Materials 2015, 8, 6154-6162; doi:10.3390/ma8095298
OPEN ACCESS
materials
ISSN 1996-1944
www.mdpi.com/journal/materials
Article
Three-Point Bending Fracture Behavior of Single Oriented
Crossed-Lamellar Structure in Scapharca broughtonii Shell
Hong-Mei Ji 1 , Wen-Qian Zhang 1 , Xu Wang 1 and Xiao-Wu Li 1,2, *
1
Institute of Materials Physics and Chemistry, College of Sciences, Northeastern University,
Shenyang 110819, China; E-Mails: [email protected] (H.-M.J.);
[email protected] (W.-Q.Z.); [email protected] (X.W.)
2
Key Laboratory for Anisotropy and Texture Engineering of Materials, Ministry of Education,
Northeastern University, Shenyang 110819, China
* Author to whom correspondence should be addressed; E-Mail: [email protected];
Tel.: +86-24-8367-8479.
Academic Editor: Juergen Stampfl
Received: 29 July 2015 / Accepted: 6 September 2015 / Published: 15 September 2015
Abstract: The three-point bending strength and fracture behavior of single oriented
crossed-lamellar structure in Scapharca broughtonii shell were investigated. The samples
for bending tests were prepared with two different orientations perpendicular and parallel
to the radial ribs of the shell, which corresponds to the tiled and stacked directions of the
first-order lamellae, respectively. The bending strength in the tiled direction is approximately
60% higher than that in the stacked direction, primarily because the regularly staggered
arrangement of the second-order lamellae in the tiled direction can effectively hinder the
crack propagation, whereas the cracks can easily propagate along the interfaces between
lamellae in the stacked direction.
Keywords: Scapharca broughtonii shell; crossed-lamellar structure; three-point bending;
strength; fracture
1. Introduction
Over billions of years of evolution, biological shells have been highly mineralized to develop into the
materials with outstanding mechanical performances. Despite the fact that most of these materials consist
Materials 2015, 8
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of ordinary inorganic calcium carbonate (CaCO3 ) and biological macromolecules, their mechanical
properties are far superior to those for the single crystals of the pure mineral [1–5].
As is well known, the crossed-lamellar structure, which is the most common structure in classes
Gastropod and Bivalvia [2,6,7], is renewed for its fantastic micro-architecture and corresponding
excellent mechanical properties. Particularly, the coarsest structures of shells including several
macrolayers arranged in either “weak” or “tough” orientations with respect to the potential crack
growth direction, have drawn a great deal of attention in recent years [2,8–13]. For instance, in
Strombus gigas shells with three macrolayers, the crossed-lamellar structures in inner, middle and
outer layers are arranged in a 0˝ /90˝ /0˝ mode, meaning that the overall arrangements of the lamellae
in the middle macrolayer have an 90˝ rotation about the axis perpendicular to the outer surface of
shell to those in the inner and outer layers. The results show that the strength is higher along the
orientation parallel to the spiral axis than that perpendicular to the spiral axis under both uniaxial
compression and three-point bending tests [10,12]. Fracturing in the “weak” and “tough” layers have
been quantitatively understood by different energy-dissipating mechanisms in Strombus gigas shells, i.e.,
crack bridging and microcracking in “tough” layer account for a larger portion of the dissipated energy,
while multiply-channel cracking in the “weak” layer just absorbs a small amount of energy [2,8,9,11].
Recently, we also performed compressive tests on Veined rapa whelk shell with two macrolayers (the
arrangement of crossed-lamellar structures are mutually perpendicular in these two layers) using four
kinds of specimens with loading axis making different angles (α = 0˝ , 30˝ , 60˝ and 90˝ ) with the spiral
lines. It was found that the interfaces of different-level lamellae in the adjacent macro-layers yield
significant effects on the mechanical behavior in a coordinated fashion [13].
