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Mechanical behaviour of 3D printed composite parts with short carbon fiber reinforcements
Journal Pre-proofs Mechanical behaviour of 3D printed composite parts with short carbon fiber reinforcements M. Somireddy, C.V. Singh, A. Czekanski PII: DOI: Reference:
S1350-6307(19)30901-X https://doi.org/10.1016/j.engfailanal.2019.104232 EFA 104232
To appear in:
Engineering Failure Analysis
Received Date: Revised Date: Accepted Date:
24 June 2019 26 August 2019 1 October 2019
Please cite this article as: Somireddy, M., Singh, C.V., Czekanski, A., Mechanical behaviour of 3D printed composite parts with short carbon fiber reinforcements, Engineering Failure Analysis (2019), doi: https://doi.org/ 10.1016/j.engfailanal.2019.104232
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Mechanical behaviour of 3D printed composite parts with short carbon fiber reinforcements M. Somireddya, C.V. Singhb,c , A. Czekanskia* a
Department of Mechanical Engineering, York University, Toronto, ON, M3J 1P3, Canada Department of Materials Science and Engineering, University of Toronto, Toronto, ON, M5S 3E4, Canada c Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, ON, M5S 3G8, Canada b
Abstract This study investigates the mechanical behaviour of 3D printed composite parts with short carbon fiber (SCF) reinforcements by conducting mechanical testing. Further, this study examines the relevance of the mechanics of traditional composite laminates for characterizing the mechanical behaviour of printed parts. Initially, test coupons were 3D printed with different layer thicknesses and printing directions. The test coupons were then subjected to mechanical testing. Test results reveal that the tensile properties of thin-layered printed parts are better than those of thick-layered printed parts, but not the interlaminar properties. Thick-layered parts performed poorly under tensile loads because of the presence of undesirable enclosed voids within the extruded fibers (extrudates). Further, laminate theory accurately predicted the mechanical behaviour of thin-layered bidirectionally printed parts. The failure surface followed the orientation of extrudates in unidirectionally printed parts. The micro-CT analysis unveiled microstructural features, such as the orientation of SCFs along the printing direction, the presence of enclosed voids within the extrudates of thick-layered parts, and a reduction in maximum length of SCF in thin-layered parts; such features influenced the final mechanical behaviour of the printed parts. Keywords: 3D printing; mechanical properties; laminate mechanics; mechanical testing; microCT 1. Introduction In recent years, the use of composite materials in the fabrication of functional parts via additive manufacturing (AM) methods has risen in popularity [1]. This trend has been fueled by the benefits of AM in tailoring the properties of 3D printed parts. The AM process is superior because it requires no tooling or special skills, production is rapid, and the design of parts is efficient, with no limitations on the geometric complexity of the final printed part. Consequently,
*
corresponding author e-mail: [email protected]
1
the development and printing of composite materials via AM is an emergent research trend [2,3]. A comprehensive work on 3D printing of composite parts is available in [4–6]. The composite materials used for printing parts have improved the mechanical, thermal, and electrical properties of the parts [7–11]. Furthermore, the properties of
parts can be tailored by altering the
mesostructure as the parts are being printed [12–16]. The 3D printing of parts using composite materials deserves special attention owing to the materials’ anisotropy. Therefore, a detailed investigation of how microstructural features influence the material properties of printed parts would guide the effective design of composite parts for 3D printing. The printed composite parts via a material-extrusion AM method, fused filament fabrication (FFF), are the main focus of this work. The material properties of the final printed part differ from those of the original material [17]. The reason for this difference is that anisotropy is introduced into the properties of the part, mainly due to the process parameters chosen [18]. The material properties are significantly influenced by two main process parameters: printing direction and layer thickness [19]. Further, the type of filament material (isotropic or composite) used in printing also introduces anisotropy into the printed parts [20]. Another factor governing the material behaviour of the final printed parts is the type of reinforcements used in the composite material [21]. The reinforcements used in composite materials to fabricate parts via FFF can be continuous fibers, short fibers, or particulates. Parts printed with continuous fibers [22–24] and short fibers [25,26] have better mechanical properties than parts made with no additional reinforcements. The orientation of reinforcements in the printed parts governs the properties of the final part [27]. The studies mentioned focused only on the influence of process parameters and reinforcements on the final properties of printed parts, but their influence on the mechanical failure behaviour of composite printed parts has not yet been investigated. The mechanical behaviour of parts fabricated with continuous fibers can be characterized using laminate mechanics [28,29]. Laminate mechanics can also be employed to characterize the mechanical behaviour of parts fabricated with only thermoplastic filament material and no additional reinforcements [30–34]. The use of laminate mechanics implies that parts printed via FFF behave like laminate structures. Thus, laminate theory can be employed to characterize the behaviour of parts subjected to different loads during stress analysis [35]. The layers of parts
2
printed with short carbon fibers (SCF) can be treated as orthotropic lamina material, and the mechanical behaviour of such parts have been characterized using laminate mechanics [36]. Delamination is one of the failure modes in laminates, and printed parts experience this failure mode [37,38]. Printing parameters have been found to influence interlayer bonding strength. The influence of layer thickness as well as reinforcements on the interlaminar properties of printed parts has not yet been explored. So far, experimental characterization of composite parts fabricated with material containing continuous fiber [22–24,28,29,37] and discontinuous fiber [19,25– 27,36,39] reinforcements has been explored, but their material modeling has not yet been attempted. Moreover, mechanics for printed parts in characterizing their mechanical failure behaviour has not been addressed in earlier works, especially for printed parts with discontinuous reinforcements. This gap is addressed in the present work. The present work characterizes the mechanical behaviour of SCF-reinforced composite parts fabricated via FFF. Mechanical test standards and materials employed for 3D printing of test coupons are then presented. Test coupons are printed uniaxially and bidirectionally with two different layer thicknesses. The test coupons are then subjected to uniaxial tensile loading to study the influence of printing direction and layer thickness on the overall mechanical behaviour of the parts. Furthermore, the test coupons are subjected to a crack opening load to assess the interlayer bonding strength of the printed parts. Laminate mechanics and laminate theories are employed to characterize the mechanical behaviour of the printed parts. Furthermore, micro-CT analysis is used to investigate the influence of SCF reinforcements on the final material properties of the printed parts. 2. Methodology 2.1 Mechanical testing The tensile test coupons were 3D printed via FFF. The printed test coupons followed the ASTM D3039 standard for tensile testing. The dimensions of the printed test coupons are listed in Table 1 and Figure 1. Axes x, y, and z represent the global coordinate system, while the local system of a single lamina is represented with 1, 2, and 3, where 1 denotes the fiber direction in a lamina, and 2 and 3 are transverse to the fiber direction, as shown in Figure 1. Additionally, test coupons (Figure 2), as per ASTM D5528, were printed to investigate the interlaminar fracture toughness of the parts under crack opening mode (mode I). 3
This study employed the mechanics of laminates to characterize the mechanical behaviour of composite parts fabricated with discontinuous fiber reinforcements. Here, printed parts are also referred to as “printed laminates” because of their laminate behaviour and further, the layers are also known as the laminae of a part. In FFF process that the filament material is melted and extruded through a fine nozzle, and then the extruded material is deposited layer upon layer on substrate to build 3D parts. The layers constitute series of tracks of extruded filament material, and the tracks of extruded material referred as ‘extrudates’ in this study. In other words, the extruded filament material is regarded as composite extrudates that form the layers, and the extrudates behave as fibers of unidirectional lamina. Commercially available composite filament material (ABS-SCF) consisting of SCF reinforcements in ABS thermoplastic was used to print the test coupons. The composite filament material spool was purchased from filament manufacturer 3DXTech (Byron Center, Michigan, USA). The main governing process parameters, layer thickness (t, in mm) and raster angle (θ, in °), are considered here to investigate their influence on the mechanical behaviour of printed parts. Raster angle represents the extrudate orientation (printing direction) for the laminate layup, while layer thickness defines the thickness of the lamina as well as the cross-sectional size of the extrudates of the printed laminates. The FFF printing process is shown in Figure 3. Thick-layered and thin-layered test coupons were printed unidirectionally and also bidirectionally by varying the process parameters. The layup order in the printed bidirectional laminates is balanced and symmetrical. The thickness of the laminae in thick-layered (t1) printed laminates is two times the thickness of laminae in thin-layered (t2) laminates, but the thickness of all layers within a laminate is equal. Table 2 shows the lamina layup order of the test coupons printed for mechanical testing. The laminate test coupons for layups 1–7 were subjected to a uniaxial tensile load along the x-axis (Figure 1), and the test coupons for layup 8 were subjected to a crack opening load (Figure 2). Table 1. Dimensions (in mm) of the tensile test coupons. Laminates Unidirectional Bidirectional
Layup No. 1 to 4 5 6, 7
L 190 127 200
W 12.7 25.4 25.4
T 2.54 2.54 2.54
TL 38 19 30
Note: L, total length; W, width; T, thickness; TL, tab length.
