52nd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference<BR> 19th
4 - 7 April 2011, Denver, Colorado
AIAA 2011-2159
Cork: Is It a Good Material for Aerospace Structures?
José M. Silva 1, Pedro V. Gamboa, C. Nunes, L. Paulo and N. Franco
Department of Aerospace Sciences, University of Beira Interior, 6201-001 Covilhã - Portugal
Cork is a natural material with remarkable energy absorption properties, which make it
an ideal candidate material when thermal insulation or vibration suppression are major
design requirements. However, little work has been done towards the use of cork based
materials in structural components. This paper envisages assessing the feasibility of using
cork composites with improved specific strength and damage tolerant properties for
aerospace applications. A review of some results and conclusions about the mechanical
behavior of cork composites will be done based on existing literature and other previous
works of the authors on this subject. In particular, two types of materials were considered:
1) a sandwich structure with a cork-epoxy agglomerate core; 2) a carbon-epoxy laminate
with embedded cork granulates. In both cases, a set of static and dynamic tests were carried
out to characterize the mechanical behavior of the material with a special emphasis on its
damage tolerant properties. These tests were replicated using a benchmark core material
commonly used in aerospace applications in order to confirm the comparative benefits of
cork based composites. A final part of this work seeks to evaluate the advantages of
combining the natural damping characteristics of cork with high performance composites
aiming at improving the aeroelastic behavior of aerospace components. In particular, the
critical flutter speed of a cork based sandwich plate was compared with other conventional
materials through a computational analysis. Regardless the type of application, results are
encouraging about the use of cork based materials in aerospace components due to their
noticeable damage tolerant and high energy absorption properties under different loading
scenarios.
Nomenclature
AIC
ARD
CFRP
FE
FOD
FRP
h
K
Lh
m
M
PMI
q
q∞
t
TPS
Vf
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
Aerodynamic Influence Coefficient matrix
Atmospheric Re-entry Demonstrator
Carbon Fibre Reinforced Plastic
Finite Element
Foreign Object Damage
Fibre Reinforced Plastic
structural deformation
stiffness matrix
aerodynamic force vector
impactor mass, plate mass [kg]
mass matrix
polymethacrylimide
generalized coordinates
freestream dynamic pressure [Pa]
time [s], thickness [m]
Thermal Protection System
flutter speed [m/s]
1
Assistant Professor, Depart. of Aerospace Sciences, University of Beira Interior, 6201-001 Covilhã, Portugal,
[email protected]
1
American Institute of Aeronautics and Astronautics
Copyright © 2011 by The authors. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.
I. Introduction
T
he increasing use of composite materials in aerospace applications is a natural result of their ability to assist in
meeting strict design demands, namely high strength to weight ratios, maximized reliability levels and good
damage tolerance characteristics. In fact, the advantages of using tailored components with optimized properties to
withstand predicted loads is a major asset that put fiber reinforced polymers (FRP) in the leading edge of other
competing class of materials since the 1970s1. Nevertheless, engineers soon realized that FRPs present some
important drawbacks that can limit its usage under certain types of operational conditions. From these, there are two
unquestionable limitations with a significant impact in a considerable number of aerospace applications: 1) FRPs,
especially those using carbon reinforcement (CFRP), have a brittle-type behavior which can constitute a serious
problem in terms of damage tolerant properties when impact loading is expected during service; 2) regardless of
their superior rigidity, composites evince a limited response when high damping factors are required under dynamic
loading, such as those resulting from aeroelastic problems or in the context of large gossamer structures in space.
