3D printed cellular structural composites with continuous ber reinforcement: A deep study on printing structural parameters and bending performance
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3D printed cellular structural composites with
continuous fiber reinforcement: A deep study on
printing structural parameters and bending
performance
Ziying Cui
Jiangnan University
Xiayan Huang
Jiangnan University
Mahyar Panahi-Sarmad
Jiangnan University
Ke Dong
Southern University of Science and Technology
Xueliang Xiao ( xiao_xueliang@jiangnan.edu.cn )
Jiangnan University
Research Article
Keywords: 3D printing, bending performance, cellular structural composites, continuous fiber
reinforcement
Posted Date: July 22nd, 2022
DOI: https://doi.org/10.21203/rs.3.rs-1857666/v1
License: This work is licensed under a Creative Commons Attribution 4.0 International License.
Read Full License
Page 1/22
Abstract
Herein, an upgraded fused deposition modeling (FDM) in a 3D printer was used to fabricate continuous
fiber-reinforced composites (CFRCs) in various shape-filled cellular structures with Kevlar fiber and
polylactic acid (PLA) as the reinforcement and matrix. The fruitful influences on the bending performance
are printing path, fiber orientation, infill density, cell length, and layer thickness which have been
systematically studied. A Finite Element (FE) model was established to compare with the experimental
results. A three-point bending test revealed that, as the effective density increased, there was an
undeniable rise in strength and stiffness of CFRCs with different infill structures. The structural
parameters like cell length are greatly responsible for strengthening bending performance; this effect was
evaluated by adjusting the fiber orientations along the stretching direction. The sample of the rhombus-
filled cellular CFRCs with a layer thickness of 0.1 mm was found to have the highest bending strength
and modulus of 53.61 MPa and 2728.41 MPa, respectively. Since the mechanical properties mostly
depend on the core shape, cell length, and layer thickness, plenty of infill structures should be designed by
a 3D printer to achieve the required bending strength and flexural modulus — this work is able to be a
paradigm guide to reduce the number of printed samples.
1. Introduction
Fiber-reinforced composites, also known as high-performance composite materials, which are new
materials feasible to fabricate via physical or chemical methods for optimizing the combination of
materials with different properties, have been increasingly applied in the automobile and aerospace
industries due to their lightweight, high-strength, and high flexural modulus [1–5]. To be more specific,
composites consist of materials with different properties to give full play to their respective advantages in
the system, leading to an overall improvement of the final product's performance compared to the
primary materials. In recent years, CFRCs have been widely used in aerospace, military industry, marine
engineering, advanced rail transit, and many other fields owing to their high specific strength,
designability, good corrosion, and fatigue resistance [6–9]. Currently, the application scale of composites
is gradually expanding, though continuous fiber composites cannot be applied to some complex
structures due to the high production cost, long cycle of manufacturing, and complex processes in
traditional manufacturing processes [10,11]. Thus, developing new composites modeling technology
seems to be necessary.
3D printing technology also referred to as additive manufacturing technology, is a process of
accumulating materials on a specific path to form printed objects, usually layer by layer, based on three-
dimensional model data [12–14]. Compared with traditional subtractive manufacturing methods, 3D
printing technology has a raw material utilization rate of nearly 100%, which has the characteristics of
low cost, short cycle, convenient operation, and a high degree of automation [15–17]. The combination of
the 3D printing procedure and fiber-reinforced composites can utterly show the manufacturing
advantages of 3D printing as well as the performance advantages of composites simultaneously [18]. In
other words, CFRCs have encompassed both advantages of the advanced 3D printing technology and
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fiber's reinforcing effects, which simplifies the process and improves the mechanical properties of fiber-
reinforced cellular composites as well. Hence, the effective manufacturing of high-performance, high-
value-added, and customized composite structures can be realized, and the application area of fiber-
reinforced composites will be further expanded [14,19–22].
