CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of our application No. 08/625,311 filed Apr. 1, 1996 (U.S. Pat. No. 5,704,402) referenced herein to obtain the benefit of such earlier filing date. 
    
    
     BACKGROUND OF THE INVENTION 
     Passive collision restraint systems typically incorporate a cushion portion which is manufactured from a woven textile material. Such cushion portion is inflated during a collision event so as to provide a barrier between a vehicle occupant and the vehicle interior structures against which that occupant would otherwise be thrown. As will be appreciated by those of skill in the art, the deployment of an air bag cushion is an extremely rapid and violent event which is initiated and completed within milliseconds. Such deployment and subsequent impact is believed to place the fabric of the cushion under substantial stresses. 
     A standard measure of performance for air bag fabrics is air permeability. These standards have historically been set and measured in relation to fabrics which are in a substantially non-tensioned environment such as in the procedure set forth by ASTM-D737. It has been postulated that typical fabrics which have historically made use of plain weave or ripstop weave constructions as are well known in the art will display fairly substantial increases in air permeability when subjected to tensional forces. It is an objective of the present invention to produce a fabric in which the air permeabilities will not substantially increase when subjected to tensions as may occur during deployment of an air bag cushion. 
     SUMMARY OF THE INVENTION 
     The above objective is accomplished according to the present invention by weaving an air bag fabric in a multiple warp harness arrangement using combinations of twill and basket weave components. Such weave constructions have been surprisingly found to not only avoid substantial increases in permeability over a range of linear tensions but to actually display a decrease in air permeability when subjected to tensions in certain ranges which may be applicable during actual air bag deployment and impact. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more readily understood from a reading of the following specification and by reference to the accompanying drawings forming a part thereof and wherein: 
     FIG. 1 is a weave diagram illustrating a potentially preferred repeating pick pattern formed using a four pick arrangement; 
     FIG. 2 is a weave diagram illustrating the multiple repeat arrangement formed by the pick pattern in FIG. 1. 
     FIG. 3 is a weave diagram illustrating an alternative pick repeat pattern according to the present invention formed using a six pick structure; 
     FIG. 4 is a weave diagram illustrating the multiple repeat arrangement of the pick pattern illustrated in FIG. 3; 
     FIG. 5 illustrates the relation between air permeability at a differential pressure of 124 Pascals across the fabric and uniform applied tension for a 315 denier construction air bag fabric formed according to both the prior art and the weave diagram of FIGS. 1 and 2. 
     FIG. 6 is a graph similar to FIG. 5 for a 630 denier fabric; 
     FIG. 7 is a graph similar to FIG. 5 wherein the Y-axis represents air permeability at a differential pressure of 3 psi; 
     FIG. 8 is a graph similar to FIG. 7 for a 630 denier fabric. 
     FIG. 9 is a graph similar to FIG. 7 for a 70 denier X 100 denier construction formed in accordance with FIGS. 3 and 4 and 
     FIG. 10 is a typical air bag arrangement in an automobile. 
    