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Figure 1. Tensile test coupons with dimensions (in mm): L, total length; W, width; t, thickness; TL, tab length; q, extrudate direction to x-axis (°).
Figure 2. Test coupon for an interlaminar fracture toughness (mode I) test. Table 2. Laminate layup orders and layer thicknesses used for printing laminate test coupons. Layup No. Raster angle (q, °) Unidirectional laminates 1 0° 2 30° 3 45° 4 60° 5 90°
Layer thickness t1 = 0.317 mm t2 = 0.158 mm × × × × ×
× × × × ×
Bidirectional laminates 6a [0°/90°]2S 6b [0°/90°]4S 7a [45°/–45°]2S 7b [45°/–45°]4S
× — × —
— × — ×
Unidirectional laminates 8a 0° 8b 0°
× —
— ×
5
Figure 3. Fused filament fabrication (3D printing) of parts. Initially, different test coupons were printed unidirectionally with varying printing direction and layer thickness. Five test coupons were printed for each test case. For example, the layup 1 test coupons can be of two different layer thicknesses (t1 and t2), resulting in a total of 10 coupons being printed for this case alone, as shown in Table 2. The raster angle represents the printing direction of the part, that is, the orientation of the extrudates of the layers in the part. For instance, layup 1 is a unidirectional laminate in that the extrudates are oriented only along the axis of loading, that is, along the x-axis, and in the other layup cases the extrudates are oriented offaxis to loading. The difference between bidirectional laminates 6a and 6b is in their layer thickness, but the layup order is the same. The layers of layup 6a are thicker than those of layup 6b. The unidirectional and bidirectional laminate test coupons mentioned in Table 2 were printed for the investigation. The other process parameters used while printing the test coupons were as follows: a printing speed of 50 mm/s, a bed temperature of 80 °C, an extruder temperature of 235 °C, 1 shell, an infill density of 100%, and an overlap between adjacent extrudates of 15%. These parameters were recommended by filament material supplier for printing good quality parts. A total of 70 test coupons for tensile testing and 10 coupons for fracture testing were printed on an Ultimaker printer. 2.2 Micro-CT scanning A micro-CT scanner is useful for obtaining intricate 3D microstructural information about a material; its working principle is shown in Figure 4 [40]. The object is positioned to rotate while exposing it to an X-ray generator for a specific period of time. The rays penetrate the object and reach the detector, and then provide the X-ray shadow 2D images containing microstructural information about the 3D object. The images are reconstructed using an algorithm [40] to carry out image analysis and obtain the desired microstructural information. The micro-CT scanning equipment used to scan the parts was a SKYSCAN 1272 (Bruker Corporation, Belgium). The scanning generates several radiographic images of the filament. Reconstruction of the images is 6
performed using Bruker’s NRecon program, which outputs the grayscale images of a cross-section of the filament. The reconstruction results can be used by other Bruker programs, including DataViewer, CT-analyser (CTAn), and CT-volume (CTVol). DataViewer is used to visualize the cross-section of the filament, CTAn to study the morphometry and also build 3D models of the microstructure, and CTVol to conduct surface rendering of 3D models of the material’s microstructure. Tomography images provide information about the constituents of the composite material, such as the matrix and reinforcements. The images reveal the size and shape of the reinforcements and their orientation as well as their distribution in the composite filament material and in the printed parts. Moreover, the scanning images are useful in the construction of the 3D geometry of the material’s microstructure. Microstructural features such as reinforcements and voids — including their size, shape, orientation, and distribution in the material of the printed parts — influence the overall material properties. Therefore, the quantification of these parameters is important for estimating the overall material properties of the printed parts.
Figure 4. Working principle of a CT scanner [40]. 3. Results and discussion 3.1 Mechanical testing Initially, uniaxial tensile tests were conducted using an MTS testing machine equipped with a 10 kN load cell. A quasi-static loading rate of 1 mm/min was applied to coupons along the x-axis to deform them, and the displacements of the coupons were recorded using a laser extensometer. The unidirectionally printed parts were subjected to tensile testing to establish the mechanics of the parts for characterizing mechanical behaviour under tensile loads. Following that step, bidirectionally printed test coupons were considered for tensile testing, and their behaviour was then characterized using laminate mechanics. 7
The test results of the unidirectionally printed parts revealed the mechanical behaviour of the printed parts and, furthermore, the influence of extrudate angle and layer thickness on this behaviour. The stiffness (Ex) and tensile strength (Xt) of the different unidirectional laminates obtained from the test results are plotted in Figure 5. The numbers on the horizontal axis of the graph represent the extrudate angle of the unidirectional laminates with respect to the x-axis. The stiffness and strength of the unidirectional thin-layered laminates is higher than that of the thicklayered laminates. The difference between the thick- and thin-layered laminates of layup 1 is higher when compared with other layup cases, where the extrudates are off-angle to the loading axis. Moreover, the strength and stiffness of the laminates of layup 1 are higher than that of the other layup cases. This indicates that the printed parts have directional properties and that the extrudates are the main load-carrying members of the parts. Therefore, the layers of the printed parts can be treated as unidirectional fiber-reinforced laminae. Furthermore, the laminae of the printed parts can be treated as transverse isotropic material. This confirms that laminate mechanics can be employed in the investigation of the mechanical behaviour of printed parts.
Figure 5. Mechanical properties of unidirectional laminates for (a) Ex (stiffness) and (b) Xt (tensile strength) for thick-layered (t1) and thin-layered (t2) laminates. The mechanical properties of the laminae of a printed laminate can be calculated from the tensile test results of laminate layups 1, 3, and 5 [41]. Table 3 presents the mechanical properties of a single lamina. The properties E1, ν12, and Xt were obtained from the results of layup 1, and E2 and Yt were calculated from the test results of layup 5. The in-plane shear properties G12 and S were obtained using the test results of layup 3. A digital image correlation (DIC) setup from LaVision GmbH was employed to measure the strain of layup 1 in order to calculate Poisson’s ratio (ν12). The elastic moduli of the lamina listed in Table 3 can define the plane constitutive 8
matrix, which is useful in the stress analysis of the printed parts. Further, the strength parameters of the lamina are useful in the failure analysis of the printed parts. Table 3. Mechanical properties of a single lamina of the printed laminates. E1, MPa E2, MPa G12, MPa ν12 Xt, MPa Yt, MPa S, MPa
Thick lamina (t1) 2684.2 ± 98.5 1545.7 ± 9.1 624.7 ± 7.1 0.34 ± 0.04 26.1 ± 0.9 14.6 ± 0.3 11.8 ± 0.6
Thin lamina (t2) 4120.4 ± 72.4 1654.3 ± 90.2 770.0 ± 21.6 0.32 ± 0.05 40.7 ± 0.6 14.9 ± 0.7 14.6 ± 1.5
Now, consider the tensile test results of the 3D printed bidirectional cross-ply and angleply layup laminates. Here, the behaviour of bidirectionally printed parts subjected to tensile loads is characterized using laminate mechanics and classical laminate theory (CLT). The constitutive relation of a lamina is written as é E1 ê1 -n n 12 21 ìs 11 ü ê ï ï ê n 12 E1 ís 22 ý = ê ï t ï ê1 -n 12n 21 î 12 þ ê 0 ê ë
n 12 E1 1 -n 12n 21 E2 1 -n 12n 21 0
ù 0 ú ú ì e11 ü úï ï 0 ú íe 22 ý ú ïg ï î 12 þ G12 ú ú û
in other form {s } = [Q]{e }
(1)
In global coordinate system, the constitutive matrix system is given as
{s } = éëQ ùû {e } where theory,
Q ij
(2)
is written as éëQ ùû = [T ] [Q ][T ] and [T ] is a transformation matrix. From laminate
the strains {e 0 } ,
-1
-T
{k} represents the in-plane strains and curvatures of a laminate,
respectively. The total strains for a laminate is written as
{e } = {e 0 } + z {k}
(3)
The resultant force (N) and moment (M) per unit width for a laminate with n number of layers are expressed as
{ N } = [ A]{e 0 } + [ B ]{k} , and {M } = [ B ]{e 0 } + [ D ]{k}
(4a,b) 9
n
where [ A] = å éëQ ùû ( zk - zk -1 ) , [ B ] = k =1
k
1 n 1 n éëQ ùû ( zk2 - zk2-1 ) , [ D ] = å éëQ ùû ( zk3 - zk3-1 ) å 3 k =1 2 k =1 k k
are stiffness
matrices of a laminate, and for a symmetric laminate [B]=[0]. Strains for a symmetric laminate subjected to only in-plane forces are given from Eq.4a as ìe xx0 ü ì N xx ü -1 ï ï 0ï ï íe yy ý = [ A] í N yy ý ïg 0 ï ïN ï î xy þ î xy þ
(5)
For uniaxial tensile testing of laminate subject to loading along x-axis, then N xx = hs xx Nyy=0 and Nxy=0 for laminate thickness h. The stress-strain relation for this case is s xx = Exx e xx0 , using the relation Eq.5, the modulus of elasticity along the x direction of the laminate is calculated as follows Exx =
