Mechanical Performance of Long-bre Reinforced Polymer Composites by 3D Printing

Additive Manufacturing (AM), also known as 3D Printing, has been around for more than 2 decades and has recently gained importance for use in direct manufacturing. The quanti�ed physical properties of materials are required by design engineers to inform and validate their designs, and this is no less true for AM that it is for traditional manufacturing methods. Recent innovation in AM has seen the emergence of long-bre composite AM technologies, such as the Mark Two (Markforged Inc, USA) system, enabling the manufacture of thermoplastic polymer composites with long-bre reinforcement. To date though, the mechanical response of the materials with respect to build parameter variation is little understood. In this project, selected mechanical properties (ultimate tensile strength – UTS and �exural modulus) of samples processed using the Mark Two printer were studied. The effect of the reinforcement type (Carbon, Kevlar®, and HSHT glass), amount of reinforcement, reinforcement lay-up orientation, and the base matrix material (Onyx and polyamide) on these properties were assessed using accepted standard test methods.


Introduction
Developments in composites compositions, and understanding the design and manufacturing using Additive Manufacturing (AM) is an important research eld today [1], which can be seen from Wohler's Report 2016, that production of end use parts represented 51% of the worldwide market using AM, with Material Extrusion being the most widely used method [2]; the reason being high demand of these materials in the aerospace and automobile sectors due to them being light-weight, having high static strength, good fatigue resistance and damage tolerance [3], [4].One of the most important applications is in light-weighting in the aerospace sector, one widely recognised being the main fuselage and the wings of the Boeing 787 being composite, and in total 50% of the material used in the aircraft is advanced composites.This reduces the maintenance requirements and cost, and increases the total reliability of the airplane, reducing the weight by 20% [5].In the automotive industry, the fuel e ciency can be increased by light-weighing cars, with the use of thermoplastic or thermoset composites instead of steel.
A weight reduction of 55% was achieved for a conventional 9 m bus [6].The energy absorption for some thermoplastic composites can be 7-8 times that of traditional steel [7], offering opportunities for improved crash protection.
There is a growing interest in the use of AM for the manufacture of composite components.The bene ts of AM, in offering exibility in design (opening up the pathway to light-weight internal structuring and for part consolidation [8]), and tool-less manufacture (removing the cost burden and extensive lead-time for tooling, especially for low volume manufacture as within the aerospace and niche vehicle land-transport sectors [9]).These capabilities make it a very attractive proposition for these sectors, but uptake has been limited to date due to the poor mechanical properties that are obtained from the polymers [10], with, until relatively recently, no capability for improving mechanical properties through their reinforcement using long or continuous bres.
Recently, long bre reinforcement of composites has been developed to a commercial offering, provided by, for example, MarkForged Inc.The technology, based on the Material Extrusion AM technology, where a lament of polymer is melted and extruded through a nozzle onto a planar platform, with addition of multiple layers forming the component.A continuous reinforcing lament is deposited onto, and embedded into, the deposited polymer tracks, thus allowing long-bre reinforcement of the polymer matrix [11].
The right balance of matrix and reinforcement materials in a polymer composites result in optimal tensile strength and stiffness for speci c applications [12].Also, these mechanical properties are affected by the various build parameters during Additive Manufacturing.Hence the selection of the right balance of composite components and optimal build parameters are equally important.Known mechanical properties are important to understand the reliability and the future performance of the part that is designed and since the MarkForged technology is relatively new, there is little available mechanical performance data available.The aim of this paper is to characterise the mechanical performance of two common matrix materials employed in Material Extrusion (polyamide (PA) and Onxy (PA-carbon bre composite) thermoplastics) with a range of bre reinforcements (High Strength High Temperature Glass Fibre -HSHT (GF), Kevlar® Fibre (KF) -poly paraphenylene terephthalamide, and Carbon Fibre (CF), and processed using a range of build parameters (number of reinforcement layers and bre-placement orientation).We aim to identify, through a statistical analysis of the material performance, the role of each material and build parameter choice on the mechanical response of the material.This will provide product designers with the tools to enable them to utilise this technology in designing products to satisfy engineering requirements.

