Structural fabric and method for making same

A structural fabric (20, 28, 40, 48) and a method for making the same is disclosed. Structural fabric (20) is comprised of a plurality of substantially parallel, uniaxial structural yarn (22) and a secondary yarn (24) for holding the structural yarns (22) in place. The structural yarns (22) are oriented at an angle skewed from both the fabric centerline (26) and a line perpendicular to the fabric centerline (26). A double biased fabric (28) is made by sewing two fabrics (20) together with secondary yarn (39). An alternate double bias fabric (40) is made by sewing together with secondary yarn (46) the two layers of a flattened helix made from structural yarn (42, 44) running spirally substantially the length of the fabric (40). A triaxial fabric (48) is made by sewing together with a secondary yarn (56) two single biased fabrics (50, 52) and a layer of longitudinally-extending structural yarn pieces (54).

TECHNICAL FIELD 
This invention relates to a structural fabric and, more particularly, to a 
fabric comprised of one or more layers having uniaxial structural yarns 
held together by sewn or knitted secondary yarn. The invention relates 
also to the method for making the fabric. 
BACKGROUND OF THE INVENTION 
Fiberglass reinforced plastic is a relatively new engineering material. 
World War II initiated a demand for fiberglass reinforced plastic as a 
construction material in radomes, aircraft parts, and marine craft. Since 
then, the material has found wide use in many other applications, 
including automobile parts, skis, and a greater variety of military and 
commercial aircraft and marine parts. 
The most common type of fiberglass reinforced plastic is comprised of 
varying ratios of glass fiber reinforcement and thermosetting polyester 
resins. Ordinarily, a catalyst is introduced into the resin to initiate 
hardening, thereby forming a solid matrix fully incapsulating the glass 
fibers. The resin provides chemical and corrosion resistance, durability, 
light weight, texture, electrical and thermal insullation, and molded-in 
color. The glass reinforcement contributes mechanical strength, rigidity, 
dimensional stability, and temperature resistance. 
Fiberglass reinforcing material has been available in continuous strand, 
chopped strand, woven roving, and matt form. The continuous strand is a 
single continuous strand used in wrapping situations to provide, for 
example, for radial burst strength in a pressure bottle. Continuous strand 
may also come in fabric form wherein a plurality of parallel, uniaxial 
structural yarns are held together by a sewing stitch or knitting stitch 
which runs perpendicular to the axis of the structural yarns. A continuous 
or uniaxial material results in high compressive and tensile strength in 
one direction. 
Chopped strands are short fibers oriented in all directions. A resulting 
laminate has equal strength in all directions. Since the fibers are less 
than two inches in length, however, the strength is not as great as it 
would be with continuous strand or woven roving. 
Woven roving is used for high strength laminates having a minimal thickness 
requirement. Woven roving is comprised of a plurality of continuous 
strands running in two directions perpendicular to each other and held 
together by weaving the one set of strands with the other. Individual 
strands are not uniaxial in woven roving fabric. 
Previously known reinforcing materials have been limited to fabrics having 
structural yarns running parallel with the centerline of the material or 
perpendicular to it. With the increased use and demand for fiberglass 
reinforced plastic, it has been found that many more applications could 
utilize the material if the structural yarn in the reinforcing fabric 
could be oriented in directions different from the Zero degree and 90 
degree directions relative to the fabrics longitudinal centerline. 
Additionally, some applications have demanded high strength in more than 
one direction, yet not all directions. Hence, a need has developed for a 
reinforcing material which can provide variable or multiple directional 
strength characteristics. The present invention addresses these problems. 
SUMMARY OF THE INVENTION 
In its simplest form, the present invention is directed to a structural 
fabric having substantially parallel, longitudinal edges with a centerline 
therebetween comprised of a plurality of substantially parallel, uniaxial 
structural yarn and means for holding each of the structural yarns in 
place with a secondary yarn. The structural yarns are oriented at an angle 
skewed from both the fabric centerline and a line perpendicular to the 
fabric centerline. Such fabric is called biased fabric. 
A composite structural fabric is comprised of a plurality of biased fabrics 
or layers wherein at least one of the layers includes a plurality of 
substantially parallel, uniaxial structural yarn. Oftentimes, the two or 
more layers of such a fabric are each comprised of a plurality of 
substantially parallel, uniaxial structural yarn. With each layer, 
however, the structural yarn ordinarily runs in a different direction. 
With this type of fabric, the holding means is comprised of secondary yarn 
sewn through all layers to hold the various layers of structural yarn 
together and in proper orientation. More commonly, a first secondary yarn 
is used to hold the structural yarns in each layer in place relative to 
each other and a second secondary yarn holds the various layers together. 
In a preferred embodiment of the more complex fabric, two layers are each 
comprised of a biased fabric. The structural yarn in the first fabric runs 
at a direction 45 degrees to the fabric centerline while the structural 
yarn in the second fabric runs 135 degrees to the centerline. Thus, the 
structural yarns in the two layers cross each other at 90 degree angles. 
