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Discontinuous Fiber Composites, Volume II
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Discontinuous Fiber Composites, Volume II

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 2020) | Viewed by 57442

Printed Edition Available!
A printed edition of this Special Issue is available here.

Special Issue Editors

Prof. Dr. Tim A. Osswald

E-Mail Website
Guest Editor
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
Special Issues, Collections and Topics in MDPI journals
Dr. Christoph Kuhn

E-Mail
Guest Editor
Volkswagen AG, Wolfsburg 38440, Germany
Interests: fiber reinforced composites; processing of thermoplastics and thermosets; sustainable material; functional polymers; hybrid/multi-material composites; process simulation; additive manufacturing
Special Issues, Collections and Topics in MDPI journals

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. Tim Osswald
Dr. Christoph Kuhn
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Journal of Composites Science is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1800 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

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
  • fiber reinforced composites
  • processing of thermoplastics and thermosets
  • sustainable material
  • functional polymers
  • hybrid/multi-material composites
  • process simulation
  • additive manufacturing

Published Papers (19 papers)

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Editorial

Jump to: Research, Review

3 pages, 193 KiB  
Editorial
Editorial for the Special Issue on Discontinuous Fiber Composites, Volume II
by Christoph Kuhn and Tim A. Osswald
J. Compos. Sci. 2021, 5(3), 71; https://doi.org/10.3390/jcs5030071 - 05 Mar 2021
Cited by 1 | Viewed by 1286
Abstract
This Special Issue on discontinuous fiber composites and its published papers, like its predecessor, give the polymer engineer and scientist an insight into challenges and research topics in the field of discontinuous fiber-reinforced composites [...] Full article
(This article belongs to the Special Issue Discontinuous Fiber Composites, Volume II)

