Agora of Additive Manufacturing (2024) paper24003

Agora of Additive Manufacturing
journal homepage: http://additive-manufacturing.or.jp/

Compressive strength of 3D-printed unidirectional carbon fiber/PA-6
composites: Effect of surface-layer printing using short carbon
fiber/PA-6 composites
Ahmad Zuhair bin Zakariaa, Akira Todorokib*, Masahito UedacaTokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 1528550, Japanb

Graduate School of Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 1528550, Japanc

Nihon University, 1-8-14 Kanda-Surugadai, Chiyoda, Tokyo, 1018308, Japan

ABSTRACTThe present study experimentally investigated the compressive strength of 3D-printed continuous carbon fiber composites.
Two types of specimens were fabricated: type A was printed with the specimen thickness direction as the layup direction,and type B was printed with the specimen width direction as the layup direction. In previous research, these specimens
were printed without surface layers (such as walls, floors, and roofs) using short carbon-fiber composite filaments, resulting
in approximately 20% lower compressive strength compared with the Markforged datasheet. In this study, the specimens
were printed with surface layers. The compressive strength showed results almost identical to those in the Markforged
datasheet. To investigate the effect of the surface layers, the fiber waviness of the specimens was examined using X-ray
computed tomography. The results indicated that the surface layers exhibited lesser waviness, leading to higher
compressive strength.
Keywords: 3D printing, Composites, Compressive strength, waviness1. Introduction
The typical compression failure mode of unidirectional carbon fiber-reinforced plastic (CFRP) is kink-band failure. This
failure mode is caused by the local shear instability of the fiber and matrix microstructure when compressive stress isapplied to the unidirectional CFRP, leading to fiber breakage within a narrow band region (1–6). For kink-band failure undercompressive loading, many researchers have reported that the initial waviness of fibers has a significant effect (1–8). Whena compressive load is applied to fibers with waviness, the waviness increases, and the matrix cannot withstand the shear
deformation between the fibers, resulting in fiber breakage and kink-band failure. In traditional CFRP made by laminating
prepregs, the waviness remains relatively stable. Consequently, the compressive strength can be reliably considered amaterial property.
Markforged has recently developed a 3D printer capable of printing continuous-fiber CFRP (cCFRP) (9). Matsuzaki
et al. developed a nozzle impregnation method to produce continuous-fiber composite materials (10). Research papers havebeen published on the tensile tests of 3D-printed cCFRP (11–13). Regarding the compressive strength of 3D-printed
continuous fiber unidirectional CFRP, Iragi et al. conducted an American Standard for Testing and Materials (ASTM)
D3410 unidirectional compression test and reported a compressive strength of 426 MPa (14). In 3D printing, the wavinessof cCFRP may vary due to printing conditions. This variation can result in the compressive strength no longer being
consistent with the material’s properties. This issue is rarely observed in CFRP produced by laminating prepregs.
* Corresponding author
Email: todoroki.a.aa@m.titech.ac.jp
Received 27 May 2024; Revised 24 July 2024; Accepted 26 July 2024
Available online 26 July 2024

