Structure and Mechanical Properties

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== ABSTRACT ==
 
  
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
 
 
Author(s): [http://www.hpc.msstate.edu/directory/information.php?eid=226 Hongjoo Rhee], [http://www.hpc.msstate.edu/directory/information.php?eid=63 Mark F. Horstemeyer], Y. Hwang, H. Lim, H. El Kadiri, W. Trim
 
 
Corresponding Author: [mailto:hrhee@cavs.msstate.edu Hongjoo Rhee]
 
 
 
== STRUCTURE ==
 
 
 
[[image:turtle01.jpg|thumb|350px|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).
 
 
 
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.
 
 
 
Figs. 3 and 4 show the microstructure observation and chemical
 
analysis results obtained from various surfaces of the turtle shell.
 
Three different layers of the outermost keratin layer, right underneath
 
the keratin layer, and the inside surface of the turtle shell carapace
 
were observed and analyzed by using an SEM and an energy dispersive
 
X-ray (EDX) spectroscopy technique, respectively. These layers
 
have different surface microstructures and chemical compositions.
 
The EDX analysis showed that the outermost keratin layer mainly
 
consists of carbon (C), oxygen (O), nitrogen (N), and sulfur (S) that are
 
main constituents of the protein. The result is not surprising since the
 
keratins are a family of fibrous structural proteins, also called scleroproteins.
 
Unlike the outermost keratin layer, right underneath the
 
keratin layer and the inside surface of the turtle shell carapace contained
 
abundant additional minerals as indicated by the presence of
 
calcium (Ca, 15–20 wt.%), phosphorous (P, 7–10 wt.%), sodium (Na),
 
chlorine (Cl), and magnesium (Mg) that are known to be main components
 
of the bone.
 
 
 
{|
 
| [[image:turtle02.jpg|thumb|250px|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.]]
 
| [[image:turtle03.jpg|thumb|195px|Fig. 3. SEM micrographs obtained from different surfaces of the turtle shell carapace;
 
(a) the outermost keratin layer, (b) underneath the keratin layer, and (c) inside
 
surface. ]]
 
| [[image:turtle04.jpg|thumb|183px|Fig. 4. Chemical analysis results obtained from different surfaces of the turtle shell
 
carapace; (a) the outermost keratin layer, (b) underneath the keratin layer, and
 
(c) inside surface.]]
 
|}
 
 
The microstructures and chemical analysis results obtained
 
from different locations of the fracture surfaces of the turtle shell
 
carapace are provided in Figs. 5 and 6. The chemical compositions
 
obtained from the exterior layers and the network (e.g. closed-cell
 
wall) region within the foam-like interior layer were quite similar
 
to those can be found in Fig. 4b–c. The fibers inside of the closedcell
 
also showed an accordant chemical composition (Fig. 6b),
 
which implies that they include “bony” fibers. The microstructure
 
observation and chemical analysis results obtained from various
 
locations of the turtle shell clearly revealed that the turtle shell
 
carapace is made of a sandwich composite structure having exterior
 
lamellar bone layers and an interior bony network of closedcell
 
fibrous foam layer.
 
 
{|
 
| [[image:turtle05.jpg|thumb|250px|Fig. 5. SEM micrographs obtained from the fracture surface of the turtle shell carapace;
 
(a) bony layers and (b) inside of the closed-cell (fibers).]]
 
| [[image:turtle06.jpg|thumb|250px|Fig. 6. Chemical analysis results obtained from the fracture surface of the turtle shell
 
carapace; (a) bony layers and (b) inside of the closed-cell (fibers). ]]
 
|}
 
 
 
 
== MECHANICAL PROPERTIES ==
 

Latest revision as of 17:26, 7 February 2011

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