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Mechanical Response of Octahedral and Octet Truss Lattice Structures Fabricated Using the CLIP Technology

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2016 International Conference on Computational Modeling, Simulation and Applied Mathematics (CMSAM 2016) ISBN: 978-1-60595-385-4

Mechanical Response of Octahedral and Octet-Truss Lattice Structures

Fabricated Using the CLIP Technology

Anil SAIGAL

1,*

, John TUMBLESTON

2

, Hendric VOGEL

2

, Courtney FOX

2

and Nikolaus MACKAY

2

1Department of Mechanical Engineering, Tufts University, 200 College Avenue,

Medford, MA 02155, USA

2

Carbon, 312 Chestnut Street, Redwood City, CA 94063, USA

*Corresponding Author

Keywords: CLIP technology, Additive manufacturing, Octahedral and octet-lattice structures.

Abstract. In the rapidly growing field of additive manufacturing (AM), the focus in recent years has shifted from prototyping to manufacturing fully functional, ultralight, ultrastiff end-use parts. This research investigates the mechanical behavior of octahedral- and octet-truss lattice structured polyacrylate fabricated using Continuous Liquid Interface Production (CLIP) technology based on 3D printing and additive manufacturing processes. Continuous Liquid Interface Production (CLIP) is a breakthrough technology that grows parts instead of printing them layer by layer. CLIP is a chemical process that carefully balances UV light and oxygen to eliminate additional mechanical steps for part delamination and resin refill. UV light triggers photopolymerization and oxygen inhibits it. By carefully balancing the interaction of light and oxygen, CLIP grows objects from a pool of resin.

Lattice structures such as the octahedral- and octet-truss lattice have recently attracted a lot of attention since they are often structurally more efficient than foams of a similar density made from the same material, and the ease with which these structures can now be produced using 3D printing and additive manufacturing. This research investigates the mechanical response under compression of octahedral- and octet-truss lattice structured polyacrylate fabricated using CLIP technology. It was found that the elastic modulus and strength of the octahedral-truss structured materials are proportional to each other over a relative density range of 0.07 to 0.35. Finally, even though the octet-truss lattice structure is a stretch-dominated unit cell structure, it was found to have a lower stiffness and strength as compared to an octahedral-truss structure of the same relative density. This can be attributed to the smaller diameter and therefore larger l/d of individual struts.

Introduction

Continuous Liquid Interface Production (CLIP) is a breakthrough technology that grows parts instead of printing them layer by layer. CLIP works by projecting light through an oxygen-permeable window into a reservoir of UV curable resin. The build platform lifts continuously as the object is grown. The heart of the clip process is a special window that is transparent to light and permeable to oxygen, much like a contact lens. By controlling the oxygen flux through the window, CLIP creates a “dead zone”—a thin layer of uncured resin between the window and the object. Establishing an oxygen-inhibited dead zone is fundamental to the clip process. This makes it possible to grow without stopping. As a continual sequence of UV images are projected, the object is drawn from the resin bath. Sophisticated software manages the entire process by controlling the variables.

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[image:2.612.207.408.138.317.2]

elongation and resilience expected of a TPE to the strength and temperature resistance of a glass-filled Nylon [1,2]. This is in contrast to the traditional 3D printing process which requires a number of mechanical steps, repeated over and over again in a layer-by-layer approach. In addition, traditionally made 3D printed parts are notoriously inconsistent [3]. The mechanical properties vary depending on the direction the parts are printed due to the layer-by-layer approach.

Figure 1. Schematic of CLIP Printer.

Background

The mechanical properties of most materials degrade rapidly as the density decreases. This loss in mechanical properties is because most engineered cellular solid materials with random porosity and low densities exhibit a quadratic or stronger relationship between Young’s modulus and density, and

strength and density. Namely, E/Es α (ρ/ρs)n and σy/σys α (ρ/ρs)n, where E is Young’s modulus, ρ is

density, σy is yield strength, and s denotes the respective bulk value of the solid constituent material

property. The power n of the scaling relationship between relative material density and the relative mechanical property depends on the material’s microarchitecture.

Mechanical metamaterials are man-made materials in which the mechanical properties are mainly defined by their structures instead of the properties of each component. Periodic cellular structures consisting of honeycomb, tetrahedral, 3D Kagome, pyramidal and octet truss arrangement of webs or struts have recently attracted a lot of attention since they have a broad range of applications including structural components, energy absorption, heat exchangers, catalyst support, filters and biomaterials [4-7].

Lattice structures with open cell intermediate face sheets between the layers of inclined struts, such as the octet-truss lattice, have recently attracted a lot of attention since they are structurally more efficient than foams of a similar density made from the same material, and the ease with which these structures can now be produced using 3D printing and additive manufacturing [8,9]. This research investigates the mechanical response of octahedral- and octet-truss lattice structured polyacrylate fabricated using Continuous Liquid Interface Production (CLIP) technology.

Experimental Methods and Materials

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(a)

(b)

Figure 2. a) Octahedral and b) Octet Structures.

The structures are approximately 25 mm x 25 mm x 25 mm. The fundamental building block for the octahedral structure is a regular octahedron whereas the octet structure has a regular octahedron as its core surrounded by eight regular tetrahedra distributed on its faces. All the strut elements have identical aspect ratios, with 12 solid rods connected at each node. The cubic symmetry of the cell’s FCC structure generates a material with approximately isotropic properties. The minimum diameter of the strut elements is 0.35 mm and increases as the relative density of the structure increases.

