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applied
sciences
Article
Impact Performance of a Plate Structure with
Coconut-Inspired Microchannels
Jie Shen 1, *, Mark Flores 2 and Andrew Sharits 2
1 Department of Computer & Information Science, University of Michigan-Dearborn,
Dearborn, MI 48128, USA
2 Air Force Research Laboratory, Wright-Patterson Air Force Base, Fairborn, OH 45433, USA;
mark.flores.7@us.af.mil (M.F.); andrew.sharits.ctr@us.af.mil (A.S.)
* Correspondence: shen@umich.edu; Tel.: +1-313-593-5680
Received: 12 November 2019; Accepted: 28 December 2019; Published: 3 January 2020
Abstract: Very few studies have been conducted in the past to reveal the underlying mechanism that
causes the high-impact performance of coconuts as a natural composite material and a structure.
The goal of this study is to understand the dominant microstructures of coconuts with respect to
mechanical impact. In this study, we use a high-resolution X-ray computed tomography (CT) machine
in scanning the microstructures and analyzing the patterns of microchannels in coconut shells. By using
the established patterns, we model a novel plate structure with coconut-inspired microchannels.
Numerical simulation elucidates a significant improvement in crashworthiness. The microchannel
design opens a path to further developing self-healing composites and intelligent composites with
sensing elements embedded in the channels.
Keywords: composites; coconut; impact performance; crashworthiness; biomimetics; industrial
applications
1. Introduction
Coconut is a natural composite product with better impact performance than human
bone structures. We still do not have full knowledge about the multiscale patterns and structures of
this composite. Although coconut fibers have been used in man-made composites, the underlying
hierarchical structure of the coconut shell was seldom investigated. There is almost no previous
effort in modeling three-dimension (3D) coconut composite patterns and structures with respect to
mechanical impact behavior. This calls for an investigation on the coconut composite patterns for
better impact performance.
2. Literature Review
Coconut fibers have been used as matrix reinforcement for polymers. A group of scientists [1–8]
evaluated the structural characteristics and mechanical performance of coir fiber/thermoset
polymer composites. Tensile and flexural strength as well as impact performance of the composites
were measured. In some studies, scanning electron microscopy or X-ray dispersion analysis
was used. Other researchers [9–13] investigated the mechanical performance of coir fiber/thermoplastic
polymer composites. The chopped dust [14,15] of coir fibers or coconut shells was also used with
polymers to form composites.
Gludovatz et al. [16] conducted an innovative study on the multiscale structure of coconut shells.
They examined the structure of coconut shells across multiple length scales through X-ray computed
tomography (CT) and focused ion-beam microscopy. Fracture toughness was investigated on the
latitudinal and longitudinal planes.
Appl. Sci. 2020, 10, 355; doi:10.3390/app10010355 www.mdpi.com/journal/applsci
Appl. Sci. 2020, 10, 355 2 of 15
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Hassan
Hassan et et al.
al.[17]
[17]investigated
investigatedthe theimprovement
improvementininshear shearstrength
strengthofofsoftsoftclay embedded
clay embedded with a
with
single
a singlecrushed
crushed coconut
coconut shell (CCS)
shell (CCS) column.
column. An Anunconfined
unconfined compression
compression testtest
waswasconducted
conductedto
determine the shear strength. The experimental results show about
to determine the shear strength. The experimental results show about 20–30% improvement in 20–30% improvement in shear
strength.
shear strength.
Schmier
Schmier et et al.
al.[18]
[18]qualitatively
qualitatively analyzed
analyzed andand visualized
visualized the functional
the functional morphology
morphology of the
of the coconut
coconut endocarp on several hierarchical levels. Based on those findings, the
endocarp on several hierarchical levels. Based on those findings, the authors presented a more precise authors presented a
more precise
evaluation ofevaluation
the toughening of themechanisms
toughening in mechanisms
the endocarp.in the endocarp.
