A functional analysis of an Additively Manufactured Bellcrank

with long fibre reinforcement

Frederic Louis Bentofrederic.bento@ist.utl.pt

Instituto Superior Tecnico, Universidade de Lisboa, Portugal

June 2019

Abstract

Additive Manufacturing (AM) is as of 2019 a mainstream manufacturing technology having evolvedfrom its rapid prototyping roots, however developments and refinements are ongoing with one in par-ticular being the production of composites.

This thesis evaluates the ability of Fused Deposition Modeling (FDM) to produce fibre reinforcedplastic components of comparable performance to those produced by conventional composite manufac-turing methods. To investigate this a Markforged Mark Two printer was used to produce a componentto replace a carbon fibre bellcrank used in a Formula SAE prototype car.

Using an iterative design for AM methodology several AM designs were designed, printed and tested,this was done based on recomendations from the printer manufacturer as early on a comparison ofsimulation and physical test deemed validation through finite element modeling invalid for AM producedparts. In the end it was not possible to produce a functional component.

Following this a comparison is done between the conventional autoclaved prepreg productionmethod and FDM to identify potential AM gains and finally an improved design methodology isproposed as well as alternative uses for this type of FDM produced composites.Keywords:Additive Manufacturing, Fibre Reinforced Plastic, Design for Additive manufacturing,Fused Deposition Modeling.

1. Introduction

Additive Manufacturing (AM) has grown steadilyfrom its early developments in the 1960s to its usetoday in manufacturing finished parts [5]. This the-sis was proposed to the author as part of project todevelop knowledge of AM on several fronts. Theinitial brief was ”a functional, economic and en-vironmental analysis of an AM produced compo-nent” with only two constraints: first the compo-nent should be connected to a mobility applicationand second it is to be manufactured using a Mark-Forged Mark Two printer. The scope of this workis to ascertain the capability of AM for the produc-tion of functional fibre reinforced parts, using a bell-crank as a case study. This case study will comparea fibre reinforced Nylon pushrod suspension bell-crank produced in AM with a currently producedpart of carbon fibre with an epoxy matrix. The goalis to achieve performance equal to the existing partusing AM. The definition and testing of this perfor-mance is a challenge due to a lack of knowledge onthe mechanical properties of AM materials, whichprecluded the use of conventional simulation toolsand ultimately led us do devise a physical testing

apparatus with which to test AM produced compo-nents. Afterwards (using an iso-performance AMdesign if achievable) a process comparison is madeto establish the advantages of AM versus the handlay-up production method of the conventional part.

1.1. History

Although some ideas and concepts concerning AMwere recorded in the late 19th century, modern AMdevelopment is considered to have begun in earnestduring the 1960s [5]. The development of AMtechnology has always followed its two originatingtechnologies CAD (computer Assisted Design) andCAM (Computer Assisted Manufacturing). duringthe 1980s (1988 - FDM) and into the 1990s (1992 -laser sinthering) many patents were filed for today’sAM technologies, the 90s also saw the appearence ofAM-specific CAD file formats such as .stl and .cli.In the 2000s a massification of 3D printing tookplace with the expiration of several patents regard-ing several older AM processes. In this decade theemmergence of desktop printers and the exchange ofknowledge made easy by the Internet saw AM reacha larger than ever audience. At the time of writing3D printing is used in a variety of applications with

1

uses in different fields growing every day.

1.2. Fused Deposition Modeling

The most common material extrusion process vari-ant, FDM (Fused Deposition Modeling) is an AMprocess in which a polymer (usually a thermoplas-tic) filament is heated to a semi-liquid state thenextruded through a nozzle onto a platform or pre-viously printed layers [3, 2]. The sucessive lay-ers of heated fillament fuse together during print-ing and solidify at room temperature [9]. Whenprinitng parts with overhangs support material isused, which is removed in post-processing. Thedimension of the nozzle determines the depositionrate of part material and the size of the small-est printable feature [3]. Aside from this severalother process parameters affect the properties of theprinted part such as layer height, extruder temper-ature, extruder pathing, the use of supports andvolumetric fill percentage. In order to adress thelack of strength of FDM parts, which limited theirapplications to prototypes, printed composites havebeen developed [15]. Composites manufactured byFDM allow for stronger printed parts when com-pared with pure thermoplastic parts by using parti-cles, fibres or nanomaterials as reinforcements. Thisallows the production of functional parts with greatdesign flexibility [15]. One way to achieve this isby adding short fibres (from the micrometre scaleto a few milimetres long) to the thermoplastic fila-ments used by an FDM printer [10]. The inclusionof chopped reinforcing fibres in printed thermoplas-tics has been shown to greatly enhance the mechani-cal properties of the printed material [11][10]. Withnoticeable improvements with as little as 1% of re-inforcing fibre (weight percentage) [13]. Howeverthe introduction of reinforcement in whatever formaggravates the problem of porosity and lack of ad-hesion between layers, limiting the amount of re-inforcing material that can be used whitout issue[10]. In addition to printing chopped fibres, FDMcan also print with long fibre reinforcement.

