Note: Descriptions are shown in the official language in which they were submitted.
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Title: Quasi-Unidirectional Fabric For Structural Applications
Technical Field
[0001] The teaching disclosed herein relates to woven quasi-unidirectional
fabrics, and in particular, to quasi-unidirectional fabrics for use in
composite
materials for structural applications.
Background
[0002] Unidirectional fabrics are fabrics in which the warp and weft yarns
form parallel layers, without the over and under crimp of a woven structure.
Without such an interwoven structure, some mechanism must be provided to
hold the layers of unidirectional yarn together to form the fabric. Some known
mechanisms include the use of resins, polymer films bonded to the individual
layers, stitching, knitted fabric layers and woven fabric layers.
[0003] When unidirectional fabric is used in the reinforcement of
composites, the composites generally have a number of layers of unidirectional
yarns with at least two of the layers being oriented transversely (at 90
degrees)
to each other. Typical arrangements include two yarn layers, with the first
layer
oriented at 0 degrees to the longitudinal direction of the fabric, and the
second
layer oriented at 90 degrees to first layer. In other arrangements, fabric
layers
may be oriented at +/-45 degrees to the longitudinal direction of the fabric.
[0004] In known composites, the yarn layers may be stitched together,
usually with stitch lines that are closely spaced together. The angle at which
the
layers of yarns are oriented with respect to each other may be varied, and the
spacing of the stitching and the length of the individual stitches may also be
varied.
[0005] However, there are a number of drawbacks with stitched fabric.
Since the stitched yarns are woven by needles that penetrate through the
structural yarn during production, considerable gaps may be formed where the
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stitches are provided. Furthermore, the penetration of the needles may cause
severe damage to the structural yarns.
[0006] Some fabrics have been produced with and without a thermoplastic
film provided between the yarn layers. With film provided between the layers,
the
fabric may be hot pressed such that the film can soften or melt during
pressing,
thus bonding with the layers and serving as a resin system in the finished
composite.
[0007] Another family of unidirectional fabrics involves impregnating a
unidirectional layer of filaments of high performance yarn with a
thermoplastic or
thermoset resin system. Two layers of the resultant prepreg can then be cross-
plied together at a predetermined angle (e.g. 90 degrees) to form a single
sheet
of material.
[0008] Unidirectional fabric may used to overcome some of the reduced
performance in structural materials caused by the crimping found in woven
fabrics. A prior art woven fabric 10 is shown in Figures 1 and 2, and includes
warp yarns 12 and weft yarns 14 that are interwoven. The warp yarns 12 and
weft yarns 14 in the fabric 10 are crimped, as each yarn is bent around other
yarns at crossover points or nodes to provide an interlocking or interwoven
structure. As a result of the crimping, less than optimal mechanical
properties
may be obtained.
[0009] Various techniques have been used to reduce the fabric crimp in
woven fabrics and to spread the crossover points apart. One way to achieve
this
is by weaving yarn in a more open construction, while retaining the weave
pattern. To achieve the desired performance, the individual yarns in the
fabric
must typically be flat and spread apart to provide an open construction for
the
fabric. Without flat, spread yarns, the interstices or gaps between the yarns
tend
to become excessive. This reduces the amount of fibre in a given volume in the
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final composite structure and tends to result in excess resin content required
to
fill in the gaps, reducing the overall performance.
[0010] Improvements in yarn manufacture and weaving technology have
allowed some high performance yarns to be woven with little or no twist and
with
a resulting flat, spread yarn orientation to address the gap issue. However,
there
is a limit to the openness of the weave that can be achieved with a woven
fabric.
As the openness increases, the fabric tends to become more of a mesh or scrim
as opposed to a fabric, and may have little or no value in structural
applications.
In addition, the fabric may become so flimsy that it cannot be handled or cut
without distorting the orientation of the yarns and ruining the fabric, making
it
difficult to work with.
[0011] Accordingly, there is a need for an improved high performance
fabric for use in structural applications that includes at least some of the
benefits
of unidirectional fabrics and woven fabrics while overcoming at least some of
the
above noted disadvantages associated with such fabrics.
