- Email: [email protected]
Fatigue behaviors of fiber-reinforced composite 3D printing
Fatigue behaviors of fiberreinforced composite 3D printing
9
Astrit Imeri, Ismail Fidan Tennessee Tech University, Cookeville, TN, United States
9.1
Introduction
Additive manufacturing (AM) is an advanced manufacturing technology that produces parts successively by adding material layer-by-layer [1]. The term 3D printing is interchangeably used with AM. Overall, it is known that AM is an evolving technology since there is a high number of research and development activities to elevate the usability of this technology. Producing complex shapes, low material waste, and no tooling requirements are just some of the advantages of AM, compared to other conventional manufacturing processes [2]. Also, an important advantage is the production of lightweight parts [3]. In terms of material usage, it is reported that 90% of the material is usable in the AM processes [4]. Due to these advantages, AM has found applications in many industries such as aerospace, biomedicine, and motorsports [5]. Parts made vary from end-user parts to prototypes and patterns [6–8]. Usually, AM produced parts are of either polymers or metals. Currently, metallic parts built with AM are expensive, while polymer parts (mostly PLA, ABS, and PC [9]) suffer from weak mechanical and thermal properties. Recent techniques like fiber-reinforced additive manufacturing (FRAM) have made it possible to make light, strong, and low-cost additively manufactured parts [10]. Fibers used in FRAM can be classified as short and continuous fibers. Short fibers have found more application in FRAM due to the easiness of use [11]. However, a printer that is capable of printing continuous FRAM parts, Markforged Mark Two (MKF), has entered the market in recent years [12]. This printer using its dual extrusion head prints the base materials (nylon or Onyx) before reinforcing them with fibers [carbon fiber (CF), fiberglass (FG), and Kevlar]. Such a unique FRAM printer has made the fiber-reinforced composite manufacturing adaptable to various industries and provided practical solutions. Fibers as reinforcement material carry the higher loads. Hence, fiber orientation in the part is important for the mechanical properties. Testing mechanical properties of parts produced by this AM method is crucial for the utilization of parts in industry to prevent mechanical failures. Statistically, 50%–90% of all failures are due to fatigue [13]. Investigating fatigue properties of FRAM materials is critical for the design stage. However, since the technology is new, not many studies have been conducted on investigating fatigue properties of continuous FRAM. In this chapter, the conducted studies are explained in detail. Generally, fatigue studies have been conducted Fatigue Life Prediction of Composites and Composite Structures. https://doi.org/10.1016/B978-0-08-102575-8.00009-7 © 2020 Elsevier Ltd. All rights reserved.
336
Fatigue Life Prediction of Composites and Composite Structures
on conventional polymers without fiber reinforcement [14, 15]. Such AM technologies are called fused filament fabrication (FFF) or fused deposition modeling (FDM). Some studies reported the tensile, flexural, and creep properties of various FRAM materials [16]. Matsuzaki et al. and Chen et al have shown that FRAM parts are superior compared to 3D printed PLA and ABS ones, in terms of yield strength [17]. Melenka et al studied the effect of increasing the volumetric fraction of continuous fibers which resulted in an increased elastic modulus of the parts [18]. Furthermore, Dickson et al performed a more thorough investigation of the tensile properties of FRAM materials by testing nylon specimens reinforced with CF, Kevlar, and FG. It was found that the specimens fail at the shoulder of the “dog-bone” specimen and it was concluded that the reason for this was the shear force due to fiber misalignment [19]. Der Klift et al also noted that continuous fibers reinforced parts contain discontinuities of fiber in each layer although not as fragmented as short fibers [20]. Creep of FRAM materials was studied by Mohammadizadeh et al., where specimens of different materials were loaded at two different temperatures [21]. When FRAM built parts are under cyclic loading, they could face catastrophic failures in the long term. Hence, generating fatigue properties data of FRAM parts under different loading conditions of various fiber patterns could benefit and broaden the knowledge base and prevent the failure. Fatigue life of parts can be improved by defining proper manufacturing parameters. This chapter provides an insight to the most recent information on how to make stronger parts which are subjected to cyclic loadings.
9.2
Materials and specimen preparations
9.2.1 3D printing equipment The equipment used for the preparation of FRAM samples is a commercially available 3D printer, MKF [12]. This printer has dual extrusion head with two nozzles that print the base and reinforcement materials, respectively. A schematic representation of the printer is shown in Fig. 9.1. As in other AM processes, to produce a part, first a digital file is needed. A solid model, designed with a computer aided design (CAD) software tool, is converted to a Standard Tessellation (STL) file. This file is then uploaded to the slicer software. Eiger is a web-based slicer of MKF. Printing settings are fixed on the slicer, which generates the numerical code for the machine to command the movement of printer heads and filaments. Specimens at the different stages are visualized in Fig. 9.2. Filaments which come in the shape of cylindrical continuous wires are packed in spools. With the aid of stepper motors, filaments are pushed to the extruders, where the temperature varies between 265°C and 270°C, enough to put nylon or Onyx in a molten state. Matrix material solidifies immediately after its extruded. Fibers have higher melting temperatures, so they are just laid down horizontally in the interior of the part area. The extrusion heads move on the horizontal plane, where the base material is
Fatigue behaviors of fiber-reinforced composite 3D printing
337
Nylon spool
Fiber spool
Extrusion head w/ nozzles
Composite part
Printing bed and platform
Fig. 9.1 Schematic representation of the continuous FRAM.
3D CAD model (solidworks)
Layer-wise assembly (eiger)
3D printed part (markforged mark two)
Fig. 9.2 Part preparation from CAD model to the 3D printed part.
338
Fatigue Life Prediction of Composites and Composite Structures
firstly printed on the periphery of the layer then it is reinforced by fibers. When the layer is concluded, the printing bed moves down so the new horizontal layer is started on the previously printed material.