Research

Jump to: Editorial, Review

11 pages, 4841 KiB  
Article
A Force-Balanced Fiber Retardation Model to Predict Fiber-Matrix-Separation during Polymer Processing
by Christoph Kuhn and Simon Wehler
J. Compos. Sci. 2020, 4(4), 165; https://doi.org/10.3390/jcs4040165 - 01 Nov 2020
Cited by 3 | Viewed by 1732
Abstract
The use of discontinuous fiber reinforced composites in injection and compression molding faces a number of challenges regarding process-induced changes in microstructure, which have a significant influence on the mechanical properties of the final component. The changes in final microstructure are caused by [...] Read more.
The use of discontinuous fiber reinforced composites in injection and compression molding faces a number of challenges regarding process-induced changes in microstructure, which have a significant influence on the mechanical properties of the final component. The changes in final microstructure are caused by complex fiber movements, such as fiber orientation, attrition and accumulation during flow. While there are existing phenomenological prediction models for both fiber orientation and attrition, the prediction of fiber accumulation due to fiber-matrix separation is currently only possible with a complex mechanistic particle simulation, which is not applicable in industrial simulations. A simplified phenomenological model, the fiber retardation model (FRM), for the prediction of fiber-matrix separation in commercially available software tools is presented in this paper. The model applies a force balance onto an interacting two phase flow of polymer melt and fiber phase and applies a retardation factor Κ to calculate the slowing and accumulation of the fiber phase. The general model is successfully applied to a simple compression molding simulation. Full article
(This article belongs to the Special Issue Discontinuous Fiber Composites, Volume II)
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50 pages, 147189 KiB  
Article
Direct Fiber Simulation of a Compression Molded Ribbed Structure Made of a Sheet Molding Compound with Randomly Oriented Carbon/Epoxy Prepreg Strands—A Comparison of Predicted Fiber Orientations with Computed Tomography Analyses
by Jan Teuwsen, Stephan K. Hohn and Tim A. Osswald
J. Compos. Sci. 2020, 4(4), 164; https://doi.org/10.3390/jcs4040164 - 31 Oct 2020
Cited by 17 | Viewed by 7700
Abstract
Discontinuous fiber composites (DFC) such as carbon fiber sheet molding compounds (CF-SMC) are increasingly used in the automotive industry for manufacturing lightweight parts. Due to the flow conditions during compression molding of complex geometries, a locally varying fiber orientation evolves. Knowing these process-induced [...] Read more.
Discontinuous fiber composites (DFC) such as carbon fiber sheet molding compounds (CF-SMC) are increasingly used in the automotive industry for manufacturing lightweight parts. Due to the flow conditions during compression molding of complex geometries, a locally varying fiber orientation evolves. Knowing these process-induced fiber orientations is key to a proper part design since the mechanical properties of the final part highly depend on its local microstructure. Local fiber orientations can be measured and analyzed by means of micro-computed tomography (µCT) and digital image processing, or predicted by process simulation. This paper presents a detailed comparison of numerical and experimental analyses of compression molded ribbed hat profile parts made of CF-SMC with 50 mm long randomly oriented strands (ROS) of chopped unidirectional (UD) carbon/epoxy prepreg tape. X-ray µCT scans of three entire CF-SMC parts are analyzed to compare determined orientation tensors with those coming from a direct fiber simulation (DFS) tool featuring a novel strand generation approach, realistically mimicking the initial ROS charge mesostructure. The DFS results show an overall good agreement of predicted local fiber orientations with µCT measurements, and are therefore precious information that can be used in subsequent integrative simulations to determine the part’s mesostructure-related anisotropic behavior under mechanical loads. Full article
(This article belongs to the Special Issue Discontinuous Fiber Composites, Volume II)
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29 pages, 15906 KiB  
Article
Calibration of Fiber Orientation Simulations for LFT—A New Approach
by Fabian Willems, Philip Reitinger and Christian Bonten
J. Compos. Sci. 2020, 4(4), 163; https://doi.org/10.3390/jcs4040163 - 30 Oct 2020
Cited by 6 | Viewed by 4759
Abstract
Short fiber reinforced thermoplastics (SFT) are extensively used due to their excellent mechanical properties and low processing costs. Long fiber reinforced thermoplastics (LFT) show an even more interesting property profile and are increasingly used for structural parts. However, their processing by injection molding [...] Read more.
Short fiber reinforced thermoplastics (SFT) are extensively used due to their excellent mechanical properties and low processing costs. Long fiber reinforced thermoplastics (LFT) show an even more interesting property profile and are increasingly used for structural parts. However, their processing by injection molding is not as simple as for SFT, and their anisotropic properties resulting from the fiber microstructure (fiber orientation, length, and concentration) pose a challenge with regard to the engineering design process. To reliably predict the structural mechanical properties of fiber reinforced thermoplastics by means of micromechanical models, it is also necessary to reliable predict the fiber microstructure. Therefore, it is crucial to calibrate the underlying prediction models, such as the fiber orientation model, within the process simulation. In general, these models may be adjusted manually, but this is usually ineffective and time-consuming. To overcome this challenge, a new calibration method was developed to automatically calibrate the fiber orientation model parameters of the injection molding simulation by means of optimization methods. This optimization routine is based on experimentally determined fiber orientation distributions and leads to optimized parameters for the fiber orientation prediction model within a few minutes. To better understand the influence of the model parameters, different versions of the fiber orientation model, as well as process and material influences on the resulting fiber orientation distribution, were investigated. Finally, the developed approach to calibrate the fiber orientation model was compared with a classical approach, a direct optimization of the whole process simulation. Thereby, the new optimization approach shows a calculation time reduced by the factor 15 with comparable error variance. Full article
(This article belongs to the Special Issue Discontinuous Fiber Composites, Volume II)
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Graphical abstract