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Ahmad Zuhair et al. used a Markforged 3D printer to fabricate cCFRP specimens with only continuous fiber filaments,
without surface layers such as walls, floors, and roofs, and conducted compression tests (15). Two types of compression tests
were performed: Type A, where the thickness direction of the specimen was the layup direction in the 3D printer, and TypeB, where the width direction of the specimen was the layup direction. Compressive strengths of 347 MPa for a 2 mm thicktype A specimen and 323 MPa for a 2 mm thick type B specimen were obtained. These strengths were significantly lower
than the compressive strength of 420 MPa in the Markforged datasheet (16). Additionally, the stress–strain diagram of type
B differed from that of type A, with the measured stress–strain curves obtained from the front and back sides deviatingfrom the initial state of the compression test. In 3D printing, the waviness of the material can vary. In the previous study
done by Ahmad Zuhair et al. (17), the cCFRP were printed without surface layers, resulting in a lower compressive strength
value compared to those in the Markforged datasheet. The absence of the surface layers may have increased the wavinessof cCFRP. In this study, we will print cCFRP with a surface layer (wall path) and compare the results with the Markforged
datasheet. This investigation focuses on assessing how changes in waviness affects the compressive strength of cCFRP.
Therefore, in this study, we used a short carbon fiber/PA-6 composite material (sCFRP) to print the surface layers.
Onyx® filament, produced by Markforged Inc., was employed to investigate the effect of the surface layer on compressive
strength. We also experimentally investigated the effect of the surface layer on the initial fiber waviness of the continuous
fibers.
2. Experimental methods
Mark Two®, produced by Markforged Inc., was used to manufacture the specimens for the cCFRP compression tests inthis study. The slicing software Eiger® was used to create print paths of the specimens. The filaments used were cCFRP(cCF/PA-6: carbon fiber, Markforged Inc.) and sCFRP (sCF/PA-6: Onyx®, Markforged Inc.).
In this study, unlike the previous report (15), we measured the compressive strength in the fiber direction of aunidirectional cCFRP specimen that has the lower surface (floor), lateral surface (wall), and upper surface (roof) made ofsCFRP filament. Two types of specimens, type A and type B, similar to those in the previous report, were prepared using
the Mark Two 3D printer. Figure 1 shows the configuration of the specimens. The specimen width is larger than that of thespecimen in the previous report (15) because it includes a wall path of sCFRP. The length of the specimen L = 78 mm, thewidth W = 12.7 mm, and the thickness t = 2 mm. One path of the wall was placed in both type A and B test specimens, andthree layers each were placed for the floor and the roof. The center parts printed using the cCFRP filament were set thesame as in the previous report (15). However, the plate thickness increased by 0.4 mm due to the printing of the floor androof, but it is expressed as 2 mm in this study.

Fig. 1 Compression test specimen configuration.

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z (Lay up direction)

NozzleFiber direction

Bed
y
t
x

L B
W
L

t

A

W

Fig. 2 Printing orientations of type A and type B specimens.

Figure 2 shows the printing orientation of the two types of specimens, A and B, fabricated using a 3D printer. In typeA, a flat specimen was printed parallel to the printing bed, making the thickness direction the layup direction during printing.
In contrast, for type B, the layup direction is in the width direction of the specimen. In Figure 2, L, W, and t represent thelongitudinal, width, and thickness directions, respectively, in the specimen coordinate system. In both types of specimens,
the direction of the continuous fibers is set in the longitudinal direction (L) of the specimen.
As mentioned earlier, sCFRP filament was used to print the wall, floor, and roof of the test specimens in this study.
Figure 3 shows the printing path for type A. Folded continuous fibers are observed at the end of the cCFRP filament
specimen. If a compressive load is applied from the end of the specimen, it will be applied to these folded continuous fibers.
For this reason, the length in the L direction was 90 mm during fabrication, and after fabrication, both ends of the specimen
were cut to a length of 78 mm for the compressive tests. To print a 2 mm-thick specimen, 16 layers of cCFRP were laminatedin the thickness direction, and 13 paths were printed in the width direction.
Figure 4 shows the printing path of the Type B specimen. When printing a single flat specimen, the contact area withthe printing bed is smaller than that corresponding to type A. Additionally, as the nozzle is higher from the printing bed, its
movement tends to cause the specimen to fall over during the printing process. To address this, we fabricated two type B
specimens by printing a pipe with a rectangular cross-section and cutting it after the printing process. This reduced the riskof falling over during printing. For the type B, 96 layers were laminated in the width direction of the specimen. Two paths
were placed on the wall, one on the front and one on the back side, creating a test piece with a thickness of 2 mm. After
printing, a woven glass fiber-reinforced epoxy (GFRP) composite material with a length of 35 mm, width of 12.7 mm, andthickness of 2.5 mm was glued as tabs, as shown in Figure 5.
The compression tests were conducted using a universal testing machine (AG-I 100kN, Shimadzu Corp.). The
compression speed is 0.5 mm/min. To measure the elastic modulus, two strain gauges were attached to the center of theback and front surfaces of the specimen. Figure 6 shows the setup used for compression testing. A jig compliant with ASTM
D695 was used for the testing.

