FABRIC WITH BALANCED MODULUS AND UNBALANCED CONSTRUCTION

Abstract: This disclosure describes fabric and a method for making fabric, for use in a flexible pressure vessel or other fabric structures, which has an unbalanced woven construction and has a balanced or nearly balanced modulus of elasticity for the end-use conditions of this fabric. Unbalanced woven fabrics that do not have balanced properties produce unequal stretching and are susceptible to premature failure. The present invention of unbalanced woven fabric construction with balanced mechanical properties such as modulus of elasticity can be used to optimize fabric strength, mass, volume, and performance characteristics and also reduce manufacturing costs for various fabric structure applications. Balanced mechanical properties for the fabric structure are achieved by altering the force of the fabric by increasing or decreasing the number of yarns in a particular direction of the fabric, by changing the strain during stretch of the fabric, or by alternative weaves of the fabric.

Description: Fabric With Balanced Modulus And Unbalanced Construction Description The present invention claims rights under 35 U.S.C. § 1 19 on Provisional application No. 60/028,896, filed on October 21 , 1996, and entitled "FABRIC WITH BALANCED MODULUS AND UNBALANCED CONSTRUCTION." The government has certain rights in this invention pursuant to Contract No. DE- AC04-76DP00789 awarded by the Department of Energy. Technical Field The present invention relates to a fabric with a balanced, nearly balanced, or optimal modulus of elasticity ("modulus") and unbalanced construction for use in various fabric structures. Specifically, the invention relates to a fabric with balanced modulus and unbalanced construction for an airbag or similar fabric structure. Background Art Fabric structures such as air bags are generally made from fabric containing a balanced woven fabric construction. Balanced woven fabric construction is obtained when the yarns in the two woven construction directions — that is, warp and fill ~ are made from the same denier material and contain the same count of yarns of equal size, strength and spacing. (The fill direction is also sometimes referred to as "weft"). These identical yarns are woven together to form a balanced weave fabric. This generally balanced construction also fortuitously results in a balanced modulus of elasticity in the warp and fill direction. Modulus of elasticity, in this usage, is the quotient of stress induced in a material divided by the strain resulting from that stress. The problem presented by prior art fabric structures is that they do not obtain balanced mechanical properties that optimize fabric mass, volume, load carrying ability, and manufacturing cost or reduce the randomness of strength for a fabric structure or flexible pressure vessel. Disclosure of the Invention The object of the invention is a fabric structure or flexible pressure vessel of unbalanced weave fabric with balanced mechanical properties to optimize the fabric mass, volume, load carrying ability, and manufacturing cost and reduce the randomness of its strength. Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. To achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention comprises a fabric made from dissimilar yams, one in warp and one in fill. The invention also comprises a fabric made from yarn stretched to balance the modulus of elasticity. This invention also comprises a fabric wherein the spaces between yams or the weave of the yams are adjusted to achieve a preferred modulus of elasticity that is approximately balanced. This invention also includes a method of manufacturing a fabric with balanced modulus and unbalanced construction. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. Brief Description of the Drawings Fig. 1 depicts weave of fabric in the warp and fill directions. Fig. 2 depicts an edge view of a plain weave fabric. Fig. 3 depicts a top view of a plain weave fabric. Fig. 4 depicts a side view of a plain weave fabric. Fig. 5 depicts an edge view of a 2 x 1 twill weave fabric. Fig. 6 depicts a top view of a twill weave fabπc. Fig. 7 depicts a side view of a twill weave fabric Best Mode for Carrying Out the Invention As disclosed herein, the fabric with near balanced or optimized modulus and unbalanced construction (unbalanced weave) concept evolved from the inventors' discovery that an unbalanced woven fabric construction expeπenced unequal stretching in the warp and fill directions during the fabπc stmcture deployment. This unequal stretching produces a skewed or sheared shape and a susceptibility to tear the fabric at relatively stiff portions in the structure such as seams, mounting areas and stitching This tearing produces premature failure in the structure, thereby reducing the efficiency of the fabric A higher fabric strength and weight was therefore required for the given fabric stmcture to avoid premature failure This increased strength and weight decreased the efficiency of the fabric stmcture for a given