20 pages, 6892 KiB  
Article
Insights into the Processing of Recycled Carbon Fibers via Injection Molding Compounding
by Jochen Wellekötter, Julia Resch, Stephan Baz, Götz Theo Gresser and Christian Bonten
J. Compos. Sci. 2020, 4(4), 161; https://doi.org/10.3390/jcs4040161 - 27 Oct 2020
Cited by 10 | Viewed by 3113
Abstract
Although fiber-reinforced plastics combine high strength and stiffness with being lightweight, major difficulties arise with high volume production and the return of manufactured parts back into the cycle of materials at the end of their lifecycles. In a novel approach, structural parts were [...] Read more.
Although fiber-reinforced plastics combine high strength and stiffness with being lightweight, major difficulties arise with high volume production and the return of manufactured parts back into the cycle of materials at the end of their lifecycles. In a novel approach, structural parts were produced from recycled material while utilizing the so-called injection molding compounding process. Recycled fibers and recycled polyamide matrix material were used by blending carbon and matrix fibers into a sliver before processing. Injection molding was then used to produce long fiber-reinforced parts through a direct fiber feed system. Recycled matrix granules were incorporated into the injection molding process by means of an injection molding compounder to investigate their influences on the mechanical properties of the parts. The findings show that the recycled fibers and matrix perform well in standardized tests, although fiber length and fiber content vary significantly and remain below expectations. Full article
(This article belongs to the Special Issue Discontinuous Fiber Composites, Volume II)
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32 pages, 10201 KiB  
Article
Comparative Analysis of the Impact of Additively Manufactured Polymer Tools on the Fiber Configuration of Injection Molded Long-Fiber-Reinforced Thermoplastics
by Lukas Knorr, Robert Setter, Dominik Rietzel, Katrin Wudy and Tim Osswald
J. Compos. Sci. 2020, 4(3), 136; https://doi.org/10.3390/jcs4030136 - 15 Sep 2020
Cited by 2 | Viewed by 3576
Abstract
Additive tooling (AT) utilizes the advantages of rapid tooling development while minimizing geometrical limitations of conventional tool manufacturing such as complex design of cooling channels. This investigation presents a comparative experimental analysis of long-fiber-reinforced thermoplastic parts (LFTs), which are produced through additively manufactured [...] Read more.
Additive tooling (AT) utilizes the advantages of rapid tooling development while minimizing geometrical limitations of conventional tool manufacturing such as complex design of cooling channels. This investigation presents a comparative experimental analysis of long-fiber-reinforced thermoplastic parts (LFTs), which are produced through additively manufactured injection molding polymer tools. After giving a review on the state of the art of AT and LFTs, additive manufacturing (AM) plastic tools are compared to conventionally manufactured steel and aluminum tools toward their qualification for spare part and small series production as well as functional validation. The assessment of the polymer tools focuses on three quality criteria concerning the LFT parts: geometrical accuracy, mechanical properties, and fiber configuration. The analysis of the fiber configuration includes fiber length, fiber concentration, and fiber orientation. The results show that polymer tools are fully capable of manufacturing LFTs with a cycle number within hundreds before showing critical signs of deterioration or tool failure. The produced LFTs moldings provide sufficient quality in geometrical accuracy, mechanical properties, and fiber configuration. Further, specific anomalies of the fiber configuration can be detected for all tool types, which include the occurrence of characteristic zones dependent on the nominal fiber content and melt flow distance. Conclusions toward the improvement of additively manufactured polymer tool life cycles are drawn based on the detected deteriorations and failure modes. Full article
(This article belongs to the Special Issue Discontinuous Fiber Composites, Volume II)
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16 pages, 8456 KiB  
Article
Validation of Fiber Breakage in Simple Shear Flow with Direct Fiber Simulation
by Tzu-Chuan Chang, Abrahán Bechara Senior, Hakan Celik, Dave Brands, Angel Yanev and Tim Osswald
J. Compos. Sci. 2020, 4(3), 134; https://doi.org/10.3390/jcs4030134 - 10 Sep 2020
Cited by 3 | Viewed by 2708
Abstract
This study aims to use particle level simulation to simulate the breakage behavior of glass fibers subjected to simple shear flow. Each fiber is represented as a chain of rods that experience hydrodynamic, interaction, and elastic effects. In order to validate the approach [...] Read more.
This study aims to use particle level simulation to simulate the breakage behavior of glass fibers subjected to simple shear flow. Each fiber is represented as a chain of rods that experience hydrodynamic, interaction, and elastic effects. In order to validate the approach of the model, the simulation results were compared to simple shear flow experiments conducted in a Couette Rheometer. The excluded volume force constants and critical fiber breakage curvature were tuned in the simulation to gain a better understanding of the system. Relaxation of the fiber clusters and a failure probability theory were introduced into the model to solve the fiber entanglement and thus, better fit the experimental behavior. The model showed agreement with the prediction on fiber length reduction in both number average length and weight average length. In addition, the simulation had a similar trend of breakage distribution compared to a loop test using glass fibers. Full article
(This article belongs to the Special Issue Discontinuous Fiber Composites, Volume II)
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15 pages, 4916 KiB  
Article
Development of Recyclable and High-Performance In Situ Hybrid TLCP/Glass Fiber Composites