Print path of the sCFRP nozzle

Fig. 3 Printing path of the type A specimen.

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sCFRP Wall path

Fig. 4 Printing path of the type B specimen.

Fig. 5 Compression test specimen with GFRP tabs.
Indenter

Guide
Guide

Specimen

L type jig

Fig. 6 Compression test setup.
The specimens used in this study were made from two materials with different mechanical properties. The
compressive strain of a specimen can be measured using an attached strain gauge. However, to determine the compressive
stress applied to cCFRP, in addition to the inner cCFRP layer, sCFRP is used for the wall layer with fibers aligned in theloading direction, and the floor and roof layers are aligned at ±45°. It is necessary to calculate this by applying the rule of
mixtures to the three types of materials, including sCFRP. In type A, the wall surrounds the specimen, and the floor androof are printed on the L–W surface using the specimen coordinates shown in Figure 2. In contrast, in type B, the floor androof are only in the L–T plane, and the L–W plane of the specimen surface becomes the wall.
Kubota et al. (18) experimentally obtained the elastic modulus of sCFRP in each direction. Volume content can bemeasured on Slice software’s Eiger®. Table 1 specifies the volume content determined and the elastic modulus of sCFRP.
As the elastic modulus of sCFRP is smaller than that of cCFRP (53 GPa (17)) and the volume fraction of the surfaces suchas the floor, roof and wall paths is very small compared to the volume fraction of cCFRP, the rule of mixtures for acomposite material composed of the three types of materials is adopted as shown in Equation (1) for simplicity.
𝜎! = 𝑉"#$%&'( 𝜎"#$%&'( + 𝑉"#$%&) 𝜎"#$%&) + 𝑉*#$%& 𝜎*#$%&(1)

Here, VsCFRP45 is the volume fraction of ±45° layer sCFRP, VsCFRP0 is the volume fraction of sCFRP whose printing pathdirection is in the L direction of the specimen, VcCFRP is the volume fraction of cCFRP, and σT is the averaged stress of thespecimen in the L direction. The compressive stress σsCFRP45 is the compressive stress of ±45° layered sCFRP, σsCFRP0 is

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the compressive stress of sCFRP with the printing path direction in the L direction of the specimen, and σcCFRP is thecompressive stress of cCFRP. The following Equation (2) holds from the elastic modulus and strain ɛ.
𝜎! = 𝑉"#$%&'( 𝐸"#$%&'( 𝜀 + 𝑉"#$%&) 𝐸"#$%&) 𝜀 + 𝑉*#$%& 𝜎*#$%&(2)

Here, e is the specimen strain measured from the strain gauge, EsCFRP45 is the elastic modulus of sCFRP printed in the inplane direction, and EsCFRP0 is the elastic modulus of sCFRP printed in the L direction of the specimen.
Table 1 Fiber volume fraction and mechanical properties to calculate compressive stress of cCFRP.
Type A

Type B𝑉"#$%&'(

0.234

0.042

𝑉"#$%&)

0.063

0.281

𝑉*#$%&

0.703

0.677

𝐸"#$%&'( (18)0.74 [GPa]𝐸"#$%&) (18)2.64 [GPa]3. Results and discussion
3.1 Compression test results
A compressive stress–strain diagram of the measured type A specimen is shown in Figure 7. Stress represents the stressapplied to the inner-layer cCFRP and is calculated using Equation (2). In Figure 7, the strains measured from the strain
gauges on both sides of the specimen are shown separately as dashed and solid lines for the five specimens. As shown in
the figure, both the solid and dashed curves are almost straight lines; however, there is a slight separation in the strain