    
     While specific embodiments of the present invention have been illustrated and will be described, it is to be understood that the invention is not limited thereto, since modifications may be made and other embodiments of the principals of this invention will occur to those skilled in the art to which the invention pertains. Rather, it is intended to cover all such alternative embodiments, procedures, and modifications thereto as may fall within the true spirit and scope of the invention as limited only by the claims herein and equivalents thereto. 
     DESCRIPTION 
     The air bag equipment as installed on a fixed portion of a car body as shown in FIG. 10 in front of a car seat plays an important role for the protection of a person in the car from injury due to the collision of the car body as the air bag is instantaneously inflated at the collision of the vehicle by the pressure of reaction gas released from an inflator, which is fixed on a steering wheel or dashboard. 
     In such air bag equipment 1, for example, chemical reaction of gas generating agent is induced by a collision signal from a collision detection sensor when the vehicle deceleration is higher than a predetermined level. It comprises an inflator 2 to inject the reaction gas into and an opening to exhaust gas from an inflatable restraint cushion or air bag 3, the base of which is fixed on the inflator 2 and which is inflated by the gas injected from the inflator 1. This air bag equipment is mounted on a fixed portion of a car body such as the central portion of the steering wheel 5 with the air bag folded up and accommodated in a pad 4. 
     Turning now to the other drawings, in FIG. 1 there is shown a four pick pattern 10 formable by means of a multiple harness weaving practice as is well know to those of skill in the art. In FIG. 2 the replication of this pick pattern across the surface of a woven fabric 11 is illustrated. Specifically, in the illustrated and potentially preferred embodiment, a fabric is formed from a four pick repeat pattern utilizing four body harnesses. For the first pick 12, first and third warp ends are preferably up while the second and fourth warp ends are down. As illustrated, this results in the pick yarn being passed below the first and third warp ends and above the second and fourth warp ends. For the second pick 14 the second and third warp ends are preferably up while the first and fourth warp ends are down. This results in the pick yarn passing over first and fourth warp yarns while passing beneath second and third warp ends. In the third pick 16, second and fourth warp ends are preferably up while first and third warp ends are down. This results in the pick yarn passing over first and third warp yarns and beneath second and fourth warp yarns. In the fourth pick, first and fourth warp ends are preferably up while second and third warp ends are down. This results in the pick yarn passing below first and fourth warp yarns and over second and third warp yarns. This same movement of the harnesses and resulting pick pattern is thereafter repeated time and again as the fabric is woven. It will be appreciated that while the manufacture of the preferred embodiment has been described in relation to a four harness system, any other system which permits the fabric to be formed with the configuration as described may likewise be utilized. 
     In FIG. 3 there is shown a six pick repeat pattern 20 according to a second embodiment of the present invention. The repeating arrangement of the pick repeat pattern of FIG. 3 across a woven fabric 21 is illustrated in FIG. 4. In the illustrated pick repeat pattern, in the first pick 22 first, third and fourth warp ends are preferably up while second, fifth and sixth warp ends are down. In the 2nd pick 24 of the illustrated pick repeat pattern, first, second and fifth warp ends are preferably up while third, fourth, and sixth warp ends are down. In the 3rd pick 26 third, fifth and sixth warp ends are preferably up while first, second, and fourth warp ends are down. In the 4th pick 28 of the repeat pattern in FIG. 3, first, third, and fourth warp ends are preferably up while second, fifth, and sixth warp ends are down. In the 5th pick 30 of the illustrated repeat pattern, first, second, and fifth warp ends are preferably up while third, fourth, and sixth warp ends are down. In the 6th pick 32 of the repeat pattern, third, fifth, and sixth warp ends are preferably up while first, second, and fourth warp ends are down. A weave diagram indicating the resultant fabric structure utilizing the pick repeat pattern illustrated and described in relation to FIG. 3 is shown in FIG. 4. As illustrated, utilization of the six pick repeat pattern produces a balanced and substantially symmetrical pattern wherein the warps across each pick are characterized by a repeating arrangement of 1 up, 1 down, 2 up, 2 down. 
     As indicated, a perceived benefit of the present invention is the production of fabrics wherein the air permeability is not substantially increased, and may actually decrease, as the fabric is subject to tensile stresses such as may occur during a collision event involving the air bag fabric. In FIG. 5 measurements of air permeability for fabric formed according to the weave diagrams of FIGS. 1 and 2 using 315 denier yarn according to the present invention and a plain weave fabric using 315 denier yarn according to the prior art at a nominal weave density of 60×60 threads per inch are compared across a range of biaxial tensions of equal magnitude in the warp and the fill directions. In FIG. 6, measurements of air permeability for fabric formed according to the weave diagrams of FIGS. 1 and 2 using 630 denier yarn according to the present invention and a plain weave fabric using 630 denier yarn according to the prior art at a nominal weave density of 41×41 threads per inch are compared across a range of biaxial tensions of equal magnitude in the warp and the fill directions. The permeabilities in FIGS. 5 and 6 were measured at a differential of 124 Pascals or approximately 0.5 inches of water across the fabric in accordance with standard testing procedures. 
     As is apparent, traditional prior art plain weave fabrics exhibit substantial increases in air permeability as applied tension is increased. Conversely, fabrics according to the present invention exhibit initial reductions in air permeability followed by only modest increases over a full range of tensions from zero to 150 pounds force per linear inch of fabric. As illustrated, in fact, there was substantially no increase in the permeability measured at a tension of 150 pounds force per linear inch over that measured with no tension on the fabric. 
     The performance curves illustrated in FIGS. 5 and 6 for prior art fabrics are believed to be fully consistent with the trends identified for such fabrics in U.S. Pat. No. 5,375,878 to Ellerbrok (incorporated by reference) wherein gas permeability is shown to be greatly increased when equal tensions are applied across the warp and the fill of prior fabric constructions. Specifically, FIGS. 1(a), 1(b) and 1(c) of the No. &#39;878 patent show dramatic increases in permeability of at least 500 percent or more when traditional fabrics are subjected to equal tensions in the warp and the fill directions in the range of from zero up to about 150 pounds force per linear inch (26.25 KN/M). 
     While most standard air permeability tests are carried out at relatively low pressures in the range of 124 Pascals, it is understood that during a collision event, internal air bag pressures may be much higher. In FIGS. 7 and 8, the permeability characteristics of fabrics formed according to the weave pattern of FIG. 1 are illustrated at differential pressures of three pounds force per square inch. In FIG. 7, permeability measurements for fabric using 315 denier yarn according to the weave pattern of FIG. 1 and a plain weave fabric using 315 denier yarn according to the prior art at a nominal weave density of 60×60 threads per inch are illustrated across a range of tensions of equal magnitude in the warp and the fill directions. In FIG. 8, permeability measurements for fabric using 630 denier yarn according to the weave pattern of FIG. 1 and a plain weave fabric using 630 denier yarn according to the prior art at a nominal weave density of 41×41 threads per inch are illustrated across a range of tensions of equal magnitude in the warp and the fill directions. 
     As illustrated in FIGS. 7 and 8, the characteristic of fabrics of the present invention whereby there is not a substantial increase in permeability over a range of applied tensions is retained even when the fabric is exposed to a high differential pressure. 
     In FIG. 9, the permeability measurements for fabric formed according to the weave diagram of FIGS. 3 and 4, using a low denier construction of 70 denier yarn in the warp and 100 denier yarn in the weft is illustrated over a range of applied tensions in comparison to a plain weave fabric using the same yarns at a nominal weave density of 180×180 threads per inch. As illustrated, a dramatic reduction in permeability at increased tensions was obtained. The present invention is thus believed to be equally applicable to standard weight fabrics using yarns of between about 300 and about 840 denier as well as to lightweight fabrics using yarn of 300 denier or less. 
     In general it is believed that any increase in the permeability of the fabric of the present invention over a range of applied tensions of equal magnitude in the warp and the fill directions of between zero and about 150 pounds force per linear inch will be substantially less than that expected from prior art fabrics, and will preferably be in the range of about fifty percent or less. In more preferred embodiments, there will be substantially no increase in permeability with applied tensions and there may actually be a decrease in such permeability of twenty percent or more. In the most preferred embodiments, the fabric of the present invention will have the above-identified permeability retention characteristics and will also exhibit initial permeabilities at no tensional loading of not more than about five SCFM/FT 2  at about 124 Pascals across the fabric and may have initial permeabilities as low as about one SCFM/FT 2  or less. 
     The invention may be further understood and appreciated by reference to the following examples which are not to be construed as unduly limiting but are rather provided to facilitate understanding. 
     EXAMPLE 1 
     An air bag fabric was woven from 315 denier warp yarn and 315 denier filling yarn according to a weave pattern as illustrated and described with respect to FIGS. 1 and 2. The yarn utilized was single ply nonhollow nylon 6,6 available from AKZO N.V. The weaving was performed on a 210 centimeter Dornier weaving machine with a 1/4 inch selvage and eight harnesses. The fabric was thereafter scoured and heatset in ordinary fashion yielding a product having physical properties as set forth in TABLE I. 
     
