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Materials Science in Additive Manufacturing 3D-printed composite auxetic structures

and energy absorbers [8,9] . It has been found that skins , The 3D-printed continuous fiber-reinforced
[10]
ceramics, graphite, zeolite, metals, and other natural composite auxetic structures possess the unified
auxetic materials exhibit natural auxetic behaviors . advantages of both continuous fibers and auxetic
[11]
Inspired by natural architectures, several types of structures. Given these desirable benefits, Dong et al.
artificial auxetic structures have been designed, which developed continuous fiber-reinforced auxetic composite
can be classified into three types: re-entrant type, chiral structures for reusable energy absorption applications [28] .
type, and rotating rigid type . With the advancement of Another study fabricated continuous fiber-reinforced
[8]
additive manufacturing, auxetic structures are fabricated re-entrant auxetic honeycomb structures through the
as mechanical metamaterials endowed with fascinating 3D printing technology by tailoring the printing path
physical properties [12-15] . to ensure better fiber path continuity [29] , and concluded
Due to the porosity and deformability, auxetic that adding continuous fibers led to dramatic increases
structures generally have lower stiffness and load- in compressive stiffness, energy absorption, and smaller
bearing capacity. To enhance the mechanical properties NPR. As disclosed earlier, the auxetic behaviors come
of auxetic materials, auxetic composites are developed. from different types of deformation mechanisms, and
The auxetic chiral structure can be 3D-printed with due to the high and anisotropic stiffness/strength of
thermoplastic polyurethane elastomer (TPU) and continuous fibers, their addition would not result in
then filled with polyurethane (PU) foam, producing consistent performance on different types of auxetic
a foam-filled chiral structure that is significantly structures.
reinforced in terms of stiffness and specific energy This study aimed to investigate the mechanical properties
absorption capacity . A study by Xue et al. [16] showed of 3D-printed continuous fiber-reinforced composite
[5]
that the composites fabricated by combining aluminum- auxetic structures. Auxetic test specimens with different
based auxetic lattice structure with polymer fillers topology and fiber path configurations were fabricated
exhibited higher elastic modulus, compressive strength, and evaluated for mechanical properties, including tensile
and energy absorption capacity, as verified in the modulus, tensile strength, and Poisson’s ratio, using uniaxial
compressive experiment. Another study showed that the tensile test. The effect of adding continuous fibers on the
printed auxetic chiral structures fabricated from high- properties of the structures is discussed.
performance composites added with chopped carbon
fibers manifested enhanced tensile modulus, strength, 2. Materials and methods
and energy absorption [17] . To investigate the mechanical properties of continuous
Continuous fiber-reinforced composites encompass fiber-reinforced composite auxetic structures, we designed
several desirable advantages, such as lightweight, high and fabricated a group of test specimens and evaluated
strength, and stiffness. They have been widely used in the their mechanical properties through a uniaxial tensile test.
fields of automobile, aircraft, and space [18-20] . 3D printing of
continuous fiber-reinforced composites provides a feasible 2.1. Design of specimens
avenue to bridge the gap between advanced materials and Three types of structures are selected as test specimens
innovative structures. Hou et al. have demonstrated the (Figure 1). Two of them are auxetic structures: the rotating
design and fabrication of a novel corrugated structure and rigid and the re-entrant types . The other is a structure
[30]
[31]
discussed the correlations among the process parameters, with a positive Poisson’s ratio named rhombus. The
structure parameters, density, fiber content, and final dimensions of the unit cell of the three specimens are 16 ×
performance of the printed specimens . Besides that, the 16 mm. The rotation angle of a single square in a rotating
[21]
manufacture of sandwich structures in different shapes rigid structure is 15°. The width of rods in re-entrant and
using a continuous carbon fiber 3D printer has also rhombus structures is 2 mm.
been attempted . Considering the requirement on path Figure 2 displays the dimensions of the test specimen.
[22]
continuity, Liu et al. proposed a path-driven design The test specimen is composed of a 4 × 7 array of unit
[23]
method to generate lattice structures with designable cells and two holders. To capture a more obvious auxetic
anisotropy and close-to-zero mean curvatures for 3D behavior and shorten the manufacturing cycle, the
printing of continuous fiber-reinforced composites. In
addition to improving mechanical properties, accentuating thickness of specimens was set as 2 mm. The test specimen
the multi-functional properties of continuous fibers such was fixed on the test machine through holders, and the
as sensing , shape morphing, [25,26] and electromagnetic uniaxial tensile loads were applied along the y direction.
[24]
interference shielding provides a new direction for Due to the property anisotropy of continuous fibers,
[27]
developing smart structures. the distribution of fiber paths has a great impact on the

Volume 2 Issue 4 (2023) 2 https://doi.org/10.36922/msam.2159