The structural yarns are held together in the individual layers by a 
secondary yarn which is either knitted or sewn to the structural yarns. A 
third layer is formed by a plurality of uniaxial structural yarns running 
substantially parallel to the centerline. The structural yarns of the 
third layer are commonly sewn with a tricot stitch to the two bias 
fabrics. Thus, a three-layer fabric having structural yarn running in 
three different directions is formed. 
The present invention is particularly advantageous in that it presents the 
plastics engineer with the versatility of using a longitudinal fabric 
having structural strength according to any angle he may designate. 
Even more importantly, more than one fabric offering the strength of 
uniaxial structural yarn may be combined to provide structural strength 
characteristics in several directions. 
Furthermore, the weight and density of the structural yarn in any one layer 
may be varied with respect to that of another layer, thereby offering even 
more engineering possibilities.

DETAILED DESCRIPTION OF THE INVENTION 
Reference is now made to the drawings wherein like reference numerals 
designate identical or corresponding parts throughout the several views. A 
single biased fabric 20 is shown in FIGS. 1 and 2. Single biased fabric 20 
is comprised of structural yarn 22 and secondary yarn 24. The plurality of 
structural yarn pieces 22 shown in FIG. 1 are spaced-apart for the sake of 
clarity. Ordinarily, structural yarns 22 would be adjacent to one another. 
All structural yarns 22 are unidirectionally oriented and are parallel to 
one another. Since structural yarns 22 are not weaved as in woven roven, 
the structural yarns 22 are uniaxial. It has been found that the uniaxial 
feature significantly enhances the strength characteristics of a fabric 20 
in the axial direction when compared with woven prior art fabrics. 
Secondary yarn pieces 24 are sewn or knitted to structural yarn pieces 22 
to hold them in place with respect to each other. Secondary yarn pieces 24 
ordinarily have a significantly smaller cross-sectional area than the 
structural yarn pieces 22. Secondary yarn pieces 24 ordinarily run 
substantially parallel to the centerline 26 of fabric 20 and are 
spaced-apart from each other. Various knitting or sewing stitches which 
are commonly known to those skilled in the art may be used. 
Structural yarn 22 is preferably a bundle of a glass fibers, although any 
structural fiber, for example, carbon or commercial proprietary fibers, 
may be used. Secondary yarn 24 is preferably made from a similar material, 
although it need not be. Secondary yarn 24 is not structural since it is 
much smaller in cross-sectional area and much more flexible than 
structural yarn 22. 
FIGS. 3 and 4 illustrate a double bias fabric 28. Double bias fabric 28 is 
comprised of an upper layer single bias fabric 30 and a lower layer single 
bias fabric 32. Fabrics 30 and 32 may be the same as fabric 20. The 
structural yarn pieces 34 of fabric 30 are oriented at an acute angle with 
respect to centerline 36 while the structural yarn pieces 38 of fabric 32 
are oriented at an obtuse angle with respect to centerline 36. Fabrics 30 
and 32 are held in place with respect to one another by secondary yarn 39. 
Note that each fabric 30 and 32 has secondary yarn (not shown) equivalent 
to secondary yarn 24 in FIG. 1 for holding the structural yarn 34 and 38, 
respectively, in place with respect to the particular layer of fabric, 
while secondary yarn 39 holds the two fabrics 30 and 32 in place relative 
to each other. The double bias feature of fabric 28 provides significant 
structural strength in the two directions represented by the directions of 
structural yarn 34 and 38. Thus, although fabric 28 is thicker than fabric 
20, it provides the significant advantage of increased structural strength 
in two directions as opposed to one. 
Double bias fabric 40, as shown in FIGS. 5 and 6, has characteristics 
similar to fabric 28, but is constructed somewhat differently. The two 
biased layers of fabric 40 are not two individual fabrics, like fabrics 30 
and 32 of fabric 28 in FIGS. 3 and 4. Rather, fabric 40 is comprised of 
continuous strands of structural yarn proceeding in a flattened helix form 
from approximately one end of fabric 40 to approximately the other end. 
For clarity, only two structural yarn strands 42 and 44 are shown in FIGS. 
5 and 6. Note that in FIG. 6 a portion of strand 44 has been cut-away to 
show the opposite edge of structural yarn piece 42. Secondary yarn pieces 
46 are sewn or knitted as described hereinbefore to hold structural yarn 
pieces 42 and 44 in place relative to one another. Ordinarily, a large 
plurality of structural yarn pieces, like 22, would proceed spirally from 
one end of fabric 40 to the other. 
A triaxial fabric 48 is shown in FIGS. 7 and 8. Triaxial fabric 48 is 
comprised of two fabric layers 50 and 52 and a third layer of uniaxial 
structural yarn 54. Layers 50 and 52 may be a fabric 40 as shown in FIGS. 
5 and 6, a fabric 28 as shown in FIGS. 3 and 4 or two biased fabrics 20 as 
shown in FIGS. 1 and 2. Structural yarn pieces 54 are held in place with 
respect to layers 50 and 52 with secondary yarn 56. Secondary yarn 56 also 
holds layers 50 and 52 with respect to one another and with respect to 
uniaxial structural yarn 54. Secondary yarn 56 is ordinarily sewn with a 
tricot stitch, commonly known to those skilled in the art. Triaxial fabric 
48 not only has the bi-directional strength characteristics of fabrics 28 
and 40, but also has significant strength characteristics along a 
direction substantially parallel with the centerline 58 of the fabric 48. 
