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Composite Structures

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Tensile and flexural behaviors of additively manufactured continuouscarbon fiber-reinforced polymer composites

Tianyu Yu, Ziyang Zhang, Shutao Song, Yuanli Bai, Dazhong Wu⁎

Department of Mechanical and Aerospace Engineering, University of Central Florida, Orlando, FL 32816, USA

A R T I C L E I N F O

Keywords:Continuous carbon fiber-reinforced polymercompositesAdditive manufacturingMicrostructureTensile and flexural behaviorsPrediction

A B S T R A C T

Continuous carbon fiber-reinforced polymer (CCFRP) composites are lightweight and strong materials that havebeen used in a wide range of applications in automotive and aerospace industries. Traditional manufacturingprocesses (e.g., lay-up and out of autoclave techniques) are not capable of fabricating complex composites.Additive manufacturing (AM) has been increasingly used to fabricate composites with complex geometries. Inthis study, CCFRP with concentric and isotropic carbon fiber infill patterns were additively manufactured usingfused deposition modeling (FDM). The microstructures of CCFRP composites with both infill patterns werecharacterized by an optical microscope and a scanning electron microscope (SEM). The void density wascharacterized for infill, solid and carbon fiber regions. Four-point flexural tests and tensile tests were conductedto evaluate the effects of carbon fiber concentration and infill patterns on both tensile and flexural behaviors. Astiffness averaging method was used to model the elastic behavior of CCFRP samples with different micro-structures by taking into account porosity in infill, solid and carbon fiber bundles. Experimental results haveshown that the specimens with the concentric carbon fiber infill pattern exhibit better flexural strength andenergy absorption capability than those with the isotropic carbon fiber infill pattern. As carbon fiber con-centration and the number of fiber rings increase, the flexural strength of the CCFRP composite increases. Thespecimen with 48.72 wt% carbon fibers showed the highest flexural strength of 270.63MPa. In comparison withchopped carbon fiber-reinforced polymer composites, the flexural strength of the CCFRP specimen was increasedby 40%. The predicted elastic moduli are in good agreement with experimental data.

1. Introduction

Fiber-reinforced polymer composites (FRPC) have been increasinglyused in the automobile and aerospace industries due to their superiormechanical properties (e.g., high strength-to-weight ratio) [1,2]. Ty-pical reinforcements used in FRPC include carbon, glass, and aramidfibers, which may be continuous or discontinuous, and carbon nano-tubes (CNTs). While several manufacturing processes have been de-veloped to fabricate discontinuous carbon fiber-reinforced polymercomposites (DCFRP), very few manufacturing techniques are capable offabricating CCFRP composites effectively and cost-efficiently due totheir unique microstructures [3–5]. It has been demonstrated that AMprocesses such as FDM are capable of fabricating FRPC with desirablemechanical properties and complex geometries [6]. Several studieshave been conducted to fabricate DCFRP as well as evaluate their me-chanical properties. For example, Ning et al. [7] found that the additionof short carbon fibers in FDM-fabricated parts made of acrylonitrilebutadiene styrene (ABS) can reduce the toughness, yield strength and

ductility of the composites, but will increase the tensile strength,Young’s modulus, flexural stress, flexural toughness and flexural mod-ulus. Porosity was a significant factor in the specimens with 10 wt%carbon fiber content. Tekinalp et al. [8] observed that as short carbonfiber content increased, porosity inside the FDM-printed beads in-creased while voids between the beads decreased. In addition, the fiberorientation was highly dependent on the build direction. Love et al. [9]evaluated the effects of the addition of chopped carbon fibers on thestrength, stiffness, thermal conductivity, and distortion of FDM-fabri-cated parts. Experimental results have shown that mixing carbon fiberswith polymers increases the strength, stiffness, and thermal con-ductivity as well as reduces the distortion of the parts significantly.However, CCFRP composites exhibit improved strength and perfor-mance compared to that of DCFRP. In automotive manufacturing, eachmanufacturing step needs to be quality-assured and fast to achieve cost-effective high-volume production [10]. Additive manufacturing pro-vides a tool for realizing complex component shapes in the automobileindustry. Matsuzaki et al. [11] fabricated continuous carbon fiber

https://doi.org/10.1016/j.compstruct.2019.111147Received 21 April 2019; Accepted 11 June 2019

⁎ Corresponding author.E-mail address: Dazhong.Wu@ucf.edu (D. Wu).

Composite Structures 225 (2019) 111147

Available online 13 June 20190263-8223/ © 2019 Elsevier Ltd. All rights reserved.