mass or volume Additionally, the thickness of a balanced construction fabric is determined by the geometrical combination of the same varns in both the warp and fill directions In contrast. in accordance with the present invention, the thickness of an unbalanced constmcuon fabric is determined by the optimum geometrical combination of different numbers and sizes of yarns in the warp and fill directions The inventors discovered that for every combination of yarns in a balanced constmction. a thinner combination of yams to achieve the same strength or a stronger combination of yam to achieve the same thickness is possible in an unbalanced construction The inventors conducted a series of structural tests that demonstrated their ability to achieve greater strength from the same fabric mass or volume by varying the constmction of the fabric, weaving dissimilar yams in the warp and fill directions, or alternating the weave of the fabric. These unbalanced woven fabπcs produced balanced or nearly balanced structural properties in the loaded or end-use condition. These unbalanced woven fabrics, therefore, were able to optimize the strength, fiber mass and volume of the fabric structures. These unbalanced woven fabrics could also achieve lower permeability from the greater packing density potential of the unequal yams. Essentially, the fibers in the resulting unbalanced woven fabric are packed more efficiently in the same volume than a conventional balanced construction fabric Additionally, the unbalanced woven fabric of the present invention, including an unequal number of machine direction and cross direction (warp and fill) fibers costs less to manufacture than a balanced constmction fabric that produces the same volume of fabric The number of fibers in the cross direction determines the amount of machine time required to produce a unit of fabπc. Thus, fewer picks made of large fibers result in balanced fabπc properties in the end use condition at a reduced cost Alternatively, the inventors discovered that changing the curvature of the ya s in the matrix of the weave would contribute to the strength of the fabric The inventors determined that they could balance the curvature of yarns by doubling the warp or lengthening the alternations between yarns The inventors conducted a series of structural tests that demonstrated that balancing the mechanical properties of the fabπc was also a function of ya curvature and that they could reduce the deviations in resultant fabπc strength by altering the fabric weave type The concept of unbalanced weave fabrics with balanced mechanical properties emerges to present a geometry which optimizes the fabric mass, volume, load carrying ability, and manufacturing cost and reduces the randomness of strength for a tabπc stmcture or flexible pressure vessel Reference will now be made in detail to the present preferred embodiments of the invention. This invention relates to a fabπc stmcture made from dissimilar or altered yams in the warp and fill directions; a fabric stmcture made from varied amounts of yams in the warp and fill directions: or a fabric made by altering the weave of the fabric to obtain a fabric with mechanical properties approximately balanced. The design of the unbalanced woven fabric of the present invention is accomplished by altering the constmction of the yams such that the mechanical properties of the final product, the fabric, are approximately balanced for the end-use conditions of the fabric. Altered construction is required because the uneven yarn size and spacing produces uneven stiffness, resulting from uneven lengths and strengths of yam in the matrix of the fabric. This uneven yam skews the amount of warp and fill stretch available under load, thereby skewing the modulus of elasticity. The yams are altered, for example, by permanently stretching them by varied amounts during production to adjust the final strength and elongation of the yams used in the finished fabric. Altering these ya s by stretching is one method of adjusting, or designing, the final modulus of elasticity of the fabric. Changing the level of stress carried by the fabric— that is, changing the capacity of the fabric— by, for example, altering the yam spacing, can also be used to adjust the modulus of elasticity. Altering the weave of the fabric is another method or manner for balancing the fabric modulus, increasing strength and reducing the randomness of strength. It is the combination of these adjustments to mechanical characteristics which determines the final modulus of the fabric. Therefore, balancing the modulus of elasticity of the fabric is achieved in three ways: first, by altering the strength of the fabric, that is, increasing or decreasing the number of yams in a particular direction of the fabric; and second, and alternatively, by changing the strain during stretch of the fabric by controlling the permanent strain of the individual yam and altering the amount of stretch of the yam during manufacture, and thus leaving a controllable amount of stretch in the