Fig. 7 Compressive stress–strain curves of type A specimens.
owing to the bending of the specimen. However, a sudden change in the strain on the front and back surfaces due to buckling
just before fracture was not observed. The average compressive strength of the type A specimen was 420 MPa, and thestandard deviation of the compressive strength was 44.4 MPa. This strength value is higher than the previous report of 347
MPa (15) without the sCFRP (Onyx®) layer and is consistent with the value in the Markforged datasheet (16).
Figure 8 shows the compressive stress-strain diagram of the measured type B specimens. Similar to the type A
specimen, both the solid and dashed curves are almost straight lines, and there is no sudden change due to buckling until
failure. Figure 9 shows a comparison between a typical example of the previous report's results (15) without a surface

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Fig. 8 Compressive stress–strain curves of type B specimen.
sCFRP layer and a typical example of the type B with an sCFRP layer in this study. As shown in Figure 9, when the sCFRP
layer is present, the strain deviation on both sides of the specimen is smaller. The average compressive strength of type B
specimen in this experiment was 456 MPa (standard deviation 54.8). This variation matches the value of the Markforged
datasheet (16). The data scattering in Fig 7 and 8 comes from the specimen fabrication. It is difficult to fabricate
unidirectional carbon fiber composites without fiber tension. This causes scattering of initial fiber waviness.
Table 2 shows the measured compressive strength and elastic modulus of each specimen compared with the previous
results without the sCFRP layer (15). The t-test was performed on the results of type A and type B with sCFRP layer, and no
significant difference was noted in both the averaged strength and elastic modulus between the two specimen types.
However, a significant difference was found when comparing the results with and without the sCFRP layer.
3.2 Observation results
Figures 10 and 11 present the observation results of the specimens after the compression test. It can be seen that kink-band
failure has occurred, and the kink band is progressing in the layup direction in both types A and B. This is consistent withthe previous results without the sCFRP layer (15) and is thought to be due to the weak lamination strength by the 3D printing
process (13).
Figures 12 and 13 show the results of X-ray computed tomography (CT) observations of the cross-sections of type A
and type B specimens before compression testing, respectively. The images were obtained using an X-ray CT imaging
device (ScaXmate-L080H; Comscantecno Co., Ltd.). Table 3 enumerates the CT scan parameters.
500
Compressive stress [MPa]With sCFRP
400With sCFRP
300 Without sCFRP
200

Without sCFRP

100
0
0 0.002 0.004 0.006 0.008 0.01
Compressive strain [-]Fig. 9 Comparison between compressive stress–strain curves of type B specimen without sCFRP and that with sCFRP.

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Table 2 Measured compressive strength and elastic modulus. Numbers in parentheses indicate the standard deviation.
Without sCFRP (Onyx®) (15)With sCFRP (Onyx®)
Strength

ModulusStrength

Modulus[MPa] (STD)[GPa] (STD)[MPa] (STD)[GPa] (STD)

Type A347(37.2)

53.3(2.1)

420 (44.4)

58.9 (3.6)

Type B

323(28.4)

52.3(4.6)456 (54.8)

59.0 (4.1)

Fig. 10 Compressive failure configurations of type A specimen printed with sCFRP.

Fig. 11 Compressive failure configurations of type B specimens printed with sCFRP.
Table 3. Parameters of X-ray CT scan (ScaXmate-L080H).
Parameters

Value

X-ray tube voltage [kV]78

X-ray tube current [µA]

26

Resolution [µm/pixel]

19.757

Number of projections

1500

Black value-4.8

White value

42.0

Figure 12 (b) shows that the almost exact rectangular cross section is maintained by the sCFRP layer on the surface.
The cCFRP was printed in two paths close to each other, and the two paths formed one set with a gap between each set. In
the absence of the sCFRP layer on the surface of Figure 12 (a), the rectangular cross-section is distorted. Although pairs ofprinting paths are recognized, there are many gaps in the entire cross-sectional view.