Summary
[0012] According to one aspect of the invention, there is provided a fabric
for use in structural applications, having a first layer of structural yarns
comprising first structural yarns aligned in a first direction, a second layer
of
structural yarns comprising second structural yarns aligned in a second
direction
at an angle relative to the first direction, the second layer of structural
yarns
disposed on top of the first layer of structural yarns, first encapsulating
yarns
disposed on the second layer of structural yarns, the first encapsulating
yarns
being aligned with the first structural yarns, and second encapsulating yarns
being aligned with the second structural yarns, the second encapsulating yarns
being alternatively woven above one of the first encapsulating yarns and below
one of the first structural yarns such that the first encapsulating yarns and
second
encapsulating yarns cooperate to secure the first layer of structural yarns to
the
second layer of structural yarns, wherein the first and second encapsulating
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yarns have tenacities and tensile moduli substantially less than tenacities
and
tensile moduli of the first and second structural yarns. In some examples,
each of
the first encapsulating yarns is overlaid on top of one of the first
structural yarns.
[0013] According to another aspect of the invention, there is provided a
structural member comprising a plurality of fabrics bonded together, each
fabric
having a first layer of structural yarns comprising first structural yarns
aligned in a
first direction, a second layer of structural yarns comprising second
structural
yarns aligned in a second direction at an angle relative to the first
direction, the
second layer of structural yarns disposed on top of the first layer of
structural
yarns, first encapsulating yarns disposed on the second layer of structural
yarns,
the first encapsulating yarns being aligned with the first structural yarns,
and
second encapsulating yarns being aligned with the second structural yarns, the
second encapsulating yarns being alternatively woven above one of the first
encapsulating yarns and below one of the first structural yarns such that the
first
encapsulating yarns and second encapsulating yarns cooperate to secure the
first layer of structural yarns to the second layer of structural yarns,
wherein the
first and second encapsulating yarns have tenacities and tensile moduli
substantially less than tenacities and tensile moduli of the first and second
structural yarns. In some examples, each of the first encapsulating yarns is
overlaid on top of one of the first structural yarns. In some examples. the
plurality
of fabrics are bonded together by a resin. In some examples, the plurality of
fabrics are bonded together by a film.
[0014] In yet another aspect of the invention, there is provided a rigid
structural member comprising a plurality of fabrics bonded together, each
fabric
having a first layer of structural yarns comprising first structural yarns
aligned in a
first direction, a second layer of structural yarns comprising second
structural
yarns aligned in a second direction at approximately 90 degrees relative to
the
first direction, the second layer of structural yarns disposed on top of the
first
layer of structural yarns, first encapsulating yarns disposed on the second
layer
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of structural yarns, the first encapsulating yarns being aligned with the
first
structural yarns, and second encapsulating yarns being aligned with the second
structural yarns, the second encapsulating yarns being alternatively woven
above
one of the first encapsulating yarns and below one of the first structural
yarns
such that the first encapsulating yarns and second encapsulating yarns
cooperate to secure the first layer of structural yarns to the second layer of
structural yarns, wherein the first and second structural yarns each have a
tenacity of at least 15 grams per denier and a tensile modulus of at least 400
grams per denier, and the first and second encapsulating yarns have a denier
between approximately 20 and approximately 1000, and the plurality of fabrics
are bonded together by at least one of a resin and a film.
[0015] Other aspects and features of the present specification will become
apparent, to those ordinarily skilled in the art, upon review of the following
description.
Brief Description of the Drawings
[0016] The drawings included herewith are for illustrating various
examples of articles, methods, and apparatuses of the present specification
and
are not intended to limit the scope of what is taught in any way. In the
drawings:
[0017] Figure 1 is a perspective view of a prior art woven fabric;
[0018] Figure 2 is an end view of the woven fabric of Figure 1;
[0019] Figure 3 is a perspective view of a prior art quasi-unidirectional
fabric;
[0020] Figure 4 is an end view of the prior art quasi-unidirectional fabric of
Figure 3;
[0021] Figure 5 is a perspective view of a quasi-unidirectional fabric
according to one embodiment of the invention;
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[0022] Figure 6 is a cross-sectional end view of the quasi-unidirectional
fabric of Figure 5 taken along line 6-6;
[0023] Figure 7 is a cross-sectional end view of the quasi-unidirectional
fabric of Figure 5 taken along line 7-7;
[0024] Figure 8 is a cross-sectional end view of the quasi-unidirectional
fabric of Figure 5 taken along line 8-8;
[0025] Figure 9 is a top view of a quasi-unidirectional fabric according to
another embodiment of the invention;
[0026] Figure 10 is a bottom view of the quasi-unidirectional fabric of
Figure 9; and
[0027] Figure 11 is a cross sectional end view of the quasi-unidirectional
fabric of Figure 9 taken along line 11-11.