The limited resilient characteristic is a well known handicap of composites that can compromise structural
integrity under impact loading, such as that inflicted by foreign object damage (FOD) in an aircraft or space debris
on a spacecraft. This problem explains the current and emergent interest of some researchers towards the
improvement of the damage tolerant properties of composite structures used in the transport industry. One promising
approach to this subject has been relying upon the use of viscoelastic materials with high energy absorption
properties in combination with resin systems with high glass transition temperatures, which means high stiffness but
low damping characteristics. House and Grant2 showed that the use of a modified epoxy resin system with more
flexible properties allowed obtaining better damage tolerant capabilities under low energy impact loading together
with higher loss factors under vibration occurrence. Also, Ruksakulpiwat et al.3 used two forms of rubber based
materials at different concentrations in polypropylene composites, which led to a significant increase in the impact
strength, in the elongation at break and even in the Young´s modulus for a limited inclusion of rubber content.
Cork appears as a natural viscoelastic candidate material to improve the energy absorption capacity of FRPs. In
fact, cork is a natural material with excellent thermal insulation and energy absorption properties which explain its
use as a preferential material, under the form of cork composites, for thermal/sound insulating purposes or vibration
attenuation in machine elements4. Extracted from the Quercus Suber L. tree, commonly known as cork oak, cork has
an alveolar cellular structure similar to that of a honeycomb, which is in the base of these properties. Consequently,
cork based materials can be considered as a viable alternative to other low weight core materials usually used in
aerospace structures, such as sandwich composites with polymeric foam cores. In some cases, the specific strength
of cork agglomerates (with a natural cellular morphology) is comparable to some rigid synthetic foams5. Some
recent works have been exploring the possibility of combining cork (in the form of powder or granulates) with
thermoplastics to take advantage of the unique damping properties of this natural material. Ben Abdallah et al.6
showed that cork can be incorporated into a polypropylene matrix to get a composite material with improved
mechanical characteristics after a convenient surface treatment to ameliorate the adhesion properties between each
phase of the composite. Also, Fernandes et al.7 concluded that the mixture of cork powder with different qualities
with thermoplastic materials using a pultrusion process, and after adding a convenient coupling agent, resulted into a
composite material with improved tensile strength and stiffness. Cork agglomerates have also been used as fillers
inside structural components to improve the energy absorption capacity under impact loading8, and results suggest
that this may become the adequate material for innovative applications that require low cost, low density and high
damage tolerance.
Recently, cork based products have also found a considerable number of applications in space structures due to
the intrinsic properties of this material, in particular its capacity to withstand very high temperatures during the reentry phase of space vehicles. In this case, cork agglomerates have been acting as a thermal protection system (TPS)
ablative material in the form of tiles covering the most part of the vehicle. NORCOAT-LIEGE is a TPS material
developed by EADS in the seventies constituted by hot-pressed cork particles agglomerated with phenolic resin9.
This material is used in the rear cone and back-cover sections of the Atmospheric Reentry Demonstrator (ARD), a
sub-orbital reentry vehicle launched in 1998 by the European Space Agency (ESA). Besides supporting high
temperatures and heat fluxes (about 2000ºC and 5MW/m2, respectively), the low density of NORCOAT-LIEGE
(ρ=480 kg/m3) is a major advantage when compared with silica-based TPS materials (ρ=1600 kg/m3). NORCOATLIEGE was also elected as a reference TPS material for the design of the ExoMars module in the context of the ESA
mission to planet Mars due to happen in 201610. In this case, cork-phenolic tiles and panels will be used for
protecting both the frontshield and back-cover areas. Ablative properties of cork have also been determinant for the
application of this material as an insulator between the metal/Kevlar case and the propellant in solid rocket boosters
(SRB)11. In fact, the combination of cork fillers with ethylene propylene diene rubber (EPDM) increases the ablative
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properties of the composite due to the gaseous byproducts of decomposition and increased amount of char
formation, which forms an excellent thermal insulation layer. The aforementioned excellent impact tolerance of cork
composites combined with the unique ablative characteristics are in the base of its recent incorporation within the
bolt catchers of the Space Shuttle solid rocket boosters.