                       TABLE 1______________________________________315 DENIER PRODUCTPROPERTY            VALUE______________________________________Weight              5.85   oz./yd..sup.2End Count           63     per inchPick Count          65     per inchWarp Tensile        530    lbs./inchFilling Tensile     512    lbs./inchWarp Elongation     46     percentFilling Elongation  42     percentWarp Tear           92     poundsFilling Tear        79     poundsGauge               0.12   inchesWarp Tongue         35     poundsFilling Tongue      41     poundsWarp Cant.          281    KG/CM.sup.2Filling Cant.       318    KG/CM.sup.2King Stiffness      1.2Mullen Burst        832    PSI______________________________________ 
    
     Permeability was then measured with uniform tension applied around a fabric sample. The results are as illustrated in FIGS. 5 and 7. 
     EXAMPLE 2 
     An air bag fabric was woven from 630 denier warp yarn and 630 denier filling yarn according to a weave pattern as illustrated and described with respect to FIGS. 1 and 2. The yarn utilized was single ply nonhollow nylon 6,6 available from DuPont de Nemours. The weaving was performed on a 210 centimeter Dornier weaving machine with a 1/4 inch selvage and eight harnesses. The greige fabric was thereafter scoured and heatset in ordinary fashion yielding a product having physical properties as set forth in TABLE 2. 
     
