U.S. patent application number 13/193776 was filed with the patent office on 2011-11-17 for composite material.
This patent application is currently assigned to AUXETIC TECHNOLOGIES LIMITED. Invention is credited to Andrew Alderson, Kim Lesley Alderson, Graham David Hudson, David Edward Skertchly.
Application Number | 20110281481 13/193776 |
Document ID | / |
Family ID | 36687617 |
Filed Date | 2011-11-17 |
United States Patent
Application |
20110281481 |
Kind Code |
A1 |
Alderson; Andrew ; et
al. |
November 17, 2011 |
COMPOSITE MATERIAL
Abstract
A composite material includes a layer of fibres conjoined to a
matrix, wherein one of the matrix and fibres has a first component
which exhibits auxetic behaviour for loading along a first
direction, and the other of the matrix and fibres has a second
component which exhibits non-auxetic behaviour for loading along
the first direction.
Inventors: |
Alderson; Andrew;
(Liverpool, GB) ; Alderson; Kim Lesley;
(Liverpool, GB) ; Hudson; Graham David; (Cheshire,
GB) ; Skertchly; David Edward; (Hampshire,
GB) |
Assignee: |
AUXETIC TECHNOLOGIES
LIMITED
Bolton
GB
|
Family ID: |
36687617 |
Appl. No.: |
13/193776 |
Filed: |
July 29, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12300229 |
Dec 6, 2010 |
7989057 |
|
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PCT/GB2007/001946 |
May 24, 2007 |
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13193776 |
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Current U.S.
Class: |
442/1 ;
156/308.2; 428/221; 428/292.1; 442/2; 442/50 |
Current CPC
Class: |
Y10T 442/10 20150401;
C08J 5/04 20130101; Y10T 428/249945 20150401; Y10T 428/249924
20150401; Y10T 442/184 20150401; Y10T 442/406 20150401; Y10T
428/24132 20150115; C08J 5/24 20130101; Y10T 442/102 20150401; Y10T
428/249921 20150401 |
Class at
Publication: |
442/1 ; 428/221;
428/292.1; 442/2; 442/50; 156/308.2 |
International
Class: |
B32B 5/02 20060101
B32B005/02; B32B 37/00 20060101 B32B037/00; B32B 5/04 20060101
B32B005/04 |
Foreign Application Data
Date |
Code |
Application Number |
May 24, 2006 |
GB |
0610272.7 |
Claims
1. A composite material comprising a layer of fibres conjoined to a
matrix, wherein one of the matrix and fibres comprises a first
component which exhibits auxetic behaviour for loading along a
first direction, and the other of the matrix and fibres comprises a
second component which exhibits non-auxetic behaviour for loading
along the first direction.
2. A composite material according to claim 1, wherein the layer of
fibres comprises the first component and the matrix comprises the
second component.
3. A composite material according to claim 1, wherein the layer of
fibres comprises the second component, and the matrix comprises the
first component.
4. A composite material according to claim 1, wherein the layer of
fibres is embedded in the matrix, partially embedded in the matrix,
or forms a separate layer in contact with the matrix.
5. A composite material according to claim 1, wherein the layer of
fibres comprises unidirectional fibres or a woven, knitted or
non-woven mesh.
6. A composite material according to claim 5, wherein the layer of
fibres comprises unidirectional fibres, and wherein the first
direction, along which loading is applied for the assessment of
auxetic behaviour, is parallel to the direction of the fibres.
7. A composite material according to claim 1, wherein the
coefficients of thermal expansion of the composite material,
measured parallel and perpendicular to the first direction, are
substantially equal.
8. A composite material according to claim 1, wherein the volume
fraction of the second component is between 60 and 70%.
9. A composite material according to claim 1, wherein the volume
fraction of the first component is less than 40%.
10. A composite material according to claim 1, wherein the
composite material additionally comprises a matrix material which
exhibits non-auxetic behaviour for loading along the first
direction.
11. A composite material according to claim 10, wherein the volume
fraction of the non-auxetic matrix material is less than 40%.
12. A composite material according to claim 1, wherein the auxetic
material is selected from auxetic thermoplastic (polyester
urethane), thermosetting (silicone rubber) and metal (copper)
foams, auxetic thermoplastic microporous polymeric cylinders (ultra
high molecular weight polyethylene (UHMWPE), polypropylene (PP),
and nylon), monofilaments (PP, nylon and polyester) and films (PP),
naturally-occurring polymers (crystalline cellulose), composite
laminates (carbon fibre-reinforced epoxy, glass fibre-reinforced
epoxy and aramid-reinforced epoxy), certain bismuth cuprate
superconducting polycrystalline compounds, 69% of the cubic
elemental metals, and naturally-occurring polymorphs of crystalline
silica (.alpha.-cristobalite and .alpha.-quartz).
13. A composite material according to claim 1, wherein the matrix
material comprises one or more polymeric materials, selected from
thermosetting polymers, thermoplastic polymers, or both
thermosetting and thermoplastic polymers.
14. A composite material according to claim 1, wherein the matrix
material further comprises one or more additional components
including any of the following either alone or in combination:
curing agent, accelerator, pigment, softener, flame retardant and
toughening agent.
15. A composite material comprising a layer of fibres and an
uncured matrix, whereby curing of the matrix will produce a
composite having a layer of fibres conjoined to a matrix, wherein
one of the matrix and fibres comprises a first component which
exhibits auxetic behaviour for loading along a first direction, and
the other of the matrix and fibres comprises a second component
which exhibits non-auxetic behaviour for loading along the first
direction.
16. A composite material according to claim 16, wherein the matrix
impregnates the layer of fibres during curing.
17. A method of making the composite material of claim 1,
comprising the steps of conjoining the layer of fibres to the
matrix.
Description
RELATED APPLICATION DATA
[0001] This application is a continuation of and claims the benefit
of U.S. application Ser. No. 12/300,229 which application, in turn,
claims the benefit of PCT/GB07/01946 which applications are
incorporated herein by reference in their entirety.
BACKGROUND
[0002] The present invention relates to a composite material and a
method for its production.
SUMMARY
[0003] A composite material is traditionally considered to be a
material system composed of a mixture or combination of two or more
micro- or macro-constituents that differ in form and chemical
composition and which are essentially insoluble in each other.
Composites are important because they possess properties that are
superior to the properties of their individual constituents.
Composite systems may be polymeric, metallic or ceramic based
systems, or some combination of these classes of materials.
Recently, composites have been developed having high and low melt
temperature constituents of the same polymer, and composites
containing constituents at the nanoscale (so-called nanocomposites)
have also been developed.
[0004] In polymeric composites, typically reinforcement materials
include glass, carbon, aramid, boron, silicon carbide and aluminium
oxide in a variety of forms including continuous fibres, short
chopped fibres, textile fabric structures and spherical inclusions.
Naturally-occurring polymer fibres such as hemp and cellulose are
also used as reinforcement materials. Common polymeric matrix
materials include thermosetting polymers such as
unsaturated-polyester, epoxy resins, phenolic resins and
polyimides, and thermoplastic polymers such as polypropylene,
polyamide, polycarbonate, polyacetols, polyetheretherketone (PEEK),
polyethylene terephtalate (PET), polyphenylene sulphide (PPS),
polyethersulphone (PES) polyetherimide (PEI), and polybutylene
terephthalate (PBT).
[0005] In ceramic composites, typically reinforcement materials
include silicon carbide, silicon nitride, boron carbide, aluminium
nitride, titanium diboride and boron nitride in a variety of forms
including continuous monofilament and multifilament tow fibres,
whiskers, platelets, and particulates. Common ceramic matrix
materials include alumina, silica, mullite, barium aluminosilicate,
lithium aluminosilicate, calcium aluminosilicate, silicon carbide,
silicon nitride, boron carbide and aluminium nitride.