There are also some shells, in which the crossed-lamellar structure is oriented merely in one
orientation (herein called single oriented crossed-lamellar structure) [14–18]. However, there is much
less information about the fracture mechanisms of single oriented crossed-lamellar structure. Thus, in
the present work, the three-point bending fracture behavior of Scapharca broughtonii shell samples
with only one macrolayer was investigated. It is expected that these studies can further reveal the
fracture mechanisms in the crossed-lamellar structure, and provide a theoretical basis for developing
biomimetic materials.
2. Experimental Section
Scapharca broughtonii shell, which is a member of the cardiidae family of the Bivalvia class, was
adopted as the target material in the present work. The shell was dried at room temperature for several
days. As shown in Figure 1a, many obvious radial ribs are arranged on the outer surface of this shell.
All investigated specimens were cut from the middle part of this shell.
The directly broken cross-sectional specimens (Figure 1a) with the fracture surfaces perpendicular or
parallel to the radial ribs (thereafter named as perpendicular or parallel orientations) were prepared for
scanning electron microscope (SEM) (Carl Zeiss, Jena, Germany) observations.
Specimens for three-point bending tests were firstly cut with a water-cooled low-speed diamond saw,
and then grounded carefully with emery papers from 600 # to 3000 #. Figure 1b shows the dimensions
(4.5 mm ˆ 1.5 mm ˆ 25 mm) of the samples, and the samples with the long side perpendicular and
parallel to the radial ribs are, herein, named as perpendicular and parallel samples respectively, as
Materials 2015, 8
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Materials 2015, 8
3
marked in Figure 1a. The three-point bending tests were conducted under a constant loading rate of
5 ˆ 10´3 mm/s
a loading
span
20 mm
on Carebending
EUT-1020
machine (with
respectively,
as with
marked
in Figure
1a.ofThe
three-point
teststesting
were conducted
undera amaximum
constant
´3
−3
load of rate
2000ofN5 and
accuracy
kN)
(Care,
as showntesting
in Figure
1c,d. (with
The
loading
× 10an mm/s
withofa 10
loading
span
of 20Tianjin,
mm onChina),
Care EUT-1020
machine
−3
strength
σbb2000
was N
calculated
using theofcommon
equationChina),
[19]: as shown in Figure 1c,d.
abending
maximum
load of
and an accuracy
10 kN)flexure
(Care, Tianjin,
The bending strength σbb was calculated using the common2flexure equation [19]:
σbb “ 3P L{2bh
(1)
2
σ bb  3PL / 2bh
(1)
where P is the maximum load, L the loading span, and b and h are the width and thickness of
where
P respectively.
is the maximum load, L the loading span, and b and h are the width and thickness of
samples,
samples,
respectively.
The obtained
bending strengths were further analyzed by means of the Weibull Equation [20]:
The obtained bending strengths were further analyzed by means of the Weibull Equation [20]:
F pV
expr´
pσ/bbσ{σ) m0 q]m s
(2)
F (qV“
) 11´ exp[
 (σ bb
(2)
0
where F(V)
F(V) is
is the
the failure
failure probability,
probability, m the Weibull modulus, and
and σ
σ0 is the characteristic strength.
Figure 1. (a) Overall view of Scapharca broughtonii shell; (b) dimensions of sample for
Figure 1. (a) Overall view of Scapharca broughtonii shell, (b) dimensions of sample for
three-point bending test, and (c,d) the experimental equipment for three-point bending tests.
three-point bending test, and (c,d) the experimental equipment for three-point bending tests.
Results and
and Discussion
Discussion
3. Results
The SEM
the the
crosscross
sections
perpendicular
and parallel
to the radial
ribsradial
of Scapharca
SEM images
imagesof of
sections
perpendicular
and parallel
to the
ribs of
broughtonii
shell
are
shown
in
Figure
2a,b,
respectively.