by Tianran Chen, Dana Kazerooni, Lin Ju, David A. Okonski and Donald G. Baird
J. Compos. Sci. 2020, 4(3), 125; https://doi.org/10.3390/jcs4030125 - 24 Aug 2020
Cited by 14 | Viewed by 2763
Abstract
By combining the concepts of in situ thermotropic liquid crystalline polymer (TLCP) composites and conventional fiber composites, a recyclable and high-performance in situ hybrid polypropylene-based composite was successfully developed. The recycled hybrid composite was prepared by injection molding and grinding processes. Rheological and [...] Read more.
By combining the concepts of in situ thermotropic liquid crystalline polymer (TLCP) composites and conventional fiber composites, a recyclable and high-performance in situ hybrid polypropylene-based composite was successfully developed. The recycled hybrid composite was prepared by injection molding and grinding processes. Rheological and thermal analyses were utilized to optimize the processing temperature of the injection molding process to reduce the melt viscosity and minimize the degradation of polypropylene. The ideal temperature for blending the hybrid composite was found to be 305 °C. The influence of mechanical recycling on the different combinations of TLCP and glass fiber composites was analyzed. When the weight fraction ratio of TLCP to glass fiber was 2 to 1, the hybrid composite exhibited better processability, improved tensile performance, lower mechanical anisotropy, and greater recyclability compared to the polypropylene reinforced by either glass fiber or TLCP alone. Full article
(This article belongs to the Special Issue Discontinuous Fiber Composites, Volume II)
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18 pages, 7420 KiB  
Article
Characterization of Mechanical Performance of Composites Fabricated Using Innovative Carbon Fiber Wet Laid Process
by Hicham Ghossein, Ahmed Arabi Hassen, Seokpum Kim, Jesse Ault and Uday K. Vaidya
J. Compos. Sci. 2020, 4(3), 124; https://doi.org/10.3390/jcs4030124 - 22 Aug 2020
Cited by 4 | Viewed by 2547
Abstract
Recent innovation in production of optimized nonwoven wet laid (WL) carbon fiber (CF) mats raised the question of optimal translation of the performance and isotropy into composites formed through these dry preforms. This work explores the mechanical behavior of composites produced from WL-CF [...] Read more.
Recent innovation in production of optimized nonwoven wet laid (WL) carbon fiber (CF) mats raised the question of optimal translation of the performance and isotropy into composites formed through these dry preforms. This work explores the mechanical behavior of composites produced from WL-CF mats in conjunction with the microstructure predicted through Object Oriented Finite Element Analysis (OOF). The mats used for the composites were prepared in two dispersion regimes using 25.4 mm long CF. The mixing regimes discussed in the author’s previous work, are identified as Method 1 for the traditional processing regime and Method 2 for the innovative regime that provided optimal nonwoven WL-CF mats. Composite panels from Method 2 mats showed a normalized tensile strength increase of 52% over those from Method 1 panels. Reproducibility analysis of composites made from Method 2 mats demonstrated a standard deviation of 2% in fiber weight content, 2% in tensile modulus and 9% in tensile strength, while composites made from Method 1 mats demonstrated a standard deviation of 5% in fiber weight content, 5% in tensile modulus and 17% in tensile strength. Systematic study of the microstructure and its analysis through OOF confirmed the isotropy translation of mats produced through method 2 to the composites. This study validated the hypothesis that optimal nonwoven mats lead to a well-balanced composite with optimal performance and that non-optimal nonwoven mats do not pack into a well-balanced composite. Full article
(This article belongs to the Special Issue Discontinuous Fiber Composites, Volume II)
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18 pages, 7851 KiB  
Article
Metamodelling of the Correlations of Preform and Part Performance for Preform Optimisation in Sheet Moulding Compound Processing
by Christian Hopmann, Jonas Neuhaus, Kai Fischer, Daniel Schneider and René Laschak Pinto Gonçalves
J. Compos. Sci. 2020, 4(3), 122; https://doi.org/10.3390/jcs4030122 - 21 Aug 2020
Cited by 3 | Viewed by 2329
Abstract
In the design of parts consisting of long-fibre-reinforced Sheet Moulding Compounds (SMC), the potential for the optimisation of processing parameters and geometrical design is limited due to the high number of interdependent variables. One of the influences on fibre orientations and therefore mechanical [...] Read more.
In the design of parts consisting of long-fibre-reinforced Sheet Moulding Compounds (SMC), the potential for the optimisation of processing parameters and geometrical design is limited due to the high number of interdependent variables. One of the influences on fibre orientations and therefore mechanical part performance is the initial filling state of the compression moulding tool, which is defined by the geometry and positioning of the SMC preform. In the past, response surface methodology and linear regression analysis were successfully used for a simulation-based optimisation of rectangular preform size and position in regard to a part performance parameter. However, the computational demand of these increase exponentially with an increase in the number of design variables, such as in the case of more complex preform geometries. In this paper, these restrictions are addressed with a novel approach for metamodelling the correlation of preform and the resulting mechanical part performance. The approach is applied to predicting the maximum absolute deflection of a plate geometry under bending load. For metamodelling, multiple neural networks (NN) are trained on a dataset obtained by process and structural simulation. Based on the discretisation of the plate geometry used in these simulation procedures, the binary initial filling states (completely filled/empty) of each element are used as inputs of the NNs. Outputs of the NNs are combined by ensemble modelling to form the metamodel. The metamodel allows an accurate prediction of maximum deflection; subsequent validation of the metamodel shows differences in predicted and simulated maximum deflection ranging from 0.26% to 2.67%. Subsequently, the metamodel is evaluated using a mutation algorithm for finding a preform that reduces the maximum deflection. Full article