[0006] In metal matrix composites, typically reinforcement
materials include tungsten, beryllium, titanium, molybdenum, boron,
graphite (carbon), alumina, silicon carbide, boron carbide and
alumina-silica in a variety of forms including continuous fibres,
discontinuous fibres, whiskers, particulates and wires. Common
metal matrix materials include aluminum, titanium, magnesium, iron
and copper alloys and superalloys.
[0007] Composite materials are typically in the form of laminates,
i.e. they are composed of a number of layers (laminae) each
containing continuous lengths of unidirectional reinforcing fibres
embedded within the matrix. Mechanical properties are optimised by
the choice of stacking sequence and orientation for a specific
application.
[0008] It is well known that the properties of advanced polymer
composites materials which are cured during fabrication at elevated
temperatures (typically 120 to 190.degree. C.) are degraded by the
residual stresses induced in the composite as the constituents,
i.e. matrix and reinforcement, shrink at different rates during
cooling to ambient temperatures (typically 20 to 30.degree.
C.).
[0009] It is also well known, that as an advanced composite heats
up and cools down the internal stresses will cause the shape of the
composite structures to distort.
[0010] In an attempt to reduce this distortion it is known to
introduce additional layers of materials which are positioned off
axis relative to the reinforcement. This process is known as
balancing. However, this has the effect of producing laminates in
which the mechanical properties may not be optimised, increases
time and cost in the manufacturing stage and also increases the
weight of the component.
[0011] An alternative approach has been to combine both positive
and negative coefficient of thermal expansion (CTE) materials
within the same composite in order to achieve on average the
desired zero or low thermal expansion of the overall composite.
Examples in this latter respect include negative axial CTE carbon
fibres within a positive CTE cyanate ester matrix for use in
casings for satellites to maintain size and shape from high
temperature launch to lower temperature space conditions. A
non-woven aramid material (negative CTE) is used to reinforce
positive CTE thermoset resin (e.g. epoxy) to produce low or zero
CTE substrates for use in printed circuit boards. Crystalline
quartz particles (negative CTE) are used within vitreous quartz
(positive CTE) to product low or zero CTE composite material for
large telescope mirror substrates and laser gyroscopes in aircraft.
Negative CTE zirconium tungstate packaging and supports are
combined with positive CTE silica fibre to produce low or zero CTE
fibre Bragg grating devices displaying constant reflected
wavelength over a range of temperatures for use in optoelectronic
systems.
[0012] However, combining negative and positive CTE materials does
have a number of disadvantages; these include: a) limited use as
there is a relative lack of negative CTE materials having the
appropriate range of other physical properties for specific
applications; b) in laminate systems there is a tendency to
increase inter laminar shear; and c) the inevitable increase in
weight and processing of the composite due to the addition of the
negative CTE material. These considerations lead to increased cost
of the final composite material.
[0013] Therefore, it is desirable to provide a composite material
whose components comprise materials having different rates of
expansion in order to minimise any distortion of the material which
results from heating up and cooling down the material. Moreover, it
is desirable that the component materials should have an
appropriate range of physical properties in order that the
composite materials can be widely used. It is also desirable to be
able to match the composite material to its surrounding structures
or to other composite materials in order to improve the performance
of the joints (being either mechanical or bonded) between the
various components of a composite material or structure of which a
composite material forms part.
[0014] According to a first aspect of the present invention there
is provided a composite material comprising a layer of fibres
conjoined to a matrix, wherein one of the matrix and fibres
comprises a first component which exhibits auxetic behaviour for
loading along a first direction, and the other of the matrix and
fibres comprises a second component which exhibits non-auxetic
behaviour for loading along the first direction.
[0015] Auxetic behaviour is defined by a Poisson's ratio, measured
in a particular direction with respect to the material, which is
negative (less than zero). As a result, when the material is
stretched in that direction by application of a tensile load, the
material expands transversely to that direction. Correspondingly,
when compressed in that direction, the material contracts
transversely to that direction. Similarly, non-auxetic behaviour is
defined by a Poisson's ratio which is positive (greater than
zero).
[0016] It will be understood that the term "first direction" is
that which the tensile load is applied, and therefore the direction
for which the auxetic behaviour is defined by the Poisson's
ratio.
[0017] It will be understood that the term "Young's modulus" is
known in the art and is a measure of stiffness. It is defined as
the ratio, for small strains, of the rate of change of stress with
strain. If Young's modulus is the same in all directions for a
material, the material is referred to as being isotropic. Materials
in which Young's modulus changes depending on which direction the
force is applied from are termed anisotropic. The SI unit of
Young's modulus is pascal (Pa) or alternatively kN/mm.sup.2, which
gives the same numeric value as gigapascals.
[0018] It will be understood that the term "Coefficient of Thermal
Expansion" is known in the art and refers to a change in a
material's dimensions due to a change in temperature. It will be
understood that materials having a positive expansion coefficient
will expand when heated, and contract when cooled. Some substances
have a negative expansion coefficient, and will expand when cooled
(e.g. freezing water).
The layer of fibres may be embedded in the matrix, partially
embedded in the matrix, or may form a separate layer in contact
with the matrix.
[0019] The layer of fibres may have any suitable construction; for
example, it may comprise bundles of unidirectional fibres, or a
woven, knitted, or non-woven mesh. Preferably, the layer of fibres
comprises unidirectional fibres or a woven, knitted or non-woven
mesh. More, preferably, the layer of fibres comprises
unidirectional fibres.
[0020] Where the layer of fibres comprises unidirectional fibres,
preferably the first direction, along which loading is applied for
the assessment of auxetic behaviour, is parallel to the direction
of the fibres.
[0021] For the avoidance of doubt, either or both of the phases
(fibre and matrix) of the composite material may comprise the first
component, the second component, or both first and second
components.
[0022] In a preferred embodiment, the layer of fibres comprises the
first component and the matrix comprises the second component.
Further preferably, the composite material comprises a layer of
fibres, some of which exhibit auxetic behaviour for loading along a
first direction and some of which exhibit non-auxetic behaviour for
loading along the first direction, embedded in a matrix which
exhibits non-auxetic behaviour for loading along the first
direction.
[0023] In a preferred embodiment, the coefficients of thermal
expansion of the composite, measured parallel and perpendicular to
the first direction, are substantially equal.
[0024] In order to control the relationship between the
longitudinal (i.e. measured parallel to the first direction) and
transverse (i.e. measured perpendicular to the first direction)
coefficients of thermal expansion of the composite, it is necessary
to select the materials of the composite having certain values of
coefficient of thermal expansion, Poisson's ratio, and Young's
modulus, and to control the volume fraction of the composite
occupied by each material.
[0025] In an alternative embodiment, the fibres comprise the second
component, and the matrix comprises the first component.
[0026] Preferably, the coefficient of thermal expansion of the
second component is lower than that of the first component, both
measured in a direction parallel to the first direction.
Preferably, the coefficient of thermal expansion of the second
component, measured in a direction parallel to the first direction,
is less than 1.times.10.sup.-5 K.sup.-1. Preferably, the
coefficient of thermal expansion of the first component, measured
in a direction parallel to the first direction, is greater than
5.4.times.10.sup.-5 K.sup.-1.
[0027] Preferably, the volume fraction of the second component is
between 60 and 70%, and more preferably is 62%. Preferably, the
volume fraction of the first component is less than 40%, more
preferably between 15 and 25%, and most preferably is 19%.
[0028] Preferably, the composite additionally comprises a matrix
material which exhibits non-auxetic behaviour for loading along the
first direction. Preferably, the volume fraction of the non-auxetic
matrix component is less than 40%, more preferably between 15 and
25%, and most preferably is 19%.