9.2.2 Specimen preparation The way of preparing the specimen is critical. There are many parameters that affect the mechanical properties. Some of the parameters are already set by the printer capability, while the others are set by the user. Among those parameters are fiber volume, layer height, fiber orientation, infill pattern, infill percentage, and other slicing parameters. Kuchipudi [21] and Imeri et al. [22], the only studies currently available in the literature, focused the research on fiber orientation and fiber volume fraction. For simplifying purposes, Kuchipudi’s work will be classified as A, while Imeri et al.’s work will be classified as B. In A, fiber orientations with respect to longitudinal axis were set at 0, 45, and 90 degree, respectively. The fiber volume fractions are 0.25 and 0.50 out of total volume. Also, the fiber-reinforced material was FG. Schematic representation of the different fiber directions is shown in Fig. 9.3. The yellow lines represent the fiber reinforcement, while the black lines represent the base matrix. White boundary lines represent the shell lines and are made of the base material. In B, concentric (C) and isotropic (I) infills were used. In concentric infill, rings are added around the conferential perimeter of the specimen. In isotropic infill, fiber is added unidirectionally, linearly in a horizontal plane. Schematic representation of the two infill types is shown in Fig. 9.4. Different types of specimens were generated with 0, 1, 2, and 3 concentric rings. The angle for the isotropic infill was zero. Fiber reinforcement materials were CF, FG, and Kevlar. The nylon direction was chosen to be rectilinear due to giving better uniaxial load performance [22].
Fig. 9.3 Schematic representation of 0-, 45-, and 90-degree fiber orientations.
Fatigue behaviors of fiber-reinforced composite 3D printing
339
Fig. 9.4 Schematic representation of concentric and isotropic infill used in B study.
R = 10 mm
H = 10 mm
W = 20 mm
10T = 50 mm L = 15 mm
T = 5 mm
Fig. 9.5 ASTM specimen dimensions [23].
Specimens in A are designed according to the ASTM D3479 standard [23]. The specimens dimensions are 250 25 2.5 mm. Specimens in B are designed according to the ASTM E606 standard [24]. Dimensions of the specimens are shown in Fig. 9.5.
9.2.3 Experimental setup To perform the experiment, a servo-hydraulic test system is needed. The test system used was a closed-loop servo-hydraulic MTS810 machine with load cell capacity of 100 kN with an accuracy on the applied load under 1%. Machine has a lower moving gripping head that performs the cycles, and an upper gripping head only for gripping purposes. Fiber-reinforced nylon composites can be crushed by the gripping pressure. High pressure can crush the specimen, while low pressure can be a reason for the specimen to slip. Hence, it is important to set up a proper gripping pressure. In this study, by trial and error it was found that the ideal gripping pressure was 4 MPa. To initiate the experiment, it is required to input the testing variables. Frequency, amplitude, and average load are all controlled through MTS’s Multipurpose Testware interface. Different loads and frequencies were applied which will be given in more details in the following sections. However, it is important to note that for any load that was applied, machine tuning was needed. Tuning was performed to understand the difference between the test parameters and the received feedback.
340
9.3
Fatigue Life Prediction of Composites and Composite Structures
Experimental analysis
In A, the specimen is first tested for maximum tensile strength then the applied highest fatigue loads are 80%, 60%, and so on. The frequency used was 15 Hz and the minimum-to-maximum load ratio R ¼ 0. Specimens are tested under these loads until failure. From the experiments various results were observed. In the 0 degree infill specimens, for both 0.25 and 0.50 fiber volume fractions the failure was brittle in the direction of the fibers. The endurance limit for 0.25 infill was 244.4 MPa, since the specimen did not fail even after 4 107 cycles. For the 0.50 infill the endurance limit was at 335.3 MPa. The specimen did not break after 9 107 cycles. Increasing the fiber volumetric fraction proved to increase the fatigue strength. In the 45 degree specimens, for 0.25 and 0.50 fiber volumetric fractions, the failure was more ductile, and the material showed large viscoplastic deformations. The endurance limits were 36.2 and 60.2 MPa for 0.25 and 0.50 fiber volume fraction, respectively. A large stretch in the material at low load levels was observed, which shows that load is acting on the base matrix. Moving forward in the 90 degree specimens, the transverse properties of the material were tested. At this direction the load is carried by the base matrix mostly. Materials showed large viscoplastic deformations and failed at much lower loads and cycles compared to the other types of specimens. From the experimental results, it is understood that the direction and volumetric fraction of FG in the nylon matrix has a high influence on the fatigue properties. Strongest results were seen in the fiber direction, while the highest ductility was noticed on the 45 degree specimens. Similarly, endurance limits are highest for 0 degree, followed by 45 degree, and 90 degree specimens, correspondingly. Further, in the experimental study in B, the frequency used was 1 cycle per second with a minimum-to-maximum load ratio of 0.1. The goal in here was to understand the effect of fiber infill type, hence the experiments were truncated after 10,000 cycles. In addition, specimens that failed in the first cycle were also eliminated to be tested in the higher loads. Specimens were grouped in concentric (C), isotropic (I), and a mix of concentric and isotropic (I + C) infills with different number of rings. The first loads level was at a maximum of 3.33 kN. Results of the number of cycles at this load are shown in Table 9.1. Table 9.1 Results of the specimens from the first load level at 3.33 kN Max load (kN)
Material
Rings
Type
Test #1
Test #2
Test #3
3.33 3.33 3.33 3.33 3.33 3.33
Carbon Carbon Kevlar Kevlar Glass Glass
2 3 2 3 2 3
C C C C C C
510 10000+ 45 383 168 644
222 – 190 72 132 561
417 – 9 181 137 644
Fatigue behaviors of fiber-reinforced composite 3D printing
341
Isotropic infill specimens all passed the 10,000 cycles milestone hence were not tabulated. Among the concentric infill specimens, the ones with three number of rings endured more cycles. Adding more rings, increases the fiber volumetric fraction and hence could result in more cycles. CF-reinforced specimen outperformed the FG and Kevlar counterparts. Next, in the second load level (10.3 kN) results are shown in Table 9.2. From the first load, the specimens with three concentric rings were tested. However, regardless of material all three concentric rings failed in the first cycle, and thus, were eliminated from testing at higher loads. Similarly, for concentric and isotropic specimen with three rings, specimens failed in the first cycle. CF- and FG-reinforced specimens with two rings and isotropic infill did not break in the first cycle, however the Kevlar-reinforced specimen did. Truncating all the specimens from the previous loads, the third load level was at a maximum of 13.8 kN. Results are presented in Table 9.3. Table 9.2 Results from the second load at 10.3–1.03 kN Max load (kN)
Material
Rings
Type
Test #1
Test #2
Test #3
10.3 10.3 10.3 10.3 10.3 10.3 10.3 10.3 10.3 10.3 10.3 10.3 10.3 10.3
Glass Glass Glass Glass Glass Carbon Carbon Carbon Kevlar Kevlar Kevlar Kevlar Carbon Glass
0 1 2 3 0 1 2 3 0 1 2 3 3 3
I I+ C I+ C I+ C I I+ C I+ C I+ C I I+ C I+ C I+ C C C
750 699 19 1 10000+ 10000+ 3753 1 10000+ 54 1 1 1 1
1161 996 26 – – – 3995 1 – 285 – – – –
989 744 31 – – – 1747 – 196 – – – –
Table 9.3 Results from the third load at 13.8–1.38 kN Max load (kN)
Material
Rings
Type
Test #1
Test #2
Test #3
13.8 13.8 13.8 13.8 13.8 13.8 13.8 13.8
Glass Glass Glass Carbon Carbon Carbon Kevlar Kevlar
0 1 2 0 1 2 0 1
I I+ C I+ C I I+ C I+ C I I+ C
5 15 1 10000+ 324 1 3 1
66 24 – – 275 – 7 –
20 38 – – 209 – 6 –
342
Fatigue Life Prediction of Composites and Composite Structures
At this relevantly high load, all the specimens with two rings combined with isotropic infill failed in the first cycle. Among Kevlar specimens, only the isotropic infill specimen endured a very small numbers of cycles. At this load, only CF with isotropic infill resisted more than 10,000 cycles. In the final load, (17.4–1.74 kN), all the specimens except the isotropic CF-reinforced specimen failed in the first cycle. Even, in these specimens cracking sound was heard in the first cycles.