constructed fabric; and third, and alternatively by alternating the weaves of the fabric and thus reducing the curvature of the yams through the fabric and matching the curvature of the warp and fill yarns, thus balancing the fabric modulus, increasing strength, and reducing the randomness of strength. The selection of the specific yarns to be used in the two directions must consider the fabric performance in both directions, the generation of interstitial shear stresses and the fiber properties in end use conditions. The density of the yarns and woven fabric is defined by the given volume and performance requirements. Once the fabric is woven with these specially selected and altered yams, testing is used to prove that the modulus is balanced. It is necessary to test the fabric in the "as used" condition to match the modulus of elasticity between the fabric construction directions. This testing usually includes exposure to relevant environments, such as combinations of temperature and loading rates. Slow or static rates of loading require static testing of the material. Dynamic loading of the fabric structure requires that similar strain-rates are induced in the test fabric to insure that the moduli are balanced for these conditions. The theoretical performance characteristics of the fabric, as defined by the yam properties and density of the woven fabric are compared to the actual characteristics achieved in testing. Modification of yarn selection or weaving is then made until the actual performance characteristics of the fabric meet the required performance in the end-use conditions. Theoretical fabric characteristics are defined based upon various procedures, such as, for example, finite element model analysis. Testing of the fabric to measure actual fabric characteristics requires putting the fabric under end-use conditions and measuring its mechanical properties in two directions, warp and fill. For example, a high speed tensile test and high strain rate testing can be performed in each direction. The tests are designed to identify the differences in the performance of the fabric in each direction. Then, by an iterative method, the fabric is redesigned so as to reach a final design where the performance of the fabric in each direction under end-use conditions is approximately the same. This invention disclosed allows the use of lighter weight fabrics in fabric stmctures such as airbags, parachutes, and balloons. The resulting fabric produced in accordance with this invention should approximately balance the mechanical properties of the fabric in two directions. This fabric can thus be made stronger without requiring stronger fibers or weight or volume. By adjusting, through iterative comparison testing, the use of dissimilar yams in dissimilar quantities in the warp and fill direction of the fabric and by minimizing the shear stresses between the warp and fill directions in the fabric from dissimilar mechanical performance an optimal balanced modulus fabric is attained. For a given strength or performance requirement, a lighter fabric requiring less volume can be used to achieve the desired result. For a set fabric weight or volume parameter, greater strength can be achieved by utilization of the present invention. Alternatively, the invention discloses the use of alternative weaves to achieve approximately balanced mechanical properties for fabric structures. Balanced mechanical properties for the fabric stmcture are achieved, for example, by adjusting the weave of the fabric to approach a balance in the curvature of the yams in each direction. This can be achieved, for example, by doubling the number of ya s in a particular direction or by lengthening the alternations between yarns, or a combination of both techniques. By altering the weave in this manner, the curvature of the yam becomes essentially the same, that is — balanced— in the over and under direction for each yam. Pictuπng the shape of the weave in two directions as essentially a rectangle, the closer that rectangle comes to a square geometry the more balanced the curvature of the yarn and the resulting mechanical properties of the fabric. Lengthening the yarn to achieve near balanced conditions by altering the weave refers to increasing the path length of the yam through the matrix of the fabric in its length direction. Crimp is the amount of kinking through the fabric. By reducing the path length of the yam the crimp or amount of kinking is reduced, reducing the curvature of the yam through the fabric matrix. The crimp or curvature of the yams in the fabric can be measured by comparing a length of yam contained in a length of fabric. The additional length of the yam required to traverse the circuitous path through the fabric matrix is a direct measure of crimp — the greater the percentage length, the greater the crimp; the smaller the percentage length, the straighter the yams and the smaller the crimp in the yam. By balancing the path length for the ya s in two directions the curvature and crimp of dissimilar yarn is made to act in a more similar or balanced manner. An example of one of the embodiments of the invention is illustrated in Figs. 5, 6, and 7. In comparison to the prior art plain weave depicted in Figs. 2, 3, and 4, an embodiment of the present invention is depicted in Figs 5, 6, and 7 which employs a 2 x 1 twill weave. The plain weave of Figs. 2. 