(This article belongs to the Special Issue Discontinuous Fiber Composites, Volume II)
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11 pages, 4212 KiB  
Article
Process Chain Optimization for SWCNT/Epoxy Nanocomposite Parts with Improved Electrical Properties
by Manuel V. C. Morais, Marco Marcellan, Nadine Sohn, Christof Hübner and Frank Henning
J. Compos. Sci. 2020, 4(3), 114; https://doi.org/10.3390/jcs4030114 - 14 Aug 2020
Cited by 2 | Viewed by 1994
Abstract
Electrically conductive nanocomposites present opportunities to replace metals in several applications. Usually, the electrical properties emerging from conductive particles and the resulting bulk values depend on the micro/nano scale morphology of the particle network formed during processing. The final electrical properties are therefore [...] Read more.
Electrically conductive nanocomposites present opportunities to replace metals in several applications. Usually, the electrical properties emerging from conductive particles and the resulting bulk values depend on the micro/nano scale morphology of the particle network formed during processing. The final electrical properties are therefore highly process dependent. In this study, the electrical resistivity of composites made from single-walled carbon nanotubes in epoxy was investigated. Three approaches along the processing chain were investigated to reduce the electrical resistivity of nanocomposites-the dispersion strategy in a three-roll mill, the curing temperature, and the application of electric fields during curing. It was found that a progressive increase in the shear forces during dispersion leads to a more than 50% reduction in the electrical resistivities. Higher curing temperatures of the nanocomposite resin also lead to a decrease of around 50% in resistivity. Furthermore, a scalable resin transfer molding set-up with gold-coated electrodes was developed and tested with different mold release agents. It has been shown that curing the material under electric fields leads to an electrical resistivity approximately an order of magnitude lower, and that the properties of the mold release agent also influence the final resistivity of different samples in the same batch. Full article
(This article belongs to the Special Issue Discontinuous Fiber Composites, Volume II)
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10 pages, 3898 KiB  
Article
Significance of Model Parameter Variations in the pARD-RSC Model
by Armin Kech, Susanne Kugler and Tim Osswald
J. Compos. Sci. 2020, 4(3), 109; https://doi.org/10.3390/jcs4030109 - 07 Aug 2020
Cited by 3 | Viewed by 1596
Abstract
This study aims to evaluate how fiber orientation results are dependent on fluctuations in input parameters, such as the average fiber length, fiber volume content, and initial alignment of fibers. The range of parameters is restricted to deviations within one specific short fiber [...] Read more.
This study aims to evaluate how fiber orientation results are dependent on fluctuations in input parameters, such as the average fiber length, fiber volume content, and initial alignment of fibers. The range of parameters is restricted to deviations within one specific short fiber reinforced thermoplastic and is not set up to investigate the differences between materials. The evaluation was conducted by a virtual shear cell based on a mechanistic modeling approach. The fiber orientation prediction model discussed is the pARD-RSC (principal anisotropic rotary diffusion-reduced strain closure) model implemented as a user routine in AUTODESK MOLDFLOW INSIGHT® (AMI®). The material investigated was discontinuous short glass fiber reinforced PBT (polybutylene-terephthalate), which is often used for housings in various industries. It is shown that variation in the input parameters, although having an influence on the fiber orientation model parameters, only affects the final orientation moderately. Full article
(This article belongs to the Special Issue Discontinuous Fiber Composites, Volume II)
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16 pages, 7941 KiB  
Article
Selective Laser Sintering of PA6: Effect of Powder Recoating on Fibre Orientation
by Tobias Heckner, Michael Seitz, Sven Robert Raisch, Gerrit Huelder and Peter Middendorf
J. Compos. Sci. 2020, 4(3), 108; https://doi.org/10.3390/jcs4030108 - 06 Aug 2020
Cited by 9 | Viewed by 2216
Abstract
In Selective Laser Sintering, fibres are strongly orientated during the powder recoating process. This effect leads to an additional increase of anisotropy in the final printed parts. This study investigates the influence of process parameter variation on the mechanical properties and the fibre [...] Read more.
In Selective Laser Sintering, fibres are strongly orientated during the powder recoating process. This effect leads to an additional increase of anisotropy in the final printed parts. This study investigates the influence of process parameter variation on the mechanical properties and the fibre orientation. A full factorial design of experiment was created to evaluate the processing parameters of recoating speed, layer thickness and laser power on the part’s modulus of elasticity. Based on the mechanical testing, computed tomography was applied to selected samples to investigate the process-induced fibre microstructure, and calculate the fibre orientation tensors. The results show increasing part stiffness in the deposition direction, with decreasing layer thickness and increasing laser power, while the recoating speed only shows little effect on the mechanical performance. This complies with computed tomography imaging results, which show an increase in fibre orientation with smaller layer thickness. With thinner layers, and hence smaller shear gaps, shear stresses induced by the roller during recoating increase significantly, leading to excessive fibre reorientation and alignment. The high level of fibre alignment implies an increase of strength and stiffness in the recoating direction. In addition, thinner layer thickness under constant laser energy density results in improved melting behaviour, and thus improved fibre consolidation, consequently further increasing the mechanical properties. Meanwhile, the parameters of recoating speed and laser power do not have a significant impact on fibre orientation within their applicable process windows. Full article