Detailed Description
[0028] Illustrated in Figures 3 and 4 is a prior art quasi-unidirectional
fabric, as described in detail in U.S. Patent Application Publication No.
2007/0099526A1 (Heerden et al.). As shown, the fabric 20 includes
unidirectional
warp yarns 22 and weft yarns 24 (referred to as "structural yarns") that are
not
woven or interlocked, but instead are provided as adjacent layers. The warp
yarns 22 can be considered to define a first structural layer, with the weft
yarns
24 defining a second structural layer stacked on top of the first structural
layer.
The warp yarns 22 are aligned with the longitudinal direction of the fabric
20, and
the weft yarns 24 are oriented at about 90 degrees to the warp yarns 22.
[0029] The fabric 20 also includes first encapsulating yarns 26 and second
encapsulating yarns 28, which are oriented at about 90 degrees with respect to
each other. As shown, the first encapsulating yarns 26 are generally parallel
to or
aligned with the warp yarns 22, while the second encapsulating yarns 28 are
generally parallel to or aligned with the weft yarns 24.
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[0030] The encapsulating yarns 26, 28 are woven together (similar to the
woven fabric 10 described above) in a manner that incorporates the structural
yarns 22, 24 into the weave, thus "encapsulating" the structural yarns 22, 24
to
hold the fabric 20 together.
[0031] As best shown in Figure 4, the first encapsulating yarns 26 are
woven above the weft yarns 24 and underneath the second encapsulating yarns
28. Similarly, the second encapsulating yarns 28 are woven under the warp
yarns 22 but pass over the first encapsulating yarns 26. Accordingly, the
encapsulating yarns 26, 28 weave the first layer of warp yarns 22 and the
second
layer of weft yarns 24 together to form the fabric 20.
[0032] However, while this weaving structure secures the fabric 20
together, it tends to result in high tension being developed in the
encapsulating
yarns 26, 28 because of a high number of bends in the encapsulating yarns 26,
28. This is generally undesirable, as it may cause damage to the encapsulating
yarns 26, 28. Furthermore, this may cause the structural yarns 22, 24 to
bundle
up, which may reduce the mechanical properties of the fabric since this
provides
more space for resin and the fiber content in the final composite part is
reduced.
[0033] Heerden et al. addresses this issue by providing a different fabric,
with the structural yarns packed very tightly together so that even if the
encapsulating yarns have a high tension and bundling of structural yarns
occurs,
there is no extra space within the woven fabric. This may be a solution for
certain applications, where the minimization of the weight of the individual
fabric
layers is not crucial. However, in other applications, such as in aerospace
composite structures, weight reduction is a primary goal, and producing such
densely packed fabrics may not be desirable.
[0034] Turning now to Figures 5 to 8, shown therein is a quasi-
unidirectional fabric 30 according to one embodiment of the present invention,
which addresses at least some of these issues. The fabric 30 includes
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unidirectional warp yarns 32 and weft yarns 34 (also referred to as
"structural
yarns") that are not woven or interlocked, but are provided as adjacent
layers.
The warp yarns 32 can be considered to define a first structural layer, with
the
weft yarns 34 defining a second layer that is stacked or disposed on top of
the
first layer.
[0035] As shown, the warp yarns 32 are oriented in a first direction, while
the weft yarns 34 are oriented in a second direction at about 90 degrees to
the
first direction. In some examples, the first direction may be in alignment
with the
longitudinal direction of the fabric 30. In other examples the first direction
may be
angularly offset from the longitudinal direction of the fabric. Furthermore,
the
angle between the first direction and the second direction need not be 90
degrees.