It is to be understood that the bias angle of the structural yarn with 
respect to the centerline of the fabric may range through angles from zero 
to 180 degrees. It is recognized, however, that fabric having structural 
yarn at 90 degrees or zero degrees with respect to the centerline of the 
fabric has been known prior to the present invention. Although the single 
bias material 20 is but a small improvement over the prior art, the double 
bias material 28 and 40 and the triaxial material 48 are very significant 
improvements. It is to be further understood that the present invention 
contemplates a single bias material like 20 being stitched or knitted 
together with other known fabrics to create multiple layered fabrics thus 
providing the characteristics of the bias material to the composite 
fabric. 
FIG. 9 illustrates two layers of biased fabric like 20 being sewn together 
with a chain-type stitch. The needle is illustrated at 60 creating the 
chain stitching with secondary yarn 40. The chain stitch is merely 
exemplary and could as well be a tricot stitch or some other stitch 
commonly known to those skilled in the art. 
Exemplary processes for making the fabrics which are in accord with the 
present invention are illustrated in FIGS. 10 and 11. FIG. 10 illustrates 
a method for making a triaxial fabric 48 like shown in FIGS. 7 and 8. A 
fabric 62 is pulled off a roll 64 by counter-rotating complimentary 
rollers 66 and 68. Roller 66 is immediately above roller 68. Fabric 62 is 
pressed and passes tightly between rollers 66 and 68. By properly 
tensioning rollers 66 and 68, the squeezing at a uniform rate between the 
rollers 66 and 68 can be utilized to change the direction of the 
continuous strand fabric 62, that is, from a fabric with structural yarn 
running either zero degrees or 90 degrees to the centerline of the fabric 
to a biased fabric 20a. 
This is illustrated in overhead view in FIG. 12, wherein fabrics 100 and 
112 are pulled off take up rolls through counter rotating pairs of rollers 
108 and 110 into knitting machine 104 at a bias. Fibers 102 are pulled 
directly off warp yarn creel 106. 
A second biased fabric 20b may be created by pressing it between a second 
set of rollers 72 and 74 as it is pulled from roll 76. 
As biased fabrics 20a and 20b are fed one on top of the other into sewing 
machine 78, a plurality of structural yarn pieces 80 are laid parallel to 
the centerlines of the fabrics and on top of the upper fabric 20b. 
Structural yarns 80 are pulled by a mechanism (not shown) in the sewing 
machine from individual yarn reels 82. A guide mechanism 84 locates each 
yarn 80 with respect to the others. The three layers--lower biased fabric 
20a, upper biased fabric 20b and the plurality of structural yarn pieces 
80--are sewn, commonly with a tricot stitch, to form a fabric 48 as shown 
in FIG. 7. 
A process illustrated in FIG. 11 may be used to create a double biased 
fabric 40 like shown in FIGS. 5 and 6. A plurality of structural yarn 
pieces 86 are pulled from individual rollers (not shown) in holder 88. The 
structural yarns 86 are threaded through openings 89 in guide member 90. A 
yarn carrier 92 is comprised of two carrier yarn chains 94 spaced-apart 
and substantially parallel with one another. Yarn catch elements 96 are 
regularly spaced along carrier yarn chains 94. Structural yarn 86 passes 
through the openings 89 in guide member 90 to catch elements 96 on the 
first yarn chain 94 nearest yarn storage holder 88. Yarn 86 is directed 
diagonally from guide member 90 to the first yarn chain 94. From the first 
yarn chain 94, yarn 86 is directed to the second yarn chain 94 at a 
different diagonal. This, of course, is accomplished by yarn carrier 90 
moving reciprocally in one direction while yarn chains 94 move 
reciprocally in a 90 degree direction relative thereto. Ordinarily, the 
yarn between guide member 90 and first yarn chain 94 is directed at an 
angle of approximately 45 degrees from the yarn between yarn carrier 90 
and yarn storage holder 88. Yarn 86 from the first yarn chain 94 to the 
second yarn chain 94 is directed at approximately 90 degrees from the yarn 
86 between guide member 90 and the first yarn chain 94. Thus, as the yarn 
carrier 92 and the guide member 90 reciprocate, structural yarn 86 is 
formed into a flattened helix and directed into sewing machine 98 for 
appropriate stitching. The resulting fabric is a double bias fabric 40 
like shown in FIGS. 5 and 6. 
As indicated, the processes are merely representative of processes which 
could be used to create the various fabrics in accordance with the present 
invention. It is to be understood, therefore, that not only the fabrics, 
but also the processes which have been set forth in the disclosure, are 
illustrative only. Hence, any changes made, especially in matters of 
method, shape, size and arrangement, to the full extent extended by the 
general meaning of the terms in which the appended claims are expressed, 
are within the principle of the invention.