(This article belongs to the Special Issue Discontinuous Fiber Composites, Volume II)
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15 pages, 5717 KiB  
Article
Measuring Fiber Length in the Core and Shell Regions of Injection Molded Long Fiber-Reinforced Thermoplastic Plaques
by Abrahán Bechara Senior and Tim Osswald
J. Compos. Sci. 2020, 4(3), 104; https://doi.org/10.3390/jcs4030104 - 31 Jul 2020
Cited by 4 | Viewed by 2701
Abstract
Long fiber-reinforced thermoplastics are an attractive design option for many industries due to their excellent mechanical properties and processability. Processing of these materials has a significant influence on their microstructure, which controls the properties of the final part. The microstructure is characterized by [...] Read more.
Long fiber-reinforced thermoplastics are an attractive design option for many industries due to their excellent mechanical properties and processability. Processing of these materials has a significant influence on their microstructure, which controls the properties of the final part. The microstructure is characterized by the fibers’ orientation, length, and concentration. Many characterization methods can capture the fiber orientation and concentration changes through the thickness in injection molded parts, but not the changes in fiber length. In this study, a technique for measuring fiber length in the core and shell regions of molded parts was proposed, experimentally verified, and used on injection molded 20 wt.% glass fiber-reinforced polypropylene plaques. The measured fiber length in the core was 50% higher than in the shell region. Comparison with simulation results shows disagreement in the shape of the through-thickness fiber length profile. Stiffness predictions show that the through-thickness changes in fiber length have little impact on the longitudinal and transverse Young’s modulus. Full article
(This article belongs to the Special Issue Discontinuous Fiber Composites, Volume II)
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22 pages, 1112 KiB  
Article
A Flow-Dependent Fiber Orientation Model
by Susanne Katrin Kugler, Argha Protim Dey, Sandra Saad, Camilo Cruz, Armin Kech and Tim Osswald
J. Compos. Sci. 2020, 4(3), 96; https://doi.org/10.3390/jcs4030096 - 22 Jul 2020
Cited by 10 | Viewed by 2729
Abstract
The mechanical performance of fiber reinforced polymers is dependent on the process-induced fiber orientation. In this work, we focus on the prediction of the fiber orientation in an injection-molded short fiber reinforced thermoplastic part using an original multi-scale modeling approach. A particle-based model [...] Read more.
The mechanical performance of fiber reinforced polymers is dependent on the process-induced fiber orientation. In this work, we focus on the prediction of the fiber orientation in an injection-molded short fiber reinforced thermoplastic part using an original multi-scale modeling approach. A particle-based model developed for shear flows is extended to elongational flows. This mechanistic model for elongational flows is validated using an experiment, which was conducted for a long fiber reinforced polymer. The influence of several fiber descriptors and fluid viscosity on fiber orientation under elongational flow is studied at the micro-scale. Based on this sensitivity analysis, a common parameter set for a continuum-based fiber orientation macroscopic model is defined under elongational flow. We then develop a novel flow-dependent macroscopic fiber orientation, which takes into consideration the effect of both elongational and shear flow on the fiber orientation evolution during the filling of a mold cavity. The model is objective and shows better performance in comparison to state-of-the-art fiber orientation models when compared to μCT-based fiber orientation measurements for several industrial parts. The model is implemented using the simulation software Autodesk Moldflow Insight Scandium® 2019. Full article
(This article belongs to the Special Issue Discontinuous Fiber Composites, Volume II)
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18 pages, 26989 KiB  
Article
Strength Prediction Sensitivity of Foamed Recycled Polymer Composite Structures due to the Localized Variability of the Cell Density Distribution
by Daniel P. Pulipati and David A. Jack
J. Compos. Sci. 2020, 4(3), 93; https://doi.org/10.3390/jcs4030093 - 16 Jul 2020
Cited by 3 | Viewed by 2094
Abstract
The need for novel methods for the reuse of post-industrial/post-consumer polymer solid wastes (PSW) is of increasing societal importance. Unfortunately, this objective is often limited due to material stream variability or insufficient load-carrying capacity of the fabricated goods. This study investigates a large [...] Read more.
