Page 62 - MSAM-2-1

Materials Science in Additive Manufacturing Energy absorption of Moore’s thin-walled structures

design. All the specimens were manufactured using fused depth) and a side length of 50 mm. In other words, the
filament fabrication (FFF), which is a low-cost additive block size (width × length × height) was 50 × 50 × 15 mm.
manufacturing technology. Quasi-static compression To achieve three relative densities, the wall thickness was
tests and loading-unloading cyclic compression tests were adjusted accordingly, as shown in Table 1.
also carried out to observe the mechanical responses and The samples were fabricated with a composite material,
evaluate the permanent deformation of the proposed onyx, using an FFF printer, MarkTwo (Markforged Inc.,
structures. Numerical models were developed and used USA). The onyx, provided by Markforged, was composed
for a parametric study to further analyze the mechanical of nylon (80 vol%) and short carbon fibers (20 vol%). The
behaviors.
carbon fibers were 200 μm in length and orientated in the
[43]
2. Methodology printing direction . An extruder of 0.4 mm in diameter
was used for filament deposition. The printing parameters
2.1. Design and fabrication of compliant structures were the same for all specimens: 0.1 mm layer height,
The novel metamaterials were inspired from a series of single-layer outline, and solid infill with a −45°/45° pattern.
space-filling infill patterns, known as Moore curves. As For each layer, the extruder travelled along the outlines
illustrated in Figure 2A, this set of curves consisted of of the structure first and filled the space in between the
th
many orders. The 0 order was constructed by connecting outlines. Therefore, the thinnest wall thickness that could
the centers of each small square from the bottom left to the be fabricated using Markforged was 0.8 mm (twice of the
bottom right. The 1 order comprised four of the 0 order resolution). Based on this principle, the smallest relative
st
th
in a smaller scale with certain rotations. By repeating the density of 20% was adopted to ensure the printing quality.
same procedure, the 2 and 3 order curves were derived In addition, the limitation of wall thickness restricted the
rd
nd
rd
(Figure 2A). By connecting the opening with a straight line fractal hierarchy up to the 3 order. The 3D-printed thin-
and smoothing the corner using cubic spline function, the walled structures with a relative density of 20%, 30%, and
cross-section centerline was constructed. All the curvatures 40% are presented in Figure 2B-D, respectively.
at the same locations were constant for all designs; the only The thickness of the fabricated specimens was measured
design parameter was wall thickness. The structures were from several regions of the cross-sections to compare
constructed by assigning certain thickness to the wall and to the design values (Table 1). Overall, the 3D-printed
extrusion along the out-of-plane direction. structures were slightly thicker than the original designs.
The first three orders of Moore curves were adopted One reason could be the high extrusion flow rate,
to investigate the influence of fractal hierarchy on energy resulting in over-extrusion of filament and a thicker print
absorption capacity. In addition, three different relative line. Within each specimen, the wall thickness was not
densities were applied, that is, 20%, 30%, and 40%, to explore consistent due to machine inaccuracy from FFF printing.
the effect of relative density on mechanical responses. All As an important printing parameter, the pressure in the
the proposed designs had a depth of 15 mm (in-plane extruder influences the filament thickness during the
A B

C D

Figure 2. (A) First three hierarchies of theoretical Moore curves. Manufactured Moore curve-inspired thin-walled structures with a relative density of
(B) 20%, (C) 30%, and (D) 40%.

Volume 2 Issue 1 (2023) 3 https://doi.org/10.36922/msam.53