[0029] The volume fraction of the first component and the matrix
material may be preferably 38% in total in the embodiment where the
matrix material and the first component are constituents of the
matrix phase.
[0030] For example, in one embodiment the composite comprises:
[0031] a non-auxetic unidirectional fibrous component having a
volume fraction of 0.62, an axial Poisson's ratio of +0.2, a
transverse Poisson's ratio of +0.28, an axial Young's modulus of
230 GPa, a transverse Young's modulus of 3 GPa, an axial
coefficient of thermal expansion of -6.times.10.sup.-7K.sup.-1, and
a transverse coefficient of thermal expansion of
7.times.10.sup.-6K.sup.-1; [0032] a non-auxetic matrix component
having a volume fraction of 0.19, an isotropic Poisson's ratio of
+0.38, an isotropic Young's modulus of 3 GPa, an isotropic
coefficient of thermal expansion of 5.4.times.10.sup.-5K.sup.-1;
and [0033] an auxetic matrix component having a volume fraction of
0.19, an isotropic Poisson's ratio of -2, an isotropic Young's
modulus of 3 GPa, an isotropic coefficient of thermal expansion of
9.61.times.10.sup.-5K.sup.-1; [0034] said composite having zero
coefficient of thermal expansion, both parallel and perpendicular
to the direction of the fibres.
[0035] In an alternative embodiment, the volume fraction of the
second component is between 60 and 70%, and more preferably is 62%.
The volume fraction of the first component may preferably be less
than 40%, more preferably less than 10%, and most preferably is
3.5%.
[0036] Preferably, the composite additionally comprises a matrix
material which exhibits non-auxetic behaviour for loading along the
first direction. The volume fraction of the non-auxetic matrix
component is between 40% and 30%, and most preferably is 34.5%.
[0037] The volume fraction of the first component and the matrix
material may be preferably 38% in total in the embodiment where the
matrix material and the first component are constituents of the
matrix phase.
[0038] For example, in the alternate embodiment, the composite
comprises [0039] a non-auxetic unidirectional fibrous component
having a volume fraction of 0.62, an axial Poisson's ratio of +0.2,
a transverse Poisson's ratio of +0.28, an axial Young's modulus of
230 GPa, a transverse Young's modulus of 3 GPa, an axial
coefficient of thermal expansion of -6.times.10.sup.-7K.sup.-1, and
a transverse coefficient of thermal expansion of
7.times.10.sup.-6K.sup.-1; [0040] a non-auxetic matrix'component
having a volume fraction of 0.3455, an isotropic Poisson's ratio of
+0.38, an isotropic Young's modulus of 3 GPa, an isotropic
coefficient of thermal expansion of 5.4.times.10.sup.-5K.sup.-1;
and [0041] an auxetic matrix component having a volume fraction of
0.0345, an isotropic Poisson's ratio of -4, an isotropic Young's
modulus of 3 GPa, an isotropic coefficient of thermal expansion of
2.86.times.10.sup.-4K.sup.-1; [0042] said composite having zero
coefficient of thermal expansion, both parallel and perpendicular
to the direction of the fibres.
[0043] The auxetic material may therefore be used to control the
thermal expansivity of a composite material.
[0044] Without wishing to be bound by theory, it is believed that,
during cure of the composite material of the second aspect of
present invention, the first and second components become linked
within the composite. The strain induced in the auxetic material
(the first component) as the composite material changes
temperature, including changes in temperature arising during
processing, causes the auxetic component to expand and contract
transverse to the first direction, in opposition to the contraction
and expansion of the non-auxetic materials (including the second
component) in the composite. As thermal strains are induced in the
composite the expansion and contraction of the auxetic component
and non-auxetic components remain in balance creating a composite
material having non coefficient of expansion or a controlled rate
of expansion in accordance with the proportion and distribution of
the auxetic material within the composite.
[0045] Particular embodiments of the composite materials of the
present invention may also exhibit one or more of the following
advantages: [0046] a) coefficients of thermal expansion equal in
the longitudinal and transverse directions (i.e. parallel and
perpendicular to the first direction); [0047] b) where the
composite materials of the present invention are in the form of
laminates, a reduction in the number of layers of material
required, relative to a laminate composite material containing no
auxetic component, as a result of the removal of the directional
dependency of thermal expansion behaviour in the laminate composite
containing the auxetic component; [0048] c) reduced levels of
residual stresses relative to prior art composite materials; [0049]
d) removal of the need for separate balancing layers, conferring
design advantages such as reduced design analysis, additional
design options, improved composite performance and reduced
composite mass; [0050] e) reduced distortion during the cooling
process; and [0051] f) improved performance of joints between the
composite materials of the present invention and surrounding
materials having different rates of expansion, relative to such
joints for materials lacking an auxetic component. The improvement
is due to the ability to match the thermal expansion behaviour of
the composite to the surrounding materials through addition of the
auxetic component within the composite material or within an
intermediate layer, such as a film adhesive, between the composite
and surrounding materials.
[0052] A variety of auxetic materials have been reported, including
auxetic thermoplastic (polyester urethane), thermosetting (silicone
rubber) and metal (copper) foams (Friis, E. A., Lakes, R. S. &
Park, J. B., J. Mater. Sci. 1988, 23, 4406); auxetic thermoplastic
microporous polymeric cylinders (ultra high molecular weight
polyethylene (UHMWPE); polypropylene (PP), and nylon) (Evans, K. E.
& Ainsworth, K. L., International Patent Application WO
91/01210, 1991; Alderson, K. L. & Evans, K. E., Polymer, 1992,
33, 4435-8; Pickles, A. P., Alderson, K. L. & Evans, K. E.,
Polymer Engineering and Science, 1996, 36, 636-42; Alderson, K. L.,
Alderson, A., Webber, R. S. & Evans, K. E., J. Mater. Sci.
Lett., 1998, 17, 1415-19), monofilaments (PP, nylon and polyester)
(Alderson, K. L., Alderson, A., Smart, G., Simkins, V. R. &
Davies, P. J., Plastics, Rubber and Composites 2002, 31(8), 344;
Ravirala, N., Alderson, A., Alderson, K. L. & Davies, P. J.,
Phys. Stat. Sol. B 2005, 242(3), 653) and films (PP) (Ravirala, N.,
Alderson, A., Alderson, K. L. & Davies, P. J., Polymer
Engineering and Science 45(4) (2005) 517), naturally-occurring
polymers (crystalline cellulose) (Peura, M., Grotkopp, I., Lemke,
H., Vikkula, A., Laine, J., Muller, M. & Serimaa, R.,
Biomacromolecules 2006, 7(5), 1521 and Nakamura, K., Wada, M.,
Kuga, S. & Okano, T. J Polym Sci B Polym Phys Ed 2004; 42,
1206), composite, laminates (carbon fibre-reinforced epoxy, glass
fibre-reinforced epoxy and aramid-reinforced epoxy) (Alderson, K.
L., Simkins, V. R., Coenen, V. L., Davies, P. J., Alderson, A.
& Evans, K. E., Phys. Stat. Sol. B 242(3) (2005) 509), certain
bismuth cuprate superconducting polycrystalline compounds (Dominec,
J., Vasek, P., Svoboda, P., Plechacek, V. & Laermans, C, Modern
Physics Letters B, 1992, 6, 1049-54), 69% of the cubic elemental
metals (Baughman, R. H., Shacklette, J. M., Zakhidov, A. A. &
Stafstrom, S., Nature, 1998, 392, 362-5), and naturally-occurring
polymorphs of crystalline silica .alpha.-cristobalite and
.alpha.-quartz) (Yeganeh-Haeri, Y., Weidner, D J. & Parise, J.