1 [ A ]11 h
(6)
-1
The elastic moduli such as E1, E2, G12, ν12 of the lamina found from the experimental tensile test results (Table 3) are used for the calculation of matrices [A], [B] and [D]. Then, Exx of the laminate can be calculated using Eq. 6. The Tsai-Hill failure criterion for the laminae for a planar stress is written as s12 X
2
-
s1s 2 X
2
+
s 22 Y
2
+
t122 S2
=1
(7)
The properties of lamina (Table 3) are useful for characterizing the mechanical behaviour of bidirectionally printed parts using classical laminate theory. CLT employs the elastic moduli of the lamina (Table 3) in the calculation of the tensile modulus of the laminate, as described above. The results for thick- and thin-layered bidirectional laminates are presented in Table 4 for crossply laminates and Table 5 for angle-ply laminates. The tensile modulus (Ex) of the laminates obtained from the experimental work was validated using CLT. Ultimate tensile strength (Ut) and strain to failure (εt) are also given in Tables 4 and 5. The difference between the values obtained using CLT and the experimental results for thick-layered bidirectional laminates is greater than that for thin-layered laminates. Moreover, the results obtained using CLT are lower than those
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obtained experimentally. The values of the properties of thin-layered bidirectional laminates are also higher than those of thick-layered laminates. The difference in the results for thick- and thinlayered laminates is due to changes in their mesostructure. The reasons for this difference are described in a later section discussing the mesostructural features observed using a micro-CT scanner. Further, finite element (FE) failure analysis of bidirectional laminates subjected to tensile loads were carried out, and the Tsai–Hill failure criterion was employed. The laminates were modeled with 2D PCOMPG finite elements in Altair HyperWorks, and then the uniaxial tensile loads were applied to the FE model. The FE model was then simulated for failure analysis; more details about FE failure modeling are available in [34]. The principal stress (σply) for first-ply failure of the laminate and the corresponding elastic ply strain (εply) are provided in Tables 4 and 5. The discrepancy between the CLT and experimental results is due to the properties used in CLT (provided in Table 3). The properties of the lamina listed in Table 3 were calculated from tensile test results for the unidirectional laminates. The properties obtained from test results for the unidirectional laminates were largely influenced by layer thickness, orientation of reinforcements, and bonding at the interface of the layers. Further, the bonding at the interface of the layers in the bidirectional laminates is not the same as that in the unidirectional laminates. A detailed microscopic investigation of these parts can reveal the reasons for the discrepancy in the experimental and CLT-based results. The following section discusses the characterization of the microstructure of the printed parts using tomographic images obtained via micro-CT scanner and also the influence of microstructural features on the material properties of the printed parts. Table 4. Mechanical properties of cross-ply laminates under tensile loading. Ex, MPa Ex, MPa (CLT) Ut, MPa σply, MPa (FE) εt εply (FE)
Thick lamina (t1) 2863.9 ± 78.7 2125.9 23.5 ±0.5 26.0 0.0158 ± 0.0006 0.0097
Thin lamina (t2) 3311.0 ± 43.1 2909.9 31.3 ± 0.4 37.8 0.0214 ± 0.0006 0.0096
Table 5. Mechanical properties of angle-ply laminates under tensile loading. Ex, MPa Ex, MPa (CLT)
Thick lamina (t1) 2094.6 ± 43.5 1733.3
Thin lamina (t2) 2330.8 ± 53.6 2150.5
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Ut, MPa σply, MPa (FE) εt εply, (FE)
21.7 ± 0.5 22.7 0.0243 ± 0.0011 0.0105
27.7 ± 0.4 31.1 0.0373 ± 0.0013 0.0105
Next, the influence of lamina layup and layer thickness on the failure behaviour of printed laminates subjected to uniaxial tensile loads is discussed. The stress–strain curves of unidirectional laminates made of two different layer thicknesses are shown in Figure 6. The thin-layered unidirectional laminates are stiffer and stronger than the thick-layered laminates. The fracture strength and fracture strain of layup 1 laminates is greater than that of the other unidirectional laminates because the orientation of extrudates in the other laminates is off-axis to loading. Also, the interface of extrudates in a layer shares a significant amount of load, and the bonding strength of the interface is lower than the strength of the extrudates. This led to the fracture occurring at the interface of the extrudates in laminate layups 2 to 5. Figure 7b shows the fracture lines of the different unidirectional laminate layups 1 to 5. The angle of the fracture lines of the laminate follow the orientation of extrudates in that laminate.
Figure 6. Stress–strain curves for (a) thick-layered and (b) thin-layered unidirectional composite laminates.
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Figure 7. (a) Tensile testing with digital image correlation setup; (b) failure line of the five different layups of composite printed test coupons. Let us now consider the failure behaviour of the bidirectional printed laminates. The stress– strain curves of the bidirectional laminates for cross-ply and angle-ply layup order are shown in Figure 8. The cross-ply laminates are stiffer and stronger than the angle-ply laminates, but the fracture strain of angle-ply laminates is higher than that of the cross-ply laminates. The thinlayered bidirectional laminates have higher fracture strength and fracture strain than the thicklayered laminates. It means that the properties of printed parts can be tailored by varying printing direction and layer thickness. The strain distribution just before propagation of the crack in the cross-ply and angle-ply laminates subjected to tensile loading is shown in Figure 9. Tensile testing of laminates with the DIC setup is shown in Figure 7a. The stress–strain curves of the laminates follow linear and nonlinear regions while deforming under tensile loading. This means that the matrix and SCF reinforcements deform elastically until the bonds break between them, and the matrix deforms plastically until complete failure of the part. Further, the layup order influences the bonding strength between the layers, meaning that bonding strength at the interface is not the same in unidirectional and bidirectional laminates. In the stress–strain curves for the laminates, note that the angle-ply laminates have higher fracture strain than the other layup order laminates. Also, the influence of bonding is significant in the unidirectional laminates with extrudates oriented off-axis to loading. Further, the properties of the lamina calculated from the tensile tests results of unidirectional laminates are also influenced by bonding strength in such laminates. Therefore, the CLT values are lower than the experimental values, as noted earlier, since CLT employs the lamina properties.
Figure 8. Stress–strain curve of (a) cross-ply and (b) angle-ply composite laminates. 13
(a)
(b)
(c)
(d)
Figure 9. Strain distribution in the printed composite laminates during uniaxial tensile testing of (a) thick-layered cross-ply laminates, (b) thin-layered cross-ply laminates, (c) thick-layered angleply laminates, and (d) thin-layered angle-ply laminates. Next, let us consider the test results for interlaminar fracture toughness of laminate layup 8. The test results reveal the interlaminar fracture toughness of the printed laminates under crack opening mode. Interlaminar fracture toughness represents the bonding strength between the layers of printed parts. Interlayer debonding (decohesion) of printed parts is also referred to as delamination, which is one of the failure modes in traditional composite laminates. Figure 10 shows a test coupon subjected to crack opening mode. Load (P) versus crack opening displacement (δ) curves for the test coupons are shown in Figure 11a, while Figure 11b illustrates the fracture toughness (GI) versus delamination length (a) of the coupons. Fracture toughness is calculated based on modified beam theory, as described in the standard. Interlaminar fracture toughness of the thin-layered printed laminates is lower than that of the thick-layered laminates because the temperature of the extruded material influences interlayer adhesion. The extruded material for thin layers carries lower heat capacity when compare to that of thick layers [42,43]. Therefore, the extruded material for thin-layered laminates was subjected to faster cooling than extruded material for thick-layered laminates. Further, the extruded material had less time to diffuse with the previously deposited material, causing poor adhesion between adjacent extrudates as well as between layers. Thus, the interlaminar bonding strength of thick-layered laminates is higher than that of thin-layered laminates. This behaviour is contrary to the behaviour of thin-layered parts under tensile loads, where the thin-layered parts have better mechanical performance than the
14
thick-layered parts. This suggests that interlaminar bonding strength of the thin-layered parts is not the reason for their higher strength and stiffness under tensile loading.