The need for novel methods for the reuse of post-industrial/post-consumer polymer solid wastes (PSW) is of increasing societal importance. Unfortunately, this objective is often limited due to material stream variability or insufficient load-carrying capacity of the fabricated goods. This study investigates a large format fiber-reinforced structural member that contains spatially varying material properties, specifically density. The application is focused on the unique features of closed-cell foamed composite structures made from recycled post-industrial/post-consumer PSW composed of High-Density Polyethylene (HDPE) and Glass Fiber Polypropylene (GFPP). The structures in this research are manufactured using a hybrid extrusion process, which involves foaming enabled by chemical blowing agents that form a fully consolidated solid outer shell and a closed-cell core. The cell distribution is inhomogeneous, in size distribution and spatial distribution, leading to significant spatial variations of the local effective stiffness. To understand the correlation between density variations and effective stiffness and strength, a low-cost method using digital imaging is introduced and integrated into a finite element subroutine. The imaging approach includes sectioning the structural member and analyzing the resulting image using various custom imaging processing techniques in the MATLAB environment. The accuracy of the imaging technique was experimentally verified using a Keyence digital microscope, and the error was found to be 3% in any given spatial feature. The processed image is then correlated to a localized density map of the cross-section using a weighted spatial averaging technique, and the local effective material properties of the foamed region are predicted using the presented micromechanical approach. The local stiffness is a function of void density, local fiber orientation, constitutive behavior of both the fiber and the matrix blend, and the non-linear response of the matrix blend. The spatially varying stiffness and nonlinear strength response at each spatial location are then integrated into a finite element subroutine within the COMSOL multiphysics environment, and results are presented for the deflection and internal stress state of the composite structure. Results indicate that the internal microstructural variations have a nominal impact on the bulk deflection profile. Conversely, results show the peak of the internal stress is increased by ∼11% as compared to the uniform core assumption, thus safe designs must consider core density spatial variations in the final product design. Full article
(This article belongs to the Special Issue Discontinuous Fiber Composites, Volume II)
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20 pages, 2749 KiB  
Article
Parameter Identification of Fiber Orientation Models Based on Direct Fiber Simulation with Smoothed Particle Hydrodynamics
by Nils Meyer, Oleg Saburow, Martin Hohberg, Andrew N. Hrymak, Frank Henning and Luise Kärger
J. Compos. Sci. 2020, 4(2), 77; https://doi.org/10.3390/jcs4020077 - 22 Jun 2020
Cited by 16 | Viewed by 3241
Abstract
The behavior of fiber suspensions during flow is of fundamental importance to the process simulation of discontinuous fiber reinforced plastics. However, the direct simulation of flexible fibers and fluid poses a challenging two-way coupled fluid-structure interaction problem. Smoothed Particle Hydrodynamics (SPH) offers a [...] Read more.
The behavior of fiber suspensions during flow is of fundamental importance to the process simulation of discontinuous fiber reinforced plastics. However, the direct simulation of flexible fibers and fluid poses a challenging two-way coupled fluid-structure interaction problem. Smoothed Particle Hydrodynamics (SPH) offers a natural way to treat such interactions. Hence, this work utilizes SPH and a bead chain model to compute a shear flow of fiber suspensions. The introduction of a novel viscous surface traction term is key to achieve full agreement with Jeffery’s equation. Careful modelling of contact interactions between fibers is introduced to model suspensions in the non-dilute regime. Finally, parameters of the Reduced-Strain Closure (RSC) orientation model are identified using ensemble averages of multiple SPH simulations implemented in PySPH and show good agreement with literature data. Full article
(This article belongs to the Special Issue Discontinuous Fiber Composites, Volume II)
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17 pages, 5981 KiB  
Article
Experimental Validation of a Direct Fiber Model for Orientation Prediction
by Sara Andrea Simon, Abrahán Bechara Senior and Tim Osswald
J. Compos. Sci. 2020, 4(2), 59; https://doi.org/10.3390/jcs4020059 - 25 May 2020
Cited by 8 | Viewed by 2439
Abstract
Predicting the fiber orientation of reinforced molded components is required to improve their performance and safety. Continuum-based models for fiber orientation are computationally very efficient; however, they lack in a linked theory between fiber attrition, fiber–matrix separation and fiber alignment. This work, therefore, [...] Read more.
Predicting the fiber orientation of reinforced molded components is required to improve their performance and safety. Continuum-based models for fiber orientation are computationally very efficient; however, they lack in a linked theory between fiber attrition, fiber–matrix separation and fiber alignment. This work, therefore, employs a particle level simulation which was used to simulate the fiber orientation evolution within a sliding plate rheometer. In the model, each fiber is accounted for and represented as a chain of linked rigid segments. Fibers experience hydrodynamic forces, elastic forces, and interaction forces. To validate this fundamental modeling approach, injection and compression molded reinforced polypropylene samples were subjected to a simple shear flow using a sliding plate rheometer. Microcomputed tomography was used to measure the orientation tensor up to 60 shear strain units. The fully characterized microstructure at zero shear strain was used to reproduce the initial conditions in the particle level simulation. Fibers were placed in a periodic boundary cell, and an idealized simple shear flow field was applied. The model showed a faster orientation evolution at the start of the shearing process. However, agreement with the steady-state aligned orientation for compression molded samples was found. Full article
(This article belongs to the Special Issue Discontinuous Fiber Composites, Volume II)
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Review