T

reinforced plastics by impregnate fibers with the plastic filament withinthe heated nozzle. An improvement of tensile strength was achievedcompared to conventional additively manufactured composites. Yanget al. [12] fabricated anisotropic CNTs reinforced nanocomposites bythe implement of electrically assisted 3D printing. In one of the 90layers specimens, each build layer was rotated 2° compared to theprevious layer. The compressive strength was significantly improvedthrough in-plane spreading of cracks and crack redirection, whichprevented the through thickness cracking.

The complex structures of CCFRP make it very challenging to modelits mechanical behavior, especially for the additively manufacturedparts. Some efforts have been made to address this issue. Lin andParandoush [6] found void formation, poor adhesion of fibers andpolymer matrix, and effective continuous fiber printing are among themost challenging issues in AM of composites. Rodriguez et al. in-vestigated the mechanical behavior of polymers during FDM process,and they proposed a mixture model by considering the FDM polymersare solid with aligned prismatic voids. An effective elastic properties ofFDM polymers can then be calculated [13]. Melenka et al. [14] mod-eled continuous Kevlar fiber-reinforced composites by using Stiffnessaveraging (VAS) method. Compared to the experimental results, thepredicted elastic modulus had a relative error of 57.5%, 6.2% and 0.1%for the 4%, 8% and 10% Kevlar fiber volume fractions. Only a limitednumber of tests were conducted. Parandoush et al. [15] implementedthe laminated object manufacturing concept into CCFRP manu-facturing. Prepreg sheets were cut, and then bonded in a layer by layermanner using a CO2 laser beam and a roller. The highest tensilestrength and flexural strength for the manufactured CCFRP achievedwere 668.3MPa and 591.16MPa, respectively. Hochard et al. [16]developed a first ply failure model for CFRP laminates. Tension andshear internal variables d1 and d2 were used in order to capture thebrittle behavior in the warp and fill directions. A traction/shear cou-pling coefficient was also implemented. The model showed good cor-relations with experiments in both static and cyclic loadings. Heß et al.[17] developed a finite element based unit-cell model to predict theelastic constants of stitched CFRP laminates. The voids produced bystitching in the individual layers in the laminates were characterized byusing micrograph analysis and were found critical in determining themechanical properties of CFRP laminates. The predicted tensile andcompressive elastic modulus had a relative error of 4.8% and 6.3%,respectively. Ogi [18] investigated the mechanical properties of ABSbased CFRP. Chopped carbon fibers with different weight fractionswere used. A macro-mechanical model was developed to predict theelastic modulus and strength of CFRPC with the consideration of fiberorientation, length and interfacial bonding in short fiber composites.Naranjo-Lozada et al. [19] investigated the tensile properties andfailure behavior of additively manufactured CCFRP specimens. A stiff-ness averaging modeling has been used to predict the elastic modulus ofCCFRP. A good predictive accuracy was achieved for specimens with11% and smaller carbon fiber volume fraction. Placing the carbon fiberstart point in the middle of the specimens was suggested to avoid tensilefracture outside the middle section.

However, little research has been reported on characterizing thefailure mode and tensile and flexural behaviors of additively manu-factured CCFRP [20,21]. In addition, no studies have been conducted totake into account the effect of voids within the carbon fiber filaments ontensile and flexural behaviors of additively manufactured CCFRP.Taking into account this effect is crucial to prediction of the mechanicalproperties of CCFRP. To fill this research gap, this paper aims tocharacterize CCFRP composites using optical microscope and scanningelectron microscope (SEM) as well as conducting tensile and flexuraltests. A stiffness averaging model is used to predict the elastic modulusof both tensile and flexural specimens. Challenges and future work havealso been discussed.

2. Materials and experimental methods

2.1. Materials

CCFRP with different carbon fiber infill densities and carbon fiberinfill patterns were fabricated using FDM. A continuous carbon fiberfilament with a diameter of 0.35mm and an Onyx filament(Markforged, Watertown, MA) were used as the reinforcement materialand the matrix material, respectively. The continuous carbon fiber fi-lament contains up to a thousand micro-carbon fibers infused with asizing agent. Onyx is made of nylon thermoplastic infused with choppedmicro-carbon fibers.