Ha
Ha et al. [19] investigated the crashworthiness characteristics of
et al. [19] investigated the crashworthiness characteristics of aa new
new bio-inspired
bio-inspired conical
conical
corrugation
corrugation tubes (CCTs). They classified the deformation of CCTs into four modes. The
tubes (CCTs). They classified the deformation of CCTs into four modes. The numerical
numerical
simulations indicatethat
simulations indicate thatthe
the
CCTsCCTs produced
produced a smoother
a smoother force–displacement
force–displacement relationship
relationship than
than straight
straight
tubes. So far, the high-impact performance of coconuts is still a mystery. The main goal of this studyof
tubes. So far, the high-impact performance of coconuts is still a mystery. The main goal is
this study
to reveal theisfundamental
to reveal the fundamental
mechanism of thismechanism
high-impactof this high-impact
performance performance
via a quantitative via a
numerical
quantitative
computation.numerical
The unique computation.
contributions Theofunique contributions
this paper include: of this paper include:
(a)
(a) WeWe designed
designed aa complete
complete set
set of
of morphological
morphological parameters
parameters to to describe
describe microchannels
microchannels inside
inside
coconut shells.
coconut shells.
(b) A series of statistical analyses were conducted in a dimensionless way to reveal the intrinsic
(b) A series of statistical analyses were conducted in a dimensionless way to reveal the intrinsic
geometric relationships of microchannels with respect to the thickness of the coconut shells.
geometric relationships of microchannels with respect to the thickness of the coconut shells.
(c) We created the first coconut-inspired plate structure with embedded microchannels.
(c) We created the first coconut-inspired plate structure with embedded microchannels.
(d) Our numerical simulations elucidate the improvement of a coconut-inspired plate in
(d) Our numerical simulations elucidate the improvement of a coconut-inspired plate in mechanical
mechanical impact performance compared to the conventional plate structure.
impact performance compared to the conventional plate structure.
The rest of this paper is organized as follows. In Section 3, the X-ray CT analysis of coconut shell
The rest of this paper is organized as follows. In Section 3, the X-ray CT analysis of coconut
samples is described. Next, a bio-inspired design of coconut composites is given in Section 4. Section
shell samples is described. Next, a bio-inspired design of coconut composites is given in Section 4.
5 provides the results of dynamic simulation and discussion, and Section 6 is related to industrial
Section 5 provides the results of dynamic simulation and discussion, and Section 6 is related to
applications of coconut composites. Some concluding remarks and future research are given in
industrial applications of coconut composites. Some concluding remarks and future research are given
Section 7.
in Section 7.
3. X-Ray CT Analysis of Coconut Shell Samples
3. X-Ray CT Analysis of Coconut Shell Samples
Our approach, as shown in Figure 1, contains the following technical components: (1)
Our approach, as shown in Figure 1, contains the following technical components: (1) microscale
microscale and mesoscale X-ray scanning of coconut specimens; (2) analysis of structures of coconut
and mesoscale X-ray scanning of coconut specimens; (2) analysis of structures of coconut shells;
shells; (3) modeling of coconut composites; and (4) dynamic impact evaluation of crashworthiness.
(3) modeling of coconut composites; and (4) dynamic impact evaluation of crashworthiness.
Figure
Figure 1.
1. Flowchart
Flowchartof
of our
our approach.
approach.
An Xradia
An XradiaVersa
Versa510 waswas
510 used in measuring
used the internal
in measuring structures
the internal of adult coconut
structures of adultshell specimens.
coconut shell
Figures 2–4Figures
specimens. present 2–4
typical camera
present images
typical of theimages
camera coconut of sample as well
the coconut as X-ray
sample CT data.
as well Figure
as X-ray CT2
showsFigure
data. the outer layer the
2 shows of fibers;
outerin Figure
layer 3, a fixture
of fibers; of the 3,
in Figure X-ray CT machine
a fixture and a CT
of the X-ray small coconutand
machine shell
a
specimen are given; and Figure 4 refers to CT data after volume reconstruction from
small coconut shell specimen are given; and Figure 4 refers to CT data after volume reconstruction two different
viewstwo
from (perspective and top).
different views (perspective and top).