B., Science, 1992, 257, 650-2; Keskar, N. R. & Chelikowsky, J.
R., Phys. Rev. B 48, 16227 (1993)). Poisson's ratios as low as -12
have been measured in the auxetic polymers (Caddock, B. D. &
Evans, K. E., J. Phys. D: Appl. Phys., 1989, 22, 1877-82),
indicating very large transverse strains (over an order of
magnitude greater than the applied longitudinal strain) are
possible.
[0053] Suitable fibres (reinforcement materials) in polymer
composites are widely known within the field and may comprise
continuous fibres, short chopped fibres, textile fabric structures
and spherical inclusions made from glass, carbon, aramid, boron,
silicon carbide and aluminium oxide. Any combination of the said
fibres and forms may be used. Nanofibres and nanotubes may also
form suitable fibres for use with the present invention. It is, of
course, recognised that other alternative polymer, metal or ceramic
materials to those identified above could be included as fibres, as
would be readily apparent to the man skilled in the art.
[0054] The matrix material of the present invention may comprise
one or more polymeric materials. The matrix material may comprise
thermosetting polymers, thermoplastic polymers, or both
thermosetting and thermoplastic polymers. Suitable thermosetting
polymer examples are well known to those skilled in the art and
include any of the following either alone or in combination: epoxy
resins, unsaturated polyester resins, phenolic resins and
polyimides. Suitable thermoplastic polymer examples are well known
to those skilled in the art and include any of the following either
alone or in combination: polypropylene, polyamide, polycarbonate,
polyacetols, polyetheretherketone (PEEK), polyethylene terephtalate
(PET), polyphenylene sulphide (PPS), polyethersulphone (PES)
polyetherimide (PEI), and polybutylene terephthalate (PBT).
[0055] The matrix material may further comprise one or more
additional components which may include any of the following either
alone or in combination: curing agents, accelerators, pigments,
softeners, flame retardants and toughening agents. The additional
components may be organic (including polymeric), inorganic
(including ceramic) or metallic in nature.
[0056] The additional components are added with the desired
properties of the composite material in mind.
[0057] The auxetic component of the present invention may be
incorporated into the fibres by way of auxetic monofilaments and
multi-filaments and/or it may be incorporated into the matrix
material.
[0058] Auxetic monofilaments and multi-filaments may be
incorporated in the form of continuous fibres, short chopped
fibres, or textile fabric structures.
[0059] The way in which the auxetic component is incorporated into
the matrix material depends upon the nature of the desired
composite material.
[0060] For example, finely divided auxetic materials may be added
to the matrix in the form of a filler. Polycrystalline aggregates
of .alpha.-cristobalite are suitable for incorporation into the
matrix in this way. The auxetic filler may also be an alternative
ceramic material, a polymer or a metal. Auxetic character may also
be incorporated into a composite material through engineering the
auxetic effect at the molecular level within the matrix itself.
Examples of auxetic molecular-level materials include liquid
crystalline polymers (He, C., Liu, P. & Griffin, A. C.,
Macromolecules, 31, 3145 (1998)), crystalline cellulose, cubic
elemental metals, zeolites, .alpha.-cristobalite, and
.alpha.-quartz.
[0061] Auxetic thermoplastic and/or thermosetting resins are known
to the skilled man and would be suitable for use as the matrix
material in the present invention.
[0062] Auxetic character may be imparted upon metal and ceramic
based composites by way of auxetic metallic and ceramic
materials.
[0063] Suitable fibres in Ceramic Matrix Composites are widely
known within the field and may comprise continuous monofilament and
multifilament tow fibres, whiskers, platelets and particulates of
silicon carbide, silicon nitride, boron carbide, aluminium nitride,
titanium diboride and boron nitride. Any combination of the said
materials and forms may be used. The auxetic component of a Ceramic
Matrix Composite may be incorporated into the fibres by way of
monofilaments and multi-filaments, whiskers, platelets and
particulates of auxetic ceramic. Known auxetic ceramics include the
.alpha.-cristobalite and .alpha.-quartz polymorphs of silica,
carbon nitride (Guo, Y. & Goddard III, W. A., Chem. Phys.
Lett., 1995, 237, 72), and certain bismuth cuprate compounds.
[0064] Matrix materials in Ceramic Matrix Composites are well known
to those skilled in the art and include oxides such as alumina,
silica, mullite, barium aluminosilicate, lithium aluminosilicate
and calcium aluminosilicate. Non-oxide ceramic matrix materials
include silicon carbide, silicon nitride, boron carbide, and
aluminium nitride. The auxetic component of a Ceramic Matrix
Composite may be incorporated into the matrix material as, for
example, finely divided auxetic ceramic materials added to the
matrix in the form of a filler. Alternatively, the ceramic matrix
may be intrinsically auxetic.
[0065] Suitable fibres in Metal Matrix Composites are widely known
within the field and may comprise continuous fibres, discontinuous
fibres, whiskers, particulates and wires of tungsten, beryllium,
titanium, molybdenum, boron, graphite (carbon), alumina, silicon
carbide, boron carbide and alumina-silica.
[0066] Matrix materials in Metal Matrix Composites materials are
well known to those skilled in the art and include aluminum,
titanium, magnesium, iron and copper alloys and superalloys.
[0067] The auxetic component of a Metal Matrix Composite may be
incorporated into the fibres by way of continuous fibres,
discontinuous fibres, whiskers, particulates and wires of auxetic
ceramic or metal material. The auxetic component of a Metal Matrix
Composite may also be incorporated into the matrix material as, for
example, finely divided auxetic ceramic or metal materials added to
the matrix in the form of a filler. Alternatively, the metal matrix
may be intrinsically auxetic. Known auxetic ceramics include the
.alpha.-cristobalite and .alpha.-quartz polymorphs of silica,
carbon nitride, and certain bismuth cuprate compounds. Known
auxetic metals include arsenic, cadmium and 69% of the cubic
elemental metals.
[0068] The present invention also provides a method for the
preparation of a composite material described herein.
[0069] According to a second aspect of the present invention there
is provided a method of making an uncured composite material of the
first aspect comprising mixing: a layer of fibres, an uncured
matrix, a first component which exhibits auxetic behaviour, and a
second component which exhibits non-auxetic behaviour.
[0070] Preferably, where the auxetic material is anisotropic, the
method according to the second aspect further includes forming the
uncured composite comprising the auxetic material having a required
orientation relative to the other components of the composite.
[0071] According to a third aspect of the present invention there
is provided a method of making a composite material comprising
forming an uncured composite material in accordance with the second
aspect, and curing the uncured composite material.
[0072] The auxetic material used for the methods of the second and
third aspect is selected for having the required properties and
used in a required quantity. The uncured composite material of the
second aspect is cured to obtain a cured composite material having
the required thermal expansivities.
[0073] In a preferred embodiment, the matrix impregnates the layer
of fibres during curing.
[0074] A typical method for the preparation of a curable composite
material of the first aspect comprises: [0075] a) laying out a
3-phase pre-preg reinforcing fibre-epoxy-auxetic material on a
supporting table. The pre-preg consists of continuous
unidirectional reinforcing fibres and continuous unidirectional
auxetic fibres in a partially cured epoxy matrix. [0076] b) cutting
out and placing pieces of the pre-preg sheet in layers on top of
each other on a tool of the required shape to form a laminate. The
layers may be placed in different directions to optimise the
properties of the composite. [0077] c) placing the constructed
laminate and tool in a vacuum bag, and applying a vacuum to remove
entrapped air from the composite part. [0078] d) placing the vacuum
bag including the composite and tooling inside an autoclave for
curing of the epoxy resin to take place. Curing conditions depend
on the particular epoxy material employed. Typically the cure cycle
lasts many hours during which the composite material is typically
heated to a temperature in the range of 120 to 190.degree. C. at a
pressure of typically 350 to 700 kPa. [0079] e) removing the vacuum
bag including the composite and tooling from the autoclave,
removing the composite and tooling from the vacuum bag, and
removing the composite part from the tooling prior to further
finishing operations.