2.2. Tensile and flexural tests

The tensile test specimen with a dog bone shape is shown in Fig. 1.The dimension of the tensile test specimen is 92.5mm in length and3.25mm in height. The tensile test specimen consists of 26 build layers.The tensile tests were conducted on an MTS Servohydraulic Testsystem. The strain rate used was 0.006mm/s. The flexural test spe-cimen with a sandwich structure was designed and fabricated accordingto ASTM D6272-17. As shown in Fig. 1(b), the dimension of the flexuraltest specimen is 130mm×15mm×3.5mm. The flexural test spe-cimen consists of 28 build layers. The flexural tests were conducted onan MTS Insight 5 electromechanical loading frame. The rate of cross-head motion was 1.5 mm/min.

2.3. Fabrication process

A dual nozzle on the print head was used to print Onyx and carbonfiber filaments during the CCFRP fabrication. The specimens consist of4 solid roof onyx layers, 4 solid floor onyx layers, and 2 solid verticalwall layers. Onyx and carbon filaments were pre-heated to a tempera-ture of 275 °C and 250 °C, respectively. For four-point flexural tests, atotal number of 54 specimens with different number of fiber layers andconcentric fiber rings were additively manufactured. The number offiber layers include 2, 4, 6, 8, 10, 12, 14, 16 and 18. The number ofconcentric fiber rings ranges from 1 to 6. The infill density of theflexural testing specimens was 50%. For tensile tests, 12 specimenswere additively manufactured with different number of fiber layers andconcentric fiber rings. The number of fiber layers includes 2, 4, 6, and 8.The number of concentric fiber rings ranges from 1 to 3. The infilldensity of the tensile test specimens was 100% solid infill. In addition, aspecimen with an isotropic carbon fiber infill pattern (0°/45°/90°/135°angle-ply) and 16 carbon fiber layers was additively manufactured forflexural testing. The flexural strength and energy absorption of thespecimen was used to compare with those with the concentric carbonfiber infill pattern.

Fig. 1. (a) Tensile test specimen and (b) flexural test specimen.

T. Yu, et al. Composite Structures 225 (2019) 111147

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3. Modeling approach

The materials deposited during the FDM process are usually inporous structures. A stiffness averaging method is used in this study topredict the elastic behavior of the additively manufactured CCFRPspecimens for both tensile and flexural testing [14]. The effective elasticproperties of such materials are calculated based on the porosity level.The compliance matrix denoted by [S] is the inverse of the stiffnessmatrix denoted by [K]. For a unidirectional material, [S] is as follows:

=

⎡

⎣

⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢

− −

− −

− −

⎤

⎦

⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥

[S]

0 0 0

0 0 0

0 0 0

0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

EνE

νE

νE E

νE

νE

νE E

G

G

G

1

1

1

1

1

1

1

212

313

121 2

323

131

232 3

23

13

12 (1)

The prismatic voids are assumed uniformly distributed within onyxmatrix and carbon fiber, and the effective elastic properties are listed asfollows [22]:

= −−

E ρ E(1 )1 1 (2)

= = −− −

E E ρ E(1 )2 3 11/2 (3)

= =− −

− + −

− −G G G

ρ ρρ ρ

(1 )(1 )(1 ) (1 )12 13

1 11/2

1 11/2 (4)

= −−

G ρ G(1 )23 11/2 (5)

= = −− −ν ν ρ ν(1 )12 13 1 (6)

= −−ν ρ ν(1 )23 1

1/2 (7)

= = = −− − −ν ν ν ρ ν(1 )21 31 32 1

1/2 (8)

where E (Elastic modulus), G (Shear modulus) and ν (Poisson’s ratio)are the elastic constants of the base materials (onyx and carbon fibers).ρ1 is void density on plane 1. The stiffness averaging matrix of a CFRPcomposite is:

∑= + +K V K V K V K[ ] [ ] [ ] [ ]ave infill infill solid solidi

n

CFi

CFi

(9)

whereVinfill,Vsolid andVCF are relative volume fraction for infill, solid andcarbon fibers. K ,infill K ,solid and KCF are the stiffness matrix for infill,solid and carbon fibers, respectively. The index i is the ith carbon fiberdirection. The average compliance matrix is =S K[ ] [ ]ave ave -1, and theelastic modulus can be calculated by = =E E, ,x S y S

1[ ]

1[ ]ave ave

11 22 and

=Ez S1

[ ]ave33 . For fiber angles that are not 0° (in the direction 1), a com-

pliance matrix transformation is needed, and the elastic modulus inthe× direction can be achieved [23].