Appl. Sci. 2020, 10, 355 3 of 15
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2020, 10,
10, xx FOR
FOR PEER
PEER REVIEW
REVIEW 33 of
of 17
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(a)
(a) (b)
(b) (c)
(c)
(d)
(d) (e)
(e) (f)
(f)
Figure
Figure2.
Figure 2.
2. Outer-layer
Outer-layer fibers
fibers of
ofan
of anadult
an adultcoconut.
adult coconut.(a)
coconut. AA complete
(a)(a) complete
A complete coconut;
coconut;
coconut;(b) An
(b)(b) opened
AnAn
opened
openedcoconut;
coconut; (c)
(c)
coconut;
(c)A
AAsurface
surfacecut;
surface cut;(d)
cut; (d)Outer-layer
(d) Outer-layerfibers;
Outer-layer fibers;(e)
fibers; (e)An
(e) Anenlarged
An enlargedview
enlarged viewof
view ofthe
of thefibers;
the fibers;(f)
fibers; (f)One
(f) Onetip
One tipof
tip ofthe
of thecoconut.
the coconut.
coconut.
(a)
(a) (b)
(b)
Figure
Figure 3.3.
3. An
An X-ray
X-ray computed
computed tomography
tomography (CT)
(CT) fixture
fixture and
and aaa coconut
coconut specimen
specimen at
at two
two orientations.
orientations.
Figure An X-ray computed tomography (CT) fixture and coconut specimen at two orientations.
(a)
(a) side
side view;
view; (b)
(b) front
front view.
view.
(a) side view; (b) front view.
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(a)
(b)
FigureFigure 4. Computed
4. Computed tomography
tomography and aand a coconut
coconut shell specimen.
shell specimen. (a) Perspective
(a) Perspective view;view;
(b) A (b) A
horizontal
horizontal cross-section.
cross-section.
In this paper, we let the outward direction of surface normal at each coconut shell be the
In this paper, we let the outward direction of surface normal at each coconut shell be the positive
positive z-coordinate direction (Figure 5); with respect to the coconut shell, the “top” refers to the
z-coordinate direction (Figure 5); with respect to the coconut shell, the “top” refers to the place of
place of junction with the outer-layer fibers, and the “bottom” denotes the location next to the
junction withcoconut
interior the outer-layer
meat layer.fibers, and
Figure the “bottom”
6 shows denotes the
five cross-section location
views next
(i.e., top to the
view) interior coconut
in different x-y
meat planes.
layer. Figure cross-section views (i.e., top view) in different x-y
From this figure, the following trends can be observed at the horizontal cross-sections. From this
6 shows five planes.
figure, the following trends can be observed at the horizontal cross-sections.
(a) Especially at places near the top horizontal cross-section, the microchannels go vertically in the z
direction (not in a horizontal plane). This helps maintain the structural integrity of coconut shells.
(b) At the middle layers, the channels are smoothly shaped, i.e., there is no sharp corner or edge for
stress concentration.
(c) At the middle layers, the channels are not located in any particular layer. In other words, different
segments of the channels stay at the different middle layers. In this way, the structural weakening
due to microchannels becomes minimum.
(d) “Horizontal Middle 1” shows (1) branching patterns, (2) small sine curve patterns, and (3) half
horizontal and half vertical patterns.
Appl. Sci. 2020, 10, 355 5 of 15
(e) “Horizontal Middle 2” demonstrates three or four branch patterns.
(f) “Horizontal Middle 3” depicts a neural network pattern.
(g) At places near the bottom layer, there are more segments of channels in the horizontal layer,
compared to the vertical direction. This may be because the bottom layer is relatively far from the
top layer where an impact force is loaded directly in an impact event.
Figure 7 demonstrates the distribution of microchannels in the side cross-sections and the top
cross-sections. The following trends can be observed:
(a) The channel density is quite uniform from the top surface to the bottom surface of the coconut shells.
(b) The cross-sectional shape of the microchannels can be described as an ellipse.