Figure 10. Printed test coupon under crack opening mode.
Fracture toughness (GI), N/mm
0.8 Thick-layered Thin-layered
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
(b) 0
a0 20 40 60 Delamination length (a), mm
80
Figure 11. Interlaminar fracture toughness results for printed laminates under mode I loading: (a) load versus displacement; (b) fracture toughness versus delamination length.
3.2 Micro-CT analysis The microstructure of the printed parts was characterized using a micro-CT scanner. Further, the influence of microstructural features on the material properties of printed parts was investigated. The parameters employed for scanning are presented in Table 6. The composite filament used for printing is first scanned. Then unidirectionally printed parts with two different layer thicknesses are scanned. Table 6. Parameters of micro-CT used for scanning 3D printed parts. Parameter
Value
15
Filter Resolution Image size X-ray voltage
X-ray intensity
None 1 μm/pixel 4904 × 3280 pixels 40 kV for filament 35 kV for thick-layered part 30 kV for thin-layered part 200 μA
The micro-CT analysis of the printed material reveals the size, shape, and distribution of the carbon fiber (CF) reinforcements and their percent volume in the printed parts. Any variation in such data between the thick-layered and thin-layered parts can affect their material behaviour. The 2D images with high resolution (1 µm/pixel) shown in this section were obtained using DataViewer, and the 3D models were obtained using CTAn as well as CTVol. The scanning results of the composite filament material reveal that the length of CF reinforcements varies between 10 and 278 µm. The diameter of the CF is around 7.5 µm, and its proportion in the filament material is around 10.60%. Next, let us consider thick-layered unidirectionally printed laminates for micro-CT analysis. A small volume of material was taken from the thick-layered 3D printed tensile test coupons for scanning. Then, tomographic images of the laminate were used to characterize the laminate’s microstructure. The cross-section of the laminate with four layers is shown in Figure 12. The mesostructure of the laminate has triangular-shaped voids (black dotted lines in Figure 12a) between adjacent layers. These voids are continuous and exist at the interfaces of all layers of the laminate; further, these voids are inherited from the printing methodology.
16
Figure 12. Thick-layered 3D printed composite part: (a) mesostructure; (b) cross-sectional view taken at X–X. The scanner reveals irregularly shaped voids (black continuous lines in Figure 12a) in the mesostructure. The voids are enclosed and present within extrudates (extruded filament) of the laminate. The enclosed voids are result of flow characteristics of extruded material and are governed by the melt temperature, printing speed and type of reinforcements. Optimal printing speed and melt temperature, and small aspect ratio of reinforcements can minimize the voids in the parts [19,44]. The presence of these enclosed voids within the extrudates affects the strength and stiffness of the thick-layered printed parts. Furthermore, the enclosed voids and the continuous interlayer voids (triangular voids in fig.12) are stress concentration points and these can lead to fail at lower stresses [8]. Therefore, the voids degrade the properties of printed parts. These are the main reasons for the lower strength and stiffness of the thick-layered printed laminates. The black areas in the image are the denser material, representing CFs, and the rest of the gray area is ABS material. The orientation and distribution of the CF reinforcements (highlighted with red continuous lines) in the thick-layered part can be seen in Figure 12b. Most of the SCFs are oriented along the extrudate direction, meaning they are aligned with the printing direction, but some SCFs are not aligned owing to the presence of enclosed voids within the extrudate. The misalignment of SCFs with printing direction near enclosed voids is due to variation of size and shape of pores during solidification of the extruded material [8]. A single extrudate of different lengths (0.2, 0.3,
17
0.4 mm) was selected for analysis by CTAn. The length of CFs in the thick-layered parts varies between 10 and 276 µm. The percentage of CF in the selected fiber volumes is around 10.50%. Now, consider the thin-layered unidirectional printed laminate for micro-CT scanning. The same CT analysis procedure was repeated for a small volume of a thin-layered laminate. The mesostructure of the laminate is shown in Figure 13a. The mesostructure has voids at the interfaces of the layers, but the enclosed voids within the extrudates are minimal when compared with those in the thick-layered laminates. Minimal internal pores in this case is due to lower clogging of SCFs in nozzle and that resulted consistent flow of material while extruding [39]. Thus, the time for trapping air bubbles in the extruded material is lower and that led to minimal voids. Therefore, the properties of thin-layered parts have a higher value than those of thick-layered parts. The orientation and distribution of CF reinforcements in the extrudate can be seen in Figure 13b. The CF reinforcements are well aligned in the printing direction compared with those in the thicklayered laminate. The length of CF reinforcements varies between 10 and 152 µm, and the reinforcements are also well dispersed in the extrudates. In these printed laminates, the maximum length of the SCFs is approximately equal to the layer thickness, and for that reason SCFs with a length greater than the layer thickness are subject to fracture while material is being deposited. The fracture occurs because the gap between the nozzle tip and the previously deposited layer is approximately equal to the layer thickness, and therefore it cannot accommodate longer SCF reinforcements. The pecent volume of CF material in the selected volume of extrudate is ~10.30%. A repetitive volume of a material (red dotted lines in Figure 13a) was considered for analysis by CTAn. The 3D model generated is shown in Figure 14. The orientation and distribution of CF reinforcememts in the thin-layered parts can be seen in Figure 14a. The 3D model file is an .stl file that can be imported into a commercial FE package for material modeling of the printed part using numerical homogenization, and it is a further step to present work and is planned in future.
18
Figure 13. Thin-layered 3D printed composite part: (a) mesostructure; (b) cross-sectional view taken at Y–Y.
400 μm
(a)
(b)
Figure 14. 3D models of the mesostructure of a thin-layered printed part obtained from micro-CT scanning: (a) orientation of short carbon fibers in the printed part; (b) representative volume element model for homogenization. 4. Conclusions In this study, 3D printed parts were treated as composite laminates, and the mechanics of laminates were employed to determine the properties of the laminae of thick-layered and thinlayered parts. These properties were then employed in CLT and Tsai–Hill failure theory to 19
characterize the mechanical behaviour of bidirectionally printed parts. CLT and failure theory accurately predicted the mechanical behaviour of thin-layered bidirectionally printed parts. Thus, laminate theories can be used in the initial design and analysis of printed laminates. The mechanical testing results of printed coupons revealed that the strength and stiffness of thinlayered printed parts are higher than that of corresponding thick-layered parts. The fracture occurred at the interface of the fibers of a layer of unidirectional laminates, and the fracture surface followed the orientation of the extrudates in that laminate. The presence of enclosed voids in the extrudates of thick-layered parts degraded the material properties of the parts. Furthermore, the enclosed voids caused improper alignment of SCF reinforcements with printing direction in the thick-layered parts. This random alignment of SCFs led to more anisotropy in these parts. However, interlayer bonding strength was higher for thick-layered than thin-layered parts. This resulted in greater interlaminar fracture toughness for thick-layered parts. This means that the thinlayered printed parts perform poorly compared with thick-layered parts where loading (compression, buckling) causes delamination. Such performance of thin-layered parts is contrary to the performance of the same parts under tensile loading. The presence of voids in the extrudates and at the interface of layers, as well as the fracture of SCFs, degraded the overall material properties of the printed parts. Proper selection of layer thickness for fabricating 3D printed parts will minimize voids within the extrudates and also the breakage of SCFs while the material is being deposited. This in turn will lead to better mechanical performance of the printed parts. Acknowledgments We thank the Lassonde School of Engineering at York University and the Natural Sciences and Engineering Research Council (NSERC) for financial support. References [1]
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Highlights: ·
Mechanics of laminates are used to characterize the mechanical behaviour of 3D printed composite parts with short carbon fiber reinforcements.
·
3D printing parts with lower layer thickness yields better tensile properties, but not better interlaminar properties.
·
3D printing parts with higher layer thickness makes them prone to having enclosed voids, which degrades material properties.