⎜ ⎟= + ⎛⎝

− + ⎞⎠

+E E

cos νE G

sin cosE

sin1 1 2 1 1x

θ θ θ θ1

4 21

2 12

2 2

2

4

(10)

4. Results and analysis

4.1. Microstructural characterization

The morphology and microstructure of the CCFRP specimens withboth carbon fiber infill patterns were characterized by two digital mi-croscopes (VHX-900, Kenyence, Japan, and Dino-Lite AM4113T,

Fig. 2. Microstructure of a typical CFRP specimen with concentric carbon fiber rings and triangular polymer infill pattern. (a) Top view of the flexural test specimen.(b) Cross-section view from A-A direction. Shells are represented as dark strips, carbon fibers are represented as dark dots, infill patterns are represented as outlineddiamonds and solid polymer layers are represented as white rectangles. (c) Top view of a build layer with triangular infill patterns (50% infill density).

Fig. 3. Schematic of the structure of the flexural test specimen. (a) Top view of a carbon fiber layer showing both infill, floor layer and continuous carbon fibers. (b)The chopped carbon fibers can be clearly seen in the onyx filament with a preferred orientation along the filament, while the continuous carbon fibers are perfectlyaligned (c) 3-D view of the macro-porosity of the infill pattern.

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Roanake, VA) and an SEM (Phenom ProX, Eindhoven, Netherlands).The digital image correlation (DIC) method was used (Vic-2D Version2009, Correlated Solutions, Inc., Columbia, SC) to track and measurecrack propagations during tensile testing. The cross section of thespecimen used in this study is shown in Fig. 2(a) and (b). The whiterectangles part represents solid polymer layers and the dark strips partrepresents shells. They are both composed of solid Onyx. The shelllayers are symmetrically fabricated around the specimen as the roof,floor and wall. Fig. 2(c) shows the microstructure within a single layer.The infill pattern (50% infill density) and the concentric continuous

carbon fibers can be clearly observed.The mesostructure (i.e., void density and infill pattern) of additively

manufactured CCFRP is critical for its mechanical behavior character-ization and modeling [22]. 2D and 3D optical images of differentmagnification were used to observe the mesostructure of the additivelymanufactured CFRP specimens. Fig. 3(a) shows the optical image of abuild layer (the first layer with continuous carbon fibers), and the floorlayer (solid onyx infill) beneath can be clearly seen. A 50% infill densitywas used, and the infill was marked with yellow arrows. With a highermagnification as shown in Fig. 3(b), the chopped short carbon fiberscan be clearly observed within the onyx filament. Most of these fibersare aligned in a preferable direction, which is the print direction. Thesechopped carbon fibers within the onyx filament will increase voiddensity due to the loose fiber-matrix bonding. The topology of themacro-voids can be observed in the 3D image as shown in Fig. 3(c), andthe layer height is about 0.125mm. With these observations, the voiddensityρ1 =50% for the infill section and the void densityρ1 =10% forthe solid section will be used (Eqs. (2)–(8)).

The continuous carbon fibers are 0.35mm filaments formed by abundle of micro carbon fibers. During 3D printing, the continuouscarbon fibers in a diameter of 0.1 mm were extruded. Within eachcontinuous carbon fiber bundle, there are up to a thousand of micro-carbon fibers (about 10 µm in diameter as shown in Fig. 4). There is alarge amount of bonding between micro-carbon fibers. There are alsomicro-voids within the bonds. Both will affect the mechanical proper-ties of CCFRP specimens. Since the stiffness of micro-carbon fibers areseveral magnitudes higher than their bonding, the bonding was taken as

Fig. 4. Micro-carbon fibers embedded within the filament showing bonding inbetween, void in the bonding can also be observed.

Fig. 5. SEM images of CCFRP specimens (a) a single filament of continuous carbon fibers on the printed CCFRP specimen (b) overhanging continuous carbon fibersbetween two filaments (c) two continuous carbon fibers on a printed layer (d) cross-section view of broken carbon fibers of a fractured CCFRP specimen.

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voids in the following modeling scheme. A void density of 60% wasused for the continuous carbon fiber section.

SEM images of different magnifications were used to examine themesostructure and microstructure of the CCFRP specimens. Fig. 5(a–c)show three SEM images of an as-printed layer of the CCFRP specimen.Fig. 5(d) shows the cross-section view of a fractured surface of CCFRPspecimen. A polymer bonded bundle of continuous micro-carbon fibers