Obviously,
most
of
areas
exhibit
Scapharca broughtonii shell are shown in Figure 2a,b, respectively. Obviously, most of areas exhibit a
crossed-lamellar structure comprising the inorganic phase of aragonite CaCO3, as demonstrated by the
X-ray diffraction (XRD) (PANalytical, Almelo, The Netherlands) analysis in Figure 2c. It is well known
Materials 2015, 8
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4
X-ray diffraction (XRD) (PANalytical, Almelo, The Netherlands) analysis in Figure 2c. It is well known
that the
the crossed-lamellar
crossed-lamellar structure
structure can
can be
be divided
divided into
into three-order
three-order lamellae,
lamellae, i.e.,
i.e., the
the first-order
first-order lamella
lamella
that
is composed
composed of
of the
the second-order
second-order lamella,
lamella, which
which consists
consists further
further of
of the
the third-order
third-order lamella
lamella [2,8–18].
[2,8–18].
is
The stereoscopic
stereoscopic schematic
schematic diagram
diagram presented
presented in
in Figure
Figure 2d
2d visually
visually illustrates
illustrates the
the architecture
architecture of
of the
the
The
Scapharca broughtonii
broughtonii shell, and it is clearly seen that the edges of the
crossed-lamellar structure in Scapharca
interesting to
to note
note that
that the
the first-order
first-order lamellae are piled up
first-order lamellae are not straight. ItIt isis interesting
along the parallel orientation (herein called stacked direction), while the first-order lamellae are tiled
along the perpendicular orientation (herein called tiled direction).
Figure 2. SEM micrographs of cross sections perpendicular (a) and parallel (b) to the
Figure 2. SEM micrographs of cross sections perpendicular (a) and parallel (b) to the
radial ribs; (c) XRD patterns of the shell; (d) Schematic illustration of the crossed-lamellar
radial ribs. (c) XRD patterns of the shell. (d) Schematic illustration of the crossed-lamellar
structure in the shell.
structure in the shell.
representative
stress—displacement
curves
and Weibull
functions
of the bending
Figure 33shows
showsthethe
representative
stress—displacement
curves
and Weibull
functions
of the
tests perpendicular
and parallel
the radial
ribs,
respectively.
It can beIt seen
thatseen
the that
50%the
fracture
bending
tests perpendicular
and to
parallel
to the
radial
ribs, respectively.
can be
50%
probabilities
(F(V) =(F(V)
50%)= of
the ofbending
tests tests
are equal
to 68.5
˘ 41.5
and
fracture
probabilities
50%)
the bending
are equal
to 68.5
± 41.5
and43.6
43.6˘± 17.2
17.2 MPa
perpendicular and
and parallel
parallelsamples,
samples,respectively.
respectively.Thus,
Thus,
is stronger
in bending
tests
for perpendicular
thisthis
shellshell
is stronger
in bending
tests with
withloading
the loading
direction
perpendicular
to the
radial
ribs,
meanvalue
valueofof bending
bending stress
stress is
the
direction
perpendicular
to the
radial
ribs,
andand
thethemean
significantly higher
higher (~60%)
(~60%) in
in this orientation than
than that in parallel orientation, which is attributed to
the extremely
extremely anisotropic
anisotropic crossed-lamellar
For the
the crossed-lamellar
crossed-lamellar structure
structure in
in aa 0°/90°/0°
0˝ /90˝ /0˝
the
crossed-lamellar structure.
structure. For
arrangement, the mean strength of the tested samples is 24.0–74.0 MPa for Strombus gigas shell [10],
and 107.5–207.0 MPa for Conus striatus shell [21]. Therefore, the bending strength is strongly
Materials 2015, 8
6158
arrangement,
the8mean strength of the tested samples is 24.0–74.0 MPa for Strombus gigas shell [10], and5
Materials 2015,
107.5–207.0 MPa for Conus striatus shell [21]. Therefore, the bending strength is strongly dependent
upon
the species
However,
Currey
and Kohn
[21] and
also Kohn
performed
bending
tests on
dependent
upon of
theshells.
species
of shells.