1.3. Fused Deposition Modeling with Long FibreReinforcement

FDM produced composites with long fibre re-inforcement are both a stronger alternative topure thermoplastic FDM and a tooless variant ofCFRTCs (Continuous Fibre Reinforced Thermo-plastic Composites) [14]. One way to print withcontinuous fibres is to use a filament of continuousfibre coated in thermoplastic (a solution adopted byMarkforged in their printers)[15]. Alternatively in-stead of having a coated fibre filament, it is possibleto mix the fibre with the thermoplastic within theprinting head [4]. AM manufacture of compositesis part of a growing effort to automate the produc-tion of composites, reducing labour costs, increasing

productivity and removing human error from theproductive process [2]. Researchers printing longfibre carbon reinforced PLA have achieved tensilestrength and Young’s Modulus 4 to 6 times greaterthan those of pure PLA specimens [15]. This im-provement is greater than what has been achievedwith short fibres, however in addition to the prob-lems of porisity and lack of adhesion between lay-ers, problems with discontinuity and irregularity ofthe reinforcing fibres were found when printing withcontinuous fibres [2]. Also these improvements werefound to be below those predicted by the rule ofmixtures [15].

Figure 1: FDM with continuous fibre reinforcement[14].

1.4. DFAM: Challenges and IssuesAM processes are found all over the spectrum ofindustrialisation with more recent or less developedtechnologies having just been validated in the lab-oratory while others are already being used to pro-duce parts for customers [1]. Expectations mustbe managed regarding AM’s design freedom as itisn’t total, just greater than that of most other pro-duction methods and should be thought of as al-lowing complex designs with fewer drawbacks thanother production methods. Regarding material us-age, more especifically waste generation, it is alsopresent in AM even though it can be less than thatof some conventional processes. AM’s design free-dom does not exempt it’s users from complying withdesign rules, on the contrary compliance with thesedesign rules is made more difficult by a lack of stan-dardisation of design methods for AM (DFAM) [7].Conversion of conventional process designs to AMis not as straightforward as converting a cad fileto stl and pressing print [16]. Currently the .stlfile type provides only the surface mesh of the ob-ject to be printed with any additional informationhaving to be inserted by the user when setting upthe print. The Additive Manufaturing File For-mat (AMF) has been proposed by an ASTM comi-tee to replace this file type with the advantage ofadding data on colour, material, texture and evensubstructure of the fabricated object [16]. CurrentCAE software is unable to perform concurrent de-

2

sign for X, as in predicting the impact one featurechange has on diferent lifecycle stages. This in prac-tice means there is no DFAM integrated framework,forcing design work to be conducted with diferentsoftware tools set up for different design objectives,making design loops single objective and discon-nected from subsequent loops, which are in turnliable to violate the rules of earlier design loops [8].The main issue stopping a wider addoption of AMin industry is the lack of development and under-standing of DFAM which limits the widespread useof AM [5].

2. PAC Fibr3D ProjectThe PAC Fibr3D project is part of a national ini-tiative aimed at fostering knowledge of AM in por-tuguese universities. As part of this project thisthesis in particular is aimed at exploring a transportrelated application of fibre reinforced AM, using aMarkforged Mark Two printer. Given the themea partnership was sought out with the university’sFSAE(Formula Society of Automotive Engineers)team. It was then agreed to develop an AM pro-duced front bell crank for their prototype vehicle.

3. Suspension Bellcrank

Figure 2: (Left) Bell crank mounted in the FST 07e;Bell crank assembly modeled in Solidworks (right).

After initial discussions with the FST Lisboaproject team and several part proposals rangingfrom fitment prototypes to a replacement wheel hubit was decided to develop an alternative to the car’scurrent bellcrank, a suspension component that iscurrently produced in carbon fibre, having previ-ously been made from alluminium. This part waschosen due to the fact that the loadings applied toit are mostly in-plane facilitating simulations andphysical testing. Furthermore this application wasdeemed by the team as a low risk means of assess-ing the suitability of fibre reinforced 3D printing toproduce functional composite parts, with the ad-vantage of having local production capability in theform of the laboratory’s Markforged printer.