3. and 4 has an effective radius of curvature 15 of the warp yam 5 what is dissimilar to. and in these figures smaller than, the effective radius of curvature 20 of the fill yam 10. In contrast, the embodiment of the present invention depicted in Figs. 5, 6, and 7 has an effective radius of curvature 60 of the waφ yams 50, 52 that is more similar and preferably approaching an identical radius of curvature 65 of the fill yam 55. In the example of Figs. 5, 6, and 7 the number of yams in the warp direction is doubled in compaπson to the pπor art plain weave depicted in Figs. 2, 3, and 4. Additionally, in the example 2 x 1 twill weave of Figs. 5, 6, and 7, the crimp and curvature in both yam directions is balanced to a greater degree than in the plain weave of Figs. 2, 3, and 4, producing a fabric stmcture with a near balanced modulus. The invention will be further clarified by the following examples, which are intended to be purely exemplary of the invention. EXAMPLES Test data from the following three versions of fabrics demonstrates the invention of obtaining additional strength from a constant mass of fabric by balancing the mechanical properties of the fabric in two directions with an unbalanced constmction. Table 1 provides the relevant data for these three fabrics. Version I This fabric was designed with conventional guidelines. 45 denier waφ yarn and 100 denier fill yarn were used. The yam properties were selected to maximize the ultimate tensile strength of each direction. Version II This fabric was designed to increase the strength of version I. The amount of yam in one direction was increased. Although static testing indicated increased strength was achieved, dynamic testing at end-use conditions and 85°C indicated that the fabric was actually weaker and that the fabric performance was much lower. Version III This fabric design used the unbalanced woven constmction with the balanced modulus design concept. The slope of the stress strain curve for both of the waφ and fill yams was made approximately parallel. For example, the modulus ratio, dividing the waφ direction modulus value by the fill direction modulus value was about .89 (or 89%) in the hot 85 °C condition. The result was that about a 50% increase in the dynamic tensile strength was seen, achieving more strength out of the same mass of fabric from the previous test versions and conventional designs. Table 1 DENIER YARN NUMBER STATIC DYNAMIC DYNAMIC ELONGATION TENSILE TENSILE TENSILE(HOT) VERSION I WARP 45 20% 183 281 229 204 FILL 100 31% 93 321 260 210 VERSION II WARP 40 31% 242 309 228 199 FILL 100 32% 92 349 255 220 VERSION III WARP 45 24% 249 356 334 302 FILL 100 24% 97 337 287 256 Test data demonstrating the invention of balancing the modulus ratio through controlling the strain of the yam during production is demonstrated in the following four fabrics listed in Table 2 below. The first fabric used conventionally produced yams, and achieved a modulus ration of 62% and 53% at ambient and hot test temperatures. The second through fourth fabrics varied combinations of waφ and fill yam properties in accordance with the present invention to affect the modulus. As is demonstrated by the results of the fourth fabric, the modulus ratio between the waφ and fill is improved by adjusting the yam properties. Here, by practicing the present invention, a greater modulus balance at both the ambient and hot conditions is achieved, measuring 80% and 82%, respectively. Table 2: Modulus Control Through Yarn Production Properties - Lightweight Yarns A similar fabric trial applying the present invention to the use of two slightly heavier denier fabrics for high temperature use was also completed. Table 3 lists the results for a 55 denier waφ and a 120 denier fill fabric. With this trial, a balancing of the fabric moduli at the hot temperature was achieved in the second test by varying the fill yams. Fabric properties at hot temperatures frequently limits the use of the fabric, and the ability to balance the moduli at high temperatures as demonstrated in this test by practicing the present invention is highly useful and important. Table 3: Modulus Control Through Yarn Production Properties - Heavier Denier Yarns Test data from the following four fabric types demonstrating the invention of obtaining balanced mechanical properties and modulus control with alternate weaves of yarns. The three fabrics chosen were a 2x1 oxford, a 2x2 basket, and a 2x1 twill. For comparison purposes, a plain weave was also woven. A 40 denier waφ by 100 denier fill was used for the trial. These fabrics were all tested in a high strain-rate machine at Sandia Labs, Livermore, California, The tests were all completed at ambient temperature.