[0036] The fabric 30 also includes first longitudinal encapsulating yarns 36
and second transverse encapsulating yarns 38. As shown, the longitudinal
encapsulating yarns 36 may be generally parallel to the warp yarns 32 (and
aligned in the longitudinal direction of the fabric 30), while the transverse
encapsulating yarns 38 may be generally parallel to the weft yarns 34 (and
generally transverse to the longitudinal direction of the fabric).
[0037] The various elements of the fabric 30 are arranged so that the
longitudinal encapsulating yarns 36 are not woven over or under any of the
structural yarns 32, 34. Rather, each of the longitudinal encapsulating yarns
36 is
provided on only one side of the fabric 30, overlaid or stacked on top of one
of
the warp yarns 32. For example, as best shown in Figure 8, each longitudinal
encapsulating yarn 36 may be overlaid on top of one of the warp yarns 32 in
general alignment with that warp yarn 32 (and passing over the weft yarns 34,
as
best shown in Figures 6 and 7). In other examples, the longitudinal
encapsulating
yarns 36 can be offset somewhat from the warp yarns 32. In some examples, the
longitudinal encapsulating yarns 36 may be all or partially received within
gaps
between adjacent warp yarns 32.
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[0038] The transverse encapsulating yarns 38 are alternatively woven
below the warp yarns 32 and above the longitudinal encapsulating yarns 36,
with
the transverse encapsulating yarns 38 engaging the longitudinal encapsulating
yarns 36 at crossover points or nodes 42. As shown, the transverse
encapsulating yarns 38 may be provided generally intermediate the weft yarns
34.
[0039] Adjacent transverse encapsulating yarns 38 have a staggered or
alternating weave, as best shown in Figures 6 and 7. For example, a first
transverse encapsulating yarn 38a passes below a first warp yarn 32a and the
adjacent first longitudinal encapsulating yarn 36a (as shown in Figure 6); the
same transverse encapsulating yarn 38a then passes over a second longitudinal
encapsulating yarn 36b and the adjacent second warp yarn 32b (as shown in
Figure 7). Conversely, a second transverse encapsulating yarn 38b (next to or
adjacent the first transverse encapsulating yarn 38a) will pass over the first
longitudinal encapsulating yarn 36a and the first warp yarn 32a (as shown in
Figure 6), and then below the second warp yarn 32b and the second longitudinal
encapsulating yarn 36b (as shown in Figure 7).
[0040] The longitudinal encapsulating yarns 36 and the transverse
encapsulating yarns 38 engage each other at the crossover points or nodes 42.
For example, as shown in Figure 8, a third transverse encapsulating yarn 38c
alternatively passes under the first warp yarn 32a and over the second warp
yarn
32b, engaging the second longitudinal encapsulating yarn 36b at node 42a.
[0041] In this manner, the encapsulating yarns 36, 38 cooperate to provide
an interwoven structure that secures the first structural layer of
unidirectional
warp yarns 32 to the second structural layer of unidirectional weft yarns 34,
holding the fabric 30 together.
[0042] This weaving structure tends to causes a reduction in the yarn
tension of the encapsulating yarns 36, 38, and may reduce the bundling or
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crimping of the structural yarns 32, 34. As a result, the structural yarns 32,
34 of
the fabric 30 tend to remain flatter, reducing the gap between the yarns 32,
34
and increasing the fiber content.
[0043] In the fabric 30 as shown, the warp yarns 32 and weft yarns 34 are
generally perpendicular with respect to each other, and the encapsulating
yarns
36, 38 are generally parallel to the warp yarns 32 and weft yarns 34,
respectively.
However, the warp yarns 32 and weft yarns 34, and the encapsulating yarns 36,
38 could be oriented at other non-perpendicular angles. Furthermore, while the
terms "warp yarn" and "weft yarn" are used herein, these terms are
specifically
not meant to be limiting, and the particular weave structure of the fabric 30
could
be used with an encapsulating yarn aligned with either the warp yarn 32 (as
shown) or alternatively with the weft yarn 34.
