In a review recently published in the open-access journal Materials, researchers discussed the recent progress in the developments of hierarchical auxetic mechanical metamaterials using additive manufacturing.
Study: Additively Manufactured Hierarchical Auxetic Mechanical Metamaterials. Image Credit: 7pic/Shutterstock.com
Complex geometry can be produced at the micro- and nanoscales using contemporary additive manufacturing techniques. This capability enables the creation of new materials with unique features from those of typical natural materials.
Numerous fresh metamaterials are currently being created for use in engineering applications. Auxetic materials are sought-after contenders for numerous industrial and technical applications because of their high-performance mechanical qualities. The struts and nodes are bent and rotated as part of the auxetic behavior's overall process, and the qualities of the resulting material depend on the geometrical parameters that determine the shape of its constituent parts.
The widespread interest in hierarchical auxetic materials can be attributed to the structures' extra benefits, such as the ability to more easily change the necessary mechanical properties. In applications where a precise set of parameters is necessary, this capability is greatly desired.
About the Study
In this study, the authors discussed the auxetic characteristics using experimental, analytical, and numerical methods. Particular consideration was given to the outcomes of studies on hierarchical auxetic materials. The extra benefits of structures, such as more flexible adjustment of the required mechanical qualities, accounted for the widespread interest in the hierarchical subclass of auxetics.
The team focused on the straight strut materials. The widespread interest in hierarchical auxetic materials could be attributed to the structures' extra benefits, such as the ability to more easily change the necessary mechanical properties. The definition of hierarchical auxetic structures was followed by a discussion of their characteristics and advantages, as reported in the literature.
The researchers examined the benefits of a number of hierarchical structures, including stronger re-entrant auxetic materials, honeycomb hierarchical structures, rotating unit auxetic materials, and re-entrant honeycomb hierarchical structures. A list of known auxetic and hierarchical auxetic features, applications, and future research opportunities for structures was also provided.
In the experiment with the identical design but with solid parts substituted with porous squares and re-entrant lattice shapes, 81% compressibility was discovered. Extreme stretchability via unit rotation increased in rectangular units from 62% to 156% for the hierarchical structure. Mechanical testing revealed that the lattice design parameters had a substantial impact on the mechanical behavior of the samples. The sample with the highest recorded Young's modulus, 383 MPa, had wires with a radius of 0.25 mm and a wire count of 29.
The brick masonry wall composed of mortar-auxetic composites had better performance in resisting the lateral impact than the unrendered wall, which resulted in a 22% reduction in lateral displacement and an eight-fold increase in energy absorption, according to simulation using the finite element modeling method. The ability to control and fine-tune the mechanical properties of auxetic metamaterials, particularly hierarchical auxetic materials, was one of their key benefits. This was made possible by the material structure design and the setting of its geometric parameters.
In conclusion, this study discussed significant findings from the study of auxetic materials. The team observed that auxetic materials are in high demand due to their higher energy absorption as compared to traditional materials. The materials were potential candidates for use in applications related to protection and damage minimization because of their capacity to reduce the impulse response, localize failure, and peak transmitted pressure. Additionally, angle-gradient auxetic materials enable the development of long-term implants that require no removal during the patients' lifetimes by addressing the primary issues with conventional bone implants.
By inclining cells of one design into the cellular structure of the same or another design, hierarchical structures reported in the literature often use two levels of the structure. For the material to behave as expected, at least one of the cells must contain auxetic characteristics.
The authors mentioned that the hierarchical auxetic materials may find use in a variety of biological applications, including implants, filtration and drug delivery systems, coronary stents, and other devices that call for high-precision tuning.
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Mazur, E., Shishkovsky, I., Additively Manufactured Hierarchical Auxetic Mechanical Metamaterials. Materials 15(16) 5600 (2022).