in the as-printed CCFRP layer can be observed in Fig. 5(a). The con-tinuous carbon fibers are packed and bonded within the bundle, whichlater was hot extruded and printed in a layer by layer fashion. Thespacing can be observed among micro-carbon fibers. Between eachbundle of continuous carbon fibers, there is a large spacing as shown inFig. 5(b), where some of the carbon fibers are hanging across thisspacing. This large spacing will weaken the carbon fiber strengthcompared to their bulk properties. A smaller spacing may benefit themechanical properties of the additively manufactured CCFRP. Fig. 5(c)shows a closer look of two single micro-carbon fibers. The diameter ofthe micro-carbon fibers is about 10 µm. It can also be observed that themicro-carbon fibers are not closely packed and thus left spacing inbetween. All of above observations support our assumptions of the ef-fective elastic properties model for the carbon fibers. The fracturedsurface of an additively manufactured CCFRP specimen clearly showsthe broken polymer matrix and broken micro-carbon fibers, some ofwhich are pullout from the polymer matrix. In summary, as shown inFigs. 4 and 5, the spacing/porosity of the additively manufacturedCCFRP specimens consists of both macro level and micro level porosity.The macro level porosity includes the matrix infill density controlledporosity, the spacing between each two carbon fiber bundles, and thespacing between a carbon fiber bundle and walls. The micro levelporosity includes the spacing among micro-carbon fibers within acarbon fiber bundle, and the voids within the polymer matrix. Theporosity of both levels contributes to weight reduction of the compo-sites but may also facilitate crack propagation. In order to understandthe mechanical behavior of the mesostructure, an effective elasticmodel based on the porosity level (Eqs. (2)–(8)) and stiffness averagingmethod (Eqs. (9) and (10)) were used to predict the elastic constants ofdifferent CCFRP specimens. The prediction results and other details areprovided in section 4.3.

A different isotropic carbon fiber infill pattern was also investigatedin this study in comparison with the concentric infill pattern. Onespecimen with angled (0°/45°/90°/135° fiber angles) isotropic carbon

Fig. 6. Microscopic images of an as-printedlayer of CCFRP specimens of differentcarbon fiber infill patterns and magnifica-tion (a) upper section of the CCFRP spe-cimen with concentric carbon fibers (b)middle section of the CCFRP specimen withconcentric carbon fibers (c) upper section ofthe CCFRP specimen with 45° isotropiccarbon fibers (d) middle section of theCCFRP specimen with 45° isotropic carbonfibers.

Fig. 7. Force-displacement curves for tensile tests of CCFRP specimens of dif-ferent carbon fiber concentration and microstructure, the lowest carbon con-centration sample L2R1 shows the highest ductility and lowest strength(0.95GPa), the highest carbon concentration sample L8R3 shows the lowestductility and highest strength (5.19GPa). Notation: LxRy where×denotes thenumber of carbon fiber layers, y denotes the number of carbon fiber rings.

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Fig. 8. Major principle strain of CCFRP specimens during tensile tests, (a)–(f) L2R3 specimen crack initiation and propagation, (g)–(h) L8R3 specimen showed morebrittle fracture with faster crack propagation.

Fig. 9. Results of four-point flexural tests.(a) Flexural strength of CCFRP specimenswith different number of carbon fiber layersand different number of carbon fiber ringswithin each layer. (b) Stress-strain curvesfor CCFRP specimens of isotropic and con-centric continuous carbon fiber infill pat-terns with six carbon fiber rings and 16carbon fiber layers (43.5 wt% carbon fi-bers).

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fiber layers was fabricated. Fig. 6 shows the optical images of the as-printed layer of both CCFRP specimens. Fig. 6(a) and (b) show theconcentric infill pattern with six concentric carbon fiber rings. Eachcarbon fiber bundle consists of hundreds of micro-carbon fibers. Theonyx in Fig. 6(a) shows a diagonal-rectangular infill pattern, which canbe seen in the middle section of the layer. As a comparison to theconcentric carbon fiber rings, Fig. 6(c) and (d) show an isotropic infillpattern with a layer of 45° fibers. The carbon fiber concentrations ofboth specimens are the same. Both infill patterns were successfullyprinted.

4.2. Tensile and flexural behaviors

To evaluate the tensile behavior of CCFRP specimens with differentcontinuous carbon fiber concentration and microstructure, twelve ten-sile tests of different CCFRP specimens were conducted, and the resultsare shown in Fig. 7. The strength and ductility of each specimen wasfound dependent on their carbon fiber concentration levels. The spe-cimens with a higher carbon fiber concentration showed a higherstrength and lower ductility. The highest strength achieved is 5.19GPafor the sample L8R3 (eight carbon fiber layers and three carbon fiberrings) with a carbon fiber concentration of 8.3 wt%, while the loweststrength is 0.95GPa for the sample L2R1 (two carbon fiber layers andone carbon fiber ring) with a carbon fiber concentration of 0.76 wt%.Achieve both high strength and ductility will be challenging but criticalfor the future research.