9.4
Statistical analysis
A full factorial ANOVA design was applied to the experimental results [25]. In ANOVA, the independent variables were load, material, number or rings, and the type of infill. The dependent variable was the number of cycles to failure. A log transformation of the dependent variable was performed due to data being counts. In addition, right and left truncations were performed due to experiments being stopped at specimens that did not fail after 10,000 cycles and the ones that failed after one cycle, respectively. Thus, leading to reducing the full-factorial ANOVA to fourway ANOVA with missing data. Since ANOVA is a robust procedure, the truncated data had very little effect on the overall power of the test. The assumptions of normality and homogeneous variations were not violated because the results were within the boundaries of classical ANOVA. There was one violation of the classical method, which was resolved. Rings and type of infill were found to overlap, which means that the two variables were correlated. Rings were found to be more influential which lead to not include the type of infill in the analysis. ANOVA results are presented in Table 9.4. From the last column, it can be understood that load, material, and rings all have significant value in the number of cycles while the interaction of load and material is slightly significant (load*material). To visualize and simplify the meaning of the numbers in Table 9.4, a group of boxplots figures for the number of cycles and each dependent variable, respectively, is provided. Starting with the number of cycles and the different load is presented in Fig. 9.6. Load 1 was not included due to the specimens tested in this load were with concentric infill. The box plots for the number of cycles are regardless of material. The thicker horizontal line in the figure represents the median. It should be noted that the median line Table 9.4 Summary of ANOVA results
Load Material Rings Load*material Residuals
Df
Sum Sq
Mean Sq
F value
p value
3 2 3 2 28
46.542 33.521 27.645 4.331 13.62
15.5141 16.7607 9.2149 2.1653 0.4864
31.894 34.4567 18.944 4.4514
3.58 1009 2.82 1008 6.57 1007 0.02096
343
4 0
2
log(cycles)
6
8
Fatigue behaviors of fiber-reinforced composite 3D printing
10.3 kN
13.8 kN
17.4 kN
Fig. 9.6 Box plot of the number of cycles vs the different types of load.
4 0
2
log(cycles)
6
8
in each of the load, is higher than the maximum number of the cycles in the other load, from right to left. This result explains the importance of the load in the variance of the number of cycles. Further, number of rings in the specimens was varied with the goal of understanding their effect. Next, the number of cycles versus the number rings is presented in Fig. 9.7.
0
1
2
Number of rings
Fig. 9.7 Box plots of number of cycles vs the number of rings.
3
344
Fatigue Life Prediction of Composites and Composite Structures
In Fig. 9.7, the median of three number of rings is higher than the other counterparts. However, analyzing the number of the cycles in Tables 9.2–9.3, the three rings specimens failed in relatively low numbers. This high median number of cycles is achieved by the concentric infill specimens. In the concentric infill the three rings number specimens gave the best results. The variance of this box plot is very high as it can be seen in the Fig. 9.7. Similarly, for two number of rings, the median is mixed between isotropic and concentric infills specimens. Finally, a preliminary S-N curve is fitted. More FG-reinforced specimens with zero rings isotropic infill were tested at different loads (14.8, 12.8, and 11.3 kN). Although more data are needed for full S-N curve, a preliminary S-N curve is shown in Fig. 9.8. Nonlinear regression was applied to the given formula: load ¼ 3547∗ exp ð0:0004∗cyclesÞ
13.3 kN 11.1 kN
Load
15.6 kN
Estimate of S-N curve with 95% confidence bands
0
200
400
600
Number of cycles
Fig. 9.8 Preliminary S-N curve for FG with zero rings isotropic infill.
800
1000
Fatigue behaviors of fiber-reinforced composite 3D printing
9.5
345
Discussion
Fiber orientation and volume fraction are critical to the mechanical properties of the FRAM parts. The higher the volume fraction the better the fatigue performance. In terms of fiber direction, specimens with similar fiber volumetric fractions but with different fiber orientation gave out different experimental results. Thus, showing that fiber orientation is influential. To visualize it, the horizontal cross-section view of the specimens can be analyzed. Stress is higher in the narrower area of the specimen, hence fiber orientation in there becomes very important. The horizontal cross-section view is shown in Fig. 9.9. Addition of rings causes the isotropic infill area to decrease. This reduction of isotropic infill area results in lower failure resistance to fatigue. In general, the failure mode of the specimens at 0 degree was fiber pull-out. At different loads, different amount of fibers pulled out were noticed. In terms of angle direction, the 45 degree deformed more than the 0 and 90 degree. In 90 and 0 degree infill specimens the break was at 90 degree. In the 45 degree infill specimens the break angle was 45 degree. Regarding fiber material type, CF specimens had a brittle failure, while FG and Kevlar specimens were more ductile. Failed specimens with different fiber reinforcing materials are shown in Fig. 9.10.
Fig. 9.9 Horizontal cross-section view of specimens with zero, one, two, and three rings, top to bottom.