In today's world, continuous fiber-reinforced polymer composite materials have become an ideal
lightweight material for manufacturing reinforced structures owing to their excellent mechanical
properties [1,13,23]. Tian et al. [24,25] introduced an FDM printing process for continuous fiber-reinforced
thermoplastic composite (CFRTPC). The results showed that the flexural strength and modulus of carbon
fiber-reinforced acrylonitrile butadiene styrene (ABS) composite samples were almost six times more
efficient than that of FDM printed pure ABS samples. In addition, the effects of printing parameters on the
interface and bending properties of continuous carbon fiber reinforced PLA composites were
comprehensively analyzed. Likewise, Dickson et al. [26] used the Mark One 3D printing system to
manufacture continuous carbon, Kevlar, and glass fibers reinforced Polyamide (PA or Nylon) composites.
They evaluated the effects of fiber orientation, fiber type, fiber volume fraction, and other factors on
composites' tensile and bending properties. The results showed that the tensile and flexural strengths of
continuous glass fiber, carbon fiber, and Kevlar fiber reinforced nylon composites are increased by 6.3
times and 5 times, respectively. Matsuzaki et al. [27–29] developed a novel FDM 3D printing method for
continuous fiber-reinforced thermoplastics. They prepared sandwich composites based on honeycomb,
circle, rectangle, and diamond core shapes and then studied the bending characteristics of the sandwich
structure composites reinforced with the carbon fiber bundles. This study indicated that the maximum
load and flexural modulus increase as the effective density increases for all core shapes, but the rhombus
core shape shows the strongest.
In the above literature, there are few studies on the structure and the design of the corresponding printing
path of CFRCs, and the influence of printing shape and cell length on the mechanical properties of
composites. Although 3D printing has many advantages over traditional CFRCs manufacturing, the
continuous fibers—introduced by 3D printing have certain limitations on additive manufacturing of
CFRCs. For example, the printing path must be continuous and non-jumping, and each layer's starting and
ending points must coincide, which controls the distribution of fibers in the composite. In addition, the
print structures and cell length are closely related to fiber orientation, fiber content, and other parameters,
which directly affect the mechanical properties of CFRCs.
In this work, continuous Kevlar fibers and PLA were utilized as reinforcement and matrix, respectively, to
achieve the printing of continuous fiber thermoplastic structure composites based on FDM 3D printing
technology. Thermoplastic PLA is one of the most commonly used FDM raw materials owing to its low
cost and low melting temperature [29–32]. As a reinforcing fiber, Kevlar fiber has constant high-
temperature stability and good mechanical properties, compensating for the insufficient impact
resistance of PLA printed objects. Here, rectangular, rhombic, and honeycomb infill lightweight cellular
structures were designed and prepared. The effects of infill shapes, cell lengths, and layer thicknesses on
the mechanical properties of continuous Kevlar fiber reinforced polylactic acid composites were also
Page 3/22
studied. Moreover, the influences of the infill density, printing paths, structural parameters, and fiber
orientation on the bending behavior were all investigated one-to-one systematically. This article aims to
explore an innovative and reasonable printing path design method (adopting an innovative and
reasonable design of printing path) and explore the effects of morphology (discussing the relationship
between morphological structure and internal fiber distribution), fiber distribution, and cell length on the
mechanical properties of CFRCs, so as to provide a new manufacturing basis for 3D printing of
continuous fiber-reinforced composites with complex structures.
2. Experimental Section
2.1 Materials and equipment
Thermoplastic polylactic acid (PLA) was used as a printing matrix (the diameter and density are 1.75 mm
and 1240 kg/m3, respectively) (Guanghua Weiye Co., Ltd, Shenzhen, China). PLA is a new type of
environmentally friendly and biodegradable material with good biocompatibility, tensile strength, and
ductility, as well as low shrinkage during printing. Kevlar fiber was utilized as a reinforcing component
(the diameter, linear density, and density are 0.025µm, 200D, and 1.44 g /cm3, respectively) (DuPont Corp,
U.S.A). PLA mainly transfers the load between fibers, and more importantly, it supports, fixes, and protects
the fiber materials. On the other side, Kevlar provides strength and toughness for the composite material.