[0080] Alternatively, another method for the preparation of a
curable composite material comprises the following: [0081] a)
applying a gel coat to an open mould. [0082] b) manually placing
reinforcing fibre incorporating auxetic fibre in the mould. The
reinforcing fibre and auxetic fibre may be in the form of a cloth
or mat. [0083] c) pouring, brusing, or spraying a resin, typically
polyester, mixed with catalysts and accelerators over and into the
reinforcing fibre-auxetic fibre plies. [0084] d) using squeegees or
rollers to wet the reinforcing fibres and auxetic fibres with the
resin, and to remove entrapped air. [0085] e) optionally adding
additional reinforcing fibre-auxetic fibre plies and resin to
increase the thickness of the part. [0086] f) curing using room
temperature curing resins, and initiating curing by a catalyst in
the resin system, which hardens the composite without external
heat.
[0087] Alternatively, there is provided a method for the
preparation of a curable composite material in the form of a hollow
cylinder comprising: [0088] a) passing the reinforcing and auxetic
fibres through a resin bath. [0089] b) winding the
resin-impregnated reinforcing and auxetic fibres on a rotating
mandrel. [0090] c) curing when sufficient layers have been applied
the component at room temperature or at elevated temperature in an
oven. [0091] d) removing the moulded composite from the
mandrel.
[0092] Alternatively, another method for the preparation of a
curable composite material comprising: [0093] a) laying out a
3-phase pre-preg reinforcing fibre-epoxy-auxetic material on a
supporting table. The pre-preg consists of continuous
unidirectional reinforcing fibres in a partially cured epoxy matrix
containing auxetic filler particles. [0094] b) cutting out and
placing pieces of the pre-preg sheet in layers on top of each other
on a tool of the required shape to form a laminate. The layers may
be placed in different directions to optimise the properties of the
composite. [0095] c) placing the constructed laminate and tool in a
vacuum bag, and applying a vacuum to remove entrapped air from the
composite part. [0096] d) placing the vacuum bag including the
composite and tooling inside an autoclave for curing of the epoxy
resin to take place. Curing conditions depend on the particular
epoxy material employed. Typically the cure cycle lasts many hours
during which the composite material is typically heated to a
temperature in the range of 120 to 190.degree. C. at a pressure of
typically 350 to 700 kPa. [0097] e) removing the vacuum bag
including the composite and tooling from the autoclave, removing
the composite and tooling from the vacuum bag, and removing the
composite part from the tooling prior to further finishing
operations.
[0098] Alternatively, another method for the preparation of a
curable composite material comprises the following: [0099] a)
applying a gel coat to an open mould. [0100] b) manually placing
reinforcing fibre in the mould. The reinforcing fibre may be in the
form of a cloth or mat. [0101] c) mixing a resin, typically
polyester, incorporating auxetic filler particles with catalysts
and accelerators, and then pouring, brushing, or spraying over and
into the reinforcing fibre plies. [0102] d) using squeegees or
rollers are used to wet the reinforcing fibres with the resin
containing the auxetic filler, and to remove entrapped air. [0103]
e) optionally adding additional reinforcing fibre plies and auxetic
filler-containing resin to increase the thickness of the part.
[0104] f) using room temperature curing resins, and initiating
curing by a catalyst in the resin system, which hardens the
composite without external heat.
[0105] Alternatively, there is provided a method for the
preparation of a curable composite material in the form of a hollow
cylinder comprising: [0106] a) passing the reinforcing fibres
through a resin bath containing auxetic filler particles within the
resin. [0107] b) winding the auxetic filler-containing
resin-impregnated reinforcing fibres on a rotating mandrel. [0108]
c) curing either at room temperature or at elevated temperature in
an oven when sufficient layers have been applied to the component.
[0109] d) removing the moulded composite from the mandrel.
[0110] It will be understood that the Poisson's ratio, Young's
modulus, and coefficient of thermal expansion are determined at
atmospheric pressure and room temperature (i.e. 20.degree. C.),
unless otherwise stated.
[0111] It is envisaged that the material of the present invention
will find utility in the following applications: [0112] a)
Composite structures where a significant reduction of weight or
increase in performance, such as load bearing capacity, is
desirable and can be achieved by reduction of the internal stresses
through the introduction of auxetic materials into the laminate.
Applications include components for aircraft, road vehicles,
off-road vehicles, military vehicles, precision machinery, boats,
ships, and submarines. [0113] b) Composites tools of improved
performance, including for example: lower cost applications where
expensive carbon fibre may be partially replaced by lower cost
auxetic fibre or filler; improved precision and longer life due to
thermal matching. [0114] c) Composite structure, containing
materials (matrix or reinforcement), which are thermally mismatched
due to elevated temperature cures. The use of auxetic constituents
enables reduced mass of the composite, reduced cost of design,
improved design performance arising from increased design freedom,
and reduced manufacturing costs and timescales. [0115] d)
Composites structures containing materials (matrix or
reinforcement) which are thermally mismatched and operate over a
considerable temperature range, including cryogenic applications.
Cryogenic structures such as cryogenic fuel tanks and spacecraft
components will benefit through reduced microcracking as a result
of the reduction in residual stresses when incorporating an auxetic
constituent within the composite. [0116] e) Composite structures
displaying enhanced stability for stability-critical applications
such as optical instruments, RF instruments and measuring
instruments. Improved stability arises through reduced
microcracking, balanced lay-ups and reduced impact of manufacturing
errors. [0117] f) Composite structures requiring zero or low CTE
behaviour, including casings for satellites to maintain size and
shape from high temperature launch to low temperature space
conditions; substrates for use in printed circuit boards; stable
structures including optical benches; large telescope mirror
substrates; laser gyroscopes in aircraft; fibre Bragg grating
devices displaying constant reflected wavelength over a range of
temperatures for use in optoelectronic systems. [0118] g) Composite
structures which require machining after moulding benefit from
containing auxetic materials in the laminate. In the current state
of the art the machining creates an imbalance in the laminate and
may induce distortion in the part. This has particular applications
for the machining of mould surfaces on composite tools. [0119] h)
Composite structures can be produced with fundamentally unbalanced
laminates by the addition of auxetic materials to the laminate.
This will have applications in parts which replace castings, or
which can be made from unbalanced pre-forms produced by hand
processes, knitting and/or weaving processes. [0120] j) By adding
auxetic materials in combination with localised unbalanced laminate
configurations it is possible to produce local areas which have a
different coefficient of thermal expansion to the component. This
can be used to create areas suitable for the fitting of components
which have a substantially different CTE, such as in metallic
bearings. [0121] k) Lower cost components can be made where
substantial quantities of the expensive high performance
reinforcement, such as carbon fibre, are replaced with a high
proportion of lower cost auxetic fibre. [0122] l) It is well known
to those versed in the art that auxetic configurations have
improved resistance to penetration, hi addition, the reduced levels
of internal stresses within the laminate resulting from the
addition of auxetic materials will increase the impact resistance
and energy absorbed during crushing. This has applications in the
production of lightweight armour and vehicle crash structures.
[0123] m) Structures which distort in response to mechanical,
thermal or electrical inputs, known as smart structures, are useful
for producing products such as aircraft of superior performance.