[0044] The fabric 30 generally does not have to be cross-plied as in
previous processes for the production of unidirectional fabrics, since two
unidirectional structural layers are joined together during the weaving
operation,
(i.e. the first structural layer comprising the warp yarns 32 and the second
structural layer comprising the weft yarns 34). Further, the unidirectional
structural yarns 32, 34 in the fabric 30 generally are not substantially
constrained
by the encapsulating yarns 36, 38 since the encapsulating yarns 36, 38
typically
comprise a low strength, low modulus yarn, as described in greater detail
below.
[0045] The fabric 30 may be woven on standard weaving looms, including
rapier, shuttle, air jet and water jet looms. It may also be produced on
knitting
machines of the type described in U.S. Patent Nos. 3,592,025 and 3,819,461, on
three dimensional weaving machines of the type described in U.S. Patent Nos.
5,465,760, 5,085,252, 6,129,122 and 5,091,245, or on equipment designed to
produce two or more unidirectional layers held together by stitching, as
described
in U.S. Patents Nos. 4,416,929, 4,550,045 and 4,484,459.
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[0046] In some examples, the structural yarns 32, 34 may be yarns having
a tenacity of about 15 grams per denier and higher, and with a tensile modulus
of
at least about 400 grams per denier. Some examples of suitable yarns could
include aramid fibers, extended chain polyethylene fibers, poly(p-phenylene-
2,6-
benzobisoxazole) (PBO) fibers, carbon, basalt and glass fibers. Other examples
include aramid and copolymer aramid fibers (produced commercially by DuPont),
Twaron Products and Teijin (under the trade names KevlarO, TwaronO, and
Technora , respectively), extended chain polyethylene fibers (produced
commercially by Honeywell, DSM, and Mitsui under the trade names Spectra ,
Dyneema , and Tekmilon , respectively), extended chain polyethylene fibers
produced in China marketed as high-intensity and high-modulus polyethylene
fiber, polyethylene fibers and films produced by Synthetic Industries and sold
under the trade name Tensylon , poly(p-phenylene-2,6-benzobisoxa-zole)
(PBO) (produced by Toyobo under the commercial name ZylonO), and Liquid
crystal polymers produced by Kuraray under the trade name Vectran . Other
suitable yarns may also be used.
[0047] The encapsulating yarns 36, 38 are generally of significantly
smaller denier than the structural yarns 34, 34 and have significantly lower
tenacities and tensile moduli. In some examples, the encapsulating yarns 36,
38
have a tenacity of less than 10 grams per denier, and a tensile modulus of
less
than 40 grams per denier. In one example, the encapsulating yarns 36, 38 are
made of polyester having a tenacity of 7.9 grams per denier, and a tensile
modulus of 39.5 grams per denier. In other examples, a vast range of suitable
yarns can be used for the encapsulating yarns 36, 38.
[0048] In some examples, the denier of the encapsulating yarns 36, 38
may range from between about 20 denier (or less), to about 1000 denier,
depending on the size of the structural yarns 32, 34 and the desired
structural
application.
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[0049] The encapsulating yarns 36, 38 are generally of a much smaller
size than the structural yarns 32, 34. The diameter of the encapsulating yarns
36,
38 may be selected based on the moduli and strength parameters of the
encapsulating yarns 36, 38. In some examples, the encapsulating yarns 36, 38
may have a diameter that is between about 2.5% and about 14% of the diameter
of the structural yarns 32, 34.
[0050] The strength and the tensile moduli of the encapsulating yarns 36,
38 may preferably be selected such that the resulting fabric 30 has structural
performance that exceeds the performance of a standard structural fabric made
using the same sizes of structural yarn but with a different weave pattern
(such
as the fabrics 10 and 20 described above).
[0051] In some examples, the encapsulating yarns 36, 38 may be selected
from a wide range of fibers. Some suitable example fibers include natural
fibers,
such as cotton, wool, sisal, linen, jute and silk. Other suitable fibers
include
manmade or synthetic fibers and filaments, such as regenerated cellulose,
rayon,
polynosic rayon and cellulose esters, synthetic fibers and filaments, such as
acrylics, polyacrylonitrile, modacrylics such as acrylonitrile-vinyl chloride
copolymers, polyamides, for example, polyhexamethylene adipamide (nylon 66),
polycaproamide (nylon 6), polyundecanoamide (nylon 11), polyolefin, for
example, polyethylene and polypropylene, polyester, for example, polyethylene
terephthalate, rubber and synthetic rubber and saran. Glass, carbon or any
other
high performance fiber may also be used as encapsulating yarns 36, 38.