4. ImplementationTo develop the new bell crank a study was con-ducted of the requirements for the part, at the

same time a design space was established by tak-ing into account all geometric constraints and po-tential interference with other elements of the car.The orginal design was then altered after consult-ing Markforged’s design recomendations. To con-firm functionality two laboratory tests were devised,based on the most demanding loads placed on thepart. In sucession PLA then carbon fibre rein-forced prototypes were tested and a final specifica-tion for the part set. After the functional analysisthe potential benefits in the production phase wellbe looked into. Finally a review of the developmentprocess for AM will be done with recomendationson how to deal with knowledge gaps in Design forAdditive Manufacturing, with a final overview ofthe work done and evaluation of the AM-spec bellcrank.

Figure 3: Schematic of the analysis methodologyused.

4.1. Conventional GeometryEach bellcrank consists of two carbon fibre platesthat are bolted to the chassis and have a spacerbetween them. They are also connected to the othersuspension components by rod end bearings. Eachplate is 3.9mm thick with a constant cross section inthe z axis (see figure 4). The plates are separated bya gap of 20mm, giving room for the rod ends andother connecting elements to operate, the overallthickness is 27.8mm.

4.2. Load Case for AM redesignThe functional requirement is a displacement mag-nitude of at most 1mm for the load cases given.

3

Figure 4: Z-axis cross section of a single bell crankplate.

From the suspension model provided and wheelloads given the reference case resulted (table 1).

Reference Load Case[N] P-Rod Spring A-R Bar Oxy Oz

LC 3360.7 3114 474.6 4920.1 32.6

Table 1: Reference load case the part must with-stand this load and only deform 0.1mm at the most.

With fully defined constraints and requirementswe are now ready to design a part to be producedby AM.

5. AM Design DevelopmentThis redesign for AM is done as a proof of conceptfor AM produced composites, with the initial focusbeing stiffness and manufacturability. For these rea-sons there are no concerns with increased mass orvolume reduction of the AM solution. Only after afunctional part is achieved with a simple redesignusing most of the available design space is an opti-misation considered.

The current bellcrank design can be modified intwo ways: geometry and material. For the geom-etry this means increasing the plate thickness orincreasing the outer radius in the hole areas thiscorresponds to the space measured between thehole diameter and the outer diameter of the partfor the chassis attachment point (Large hole) andpushrod, spring-damper and anti-roll bar (Smallholes) respectively, this quantifies the amount ofspace around both the small and large holes whichultimately limits the amount of fibre that can be in-serted in those areas. Material-wise apart from theinitial PLA prototype it made little sense to useany reinforcement other than carbon fibre, as thematerial used in the original part is carbon fibre.With the geometric conditions previously stated wecan only increase the plate thickness inward, withthis in mind a maximum thickness for each plateof 5mm was considered a good balance of increasedmaterial and not affecting the suspension geometry

significantly, meaning it could be bolted to the carwithout any other component of the car being mod-ified. Looking now at the cross section of the platesalong the z axis and in particular at the hole regionsan outer radius such that there is a gap of 14mmbetween the hole and the outer face of the platewas deemed suficient, meaning an outer radius of20mm around the smaller holes and 42mm aroundthe larger one. When planning to exploit the interplate space the first constraint is to assure the free-dom of movement of the rod ends by maintaininga radius of 15mm around the small holes betweenplates. In the conventional design around the chas-sis mounting hole there is space taken up by thespacer but if another way to ensure the gap betweenplates was found it could be removed and the bell-crank simply bolted to the chassis using the samewashers, allowing the use of all the space aroundthe chassis connection hole. This opens up the pos-siblility of a single piece bell crank to be made.

5.1. Laboratory Test 1 - Pushrod attachmentTo test the deformation in the part due to the loadson the small holes a test rig was set up based on themost demanding load case in which the push rodtransmits 3.3 kN away from the bell crank. At firstglance it is expected that the hole region where thepush rod connects will suffer the highest loads withthe smallest amount of material to bear it, mean-ing it should fail before other areas where the loadsare smaller and where in some cases there is morematerial. In light of this a test rig was set up totest the push rod connection hole with a force ap-plied in a similar direction to the real case. The testrig (Figure 4.4) fixes both plates of the bellcrank inposition at points O and S, and applies a force atpoint P, leaving point A free. The test consists ofapplying force at point P while measuring the forceapplied and the displacement. The target set forthe part is to withstand a force of at least 3.3kNwhilst deforming less than 1mm.