Apart from space applications, little effort has been done towards the use of cork composites in high
performance aircraft structural components benefiting from the remarkable intrinsic properties of this natural
material. Recently, a preliminary approach to this possibility has been investigated by Castro et al.12, who developed
a cork-epoxy composite with improved mechanical properties. Static flexural tests carried out in the course of this
work allowed confirming the superior specific strength of this optimized material when compared to conventional
cork agglomerates. On the other hand, preliminary results from low energy impact tests suggest that cork-epoxy
agglomerates are a viable alternative to other low weight core materials commonly used in sandwich components,
such as high performance polymeric foams. In fact, cork-epoxy cores evinced a superior energy absorption capacity
with minimum damage occurrence, resulting in better crashworthiness properties when impact loading is expected
during aircraft operation (such as FOD).
Another interesting application of cork granulates will be explored by the authors in the present work, envisaging
embedding this viscoelastic material in CFRP laminates aiming at improving the limited resilience properties of high
performance resin (matrix) systems, as mentioned before. Results from a set of static and dynamic experimental
tests will be discussed to assess the feasibility of this hybrid cork composite, since a trade-off must be considered in
terms of the energy absorption improvement and the likely specific strength reduction caused by the inclusion of
cork granulates in the laminate. Finally, this work also intends to explore the use of cork-epoxy cores in a sandwich
configuration in order to increase the structural eigenvalues and, consequently, critical flutter speed and frequency
on a flat panel. This will confirm the intrinsic influence of cork, as a natural viscoelastic material, for the
enhancement of the damping properties of FRP structural components whose dynamic response is of utmost
importance under specific operational conditions of air and space vehicles, acting as a passive and efficient solution
to overcome aeroelastic and other vibration induced problems.
II. Experimental Tests and Materials
A. Materials
As mentioned before, a new type of cork-epoxy composite with enhanced mechanical properties was developed
in a previous work of the authors12 aiming at obtaining better overall mechanical properties when compared to
conventional cork agglomerates. In this work, an optimized process has been used to find the best agglomeration
method and the right cork/resin ratio. In order to do so, different levels of compacting pressures, granulate sizes and
resin weight fractions were considered. Figure 1 shows a schematic representation of the fabrication process of the
cork-epoxy agglomerates. The fabrication variables were optimized envisaging the best mechanical strength with a
minimum weight, which led to a final density of 260kg/m3 (in the case of a 30mm thick agglomerate).
Cork-epoxy composites were used as a core of a sandwich component in combination with two carbon-epoxy
facesheets which were obtained through a hand layup process consisting of three plies of a 196g/m2 carbon fiber
fabric with orientation [0º/90º/0º]. The dimensions of sandwich specimens were chosen taking into account the
requirements of the impact and residual strength tests. However, there is no specific standard for the former type of
tests in the particular case of sandwich components. Therefore, it was decided to follow the orientations of both
ASTM C393/C393M13 and ASTM D7136/D7136M14 standards with minor adjustments, since it was necessary to
obtain the shear strength of the cork-epoxy core (before and after impact) by means of flexural tests.
Table 1 presents the maximum shear and direct stress values as obtained from three-point flexural tests for
conventional cork agglomerates and cork-epoxy agglomerates. For comparative purposes, these tests were
reproduced with a different core material (maintaining the same thickness), namely a polymethacrylimide foam
(Rohacell®), which is commonly used in aerospace components. These results show that the highest maximum core
shear stress is verified for the cork-epoxy agglomerate cores, which is 38% to 56 % higher than Rohacell and 4 to 7
times higher than the commercial cork agglomerates. Moreover, cork-epoxy sandwich panels clearly present higher
direct stress values when compared with the commercially available conventional cork agglomerates.
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Cork
granulates
+
Table 1. Maximum core shear stress and face bending
stress for different core materials in sandwich specimens
(average values).
