Structure and Mechanical Properties

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The multiscale structure, materials properties, and mechanical responses of the turtle shell (Terrapene carolina) were studied to understand the fundamental knowledge of naturally occurring biological penetrator-armor systems. The structure observation and chemical analysis results revealed that the turtle shell carapace comprises a multiphase sandwich composite structure of functionally graded material having exterior bone layers and a foam-like bony network of closed-cells between the two exterior bone layers. Although the morphology was quite different, the exterior bone layers and interior bony network possessed comparable hardness and elastic modulus values of ~1 GPa and ~20 GPa, respectively. Compression and flexure test results showed a typical nonlinear deformation behavior recognizant of man-made foams. The mechanical test results revealed that the interior closed-cell foam layer plays a significant role on the overall deformation behavior of the turtle shell. The finite element analysis simulation results showed comparable agreement with the actual experimental test data. This systematic study could provide fundamental understanding for structure-property phenomena and biological pathways to design bio-inspired synthetic composite materials


Fig. 1. Multiscale hierarchy and structure of the turtle shell; (a) a morphology of the turtle shell carapace, (b) a costal scute showing the successive growth pattern, (c) a crosssectional view of the carapace showing composite layers, (d) an SEM micrograph of a fracture surface, (e) an SEM micrograph of a cell structure, and (f) an SEM micrograph of a fibrous structure inside of the cell.
 Structure observations on the turtle shell revealed a multiphase

composite material that is arranged by a multiscale hierarchy. Such a multiscale hierarchical structure of the turtle shell carapace is depicted in Fig. 1. The turtle shell comprises a series of connected individual plates covered with a layer of horny keratinized scutes (Fig. 1a–b). The scutes are made up of a fibrous protein called keratin that also comprises the scales of other reptiles [5]. These scutes overlap the seams between the shell bones and serve to reinforce the overall protection to the shell. The carapace is made of a sandwich composite structure of functionally graded material (FGM) having relatively denser exterior layers and an interior fibrous foam-like layer (Fig. 1c–d). SEM micrographs clearly revealed such fibrous structure inside of the cell (Fig. 1e–f).

Fig. 2. (a) A side sectional view and (b) a top sectional view of the turtle shell carapace coupon obtained from X-ray CT single slice scan showing randomly distributed closed-cell pores within the foam-like interior layer.
 The internal structure of the turtle shell was nondestructively

observed by using an X-ray computed tomography (CT) and obtained images are provided in Fig. 2. The X-ray CT was carried out by using a v|tome|x by phoenix|x-ray. The X-ray CT images clearly showed that the pores within the interior foam-like layer of the turtle shell carapace were closed-cell type and randomly distributed. In addition, the results obtained from the in-house image analyzer software revealed that the porosity levels of the relatively denser exterior, interior foamlike layer, and whole turtle shell carapace including all three layers were 6.86%, 65.5%, and 48.9%, respectively.

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