Claims: The summary of test results is shown in Table 4. The strongest waφ direction was displayed by the 2x1 twill, which was also the strongest fill direction. As demonstrated by comparison, the initial .70 modulus ratio for the plain weave fabric improved to 1.07 and .97 respectively for the 2x1 oxford and 2x1 twill weaves, practically achieving the optimal desired balanced ratio of 100%. This result was achieved by a balancing that is purely a function of yam curvature. Additionally, the scatter of strength data was reduced in the alternative weaves, in comparison to the plain weave. For these tests the 2x1 oxford and 2x1 twill appear the most balanced. Table 4: Alternate Weave Trial High Strain-Rate Results FABRIC TYPE STRENGTH STRAIN MODULUS MODULUS RATIO (lbs/in) (% at break) (lb/in) (Ewarp/Efill) 64345 Plain Weave 0.70 warp 89 37 239 fill 100 29 343 64346 2x1 Oxford Weave 1.07 warp 94 35 267 fill 93 37 250 64347 2x2 Basket Weave 1.22 warp 93 29 321 fill 87 33 264 64348 2x1 Twill Weave 0.97 warp 96 29 329 fill 109 32 339 It will be apparent to those skilled in the art that various modifications and variations can be made in the fabric stmcture of the present invention and in constmction of this fabric without departing from the scope or spirit of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims. Claims 1. A fabric stmcture comprising: a fabric having approximately a balanced modulus of elasticity in its end-use condition including two dissimilar yams, each woven in one of two directions with one direction being waφ and one direction being fill. 2. The fabric stmcture of claim 1 , wherein the yams are greater in number in one direction than the other. 3. The fabric stmcture of claim 1 , wherein at least the yarn in one direction is stretched in an altered amount to adjust the modulus of elasticity of the fabric. 4. A fabric stmcture comprising: a fabric having approximately a balanced modulus of elasticity in its end-use condition, including yams woven in two directions, one direction being waφ and one direction being fill, and having yams greater in number in one direction than the other. 5. The fabric stmcture of claim 2, wherein the yarns are woven in a twill weave. 6. The fabric stmcture of claim 5 wherein the yams are woven with two yams in one direction and one yam in the other direction. 7. The fabric stmcture of claim 4, wherein the yams are woven in a twill weave. 8. The fabric stmcture of claim 7 wherein the yarns are woven with two yams in one direction and one yam in the other direction. 9. The fabric stmcture of claim 2 wherein the yams are woven in an oxford weave. 10. The fabric stmcture of claim 9 wherein the yams are woven with two yams in one direction and one yam in the other direction. 11. The fabric structure of claim 4 wherein the yarns are woven in an oxford weave. 12. The fabric structure of claim 11 wherein the yams are woven with two yarns in one direction and one yam in the other direction. 13. The fabric stmcture of claim 2 wherein the yarns are woven in a basket weave. 14. The fabric stmcture of claim 4 wherein the yams are woven in a basket weave. 15. A method of manufacturing fabric wherein dissimilar yams in two directions, one direction being waφ and one direction being fill, are woven together to balance the modulus of elasticity of the fabric in the waφ and fill direction under end-use conditions. 16. The method of claim 15 wherein the method includes stretching at least one of the yams before weaving them together to balance the modulus of elasticity of the fabric in the waφ and fill direction under end-use conditions. 17. The method of claim 15 wherein the fabric comprises yams in two directions and the yams have a plurality of spaces between them, wherein the method includes adjusting the spaces between the yams to achieve an approximately balanced modulus of elasticity. 18. The method of claim 15 wherein the fabric comprises a matrix of woven yams in two directions and the yams in the weave have a curvature and wherein the method includes adjusting the curvature of the yarns to achieve an approximately balanced modulus of elasticity. 19. The method of claim 18 wherein adjusting the curvature of yams is accomplished by weaving more yams in one direction than the other. 20. The method of claim 18 wherein the yams have a path length and the method includes adjusting the curvature of yams by varying the path length of the yam.

Inventors: GWINN KENNETH W, HOLDER JOSEPH P, GWINN, KENNETH, W., HOLDER, JOSEPH, P.
Applicants: GWINN KENNETH W, HOLDER JOSEPH P, PRECISION FABRICS GROUP INC, SANDIA NATIONAL LAB, GWINN, KENNETH, W., HOLDER, JOSEPH, P., PRECISION FABRICS GROUP, INC., SANDIA NATIONAL LABORATORIES
Application year: 1997
Main classification: B60R21
IPC codes: B60R21, D03D1