The equipment used is a modified FDM 3D printer (Createbot MID250, KaiNing Electric Co. Ltd., Wuxi,
China). The schematic representation of the printing procedure is shown in Fig. 1(a). The continuous fiber
was guided—with a plastic tube on the side of the print head—into the print head, which led to the
simultaneous printing of continuous fiber and thermoplastic polymers. The thermoplastic matrix in the
heating block was heated and melted during the printing process. The continuous fiber was led by the
plastic tube, as mentioned above, and then held together with the thermoplastic matrix in the molten
state. With the extrusion motor's action, the matrix and continuous fiber were extruded from the nozzle
together and then immediately solidified and adhered to the printing platform. At the same time,
continuous fiber was pulled out of the plastic tube to achieve 3D printing of CFRCs. After that, a universal
electronic material testing machine (Instron 3385H, Instron Corp., London, U.K) equipped with a 10 KN
load cell was used to measure the mechanical properties of the printed CFRCs samples in accordance
with ASTM D790-07 (Standard Test Method for Flexural Properties of Unreinforced and Reinforced
Plastics and Electrical Insulating Materials).
2.2 Experimental methods
2.2.1 Fabrication of CFRCs
First, 3D models of the printed structures were constructed by using some drawing software such as CAD,
Solidworks, etc. In this work, rectangle, rhombus- and honeycomb-filled cellular structures were designed
and printed in order to study the effect of different infill shapes on the bending properties of CFRCs. The
Page 4/22
dimensions of these samples were all set to 120 mm length (L) × 20 mm height (H) × 2 mm thickness (T).
This paper also studied the effect on the bending performance of the rhombus-filled CFRCs with cell
lengths of 10, 15, 20, 30, and 40 mm (with a constant layer thickness of 0.5 mm). The nozzle coordinates,
printing speed, printing path, extrusion volume, and other parameters were set by writing G-code in
Repetier (the paths and other parameters of all printed samples can be found in the supporting
information), as shown in Fig. 1(f). The design of the printing paths became more significant due to the
addition of continuous fibers. When the nozzle moves in a pre-determined path without extrusion, PLA
will not be extruded, but the Kevlar fiber must be continuously pulled out of the nozzle, which means the
printing process is continuous, and the overlapped paths are unavoidable for the designed structures
because of the "none-skipped" printing process. The printing was completed by accumulating layer by
layer; hence, it was only needed to design the path of one layer. The printing paths (arrows represent the
printing direction) of lightweight cellular structures are listed in Fig. 1(c)-(e).
To understand more clearly, the printing paths of all three filling patterns are divided into several steps,
and each step of the printing starts from the "starting point", but the whole printing process is continuous.
The combined graph of each path is displayed at the bottom of the path diagram. The movement of the
nozzle from the start point to the end of a printing layer is regarded as a cycle. Three identical cycles are
superimposed on the printed structure by adjusting the coordinates of the nozzle on Z-axis in the G-code,
and a complete printing structure is obtained. Eventually, the designed G-code program is imported into
the 3D printer. It will automatically start printing after the printer is heated to the set temperature.
The extrusion volume of the nozzle will affect the combination of PLA and the continuous fiber, the
bonding of the composite material and printing platform, and the fusion interface between layers, which
further affects the structural morphology and mechanical properties of CFRCs. The nozzle repeatedly
moves along the overlapping paths during the printing process, and the final size of the sample will be
slightly wider than the overlapping paths. Therefore, it is necessary to set different extrusion volumes to
ensure the consistency of the sample size and the stability of the structure. The extrusion volume of the
nozzle can be calculated based on the three equations,
4w · h · l
E0 =
π · d2
1
4d · h · l
E 1 = 130% ×
π · d2
2
E2 = E0 − E1
3
Page 5/22
Where w is the width of the printed lines, h represents the layer thickness, l denotes the length of the
printed strut, and d shows the nozzle diameter.
The amount of extrusion volume per unit length (E0) of the normal path can be calculated according to
Eq. 1. When the nozzle passes between the overlapping paths for the first cycle of extrusion (E1), the
printed strut's width is set to 130% of the nozzle diameter (based on Eq. 2) in order to obtain an excellent
quality of the interface. When the nozzle passes for the second time between the overlapping path, E1 can
be subtracted from E0, and the remaining value is the amount of extrusion between the overlapping path
(E2), under the calculation of Eq. 3. Eventually, it is possible to optimize the expected sample size as
much as possible with the aid of the aforementioned equations.
