Cholla cactus wood (Cylindropuntia imbricata): Hierarchical structure and micromechanical properties.

The Cholla cactus is a species of cacti that survives in arid environments and produces a unique mesh-like porous wood. In this article, we present a comprehensive investigation on the hierarchical structure and micromechanical properties of the Cholla cactus wood. Multiple approaches consisting of X-ray tomography, scanning electron microscopy, scanning probe microscopy, nanoindentation, and finite element simulations were used to gain insight into the structure, property, and design principles of the Cholla cactus wood. The microstructure of the Cholla cactus wood consists of different components, including vessels, rays, and fibers. In the present study, we quantitatively describe the structure of each of these wood components and their likely functions, both from the perspective of biological and mechanical behavior. Nanoindentation experiments revealed for the first time that the cell walls of the fibers exhibit stiffness and hardness higher than those of rays. Furthermore, the idea of making porous, thin-walled cylinders was abstracted from the design of vessel elements, and the structures inspired by this principle were studied in tensile and torsional loading conditions using finite element simulations. Finite element simulations revealed that the utilization of a larger volume of material to carry the load leads to an increase in toughness of these structures, and thus suggested that the pores should be architected to maximize the distribution of load. STATEMENT OF SIGNIFICANCE: The Cholla cactus wood possess a unique hierarchical structure that enables it to thrive in arid environments. Our correlative microscopy approach reveals incredible strategies that individual wood components exhibit to enable the survival of Cholla cactus in extreme environments. The present work quantifies the microstructure and mechanical properties of this very interesting natural system. We further investigate a design principle inspired by the vessel elements, one of the wood components of Cholla cactus, using finite element simulations. The study presented here advances our understanding of the structural significance of Cholla cactus and potentially other desert plants and will further help design architected structural materials.

In situ investigations of failure mechanisms of silica fibers from the venus flower basket (Euplectella Aspergillum).

The fibers of the deep-sea sponge Euplectella aspergillum exhibit exceptional mechanical properties due to their unique layered structure at a micrometer length scale. In the present study, we utilize a correlative approach comprising of in situ tensile testing inside a scanning electron microscope (SEM) and post-failure fractography to precisely understand mechanisms through which layered architecture of fibers fracture and improves damage tolerance in tensile loading condition. The real-time observation of fibers in the present study confirms for the first time that the failure starts from the surface of fibers and proceeds to the center through successive layers. The concentric layers surrounding the central core sacrifice themselves and protect the central core through various toughening mechanisms like crack deflection, crack arrest, interface debonding, and fiber pullout. STATEMENT OF SIGNIFICANCE: Biological materials often exhibit multiscale hierarchical structures that can be incorporated into the design of next generation of engineering materials. The fibers of deep-sea sponge E. aspergillum possess core-shell like layered architecture. Our in situ study reveals astounding strategies by which this architecture delays the fracture of the fiber. The core-shell architecture of these fibers behaves like fiber-reinforced ceramic matrix composite, where the outer shells act as a matrix and the central core acts as a fiber. The outer shells take the environmental brunt and scarify themselves to protect the central core. The precise understanding of damage evolution presented here will help to design architected materials for load-bearing applications.

The Comparative approach to bio-inspired design: integrating biodiversity and biologists into the design process.

Biodiversity provides a massive library of ideas for bio-inspired design, but the sheer number of species to consider can be daunting. Current approaches for sifting through biodiversity to identify relevant biological models include searching for champion adapters that are particularly adept at solving a particular design challenge. While the champion adapter approach has benefits, it tends to focus on a narrow set of popular models while neglecting the majority of species. An alternative approach to bio-inspired design is the comparative method, which leverages biodiversity by drawing inspiration across a broad range of species. This approach uses methods in phylogenetics to map traits across evolutionary trees and compare trait variation to infer structure-function relationships. Although comparative methods have not been widely used in bio-inspired design, they have led to breakthroughs in studies on gecko-inspired adhesives and multifunctionality of butterfly wing scales. Here we outline how comparative methods can be used to complement existing approaches to bioinspired design, and we provide an example focused on bio-inspired lattices, including honeycomb and glass sponges. We demonstrate how comparative methods can lead to breakthroughs in bio-inspired applications as well as answer major questions in biology, which can strengthen collaborations with biologists and produce deeper insights into biological function.

Bioinspired Honeycomb Core Design: An Experimental Study of the Role of Corner Radius, Coping and Interface.

The honeybee's comb has inspired the design of engineering honeycomb core that primarily abstract the hexagonal cell shape and exploit its mass minimizing properties to construct lightweight panels. This work explored three additional design features that are part of natural honeybee comb but have not been as well studied as design features of interest in honeycomb design: the radius at the corner of each cell, the coping at the top of the cell walls, and the interface between cell arrays. These features were first characterized in natural honeycomb using optical and X-ray techniques and then incorporated into honeycomb core design and fabricated using an additive manufacturing process. The honeycomb cores were then tested in out-of-plane compression and bending, and since all three design features added mass to the overall structure, all metrics of interest were examined per unit mass to assess performance gains despite these additions. The study concluded that the presence of an interface increases specific flexural modulus in bending, with no significant benefit in out-of-plane compression; coping radius positively impacts specific flexural strength, however, the corner radius has no significant effect in bending and actually is slightly detrimental for out-of-plane compression testing.

An Examination of the Low Strain Rate Sensitivity of Additively Manufactured Polymer, Composite and Metallic Honeycomb Structures.

The characterization of additively manufactured cellular materials, such as honeycombs and lattices, is crucial to enabling their implementation in functional parts. One of the characterization methods commonly employed is mechanical testing under compression. This work focuses specifically on the dependence of these tests to the applied strain rate during the test over low strain rate regimes (considered here as 10-6 to 10-1 s-1). The paper is limited to the study of strain the rate dependence of hexagonal honeycomb structures manufactured with four different additive manufacturing processes: one polymer (fused deposition modeling, or material extrusion with ABS), one composite (nylon and continuous carbon fiber extrusion) and two metallic (laser powder bed fusion of Inconel 718 and electron beam melting of Ti6Al4V). The strain rate sensitivities of the effective elastic moduli, and the peak loads for all four processes were compared. Results show significant sensitivity to strain rate in the polymer and composite process for both these metrics, and mild sensitivity for the metallic honeycombs for the peak load. This study has implications for the characterization and modeling of all mechanical cellular materials and makes the case for evaluation and if appropriate, inclusion, of strain rate effects in all cellular material modeling.

Four Questions in Cellular Material Design.

The design of cellular materials has recently undergone a paradigm shift, enabled by developments in Additive Manufacturing and design software. No longer do cellular materials have to be limited to traditional shapes such as honeycomb panels or stochastic foams. With this increase in design freedom comes a significant increase in optionality, which can be overwhelming to the designer. This paper aims to provide a framework for thinking about the four key questions in cellular material design: how to select a unit cell, how to vary cell size spatially, what the optimal parameters are, and finally, how best to integrate a cellular material within the structure at large. These questions are posed with the intent of stimulating further research that can address them individually, as well as integrate them in a systematic methodology for cellular material design. Different state-of-the-art solution approaches are also presented in order to provoke further investigation by the reader.