24
Declaration of interest Article Title: Mechanical behaviour of 3D printed composite parts with short carbon fiber reinforcements Authors: Madhukar Somireddy, Chandra Veer Singh, Aleksander Czekanski Declarations of interest: ‘none'
S1350-6307(19)30901-X https://doi.org/10.1016/j.engfailanal.2019.104232 EFA 104232
To appear in:
Engineering Failure Analysis
Received Date: Revised Date: Accepted Date:
24 June 2019 26 August 2019 1 October 2019
Please cite this article as: Somireddy, M., Singh, C.V., Czekanski, A., Mechanical behaviour of 3D printed composite parts with short carbon fiber reinforcements, Engineering Failure Analysis (2019), doi: https://doi.org/ 10.1016/j.engfailanal.2019.104232
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Mechanical behaviour of 3D printed composite parts with short carbon fiber reinforcements M. Somireddya, C.V. Singhb,c , A. Czekanskia* a
Department of Mechanical Engineering, York University, Toronto, ON, M3J 1P3, Canada Department of Materials Science and Engineering, University of Toronto, Toronto, ON, M5S 3E4, Canada c Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, ON, M5S 3G8, Canada b
Abstract This study investigates the mechanical behaviour of 3D printed composite parts with short carbon fiber (SCF) reinforcements by conducting mechanical testing. Further, this study examines the relevance of the mechanics of traditional composite laminates for characterizing the mechanical behaviour of printed parts. Initially, test coupons were 3D printed with different layer thicknesses and printing directions. The test coupons were then subjected to mechanical testing. Test results reveal that the tensile properties of thin-layered printed parts are better than those of thick-layered printed parts, but not the interlaminar properties. Thick-layered parts performed poorly under tensile loads because of the presence of undesirable enclosed voids within the extruded fibers (extrudates). Further, laminate theory accurately predicted the mechanical behaviour of thin-layered bidirectionally printed parts. The failure surface followed the orientation of extrudates in unidirectionally printed parts. The micro-CT analysis unveiled microstructural features, such as the orientation of SCFs along the printing direction, the presence of enclosed voids within the extrudates of thick-layered parts, and a reduction in maximum length of SCF in thin-layered parts; such features influenced the final mechanical behaviour of the printed parts. Keywords: 3D printing; mechanical properties; laminate mechanics; mechanical testing; microCT 1. Introduction In recent years, the use of composite materials in the fabrication of functional parts via additive manufacturing (AM) methods has risen in popularity [1]. This trend has been fueled by the benefits of AM in tailoring the properties of 3D printed parts. The AM process is superior because it requires no tooling or special skills, production is rapid, and the design of parts is efficient, with no limitations on the geometric complexity of the final printed part. Consequently,
*
corresponding author e-mail: [email protected]
1
the development and printing of composite materials via AM is an emergent research trend [2,3]. A comprehensive work on 3D printing of composite parts is available in [4–6]. The composite materials used for printing parts have improved the mechanical, thermal, and electrical properties of the parts [7–11]. Furthermore, the properties of
parts can be tailored by altering the
mesostructure as the parts are being printed [12–16]. The 3D printing of parts using composite materials deserves special attention owing to the materials’ anisotropy. Therefore, a detailed investigation of how microstructural features influence the material properties of printed parts would guide the effective design of composite parts for 3D printing. The printed composite parts via a material-extrusion AM method, fused filament fabrication (FFF), are the main focus of this work. The material properties of the final printed part differ from those of the original material [17]. The reason for this difference is that anisotropy is introduced into the properties of the part, mainly due to the process parameters chosen [18]. The material properties are significantly influenced by two main process parameters: printing direction and layer thickness [19]. Further, the type of filament material (isotropic or composite) used in printing also introduces anisotropy into the printed parts [20]. Another factor governing the material behaviour of the final printed parts is the type of reinforcements used in the composite material [21]. The reinforcements used in composite materials to fabricate parts via FFF can be continuous fibers, short fibers, or particulates. Parts printed with continuous fibers [22–24] and short fibers [25,26] have better mechanical properties than parts made with no additional reinforcements. The orientation of reinforcements in the printed parts governs the properties of the final part [27]. The studies mentioned focused only on the influence of process parameters and reinforcements on the final properties of printed parts, but their influence on the mechanical failure behaviour of composite printed parts has not yet been investigated. The mechanical behaviour of parts fabricated with continuous fibers can be characterized using laminate mechanics [28,29]. Laminate mechanics can also be employed to characterize the mechanical behaviour of parts fabricated with only thermoplastic filament material and no additional reinforcements [30–34]. The use of laminate mechanics implies that parts printed via FFF behave like laminate structures. Thus, laminate theory can be employed to characterize the behaviour of parts subjected to different loads during stress analysis [35]. The layers of parts
2
printed with short carbon fibers (SCF) can be treated as orthotropic lamina material, and the mechanical behaviour of such parts have been characterized using laminate mechanics [36]. Delamination is one of the failure modes in laminates, and printed parts experience this failure mode [37,38]. Printing parameters have been found to influence interlayer bonding strength. The influence of layer thickness as well as reinforcements on the interlaminar properties of printed parts has not yet been explored. So far, experimental characterization of composite parts fabricated with material containing continuous fiber [22–24,28,29,37] and discontinuous fiber [19,25– 27,36,39] reinforcements has been explored, but their material modeling has not yet been attempted. Moreover, mechanics for printed parts in characterizing their mechanical failure behaviour has not been addressed in earlier works, especially for printed parts with discontinuous reinforcements. This gap is addressed in the present work. The present work characterizes the mechanical behaviour of SCF-reinforced composite parts fabricated via FFF. Mechanical test standards and materials employed for 3D printing of test coupons are then presented. Test coupons are printed uniaxially and bidirectionally with two different layer thicknesses. The test coupons are then subjected to uniaxial tensile loading to study the influence of printing direction and layer thickness on the overall mechanical behaviour of the parts. Furthermore, the test coupons are subjected to a crack opening load to assess the interlayer bonding strength of the printed parts. Laminate mechanics and laminate theories are employed to characterize the mechanical behaviour of the printed parts. Furthermore, micro-CT analysis is used to investigate the influence of SCF reinforcements on the final material properties of the printed parts. 2. Methodology 2.1 Mechanical testing The tensile test coupons were 3D printed via FFF. The printed test coupons followed the ASTM D3039 standard for tensile testing. The dimensions of the printed test coupons are listed in Table 1 and Figure 1. Axes x, y, and z represent the global coordinate system, while the local system of a single lamina is represented with 1, 2, and 3, where 1 denotes the fiber direction in a lamina, and 2 and 3 are transverse to the fiber direction, as shown in Figure 1. Additionally, test coupons (Figure 2), as per ASTM D5528, were printed to investigate the interlaminar fracture toughness of the parts under crack opening mode (mode I). 3
This study employed the mechanics of laminates to characterize the mechanical behaviour of composite parts fabricated with discontinuous fiber reinforcements. Here, printed parts are also referred to as “printed laminates” because of their laminate behaviour and further, the layers are also known as the laminae of a part. In FFF process that the filament material is melted and extruded through a fine nozzle, and then the extruded material is deposited layer upon layer on substrate to build 3D parts. The layers constitute series of tracks of extruded filament material, and the tracks of extruded material referred as ‘extrudates’ in this study. In other words, the extruded filament material is regarded as composite extrudates that form the layers, and the extrudates behave as fibers of unidirectional lamina. Commercially available composite filament material (ABS-SCF) consisting of SCF reinforcements in ABS thermoplastic was used to print the test coupons. The composite filament material spool was purchased from filament manufacturer 3DXTech (Byron Center, Michigan, USA). The main governing process parameters, layer thickness (t, in mm) and raster angle (θ, in °), are considered here to investigate their influence on the mechanical behaviour of printed parts. Raster angle represents the extrudate orientation (printing direction) for the laminate layup, while layer thickness defines the thickness of the lamina as well as the cross-sectional size of the extrudates of the printed laminates. The FFF printing process is shown in Figure 3. Thick-layered and thin-layered test coupons were printed unidirectionally and also bidirectionally by varying the process parameters. The layup order in the printed bidirectional laminates is balanced and symmetrical. The thickness of the laminae in thick-layered (t1) printed laminates is two times the thickness of laminae in thin-layered (t2) laminates, but the thickness of all layers within a laminate is equal. Table 2 shows the lamina layup order of the test coupons printed for mechanical testing. The laminate test coupons for layups 1–7 were subjected to a uniaxial tensile load along the x-axis (Figure 1), and the test coupons for layup 8 were subjected to a crack opening load (Figure 2). Table 1. Dimensions (in mm) of the tensile test coupons. Laminates Unidirectional Bidirectional
Layup No. 1 to 4 5 6, 7
L 190 127 200
W 12.7 25.4 25.4
T 2.54 2.54 2.54
TL 38 19 30
Note: L, total length; W, width; T, thickness; TL, tab length.