346
Fatigue Life Prediction of Composites and Composite Structures
Fig. 9.10 FG, CF, and Kevlar fiber-reinforced broken specimens at 10.3 kN.
Fig. 9.11 CF, Kevlar, and FG broken specimens at maximum load of 13.8 kN.
In study B in general, similar breaking patterns among different materials were noticed at the same load. For instance, in a higher load, the fiber pull-out was faster and in higher volume for all the materials. Broken specimens are shown in Fig. 9.11. Comparing Figs. 9.10 and 9.11, it can be seen the difference in the amount fiber pull-out.
Fatigue behaviors of fiber-reinforced composite 3D printing
9.6
347
Conclusions and outlook
The current research studies conducted on fatigue properties showed that 0 degree specimens perform best in uniaxial tension-tension loads, with zero ring carbon-fiber being the strongest. Also, 45 degree specimens turned out to be the most ductile. Increasing fiber volume fraction increases the fatigue performance. The information presented in this chapter can serve as a knowledge base for the fatigue properties of continuous fiber-reinforced AM parts. Since continuous FRAM is a 4-year-old technique, the conducted research is preliminary. Future studies could include testing at different orientations, mixing fiber orientations, and fiber materials. Furthermore, to improve the interfacial bonding of the fibers and base matrix different types of coatings could be applied and investigated. Also, analytical models could be developed to predict the fatigue life of FRAM components. Finally, automated fiber placement for better properties could be developed with the help of topology optimization. Specifically, the angle direction of fibers for different applications could be optimized.
Acknowledgments This work is part of a larger project funded by the Advanced Technological Education Program of the National Science Foundation, DUE #1601587. The funding provided by the National Science Foundation is greatly appreciated.
References [1] I. Gibson, D.W. Rosen, B. Stucker, Additive Manufacturing Technologies: Rapid Prototyping to Direct Digital Manufacturing, Springer, 2010. [2] G.J.M. Krijnen, 3D printing of functional structures, Vonk 34 (1) (2016) 10–20. [3] R. Huang, et al., Energy and emissions saving potential of additive manufacturing: the case of lightweight aircraft components, J. Clean. Prod. 135 (2016) 1559–1570. [4] Y. Swolfs, S.T. Pinho, Designing and 3D printing continuous fibre-reinforced composites with a high fracture toughness, in: American Society for Composites Thirty-First Technical Conference, 2016. [5] S. Rawal, J. Brantley, N. Karabudak, Additive manufacturing of Ti-6Al-4V alloy components for spacecraft applications, in: RAST 2013—Proceedings of 6th International Conference on Recent Advances in Space Technologies, 2013. [6] N.A. Russell, J. Floyd, J. Caston, M.R. Villalpando, I. Fidan, Project InnoDino: additively innovative dinosaur design and manufacturing, Int. J. Rapid Manuf. 6 (4) (2017) 262–278. [7] T. Fresques, D. Cantrell, I. Fidan, The development of a framework between the 3D printed patterns and sand-cast work pieces, Int. J. Rapid Manuf. 5 (2) (2015) 170–185. [8] J. Watson, F. Vondra, I. Fidan, The development of a framework for 3D printing, casting, and entrepreneurship, in: 2017 ASEE Annual Conference & Exposition, 2017. [9] P. Dudek, FDM 3D printing technology in manufacturing composite elements, Arch. Metall. Mater. (2013). [10] T. Page, Technology forecast for composite materials in product design, i-manager’s J. Mater. Sci. 3 (4) (2016) 5–19.
348
Fatigue Life Prediction of Composites and Composite Structures
[11] Z. Quan, et al., Additive manufacturing of multi-directional preforms for composites: opportunities and challenges, Mater. Today 18 (2015) 503–512. [12] Markforged.com, About Us: 3D Printer Manufacturer & Technology Company j Markforged [online], 2019. Available at: https://markforged.com/about/. (Accessed 20 May 2019). [13] R. Stephens, A. Fatemi, R. Stephens, H. Fuchs, Metal Fatigue in Engineering, John Wiley & Sons Inc, 2000. [14] T. Letcher, M. Waytashek, Material property testing of 3D-printed specimen in PLA on an entry-level 3D printer, in: Volume 2A: Advanced Manufacturing, 2014. [15] M. Fischer, V. Sch€oppner, Fatigue behavior of FDM parts manufactured with Ultem 9085, JOM 69 (2017) 563–568. [16] I. Fidan, et al., The trends and challenges of fiber reinforced additive manufacturing, J. Adv. Manuf. Technol. (2019) 1–18. [17] R. Matsuzaki, et al., Three-dimensional printing of continuous-fiber composites by in-nozzle impregnation, Sci. Rep. 6 (2016)23058. [18] G.W. Melenka, B.K.O. Cheung, J.S. Schofield, M.R. Dawson, J.P. Carey, Evaluation and prediction of the tensile properties of continuous fiber-reinforced 3D printed structures, Compos. Struct. 153 (2016) 866–875. [19] A.N. Dickson, J.N. Barry, K.A. McDonnell, D.P. Dowling, Fabrication of continuous carbon, glass and Kevlar fibre reinforced polymer composites using additive manufacturing, Addit. Manuf. 16 (2017) 146–152. [20] F. Van Der Klift, Y. Koga, A. Todoroki, M. Ueda, Y. Hirano, R. Matsuzaki, 3D printing of continuous carbon fibre reinforced thermo-plastic (CFRTP) tensile test specimens, Open J. Compos. Mater. 6 (2016) 18–27. [21] M. Mohammadizadeh, I. Fidan, M. Allen, A. Imeri, Creep behavior analysis of additively manufactured fiber-reinforced components, Int. J. Adv. Manuf. Technol. 99 (2018) 1225–1234. [22] M. Fernandez-Vicente, W. Calle, S. Ferrandiz, A. Conejero, Effect of infill parameters on tensile mechanical behavior in desktop 3D printing, 3D Print. Addit. Manuf. 3 (2016) 183–192. [23] D3479/D3479M-12, Standard Test Method for Tension-Tension Fatigue of Polymer Matrix Composite. 2012. [24] ASTM International, E606/E606M-12, Standard Test Method Strain-Controlled Fatigue Testing, ASTM International, West Conshohocken, PA, 2012. [25] A. Imeri, I. Fidan, M. Allen, D.A. Wilson, S. Canfield, Fatigue analysis of the fiber reinforced additively manufactured objects, Int. J. Adv. Manuf. Technol. 98 (2018) 2717–2724.
