Experiments-Structure-Property Relations

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Abstract

Multiscale hierarchical structures, materials properties, and mechanical behaviors of the nine-banded armadillo (Dasypus novemcinctus) shell were studied to provide fundamental knowledge for understanding biological composite systems. The nine-banded armadillo’s dermal shell is characterized into three regions: the forward, band, and rear shells. The forward and rear shells comprise a sandwich composite structure of functionally graded material having relatively denser exterior bony layers and an interior bony network of foam. The forward and rear shell’s strength (~ 1500MPa) was greater than the intermediate band shell (~ 500MPa). The band shell revealed a more complicated structure where adjacent bands are partially overlapped and connected with each other to provide flexibility in addition to protection. Hardness tests showed that the top surfaces of each shell had hardness (~ Hv50) greater than the front and side surfaces (~ Hv40). Compression test results on the forward and rear shells showed a typical nonlinear deformation behavior similar to synthetic foams, where microbuckling is a key inelastic deformation mechanism. A comparison and contrasting study of the structure-property relations between the armadillo shell and other biological structural materials could provide fundamental understandings for deformation mechanisms that can lead to the development of novel bio-inspired safety system design methodologies.

Introduction

Many scientists and engineers today are extensively studying nature to gain design insights and methodologies that will help create novel materials and design inspiration. The solutions inspired by natural selection are often a good head start in the search for answers to scientific and engineering problems. Biomimetics is the application of biological methods and systems found in nature to the study and design of engineering systems and modern technology. This new field of science encompasses biologically inspired and mimicked technologies and represents the study and imitation of nature’s methods, designs, and processes [1-3]. Bio-inspired research or biomimetics involves the study and distillation of principles and functions found in biological systems; application of this knowledge produces novel and exciting basic technologies and new

approaches to solve scientific problems.
Armadillo

Most biological materials are complex composites that are hierarchically structured. In nature, a hierarchy of different structural composite structures can be found in various animals, such as sea shells, crab cuticle, turtle shell, ram horn, elk antler, etc [4-5]. These biological structural systems are typically designed for strength, stiffness, and energy absorption (resistance to penetration events). In our study, we are exclusively focusing on the structures, materials properties, and mechanical behavior of the armadillo shell and characterizing its role for protection.

Armadillos are small placental mammals of the order Cingulata, known for having a leathery armor shell. There are approximately 10 extant genera and around 20 extant species of armadillo, some of which are distinguished by the number of bands on their armor. Nine-banded armadillos (Dasypus novemcinctus) are found in South, Central, and North America and have the largest range of any extant species of armadillo. In the United States, the sole resident armadillo is the nine-banded armadillo. Like many other armadillos, the nine-banded armadillo is covered by an outer body armor made up of bony plates covered in a leathery keratinous skin. The armor is formed by plates of dermal bone covered in relatively small, overlapping epidermal scales called scutes composed of bone with a covering of keratin. In most species, there are rigid shields over the shoulders and hips with a number of bands separated by a flexible skin covering the back and flanks. These osteodermal scales provide a hard but flexible covering and additional bony/keratinous scales partially cover the top of the head, the upper parts of the limbs, and the tail. The underside of the nine-banded armadillo lacks armored protections and is simply covered with soft skin and fur [6-9].

Several studies on the characteristics of leprosy in the armadillo and human contraction from infected armadillos have been reported in the literature since the armadillos are highly endemic, natural hosts of leprosy, and the only mammals besides humans to suffer from lepromatous leprosy [10-15]. However, the structure and mechanical responses of the armadillo shells surprisingly have not been studied, although they possess superior armor behavior against environmental threats. Joyce et al. [16] reported that the vertically oriented dorsal ribs of the primitive turtle lineage, Chinlechelys tenertesta, obtained from a thin-shelled fossil from the Upper Triassic era are only poorly associated with the overlying costal bones indicating that these two structures are independent ossifications in basal turtles. They hypothesized that turtles ultimately originated from something that looked like an armadillo. The main objective of the present study is to quantify the structure-property relationships of the armadillo shell based on the materials and geometric characteristics. Since no structure-property relations have been analyzed on the armadillo shell, there exists a lack of experimental database of such biological structural material. Moreover, there is no systematic research on the nine-banded armadillo shell to understand biological pathways to create bio-inspired synthetic counterparts. Therefore, a comparison-contrasting study of the structure-property relations between the armadillo shell and other biological structural materials, especially a turtle shell, could provide understanding of mechanisms that can lead to the development of novel bio-inspired safety system design methodologies.

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