4
Figure 1. Tensile test coupons with dimensions (in mm): L, total length; W, width; t, thickness; TL, tab length; q, extrudate direction to x-axis (°).
Figure 2. Test coupon for an interlaminar fracture toughness (mode I) test. Table 2. Laminate layup orders and layer thicknesses used for printing laminate test coupons. Layup No. Raster angle (q, °) Unidirectional laminates 1 0° 2 30° 3 45° 4 60° 5 90°
Layer thickness t1 = 0.317 mm t2 = 0.158 mm × × × × ×
× × × × ×
Bidirectional laminates 6a [0°/90°]2S 6b [0°/90°]4S 7a [45°/–45°]2S 7b [45°/–45°]4S
× — × —
— × — ×
Unidirectional laminates 8a 0° 8b 0°
× —
— ×
5
Figure 3. Fused filament fabrication (3D printing) of parts. Initially, different test coupons were printed unidirectionally with varying printing direction and layer thickness. Five test coupons were printed for each test case. For example, the layup 1 test coupons can be of two different layer thicknesses (t1 and t2), resulting in a total of 10 coupons being printed for this case alone, as shown in Table 2. The raster angle represents the printing direction of the part, that is, the orientation of the extrudates of the layers in the part. For instance, layup 1 is a unidirectional laminate in that the extrudates are oriented only along the axis of loading, that is, along the x-axis, and in the other layup cases the extrudates are oriented offaxis to loading. The difference between bidirectional laminates 6a and 6b is in their layer thickness, but the layup order is the same. The layers of layup 6a are thicker than those of layup 6b. The unidirectional and bidirectional laminate test coupons mentioned in Table 2 were printed for the investigation. The other process parameters used while printing the test coupons were as follows: a printing speed of 50 mm/s, a bed temperature of 80 °C, an extruder temperature of 235 °C, 1 shell, an infill density of 100%, and an overlap between adjacent extrudates of 15%. These parameters were recommended by filament material supplier for printing good quality parts. A total of 70 test coupons for tensile testing and 10 coupons for fracture testing were printed on an Ultimaker printer. 2.2 Micro-CT scanning A micro-CT scanner is useful for obtaining intricate 3D microstructural information about a material; its working principle is shown in Figure 4 [40]. The object is positioned to rotate while exposing it to an X-ray generator for a specific period of time. The rays penetrate the object and reach the detector, and then provide the X-ray shadow 2D images containing microstructural information about the 3D object. The images are reconstructed using an algorithm [40] to carry out image analysis and obtain the desired microstructural information. The micro-CT scanning equipment used to scan the parts was a SKYSCAN 1272 (Bruker Corporation, Belgium). The scanning generates several radiographic images of the filament. Reconstruction of the images is 6
performed using Bruker’s NRecon program, which outputs the grayscale images of a cross-section of the filament. The reconstruction results can be used by other Bruker programs, including DataViewer, CT-analyser (CTAn), and CT-volume (CTVol). DataViewer is used to visualize the cross-section of the filament, CTAn to study the morphometry and also build 3D models of the microstructure, and CTVol to conduct surface rendering of 3D models of the material’s microstructure. Tomography images provide information about the constituents of the composite material, such as the matrix and reinforcements. The images reveal the size and shape of the reinforcements and their orientation as well as their distribution in the composite filament material and in the printed parts. Moreover, the scanning images are useful in the construction of the 3D geometry of the material’s microstructure. Microstructural features such as reinforcements and voids — including their size, shape, orientation, and distribution in the material of the printed parts — influence the overall material properties. Therefore, the quantification of these parameters is important for estimating the overall material properties of the printed parts.
Figure 4. Working principle of a CT scanner [40]. 3. Results and discussion 3.1 Mechanical testing Initially, uniaxial tensile tests were conducted using an MTS testing machine equipped with a 10 kN load cell. A quasi-static loading rate of 1 mm/min was applied to coupons along the x-axis to deform them, and the displacements of the coupons were recorded using a laser extensometer. The unidirectionally printed parts were subjected to tensile testing to establish the mechanics of the parts for characterizing mechanical behaviour under tensile loads. Following that step, bidirectionally printed test coupons were considered for tensile testing, and their behaviour was then characterized using laminate mechanics. 7
The test results of the unidirectionally printed parts revealed the mechanical behaviour of the printed parts and, furthermore, the influence of extrudate angle and layer thickness on this behaviour. The stiffness (Ex) and tensile strength (Xt) of the different unidirectional laminates obtained from the test results are plotted in Figure 5. The numbers on the horizontal axis of the graph represent the extrudate angle of the unidirectional laminates with respect to the x-axis. The stiffness and strength of the unidirectional thin-layered laminates is higher than that of the thicklayered laminates. The difference between the thick- and thin-layered laminates of layup 1 is higher when compared with other layup cases, where the extrudates are off-angle to the loading axis. Moreover, the strength and stiffness of the laminates of layup 1 are higher than that of the other layup cases. This indicates that the printed parts have directional properties and that the extrudates are the main load-carrying members of the parts. Therefore, the layers of the printed parts can be treated as unidirectional fiber-reinforced laminae. Furthermore, the laminae of the printed parts can be treated as transverse isotropic material. This confirms that laminate mechanics can be employed in the investigation of the mechanical behaviour of printed parts.
Figure 5. Mechanical properties of unidirectional laminates for (a) Ex (stiffness) and (b) Xt (tensile strength) for thick-layered (t1) and thin-layered (t2) laminates. The mechanical properties of the laminae of a printed laminate can be calculated from the tensile test results of laminate layups 1, 3, and 5 [41]. Table 3 presents the mechanical properties of a single lamina. The properties E1, ν12, and Xt were obtained from the results of layup 1, and E2 and Yt were calculated from the test results of layup 5. The in-plane shear properties G12 and S were obtained using the test results of layup 3. A digital image correlation (DIC) setup from LaVision GmbH was employed to measure the strain of layup 1 in order to calculate Poisson’s ratio (ν12). The elastic moduli of the lamina listed in Table 3 can define the plane constitutive 8
matrix, which is useful in the stress analysis of the printed parts. Further, the strength parameters of the lamina are useful in the failure analysis of the printed parts. Table 3. Mechanical properties of a single lamina of the printed laminates. E1, MPa E2, MPa G12, MPa ν12 Xt, MPa Yt, MPa S, MPa
Thick lamina (t1) 2684.2 ± 98.5 1545.7 ± 9.1 624.7 ± 7.1 0.34 ± 0.04 26.1 ± 0.9 14.6 ± 0.3 11.8 ± 0.6
Thin lamina (t2) 4120.4 ± 72.4 1654.3 ± 90.2 770.0 ± 21.6 0.32 ± 0.05 40.7 ± 0.6 14.9 ± 0.7 14.6 ± 1.5
Now, consider the tensile test results of the 3D printed bidirectional cross-ply and angleply layup laminates. Here, the behaviour of bidirectionally printed parts subjected to tensile loads is characterized using laminate mechanics and classical laminate theory (CLT). The constitutive relation of a lamina is written as é E1 ê1 -n n 12 21 ìs 11 ü ê ï ï ê n 12 E1 ís 22 ý = ê ï t ï ê1 -n 12n 21 î 12 þ ê 0 ê ë
n 12 E1 1 -n 12n 21 E2 1 -n 12n 21 0
ù 0 ú ú ì e11 ü úï ï 0 ú íe 22 ý ú ïg ï î 12 þ G12 ú ú û
in other form {s } = [Q]{e }
(1)
In global coordinate system, the constitutive matrix system is given as
{s } = éëQ ùû {e } where theory,
Q ij
(2)
is written as éëQ ùû = [T ] [Q ][T ] and [T ] is a transformation matrix. From laminate
the strains {e 0 } ,
-1
-T
{k} represents the in-plane strains and curvatures of a laminate,
respectively. The total strains for a laminate is written as
{e } = {e 0 } + z {k}
(3)
The resultant force (N) and moment (M) per unit width for a laminate with n number of layers are expressed as
{ N } = [ A]{e 0 } + [ B ]{k} , and {M } = [ B ]{e 0 } + [ D ]{k}
(4a,b) 9
n
where [ A] = å éëQ ùû ( zk - zk -1 ) , [ B ] = k =1
k
1 n 1 n éëQ ùû ( zk2 - zk2-1 ) , [ D ] = å éëQ ùû ( zk3 - zk3-1 ) å 3 k =1 2 k =1 k k
are stiffness
matrices of a laminate, and for a symmetric laminate [B]=[0]. Strains for a symmetric laminate subjected to only in-plane forces are given from Eq.4a as ìe xx0 ü ì N xx ü -1 ï ï 0ï ï íe yy ý = [ A] í N yy ý ïg 0 ï ïN ï î xy þ î xy þ
(5)
For uniaxial tensile testing of laminate subject to loading along x-axis, then N xx = hs xx Nyy=0 and Nxy=0 for laminate thickness h. The stress-strain relation for this case is s xx = Exx e xx0 , using the relation Eq.5, the modulus of elasticity along the x direction of the laminate is calculated as follows Exx =