2.2.2 Three-point bending test
A set of three-point bending tests were conducted to evaluate the bending resistance and stiffness of the
printed cellular CFRCs, as shown in Fig. 2(a). The tests were carried out with a span of 70 mm and a
loading rate of 2.0 mm/min. The computer connected to the equipment would record the loaded force (F),
the downward displacement (S), and the running time (T). The related load-displacement curves were
generated after the completion of the test, as shown in Fig. 2(b). The tests were performed on the CFRCs
filled with different core cellular shapes (rectangle, rhombus, and honeycomb), as well as rhombic infill
CFRCs with various cell lengths (10mm,15mm,20mm,30mm,40mm). The three-point bending tests were
repeated three times for each of the three types of core shapes and the five types of cell lengths to ensure
the data's validity. The flexural modulus (E) and the bending strength (σ) can be reasonably calculated by
the following equations[27–29],
l3
E=
4HT 3 ( ) ΔF
ΔS
4
3F × l
σ=
2HT 2
5
Where l is the distance between the supporting points, H is the specimen width, T is the specimen
thickness, F is the applied load, and S is the deflection.
2.2.3 Finite element modeling
The ABAQUS was used to establish the mechanical models corresponding to the printed structures in the
experiment and to simulate the three-point bending process of CFRCs. In finite element modeling, the
CFRCs of the three infill shapes have the same dimensions with a layer thickness of 0.5 mm. Among
them, the element length of the rhombus-filled CFRCs is 20 mm. In fact, the diameter of the Kevlar fiber is
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too small to be simulated successfully, so the width of the fiber bundle was set to be 0.5 mm while the
width of the printing line was 1.5 mm. The pure matrix and reinforced fibers were considered, respectively,
in analytical modeling and mechanical simulations (input parameters of the finite element model are
summarized in the supporting information). The displacement control boundary was applied along the
direction consistent with the three-point bending test (span of 70 mm, loading rate of 2.0 mm/min− 1).
3. Results And Discussions
3.1. Three-point bending test
The schematic diagrams of the three-point bending test are shown in Fig. 2(a), and photographs taken
during and after the three-point bending test are shown in Fig. 2(b). The photograph taken during the
interval of O ~ A illustrates the bending deformation of the CFRCs sample under the applied load, and the
result of the three-point bending test is shown in the other photograph. As can be seen from the
photographs, the top surface of the sample was compressed during the test while the bottom surface
was stretched. When the sample broke, the crack initially occurred in the bottom surface under the
indenter. The maximum load was used to evaluate the bending strength for facile measurement.
Figure 2(b) exhibits the load-displacement curve obtained from the bending test. The curve grows linearly
in the interval of O ~ A, which can be called the elastic deformation stage. Thereafter, the curve enters the
yielding-like stage after a linear growth. Due to the gradual failure process, the load is nonlinearly
deformed in the A ~ B interval, which means that at this stage, the PLA has broken, but the fiber remains
intact. Indeed, this process benefits from the full potential of Kevlar fibers. The curve drops rapidly after
point B, indicating that the fiber has broken and the sample is completely demolished. The failure points
are different due to the various core shapes, which is why the fracture point was not directly below the
loading point. All in all, the three-point bending process for the printed composite is found to have three
bending stages: the elastic deformation stage, the yield-like stage, and the complete failure stage, and the
yield-like stage last the longest.
3.2. Bending behavior through FE modeling
The FE simulated deformation is shown in Fig. 3. Taking honeycomb infill CFRC as a study case, its finite
element analysis of three-point bending test was carried out to indicate the bending behavior of Fig. 2(b).
At the beginning of loading, the specimen underwent elastic deformation, and the stress increased
linearly with the increase of strain. After the specimen reached the yield-like point, the stress no longer
increased gradually, while the specimen still had to bend deformation. Eventually, the bending stress
decreased sharply after the maximum value, indicating that the specimen had fractured. The simulation
results fully verified the rationality of the three-point bending test experimentally, and the simulated
deformation behavior was in complete agreement with the experimental observation in Section 3.1.