Epoxy resin
system
Core material
Conventional cork agglomerate
Cork-epoxy agglomerate
PMI foam (Rohacell)
Compression moulding in a
hydraulic press
(p=50-60 bar)
τmax [MPa]
0.2332
0.9365
0.6007
σmax [MPa]
24.22
54.03
62.38
The other type of specimens used in this work consisted in
a carbon-epoxy laminate fabricated in autoclave with
embedded cork granulates with two sizes: small (0.5mm
average) and medium (0.9mm average). Twenty eight plies of
Curing stage at a heating chamber
a high strength prepreg were manually stacked, which resulted
o
(T=80 C; at 2 hours)
into a 5mm thick laminate suitable for impact testing. All the
plies were oriented parallel to the longitudinal axis of the
specimens, and cork granulates were embedded between the
4th-5th plies and 23rd-24th plies.
Embedded cork granulates can act as elastic inclusions
within the laminate and therefore cause a detrimental effect in
the overall limit strength of the composite. This effect was
confirmed through three-point flexural tests carried out
according to ASTM D79015. In this case, a rectangular
Cork-epoxy agglomerate
specimen geometry was used (150mm x 25mm) and a total of
10 plies were considered with cork granulates placed between
Figure 1. Schematic representation of the
the 3rd-4th plies and the 7th-8th plies.
fabrication process of the cork-epoxy
The graphs of Fig. 2 show the comparative reduction in the
agglomerates.
ultimate flexural strength and elastic modulus of both types of
laminates, i.e., with and without cork. As we can see, the presence of cork inside the laminates causes a decrease in
both static limits of the carbon-epoxy composites, namely 19.8% (Fig. 2a) and 7.9% (Fig. 2b).
2000
120
112,9
106
1500
1316,3
1000
500
0
Elastic modulus, Mpa
Ultimate strength, MPa
1628
90
60
30
0
Without cork
With cork
Without cork
Type of specimens
With cork
Type of specimen
Figure 2. Effect of the embedded cork granulates in the mechanical strength of the laminates: a) ultimate
strength; b) elastic modulus.
B. Experimental tests
The damage tolerance capability of sandwich specimens was performed from a residual strength characterization
after impact based on four-point flexural tests. This choice was due to the fact that these tests allow characterizing
various types of damage mechanisms, in particular shear loads in the core material and compression or tension loads
in the facesheets with or without impact damage. A digitally controlled servo-hydraulic machine with a 100kN load
cell has been used for this purpose. A particular attention was paid towards the potential damage caused by the
contact of the hemispherical actuators of the machine and the material, which was minimized with small rubber pads
in the interface region.
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Impact tests were performed using a drop tower apparatus with a free-falling mass connected to a 20mm
hemispherical impactor. The variation of the energy of impact was achieved through the choice of different
departure heights of the falling mass. Impact loads were acquired with a piezoelectric force transducer located
between the impactor and the load carriage. Specimens were constrained in each lateral side of a fixed support on
the impact apparatus. The point of impact was chosen according to the intended damage zone for each type of
specimen, i.e., the middle-point of the laminate plates or the middle-point of the semi-span distance of sandwich
plates. This latter choice was due to the requirements of the residual strength flexural tests.
III. Computational analysis
A preliminary approach to the damping effect of cork in the flutter prevention was also addresses in this work. A
computational analysis of the aeroelastic properties of a 500mm x 150mm sandwich plate with a cork core and
carbon-epoxy facesheets was carried out. A 1mm thick cork-epoxy agglomerate was used whereas the facesheets
were made of 0.2mm carbon/epoxy plain weave skins.
These results were compared with other types of materials: a 0.75mm aluminum alloy plate and a 1.4mm carbonepoxy plate. These thicknesses were chosen such that the flight envelope of the three structures would be
approximately the same in order to focus the analysis on the weight of the components. The computational model
was built using the finite element (FE) code ANSYS®, and the aerodynamic coupling was performed using
ZAERO®. Cantilever boundary conditions were defined and a modal analysis was performed in order to retrieve the
natural frequencies and mode shapes of the structure. The determination of the modes of vibration in the natural
frequencies is an important design issue, since the deformation of the plate is maximum during these modes.
