Light but tough bio-inherited materials: Luffa sponge based nickel-plated composites

Analysis and Simulation of the Compressive Strength of Bioinspired Lightweight Structures Manufactured by a Stereolithography 3D Printer

The use of metamaterials is a good alternative when looking for structures that can withstand compression forces without increasing their weight. In this sense, using nature as a reference can be an appropriate option to design this type of material. Therefore, in this work, a comparative study of a selection of eight representative models of a wide variety of existing solutions, both bioinspired and proposed by various researchers, is presented. These models have been manufactured using stereolithography (SLA) printing, which allows complex geometries to be obtained in a simple way that would be more complicated to achieve by other procedures. Additionally, the manufacturing cost of each model has been determined. The compression tests of the different models have made it possible to evaluate the breaking force and its corresponding deformation. Likewise, a finite element analysis of the manufactured models has been carried out to simulate their behavior under compression, achieving results very similar to those obtained in the experimental tests. In this way, it has been concluded that, among the three-dimensional patterns, the structure called "3D auxetic" is the one that supports the greatest breaking force due to the topographic characteristics of its bar structure. Similarly, among the two-dimensional patterns, the structure called "Auxetic 1", with a topography based on curves, is capable of supporting the greatest deformation in the compression direction before breaking. Moreover, the highest resistance-force-to-cost ratio has been obtained with a "3D auxetic" structure.

Lightweight, compression-resistant cellular structures inspired from the infructescence of Liquidambar formosana

In order to adapt to the environment, plants have evolved many structural designs to improve material utilization. The head infructescence can be described as the Fibonacci sequence, in consistent with plant developmental biology. The lignified framework inside the head infructescence possesses idiographic structural designs that optimize maximum energy efficiency, growing space, seed spreading probability, and enhance the mechanical behavior of the infructescences. In this study, the hierarchical structure and mechanical properties of the infructescence of Liquidambar formosana, commonly called Formosan gum, were investigated. Liquidambar formosana has maple-like leaves and burr-like infructescences. The buckyball-like framework inside infructescence consists of chambers (cells), which support the whole structure under compression. Inspired by the framework, we proposed three models: Thomson model based on the lowest potential energy state, Poisson disc model indicated random distribution, and spherical Fibonacci model represented plant development. Three-dimensional physical entities of these models were fabricated by additive manufacturing. We discovered that under compression testing, these models appear different mechanical properties and deformation mechanisms based on their structures. Spherical Fibonacci model provides superior mechanical properties compared to Thomson and Poisson disc models due to its unique structural design. It is the first time that spherical Fibonacci model brought into the bio-inspired mechanics models through structural analysis and finite element method. The unique construction of Liquidambar formosana has great potential in the designs of novel lightweight, anti-buckling composites, and bio-inspired architectures.

Recent Developments in Luffa Natural Fiber Composites: Review

Natural fiber composites (NFCs) are an evolving area in polymer sciences. Fibers extracted from natural sources hold a wide set of advantages such as negligible cost, significant mechanical characteristics, low density, high strength-to-weight ratio, environmental friendliness, recyclability, etc. Luffa cylindrica, also termed luffa gourd or luffa sponge, is a natural fiber that has a solid potential to replace synthetic fibers in composite materials in diverse applications like vibration isolation, sound absorption, packaging, etc. Recently, many researches have involved luffa fibers as a reinforcement in the development of NFC, aiming to investigate their performance in selected matrices as well as the behavior of the end NFC. This paper presents a review on recent developments in luffa natural fiber composites. Physical, morphological, mechanical, thermal, electrical, and acoustic properties of luffa NFCs are investigated, categorized, and compared, taking into consideration selected matrices as well as the size, volume fraction, and treatments of fibers. Although luffa natural fiber composites have revealed promising properties, the addition of these natural fibers increases water absorption. Moreover, chemical treatments with different agents such as sodium hydroxide (NaOH) and benzoyl can remarkably enhance the surface area of luffa fibers, remove undesirable impurities, and reduce water uptake, thereby improving their overall characteristics. Hybridization of luffa NFC with other natural or synthetic fibers, e.g., glass, carbon, ceramic, flax, jute, etc., can enhance the properties of the end composite material. However, luffa fibers have exhibited a profuse compatibility with epoxy matrix.

Mechanical properties and energy absorption characteristics of tropical fruit durian (Durio zibethinus)

The paper presents for the first time the material properties and energy absorption capacity of durian shells with an attempt to use as an alternative sustainable material and mimic their structural characteristics to design a bio-inspired structure for protective packaging applications. A series of quasi-static compression tests were carried out to determine Young's modulus and bioyield stress of the durian shells as well as their energy absorption capacity. The mesocarp layers and thorns are interesting parts for investigating their energy absorption characteristics because they play an important role in protecting the flesh of durians during their drop impact onto the ground. The mesocarp layers of the shell were subjected to axial and lateral compression while the thorn specimens were compressed under axial loading with an increasing number of thorns. The results showed that the densification strain, plateau stress and specific energy absorption of the mesocarp layer under lateral loading is higher than that under axial loading. Furthermore, the compression tests on the thorns demonstrated that an increase in the number of thorns helped to absorb more energy and the specific energy absorption of the thorns was nearly two times higher than that of the mesocarp layer under the axial loading. In addition, the cyclic loading of the thorns showed that the extent of reversibility of deformation in the thorns decreases from 32% at the first cycle to around 10% at the 9th-cycle. Finally, the microstructure of the thorn and mesocarp layer was investigated to explain the experimental observation. The results indicated that the spherical shape associated with the thorns and mesocarp materials displayed an excellent energy absorption efficiency that can be mimicked to design an effective bio-inspired absorber for packing applications.

Tunable Energy Absorption Characteristics of Architected Honeycombs Enabled via Additive Manufacturing

Tailoring of material architectures in three-dimensions enabled by additive manufacturing (AM) offers the potential to realize bulk materials with unprecedented properties optimized for location-specific structural and/or functional requirements. Here we report tunable energy absorption characteristics of architected honeycombs enabled via material jetting AM. We realize spatially tailored 3D printed honeycombs (guided by FE studies) by varying the cell wall thickness gradient and evaluate experimentally and numerically the energy absorption characteristics. The measured response of architected honeycombs characterized by local buckling (wrinkling) and progressive failure reveals over 110% increase in specific energy absorption (SEA) with a concomitant energy absorption efficiency of 65%. Design maps are presented that demarcate the regime over which geometric tailoring mitigates deleterious global buckling and collapse. Our analysis indicates that an energy absorption efficiency as high as 90% can be achieved for architected honeycombs, whereas the efficiency of competing microarchitected metamaterials rarely exceeds 50%. The tailoring strategy introduced here is easily realizable in a broad array of AM techniques, making it a viable candidate for developing practical mechanical metamaterials.