Page 7/22During bending, FE simulation is a good way to understand the happening point of the fracture and the
fiber status during fracturing. At the same time, the FE simulation results can also be employed to
compare with experimental data. As shown in Fig. 3(e) and Fig. 3(g), the stress was observed mainly to
concentrate at the loading point and the fiber interlacement point in the simulation, which is nothing to do
with the filling structure like rhomboid or rectangular shapes. In fact, this point coincides with the highest
bending moment section in the 3-point bending test. This is almost the same as the tested fracture
behavior of the sample, as shown in Fig. 3(f) and Fig. 3(h). The tested and simulated results both indicate
that Kevlar fibers undertake the most in-plane stress in the fiber axial direction and play a major role in
resisting the out-of-plane bending deformation.
3.3. Reinforcement effect of Kevlar fiber
A few pure PLA control samples were prepared to compare the bending performance with the continuous
fiber-reinforced cellular structures to make a thorough inquiry of Kevlar fiber's reinforcement effect in the
PLA matrix. As shown in Fig. 4, a comparison was made between the pure PLA printed samples and the
CFRCs with the same structural parameters. The results showed that the continuous fibers of Kevlar
significantly increased the bending strength and flexural modulus. The strength of the rectangle-,
rhombus- and honeycomb-filled cellular CFRCs increased by 17.89%, 36.45%, and 49.51%, and the
flexural modulus increased by 20.34%, 41.14%, and 54.75%, respectively. Moreover, it can be clearly
stated that—from the histogram in Fig. 4(a)—compared with pure PLA structures, the addition of Kevlar
fiber increases the disparity in bending performance of three types of cellular CFRCs. The difference in
bending strength between the rhombus-filled structure and the honeycomb-filled structure increased from
7.70 MPa to 9.71 MPa, and the difference in flexural modulus also increased from 172.40 MPa to 192.72
MPa. Especially when comparing rhombus and rectangle-filled structures, the bending strength and the
flexural modulus for the pure PLA samples are almost the same. However, for the CFRCs samples, it can
be seen from the figure that the bending performance of the rhombus-filled CFRCs is better than that of
the rectangle-filled CFRCs. It is worth noting that Kevlar fiber has high strength, high modulus, good
toughness, and constant high-temperature stability [6]. As can be seen, the combination of Kevlar fiber
and the polymer matrix can considerably improve the bending performance of CFRCs.
After the three-point bending test, a scanning electron microscope (SEM, JEOL Model JSM-6490, Japan)
was used to observe the interface morphologies between the Kevlar fiber and the PLA matrix and
fractured cross-sectional characteristics of CFRCs. Since CFRCs are accumulated layer by layer, the
morphology of the extruded layers can be vividly seen in Fig. 4(b), and there are four layers in total. From
Fig. 4(c), since the fibers and the molten matrix are extruded together, it can be found that the molten
matrix covers the fibers, and most of the fibers have been firmly embedded in PLA matrix, which is
conducive to the formation of good interfacial adhesion. Figure 4(d) shows the fractured cross-sectional
view of CFRCs after the 3-point bending test without obvious delamination, indicating that the printed
layers are fully integrated. The fibers embedded in the matrix are not as brittle as PLA due to the good
toughness of Kevlar fiber. Therefore, the breakage of the fibers in the 3-point bending test is irregular, and
the broken fibers appear very disorderly in the fractured cross-section.
Page 8/223.4. Infill patterns and their effect on bending performance
To discuss the bending performance of CFRCs, filled with different core cellular shapes such as rectangle,
rhombus, and honeycomb, the Load-Displacement curves for each infill pattern were measured as
depicted in Fig. 5(a). The bending strength and the flexural modulus of different core shapes can also be
seen in Fig. 4(a). Here, the rhombus-filled CFRCs endured an enormous load in the three-point bending
test, and the breaking point appeared the latest in time. Also, whether it is a pure PLA sample or a CFRCs
sample, the rhombus infill cellular structures manifest the best bending performance in comparison with
other designed structures, including the maximum force, bending strength, and flexural modulus, then
followed by rectangle and honeycomb infill shapes, respectively.
