavior and relevant process effects. Virtual process design is thus highly important for the composite manufacturing industry for creating high quality parts. Although thermoset injection molding shows a more complex flow behavior, state of the art molding simulation software typically focusses on thermoplastic injection molding. To overcome this gap in virtual process prediction, the present work proposes a finite volume (FV) based simulation method, which models the multiphase flow with phase-dependent boundary conditions. Compared to state-of-the-art Finite-Element-based approaches, Finite-Volume-Method (FVM) provides more adequate multiphase flow modeling by calculating the flow at the cell surfaces with an Eulerian approach. The new method also enables the description of a flow region with partial wall contact. Furthermore, fiber orientation, curing and viscosity models are used to simulate the reinforced reactive injection molding process. The open source Computational-Fluid-Dynamics (CFD) toolbox OpenFOAM is used for implementation. The solver is validated with experimental pressure data recorded during mold filling. Additionally, the simulation results are compared to commercial Finite-Element-Method software. The simulation results of the new FV-based CFD method fit well with the experimental data, showing that FVM has a high potential for modeling reinforced reactive injection molding. Full article

(This article belongs to the Special Issue Discontinuous Fiber Composites)

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6512 KiB

Open AccessArticle

Simulative Prediction of Fiber-Matrix Separation in Rib Filling During Compression Molding Using a Direct Fiber Simulation

by Christoph Kuhn, Ian Walter, Olaf Täger and Tim Osswald

J. Compos. Sci. 2018, 2(1), 2; https://doi.org/10.3390/jcs2010002 - 28 Dec 2017

Cited by 19 | Viewed by 3684

Abstract

Compression molding of long fiber reinforced composites offers specific advantages in automotive applications due to the high strength to weight ratio, the comparably low tooling costs and short cycle times. However, the manufacturing process of long fiber composite parts presents a range of challenges. The phenomenon of fiber matrix separation (FMS) is causing severe deviations in fiber content, especially in complex ribbed structures. Currently, there is no commercial software that is capable to accurately predict FMS. This work uses a particle level mechanistic model to study FMS in a rib filling application. The direct fiber simulation (DFS) is uniquely suited to this application due to its ability to model individual fibers and their bending, as well as the interaction amongst fibers that leads to agglomeration. The effects of mold geometry, fiber length, viscosity, and initial fiber orientation are studied. It is shown that fiber length and initial fiber orientation have the most pronounced effects on fiber volume percentage in the ribs, with viscosity and part geometry playing a smaller role. Full article

(This article belongs to the Special Issue Discontinuous Fiber Composites)

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4899 KiB

Open AccessArticle

Investigation of Quasi-Static and Dynamic Material Properties of a Structural Sheet Molding Compound Combined with Acoustic Emission Damage Analysis

by Anna Trauth, Pascal Pinter and Kay André Weidenmann

J. Compos. Sci. 2017, 1(2), 18; https://doi.org/10.3390/jcs1020018 - 14 Dec 2017

Cited by 22 | Viewed by 4567

Abstract

Sheet molding compounds (SMC) are discontinuously fiber-reinforced thermosets, attractive to the automotive industry due to their outstanding specific strength and stiffness, combined with a cost efficient manufacturing process. Increasingly important for structural components, a structural SMC-based improved resin formulation featuring no fillers is investigated in this study. The influence of fiber volume content, fiber length, and manufacturing induced fiber orientation on quasi-static and dynamic mechanical properties of vinylester-based SMC is characterized. Stiffness and strength increased with increasing fiber volume content for tensile, compression, and flexural loadings. Fiber length distribution did not significantly influence the mechanical properties of the material. The movement of the conveyor belt leads to an anisotropic fiber orientation and orientation-dependent mechanical properties. Acoustic emission coupled with machine learning algorithms enabled the investigation of the damage mechanisms of this discontinuous glass fiber SMC. The acoustic emission analysis was validated with micro computed tomography of damaged specimens. The dominant failure mechanisms of the SMC exposed to bending loading were matrix cracking and interface failure. Full article

(This article belongs to the Special Issue Discontinuous Fiber Composites)

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