Figure 5: Set up for the Pushrod Attachment Test.

After connecting the PLA part to the testing ap-

4

paratus, the part was loaded with a deformationspeed of 0.5mm/min, this speed was used for bothof the carbon fibre part tests as well. During thetest the highest recorded load before part failurewas 1627N with a corresponding displacement of1.28mm. The force required to cause a 1mm dis-placement of the part was only 1381N. This doesnot meet the requirements set.

Figure 6: Load-Displacement curve for the initialPLA part. Peak Load: 1627N Maximum displace-ment: 1.28mm Test done at 0.5mm/minute defor-mation rate.

The first carbon reinforced part (CF05) withplate thickness 5mm was then tested, achieving apeak load of 3.5kN and maximum deformation of2.95mm.

Figure 7: Load-Displacement curve for the carbonreinforced part with 5mm thickness. Peak Load:3.5kN Maximum displacement: 2.95mm Test doneat 0.5mm/minute deformation rate.

The second carbon part (CF10) with thickness10mm was tested, achieving a peak load of 3.5kNand maximum deformation of 1.96mm.

5.2. Test 2 - Chassis attachmentTo replicate the largest simulated chassis attach-ment load an experiment was devised in which thebell crank was fixed to the test bench while the re-maining holes are loaded vertically (fig 9). The idea

Figure 8: Load-Displacement curve for the carbonreinforced part with 10mm thickness. Peak Load:3.5kN Maximum displacement: 1.96mm Test doneat 0.5mm/minute deformation rate.

Pushrod Attachment TestsLoad (kN) Displacement

Target 3.3 <1 (mm)

PLA 1.6 1.28 (mm)Carbon (t=5mm) 3.3 2.72 (mm)

Carbon (t=10mm) 3.3 1.89 (mm)

Table 2: Results for pushrod attachment tests com-pared to the target results.

here is to concentrate the load on the large hole(chassis attachment). The peak load for this test is5kN in plane with the bell crank. For the test ofCF10 the maximum load was mistakenly increasedto 5.6kN, this highlights one of the disadvantages ofusing the VIC software: since it isn’t connected tothe tensile test machine one is unable to find thedisplacement of intermediate loads, the only oneobtainable is that of the last point filmed, whichcorresponds to the maximum load. This means wedo not have the displacement of CF10 when loadedwith 5kN only for 5.6kN.

Figure 9: Set up for the Chassis Attachment Test(camera in the foreground).

From the test of the first carbon bell crank CF05a maximum displacement magnitude of 1.52mm wasreached (fig 10). It should be noted that due to

5

problems with lighting the displacement field ob-tained from Vic-2D 2009 does not cover the wholepart. However since the largest displacement mea-sured is larger than the maximum allowed by thespecification, the true maximum displacement is ir-relevant as the part is already considered inade-quate by the maximum displacement criteria.

(a) X displacement (b) Y displacement

Figure 10: Maximum displacement in X and Y di-rections for 3 point test of CF05. Due to problemswith the camera the displacement field obtaineddoes not cover the entire part, it does however serveto give an idea of the displacement magnitude forthe test load.

The test of the second bell crank (CF10) allowedthe capture of most of the parts displacement field,the displacement magnitudes were measured in agreater area than for CF05, which might explainthe greater than expected displacement.

(a) X displacement (b) Y displacement

Figure 11: Maximum displacement in X and Y di-rections for 3 point test of CF10.

Chassis Attachment TestsLoad (kN) Displacement

Target 5 <1 (mm)

CF05 (t=5mm) 5 1.5 (mm)

CF10 (t=10mm) 5.6 1.5 (mm)

Table 3: Results for chassis attachment tests com-pared to the target results.

The carbon bell cranks were both able to re-sist the maximum loads, however the maximumdisplacement was greater than what was deemedacceptable by the project team (1mm of displace-ment).

5.3. Simulation of the Pushrod Attachment TestThis test was simulated as the load applicationpoint is well known, in this case, the pushrod at-tachment. As in the laboratory test the chassis at-

Figure 12: Set up of the simulation in Simens NX,with the fixed point in blue and the applied load onthe pushrod connection in red.

tachment point is fixed (large hole). The load tobe simulated is 1.2 kN, this was chosen because itis still in the elastic range of the load-displacementcurve for the tensile test of the real part and giventhat the simulation software works best in the elas-tic range this should yield the most valid compari-son between reality and simulation.