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In type B of Figure 13, it can be seen that in case (a) without sCFRP, the surface exhibits severe unevenness.
Additionally, gaps are observed between the laminated layers, indicating extremely weak bonding between the layers.
Many gaps are also present between the paths. In case (b), which uses sCFRP, the gaps between paths are smaller thanthose in case (a). This suggests that the use of sCFRP stabilizes the paths of cCFRP during printing, reducing surface
irregularities and gaps between paths.

Pair of paths

Gap(a) Without sCFRP
Pair of paths
Gap(b) With sCFRP
Fig. 12. Observations of the cross-section of the type A specimens.

(a) Without sCFRP(b) With sCFRP
Fig. 13 Observations of the cross-section of the type B specimens.

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The elastic modulus in Table 2 is almost equivalent to the elastic modulus of 62 GPa reported by Markforged (16) whenusing sCFRP. This indicates that the low strength and low elastic modulus observed when sCFRP is not used are due to theeffect of voids caused by the gaps. Additionally, the difference in strain behavior of type B between the front and back sides
without sCFRP (Figure 9) is likely ascribable to the difference in unevenness between the front and back sides as shown in
Figure 13(a). This unevenness appears to be caused by bending deformation, possibly due to positional errors in the pathof the continuous fibers and the difference in the elastic modulus of the outermost layer on the front and back sides.
Budiansky (14) demonstrated the importance of initial waviness in the axial compressive strength of cCFRP. Takahashi
et al. (8) showed in finite element simulations that the average waviness of a cluster of unidirectional carbon fibers isessential, and the effect of the maximum deflection on compressive strength is small. Unfortunately, the resolution of the
X-ray CT used in this study does not allow us to measure the exact waviness of individual carbon fibers. Therefore, wemeasured the maximum measurable waviness of the X-ray CT scanner used in this study and qualitatively evaluated theinitial waviness. We hypothesize that the maximum fiber waviness is positively correlated with the average fiber wavinessin this study. Figures 14 (a)–(d) show the measurement results of maximum waviness with and without sCFRP in types
A and B. Table 4 specifies the maximum measured waviness values.
ZX

YZ

ZX

YZ

Maximum
initialdeflection

Maximum
initialdeflection(a) Type A without sCFRPZXYZ(b) Type A with sCFRP
ZX

YZMaximum
initialdeflection
Maximum
initialdeflection(c)Type B without sCFRP(d) Type B with sCFRP

Fig. 14 Measurements of the maximum initial waviness of carbon fibers using X-ray CT.

Table 4 Angle of the waviness of the most prominently undulated cCFRP paths.
Specimen typeType A without sCFRP

Angle of waviness [deg]13.8

Compressive strength [MPa]347Type A with sCFRP

6.07

420

Type B without sCFRP

9.99

323

Type B with sCFRP

4.48

456

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From the results in Table 4, it can be seen that using sCFRP in both types A and B suppresses waviness by about half.
Additionally, type B without sCFRP, which has the lowest compressive strength, exhibits the highest waviness. Conversely,
the case with type B with sCFRP, which has the highest compressive strength, shows the lowest waviness. While thismaximum waviness value does not directly provide the compressive strength, it qualitatively represents the magnitude of
the compressive strength. Therefore, by printing sCFRP walls, floors, and roofs, the paths of cCFRP are able to suppress
waviness and improve compressive strength.
4. Conclusions
In this study, 3D-printed cCFRP was fabricated using two different orientations, and the effect of walls, floors, and roofs
printed by sCFRP on the compressive properties was investigated. Cross-sectional observations were performed using Xray CT to identify the cause of the decrease in compressive strength. The following conclusions were obtained:(1) Creating a surface with sCFRP on 3D-printed cCFRP improves the compressive strength and compressive elastic
modulus.
(2) The kink band during compression failure of 3D-printed cCFRP always evolves in the layup direction.
(3) Printing sCFRP walls, floors, and roofs makes it difficult for cCFRP paths to move, suppresses waviness, and improvescompressive strength.
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