9
Astrit Imeri, Ismail Fidan Tennessee Tech University, Cookeville, TN, United States
9.1
Introduction
Additive manufacturing (AM) is an advanced manufacturing technology that produces parts successively by adding material layer-by-layer [1]. The term 3D printing is interchangeably used with AM. Overall, it is known that AM is an evolving technology since there is a high number of research and development activities to elevate the usability of this technology. Producing complex shapes, low material waste, and no tooling requirements are just some of the advantages of AM, compared to other conventional manufacturing processes [2]. Also, an important advantage is the production of lightweight parts [3]. In terms of material usage, it is reported that 90% of the material is usable in the AM processes [4]. Due to these advantages, AM has found applications in many industries such as aerospace, biomedicine, and motorsports [5]. Parts made vary from end-user parts to prototypes and patterns [6–8]. Usually, AM produced parts are of either polymers or metals. Currently, metallic parts built with AM are expensive, while polymer parts (mostly PLA, ABS, and PC [9]) suffer from weak mechanical and thermal properties. Recent techniques like fiber-reinforced additive manufacturing (FRAM) have made it possible to make light, strong, and low-cost additively manufactured parts [10]. Fibers used in FRAM can be classified as short and continuous fibers. Short fibers have found more application in FRAM due to the easiness of use [11]. However, a printer that is capable of printing continuous FRAM parts, Markforged Mark Two (MKF), has entered the market in recent years [12]. This printer using its dual extrusion head prints the base materials (nylon or Onyx) before reinforcing them with fibers [carbon fiber (CF), fiberglass (FG), and Kevlar]. Such a unique FRAM printer has made the fiber-reinforced composite manufacturing adaptable to various industries and provided practical solutions. Fibers as reinforcement material carry the higher loads. Hence, fiber orientation in the part is important for the mechanical properties. Testing mechanical properties of parts produced by this AM method is crucial for the utilization of parts in industry to prevent mechanical failures. Statistically, 50%–90% of all failures are due to fatigue [13]. Investigating fatigue properties of FRAM materials is critical for the design stage. However, since the technology is new, not many studies have been conducted on investigating fatigue properties of continuous FRAM. In this chapter, the conducted studies are explained in detail. Generally, fatigue studies have been conducted Fatigue Life Prediction of Composites and Composite Structures. https://doi.org/10.1016/B978-0-08-102575-8.00009-7 © 2020 Elsevier Ltd. All rights reserved.
336
Fatigue Life Prediction of Composites and Composite Structures
on conventional polymers without fiber reinforcement [14, 15]. Such AM technologies are called fused filament fabrication (FFF) or fused deposition modeling (FDM). Some studies reported the tensile, flexural, and creep properties of various FRAM materials [16]. Matsuzaki et al. and Chen et al have shown that FRAM parts are superior compared to 3D printed PLA and ABS ones, in terms of yield strength [17]. Melenka et al studied the effect of increasing the volumetric fraction of continuous fibers which resulted in an increased elastic modulus of the parts [18]. Furthermore, Dickson et al performed a more thorough investigation of the tensile properties of FRAM materials by testing nylon specimens reinforced with CF, Kevlar, and FG. It was found that the specimens fail at the shoulder of the “dog-bone” specimen and it was concluded that the reason for this was the shear force due to fiber misalignment [19]. Der Klift et al also noted that continuous fibers reinforced parts contain discontinuities of fiber in each layer although not as fragmented as short fibers [20]. Creep of FRAM materials was studied by Mohammadizadeh et al., where specimens of different materials were loaded at two different temperatures [21]. When FRAM built parts are under cyclic loading, they could face catastrophic failures in the long term. Hence, generating fatigue properties data of FRAM parts under different loading conditions of various fiber patterns could benefit and broaden the knowledge base and prevent the failure. Fatigue life of parts can be improved by defining proper manufacturing parameters. This chapter provides an insight to the most recent information on how to make stronger parts which are subjected to cyclic loadings.
9.2
Materials and specimen preparations
9.2.1 3D printing equipment The equipment used for the preparation of FRAM samples is a commercially available 3D printer, MKF [12]. This printer has dual extrusion head with two nozzles that print the base and reinforcement materials, respectively. A schematic representation of the printer is shown in Fig. 9.1. As in other AM processes, to produce a part, first a digital file is needed. A solid model, designed with a computer aided design (CAD) software tool, is converted to a Standard Tessellation (STL) file. This file is then uploaded to the slicer software. Eiger is a web-based slicer of MKF. Printing settings are fixed on the slicer, which generates the numerical code for the machine to command the movement of printer heads and filaments. Specimens at the different stages are visualized in Fig. 9.2. Filaments which come in the shape of cylindrical continuous wires are packed in spools. With the aid of stepper motors, filaments are pushed to the extruders, where the temperature varies between 265°C and 270°C, enough to put nylon or Onyx in a molten state. Matrix material solidifies immediately after its extruded. Fibers have higher melting temperatures, so they are just laid down horizontally in the interior of the part area. The extrusion heads move on the horizontal plane, where the base material is
Fatigue behaviors of fiber-reinforced composite 3D printing
337
Nylon spool
Fiber spool
Extrusion head w/ nozzles
Composite part
Printing bed and platform
Fig. 9.1 Schematic representation of the continuous FRAM.
3D CAD model (solidworks)
Layer-wise assembly (eiger)
3D printed part (markforged mark two)
Fig. 9.2 Part preparation from CAD model to the 3D printed part.
338
Fatigue Life Prediction of Composites and Composite Structures
firstly printed on the periphery of the layer then it is reinforced by fibers. When the layer is concluded, the printing bed moves down so the new horizontal layer is started on the previously printed material.
