AIRBAG WITH NON-CIRCULAR VENT HOLE

Abstract: This disclosure describes an airbag in which non-circular vent holes replace traditional circular vent holes. Vent holes, in general, provide a geometric asymmetry in the airbag pattern and induce a stress concentration in the geometry. Investigation of airbag deployments has shown that circular vent holes elongate during the early stages of a deployment. This causes stress concentrations at the two circumferential points of the circle as the fabric is strained. Based on this discovery, the inventors determined that a non-circular vent hole, such as an ellipse or rectangle would be advantageous during the early deployment since this non-circular vent hole has assumed a more natural position during this early straining, thereby not inducing additional straining in this region.

Description: AIRBAG WITH NON-CIRCULAR VENT HOLE BACKGROUND OF THE INVENTION This application is based on Provisional Application No. 60/021,500 filed July 10, 1996 which was also based on Provisional Application No. 60/020,851 filed June 28, 1996. Field of the Invention The invention relates to vent holes in flexible pressure vessel structures. Specifically, the invention relates to non-circular vent holes in airbags. Description of the Related Art Airbag vents are generally a circular hole (or perhaps two holes in some designs) usually located somewhere on the back panel of an airbag. These vents deflate the airbag by providing a path for the inflator gas to exit the airbag. The deflating airbag decelerates the occupant at an acceptable rate during an impact (i.e., pressure in the airbag increases as the occupant impacts into it and the vent acts to relieve or better control the internal pressure). Without an inflator gas vent of some type, the airbag would generally be very stiff, leading to higher deceleration forces exerted on the occupant and would likely rebound the occupant into the seat, potentially causing whiplash injuries. The vent also quickly deflates the airbag in case of accidental deployment. The geometry of a vent has two primary design considerations. First, it must be a reasonably efficient shape to permit the inflator gas to flow through it. The circular vent was probably developed under this criterion, as it presents the optimum area to perimeter ratio for two-dimensional geometries. However, a second design criterion is an efficient structural shape, which is certainly dependent on the geometry and construction of the particular airbag. The non-circular shapes described below attempt to present a geometry which is both structurally efficient and provides a reasonable flow path. SUMMARY OF THE INVENTION As disclosed herein, the non-circular vent concept evolved from the inventors' discovery that the traditional circular vent hole experiences significant elongation during the early stages of module deployment. The inventors conducted a series of static deployment module tests that demonstrated airbag failure initiating at the circular geometry vent when the vent reinforcement (called a "doubler" in the industry) was eliminated from the airbag construction. This airbag failure, coupled with the desire to eliminate the vent doubler from the airbag construction, inspired the concept of non-circular vent geometries. From a structural viewpoint, the vent is a geometric discontinuity in the fabric membrane and presents a stress concentration as the airbag is pressurized (i.e., the vent is a void in the structure and the load must be supported by the locally surrounding fabric, hence the concentration of stress). Shear stress (which is detrimental to the fabric structure) is necessarily induced in the fabric surrounding the vent as the fabric distributes the loading to account for the missing fabric. The vent hole increases the flexibility of the airbag structure, which causes the fabric around the hole to stretch. At some point during this stretching, the fabric will begin to resist. The fabric, however, has only a limited amount that it can stretch before failure. The inventors have found that the fabric surrounding the vent hole stretches quite easily as the hole is initially deformed from a circle to an ellipse. Only after it has been initially deformed, does the fabric begin to significantly resist stretching. By this time, however, much of the fabric's available "stretch" has been exhausted, and the resistance can only be maintained over a short period of final elongation to failure. The concept of a non-circular vent hole emerges to present a geometry which minimizes the flexibility caused by the void in the fabric structure. (Because no fabric extends across the vent hole, it is necessarily more flexible than if fabric were present. This zone of greater flexibility disturbs the load distribution in the fabric, forcing the surrounding fabric to carry additional load and therefore additional stress.) In particular, if the constructed vent geometry is similar to the strained vent geometry during the actual deployment, then the induced stress can be significantly less. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates an elliptical vent hole according to one embodiment of the invention. Fig. 2 illustrates a diamond shaped vent hole according to a second embodiment of the invention. Fig. 3 illustrates an inverted triangular shaped vent hole. Fig. 4 illustrates a triangular shaped vent hole. Fig. 5 is a rear view of an airbag having an elliptical vent hole. Fig. 6 is a rear view of an airbag having an inverted triangular shaped vent hole. Fig. 7 illustrates a vent hole having dimensions selected in accordance with the orthotropic properties of an airbag material. Detailed Description of the Drawings The elliptical shaped vent shown in Figure 1 evolved from the discovery that a circular vent elongated to oval shape during the module deployment. By using an ellipse, the fabric is positioned to carry load more efficiently, removing the flexibility required to transition from the circular hole to the ellipse. Thus, the fabric already being in the "deformed" condition immediately resists stretching, thereby allowing the fabric to exhaust its entire "stretching capability," resisting deformation and tearing. This provides a more efficient and sturdier construction. To take advantage of the continuous strain of the fabric, the vent hole shape need not be limited to an ellipse. A slit, rectangular or any generally elongated shape will also provide immediate, continuous resistance to stretching and tearing. The direction of elongation will preferably be in the direction parallel to the axis of lowest modulus of elongation (normally between the warp and fill, or weaving directions). For example. Figure 5 shows a generally square airbag. The greatest elongation is generally parallel to the axial direction of inflation (i.e., in the direction of the driver as the airbag inflates). It is possible, however, and within the scope of this disclosure, that the greatest elongation may be encountered in some other direction. This would typically depend on the geometry of the airbag and the orientation of the fabric and may readily be determined. The ellipse is advantageous from the standpoint that the smooth, continuous periphery does not present stress concentration regions like the corners of a rectangle or the ends of a slit. The rectangular corners or slit's edges may, however, be rounded to decrease the stress concentration factors at these regions. The long axis of the ellipse is oriented toward the bias fabric direction, i.e., between the warp and fill directions. The diamond shaped vent geometry shown in Figure 2 takes advantage of a different strengthening mechanism and parallels the warp and fill directions of the fabric weave. By doing so, the inventors apply a principle of orthogonality for load bearing on the vent region similar to that applied to the main diagonal seams of the airbag. The diamond is oriented such that the waφ and fill construction directions are normal to the "flats" of the diamond. This design forces the fabric to carry load in its optimum configuration — in the direction of the warp and fill. The corners of the diamond shape may also be rounded to minimize the tendency of fabric structures under stress to tear at a sharp corner. The triangle and inverted triangle shapes (with rounded corners) shown in Figures 3 and 4 also have edges which parallel the fabric waφ and fill lines and provide an acceptable flow area. In particular, the inverted triangle provides a shape similar to the constructed geometry of the airbag shown in Figure 6. One final observation concerning the vent geometries. Assuming the modulus of the fabric is orthotropic (for example, the waφ modulus is typically lower than the fill modulus for the unbalanced construction fabrics which are sometimes used), the initial shape of the vent may not be as symmetric as the previously shown examples. That is, assuming the fabric experiences greater elongation along the waφ direction, the diamond shape (for example) may be initially more rectangular than square, so that the final deformed shape is symmetric. The initial shapes of the vent can be determined by scaling the vent dimensions according to the actual moduli of the fabric construction. An example of this is shown in Figure 7, where the ratio of the short length to the long length is approximately equal to the ratio of the modulus of elongation in the direction parallel to the short length to the modulus in the direction parallel to the long length. EXAMPLE Four airbags were constructed from a 45x100 denier calendared nylon fabric and deployed at 90°C from a T300 module with a DI-1 inflator. The first two airbags had an elliptical shaped vent hole, and the second two airbags had a diamond shaped vent hole. The elliptical geometry was constructed with a major axis of 28.5 mm and a minor axis of 20.1 mm (i.e., approximately a 45°ellipse). The diamond geometry was constructed as a 22 mm square, however, the corners were rounded with a 5 mm radius. Both vent geometries were reinforced with a 420 denier vent doubler which was stitched into place by a 5 mm wide zigzag stitch (which is offset from the edge of the vent by 5 mm). All the tests were successfully deployed and post-test examination exhibited no structural distress at the vent region. Comparable circular vents for this test series, however, showed heavy combing at the vent. A second test series was conducted. Again, the elliptical and diamond shape vents were tested. For these tests, the module and test conditions were identical to the previous testing, however, the airbag was constructed from the 100x200 denier calendared nylon fabric. The vents were dimensioned as described previously, however, no vent doubler was utilized. Of the four tests conducted, three proceeded successfully to completion. One test had to be eliminated when the corner of the module cover apparently snagged the vent hole during the early stages of deployment and ripped the vent. The remaining vents, two diamond and the eliptical vent, deployed and showed no signs of structural distress. The vent holes of the instant invention may take on many sizes and shapes, so long as they take advantage of one or both of the strengthening mechanisms described herein. Although, ideally suited for light-weight (i.e. having a denier less than 420) airbags, the non- circular vent holes may also be used in heavy or intermediate-weight fabric airbags.

