Experiments-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 <ref> D.R Katti, S.M. Pradhan, K.S. Katti, Rev. Adv. Mater Sci. 6(2004) 162.</ref>. 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|275px|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|215px|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|210px|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.]]
 
|}
 
[[image:turtle05_06.jpg|thumb|right|300px|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). 
 
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). ]]
 
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.
 
 
== Mechanical Properties ==
 
 
[[image:turtle07.jpg|thumb|400px|Fig. 7. Indentation test results obtained from (a) nano-indentation and (b) Vickers hardness tests on the side surface of the turtle shell carapace. ]]
 
[[image:turtle08.jpg|thumb|400px|Fig. 8. Quasi-static compression test results on the turtle shell carapace coupon specimens under various strain rates and specimen geometries; (a) stress versus strain curves
 
and (b) specific energy absorption as a function of density. ]]
 
 
Experimental results obtained from the nano- and microindentation
 
tests on the side surfaces of the turtle shell carapace
 
are provided in Fig. 7. The results showed that the exterior layers
 
and interior bony closed-cell walls possess comparable hardness
 
and modulus values. Hardness and elastic modulus values obtained
 
from the nano-indentation tests ranged from 0.8–1.1 GPa and 18.3–
 
24.8 GPa, respectively; whereas, the average hardness value obtained
 
from the Vickers hardness tests was about Hv100 that
 
corresponds to 0.98 GPa. There were small variations in hardness
 
and elastic modulus values from experiments due to the roughness
 
of the specimen. The nano-indentation test results reflect highly
 
localized micromechanical properties that may contain porous or
 
impurities in its texture. Since the regions of indentation are so
 
small that local impurities or defects can induce uncertainties in the
 
measurements. This effect is minimized under Vickers hardness
 
test set-up and the exterior layers and closed-cell walls within an
 
interior layer possessed comparable hardness values.
 
 
 
For quasi-static compression tests, two different types of coupon
 
specimens including all three layers and then only a bony exterior
 
layer were prepared. The effect of strain rate on the mechanical
 
behavior of the turtle shell was compared with respect to the different
 
density levels and the raw data obtained from the tests is illustrated in
 
Fig. 8a. The lower five curves (represented by lines with symbols)
 
were obtained from the test specimens including all three layers (two
 
exterior and an interior layers); whereas the upper six curves were
 
obtained from the specimens only containing a relatively denser exterior
 
layer. Top three curves (in symbols) among those six curves
 
were obtained from thinner specimens and the bottom three curves
 
(in lines) represent thicker specimens. The thickness difference
 
between those two regimes was about 15%. The favorable deformation
 
mechanism of the turtle shell carapace under quasi-static compression
 
test conditions can be explained by importing that of synthetic
 
foams and/or honeycombs since fundamental structures of the test
 
specimens are similar to those of such cellular solids. At small strains,
 
the specimens were deformed in a linear elastic manner due to the cell
 
wall bending<ref>L.J. Gibson, M.F. Ashby, Cellular Solids: Structure and Properties – Second Edition. Cambridge University Press, Cambridge, U.K., 1997.</ref>. Soon after the initial linear elastic deformation, a
 
plateau of deformation was reached, because of the buckling of the
 
cell walls. After such a plateau of deformation, another period of linear
 
deformation was proceeded since a densification occurred resulting
 
in a rapid increase of compressive stress. When comparing the specimens
 
containing the exterior region only, the thicker specimens
 
showed a similar deformation yet much weaker behavior than those
 
can be observed in the specimens including all three layers; whereas
 
the thinner specimens showed almost a linear compressive deformation
 
behavior simply because of the density and structure differences.
 
Most of discernible pores within exterior layer are distributed near
 
the region between the exterior layer and interior foam-like layer.
 
Fig. 8b provides the comparison of specific energy absorption obtained
 
from the quasi-static compression test results (Fig. 8a). Density
 
and porosity levels of the test specimens were already considered
 
in this normalized data. The energy absorption ability of the turtle
 
shell carapace increased with increasing strain rate for a given density
 
level. The composite layers including all three layers showed better
 
energy absorption ability compared to the exterior layer for any given
 
strain rate. In addition, such composite layers possessed a considerable
 
amount of plateau of deformation that is a model index of good
 
energy absorbing materials. The combining information of these two
 
plots in Fig. 8 is very important to design the optimum energy absorbing
 
composite material. For example, composite foam materials
 
can be tailored to give the best combination of properties for a given
 
package by choosing the right combination of the cell wall materials,
 
relative density, reinforcement phases, and so on.
 
 
== References ==
 
<references/>
 
 
 
 
Citation: H. Rhee, M.F. Horstemeyer, Y. Hwang, H. Lim, H. El Kadiri, and W. Trim, “A study on the structure and mechanical behavior of the ''Terrapene carolina'' carapace: a pathway to design bio-inspired synthetic composites,” Materials Science and Engineering C 29 (2009) 2333-2339.
 
 
[[Category: Biomaterials]]
 
[[Category: Macroscale]]
 

Latest revision as of 13:46, 1 June 2015

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