Generally, aeroelastic phenomena can be avoided by increasing the natural frequencies so that resonance will not
occur. We can see from the results in Table 2 that the carbon/cork sandwich configuration leads to an increase in
these frequencies since the lower density of the core allows an overall reduced mass without compromising the
stiffness of the structure.
Table 2. Natural frequencies of the first vibration modes for the three types of materials.
Vibration mode
1
2
3
4
Aluminum
2.46
15.48
16.54
43.81
Natural frequencies, Hz
CFRP
4.61
18.86
28.86
61.99
Cork sandwich
5.97
24.36
37.38
80.03
The structural modal data were then exported to ZAERO® to perform the aerodynamic coupling, considering the
conditions defined in the input file, namely aerodynamic mesh density, sea level conditions, etc. Figure 3 presents a
schematic representation of the computational method implemented in this work. Two basic methods are used to
calculate the flutter boundary from the aeroelastic equations of motion: the k and g-methods. Each method applies
the assumption that, at the flutter boundary, one of the natural vibration modes of the system will become neutrally
stable and produce simple harmonic motion and the other modes remain stable.
Results for the flutter speeds of the three types of materials are presented in Table 3, which were determined
considering a zero damping condition. Although the three plates are limited by the same flight envelope, the cork
sandwich ensures a lighter structure and, therefore, a higher ratio between the flutter speed and the mass of the
component. One straightforward explanation for this is the higher second moment of area provided by the sandwich
configuration, which shifts the natural frequencies of the first bending and torsion modes (the most critical ones) to
higher values, as quantified in Table 2. Nevertheless, one can speculate about another possible explanation for this
frequency shift, as the viscoelastic properties of the cork core may contribute to a higher level of damping than that
provided by the other types of materials. Despite of having a sound probability, this hypothesis requires further
investigation based on the experimental characterization of the dynamic response of cork-epoxy composites.
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American Institute of Aeronautics and Astronautics
Figure 3. Computational method for the determination of the flutter speed.
Table 3. Flutter analysis results for the three plates made with different materials
Vf (g-method),
m/s
Vf (k-method),
m/s
Rigidity,
N/m2
Mass,
kg
Vf /Mass,
m.s-1/kg
Aluminium
(0.75mm)
CFRP
(1.4mm)
Sandwich
(1.4mm)
19.98
20.87
19.92
20.00
20.90
19.80
0.363
1.44
0.916
0.158
0.116
0.065
125.8
180.17
306.5
IV. Experimental results and discussion
As mentioned before, the damage tolerant characteristics of cork based composites were assessed from impact
tests followed by residual strength flexural tests. Figure 4 presents the Force vs Time impact response of sandwich
specimens with 30mm core considering two energy levels: 5 Joule and 20 Joule. For the sake of comparison, these
curves are overlaid with those regarding sandwich specimens with PMI core material with identical thickness. An
immediate conclusion follows from the observation of these graphics, i.e., cork based materials have a smoother
response when compared to PMI foam, which is evident from the reduced amplitude of oscillations of the former
after the impact event. This strengthens the idea of the higher energy absorption capacity of cork composites with
lesser damage. On the contrary, the impact behavior of PMI foam evince a clear reduction in the force curve after
the impact peak, which is a direct consequence of the occurrence of a damage mechanism, which can be either
related with the facesheet or core materials. Another interesting conclusion that can be taken from Fig. 4 is the time
duration of the Force vs Time curves, which shows a wider plateau for the PMI foam, especially in the case of the
highest energy level of impact (almost twice of that of cork-epoxy core). This fact seems to be related to the higher
level of absorbed energy by the foam material, since in this case the impactor would cause some form of damage in
the facesheet or core materials (or both) with a permanent deformation, which would mean that a downward
movement (i.e., perforation) would most likely occur. On the other hand, a shorter time of contact during the impact
event would mean a higher amount of elastic energy in the form of vibrations, elastic deformation and an eventual
rebound of the impactor. This assumption is corroborated by the Force vs Displacement curve in the graph of Fig. 5,
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which is representative of the impactor movement for the higher energy level (20 Joule). As we can observe, the
displacement of the impactor is smaller in the the cork-epoxy core specimens, and a rebound took place in all cases
(Fig. 5a). However, the displacement is considerably higher in the case of sandwich specimens with PMI cores, and
it was found that the impactor did not completely rebound after reaching the surface of the material, which is a clear
indicator of the total energy absorption by the PMI foam in the form of damage (Fig. 5b).