Claims: Claims; 1. A flexible pressure vessel comprising: an inflatable chamber having at least one wall; and a non-circular vent hole in said wall. 2. The flexible pressure vessel of claim 1, wherein; said vent hole is an ellipse. 3. The flexible pressure vessel of claim 1, wherein; said vent hole is a diamond. 4. The flexible pressure vessel of claim 3, wherein; said diamond has rounded corners. 5. The flexible pressure vessel of claim 1, wherein; said vent hole is a rectangle. 6. The flexible pressure vessel of claim 5, wherein; said rectangle has rounded corners. 7. The flexible pressure vessel of claim 1 , wherein; said vent hole is a triangle. 8. The flexible pressure vessel of claim 7, wherein; said triangle has rounded corners. 9. The flexible pressure vessel of claim 7, wherein; said triangle is inverted. 10. The flexible pressure vessel of claim 9, wherein; said inverted triangle has rounded corners. 1. The flexible pressure vessel of claim 1 , wherein; said vent hole is a slit.

Inventors: GWINN KENNETH W, NELSEN JAMES M, GWINN, KENNETH, W., NELSEN, JAMES, M.
Applicants: GWINN KENNETH W, NELSEN JAMES M, GWINN, KENNETH, W., NELSEN, JAMES, M.
Application year: 1997
Main classification: B60R21
IPC codes: B60R21