During the three-point bending test, the indenter continuously produced downward displacement, and the
sample had a certain bending deformation with the increase of displacement and load. In Fig. 5(b), the
angle α between the hypotenuse and the edge of the rhombus structure is 45°, and the angle β of the
honeycomb structure is 60°. In this way, the rhombus core has more components in the x-axis direction of
the fiber orientation, parallel to the edge of the rhombus structure. That means more fibers will be
assigned to resist the downward bending deformation. By contrast, the honeycomb core has more angle
components in the y-axis direction, which is vertical to the load direction. Therefore, the rhombus infill
CFRCs has better mechanical properties than the honeycomb CFRCs. As for the rectangular structure,
more fibers are distributed on the x-axis; thus, the mechanical properties of rectangular infill CFRCs are
also better than that of the honeycomb structures.
In addition to another printing parameter of fiber orientation that affects mechanical properties, the
interweaving structure of fibers in CFRCs, which is determined by the printing path, is also an essential
factor. In this work, to fabricate the samples with better stability, the fibers incorporated in the CFRCs were
interwoven as much as possible while designing the printing paths. For instance, in the honeycomb and
rhombic infill patterns of CFRCs in Fig. 1(d) and (e), we chose to adopt a path like a trapezoid, which
leads to avoiding the appearance of weak points as much as possible. In the rectangular infill CFRCs,
although the fibers are interlaced in the y-axis direction when the fibers change direction (due to the
existence of the 90° angle), the appearance of the weak point is inevitable, as shown in Fig. 5(b).
Table 1
Performance comparison of CFRCs with different infill patterns
Infill pattern Maximum load Flexural Effective density F/ρ E/ρ
modulus
Fm (N) (kg/m3) (KN·mm3/g) (KN·m/kg)
Ef (MPa)
Honeycomb 14.40 558.87 375.00 38.40 1490.32
Rectangle 18.70 738.00 452.08 41.36 1632.45
Rhombus 20.76 984.40 513.13 40.46 1918.42
Page 9/22Some mechanical properties with tested mean values of CFRCs for three types of infill structures were
tabulated in Table 1. The overall trend illustrates that the maximum load and flexural modulus increase
with the increment of effective density. In this table, F/ρ and E/ρ parameters indicate the maximum load
and flexural modulus divided by the effective density, respectively. Comparing the maximum
load/effective density (F/ρ) of CFRCs with different infill core shapes, the rectangle-filled CFRCs have the
highest value, followed by rhombus and honeycomb structures, respectively. Although the maximum load
applied on the rhombus infill CFRCs is higher than that of the rectangle, the effective density of the
rhombus core shape is also more significant than that of the rectangle core shape. In terms of designing
the printing path, if the nozzle in the rectangle-filled structure moves along the right-angle side, then the
nozzle in the rhombus-filled structure moves along the hypotenuse. Therefore, the total extrusion of the
rhombus-filled structure is more in amount than that of the rectangle-filled structure, and the mass of the
rhombus-filled samples is larger. In addition, since the samples have the same length, width, and
thickness (120 mm × 20 mm × 2 mm), the effective density of the rhombus core shape is larger than that
of the rectangle core shape. When the flexural modulus/effective density (E/ρ) is compared, the rhombus
core shape has the highest value, followed by the rectangle and honeycomb core shapes. Therefore, the
effective density has a positive effect on the bending performance of CFRCs. The effective density of
each sample can be calculated using Eq. (6).
M
ρ=
LHT
6
Here, ρ is the effective density, M is the mass, L is the specimen length, H is the specimen width, and T is
the specimen thickness.
3.5. Effect of Cell length effect on bending performance
The length of the printing unit is an important parameter when considering the flexibility of CFRCs. In this
study, the cell length of CFRCs was set to be 10, 15, 20, 30, and 40 mm, as shown in Fig. 6(a), and other
parameters were kept precisely the same. A smaller cell length will consume more material to print a
sample and have a higher infill density. Figure 6(b) depicts the fiber content and effective density of
rhombus-filled CFRCs with different cell lengths. During the printing process of CFRCs, the continuous
fiber and PLA matrix are extruded from the nozzle simultaneously; therefore, it is only required to consider
the total length of the printing path to calculate the fiber content. The load-displacement line diagrams
were obtained from the three-point bending tests for cell length, bending strength, and flexural modulus of
the printed rhombus-filled CFRCs samples with different cell lengths from 10 mm to 40 mm; the results
are shown in Fig. 6(c) and (d). In Fig. 6(e), the parameters of F/ρ and E/ρ are used to evaluate the
stiffness of CFRCs with different cell lengths. As the cell length increases, the effective density of the
CFRCs decreases—based on Eq. (6)—from 661.81 to 456.95 kg/m3.