Digital Image Correlation (DIC) analysis was conducted during thetensile testing for the 2D strain measurement of CCFRP specimens.Fig. 8(a)–(f) show the major principle strain of L2R3 specimen (carbonfiber concentration of 2.14 wt%), and Fig. 8(g)–(h) show the majorprinciple strain of L8R3 specimen, during the tensile testing for thecrack initiation and propagation. A similar observation can be made asin the force-displacement curves, the L2R3 specimen showed highductility due to its low carbon fiber concentration. In addition, an in-clined crack angle of 45° with respect to the axial direction was ob-served in the direction where the maximum shear stress presents. The

L8R3 specimen showed lower ductility due to its high carbon fiberconcentration. During DIC measurement, the sampling frequency is1 Hz (one picture was taken in each second) and Fig. 8(g) and (h) weretaken consecutively. It can be observed that the crack propagation ismuch faster for the specimen L8R3 than others. CCFRP fabricationthrough additive manufacturing with both high ductility and strengthare very attractive and will be investigated in the future research, whichis out of the scope of the current study.

To evaluate the flexural strength of CCFRP specimens with differentcontinuous carbon fiber concentration and microstructure, 4-pointflexural tests were conducted. The stress σ for a four-point flexural testis defined as

=σ FLbd

34 2 (11)

whereF is the load at the fracture point, L is the length of the supportspan, b is the width and d is the thickness of the test specimen. Thisstress is the stress in the outermost carbon fibers of flexural specimens.

The flexural tests results are shown in Fig. 9. For the specimen with18 fiber layers, as the number of carbon fiber rings increased from 1 to6, the flexural strength increased from 69.06MPa to 270.63MPa. Forthe specimen with 6 carbon fiber rings, as the number of carbon fiberlayers increased from 2 to 18, the flexural strength increased from81.97MPa to 270.63MPa. For the specimen with only one and twocarbon fiber rings, the number of carbon fiber layers has little impact onflexural strength because these fiber rings were 3D printed at the outer

Table 1Elastic constants and strength of onyx and continuous carbon fiber filament.

Material property Onyx Carbon fiber

Elastic modulus E1 (GPa) 1.4 60Elastic modulus E2 (GPa) 1.4 2Poisson’s ratio ν12 0.35 0.33Poisson’s ratio ν23 0.35 0.1Tensile Strength (MPa) 36 800Flexural Strength (MPa) 81 540

Fig. 10. Actual CFRP elastic modulus between measured in experiments versus predicted values: (a) tensile modulus, (b) flexural modulus.

Table 2Relative errors of predicted CFRP elastic modulus for tensile tests.

Number of carbon fiber layers Relative error

2 25.95%4 11.30%6 8.70%8 13.97%

Table 3Relative errors of predicted CFRP elastic modulus for flexural tests.

Number of carbon fiber layers Relative error

2 57.86%4 49.56%6 31.87%8 19.89%10 25.66%12 20.76%14 15.99%16 21.21%

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boundary of the specimen. As the number of carbon fiber rings in-creased, the impact of the number of carbon fiber layers on the flexuralstrength increased. The flexural strength of CCFRP specimen fabricatedby FDM is 40% higher than that of chopped carbon fiber reinforcedpolymer composites (one of the DCFRPs) for the 5 wt% carbon fiberconcentration case [7], which demonstrated CCFRP is a better candi-date in high-strength applications compared to DCFRP. The flexuralbehavior for different carbon fiber pattern was shown in Fig. 9(b). Thespecimen with the concentric infill pattern shows a higher flexuralstrength than the isotropic infill pattern, and both specimens consist of16 continuous carbon fiber layers and 43.5 wt% carbon fiber con-centration. Additionally, the isotropic specimen failed at a smallerstrain around 3%, while the concentric specimen failed at a higherstrain around 4%, which shows a higher energy absorption capacity.According to the Eqs. (9) and (10), the angled fiber layers have lowerelastic modulus E1 compared with the unidirectional (fiber angles areall of 0°) reinforced CCFRP. This renders a higher elastic modulus E1and thus high energy absorption capability for CCFRP specimen of theconcentric carbon fiber rings (fiber angle all 0°). However, the angledCCFRP specimen will have better performance in other properties (e.g.transverse tensile modulus and shear resistance).

4.3. Prediction of elastic modulus

In order to predict the elastic behavior of each CCFRP specimen,effective elastic properties based on the porosity level and stiffnessaveraging method were used in this study. The elastic properties of basematerials Onyx and carbon fibers of 100% density are listed in Table 1.