Additive Manufacture Of Thermoplastic Composite Components
Additive Manufacturing (AM) is widely used for the fabrication of polymer components ranging from prototypes to end-use parts [13].A wide range of AM technologies have been developed including Vat Polymerisation [14], Binder Jetting [15], Material Jetting [16] and Material Extrusion [17].The latter is widely used due to its ease of use, low cost and ubiquity within the market-place [18].The Material Extrusion process consists of a thermoplastic lament feed into a heated nozzle, where it is melted and deposited onto planar platform under CNC (Computer Numerical Control).After the manufacture of one layer of the component, the platform lowers (or the nozzle is raised) by one-layer thickness (typically 0.05-0.2mm) and the next layer is deposited.This is continued until the product is complete.
Non-reinforced thermoplastics used in this process suffer from poor mechanical properties [10], which as limited the widespread uptake of this technology for end-use products.Some attempt has been made to improve properties, e.g through the use of chopped bre reinforcement, such as glass bre and polypropylene (PP) bre in PP matrix, showing 30 and 40% improvement in UTS compared to pure PP [19], and 5 wt% of either vapour grown carbon bres (VGCFs) and single wall carbon nanotubes (SWNTs) in an acrylonitrile butadiene styrene (ABS) providing a 18% and 31% improvement in UTS [20].The larger challenge of incorporating continuous or long bre reinforcement into AM polymer composites offers the opportunity for signi cantly larger improvements in mechanical properties.This has been achieved by a number of researchers, namely, Matsuzaki et al. [21], developed in-nozzle impregnation of carbon and other bres in polylactic acid (PLA), with large increases in UTS and exural modulus observed.Addition of reinforcing bres into the deposited polymer layers was an approach taken by Mori et al. [22], with the embedding of Kevlar® bres into a nylon matrix using a desktop Material Extrusion system, improving strength and fatigue properties.Van Der Klift et al. [23] studied the mechanical behaviour of 6 layers of carbon bre embedded in a polyamide matrix using a commercial Mark One AM system (Markforged Inc, USA), observing a 9x increase in UTS over the unreinforced material, with similar studies by Dickson et al. [12] extending the study to the effect of Kevlar® reinforcement, with up to 6.3x increase in UTS and exural strength observed.
There is thus a growing body of work in observing the effect of continuous bre reinforcement of AM thermoplastic composites, providing evidence of useful and signi cant increases in mechanical properties.These studies though are, to date, lacking any use of experimental design accompanied by statistical analysis to elucidate the individual effects of material choice and build parameters on the mechanical response of the materials.This paper aims to provide product designers, and users of commercial continuous bre AM technology, with a knowledge base capturing the impact and statistical signi cance of the choice of base material, the reinforcement material, and some key build parameters; allowing them to make informed decisions in the choice of materials and parameters to meet their design needs.

Materials
Mechanical test samples were manufactured using a Mark Two Material Extrusion system (Markforged Inc, USA).The base materials employed were nylon, a proprietary polyamide composition from Markforged) and Onyx, a proprietary short carbon bre reinforced polyamide composite from MarkForged.Both laments were 1.75 mm diameter, and were kept in a dry box prior to, and during use.
The reinforcing laments used were glass Fibre (GF) -0.3 mm bundles, carbon Fibre (CF) -0.35 mm bundles, and Kevlar® Fibre (KF) -0.3 mm bundles, all supplied by MarkForged Inc. Tensile and exural test specimens were manufactured, based on standard designs; ASTM D3039 [24] for tensile samples and ASTM D7264 [25] for exural samples.ASTM D3039 was chosen over an ISO standard as it allows for easier set-up to avoid exure of the sample under test [26].ASTM 7264 (four-point bend testing) was preferred over three-point bend testing as it has been shown that the error in the exural modulus from 3point is 5% higher than 4-point [27].The speci cations of the samples are given in Table 1.ASTM D3039 requires the addition of tabs at the sample ends to avoid damage due to test grips.The standard allows for the printing of the tabs during sample manufacture, as used by Giannakis [28], however, the more widely accepted method of using adhesion of tabs was employed to reduce AM costs and time.Tabs were applied following ASTM D3039, using NEMA standards Grade G-10/FR4 composite (PAR Group Ltd, UK), 30 mm tab length and a bevel angle of 90 o , also recommended by Hodgkinson [26].The tabs were adhesively bonded to the specimens using a heat-cured epoxy VTA 260 (Cytec Industrial Materials (Derby) Ltd, UK).The samples were vacuum bagged and heated at 65 o C for 16hrs to effect a cure.