5.3.1 PLA isotropic model

Since the PLA part was printed with 100% fill ofpure PLA it will considered an isotropic and ho-mogenous material with a Young’s Modulus be-tween 2.8 and 4.7 GPa [12, 6]. This was thoughtto be a good first approach as it should give agood estimate on where these simplifacations com-pare with reality. To bracket the range of Youngsmodulae found in research two isotropic simulationsa ”strong PLA” simulation with E=4.7GPa and a”weak PLA” simulation with E=2.8GPa. A finiteelement model was thus established using tetrahe-dral elements with 10 nodes and with an elementsize of 1mm, the resulting mesh has approximatelly80000 elements.

Figure 13: Displacement of the simulated part usinga reference load of 1.2kN modeled as isotropic andusing ”weak PLA” properties (E=2.8GPa) from re-search on FDM printed PLA.

6

Figure 14: Displacement of the simulated part usinga reference load of 1.2kN modeled as isotropic andusing ”strong PLA” properties (E=4.7GPa) fromresearch on FDM printed PLA.

5.3.2 PLA laminate theory model

Using lamina constants from research on printedPLA [12], it was possible to model the part as a”composite” with alternating laminas of 0/90 ori-entation of orthotropic PLA. A 2D mesh with thebell crank plate was generated using square ele-ments with 4 nodes and an element size of 1mm,the resulting mesh has 4522 elements.

Figure 15: Displacement of the simulated part us-ing a reference load of 1.2kN modeled using lami-nate theory and lamina date from research on FDMprinted PLA[12].

5.3.3 Simulation Comparison

Pushrod Attachment test simulationsPart and material model Displacement

Reference PLA lab test 0.871 (mm)

PLA (isotropic E=2.8GPa) 0.139 (mm)

PLA (isotropic E=4.7GPa) 0.082 (mm)

PLA (laminate approach) 0.0985 (mm)

Table 4: Results for the simulation of the pushrodattachment test of the PLA prototype and the cor-responding results obtained in the laboratory.

All simulations overestimated the stiffness of thepart with the most compliant simulation yieldinga displacement of only 15% of what was measuredin the laboratory. This results meets the expecta-tions of inadequate simulation tools for this type ofmaterial.

6. ConclusionsThe objective of this thesis was to determine thefeasibility of using AM produced composites in de-manding structural roles. The AM produced partswere unable to match conventional composites instiffness although they were comparable in strengthas none of the composite parts failed in testing.Thus we conclude that at this time AM producedcomposites are not yet suited to highly demandingapplications.

7. Dealing with AM knowledge gapsIn the course of this work the knowledge gaps inAM became apparent, these gaps stem from AMstill being in its infancy in terms of engineering de-sign methodologies. With time one expects thatresearch and practical experience will coalesce torefine DFAM methods. The main issues found re-lating to knowledge gaps were:

• Lack of detailed material models, with littleknowledge of elastic properties. These gaps inmaterial knowledge lead to a lack of appropri-ate simulation tools meaning that validating acomponent must be done by mechanical test-ing.

• Despite the guidelines given by manufacturersprinting issues were found leading to changesin the design to improve printability which re-sulted in parts that compromise performancefor printability, specifically having to alternatefibre layers with Nylon to avoid printing errors,this reduced the maximum amount of fibre thatcould be added to the part limiting its strength.

7.1. Proposed Design for Additive manufacturingmethodology

From the experience gained in developing this partfor AM a few recomendations regarding DFAM canbe made. This takes the form of a sample partdesign path, that covers the different design choicesavailable when using AM.

To make the most of AM’s design freedom its useshould be considered early on in the design stage asmultiple parts can be combined into one, reducingcomplexity and avoiding the need for fasteners orother joining methods. Loads should be carefullyand precisely defined to avoid over-engineering thepart. At the time of writing AM parts must bevalidated by physical testing. Table 5 shows themain steps in developing an AM produced part.

7

Idealized AM part development path

Part Purpose

GeometryMock-Up

Geometry validating proto-type made from a inexpen-sive material, allowing forearly detection of fitting,printing or other functionalproblems.

MaterialBenchmark

Using the same basic ge-ometry several parts areprinted with different ma-terials to find the best ma-terial for the componentunder development, withphysical testing replicatingservice loads. This mustbe done as currently simula-tion tools are unsatisfactoryand to detect any printingissues.