(a)
(b)
Figure 4. Force-Time curves obtained for sandwich specimens with cork-epoxy or PMI cores considering two
levels of energy of impact; a) 5 Joule; b) 20 Joule.
(a)
(b)
Figure 5. Force-Displacement curves obtained for the sandwich specimens with the two types of core materials
and considering the highest energy level of impact (20 Joule): a) cork-epoxy; b) PMI foam.
The four-point flexural residual strength tests allow quantifying the effect of damage as a function of the energy
level of impact (5 Joule or 20 Joule) and the core material (cork-epoxy or PMI foam). Table 4 presents the ultimate
loads as obtained for both impacted and non-impacted specimens. These results are clear about a noticeable strength
reduction after impact in the PMI sandwiches, which can reach about 19% in the case of the 20J impact event. For
the same energy level, cork based sandwiches do not seem to be affected by the impact in terms of residual strength
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reduction. In fact, the residual flexural strength obtained for this type of material was unexpectedly higher than that
for non-impacted specimens, which is a rather interesting result without any apparent solid explanation. However,
this behavior is definitely an additional confirmation of the high energy recovery capacity of cork-epoxy cores,
whose elastic behavior is reflected into a protective effect of the facesheet materials with a consequent reduction in
the area of damage.
Table 4. Load limit obtained for the different core materials from the post-impact residual strength
bending tests (t = 30mm thickness).
Material
without impact, kN
5J impact, kN
Cork-epoxy
PMI foam
3.24
4.58
3.53
3.22
Force variation
(5J vs no-impact)
+8.9%
-29.7%
20J impact, kN
3.70
3.72
Force variation
(20J vs no-impact)
+14.2%
-18.8%
Area of damage, mm2
A quantitative analysis of the damage extension in the impacted facesheets for both types of core materials was
carried out with an image editing software coupled with a low magnification microscopic device. The evolution of
the damage region with the impact energy increase is represented by the graph in Fig. 6. From these results, it
becomes clear that cork-epoxy core
sandwiches have a smaller damaged area
400
relative to that obtained for PMI foams
considering identical energy levels of
impact. In fact, for the two highest energy
300
levels (15 and 20 Joule), there was a
complete perforation of the facesheet
200
material in the sandwich specimens with
Cork-epoxy
PMI core and a detainment of the
PMI foam
impactor head within the core region
100
occurred in some tests at the highest
energy level (20 Joule), as we can see
from Fig. 5(b). On the contrary, the
0
energy absorption capacity of the cork5
10
15
20
epoxy cores prevented the full perforation
Impact energy levels, Joule
of the facesheets, even in the case of the
higher energy levels, and impactor
Figure 6. Damage evolution as a function of energy of impact
rebound took place in all tests.
for the two types of core materials.
An identical beneficial effect of cork was
found in the case of the cork-epoxy
laminates described in Section II.
However, for this type of material, the energy absorption capacity is less noticeable than that for the cork-epoxy
cores. Nevertheless, the behavior of the cork-laminates under impact shows an apparently smoother response and
less oscillation peaks associated with resonance, which can be attributed to the intrinsic damping properties of the
cork granules.
Figure 7 presents the Force vs Time response for the two types of cork granulate size (small and medium)
compared with a plain carbon-epoxy laminate. This graph was obtained for the minimum energy level of impact (5
Joule). In addition to the aforementioned damping characteristics, the introduction of the cork granulates within the
laminate causes a slight delay in the occurrence of the first peak force decrease, which has been related to the onset
of delamination damage by other authors in similar materials16.