(c) Smaller channels tend to take a circular shape, while larger channels are of elliptical shape.
(d) The long axis of microchannels is roughly perpendicular to the direction of surface normal of the
coconut shells.
The statistical characteristics of microchannels are analyzed with respect to the following features:
(1) The lengths of long and short half-axes of an ellipse representing a microchannel, rlong (mm) and
rshort (mm)
(2) A ratio of rshort to rlong
rshort +rlong
(3) An average radius of a channel, ra = 2 (mm)
(4) Thickness of coconut shell, t (mm)
(5) A ratio of ra to t
(6) Distance from the top surface to the location of a channel, dtop (mm)
(7) A ratio of dtop to t
(8) Amplitude, a (mm), and period, T (mm), of a microchannel in the form of a sine curve
(9) A ratio of a to T
(10) A ratio of T to t
Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 17
Figure
Figure 5. 5.Definition
Definitionof
of aa coconut
coconut coordinate
coordinatesystem.
system.
(a) Especially at places near the top horizontal cross-section, the microchannels go vertically in the
z direction (not in a horizontal plane). This helps maintain the structural integrity of coconut
shells.
(b) At the middle layers, the channels are smoothly shaped, i.e., there is no sharp corner or edge for
stress concentration.
(c) At the middle layers, the channels are not located in any particular layer. In other words,
different segments of the channels stay at the different middle layers. In this way, the structural
weakening due to microchannels becomes minimum.
(d) “Horizontal Middle 1” shows (1) branching patterns, (2) small sine curve patterns, and (3) half
horizontal and half vertical patterns.
horizontal and half vertical patterns.
(e) “Horizontal Middle 2” demonstrates three or four branch patterns.
(f) “Horizontal Middle 3” depicts a neural network pattern.
(g) At places near the bottom layer, there are more segments of channels in the horizontal layer,
Appl. compared to the vertical direction. This may be because the bottom layer is relatively far from
Sci. 2020, 10, 355 6 of 15
the top layer where an impact force is loaded directly in an impact event.
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(c) (d)
(e)
Figure
Figure 6. 6. Horizontal
Horizontal (x-y(x-y plane)
plane) cross-sections
cross-sections of a coconut
of a coconut shell. (a) Top(a)
shell. Top surface;
surface; (b) Horizontal
(b) Horizontal middle
1;middle 1; (c) Horizontal
(c) Horizontal middle 2; middle 2; (d) Horizontal
(d) Horizontal middle
middle 3; (e) 3; (e)
Bottom Bottom surface.
surface.
Figure 7 demonstrates the distribution of microchannels in the side cross-sections and the top
cross-sections. The following trends can be observed:
(a) The channel density is quite uniform from the top surface to the bottom surface of the coconut
shells.
(b) The cross-sectional shape of the microchannels can be described as an ellipse.
(c) Smaller channels tend to take a circular shape, while larger channels are of elliptical shape.
Appl. Sci. 2020, 10, 355 7 of 15
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(a) (b)
(c) (d)
(e)
Figure 7. Side (x-z plane) and front (y-z plane) cross-sections of a coconut shell. (a) Side cross-section
Figure 7. Side (x-z plane) and front (y-z plane) cross-sections of a coconut shell. (a) Side cross-section 1;
1; (b) Side cross-section 2; (c) Side cross-section 3; (d) Front cross-section 1; (e) Front cross-section 2.
(b) Side cross-section 2; (c) Side cross-section 3; (d) Front cross-section 1; (e) Front cross-section 2.
The statistical characteristics of microchannels are analyzed with respect to the following
a circular channel, rshort = rlong , and with respect to an elliptical channel, rshort rlong . The results
Forfeatures:
of statistical analyses (based on approximate normal distributions) in Figure 8 indicate that the mean
(1) The lengths of long and short half-axes of an ellipse representing a microchannel, (mm)
values of the radius ratio, ra /t and dtop /t are 0.54, 0.06, and 0.58, respectively. Figure 9 indicates that the
and (mm)
mean values(2) Aof a ratio
ratio of of sine
to amplitude and period to the shell thickness are respectively 0.32 and 1.92.
rshort /rlong = 0.54 means that the channels are of an elliptical shape and its long axis is about twice as
long as its short axis; with dtop /t = 0.06, the diameter of the microchannels is approximately 1/10th of
the coconut shell thickness; when dtop /t = 0.58, it indicates that the middle layers are the center place of
a distribution. Actually, the distribution in Figure 8c is more like a uniform distribution across the
entire coconut thickness except at the places near the top (dtop /t = [0.0, 0.1]).
