Design, fabrication, and characterization of hierarchical mechanical metamaterials

Abstract

Natural mechanical materials, such as bamboo and bone, often exhibit superior specific mechanical properties due to their hierarchical porous architectures. Using the principle of hierarchy as inspiration can facilitate the development of hierarchical mechanical metamaterials (HMMs) across multiple length scales via 3D printing. In this work, we propose self-similar HMMs that combine octet-truss (OCT) architecture as the first and second orders, with cubic architecture as the third or more orders. These HMMs were fabricated using stereolithography 3D printing, with the length sizes ranging from approximately 200 µm to the centimeter scale. The compressive stress–strain behaviors of HMMs exhibit a zigzag characteristic, and the toughness and energy absorption can be significantly enhanced by the hierarchical architecture. The compressive moduli are comparable to that of natural materials, and the strengths are superior to that of most polymer/metal foams, alumina hollow/carbon lattices, and other natural materials. Furthermore, the flexural stress–strain curves exhibit a nonlinear behavior, which can be attributed to the hierarchical architecture and local damage propagation. The relatively high mechanical properties can be attributed to the synergistic effect of the stretch-dominated OCT architecture and the bending-dominated cube architecture. Lastly, an ultralight HMM-integrated unmanned aerial vehicle (HMM-UAV) was successfully designed and printed. The HMM-UAV is ~85% lighter than its bulk counterpart, remarkably extending the flight duration time (~53%). This work not only provides an effective design strategy for HMMs but also further expands the application benchmark of HMMs.

Graphical abstract

Keywords

3D printing / mechanical metamaterials / hierarchical architecture / mechanical behavior / unmanned aerial vehicle (UAV)

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Nomenclature

Abbreviations
BCC	Body center cubic
FDM	Fused deposition modeling
FE	Finite element
HMM	Hierarchical mechanical metamaterial
HMM-UAV	HMM-integrated unmanned aerial vehicle
MTS	Material testing system
OCT	Octet-truss
SLA	Stereolithography
SLM	Selective laser melting
STL	Standard Tessellation Language
TPL	Two-photon lithography
UAV	Unmanned aerial vehicle
Variables
b	Width of the sample
C	Correlation coefficient
d	Thickness of the sample
D	Deflection at the center of the sample
E	Effective Young’s moduli of the HMMs
Eb	Effective Young’s moduli of base material
F	Load recorded by the force transducer
l	Length of each strut
L	Support span length
σ	Effective stress of the HMMs
σb	Effective stress of the base material
ε	Flexural strain
\rho	Density of the HMMS
{\rho }_{\text{b}}	Density of the base material
R	Strut diameter
\overline{R}	Equivalent strut diameter

Acknowledgement

The authors would like to gratefully acknowledge the financial support of the National Natural Science Foundation of China (Grant No. 51905350).

Electronic Supplementary Material

The supplementary material can be found in the online version of this article at https://doi.org/10.1007/s11465-023-0776-9 and are accessible to authorized users.

Conflict of Interest

The authors declare that they have no conflict of interest.

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