Results and Discussion

Figure 3 shows the stress strain behavior of the solid, octahedral and octet lattice structures fabricated using the CLIP technology. The compressive mechanical behavior of the lattice structures observed is typical of cellular structures which includes a region of nominally elastic response, yielding, plastic strain hardening to a peak in strength, followed by a drop in flow stress to a plateau region and finally rapid hardening associated with contact of the deformed struts with each other as part of densification.

(a) (b) (c) Figure 3. Stress-Strain Behavior of the a) Solid, b) Octahedral and c) Octet Structures.

Table 1 summarizes the measured relative density, compressive modulus and compressive strength of the solid, octahedral and octet structures. Based on ASTM D1621, the yield strength of the solid was determined to be the stress at 10% deformation. It is quite clear that even though these lattice structures are quite light, these structures also have significantly reduced modulus and strength.

0 20 40 60

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[image:4.612.99.513.442.600.2]

In addition, it was found that Eoctahedral/Eoctet = (ρoctahedral/ρoctet)1.98 and σoctahedral/σoctet = (ρoctahedral/ρoctet)2.04. The value of n that satisfies the equations E/Es α (ρ/ρs)n and σy/σys α (ρ/ρs)n is found to be between 2.6 and 2.9. As such, it appears that both these structures at these low relative densities deform predominantly as a result of bending and/or buckling of the struts.

Table 1. Measured Properties of Different Structures.

Solid Octahedral Octet

Modulus (MPa) 520.40 0.398 1.991

Strength (MPa) 32.94 0.025 0.130

Relative Density 1.000 0.065 0.148

Sp. Modulus 444.74 5.20 11.52

Sp. Strength 28.15 0.33 0.75

Based on the data, it is not clear if the measured increase in strength and stiffness of the octet structure relative to the octahedral structure is due to the increase in relative density of the structure or due the different structures. Hence, five different specimens consisting of octahedral structure with relative densities ranging approximately from 0.07 to 0.35 were fabricated and tested along with an octet structure with a relative density of 0.27.

Figure4 shows the stiffness and strength of the octahedral and octet structures, respectively, as a function of the relative density of the structure. It was found that the Young’s modulus and strength of the octahedral-truss structured materials are proportional to each other over the relative density range of 0.07 to 0.35. The ratio of elastic modulus to strength was between 16 and 18 over the entire range and about the same as that for the bulk material. Finally, even though the octet-truss lattice structure is a stretch-dominated unit cell structure, it did not behave as such in this investigation and was found to have a lower stiffness and strength as compared to an octahedral-truss structure of the same relative density. This can be attributed to the smaller diameter and therefore larger l/d of individual struts.

(a) (b)

Figure 4. a) Elastic Modulus and b) Strength of Octahedral and Octet Structures as a function of Relative Density.

Conclusions

The compressive stress-strain behavior of the octahedral and octet lattice structures observed is typical of cellular structures. Even though these lattice structures are quite light, these structures also have significantly reduced modulus and strength. It appears that both these structures at low relative densities deform predominantly as a result of bending and or buckling of the struts. It was found that the stiffness and strength of the octet-truss structureis lower as compared to an octahedral-truss structure of the same relative density.

0 50 100 150

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Relative Density

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References

[1]J.R. Tumbleston, D. Shirvanyants, N. Ermoshkin, et al., March 2015, "Continuous Liquid

Interface Production of 3D Objects,” Science, Vol. 347(6228), pp. 1349-1352.

[2]www.carbon3D.com, 2016.

[3]C.T. Mueller, A. Irani and B.E. Jenett, 2014, “Additive Manufacturing of Structural Prototypes

for Conceptual Design,” Proceedings of the IASS-SLTE Symposium, Shells, Membranes and Spatial Structures: Footprints, Brasil and Pauletti (Eds.), Brasilia, Brazil, September 15-19, 2014.

[4]K. Ando and H. Onda, April 1999, “Mechanism for Deformation of Wood as a Honeycomb

Structure I: Effect of Anatomy on the Initial Deformation Process,” Journal of Wood Science, Vol. 45(2), pp. 120-126.

[5]X. Zheng, H. Lee, T.H. Weisgraber, etal., June 2014, “Ultralight, Ultrastiff Mechanical

Metamaterials,” Science, Vol. 344, Issue 6190, pp. 1373-1377.

[6]E.B. Duoss, T.H. Weisgraber, K. Hearon, etal., 2014, “Three-Dimensional Printing of

Elastomeric, Cellular Architectures with Negative Stiffness,”Advanced Functional Materials, Vol. 24, pp. 4905-4913.

[7]D. Jang, L.R. Meza, F. Greer and J.R. Greer, October 2013, “Fabrication and Deformation of

Three-Dimensional Hollow Ceramic Nanotubes,”Nature Materials, Vol. 12, pp. 893-898.

[8]V.S. Deshpande, N.A. Fleck and M. F. Ashby, 2001, “Effective Properties of the Octet-Truss

Lattice Materials,” Journal of the Mechanics and Physics of Solids, Vol. 49, pp. 1747–1769.

[9]L. Dong, V. Deshpande and H. Wadley, 2015, “Mechanical Response of Ti-6Al-4V Octet-Truss

Figure

Figure 1. Schematic of CLIP Printer.
Table 1. Measured Properties of Different Structures.

References

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