1 [ A ]11 h
(6)
-1
The elastic moduli such as E1, E2, G12, ν12 of the lamina found from the experimental tensile test results (Table 3) are used for the calculation of matrices [A], [B] and [D]. Then, Exx of the laminate can be calculated using Eq. 6. The Tsai-Hill failure criterion for the laminae for a planar stress is written as s12 X
2
-
s1s 2 X
2
+
s 22 Y
2
+
t122 S2
=1
(7)
The properties of lamina (Table 3) are useful for characterizing the mechanical behaviour of bidirectionally printed parts using classical laminate theory. CLT employs the elastic moduli of the lamina (Table 3) in the calculation of the tensile modulus of the laminate, as described above. The results for thick- and thin-layered bidirectional laminates are presented in Table 4 for crossply laminates and Table 5 for angle-ply laminates. The tensile modulus (Ex) of the laminates obtained from the experimental work was validated using CLT. Ultimate tensile strength (Ut) and strain to failure (εt) are also given in Tables 4 and 5. The difference between the values obtained using CLT and the experimental results for thick-layered bidirectional laminates is greater than that for thin-layered laminates. Moreover, the results obtained using CLT are lower than those
10
obtained experimentally. The values of the properties of thin-layered bidirectional laminates are also higher than those of thick-layered laminates. The difference in the results for thick- and thinlayered laminates is due to changes in their mesostructure. The reasons for this difference are described in a later section discussing the mesostructural features observed using a micro-CT scanner. Further, finite element (FE) failure analysis of bidirectional laminates subjected to tensile loads were carried out, and the Tsai–Hill failure criterion was employed. The laminates were modeled with 2D PCOMPG finite elements in Altair HyperWorks, and then the uniaxial tensile loads were applied to the FE model. The FE model was then simulated for failure analysis; more details about FE failure modeling are available in [34]. The principal stress (σply) for first-ply failure of the laminate and the corresponding elastic ply strain (εply) are provided in Tables 4 and 5. The discrepancy between the CLT and experimental results is due to the properties used in CLT (provided in Table 3). The properties of the lamina listed in Table 3 were calculated from tensile test results for the unidirectional laminates. The properties obtained from test results for the unidirectional laminates were largely influenced by layer thickness, orientation of reinforcements, and bonding at the interface of the layers. Further, the bonding at the interface of the layers in the bidirectional laminates is not the same as that in the unidirectional laminates. A detailed microscopic investigation of these parts can reveal the reasons for the discrepancy in the experimental and CLT-based results. The following section discusses the characterization of the microstructure of the printed parts using tomographic images obtained via micro-CT scanner and also the influence of microstructural features on the material properties of the printed parts. Table 4. Mechanical properties of cross-ply laminates under tensile loading. Ex, MPa Ex, MPa (CLT) Ut, MPa σply, MPa (FE) εt εply (FE)
Thick lamina (t1) 2863.9 ± 78.7 2125.9 23.5 ±0.5 26.0 0.0158 ± 0.0006 0.0097
Thin lamina (t2) 3311.0 ± 43.1 2909.9 31.3 ± 0.4 37.8 0.0214 ± 0.0006 0.0096
Table 5. Mechanical properties of angle-ply laminates under tensile loading. Ex, MPa Ex, MPa (CLT)
Thick lamina (t1) 2094.6 ± 43.5 1733.3
Thin lamina (t2) 2330.8 ± 53.6 2150.5
11
Ut, MPa σply, MPa (FE) εt εply, (FE)
21.7 ± 0.5 22.7 0.0243 ± 0.0011 0.0105
27.7 ± 0.4 31.1 0.0373 ± 0.0013 0.0105
Next, the influence of lamina layup and layer thickness on the failure behaviour of printed laminates subjected to uniaxial tensile loads is discussed. The stress–strain curves of unidirectional laminates made of two different layer thicknesses are shown in Figure 6. The thin-layered unidirectional laminates are stiffer and stronger than the thick-layered laminates. The fracture strength and fracture strain of layup 1 laminates is greater than that of the other unidirectional laminates because the orientation of extrudates in the other laminates is off-axis to loading. Also, the interface of extrudates in a layer shares a significant amount of load, and the bonding strength of the interface is lower than the strength of the extrudates. This led to the fracture occurring at the interface of the extrudates in laminate layups 2 to 5. Figure 7b shows the fracture lines of the different unidirectional laminate layups 1 to 5. The angle of the fracture lines of the laminate follow the orientation of extrudates in that laminate.
Figure 6. Stress–strain curves for (a) thick-layered and (b) thin-layered unidirectional composite laminates.
12
Figure 7. (a) Tensile testing with digital image correlation setup; (b) failure line of the five different layups of composite printed test coupons. Let us now consider the failure behaviour of the bidirectional printed laminates. The stress– strain curves of the bidirectional laminates for cross-ply and angle-ply layup order are shown in Figure 8. The cross-ply laminates are stiffer and stronger than the angle-ply laminates, but the fracture strain of angle-ply laminates is higher than that of the cross-ply laminates. The thinlayered bidirectional laminates have higher fracture strength and fracture strain than the thicklayered laminates. It means that the properties of printed parts can be tailored by varying printing direction and layer thickness. The strain distribution just before propagation of the crack in the cross-ply and angle-ply laminates subjected to tensile loading is shown in Figure 9. Tensile testing of laminates with the DIC setup is shown in Figure 7a. The stress–strain curves of the laminates follow linear and nonlinear regions while deforming under tensile loading. This means that the matrix and SCF reinforcements deform elastically until the bonds break between them, and the matrix deforms plastically until complete failure of the part. Further, the layup order influences the bonding strength between the layers, meaning that bonding strength at the interface is not the same in unidirectional and bidirectional laminates. In the stress–strain curves for the laminates, note that the angle-ply laminates have higher fracture strain than the other layup order laminates. Also, the influence of bonding is significant in the unidirectional laminates with extrudates oriented off-axis to loading. Further, the properties of the lamina calculated from the tensile tests results of unidirectional laminates are also influenced by bonding strength in such laminates. Therefore, the CLT values are lower than the experimental values, as noted earlier, since CLT employs the lamina properties.
Figure 8. Stress–strain curve of (a) cross-ply and (b) angle-ply composite laminates. 13
(a)
(b)
(c)
(d)
Figure 9. Strain distribution in the printed composite laminates during uniaxial tensile testing of (a) thick-layered cross-ply laminates, (b) thin-layered cross-ply laminates, (c) thick-layered angleply laminates, and (d) thin-layered angle-ply laminates. Next, let us consider the test results for interlaminar fracture toughness of laminate layup 8. The test results reveal the interlaminar fracture toughness of the printed laminates under crack opening mode. Interlaminar fracture toughness represents the bonding strength between the layers of printed parts. Interlayer debonding (decohesion) of printed parts is also referred to as delamination, which is one of the failure modes in traditional composite laminates. Figure 10 shows a test coupon subjected to crack opening mode. Load (P) versus crack opening displacement (δ) curves for the test coupons are shown in Figure 11a, while Figure 11b illustrates the fracture toughness (GI) versus delamination length (a) of the coupons. Fracture toughness is calculated based on modified beam theory, as described in the standard. Interlaminar fracture toughness of the thin-layered printed laminates is lower than that of the thick-layered laminates because the temperature of the extruded material influences interlayer adhesion. The extruded material for thin layers carries lower heat capacity when compare to that of thick layers [42,43]. Therefore, the extruded material for thin-layered laminates was subjected to faster cooling than extruded material for thick-layered laminates. Further, the extruded material had less time to diffuse with the previously deposited material, causing poor adhesion between adjacent extrudates as well as between layers. Thus, the interlaminar bonding strength of thick-layered laminates is higher than that of thin-layered laminates. This behaviour is contrary to the behaviour of thin-layered parts under tensile loads, where the thin-layered parts have better mechanical performance than the
14
thick-layered parts. This suggests that interlaminar bonding strength of the thin-layered parts is not the reason for their higher strength and stiffness under tensile loading.