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The fatigue strength is another key
design requirement of aerospace
components, which can be regarded as
the tolerance of the material to
withstand a cyclic propagation of
damage. In the particular case of FRPs,
fatigue damage is a complex
phenomenon resulting from the
combination
of
different
ruin
mechanisms, such as fiber breakage,
matrix cracking and delaminations. On
the other hand, the presence of an
elastic inclusion within a composite
laminate can be regarded as a stress
concentration region where a crack
initiation process is most probable.
Therefore, one goal of the present
work is to find out any possible
Figure 7. Force-Time curves obtained from impact testing of carbondetrimental effect regarding the fatigue
epoxy laminates, with and without cork granulates (energy of impact =
performance of the laminates caused
5Joule).
by the presence of the cork granules.
In order to do so, a set of fatigue tests
was performed envisaging obtaining a S-N plot for the two types of materials, i.e., laminates with and without cork
granulates. The results of these fatigue tests are expressed in Fig. 8. Clearly, the two curves are very similar, which
means that the fatigue life is identical for the two types of materials (and considering the same level of stress)
regardless of the use of embedded cork granulates. This fact is also a good indicator of the viscoelastic behavior of
cork since its presence inside the laminate seems to promote an increase in energy absorption and therefore reducing
the energy available for the cyclic propagation of damage.
1
Carbon-epoxy
C a rbono/E poxy
C a rbono/E powith
xy cocork
m C ortiça
Carbon-epoxy
0 .9 5
0 .9
0 .8 5
0 .8
0 .7 5
0 .7
0 .6 5
0 .6
0 .5 5
0 .5 3
10
1 04
1 05
10 6
1 07
N .º C iclos log(N )
Cycles, N
Figure 8. S-N curve for carbon-epoxy laminate specimens with and without cork granulates.
Despite these encouraging results regarding the effect of the viscoelastic properties of cork on the improvement
of the damage tolerance characteristics of composite laminates, further research is necessary to extend the
characterization of the mechanical properties of this hybrid material based on more additional tests. In particular, a
quantitative analysis of the damping factor of cork based laminates would be of utmost importance to corroborate
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the results presented in this work and therefore validate the use of this type of materials in components where the
vibration suppression is a major requirement.
V. Conclusions
The main objective of this paper was to explore the possibility of using cork based composites for structural
components in the context of aerospace applications. Cork is a natural material with remarkable properties, namely
the high energy absorption and damping capabilities, but its specific strength is still far from that required for the
design of aircraft structures. However, the results in this paper are encouraging regarding the improvement of the
mechanical properties of cork based composites in the form of sandwich components with a cork-epoxy core or a
carbon-epoxy laminate with embedded cork granulates. A set of experimental tests were carried out aiming at
assessing the damage tolerant features of this type of materials under low energy impact loading, which allowed to
confirm that the use of cork-epoxy cores leads to a superior energy absorption with a lesser damage extension of
both the core and facesheet materials of the sandwich components. This fact is supported either by the results of the
residual strength tests after impact, showing that cork-epoxy cores withstand higher values of ultimate load when
compared with PMI specimens, or by the observation of a smaller damage area caused by impact for the same
energy level. In the case of composite laminate specimens, it has been shown that the use of embedded cork
granulates within the laminate improves the resilient properties of this usually brittle type material under impact
loading. Fatigue tests also allowed to exclude a possible detrimental effect resulting from the inclusion of the cork
granulates which could promote crack initiation sites under dynamic loading.
Another part of this work envisaged looking at a possible positive influence of the viscoelastic properties of cork
for the suppression of flutter in a plate, which would allow for the design of a lighter alternative regarding other
types of materials. Preliminary results from a computational analysis indicate that cork based composites have a
higher ratio between the flutter speed and the mass of the plate, which means that this can be a viable and lighter
passive solution for the same flight envelope. However, further investigation is required in order to quantify the
increase of the damping features as a direct consequence of the use of cork either on sandwich or laminate
composites.
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