[0052] Staple yarns may also be used and may include any of the above
fibers, low denier staple yarns or any combination of these yarns. Staple
yarns
may be used particularly where the base properties of continuous filament
yarns
exceed the maximum allowable properties required in a quasi-unidirectional
fabric. Staple yarns, by the discontinuous nature of their filaments that form
the
yarn, tend to have much lower tensile and modulus properties as opposed to
yarns composed of continuous filaments.
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[0053] The performance of the final fabric 30 is generally a function of the
properties of the encapsulating yarns 36, 38 and the structural yarns 32, 34.
In
composite structures, maximizing the amount of the structural fibres in a
given
volume tends to be of primary importance, as higher fibre volume fraction
generally signifies higher structural properties. Therefore, in some examples
it
may be desirable that the encapsulating yarns 36, 38 have a denier that is as
low
as practical to weave the fabric 30.
[0054] The pick count of a structural fabric used in an application without a
resin can be calculated from the maximum tightness that can be woven in a
plain
weave fabric of 100% structural yarn. The yarn count in the structural yarn
per
inch should be about 50% of this value plus or minus two picks for optimal
strength. The pick count may vary from this value but the structural
properties
may decrease.
[0055] Prior art quasi-unidirectional woven fabric, such as the Heerden
fabric 20 described above, tends to have yarn-bundling or crimping issues due
to
relatively high encapsulating yarn tension. The fabric 30 disclosed herein
tends
to overcome these issues by providing the fabric construction with lower
encapsulating yarn tension.
[0056] In the fabric 30 shown in Figures 5-8, it may be desirable to
minimize the weight of the encapsulating yarns 36, 38 as a percent of the
total
weight of the fabric 30, since the encapsulating yarns 36, 38 may not
contribute
as much to the structural strength of the fabric 30 as the structural yarns
32, 34.
Conversely, an increased amount of encapsulating yarn may result in a more
durable, stable fabric, however, the fabric will be heavier and may have
reduced
structural properties due to the increased constraints of the structural
yarns.
[0057] In some examples, the lowest denier, lowest strength encapsulating
yarn that can be woven and which satisfies all of the requirements for a
particular
application is preferred as an encapsulating yarn.
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[0058] The fabric 30 described herein may be further processed after it
has been woven to form a structural member or panel. For example, the fabric
may be fabricated into a prepreg using a film or a wet resin, for example.
Depending on the application, the film or resin may be applied to one side of
the
fabric, the fabric may be totally impregnated with a resin, or the film may be
worked into the fabric. In some examples, the film or resin may be a
thermoplastic or a thermoset resin. Generally, any resin or film that can be
used
to create a prepreg may be used with this fabric. Two or more layers of the
fabric
30 may also be laminated together to create a multi-layer fabric.
[0059] Turning now to Figures 9, 10 and 11, shown therein is a fabric 50
according to another embodiment of the invention. The fabric 50 has yarn
pattern
similar to fabric 30 described above, and includes a first structural layer of
unidirectional warp yarns 52, and a second structural layer of unidirectional
weft
yarns 54 stacked or disposed on top of the first structural layer.
[0060] As shown, the warp yarns 52 are oriented in a first direction, while
the weft yarns 54 are oriented in a second direction at an angle to the first
direction. In some embodiments, the angle between the first direction and the
second direction is about 90 degrees. In some examples, the first direction
may
be in alignment with the longitudinal direction of the fabric 50, but other
orientations could also be used.
[0061] As best shown in Figure 11, in this example, the warp yarns 52 and
weft yarns 54 are generally flat unidirectional yarns (as opposed to the more
circular warp yarns 32 and weft yarns 54 illustrated above in Figures 5-8). As
a
result the overall fabric 50 tends to have a very flat configuration.
[0062] The fabric 50 also includes first longitudinal encapsulating yarns 56
and second transverse encapsulating yarn 58. As shown, the longitudinal
encapsulating yarns 56 are generally parallel to the warp yarns 52 (and
aligned
in the longitudinal direction of the fabric 30), while the transverse
encapsulating
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yarns 58 are generally parallel to the weft yarns 54 (and generally transverse
to
the longitudinal direction of the fabric 50).