The addition of auxetic materials to a composite laminate used in a
smart structure reduces the cost and complexity of the design since
thermal balancing issues can be ignored, and enables the
optimisation and adjustment of the laminate to respond to the
mechanical, thermal or electrical input.
BRIEF DESCRIPTION OF THE DRAWINGS
[0124] The present invention will now be described further, by way
of example only, and with reference to the following drawings in
which:
[0125] FIG. 1 shows a diagrammatic representation of a
unidirectional composite laminate according to the prior art;
[0126] FIG. 2 shows a diagrammatic representation of a
unidirectional composite laminate according to the present
invention;
[0127] FIG. 3 shows a graph depicting coefficients of thermal
expansion as a function of the Poisson's ratio of the third, phase,
for the laminate of FIG. 2;
[0128] FIG. 4 shows a graph depicting the Poisson's ratio and
coefficient of thermal expansion of the third phase as a function
of volume fraction of the reinforcing fibre for the laminate of
FIG. 2 with equal volume fractions for the non-auxetic matrix and
3.sup.rd auxetic phase;
[0129] FIG. 5 shows a graph depicting the Poisson's ratio and
coefficient of thermal expansion of the third phase as a function
of volume fraction of the reinforcing fibre for the laminate of
FIG. 2 with the volume fraction of the 3.sup.rd auxetic phase equal
to 10% of the volume fraction of the non-auxetic matrix;
[0130] FIG. 6 shows a graph depicting the length as a function of
time for an auxetic polypropylene fibre undergoing a heating cycle
from 30.degree. C. to 80.degree. C. and back to 30.degree. C.;
[0131] FIG. 7 shows a graph depicting coefficients of thermal
expansion as a function of the Young's modulus of the third phase
for the laminate of FIG. 2;
[0132] FIG. 8 shows a Finite Element Model (FEM) of a 3-phase
composite comprising a central reinforcing fibrous phase surrounded
by a matrix phase and a 3.sup.rd (fibrous) phase located at each
corner of the repeat unit;
[0133] FIG. 9 sows a FEM model of axial strains acting on the
reinforcing fibrous phase as a result of heating of the composite
up to 120.degree. C.;
[0134] FIG. 10 shows a FEM model of axial strains acting on the
non-auxetic matrix phase as a result of heating of the composite up
to 120.degree. C.;
[0135] FIG. 11 shows a FEM model of axial strains acting on the
3.sup.rd phase as a result of heating of the composite up to
120.degree. C.;
[0136] FIG. 12 shows a FEM model of transverse (z direction)
strains acting on a non-auxetic 3.sup.rd phase as a result of
heating of the composite up to 120.degree. C.;
[0137] FIG. 13 shows a FEM model of transverse (z direction)
strains acting on an auxetic 3.sup.rd phase as a result of heating
of the composite up to 120.degree. C.;
[0138] FIG. 14 shows a FEM model of transverse (z direction)
stresses acting on a 2-phase composite (comprising a central
reinforcing fibrous phase surrounded by a non-auxetic matrix) as a
result of heating of the composite up to 150.degree. C.; and
[0139] FIG. 15 shows a FEM model of transverse (z direction)
stresses acting on a 3-phase composite (comprising a central
reinforcing fibrous phase surrounded by a non-auxetic matrix with
an auxetic 3.sup.rd phase) as a result of heating of the composite
up to 150.degree. C.
DETAILED DESCRIPTION
[0140] FIG. 1 shows a composite laminate material 1 according to
the prior art. The composite material 1 comprises two layers of
carbon fibre reinforcement 2 and three layers of epoxy matrix
component 3. The carbon fibre reinforcement layers 2 are arranged
between the epoxy matrix component layers 3.
[0141] FIG. 2 shows a composite laminate material 4 of the present
invention. The composite material 4 comprises carbon fibre
reinforcement layers 5, and epoxy matrix component layers 6. The
composite material also comprises an auxetic component layer 7
which is located between the carbon fibre reinforcement layers
5.
[0142] The following text further illustrates the present invention
by comparing the anisotropic thermal expansion and residual stress
behaviour for the prior art composite materials of the type shown
in FIG. 1, and composite materials of the present invention of the
type shown in FIG. 2.
[0143] Thermal Expansion Behaviour
[0144] Prior Art Composite Material.
[0145] If the reinforcing fibres 2 of the composite material 1 of
FIG. 1 are assumed to be intimately bonded interfaces, then the
thermal expansion coefficients along and transverse to the fibre
layer 2 direction (X.sub.1) are known to be reproduced well by the
following equations (Kollar, L. P. & Springer, G. S., Mechanics
of Composite Structures, Cambridge, pp. 443-444):
.alpha. 1 = V f E f 1 .alpha. f 1 + V m E m .alpha. m V f E f 1 + V
m E m ( 1 ) .alpha. 2 = V f .alpha. f 2 + V m .alpha. m + V f v f
12 ( .alpha. f 1 - .alpha. 1 ) + V m v m ( .alpha. m - .alpha. 1 )
( 2 ) ##EQU00001##
[0146] where: [0147] .alpha..sub.1 and .alpha..sub.2 are the
coefficients of thermal expansion of the composite material 1 along
and transverse to the fibre layer 2 direction, respectively, [0148]
V.sub.f and V.sub.m are the fibre layer 2 and matrix 3 volume
fractions, respectively, [0149] E.sub.f1 and E.sub.m are the fibre
layer 2 axial modulus and the matrix layer 3 Young's modulus,
respectively, [0150] A.sub.f1, .alpha..sub.f2 and .alpha..sub.m are
the fibre 2 axial, fibre 2 radial and matrix 3 coefficients of
thermal expansion, respectively, and [0151] v.sub.f12 and v.sub.m
are the fibre 2 axial and matrix 3 Poisson's ratios,
respectively.
[0152] Using typical values of the parameters for carbon fibre as
the fibre layers 2 and epoxy resin as the matrix 3 (V.sub.f=0.62,
V.sub.m=0.38, E.sub.f1=230 GPa, E.sub.m=3 GPa,
.alpha..sub.f1=-6.times.10.sup.-7K.sup.-1
.alpha..sub.f2=7.times.10.sup.-6K.sup.-1,
.alpha..sub.m=5.4.times.10.sup.-5K.sup.-1, v.sub.f12=+0.2 and
v.sub.m=+0.38), equations (1) and (2) yield values for the
coefficients of thermal expansion of the composite of
.alpha..sub.1=-1.67.times.10.sup.-7K.sup.-1 and
.alpha..sub.2=3.26.times.10.sup.-5K.sup.-1, clearly demonstrating
the anisotropic nature of the thermal expansivity of the composite
material 1 of FIG. 1.
[0153] Composite Material of the Present Invention.
[0154] The established analytical models as shown by equations (1)
and (2) can be extended to include the presence of a third phase,
and the coefficients of thermal expansion of the 3-phase composite
material 4 of FIG. 2 are given by:
.alpha. 1 = V f E f 1 .alpha. f 1 + V m E m .alpha. m + V a E a
.alpha. a V f E f 1 + V m E m + V a E a ( 3 ) .alpha. 2 = V f
.alpha. f 2 + V m .alpha. m + V a .alpha. a + V f v f 12 ( .alpha.
f 1 - .alpha. 1 ) + V m v m ( .alpha. m - .alpha. 1 ) + V a v a (
.alpha. a - .alpha. 1 ) ( 4 ) ##EQU00002##
[0155] where: [0156] V.sub.a is the volume fraction of the third
auxetic phase 7, [0157] E.sub.a is the Young's modulus of the third
auxetic phase 7, [0158] .alpha..sub.a is the coefficient of thermal
expansion of the third phase 7, and [0159] v.sub.a is the Poisson's
ratio of the third phase 7, [0160] with the other symbols as
already defined for equations (1) and (2).