9.2.2 Specimen preparation The way of preparing the specimen is critical. There are many parameters that affect the mechanical properties. Some of the parameters are already set by the printer capability, while the others are set by the user. Among those parameters are fiber volume, layer height, fiber orientation, infill pattern, infill percentage, and other slicing parameters. Kuchipudi [21] and Imeri et al. [22], the only studies currently available in the literature, focused the research on fiber orientation and fiber volume fraction. For simplifying purposes, Kuchipudi’s work will be classified as A, while Imeri et al.’s work will be classified as B. In A, fiber orientations with respect to longitudinal axis were set at 0, 45, and 90 degree, respectively. The fiber volume fractions are 0.25 and 0.50 out of total volume. Also, the fiber-reinforced material was FG. Schematic representation of the different fiber directions is shown in Fig. 9.3. The yellow lines represent the fiber reinforcement, while the black lines represent the base matrix. White boundary lines represent the shell lines and are made of the base material. In B, concentric (C) and isotropic (I) infills were used. In concentric infill, rings are added around the conferential perimeter of the specimen. In isotropic infill, fiber is added unidirectionally, linearly in a horizontal plane. Schematic representation of the two infill types is shown in Fig. 9.4. Different types of specimens were generated with 0, 1, 2, and 3 concentric rings. The angle for the isotropic infill was zero. Fiber reinforcement materials were CF, FG, and Kevlar. The nylon direction was chosen to be rectilinear due to giving better uniaxial load performance [22].
Fig. 9.3 Schematic representation of 0-, 45-, and 90-degree fiber orientations.
Fatigue behaviors of fiber-reinforced composite 3D printing
339
Fig. 9.4 Schematic representation of concentric and isotropic infill used in B study.
R = 10 mm
H = 10 mm
W = 20 mm
10T = 50 mm L = 15 mm
T = 5 mm
Fig. 9.5 ASTM specimen dimensions [23].
Specimens in A are designed according to the ASTM D3479 standard [23]. The specimens dimensions are 250 25 2.5 mm. Specimens in B are designed according to the ASTM E606 standard [24]. Dimensions of the specimens are shown in Fig. 9.5.
9.2.3 Experimental setup To perform the experiment, a servo-hydraulic test system is needed. The test system used was a closed-loop servo-hydraulic MTS810 machine with load cell capacity of 100 kN with an accuracy on the applied load under 1%. Machine has a lower moving gripping head that performs the cycles, and an upper gripping head only for gripping purposes. Fiber-reinforced nylon composites can be crushed by the gripping pressure. High pressure can crush the specimen, while low pressure can be a reason for the specimen to slip. Hence, it is important to set up a proper gripping pressure. In this study, by trial and error it was found that the ideal gripping pressure was 4 MPa. To initiate the experiment, it is required to input the testing variables. Frequency, amplitude, and average load are all controlled through MTS’s Multipurpose Testware interface. Different loads and frequencies were applied which will be given in more details in the following sections. However, it is important to note that for any load that was applied, machine tuning was needed. Tuning was performed to understand the difference between the test parameters and the received feedback.
340
9.3
Fatigue Life Prediction of Composites and Composite Structures
Experimental analysis
In A, the specimen is first tested for maximum tensile strength then the applied highest fatigue loads are 80%, 60%, and so on. The frequency used was 15 Hz and the minimum-to-maximum load ratio R ¼ 0. Specimens are tested under these loads until failure. From the experiments various results were observed. In the 0 degree infill specimens, for both 0.25 and 0.50 fiber volume fractions the failure was brittle in the direction of the fibers. The endurance limit for 0.25 infill was 244.4 MPa, since the specimen did not fail even after 4 107 cycles. For the 0.50 infill the endurance limit was at 335.3 MPa. The specimen did not break after 9 107 cycles. Increasing the fiber volumetric fraction proved to increase the fatigue strength. In the 45 degree specimens, for 0.25 and 0.50 fiber volumetric fractions, the failure was more ductile, and the material showed large viscoplastic deformations. The endurance limits were 36.2 and 60.2 MPa for 0.25 and 0.50 fiber volume fraction, respectively. A large stretch in the material at low load levels was observed, which shows that load is acting on the base matrix. Moving forward in the 90 degree specimens, the transverse properties of the material were tested. At this direction the load is carried by the base matrix mostly. Materials showed large viscoplastic deformations and failed at much lower loads and cycles compared to the other types of specimens. From the experimental results, it is understood that the direction and volumetric fraction of FG in the nylon matrix has a high influence on the fatigue properties. Strongest results were seen in the fiber direction, while the highest ductility was noticed on the 45 degree specimens. Similarly, endurance limits are highest for 0 degree, followed by 45 degree, and 90 degree specimens, correspondingly. Further, in the experimental study in B, the frequency used was 1 cycle per second with a minimum-to-maximum load ratio of 0.1. The goal in here was to understand the effect of fiber infill type, hence the experiments were truncated after 10,000 cycles. In addition, specimens that failed in the first cycle were also eliminated to be tested in the higher loads. Specimens were grouped in concentric (C), isotropic (I), and a mix of concentric and isotropic (I + C) infills with different number of rings. The first loads level was at a maximum of 3.33 kN. Results of the number of cycles at this load are shown in Table 9.1. Table 9.1 Results of the specimens from the first load level at 3.33 kN Max load (kN)
Material
Rings
Type
Test #1
Test #2
Test #3
3.33 3.33 3.33 3.33 3.33 3.33
Carbon Carbon Kevlar Kevlar Glass Glass
2 3 2 3 2 3
C C C C C C
510 10000+ 45 383 168 644
222 – 190 72 132 561
417 – 9 181 137 644
Fatigue behaviors of fiber-reinforced composite 3D printing
341
Isotropic infill specimens all passed the 10,000 cycles milestone hence were not tabulated. Among the concentric infill specimens, the ones with three number of rings endured more cycles. Adding more rings, increases the fiber volumetric fraction and hence could result in more cycles. CF-reinforced specimen outperformed the FG and Kevlar counterparts. Next, in the second load level (10.3 kN) results are shown in Table 9.2. From the first load, the specimens with three concentric rings were tested. However, regardless of material all three concentric rings failed in the first cycle, and thus, were eliminated from testing at higher loads. Similarly, for concentric and isotropic specimen with three rings, specimens failed in the first cycle. CF- and FG-reinforced specimens with two rings and isotropic infill did not break in the first cycle, however the Kevlar-reinforced specimen did. Truncating all the specimens from the previous loads, the third load level was at a maximum of 13.8 kN. Results are presented in Table 9.3. Table 9.2 Results from the second load at 10.3–1.03 kN Max load (kN)
Material
Rings
Type
Test #1
Test #2
Test #3
10.3 10.3 10.3 10.3 10.3 10.3 10.3 10.3 10.3 10.3 10.3 10.3 10.3 10.3
Glass Glass Glass Glass Glass Carbon Carbon Carbon Kevlar Kevlar Kevlar Kevlar Carbon Glass
0 1 2 3 0 1 2 3 0 1 2 3 3 3
I I+ C I+ C I+ C I I+ C I+ C I+ C I I+ C I+ C I+ C C C
750 699 19 1 10000+ 10000+ 3753 1 10000+ 54 1 1 1 1
1161 996 26 – – – 3995 1 – 285 – – – –
989 744 31 – – – 1747 – 196 – – – –
Table 9.3 Results from the third load at 13.8–1.38 kN Max load (kN)
Material
Rings
Type
Test #1
Test #2
Test #3
13.8 13.8 13.8 13.8 13.8 13.8 13.8 13.8
Glass Glass Glass Carbon Carbon Carbon Kevlar Kevlar
0 1 2 0 1 2 0 1
I I+ C I+ C I I+ C I+ C I I+ C
5 15 1 10000+ 324 1 3 1
66 24 – – 275 – 7 –
20 38 – – 209 – 6 –
342
Fatigue Life Prediction of Composites and Composite Structures
At this relevantly high load, all the specimens with two rings combined with isotropic infill failed in the first cycle. Among Kevlar specimens, only the isotropic infill specimen endured a very small numbers of cycles. At this load, only CF with isotropic infill resisted more than 10,000 cycles. In the final load, (17.4–1.74 kN), all the specimens except the isotropic CF-reinforced specimen failed in the first cycle. Even, in these specimens cracking sound was heard in the first cycles.