9 indicates that the mean values of a ratio of sine amplitude and period to the shell thickness are
respectively 0.32 and 1.92. / = 0.54 means that the channels are of an elliptical shape and
its long axis is about twice as long as its short axis; with / = 0.06, the diameter of the
microchannels is approximately 1/10th of the coconut shell thickness; when /t = 0.58, it
indicates that the middle layers are the center place of a distribution. Actually, the distribution in
Appl. Sci. Figure 8c355
2020, 10, is more like a uniform distribution across the entire coconut thickness except at the places 8 of 15
near the top ( /t = [0.0, 0.1]).
Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 17
(a)
(b)
(c)
Figure Figure 8. Statistical characteristics of microchannels of a coconut shell with a thickness of 2.88 mm
8. Statistical characteristics of microchannels of a coconut shell with a thickness of 2.88 mm
(dimensions of the coconut shell specimen in x, y, and z: 18.6 mm, 18.98 mm, and 18.9 mm). (a)
(dimensions of the coconut shell specimen in x, y, and z: 18.6 mm, 18.98 mm, and 18.9 mm). (a) Radius
Radius ratio; (b) Average radius/thickness; (c) Distance to top/thickness.
ratio; (b) Average radius/thickness; (c) Distance to top/thickness.
Appl. Sci. 2020, 10, 355 9 of 15
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(a)
(b)
Figure
Figure 9. Additional
9. Additional statisticalcharacteristics
statistical characteristics of
of microchannels
microchannels inin
thethe
form of aofsine
form curve.
a sine (a) Sine
curve. (a) Sine
amplitude/thickness; (b) Sine period/thickness.
amplitude/thickness; (b) Sine period/thickness.
4. Bio-Inspired
4. Bio-Inspired Design
Design of of CoconutComposites
Coconut Composites
Based on the analysis results in Section 3, we decide to use the following patterns for
Based on the analysis results in Section 3, we decide to use the following patterns for constructing
constructing a coconut-inspired plate: (1) radius ratio = 0.5, i.e., elliptical channels with = 0.5
a coconut-inspired plate: (1) radius ratio = 0.5, i.e., elliptical channels with rshort = 0.5 rlong ;th(2) the ratio
; (2) the ratio of to t = 0.05, which means that the diameter of microchannels is 1/10 of the
of ra to t = 0.05, which means that the diameter of microchannels is 1/10th of the plate thickness;
plate thickness; (3) the distribution of microchannels follows an approximate uniform distribution,
(3) the
as distribution
illustrated in of themicrochannels follows
right image of Figure 6; an approximate
(4) the uniform
layout of each distribution,
microchannel as illustrated in
is an approximate
the right imageexcept
sine curve, of Figure 6; places
at the (4) thenear
layout
the of
topeach microchannel
of the is an
plate where the approximate
channels sine curve,
grow vertically except
in the
at thedirection
places of near
platethesurface
top ofnormal;
the plate
and where the channels
(5) the long grow
axis of each verticallyis in
microchannel the direction
perpendicular of plate
to the
surface normal;
direction and
of the (5) the
surface long axis of each microchannel is perpendicular to the direction of the
normal.
Figure
surface normal. 10 shows a design of coconut-inspired plate model based on the aforementioned
patterns. The left image refers to a computer-aided
Figure 10 shows a design of coconut-inspired design
plate (CAD)
model model,
based andaforementioned
on the the right image is a
patterns.