[0161] One or more properties of the 3.sup.rd auxetic phase 7 may
be varied in order to achieve equal thermal expansivities along and
transverse to the fibre layer 5 direction, including the
possibility of (near) zero thermal expansion. For example, assuming
all other properties of the 3.sup.rd auxetic phase 7 are the same
as the epoxy matrix 6 properties, and that the epoxy 6 and third
auxetic phase 7 have equal volume fractions of 0.19 (i.e. the fibre
layer 5 volume fraction is 0.62), then equal and near zero thermal
expansivities are achieved along and transverse to the fibre layer
5 direction by choosing a 3.sup.rd auxetic phase 7 having a
Poisson's ratio near to -3. This is shown in FIG. 3 which shows the
Poisson's ratio and coefficient of thermal expansion of the
3.sup.rd auxetic phase 7 as a function of the volume fraction of
the reinforcing fibre 5, where the volume fraction of the 3.sup.rd
auxetic phase is equal to 10% of the volume fraction of the
non-auxetic matrix 6. Polymeric auxetic materials are known with
Poisson's ratios as low as -12.
[0162] Where the coefficient of thermal expansion is zero, both
along and transverse to the fibre 6 direction, the following
relations hold for the coefficient of thermal expansion and
Poisson's ratio of the third phase 7:
.alpha. a = - ( V f E f 1 .alpha. f 1 + V m E m .alpha. m ( 1 - V f
- V m ) E a ) ( 5 ) v a = V f [ .alpha. f 2 + .alpha. f 1 ( v f 12
- E f 1 E a ) ] + V m .alpha. m ( 1 + v m - E m E a ) V f E f 1 E a
.alpha. f 1 + V m E m E a .alpha. m ( 6 ) ##EQU00003##
[0163] Equation (5) enables the relative amounts and properties of
the non-auxetic reinforcement 5 and matrix phases 6 to be carefully
selected to match the available coefficient of thermal expansion
and Young's modulus of the third (auxetic) phase 7, and vice versa.
Equation (6) provides for the selection of the appropriate sign and
magnitude of Poisson's ratio for the third phase 7 based on the
relative proportions and properties of the non-auxetic
reinforcement 5 and matrix phases 6 and the Young's modulus of the
third phase 7.
[0164] The coefficient of thermal expansion and Poisson's ratio of
the third phase 7 are shown in FIG. 4 as a function of volume
fraction of the reinforcing phase 5 for the proportions and
properties of the constituents as defined above. For realistic
reinforcing fibre 5 volume fractions in laminate composite systems
4 (V.sub.f=0.6 to 0.7) the coefficient of thermal expansion of the
third phase 7 is of the order of 1.times.10.sup.-4 K.sup.-1. The
Poisson's ratio of the third phase 7 for Vf in the range 0.6 to 0.7
is of the order of -2.
[0165] Alternatively, it may be desirable to have a low volume
fraction of the 3.sup.rd auxetic phase 7. The coefficient of
thermal expansion and Poisson's ratio of the third phase are shown
in FIG. 5 as a function of volume fraction of the reinforcing phase
5 for the properties of the constituents as defined above, with the
volume fraction of the 3.sup.rd phase 7 equal to 10% of the volume
fraction of the matrix phase 6. For realistic reinforcing fibre 5
volume fractions in laminate composite systems 4 (V.sub.f=0.6 to
0.7) the coefficient of thermal expansion of the third phase 7 is
of the order of 3.times.10.sup.-4 K.sup.-1. The Poisson's ratio of
the third phase 7 for V.sub.f in the range 0.6 to 0.7 is of the
order of -4.
[0166] Auxetic polymers are known with Poisson's ratios in the
range 0 to -12. A coefficient of thermal expansion in the range
1.times.10.sup.-4 to 3.times.10.sup.-4 K.sup.-1, for high and low
auxetic fibre volume fractions respectively, is typical of many
polymers and corresponds to the coefficient of expansion measured
for auxetic polypropylene fibres of 2.times.10.sup.-4 K.sup.-1.
[0167] FIG. 6 show a graph depicting the length as a function of
time for an auxetic polypropylene fibre undergoing a heating cycle
from 30.degree. C. to 80.degree. C. and back to 30.degree. C. At
80.degree. C. the fibre undergoes an extension of 0.14 mm from an
initial length of 13 mm at 30.degree. C. This corresponds to a
strain of 0.01 over a 50.degree. C. (50 K) temperature increase,
yielding a coefficient of thermal expansion of 2.times.10.sup.-4
K.sup.-1 for the fibre.
[0168] Alternatively, for a 3.sup.rd auxetic phase 7 possessing a
Poisson's ratio of v.sub.a=-0.6 (typical of the auxetic polymeric
fibres reported in the literature), with all other parameters
except Young's modulus as above, and having equal 3<rd> phase
7 and matrix 6 volume fractions, equal (but non-zero) thermal
coefficients of expansion are realised for the composite material 4
when the Young's modulus of the 3.sup.rd auxetic phase 7 is of the
order of the axial Young's modulus of the reinforcing (carbon)
fibre 5. This is shown by FIG. 7 which is a graph of coefficients
of thermal expansion as a function of the Young's modulus of the
third phase 7 for the composite material 4 of FIG. 2.
[0169] Residual Stresses
[0170] The two-phase carbon-epoxy composite structure 1 shown
schematically in FIG. 1 will typically be cured at elevated
temperatures, and subsequently cooled to ambient temperatures.
During cooling the matrix 3 and reinforcement 2 shrink at different
rates. This gives rise to a thermally induced mechanical stress on
each component.
[0171] For longitudinal expansion (i.e. along the fibre 2
direction), the carbon fibre 2 undergoes little thermal expansion
or contraction upon cooling due to the near zero coefficient of
thermal expansion of the fibre 2 in this direction. The epoxy 3, on
the other hand, has a large positive coefficient of thermal
expansion, and therefore contracts in length. However, whilst the
interface between the epoxy 3 and carbon 2 is intact, then the
higher modulus carbon fibre 2 constrains the lower modulus epoxy
matrix 3 from contraction, and so the thermal loading is converted
to a mechanical tensile stress on the matrix 3. A tensile stress
along the fibre 2 direction tends to cause the epoxy 3 to contract
transversely (due to the positive Poisson's ratio of epoxy),
leading to a build up of residual stress at the fibre 2-matrix 3
interface and therefore a degradation of the mechanical properties
of the composite material 1.
[0172] For a 3-phase composite material 4 shown schematically in
FIG. 2, the conversion of thermal strain to mechanical stress on
the constituents would tend to put both the epoxy 6 and auxetic 7
(3.sup.rd auxetic phase) under tensile stress in the fibre
direction 5 as the composite 4 cools. The auxetic phase 7 will
expand in the transverse direction as a result of the negative
Poisson's ratio, opposing the tendency of the epoxy 6 to contract.
This will give rise to a reduction in the residual stresses within
the composite 4, and therefore reduce the degradation of the
mechanical properties that would otherwise occur in the 2-phase
composite 1 as shown in FIG. 1.
[0173] Similarly, during heating of a 3-phase composite 4, the
matrix 6, auxetic 7, and reinforcement 5 phases expand at different
rates. Again, the carbon fibre 5 undergoes little thermal expansion
or contraction upon heating due to the near zero coefficient of
thermal expansion of the fibre 5 in this direction. The epoxy 6 and
auxetic 7 (3.sup.rd) phases, on the other hand, have large positive
coefficients of thermal expansion and so try to increase in length.