Jump to: Editorial, Research

21 pages, 1221 KiB  
Review
Fiber Orientation Predictions—A Review of Existing Models
by Susanne Katrin Kugler, Armin Kech, Camilo Cruz and Tim Osswald
J. Compos. Sci. 2020, 4(2), 69; https://doi.org/10.3390/jcs4020069 - 08 Jun 2020
Cited by 38 | Viewed by 4938
Abstract
Fiber reinforced polymers are key materials across different industries. The manufacturing processes of those materials have typically strong impact on their final microstructure, which at the same time controls the mechanical performance of the part. A reliable virtual engineering design of fiber-reinforced polymers [...] Read more.
Fiber reinforced polymers are key materials across different industries. The manufacturing processes of those materials have typically strong impact on their final microstructure, which at the same time controls the mechanical performance of the part. A reliable virtual engineering design of fiber-reinforced polymers requires therefore considering the simulation of the process-induced microstructure. One relevant microstructure descriptor in fiber-reinforced polymers is the fiber orientation. This work focuses on the modeling of the fiber orientation phenomenon and presents a historical review of the different modelling approaches. In this context, the article describes different macroscopic fiber orientation models such as the Folgar-Tucker, nematic, reduced strain closure (RSC), retarding principal rate (RPR), anisotropic rotary diffusion (ARD), principal anisotropic rotary diffusion (pARD), and Moldflow rotary diffusion (MRD) model. We discuss briefly about closure approximations, which are a common mathematical element of those macroscopic fiber orientation models. In the last section, we introduce some micro-scale numerical methods for simulating the fiber orientation phenomenon, such as the discrete element method (DEM), the smoothed particle hydrodynamics (SPH) method and the moving particle semi-implicit (MPS) method. Full article
(This article belongs to the Special Issue Discontinuous Fiber Composites, Volume II)
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