[0063] The longitudinal encapsulating yarns 56 are not woven over or
under any of the structural yarns 52, 54. Rather, each of the longitud'+nal
encapsulating yarns 56 is provided on only one side of the fabric 50, overlaid
or
stacked on top of one of the warp yarns 52. For example, as best shown in
Figure 11, each longitudinal encapsulating yarn 56 is overlaid on top of one
of
the warp yarns 52 in general alignment with that warp yarn 52. Furthermore, as
shown, the transverse encapsulating yarns 58 are alternatively woven below the
warp yarns 52 and above the longitudinal encapsulating yarns 56, the
transverse
encapsulating yarns 58 engaging the longitudinal encapsulating yarns 56 at
crossover points or nodes 62.
[0064] The warp yarns 52 are spaced apart a first distance dl, while the
weft yarns are spaced apart a second distance Q. The first distance dl and the
second distance d2 may be selected to provide a desired density to the fabric
50.
In some examples, the first distance d, may be selected to be as small as
possible Qust large enough to accommodate the crimp in the transverse
encapsulating yarns 58 as they pass over and under adjacent warp yarns 52).
Furthermore, the second distance d2 may be selected to be as small as possible
(slightly larger than the width of the transverse encapsulating yarns 58).
EXAMPLE
[0065] A comparison was made between a known fabric, FR CAA A101
100.0 0000, 230 g/m2 3K Toray T300 carbon 14x14, marketed by Barrday Inc.
as SentinelTM fabric, and a concept quasi-unidirectional fabric 230 g/m2 3K
Toray
T300 carbon 14x14, using the same materials as the Sentinel fabric but with
the
structure as shown in Figures 9-11. Both fabrics were treated with the same
resin, namely Epoxy NB301.
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[0066] During testing, the Sentinel fabric showed a tensile strength of 768
MPa, while the concept fabric showed a tensile strength of 807 MPa. This
represents a performance increase of about 5.1% in the tensile strength.
Furthermore, the Sentinel fabric showed a flexural strength of 701 MPa, while
the
concept fabric showed a flexural strength of 931 MPa. This represents a
performance increase of about 32.8%.
[0067] Thus, the fabric 50 tends to provide improved performance in at
least two areas, namely tensile strength and flexural strength, over prior art
quasi-unidirectional fabrics.
[0068] The fabrics described herein can be designed to produce a quasi-
unidirectional fabric for use in various structural applications, such in
rigid panels
for use in the aerospace industry. The fabrics may be used by themselves or in
combination with various other fabrics and materials to produce structural
elements. Such other fabrics may include woven fabrics made of aramids,
polyethylene, poly(p-phenylene-2,6-benzobisoxazole) (PBO) fibers, carbon
fibres, and basalt or glass fibers. The other fabrics may also include various
unidirectional products based on known unidirectional technology where the
structural fibers are aramids, polyethylene or poly(p-phenylene-2,6-
benzobisoxazole) (PBO).
[0069] The fabrics described herein may generally be used in any
combination with the materials listed above and may replace any one material
or
combination of materials in an existing structural fabric. In addition, the
fabrics
described herein may be laminated together or laminated with films to produce
structural elements for various applications, including rigid and resilient
applications. The proportions of each material selected and the design of the
structural elements may vary depending on the intended application (i.e.,
particular specifications for aerospace applications).
CA 02622685 2008-02-22
-17-
[0070] In some example fabrics, the ratio of the diameter of the
encapsulating yarns to the diameter of the structural yarns should be as low
as
possible (with all other factors being equal) to allow for tight packing of
the
structural yarns. Generally, the smaller the diameter of the encapsulating
yarn
and/or the more deformable the encapsulating yarn is, the less potential there
is
for the encapsulating yarn either to constrain or to impart undesired crimp
into
the structural yarns and thereby adversely affect structural performance.
[0071] While the above description provides examples of one or more
fabrics, processes or apparatuses, it will be appreciated that other fabrics,
processes or apparatuses may be within the scope of the present description as
interpreted by one of skill in the art.