However, the higher modulus carbon fibre 5 constrains the lower
modulus epoxy matrix 6 and auxetic phase 7 from extension, and so
the thermal loading is converted to a mechanical compressive stress
on the matrix 6 and auxetic phases 7 in the fibre 5 direction. As a
result, large compressive strains develop in the epoxy 6 and
auxetic phases 7 along the fibre 5 direction, relative to the near
zero axial strain in the carbon fibre 5 itself. A compressive
stress along the fibre 5 direction causes the epoxy 6 to expand
transversely (due to the positive Poisson's ratio of epoxy 6) and
the auxetic phase 7 to contract in the transverse direction (as a
result of the negative Poisson's ratio). Once again, there is a
reduction in the residual stresses within the composite 4, and
therefore a reduction in the degradation of the mechanical
properties that would otherwise occur in a 2-phase composite 1 as
shown in FIG. 1.
[0174] For the following finite element modelling FEM FIGS. 8-15,
the key shown on the figures identifies areas of higher compressive
strain or stress as those corresponding to the shading shown on the
left side of the key. Areas of lower compressive (or in some cases
tensile) strain or stress are shown by shading corresponding to
that on the right side of the key.
[0175] FIG. 8 shows an FEM of a 3 phase composite material of the
type shown in FIG. 2 undergoing heating from 0.degree. C. to
120.degree. C. FIG. 8 shows the 3-phase unit-cell 80 used in the
FEM simulations, the unit-cell 80 comprising: [0176] a non-auxetic
unidirectional reinforcing fibrous component 81 having a volume
fraction of 0.62, an axial Poisson's ratio of +0.2, an axial
Young's modulus of 230 GPa, an axial coefficient of thermal
expansion of -6.times.10.sup.-7K.sup.-1, and a transverse
coefficient of thermal expansion of 7.times.10.sup.-6K.sup.-1;
[0177] a non-auxetic matrix component 82 having a volume fraction
of 0.19, an isotropic Poisson's ratio of +0.38, an isotropic
Young's modulus of 3 GPa, an isotropic coefficient of thermal
expansion of 5.4.times.10.sup.-5K.sup.-1; and [0178] a third phase
unidirectional fibrous component 83 having a volume fraction of
0.19, an isotropic Young's modulus of 3 GPa, and an isotropic
coefficient of thermal expansion of
5.4.times.10.sup.-5K.sup.-1.
[0179] FIGS. 9, 10 and 11 show FEMs of the individual components
making up a unit cell of the type shown in FIG. 8. FIG. 9 shows an
FEM of a reinforcing fibre 90. FIG. 10 shows an FEM of the matrix
100, and FIG. 11 shows an FEM of the 3.sup.rd auxetic phase 110.
The strains that develop upon heating to 120.degree. C. along the
axial (fibre) direction in the reinforcing fibre 90, non-auxetic
matrix 100 and the third phase 110, clearly showing compressive
strains developing in the matrix 100 and third phase constituents
110, and that these strains are approximately 2 orders of magnitude
larger than those that develop in the reinforcing fibre phase
90.
FIGS. 12 and 13 show FEMs of transverse strains acting on a
non-auxetic 3.sup.rd phase 120 and 130 due to heating a composite
material up to 120.degree. C. The strains that develop in the
transverse z direction perpendicular to the axial direction for the
3.sup.rd phases 120 and 130 have a Poisson's ratio of +0.38 (i.e.
same as the non-auxetic matrix phase) and -0.6 (i.e. auxetic),
respectively.
[0180] FIGS. 12 and 13 clearly show the transverse deformation of
the 3.sup.rd phase 120 and 130 depend on the sign of the Poisson's
ratio of the material, undergoing transverse expansion and
contraction for non-auxetic and auxetic constituents respectively,
as a result of the axial compression which builds up on heating
when compared to FIG. 11.
[0181] FIGS. 14 and 15 show the transverse (z direction) stresses
which develop within a 2-phase 140 and 3-phase 150 composite system
upon heating to 150.degree. C.
[0182] The 2-phase composite 140 used for FIG. 14 is of the type
shown in FIG. 1 and, comprises: [0183] a non-auxetic unidirectional
reinforcing fibrous component 141 having a volume fraction of 0.62,
an axial Poisson's ratio of +0.2, an axial Young's modulus of 230
GPa, an axial coefficient of thermal expansion of
-6.times.10.sup.-7K.sup.-1, and a transverse coefficient of thermal
expansion of 7.times.10.sup.-6K.sup.-1; and [0184] a non-auxetic
matrix component 142 having a volume fraction of 0.38, an isotropic
Poisson's ratio of +0.38, an isotropic Young's modulus of 3 GPa,
and an isotropic coefficient of thermal expansion of
5.4.times.10.sup.-5K.sup.-1.
[0185] The 3-phase composite 150 used for FIG. 15 is of the type
shown in FIG. 2, and comprises: [0186] a non-auxetic unidirectional
reinforcing fibrous component 151 having a volume fraction of 0.62,
an axial Poisson's ratio of +0.2, an axial Young's modulus of 230
GPa, an axial coefficient of thermal expansion of
-6.times.10.sup.-7K.sup.-1, and a transverse coefficient of thermal
expansion of 7.times.10.sup.-6K.sup.-1; [0187] a non-auxetic matrix
component 152 having a volume fraction of 0.19, an isotropic
Poisson's ratio of +0.38, an isotropic Young's modulus of 3 GPa, an
isotropic coefficient of thermal expansion of
5.4.times.10.sup.-5K.sup.-1; and [0188] an auxetic unidirectional
fibrous component 153 having a volume fraction of 0.19, an
isotropic Poisson's ratio of -0.6, an isotropic Young's modulus of
0.3 GPa, and an isotropic coefficient of thermal expansion of
8.5.times.10.sup.-5K.sup.-1.
[0189] FIGS. 14 and 15 clearly show a reduction of the residual
compressive stresses is achieved in the 3-phase composite 150 when
compared to the 2-phase composite 140. This is due to the presence
of the 3.sup.rd auxetic phase 153.
[0190] An example of the enhancement due to an auxetic phase in a
multi-component composite of the type shown in FIG. 2 undergoing a
directly applied mechanical load has been demonstrated in tests
where a single auxetic fibre embedded in epoxy resin was found to
require twice the force and three times the energy to extract the
fibre from the epoxy in comparison to the equivalent non-auxetic
fibre of the type shown in FIG. 1. In the current invention, the
effect is achieved not by a directly applied mechanical load, but
through conversion of thermal strain to mechanical stress during
cooling and/or heating of the composite.
Method of Making the Composite Material
Example 1
[0191] A composite system of the type shown in FIG. 2 was prepared
comprising auxetic polypropylene fibre embedded within a softened
cold-cure epoxy matrix. The auxetic fibres were produced using melt
extrusion of grade PB0580 polypropylene powder produced by
Plast-Labor S.A. and supplied by Univar plc. The cold-cure epoxy
resin used was Araldite LY 5052 with hardener HY 5084. Dibutyl
phthalate was added to the resin as an inhibitor to cross-linking
during the curing process, thus enabling careful control of the
degree of cross-linking in the final produced composite system of
the type shown in FIG. 2.
Example 2
[0192] A composite system of the type shown in FIG. 2 was prepared
comprising auxetic polypropylene fibre and glass reinforcing fibre
embedded within a cold-cure epoxy matrix. The auxetic fibres were
produced using melt extrusion of grade PB0580 polypropylene powder
produced by Plast-Labor S.A. and supplied by Univar pic. The glass
fibre was provided by PPM Glass. The cold-cure epoxy resin was
Araldite LY 5052 with hardener HY 5052 supplied by Huntsman of the
type shown in FIG. 2. It is of course to be understood that the
invention is not intended to be restricted to the details of the
above embodiments which are described by way of example only.
* * * * *