Parameter Selection
A number of the available build parameters de ning the structure of the samples (Fig. 1) were set at constant levels.These were 'wall layers' -the number of layers of bre reinforcement in the walls, ' oor layers' -the number of reinforcement layers in the oor, and 'ceiling layers -the number of The parameters selected for variable analysis were base material, reinforcement material, number of reinforcement layers and reinforcement orientation.Base materials were Nylon and Onyx (Markforged Inc, USA), and reinforcement materials were GF, CF and KF (MarkForged Inc, USA).Two values for number of reinforcement layers were selected (4 and 12), which are at allowable extremes for both sample thicknesses.
The reinforcement orientation ( bre layup angle) also affects the mechanical properties of the composites [29].A unidirectional angle orientation has higher strength but only in the direction of the bre.However, to allow wider application in products where multi-directional strength is required, a bidirectional lay-up is desirable [32], and (0 0 ,45 0 ) and (0 0 ,90 0 ) were selected.

Sample Manufacture
Samples were manufactured using a Mark Two AM system (MarkForged Inc, USA) using the materials identi ed in 3.1.1and parameters identi ed in 3.1.2.A standard full-factorial design of experiment was employed to allow statistical variation of parameters to be assessed.A multilevel design was used, with 4 factors (three 2-level and one 3-level) and 5 replicates.A summary of the DoE structure is given in Table 2. Samples were also manufactured for un-reinforced Nylon and Onxy (5 of each) to provide baseline performance comparison.Fibre Orientation (F) 2 (0 0 ,45 0 ), (0 0 ,90 0 )

Sample Evaluation
For both tensile and exural testing, testing was performed at 20-22 0 C. Extension was measured using an Instron 50 mm gauge length extensometer (Instron, USA).

Tensile Test Measurement
Tensile testing was performed using an Instron 3367 tensile testing rig (Instron, USA), with a 30kN load cell with a cross-head speed of 2 mm/min (as per ASTM D3390).An extensometer (Instron, USA) with 50 mm gauge length was used.The dimensions of each sample were taken at three places using a digital micrometre screw gauge with measurement accuracy ± 0.001 mm.

Flexural Test Measurement
4-point exural testing was performed using an Instron 5800R (Instron, USA), with a 100kN load cell.A cross-head speed of 1 mm/min was employed as per ASTM 7264.The span length was set to 128 mm, corresponding to a 32:1 span to thickness ratio as per the standard.Flexural modulus was calculated for the slope of the stress-strain curve at 0.15% strain level.

Results And Analysis
The data from the mechanical testing was analysed within a statistical analysis software, Minitab v19 (Minitab LLC, USA).An Analysis of Variance was performed to provide the size of each parameter effect (F-Value) and the signi cance of each parameter effect (P-Value).The results of the statistical analysis for the Ultimate Tensile Strength (UTS) and Flexural Modulus (E) are given in Table 3 (UTS) and Table 4 (E).The Pareto charts for the standardised parameter effects for UTS and E are given in Fig. 3 and Fig. 4.
Parameters signi cant at 95% (α < 0.05) are those with a Standardised Effect > than the red line.The red line is drawn at t, where t is the (1 -α / 2) quantile of a t-distribution with degrees of freedom equal to the degrees of freedom for the error term (2.0 for UTS and 1.98 for E).The Main Effects Plots for each parameter and level are given in Fig. 5 (UTS) and Fig. 6 (E).These indicate the average change in the UTS and E when selecting a particular parameter level.Tables 5 and 6 show the calculated differential mean UTS values (as a % of the global average UTS (dotted line on Fig. 5)) and mean E values (as a % of the global average E (dotted line on Fig. 6)) for each of the parameter levels.The Cost Ratio ($/MPa) for UTS, and UTS for each of the 24 composites and for the un-reinforced Nylon and Onyx (average of 5 samples for each) is given in Fig. 7.The Cost Ratio ($/GPa for E, and E for each of the 24 composites and for the un-reinforced Nylon and Onyx (average of 5 samples for each) is given in Fig. 8.The cost for manufacture used to calculate the Cost Ratio values was calculated using the material usage (matrix and reinforcement) recorded from the Mark Two printer after each build (and divided by 5 to obtain cost / part).