                       TABLE 2______________________________________630 DENIER PRODUCTPROPERTY            VALUE______________________________________Weight              8.38   oz./yd..sup.2End Count           46     per inchPick Count          48     per inchWarp Tensile        755    lbs./inchFilling Tensile     760    lbs./inchWarp Elongation     43     percentFilling Elongation  45     percentWarp Tear           179    poundsFilling Tear        200    poundsGauge               0.17   inchesWarp Tongue         61     poundsFilling Tongue      72     poundsWarp Cant.          140    KG/CM.sup.2Filling Cant.       148    KG/CM.sup.2King Stiffness      1.3Mullen Burst        1,132  PSI______________________________________ 
    
     Permeability was measured with uniform stress applied around a fabric sample. The results of the permeability measurement are as shown in FIGS. 6 and 8. 
     EXAMPLE 3 
     An air bag fabric was woven from a 420 denier warp yarn and a 420 denier filling yarn according to a weave pattern as illustrated and described with respect to FIGS. 1 and 2. The yarn utilized was single ply nonhollow nylon 6,6 available from AKZO N.V.. The weaving was performed on a 210 centimeter Dornier weaving machine with a 1/4 inch selvage and eight harnesses. The fabric was thereafter scoured and heatset in ordinary fashion yielding a product having physical properties as set forth in TABLE 3. 
     
                       TABLE 3______________________________________420 DENIER PRODUCTPROPERTY            VALUE______________________________________Weight              6.93   oz./yd..sup.2End Count           56     per inchPick Count          57     per inchWarp Tensile        562    lbs./inchFilling Tensile     551    lbs./inchWarp Elongation     43     percentFilling Elongation  47     percentWarp Tear           135    poundsFilling Tear        114    poundsGauge               0.14   inchesWarp Tongue         51     poundsFilling Tongue      53     poundsWarp Cant.          187    KG/CM.sup.2Filling Cant.       164    KG/CM.sup.2King Stiffness      1Mullen Burst        898    PSI______________________________________ 
    
     EXAMPLE 4 
     A lightweight air bag fabric was woven from a 70 denier warp yarn and a 100 denier filling yarn according to a weave pattern as illustrated and described with respect to FIGS. 3 and 4. The yarn utilized was single ply nylon 6,6 available from Du Pont de Nemours. The weaving was performed on a 190 centimeter Nissan air-jet weaving machine with a 1/4 inch selvage and six harnesses. The fabric was thereafter secured and heatset in ordinary fashion yielding a product having physical properties as set forth in TABLE 4. 
     
                       TABLE 4______________________________________FABRIC TEST RESULTSPROPERTY          VALUE______________________________________End Count         197    per inchPick Count        127    per inchWeight            3.73   oz./yd..sup.2Thickness         8      millsWarp Tensile      305    lbs. force/inchFilling Tensile   360    lbs. force/inchWarp Tear         10.5   lbs. forceFilling Tear      10.4   lbs. force______________________________________ 
    
     Permeability was then measured with uniform stress applied around a fabric sample. The results of the permeability measurement are as shown in FIG. 9. Tests were terminated at about eighty pounds force due to the light nature of the fabric. 
     As can be seen, the present invention provides a fabric which has substantially improved retention of air permeability characteristics over that which is previously available in prior art fabrics. Such fabrics may be either of light weight or standard weight construction. While preferred embodiments of the invention have been illustrated and described, using specific terms, such description as been for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit and scope of the invention which is defined and limited only by the appended claims and equivalents thereto.
A stable permeability fabric and inflatable restraint cushion formed therefrom. The air permeability of the fabric is initially low and does not substantially increase when the fabric is subjected to biaxial loading in both the warp and the fill directions. This permeability profile permits the restraint cushion formed from the fabric to maintain stable permeability characteristics during deployment.