9.4
Statistical analysis
A full factorial ANOVA design was applied to the experimental results [25]. In ANOVA, the independent variables were load, material, number or rings, and the type of infill. The dependent variable was the number of cycles to failure. A log transformation of the dependent variable was performed due to data being counts. In addition, right and left truncations were performed due to experiments being stopped at specimens that did not fail after 10,000 cycles and the ones that failed after one cycle, respectively. Thus, leading to reducing the full-factorial ANOVA to fourway ANOVA with missing data. Since ANOVA is a robust procedure, the truncated data had very little effect on the overall power of the test. The assumptions of normality and homogeneous variations were not violated because the results were within the boundaries of classical ANOVA. There was one violation of the classical method, which was resolved. Rings and type of infill were found to overlap, which means that the two variables were correlated. Rings were found to be more influential which lead to not include the type of infill in the analysis. ANOVA results are presented in Table 9.4. From the last column, it can be understood that load, material, and rings all have significant value in the number of cycles while the interaction of load and material is slightly significant (load*material). To visualize and simplify the meaning of the numbers in Table 9.4, a group of boxplots figures for the number of cycles and each dependent variable, respectively, is provided. Starting with the number of cycles and the different load is presented in Fig. 9.6. Load 1 was not included due to the specimens tested in this load were with concentric infill. The box plots for the number of cycles are regardless of material. The thicker horizontal line in the figure represents the median. It should be noted that the median line Table 9.4 Summary of ANOVA results
Load Material Rings Load*material Residuals
Df
Sum Sq
Mean Sq
F value
p value
3 2 3 2 28
46.542 33.521 27.645 4.331 13.62
15.5141 16.7607 9.2149 2.1653 0.4864
31.894 34.4567 18.944 4.4514
3.58 1009 2.82 1008 6.57 1007 0.02096
343
4 0
2
log(cycles)
6
8
Fatigue behaviors of fiber-reinforced composite 3D printing
10.3 kN
13.8 kN
17.4 kN
Fig. 9.6 Box plot of the number of cycles vs the different types of load.
4 0
2
log(cycles)
6
8
in each of the load, is higher than the maximum number of the cycles in the other load, from right to left. This result explains the importance of the load in the variance of the number of cycles. Further, number of rings in the specimens was varied with the goal of understanding their effect. Next, the number of cycles versus the number rings is presented in Fig. 9.7.
0
1
2
Number of rings
Fig. 9.7 Box plots of number of cycles vs the number of rings.
3
344
Fatigue Life Prediction of Composites and Composite Structures
In Fig. 9.7, the median of three number of rings is higher than the other counterparts. However, analyzing the number of the cycles in Tables 9.2–9.3, the three rings specimens failed in relatively low numbers. This high median number of cycles is achieved by the concentric infill specimens. In the concentric infill the three rings number specimens gave the best results. The variance of this box plot is very high as it can be seen in the Fig. 9.7. Similarly, for two number of rings, the median is mixed between isotropic and concentric infills specimens. Finally, a preliminary S-N curve is fitted. More FG-reinforced specimens with zero rings isotropic infill were tested at different loads (14.8, 12.8, and 11.3 kN). Although more data are needed for full S-N curve, a preliminary S-N curve is shown in Fig. 9.8. Nonlinear regression was applied to the given formula: load ¼ 3547∗ exp ð0:0004∗cyclesÞ
13.3 kN 11.1 kN
Load
15.6 kN
Estimate of S-N curve with 95% confidence bands
0
200
400
600
Number of cycles
Fig. 9.8 Preliminary S-N curve for FG with zero rings isotropic infill.
800
1000
Fatigue behaviors of fiber-reinforced composite 3D printing
9.5
345
Discussion
Fiber orientation and volume fraction are critical to the mechanical properties of the FRAM parts. The higher the volume fraction the better the fatigue performance. In terms of fiber direction, specimens with similar fiber volumetric fractions but with different fiber orientation gave out different experimental results. Thus, showing that fiber orientation is influential. To visualize it, the horizontal cross-section view of the specimens can be analyzed. Stress is higher in the narrower area of the specimen, hence fiber orientation in there becomes very important. The horizontal cross-section view is shown in Fig. 9.9. Addition of rings causes the isotropic infill area to decrease. This reduction of isotropic infill area results in lower failure resistance to fatigue. In general, the failure mode of the specimens at 0 degree was fiber pull-out. At different loads, different amount of fibers pulled out were noticed. In terms of angle direction, the 45 degree deformed more than the 0 and 90 degree. In 90 and 0 degree infill specimens the break was at 90 degree. In the 45 degree infill specimens the break angle was 45 degree. Regarding fiber material type, CF specimens had a brittle failure, while FG and Kevlar specimens were more ductile. Failed specimens with different fiber reinforcing materials are shown in Fig. 9.10.
Fig. 9.9 Horizontal cross-section view of specimens with zero, one, two, and three rings, top to bottom.
346
Fatigue Life Prediction of Composites and Composite Structures
Fig. 9.10 FG, CF, and Kevlar fiber-reinforced broken specimens at 10.3 kN.