Figure 10. Printed test coupon under crack opening mode.
Fracture toughness (GI), N/mm
0.8 Thick-layered Thin-layered
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
(b) 0
a0 20 40 60 Delamination length (a), mm
80
Figure 11. Interlaminar fracture toughness results for printed laminates under mode I loading: (a) load versus displacement; (b) fracture toughness versus delamination length.
3.2 Micro-CT analysis The microstructure of the printed parts was characterized using a micro-CT scanner. Further, the influence of microstructural features on the material properties of printed parts was investigated. The parameters employed for scanning are presented in Table 6. The composite filament used for printing is first scanned. Then unidirectionally printed parts with two different layer thicknesses are scanned. Table 6. Parameters of micro-CT used for scanning 3D printed parts. Parameter
Value
15
Filter Resolution Image size X-ray voltage
X-ray intensity
None 1 μm/pixel 4904 × 3280 pixels 40 kV for filament 35 kV for thick-layered part 30 kV for thin-layered part 200 μA
The micro-CT analysis of the printed material reveals the size, shape, and distribution of the carbon fiber (CF) reinforcements and their percent volume in the printed parts. Any variation in such data between the thick-layered and thin-layered parts can affect their material behaviour. The 2D images with high resolution (1 µm/pixel) shown in this section were obtained using DataViewer, and the 3D models were obtained using CTAn as well as CTVol. The scanning results of the composite filament material reveal that the length of CF reinforcements varies between 10 and 278 µm. The diameter of the CF is around 7.5 µm, and its proportion in the filament material is around 10.60%. Next, let us consider thick-layered unidirectionally printed laminates for micro-CT analysis. A small volume of material was taken from the thick-layered 3D printed tensile test coupons for scanning. Then, tomographic images of the laminate were used to characterize the laminate’s microstructure. The cross-section of the laminate with four layers is shown in Figure 12. The mesostructure of the laminate has triangular-shaped voids (black dotted lines in Figure 12a) between adjacent layers. These voids are continuous and exist at the interfaces of all layers of the laminate; further, these voids are inherited from the printing methodology.
16
Figure 12. Thick-layered 3D printed composite part: (a) mesostructure; (b) cross-sectional view taken at X–X. The scanner reveals irregularly shaped voids (black continuous lines in Figure 12a) in the mesostructure. The voids are enclosed and present within extrudates (extruded filament) of the laminate. The enclosed voids are result of flow characteristics of extruded material and are governed by the melt temperature, printing speed and type of reinforcements. Optimal printing speed and melt temperature, and small aspect ratio of reinforcements can minimize the voids in the parts [19,44]. The presence of these enclosed voids within the extrudates affects the strength and stiffness of the thick-layered printed parts. Furthermore, the enclosed voids and the continuous interlayer voids (triangular voids in fig.12) are stress concentration points and these can lead to fail at lower stresses [8]. Therefore, the voids degrade the properties of printed parts. These are the main reasons for the lower strength and stiffness of the thick-layered printed laminates. The black areas in the image are the denser material, representing CFs, and the rest of the gray area is ABS material. The orientation and distribution of the CF reinforcements (highlighted with red continuous lines) in the thick-layered part can be seen in Figure 12b. Most of the SCFs are oriented along the extrudate direction, meaning they are aligned with the printing direction, but some SCFs are not aligned owing to the presence of enclosed voids within the extrudate. The misalignment of SCFs with printing direction near enclosed voids is due to variation of size and shape of pores during solidification of the extruded material [8]. A single extrudate of different lengths (0.2, 0.3,
17
0.4 mm) was selected for analysis by CTAn. The length of CFs in the thick-layered parts varies between 10 and 276 µm. The percentage of CF in the selected fiber volumes is around 10.50%. Now, consider the thin-layered unidirectional printed laminate for micro-CT scanning. The same CT analysis procedure was repeated for a small volume of a thin-layered laminate. The mesostructure of the laminate is shown in Figure 13a. The mesostructure has voids at the interfaces of the layers, but the enclosed voids within the extrudates are minimal when compared with those in the thick-layered laminates. Minimal internal pores in this case is due to lower clogging of SCFs in nozzle and that resulted consistent flow of material while extruding [39]. Thus, the time for trapping air bubbles in the extruded material is lower and that led to minimal voids. Therefore, the properties of thin-layered parts have a higher value than those of thick-layered parts. The orientation and distribution of CF reinforcements in the extrudate can be seen in Figure 13b. The CF reinforcements are well aligned in the printing direction compared with those in the thicklayered laminate. The length of CF reinforcements varies between 10 and 152 µm, and the reinforcements are also well dispersed in the extrudates. In these printed laminates, the maximum length of the SCFs is approximately equal to the layer thickness, and for that reason SCFs with a length greater than the layer thickness are subject to fracture while material is being deposited. The fracture occurs because the gap between the nozzle tip and the previously deposited layer is approximately equal to the layer thickness, and therefore it cannot accommodate longer SCF reinforcements. The pecent volume of CF material in the selected volume of extrudate is ~10.30%. A repetitive volume of a material (red dotted lines in Figure 13a) was considered for analysis by CTAn. The 3D model generated is shown in Figure 14. The orientation and distribution of CF reinforcememts in the thin-layered parts can be seen in Figure 14a. The 3D model file is an .stl file that can be imported into a commercial FE package for material modeling of the printed part using numerical homogenization, and it is a further step to present work and is planned in future.
18
Figure 13. Thin-layered 3D printed composite part: (a) mesostructure; (b) cross-sectional view taken at Y–Y.
400 μm
(a)
(b)
Figure 14. 3D models of the mesostructure of a thin-layered printed part obtained from micro-CT scanning: (a) orientation of short carbon fibers in the printed part; (b) representative volume element model for homogenization. 4. Conclusions In this study, 3D printed parts were treated as composite laminates, and the mechanics of laminates were employed to determine the properties of the laminae of thick-layered and thinlayered parts. These properties were then employed in CLT and Tsai–Hill failure theory to 19
characterize the mechanical behaviour of bidirectionally printed parts. CLT and failure theory accurately predicted the mechanical behaviour of thin-layered bidirectionally printed parts. Thus, laminate theories can be used in the initial design and analysis of printed laminates. The mechanical testing results of printed coupons revealed that the strength and stiffness of thinlayered printed parts are higher than that of corresponding thick-layered parts. The fracture occurred at the interface of the fibers of a layer of unidirectional laminates, and the fracture surface followed the orientation of the extrudates in that laminate. The presence of enclosed voids in the extrudates of thick-layered parts degraded the material properties of the parts. Furthermore, the enclosed voids caused improper alignment of SCF reinforcements with printing direction in the thick-layered parts. This random alignment of SCFs led to more anisotropy in these parts. However, interlayer bonding strength was higher for thick-layered than thin-layered parts. This resulted in greater interlaminar fracture toughness for thick-layered parts. This means that the thinlayered printed parts perform poorly compared with thick-layered parts where loading (compression, buckling) causes delamination. Such performance of thin-layered parts is contrary to the performance of the same parts under tensile loading. The presence of voids in the extrudates and at the interface of layers, as well as the fracture of SCFs, degraded the overall material properties of the printed parts. Proper selection of layer thickness for fabricating 3D printed parts will minimize voids within the extrudates and also the breakage of SCFs while the material is being deposited. This in turn will lead to better mechanical performance of the printed parts. Acknowledgments We thank the Lassonde School of Engineering at York University and the Natural Sciences and Engineering Research Council (NSERC) for financial support. References [1]
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Highlights: ·
Mechanics of laminates are used to characterize the mechanical behaviour of 3D printed composite parts with short carbon fiber reinforcements.
·
3D printing parts with lower layer thickness yields better tensile properties, but not better interlaminar properties.
·
3D printing parts with higher layer thickness makes them prone to having enclosed voids, which degrades material properties.
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Declaration of interest Article Title: Mechanical behaviour of 3D printed composite parts with short carbon fiber reinforcements Authors: Madhukar Somireddy, Chandra Veer Singh, Aleksander Czekanski Declarations of interest: ‘none'