However,
Currey
[21] three-point
also performed
three-point
˝
Conus
which also
threewhich
macrolayers
withthree
the crossed-lamellar
structure
in a 0 /90˝ /0˝
bendingmiles
testsshell,
on Conus
mileshasshell,
also has
macrolayers with
the crossed-lamellar
arrangement.
Half of whose
test pieces
were
prepared
grinding
the inner
andoff
they
structure in a 0°/90°/0°
arrangement.
Half
of whose
testby
pieces
wereoff
prepared
by layer,
grinding
thefound
inner
that
the
mean
strength
for
the
Conus
miles
shell
samples
with
three
macrolayers
is
54.1
MPa,
but
for
layer, and they found that the mean strength for the Conus miles shell samples with three macrolayers
those
twobut
maccrolayers
it is 194.8
MPa. Thus, the
the crossed-lamellar
structure
is 54.1with
MPa,
for those with
two maccrolayers
it isarrangement
194.8 MPa.ofThus,
the arrangement
of the
also
plays
an
important
role
in
the
bending
mechanical
properties.
crossed-lamellar structure also plays an important role in the bending mechanical properties.
Figure
bending
stress—displacement
curves
and Weibull
plots of plots
bending
Figure 3.3.Representative
Representative
bending
stress—displacement
curves
and Weibull
of
strength
of
Scapharca
broughtonii
shell
samples
with
two
directions
under
three-point
bending strength of Scapharca broughtonii shell samples with two directions under
bending
tests.
three-point
bending tests.
Figure
gives the
Figure 44 gives
the fracture
fracture surface
surface morphologies
morphologies observed
observed along
along different
different directions
directions of
of bending
bending
samples.
Theirregular
irregularzigzag
zigzag
fracture
features
clearly
observed
the thickness
and
samples. The
fracture
features
are are
clearly
observed
both both
alongalong
the thickness
and width
directions
on theonperpendicular
samples
(Figure
4a,c),
as as
compared
samples
width
directions
the perpendicular
samples
(Figure
4a,c),
comparedtotothose
thosefor
for parallel
parallel samples
(Figure 4b,d).
4b,d). From
rough, and
and the
the
(Figure
From Figure
Figure 4e,g,
4e,g, itit can
can be
be detected
detected that
that the
the fracture
fracture pattern
pattern is
is quite
quite rough,
fracture paths
lamellae show
show evidently
evidently step-like
step-like features
features in
in perpendicular
perpendicular samples.
samples.
fracture
paths between
between first-order
first-order lamellae
In contrast,
smooth, and
lamellae comprising
comprising
In
contrast, the
the fracture
fracture surface
surface is
is relatively
relatively smooth,
and partially
partially tiled
tiled first-order
first-order lamellae
successive third-order
third-order lamellae
lamellae are
are obviously
obviously presented
presented in
in parallel
parallel samples,
samples, as
as indicated
indicated in
in Figure
Figure 4f,h.
4f,h.
successive
In Scapharca
broughtonii shell,
shell, considering
considering the
the potential
potential catastrophic
catastrophic crack
crack propagation
propagation direction
direction
In
Scapharca broughtonii
under three-point
three-point bending
bending tests,
tests, these
and parallel
parallel samples
samples can
can be
be further
further described
described as
as
under
these perpendicular
perpendicular and
the “tough”
“tough” and
and “weak”
“weak” samples,
samples, respectively.
respectively. Specifically,
in the
the “tough”
“tough” sample,
sample, cracks
cracks propagate
propagate
the
Specifically, in
along the
the preferred
preferred direction,
direction, namely,
the interfaces
interfaces between
between the
the second-order
second-order lamellae,
lamellae, as
as shown
shown in
in
along
namely, the
Figure 5a.