Fig. 9.11 CF, Kevlar, and FG broken specimens at maximum load of 13.8 kN.
In study B in general, similar breaking patterns among different materials were noticed at the same load. For instance, in a higher load, the fiber pull-out was faster and in higher volume for all the materials. Broken specimens are shown in Fig. 9.11. Comparing Figs. 9.10 and 9.11, it can be seen the difference in the amount fiber pull-out.
Fatigue behaviors of fiber-reinforced composite 3D printing
9.6
347
Conclusions and outlook
The current research studies conducted on fatigue properties showed that 0 degree specimens perform best in uniaxial tension-tension loads, with zero ring carbon-fiber being the strongest. Also, 45 degree specimens turned out to be the most ductile. Increasing fiber volume fraction increases the fatigue performance. The information presented in this chapter can serve as a knowledge base for the fatigue properties of continuous fiber-reinforced AM parts. Since continuous FRAM is a 4-year-old technique, the conducted research is preliminary. Future studies could include testing at different orientations, mixing fiber orientations, and fiber materials. Furthermore, to improve the interfacial bonding of the fibers and base matrix different types of coatings could be applied and investigated. Also, analytical models could be developed to predict the fatigue life of FRAM components. Finally, automated fiber placement for better properties could be developed with the help of topology optimization. Specifically, the angle direction of fibers for different applications could be optimized.
Acknowledgments This work is part of a larger project funded by the Advanced Technological Education Program of the National Science Foundation, DUE #1601587. The funding provided by the National Science Foundation is greatly appreciated.
References [1] I. Gibson, D.W. Rosen, B. Stucker, Additive Manufacturing Technologies: Rapid Prototyping to Direct Digital Manufacturing, Springer, 2010. [2] G.J.M. Krijnen, 3D printing of functional structures, Vonk 34 (1) (2016) 10–20. [3] R. Huang, et al., Energy and emissions saving potential of additive manufacturing: the case of lightweight aircraft components, J. Clean. Prod. 135 (2016) 1559–1570. [4] Y. Swolfs, S.T. Pinho, Designing and 3D printing continuous fibre-reinforced composites with a high fracture toughness, in: American Society for Composites Thirty-First Technical Conference, 2016. [5] S. Rawal, J. Brantley, N. Karabudak, Additive manufacturing of Ti-6Al-4V alloy components for spacecraft applications, in: RAST 2013—Proceedings of 6th International Conference on Recent Advances in Space Technologies, 2013. [6] N.A. Russell, J. Floyd, J. Caston, M.R. Villalpando, I. Fidan, Project InnoDino: additively innovative dinosaur design and manufacturing, Int. J. Rapid Manuf. 6 (4) (2017) 262–278. [7] T. Fresques, D. Cantrell, I. Fidan, The development of a framework between the 3D printed patterns and sand-cast work pieces, Int. J. Rapid Manuf. 5 (2) (2015) 170–185. [8] J. Watson, F. Vondra, I. Fidan, The development of a framework for 3D printing, casting, and entrepreneurship, in: 2017 ASEE Annual Conference & Exposition, 2017. [9] P. Dudek, FDM 3D printing technology in manufacturing composite elements, Arch. Metall. Mater. (2013). [10] T. Page, Technology forecast for composite materials in product design, i-manager’s J. Mater. Sci. 3 (4) (2016) 5–19.
348
Fatigue Life Prediction of Composites and Composite Structures
[11] Z. Quan, et al., Additive manufacturing of multi-directional preforms for composites: opportunities and challenges, Mater. Today 18 (2015) 503–512. [12] Markforged.com, About Us: 3D Printer Manufacturer & Technology Company j Markforged [online], 2019. Available at: https://markforged.com/about/. (Accessed 20 May 2019). [13] R. Stephens, A. Fatemi, R. Stephens, H. Fuchs, Metal Fatigue in Engineering, John Wiley & Sons Inc, 2000. [14] T. Letcher, M. Waytashek, Material property testing of 3D-printed specimen in PLA on an entry-level 3D printer, in: Volume 2A: Advanced Manufacturing, 2014. [15] M. Fischer, V. Sch€oppner, Fatigue behavior of FDM parts manufactured with Ultem 9085, JOM 69 (2017) 563–568. [16] I. Fidan, et al., The trends and challenges of fiber reinforced additive manufacturing, J. Adv. Manuf. Technol. (2019) 1–18. [17] R. Matsuzaki, et al., Three-dimensional printing of continuous-fiber composites by in-nozzle impregnation, Sci. Rep. 6 (2016)23058. [18] G.W. Melenka, B.K.O. Cheung, J.S. Schofield, M.R. Dawson, J.P. Carey, Evaluation and prediction of the tensile properties of continuous fiber-reinforced 3D printed structures, Compos. Struct. 153 (2016) 866–875. [19] A.N. Dickson, J.N. Barry, K.A. McDonnell, D.P. Dowling, Fabrication of continuous carbon, glass and Kevlar fibre reinforced polymer composites using additive manufacturing, Addit. Manuf. 16 (2017) 146–152. [20] F. Van Der Klift, Y. Koga, A. Todoroki, M. Ueda, Y. Hirano, R. Matsuzaki, 3D printing of continuous carbon fibre reinforced thermo-plastic (CFRTP) tensile test specimens, Open J. Compos. Mater. 6 (2016) 18–27. [21] M. Mohammadizadeh, I. Fidan, M. Allen, A. Imeri, Creep behavior analysis of additively manufactured fiber-reinforced components, Int. J. Adv. Manuf. Technol. 99 (2018) 1225–1234. [22] M. Fernandez-Vicente, W. Calle, S. Ferrandiz, A. Conejero, Effect of infill parameters on tensile mechanical behavior in desktop 3D printing, 3D Print. Addit. Manuf. 3 (2016) 183–192. [23] D3479/D3479M-12, Standard Test Method for Tension-Tension Fatigue of Polymer Matrix Composite. 2012. [24] ASTM International, E606/E606M-12, Standard Test Method Strain-Controlled Fatigue Testing, ASTM International, West Conshohocken, PA, 2012. [25] A. Imeri, I. Fidan, M. Allen, D.A. Wilson, S. Canfield, Fatigue analysis of the fiber reinforced additively manufactured objects, Int. J. Adv. Manuf. Technol. 98 (2018) 2717–2724.