However, since the angle α decreases with the increase of the cell length, as shown in Fig. 5(b), the fiber
orientation in the rhombus infill CFRCs increases in the x-axis direction. Hence, more fibers participate in
Page 10/22resisting the bending deformation of the structure. Therefore, the bending strength and modulus
increased from 24.14 MPa to 29.61 MPa, and 864.06 MPa to 1115.09 MPa, respectively. In addition,
comparing the samples with cell lengths ranging from 10 to 40 mm, the samples' effective densities were
decreased by 30.95%, while F/ρ and E/ρ were increased by 75.77% and 86.90%, respectively. Therefore, it
can be said that the cell length has a positive effect on the bending performance of lightweight cellular
CFRCs. The mechanical properties of rhombus infill CFRCs with a cell length of 40 mm are much better
than that of 10 mm.
3.6. Effect of printed layer thickness on bending
performance
The printed layer thickness is an important parameter in determining the fiber content in the composite
material of fiber-reinforced truss structure. When the thickness of the printing structure is constant, the
number of printing layers required to complete the whole printing process is determined by the layer
thickness. In this experiment, the layer thickness of CFRCs was set to be 0.1 mm, 0.3 mm, 0.5 mm, and
0.7 mm, and the corresponding printing layers were 20, 7, 4, and 3, respectively, as shown in Fig. 7(a).
The characteristic parameters bending properties of rhombus-filled CFRCs samples with different printing
layer thicknesses are shown in Fig. 7(b)-(e). With the increase of the printing layer from 0.1 mm to 0.7
mm, the fiber content decreased from 14.82–2.39% due to a decrease in printing layer. The number of
fibers in each layer is only related to the printing path, so the number of fibers is the same for each
printing layer. Therefore, the smaller the printing layer thickness, the more printing layers, and the greater
the fiber content. Kevlar fiber can considerably improve the bending performance of CFRCs, so when the
layer thickness is 0.1 mm, the bending performance of the sample is the best, and the bending strength
and bending modulus are 53.61 MPa and 2728.41 MPa, respectively. With the increase of the printing
layer thickness, the contact pressure of the printing nozzle on the printing material decreases, and the
adhesion quality between layers deteriorates. The parameters of F/ρ and E/ρ of CFRCs with different
printing layer thicknesses are shown in Fig. 7(e). From the overall trend, with the increase of the layer
thickness, the value of F/ρ decreases from 68.88 to 41.14, and the value of E/ρ also decreases from
4600.56 to 1986.24. Therefore, with the increase of the printing layer thickness, the stiffness of CFRCs
gradually decreases, and the bending performance of the CFRCs gradually weakens.
4. Conclusions
In this work, Kevlar fiber and PLA matrix were both fed into the print nozzle of a continuous-fiber-based
3D printer simultaneously to prepare Kevlar continuous fiber-reinforced structural composite. By
comparing the mechanical properties of printed samples based on pure PLA and CFRCs with the same
structure, the enhancement effect of Kevlar fiber on the 3D printed lightweight cellular structures was
discussed. Furthermore, it was verified that Kevlar fiber is an ideal lightweight material for the fabrication
of reinforced structures. The three-point bending test results demonstrated that the mechanical properties
of CFRCs depend on the infill shapes to a large extent. By comparing the bending performance of CFRCs
Page 11/22with different infill shapes (rectangle, rhombus, and honeycomb), the relationships between fiber
orientation, printing path, infill density, and bending performance of CFRCs were studied.
