1 Research Background
Materials with tunable elastic properties offer tremendous possibilities for smart machines, robots, aircraft and other systems. However, even if a phase change is induced, the elasticity of conventional materials can hardly be changed or tuned during operation. Mechanical metamaterials are artificially constructed materials whose properties exceed those of traditional materials. Adjustable elasticity is possible in reconfigurable metamaterials , but continuous adjustability in existing designs is plagued by problems such as structural instability, weak robustness, plastic failure, and slow response.
2 Research results
Fang Xin and Wen Jihong of the National University of Defense Technology, Hong Kong Polytechnic University Cheng Li, and Karlsruhe Institute of Technology in Germany Peter Gumbsch et al. jointly reported a metamaterial design paradigm using gears with encoded stiffness gradients as Component elements, and organize clusters of gears to fulfill multiple functions. The design achieves continuously adjustable elastic properties while maintaining stability and robustness even under heavy loads. This gear-based metamaterial has excellent properties, such as continuous modulation of Young's modulus over two orders of magnitude, shape deformation between ultrasoft and solid, and fast response. The research puts fully programmable materials and adaptive robots within reach. The research work related to was published in the top international journal "Nature Materials" under the title "Programmable gear-based mechanical metamaterials".
3 Graphic Express
Researchers have designed a new paradigm. First, adjustability is achieved by assembling elements with built-in stiffness gradients. Secondly, coupling between elements must comply with large deformations. Achieving such a tunable yet strong material requires ensuring tunability under large forces and robust controllability while avoiding plastic deformation during tuning. This variable but strong coupling is achieved through a gear set. The gears smoothly transfer rotational and heavy compressive loads due to reliable gear meshing. Alternatively, stiffness gradients can be built into a single gear body or achieved through stepped gear assemblies. Gear sets can be assembled into manifolds and can be regularly arranged as cells to form metamaterials (Fig. 1c). Since many gear assembly architectures exist, the proposed design concept is very general.
Figure 1. Design concept for mechanical metamaterials
The first prototype was created using a tightly coupled periodic gear and two lattice frameworks to arrange the gears into a simple quadratic pattern (Fig. 2a). Face gears contain hollow sections. Two elastic arms are formed on the outside, and their radial thickness changes smoothly with the rotation angle θ. Under compressive load, the deformation of the arm is mainly bending (Fig. 2c). Adjustability depends on the shape of the built-in hollow section. Inspired by the Chinese Tai Chi diagram, Figure 2b features spiral directions that provide smooth changes and polarity. In any two meshing gears, the direction of rotation is opposite. In addition, the spiral directions of the front and back Tai Chi patterns are opposite. Therefore, the meshing pattern of a pair of gears has two poles.
Figure 2. Mechanical metamaterial based on Tai Chi gear
Researchers have proven that even at the microscale, gear-based integrated metamaterials can be directly manufactured through 3D printing. The main challenge with this kind of integrated manufacturing is ensuring that the meshing teeth do not fuse together, but still effectively participate in the meshing. To solve this problem, a small gap is retained between the meshing tooth surfaces in the digital model of the assembly. The researchers used projection microstereolithography 3D printing technology to create an integrated micro-metamaterial composed of 5×6 Tai Chi gears (Figure 2h). The diameter and tooth thickness of the Tai Chi gear are 3.6 mm and 235 μm respectively, and the thickest arm is 75 μm. The microgears are arranged in P+ (0°), and the minimum gap remaining between the teeth is 32 μm. The sample is made of photosensitive resin with a Young's modulus of 3.5 GPa. Using this integrated design strategy, gear-based metamaterials can be scaled up in size and quantity of gears with appropriate high-resolution large-scale 3D printing equipment.
Figure 3. Metamaterial consisting of a planetary gear system
The first metamaterial is only adjustable under compressive load. The tensile load is borne by the frame, and the tensile modulus of and is Et=Ef. For strong metamaterials, both the compressive and tensile modulus can be adjusted while maintaining structural integrity, which can be achieved by organizing the planetary gear system into cells (Figure 3a). The cell contains six gears: an internal ring gear, a central sun gear and two pairs of planetary gears A1–A2 and B1–B2, where the gear centers A1–O–A2 and B1–O–B2 are collinear. Using this cluster of gears, the researchers created a layered, strong metamaterial whose adjustability comes from the relative rotation of the gears within the cells. The ring thickness is uniform, and adjacent rings are rigidly connected to in the secondary lattice, thus ensuring structural integrity. The teeth prevent relative sliding between the two gears, even under tension. For the assembled metamaterial, all sun gears are connected via shafts to transmission gears, which are tightly coupled. Therefore, robust reconstruction of all cellular modes can be achieved by rotating the transmission gear.
Figure 4. Strong or ultrasoft metamaterials under shear
Interestingly, the metamaterial in Figure 2a remains stable under compressive stress and displays large stiffness when sheared. One of the factors supporting stability is the uneven loading of the meshing teeth at different points, which can cause bending deformations that clamp the teeth tightly together. The shear modulus of the metamaterial G=Gg+Gf is composed of the shear modulus generated by the gear (Gg) and the frame (Gf). Shear forces induce gear rotation and planetary rotation.
Researchers proposed several scenarios to demonstrate the broad application potential of gear-based metamaterials. For robots, adjustable stiffness legs/actuators provide high stiffness to stably support heavy objects while walking, and low stiffness to provide shock-absorbing protection when jumping or running. Similar adjustable stiffness isolators are needed in aero-engine pylon systems to maintain optimal performance and efficiency during different phases of flight. Gear-based fast-response metamaterials that may produce sensitive variable-stiffness skins have been attracting attention. Furthermore, the resonator with tunable stiffness is a key component in programmable metamaterials for wave manipulation. Therefore, gear-based programmable metamaterials can help realize a wide range of smart machines.
Figure 5. Characteristics of active mechanical metamaterials
4 Conclusion and outlook
Research shows that gear-based mechanical metamaterials can maintain stability, strength and high load-bearing capacity, while having in-situ adjustability, strong programmability and easy implementation. The gear set provides a wide design space, allowing for customizable properties of the metamaterial. In addition to the demonstrated Young's modulus, shape deformation and impact protection, the adjustability can be extended to other elastic properties such as shear modulus, Poisson's ratio , strength, deformation mode, and even damping coefficient . 3D metamaterials can also be envisioned by using bevel gears , assembling face gears into a hierarchy as shown in Figure 3, or synthesizing different types of gears. Integrated manufacturing links these adjustability to produce robust, multipurpose devices. Taking micro-metamaterials as an example, gear-based metamaterials can be further miniaturized and expanded through high-resolution and large-scale 3D printing.
In summary, this study proposes and demonstrates an unconventional design paradigm for programmable dynamic metamaterials through the variable but strong coupling and built-in variability of gears. From conceptual prototypes to mechanical analysis, gear-based mechanical metamaterials demonstrate flexible tunability and integrated fabrication at both macro and micro scales and demonstrate a wide range of potential applications. This research broadens the horizons for designing fully programmable materials, thus promoting their exploration in practical applications.
5 Literature
Literature link:
https://www.nature.com/articles/s41563-022-01269-3
Original literature:
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