Discussion
As can be seen from Tables 3 and 4 and Figs. 3 and 4, for both UTS and E, all four main parameters are statistically signi cant (Standard Effect is higher than the signi cance line for 95% signi cance) and can control the mechanical response of the material, although the number of layers is highly signi cant for both UTS and E, and is signi cantly more important than any other parameter.It is surprising that the type of reinforcement has such a small effect on the mechanical response, with the Standardised Effect on UTS for 'reinforcement' being only 27% of that for 'layers' (Fig. 3), although for E, the effect is more signi cant, being 70% of that for 'layers' (Fig. 4).The type of matrix material has been shown to have very little effect on the mechanical response, with the Standardised Effect for 'matrix' being only 12% of that for 'layers' for UTS (Fig. 3) and only 5% of that for 'layers' for E (Fig. 5).Also, for E, the 'matrix' parameter is only just signi cant (2.63) compared to the signi cance limit of 1.98.Thus, lower-cost, and more abundant Nylon matrix material offers similar performance to the more expensive Onyx.'Orientation' has a standardized effect of only 3% for UTS and 19% for E compared to those for 'layers'.
The selection of parameter levels for main parameters can be informed from the Main Effects Plots (Figs. 5 and 6 and Tables 5 and 6).These indicate the change in observed average (over all samples) mechanical response as a result in change in parameter level.From Figs. 5 and 6 and Tables 5 and 6 it is clearly evident that the results for UTS and E follow similar, but not identical trends.For 'matrix' parameter, both UTS and E, selecting Onxy over Nylon does provide a small increase in both UTS and E, but, this is only (UTS) from 5.58% below global mean to 11.66% above (Table 5) and (E) from − 1.55% below global mean to 1.55% above (Table 6).Thus, optimal properties are achieved by selecting Onyx over Nylon as matrix material, although, as discussed later, the improvements in mechanical response may bot outweigh the extra cost incurred.
For 'reinforcement' parameter, there is a large effect in moving from GF to CF, with large increases in both UTS (from 0.14% below global mean to 22.06% above -Fig.4 and Table 5) and E (from 0.12% below global mean to 42.75% above -Fig.5 and Table 6).There is a nearly equally sized decrease in properties for both UTS and E when selecting KF, shifting to 22.94% below global mean (UTS) and 16.98% below global mean (E).Thus, optimal properties are obtained by selecting CF as reinforcement material.
As discussed initially above, the 'layers' parameter has the largest in uence over the mechanical properties.From the Main Effect plots for UTS and E (Figs. 5 and 6), moving from 4 layers to 12 layers provides a improvement in UTS from 46.52% below global mean to 46.56% above global mean; the same change results in raising E from 30.76% below global mean to 30.76% above global mean.Thus, optimal properties are obtained by using a higher number of reinforcement layers, and, as we see later, is an affordable choice.
The Main Effect plots also demonstrate the marginal improvement in moving from a (0,45) pattern to a (0,90) pattern, with UTS increasing from 1.23% below global mean to 1.27% above global mean (Table 5), and E increasing from 5.78% below global mean to 5.80% above global mean (Table 6).This small effect may be due to the similarity between the two lay-up patterns.Higher performance was achieved by Klift et al [23], achieving 400 MPa (σ = 20.35)UTS for Nylon-Carbon samples, using a concentric ring lay-up, with our Nylon-CF only achieving 249 ± 6 MPa.Although the results are not directly comparable as Klift et al. used an increased number of reinforcement layers of 16, compared to 12 in our research.It is clear though from this research that a (0,90) pattern does help to optimise the UTS and E and has no signi cant effect on the cost (as we see later).
It can be deduced from the analysis of cost verses strength (Fig. 7) and stiffness (Fig. 8) that the lowest cost to strength ratio (most desirable) is achieved using a GF reinforced Nylon with 12 layers and (0,90) lay-up, at 0.0221 ± 0.0002 $MPa − 1 , and achieving 220 ± 2 MPa UTS (79 ± 1% of highest UTS, achieved using CF reinforced Onyx with 12 layers and (0,90) lay-up.The equivalent material using (0,45) only achieves a UTS of 187 ± 4 MPa (67 ± 1% of highest UTS) at a cost to strength ratio of 0.0261 ± 0.0005 $MPa − 1 .The highest performing composite, 278 ± 4 MPa (CF reinforced Onyx with 12 layers and (0,90) lay-up) has a cost to strength ratio of 0.0303 ± 0.0004 $MPa − 1 , 37 ± 2% higher cost than the optimum cost to strength ratio material.Thus, for a 21 ± 1% increase in strength, a 37 ± 2% increase in cost is incurred, making the most cost-effective material option highly attractive for all but the most demanding applications.
The stiffest material is CF reinforced Onyx with 12 layers and (0,90) (15.5 ± 0.3 GPa), and is 2nd most cost-effective material (0.49 ± 0.06 $GPa − 1 ) (Fig. 8).The most cost-effective material is GF reinforced Nylon with 12 layers and (0,45) (0.40 ± 0.02 $GPa − 1 ), which is 20 ± 3% lower cost but only retains 39 ± 2% of the stiffness of the stiffest material.It is therefore only practical to use this most cost-effective material where high stiffness is not a design requirement.Only three materials have E > 10 GPa, and these are all CF reinforced with 12 layers.It is therefore practical to use the stiffest material (CF reinforced Onyx with 12 layers and (0,90)) for all but the most cost-sensitive applications.KF does not provide any technical advantage over CF and is also not competitive economically.