The effective elastic constants for each material were calculatedusing Eqs. (2)–(8). The elastic modulus of 12 CCFRP tensile specimensand 54 CCFRP flexural specimens with concentric carbon fiber ringswere predicted using stiffness averaging method (Eq. (9)). The predic-tion results were shown in Fig. 10 (a) and (b) (sample index is as-cending by the number of rings and the number of carbon fiber layers).For the tensile modulus comparison as shown in Fig. 10(a), it can beseen that for the specimens with a small number of carbon fiber layers(2 and 4), the predicted modulus is slightly higher than the experi-mental results. However, the predicted modulus becomes lower thanthe experimental results with the increase of carbon fiber layers. Thelowest relative error achieved is 8.70% for 6 carbon fiber layers spe-cimens (as shown in Table 2).

For the flexural tests comparison, it can be seen that for the sampleswith a small number of carbon fiber layers (2 and 4), the predictedmodulus is higher than the experimental results, which is due to the lowreinforcement effect of few carbon fibers. For these kind of specimens,

the cracks generated from the outer layers can shear the carbon fibersmore easily. This local effect was not taken into account in the currentmodel. With more carbon fiber layers printed within the specimens, thepredicted values are closer to the testing results. The relative errors ofthe predicted elastic modulus for different number of carbon fiberlayers are shown in Table 3. When the number of carbon fiber layersexceeds eight, the relative error stays around 20%, and the lowest re-lative error achieved is about 15.99% for the specimens with 14 layersof carbon fibers. A detailed examination of crack growth during theflexural testing may be helpful to better understand the high relativeerror in small number of carbon fiber layers, and this will be in-vestigated in the future research. Fig. 11. (a) and (b) show the predictedagainst actual elastic modulus for tensile and flexural tests, respec-tively. The results have shown that the predictive results using stiffnessaveraging method fit the experimental data very well.

5. Conclusions

In this study, CCFRP with concentric and isotropic carbon fiber infillpatterns were additively manufactured using FDM. The macro porosityand micro porosity of the CCFRP specimens were observed in the SEMimages. Effective elastic properties based on the porosity level and thestiffness averaging method were used to predict the elastic modulus ofthe CCFRP specimens.

1. Experimental results have shown that the specimens with the con-centric carbon fiber infill pattern showed a higher flexural strengthand energy absorption capability than those with the isotropiccarbon fiber pattern and 43.5 wt% carbon fibers. A 40% increase inflexural strength was achieved in CCFRP compared to choppedcarbon fiber reinforced polymer composites with 5 wt% carbon fi-bers. The highest strength of 270.63MPa was achieved for thespecimen with 48.72 wt% carbon fibers.

2. Microstructural characterization was conducted on the additivelymanufactured CCFRP specimens. The fracture surfaces and largespacing of the CCFRP specimens within the polymer infill andcarbon fiber bundles were observed. These spacing and voids have asignificant impact on predicting the elastic properties as well as thetensile and flexural behaviors of the CCFRP specimens.

3. It was found that the strength and ductility of each specimen weredependent on their carbon fiber concentration levels. The specimenswith a higher carbon fiber concentration showed a higher strengthand lower ductility.

4. Additively manufactured angle-ply CCFRP specimens exhibitedlower elastic modulus in the longitudinal direction. However, angle-

Fig. 11. Actual CFRP tensile modulus measured in experiments versus predicted values: (a) tensile modulus, (b) flexural modulus.

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ply CCFRP specimens may exhibit enhanced strength in other di-rections. The concentric fiber rings were found to be more beneficialin the flexural strength reinforcement.

5. The stiffness averaging method was demonstrated to be effective inpredicting the elastic modulus of additively manufactured CCFRP bytaking into account porosity in infill, solid and carbon fiber bundles.The predicted elastic moduli are in good agreement with experi-mental data. The lowest relative error achieved is about 8.70% and15.99% for both types of tests, respectively.

Acknowledgments

We would like to thank Dr. Ali Gordon for his valuable and con-structive suggestions during the development of this work. We wouldalso like to thank Mr. Aaron Santomauro for conducting the flexuraltests.

Data availability

The data that support the findings of this study are available onrequest from the corresponding author.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.compstruct.2019.111147.

References

[1] Friedrich K, Almajid AA. Manufacturing aspects of advanced polymer compositesfor automotive applications. Appl Compos Mater 2013;20(2):107–28.