5a. However,
However,ininthethe
adjacent
first-order
lamellae,
the second-order
lamellae
are arranged
by˝
Figure
adjacent
first-order
lamellae,
the second-order
lamellae
are arranged
by ˘ 45
± 45° orientation with respect to the loading direction; in this case, cracks propagate first along the
preferred direction (interfaces between the second-order lamellae), but they are subsequently arrested
Materials 2015, 8
6159
orientation
with respect
to the loading direction; in this case, cracks propagate first along the preferred6
Materials 2015,
8
direction (interfaces between the second-order lamellae), but they are subsequently arrested by the
second-order
lamellae
perpendicular
to the to
crack
in theinadjacent
first-order
lamella
(see
by the second-order
lamellae
perpendicular
the propagation
crack propagation
the adjacent
first-order
lamella
the
drawing
in Figure
5a). In5a).
contrast,
sequential
crackingcracking
along thealong
weakthe
interface
between
(seesectional
the sectional
drawing
in Figure
In contrast,
sequential
weak interface
first-order
lamellae occurs
during
bending
in the “weak”
(Figure 5b),
causing
between first-order
lamellae
occurs
during deformation
bending deformation
in theorientation
“weak” orientation
(Figure
5b),
that
onlythat
a relatively
little energy
completely
fracture the
shell.the shell.
causing
only a relatively
littlecan
energy
can completely
fracture
Figure 4. SEM images of the fracture planes of perpendicular (a,c,e,g) and parallel (b,d,f,h)
Figure 4. SEM images of the fracture planes of perpendicular (a,c,e,g) and parallel
samples of Scapharca broughtonii shell.
(b,d,f,h) samples of Scapharca broughtonii shell.
Compared
Compared with
with the
the fracture
fracture behavior
behavior of
of the
the “weak”
“weak” layer
layer in
in Strombus
Strombus gigas
gigas shell
shell including
including three
three
macrolayers
channel
cracks
observed
in the
Scapharca
broughtonii
shell.
macrolayers [2,8,9,11],
[2,8,9,11],there
thereare
arenono
channel
cracks
observed
in current
the current
Scapharca
broughtonii
The
experimental
resultsresults
indicate
that the
energy-dissipating
mechanism
in Strombus
gigas gigas
shell
shell.above
The above
experimental
indicate
that
the energy-dissipating
mechanism
in Strombus
with
suggested
by Kessler
et al. [8]
notdoes
apply
the current
oriented
shell three
with macrolayers
three macrolayers
suggested
by Kessler
et does
al. [8]
notto apply
to thesingle
current
single
crossed-lamellar
structure structure
in Scapharca
broughtonii
shell, where
cracks cannot
form inform
the
oriented crossed-lamellar
in Scapharca
broughtonii
shell,channel
where channel
cracks cannot
“weak”
orientation
duringduring
bending
deformation.
Specifically,
in the current
Scapharca
broughtonii
shell,
in the “weak”
orientation
bending
deformation.
Specifically,
in the current
Scapharca
broughtonii
the
crossed-lamellar
structurestructure
is oriented
single direction,
in the
deficiency
of channel
cracks
shell,
the crossed-lamellar
is inoriented
in singleresulting
direction,
resulting
in the
deficiency
of
channel cracks in “weak” orientation due to lacking of the arrestment of the “tough” layer.
Accordingly, the primary fracture resistance (or mechanism) in single oriented crossed-lamellar
Materials 2015, 8
6160
in “weak”2015,
orientation
due to lacking of the arrestment of the “tough” layer. Accordingly, the primary
Materials
8
7
fracture resistance (or mechanism) in single oriented crossed-lamellar structure is closely related to the
fact that the
staggered
arrangement
second-order
lamellae
among theofadjacent
first-order
structure
is regularly
closely related
to the
fact that of
thethe
regularly
staggered
arrangement
the second-order
lamellae among
can effectively
hinder
the cracklamellae
propagation.
lamellae
the adjacent
first-order
can effectively hinder the crack propagation.