Conclusions
The mechanical capabilities of a continuous bre thermoplastic composite AM technology (Mark Two, MarkForged Inc, USA) has explored for all the continuous-bre reinforcement options available (glass, carbon and Kevlar®) and have demonstrated that it has the capability to engage in engineering applications provided the correct choice of process (printing) parameters and materials is made.
The effect of the key process parameters of: matrix polymer, reinforcement bre, level of reinforcement and reinforcement lay-up, upon the strength (UTS) and stiffness (E) were evaluated using a DoE and ANOVA methodology, per approved ASTM standards.The following conclusions can be made: 1. 'Layers' and 'Reinforcement' play a very signi cant role in determining both tensile and exural properties, with Layers being signi cantly more important in determining UTS than any other parameter (73% higher standardized effect than 'reinforcement').
2. The bre layup angle and matrix material have a minor effect on the mechanical properties.'Matrix' having a standardized effect only 12% that of 'layers' for UTS and 5% that of layers for E; 'orientation' having a standardized effect only 3% that of 'layers' for UTS and 19% that of layers for E. 3. Optimal properties are obtained by selecting CF as reinforcement material, using a higher number of reinforcement layers (demonstrated to be an affordable choice) and by selecting Onyx over Nylon as matrix material, although the improvements in mechanical response may not outweigh the extra cost incurred.4. For UTS, the size of effect of 'reinforcement' was found to be CF > GF > KF; while for E it was CF > KF > GF. 5. GF can improve the tensile and exural modulus of Nylon by 1340±293% and 876±52% and of Onyx by 1390±234% and 569±37%. .CF can improve the tensile and exural modulus of Nylon by 1472±187% and 2024±107% and of Onyx by 1400±153% and 1214±81%.7. KF can improve the tensile and exural modulus of Nylon by 960±243% and 1027±53% and of Onyx by 810±83% and 627±37%.

Figure 2 Build
Figure 2

Figure 5 Main
Figure 5

Figure 6 Main
Figure 6

Table 1
Tensile and Flexural Test Sample Speci cations.

Table 2
Summary of Parameters Employed and Levels Set for Sample Manufacture.

Table 3 ANOVA
Results for Main Parameters and All Interactions for UTS.

Table 4 ANOVA
Results for Main Parameters and All Interactions for E.

Table 5
Main Effect Changes on Average UTS.