The left image refers to a computer-aided design (CAD) model, and the right image is a finite
element mesh. This is only one example of applying the above patterns. There are many possible
Appl. Sci. 2020, 10, 355 10 of 15
Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 17
finite element mesh. This is only one example of applying the above patterns. There are many
designs to be derived from the aforementioned patterns, and it depends upon other scientists and
possible designs to be derived from the aforementioned patterns, and it depends upon other
engineers to explore
scientists in the future
and engineers for in
to explore various applications.
the future for various applications.
(a)
Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 17
(b)
(c)
Figure 10. Design of a coconut-inspired plate model with microchannels. (a) Defining a path for a
Figure 10. Design of a coconut-inspired plate model with microchannels. (a) Defining a path for a
microchannel; (b) A microchannel network; (c) A finite element model.
microchannel; (b) A microchannel network; (c) A finite element model.
5. Dynamic Impact Simulation and Discussion
A pendulum impact test is established, as shown in Figure 11. The steel ball has a reference
point as its rotational center. The displacements at the boundaries of a plate are fixed along all three
coordinate directions as well as three rotational directions. The plate is oriented with its surface
normal pointing horizontally. We consider a generic material for the plate with Young’s modulus =
70 GPa and Poisson’s ration = 0.34. The exact values of material parameters are not important here,
because we are only interested in a comparison of impact performance. The steel ball is 45 degrees(c)
Figure 10. Design of a coconut-inspired plate model with microchannels. (a) Defining a path for 11
Appl. Sci. 2020, 10, 355
a of 15
microchannel; (b) A microchannel network; (c) A finite element model.
5. Dynamic Impact Simulation and Discussion
established, as shown
A pendulum impact test is established, shown inin Figure
Figure 11.
11. The steel ball has a reference
center. The displacements at the boundaries of a plate are fixed along all three
point as its rotational center.
coordinate directions
coordinate directionsasaswell
wellas as three
three rotational
rotational directions.
directions. The plate
The plate is oriented
is oriented with itswith its surface
surface normal
normal pointing
pointing horizontally.
horizontally. We consider
We consider a generica material
generic material for the
for the plate withplate with Young’s
Young’s = 70 GPa=
modulusmodulus
70 GPa
and Poisson’s ration =ration
and Poisson’s = 0.34.
0.34. The The
exact exactof
values values of material
material parameters
parameters are not important
are not important here,
here, because
because
we we interested
are only are only interested in a comparison
in a comparison of impactofperformance.
impact performance.
The steelTheball steel
is 45 ball is 45from
degrees degrees
the
from the
plate, plate,
with with angular
its initial angular=velocity
its initialvelocity = 750 radians/second
750 radians/second and its
and its weight = weight
1.5 kg. = 1.5 kg.
Figure
Figure 11. A pendulum
11. A pendulum impact
impact test.
test.
Figure 12c shows about 40% improvement in reducing the impact acceleration of a plate with
coconut-inspired microchannels (Figure 10) compared to a regular plate with the same material
parameters and impact setting. Figure 13 shows that the increase in strain energy and the decrease in
acceleration are one underlying reason for the coconut-inspired plate to absorb more impact energy,
leading to better crashworthiness, i.e., better protection to occupants if the plate serves as a protection
layer. The strain energy is defined as
Z
1
Ue = {σ}T {ε}dV (1)
2 V
n o n o
where the stress vector {σ}T = σxx σ yy σzz σxy σxz σ yz and elastic strain {ε}T = εxx ε yy εzz εxy εxz ε yz .
V refers to the volume of the model. The material is modeled with both plastic strain and elastic strain.parameters and impact setting. Figure 13 shows that the increase in strain energy and the decrease in
acceleration are one underlying reason for the coconut-inspired plate to absorb more impact energy,
leading to better crashworthiness, i.e., better protection to occupants if the plate serves as a
protection layer. The strain energy is defined as
1
= (1)
2
Appl. Sci. 2020, 10, 355 where the stress vector = and elastic strain = 12 of 15
. V refers to the volume of the model. The material is modeled with both
plastic strain and elastic strain.