Figure 5.
Schematics of
of the
the principal
principal fracture
fracture mechanisms
mechanisms in
in Scapharca
Scapharca broughtonii
broughtonii shell
shell
Figure
5. Schematics
samples prepared
prepared perpendicular
perpendicular (a)
samples
(a) and
and parallel
parallel (b)
(b) to
to the
the radial
radial ribs.
ribs.
Conclusions
4. Conclusions
a typical
crossed-lamellar
structure,
which
is oriented
in single
Scapharca broughtonii
broughtoniishell
shellpresents
presents
a typical
crossed-lamellar
structure,
which
is oriented
in
direction.
As theAsthree-point
bending
sample
is prepared
parallel
to the
ribs,ribs,
the the
long
axisaxis
of
single
direction.
the three-point
bending
sample
is prepared
parallel
to radial
the radial
long
sample
is exactly
the the
stacked
direction
of the
lamellae,
for which
the cracks
propagate
along
of
sample
is exactly
stacked
direction
of fist-order
the fist-order
lamellae,
for which
the cracks
propagate
the interfaces
betweenbetween
lamellaelamellae
to cause easily
a rapid
fracture.
Asfracture.
the sample
prepared
along
the interfaces
to cause
easily
a rapid
Asisthe
sampleperpendicular
is prepared
to the radial ribs,
theradial
long ribs,
axis the
of sample
is exactly
theistiled
direction
of the
fist-order
lamellae;
in
perpendicular
to the
long axis
of sample
exactly
the tiled
direction
of the
fist-order
lamellae;
in this
case, staggered
the regularly
staggeredofarrangement
second-order
lamellae block
can effectively
this case, the
regularly
arrangement
second-orderoflamellae
can effectively
the crack
block
the crack
propagation, increasing
thus significantly
increasing
the resistance
propagation,
thus significantly
the resistance
to fracture.
Therefore, tothefracture.
bending Therefore,
strength is
the
bending strength
is significantly
higher (~60%)
in the than
perpendicular
orientation
than that in the
significantly
higher (~60%)
in the perpendicular
orientation
that in the parallel
orientation.
parallel orientation.
Acknowledgments
Acknowledgments
This work was financially supported by the Fundamental Research Funds for the Central University
of China
underwas
Grant
nos. N110105001
N120505001,
and
partiallyFunds
by thefor
National
Natural
Science
This work
financially
supported and
by the
Fundamental
Research
the Central
University
Foundation
of China
(NSFC)
under Grant
51231002,
andthe
51571058.
Xiao-Wu
Li is
of
China under
Grant nos.
N110105001
and nos.
N120505001,
and51271054
partially by
National Natural
Science
grateful for this
support.
Foundation
of China
(NSFC) under Grant nos. 51231002, 51271054 and 51571058. Xiao-Wu Li is
grateful for this support.
Author Contributions
Author Contributions
Xiao-Wu Li designed the scope of the paper. Hong-Mei Ji, Wen-Qian Zhang and Xu Wang
participated
sample the
preparation
experiments.
Hong-Mei
Ji and Zhang
Xiao-Wu
analyzed
Xiao-Wu in
Lithe
designed
scope ofand
theallpaper.
Hong-Mei
Ji, Wen-Qian
andLiXu
Wang
the experimental
Ji and
drafted
the initial manuscript
and
Li shaped
it into the
participated
in theresults.
sampleHong-Mei
preparation
all experiments.
Hong-Mei
Ji Xiao-Wu
and Xiao-Wu
Li analyzed
final form. results. Hong-Mei Ji drafted the initial manuscript and Xiao-Wu Li shaped it into the
experimental
final form.
Materials 2015, 8
6161
Conflicts of Interest
The authors declare no conflict of interest.
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