ProductionPart

Using the chosen materialthis part has its design opti-mized (geometry, print set-tings, reinforcement distri-bution, part orientation inthe print bed) for fast andreliable priting.

OptimizedPart

Further optimized partachieving the same func-tional requirements asthe production part withreduced material consump-tion and equal or smallerproduction time. Essen-tially an equal performingdesign with productivitygains.

Table 5: Milestone parts in the hypothectical de-velopment of an AM solution.

8. Closing Remarks

AM is still in growth with new developments inmaterials, print technologies and software apear-ing constantly. For our application improvementsto FDM with long fibre reinforcement could allowhigher fibre volume percentages, improving mate-rial strength [4].

Acknowledgements

This work would not be possible without the guid-ance of Professors Bruno Soares and Paulo Peas. Agreat deal of thanks is owed to Diogo Rechena andPedro Pereira for their essential contrinutions thatmade the lab work possible.

References[1] AM Platform. Additive Manufacturing :

Strategic Reseach Agenda. AM Platform,pages 1–64, 2014.

[2] J. Frketic, T. Dickens, and S. Ramakrish-nan. Automated manufacturing and processingof fiber-reinforced polymer (FRP) composites:An additive review of contemporary and mod-ern techniques for advanced materials manu-facturing. Additive Manufacturing, 14:69–86,2017.

[3] I. Gibson, D. Rosen, and B. Stucker. AdditiveManufacturing Technologies. 2010.

[4] B. Jackson. Northrop Grumman grantedpatent for composite fiber 3D printing. Onlinepublishing, 2019. Accessed: 2019-02-26.

[5] M. Kathryn, G. Moroni, T. Vaneker, G. Fadel,R. I. Campbell, I. Gibson, A. Bernard,J. Schulz, P. Graf, B. Ahuja, and F. Mar-tina. CIRP Annals - Manufacturing Tech-nology Design for Additive Manufacturing :Trends , opportunities , considerations , andconstraints. CIRP Annals - ManufacturingTechnology, 65(2):737–760, 2016.

[6] A. Lanzotti, M. Grasso, G. Staiano, andM. Martorelli. The impact of process param-eters on mechanical properties of parts fabri-cated in PLA with an open-source 3-D printer.2015.

[7] M. Leary, L. Merli, F. Torti, M. Mazur, andM. Brandt. Optimal topology for additivemanufacture : A method for enabling addi-tive manufacture of support-free optimal struc-tures. Materials and Design, 63:678–690, 2014.

[8] L. Li, J. Liu, Y. Ma, R. Ahmad, andA. Qureshi. Advanced Engineering Informat-ics Multi-view feature modeling for design-for-additive manufacturing. Advanced Engineer-ing Informatics, 39(November 2018):144–156,2019.

[9] T. D. Ngo, A. Kashani, G. Imbalzano, K. T.Nguyen, and D. Hui. Additive manufacturing(3D printing): A review of materials, methods,applications and challenges. Composites PartB: Engineering, 143(December 2017):172–196,2018.

[10] F. Ning, W. Cong, J. Qiu, J. Wei, and S. Wang.Additive manufacturing of carbon fiber rein-forced thermoplastic composites using fuseddeposition modeling. Composites Part B: En-gineering, 80:369–378, 2015.

8

[11] P. Parandoush and D. Lin. A review on ad-ditive manufacturing of polymer-fiber compos-ites. Composite Structures, 182:36–53, 2017.

[12] Y. Song, Y. Li, W. Song, K. Yee, K. Lee, andV. L. Tagarielli. Measurements of the mechani-cal response of unidirectional 3D-printed PLA.123:154–164, 2017.

[13] D. Thomas. Developing enhanced carbonnanotube reinforced composites for full-scale3D printed components. Reinforced Plastics,62(4):212–215, 2018.

[14] X. Tian, T. Liu, C. Yang, Q. Wang, and D. Li.Interface and performance of 3D printed con-tinuous carbon fiber reinforced PLA compos-ites. Composites Part A: Applied Science andManufacturing, 88:198–205, 2016.

[15] X. Wang, M. Jiang, Z. Zhou, J. Gou, andD. Hui. 3D printing of polymer matrix com-posites: A review and prospective. CompositesPart B: Engineering, 110:442–458, 2017.

[16] S. Yang and Y. F. Zhao. Additivemanufacturing-enabled design theory andmethodology : a critical review. pages 327–342, 2015.

9