[2] Williams G, Trask R, Bond I. A self-healing carbon fibre reinforced polymer foraerospace applications. Compos A Appl Sci Manuf 2007;38(6):1525–32.

[3] Pinho L, Carou D, Davim J. Comparative study of the performance of diamond-coated drills on the delamination in drilling of carbon fiber reinforced plastics:assessing the influence of the temperature of the drill. J Compos Mater2016;50(2):179–89.

[4] Henerichs M, Voss R, Kuster F, Wegener K. Machining of carbon fiber reinforcedplastics: Influence of tool geometry and fiber orientation on the machining forces.CIRP J Manuf Sci Technol 2015;9:136–45.

[5] Soutis C. Carbon fiber reinforced plastics in aircraft construction. Mater Sci Eng, A

2005;412(1–2):171–6.[6] Parandoush P, Lin D. A review on additive manufacturing of polymer-fiber com-

posites. Compos Struct 2017;182:36–53.[7] Ning F, Cong W, Qiu J, Wei J, Wang S. Additive manufacturing of carbon fiber

reinforced thermoplastic composites using fused deposition modeling. Compos BEng 2015;80:369–78.

[8] Tekinalp HL, Kunc V, Velez-Garcia GM, Duty CE, Love LJ, Naskar AK, et al. Highlyoriented carbon fiber–polymer composites via additive manufacturing. Compos SciTechnol 2014;105:144–50.

[9] Love LJ, Kunc V, Rios O, Duty CE, Elliott AM, Post BK, et al. The importance ofcarbon fiber to polymer additive manufacturing. J Mater Res 2014;29(17):1893–8.

[10] Henning F, Karger L, Dorr D, Schirmaier FJ, Seuffert J, Bernath A. Fast processingand continuous simulation of automotive structural composite components.Compos Sci Technol 2019;171:261–79.

[11] Matsuzaki R, Ueda M, Namiki M, Jeong T-K, Asahara H, Horiguchi K, et al. Three-dimensional printing of continuous-fiber composites by in-nozzle impregnation. SciRep-Uk 2016;6:23058.

[12] Yang Y, Chen Z, Song X, Zhang Z, Zhang J, Shung KK, et al. Biomimetic anisotropicreinforcement architectures by electrically assisted nanocomposite 3D printing. AdvMater 2017;29(11):1605750.

[13] Rodriguez JF, Thomas JP, Renaud JE. Mechanical behavior of acrylonitrile buta-diene styrene fused deposition materials modeling. Rapid Prototyping Journal2003;9(4):219–30.

[14] Melenka GW, Cheung BKO, Schofield JS, Dawson MR, Carey JP. Evaluation andprediction of the tensile properties of continuous fiber-reinforced 3D printedstructures. Compos Struct 2016;153:866–75.

[15] Parandoush P, Zhou C, Lin D. 3D printing of ultrahigh strength continuous carbonfiber composites. Adv Eng Mater 2018:1800622.

[16] Hochard C, Payan J, Bordreuil C. A progressive first ply failure model for woven plyCFRP laminates under static and fatigue loads. Int J Fatigue 2006;28(10):1270–6.

[17] Heß H, Roth YC, Himmel N. Elastic constants estimation of stitched NCF CFRPlaminates based on a finite element unit-cell model. Compos Sci Technol2007;67(6):1081–95.

[18] Ogi K, Nishikawa T, Okano Y, Taketa I. Mechanical properties of ABS resin re-inforced with recycled CFRP. Adv Compos Mater 2007;16(2):181–94.

[19] Lozada JN, Ahuett-Garza H, Castañón PO, Verbeeten WM, González DS. Tensileproperties and failure behavior of chopped and continuous carbon fiber compositesproduced by Additive Manufacturing. Addit Manuf 2019.

[20] Ning F, Cong W, Hu Y, Wang H. Additive manufacturing of carbon fiber-reinforcedplastic composites using fused deposition modeling: effects of process parameterson tensile properties. J Compos Mater 2017;51(4):451–62.

[21] Goh G, Dikshit V, Nagalingam A, Goh G, Agarwala S, Sing S, et al. Characterizationof mechanical properties and fracture mode of additively manufactured carbonfiber and glass fiber reinforced thermoplastics. Mater Des 2018;137:79–89.

[22] Rodríguez JF, Thomas JP, Renaud JE. Mechanical behavior of acrylonitrile buta-diene styrene fused deposition materials modeling. Rapid Prototyping Journal2003;9(4):219–30.

[23] Reddy JN. Mechanics of laminated composite plates and shells: theory and analysis.CRC Press; 2004.

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