Appl. Sci. 2020, 10, x FOR PEER REVIEW (a) 14 of 17
(b)
(c)
Figure 12. Comparison between a coconut-inspired plate and a regular plate with respect to impact
Figure 12. Comparison between
performance. a coconut-inspired
(In the scientific plate and
notation, 3.913e+09 or 3.913E+09 a regular
represents 3.913 × 109plate with respect to impact
; other floating
values follow the same rule.) (a) Distribution of acceleration (m/s ); (b) Distribution of von Mises
performance. (In the scientific notation, 3.913e+09
stress (N/mm ); (c) Acceleration reduction.
or 3.913E+09 represents 3.913 × 109 ; other floating
values follow the same rule.) (a) Distribution of acceleration (m/s2 ); (b) Distribution of von Mises
stress (N/mm2 ); (c) Acceleration reduction.Appl. Sci. 2020, 10, 355 13 of 15
Appl. Sci. 2020, 10, x FOR PEER REVIEW 15 of 17
(a)
(b)
Figure
Figure 13.13. Comparison
Comparison of
ofstrain
strainenergy
energyduring a pendulum
during impact
a pendulum test. (a)
impact A regular
test. plate; (b)plate;
(a) A regular A
coconut-inspired plate.
(b) A coconut-inspired plate.
6. Industrial
6. Industrial Applications
Applications
Coconut
Coconut shells
shells havehave
beenbeen utilized
utilized in making
in making industrial
industrial components
components suchparts.
such as auto as auto parts.
Automakers
Automakers
incorporate the incorporate thetococonut
coconut fibers producefibers to composite
useful produce useful composite
components components
[20,21]. The auto[20,21]. The
parts include
auto parts include the load floor, package shelf, console armrests, structural guards, rear
the load floor, package shelf, console armrests, structural guards, rear deck lid, applique brackets, deck lid,
applique brackets, side-door cladding, interior door panels, compartment liner,
side-door cladding, interior door panels, compartment liner, radiator end tank, and veneer. radiator end tank,
and veneer.
Coconut husks was also used for making a stronger, lighter paddleboard [22]. By absorbing
Coconut husks was also used for making a stronger, lighter paddleboard [22]. By absorbing less
less epoxy than other materials, coconut fibers reduce the weight of a paddleboard with Australian
epoxy than other materials, coconut fibers reduce the weight of a paddleboard with Australian
brand NSP. It decreases the weight of the board by 30% and eliminates toxic materials by 40%.
brand NSP. It decreases the weight of the board by 30% and eliminates toxic materials by 40%.
7. Conclusions and Future Perspective
In this study, the following conclusions can be drawn:
(a) We digitally uncover a network of microchannels inside coconut shells as one underlying reason
for the high-impact performance of coconuts.Appl. Sci. 2020, 10, 355 14 of 15
(b) We designed five dimensionless ratios to describe the intrinsic geometric relation between
microchannels and the thickness of coconut shells. These dimensionless parameters are
scale-invariant and will therefore facilitate coconut-inspired designs.
(c) The cross-section of microchannels is approximated by an ellipse. A series of statistical analyses
reveal the mean values of microchannel morphology with respect to coconut shell thickness.
(d) In addition, we numerically demonstrate the significant improvement (about 40%) in reducing
the impact acceleration with a coconut-inspired plate. The design patterns of microchannels can
be potentially applied in various applications.
The microchannels may be further utilized as the space of healing agents for self-healing composites
or the space of sensing elements for intelligent composites with self-sensing capability.
Author Contributions: J.S. conducted the simulation work; M.F. and A.S. provided guidance on X-ray CT scanning.
All authors have read and agreed to the published version of the manuscript.
Funding: This study was supported in part by an Air Force Summer Faculty Fellowship and U.S. National Science
Foundation grants (DMI-0514900, CMMI-0721625, ECCS-1039563, and IIP-1445355).
Acknowledgments: Help in impact simulation from Abaqus Acumen is sincerely appreciated.
Conflicts of Interest: The authors declare no conflict of interests.
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