In this article, the rhombus-filled CFRCs have better bending resistance as compared to the other samples
filled with honeycomb and rectangular patterns. Also, the rhombic structure has more fiber orientation,
which leads to more resistance to bending. Moreover, benefitting from the innovative design of the
printing path, fibers are evenly distributed and overlapped in the CFRCs, with few weak spots in the
printed structure. Based on rhombus-filled CFRCs, the influence on characteristic parameters and
mechanical properties of rhombus infill cellular CFRCs with different cell lengths were systematically
studied. The results exhibited that as the cell length increases, the CFRCs perform not only higher bending
strength and flexural modulus but also lighter mass, which provides a crucial basis for the application of
such materials. It is crystal clear that lightweight cellular materials will have tremendous development
potential in the near future. Noteworthy, this article provides a basis for the study of CFRCs with various
infill patterns and different structural parameters.
Declarations
Author Contributions Statement: Z. Cui performed the experiment, analysed the data and wrote up the
manuscript. X. Huang contributed significantly to the analysis and manuscript preparation. M. Panahi-
Sarmad assisted with the analysis of data and results with constructive discussions. K. Dong contributed
to the conception and idea of the study. X. Xiao revised and confirmed the final manuscript. All authors
reviewed and agreed to publish the manuscript.
Funding: This work was financially supported by the National Natural Science Foundation of China
(Grant No. 51703083), the Project "Fibre materials and products for emergency support and public safety"
from Jiangsu New Horizon Advanced Functional Fibre Innovation Centre Co. Ltd. (Grant No. 2020-
fx020026).
Conflict of Interest: The authors declare no conflict of interest.
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Figures
Page 15/22Figure 1
3D printing of CFRCs. (a) The schematic representation of the printing procedure, (b) the printing process
of rhombus-filled CFRCs with a cell length of 30 mm. (c) Print paths of a single layer of rectangular infill
CFRCs, (d) print paths of a single layer of honeycomb infill CFRCs, (e) print paths of a single layer of
rhombus infill CFRCs, and (f) the print model(left) and the G-code edited in Repetier software.
Page 16/22Figure 2
(a) Schematic diagrams of the three-point bending test, (b) a load-displacement curve obtained from the
three-point bending test, and photos were taken during the test.
Page 17/22Figure 3
(a)-(d) Finite Element (FE) simulation of bending behavior of a honeycomb-filled cellular CFRC: (a)
original state, (b) elastic state, (c) yield-like state, (d) fracture stage, FE models of (e) rhombus-filled and
(f) rectangle-filled CFRCs, and experimental test of samples of (g) rhombus-filled and (h) rectangle-filled
CFRCs.
Page 18/22Figure 4
(a) Mechanical properties of printed CFRCs and pure PLA samples with different infill patterns, (b)-(d)
SEM images of morphological characterization of CFRCs, (b) Side view of CFRCs, (c) interface of Kevlar
fiber and PLA matrix, and (d) fractured cross-section of CFRCs after 3-point bending test. Note: All
samples are printed under the same conditions: layer thickness of 0.5 mm; printing speed of 60 mm/min;
nozzle diameter of 1 mm; nozzle temperature of 205 °C; printing platform temperature of 50 °C.
Figure 5
Page 19/22Mechanical properties of 3D printed cellular CFRCs with different core shapes. (a) Load–displacement
curves were obtained from the three-point bending test and (b) samples of 3D printed CFRCs and their
partially enlarged images. Note that the curves in different colors represent the fiber orientation in CFRCs.
Figure 6
Page 20/22(a) Rhombus-filled cellular structures with different cell lengths of 10, 15, 20, 30, and 40 mm, respectively,
(b)-(e) Characteristic parameters and mechanical properties of rhombus-filled CFRCs with cell lengths
from 10 mm to 40 mm, (b) fiber content and effective density, (c) load-displacement curves, (d) bending
strength and flexural modulus, (e) maximum force divided by density, and flexural modulus divided by the
density of different cell length.
Figure 7
Page 21/22(a) Rhombus-filled cellular structures with different layer thicknesses of 0.7, 0.5, 0.3, and 0.1 mm,
respectively, (b)-(e) Characteristic parameters and mechanical properties of rhombus-filled CFRCs with
different layer thicknesses of 0.1, 0.3, 0.5, 0.7 mm respectively, (b) fiber content and effective density, (c)
load-displacement curves, (d) bending strength and flexural modulus, (e) maximum force divided by
density, and flexural modulus divided by the density of different layer thickness.
Supplementary Files
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