Weight-reducing composites for auto air-bag inflators.
Vehicle weight, an important issue in automotive design, promises to become more critical as fleet mileage standards continue to increase. At the same time, vehicle safety has been improved by the incorporation of driver-side air-bag modules, and by the mid-'90s, most new vehicles will also have passenger-side air bags, as a result of new regulations mandating passive restraint systems. Inflators for passenger-side air bags, currently made of low-carbon steel, have proven successful under all air-bag-deployment conditions, and low-weight aluminum inflators will soon be introduced. But even lower-weight inflators will be required for future automotive air-bag applications as mileage standards place further constraints on vehicle weight.
This article describes the development of a passenger-side air-bag inflator in which replacement of the steel body with a composite body achieved a 65% weight saving. The composite inflator also lends itself to modular fabrication with low-cost tooling, making the device faster and easier to manufacture. Fewer manufacturing steps are required, and more components can be built in-house, leading to improved quality control and lower overall cost. The performance requirements, design analysis, and fabrication techniques are discussed, and the tests performed with the device are summarized. Although some information must remain proprietary, enough details are provided for readers to understand the advantages of the composite-inflator concept.
Replacement of other metal components with composites could result in even greater weight reduction. Research is now under way to complete development of the composite inflator and explore its full potential.
Automotive air bags are actuated by crash sensors, which send an electrical impulse to the inflator when a significant frontal collision occurs. Figure 1 shows the basic components of a passenger-side air-bag module. The inflator is a tubular pressure vessel containing an ignitor, several grains (pellets) of gas generant, and a filter surrounded by metal burst foil. The ignitor receives the electrical impulse, passes it through a heating filament, and actuates a small pyrotechnic charge. The charge ignites the gas generant, which decomposes very quickly at a carefully controlled rate, producing nearly pure nitrogen gas.
Although initially at a high temperature, the nitrogen cools rapidly as it expands, so that the fully deployed bag is only slightly above room temperature. In passenger-side air bags, the entire inflation sequence takes place within 50 to 65 milliseconds. By comparison, it takes 200 milliseconds, or 0.2 sec, for the human eye to blink.
Fiber-reinforced composites are already being applied in other types of pressure vessels such as compressed-gas cylinders. Even though air bags are stored at ambient conditions in the vehicle, the inflators reach much greater pressures and temperatures than most pressure vessels during deployment.
Also, air-bag inflators are capped by a separate piece at each end; these end caps must be easy to install and must be able to withstand air-bag deployment forces. If the caps are an integral part of the pressure vessel structure, stress concentrations must not be induced at the cylinder/end-cap juncture during deployment.
Air-bag inflators also differ from other pressure vessels in the location of their gas-discharge nozzles. Most pressure vessels discharge gas from one nozzle located at the end of the cylinder. Air-bag inflators release gas through several nozzles located throughout the cylindrical section.
The inflator design was approached by dividing the device into two parts--the cylindrical section and the end caps. Analysis and design of the cylindrical section was relatively straightforward compared with that of the end caps. Because of the known notch sensitivity of composite materials, the design and fabrication of the inflator's gas-discharge nozzles became a critical issue for analysis.
Several design techniques were applied, including netting analysis, burst-strength analysis, and detailed finite-element analysis of the end caps and nozzle openings. The results indicated that it would be feasible to design and fabricate a low-weight air-bag inflator from high-strength, low-density composite materials.
A carbon-fiber-reinforced vinyl ester resin was selected for the composite inflator body. Braiding and resin transfer molding were identified as practical fabrication techniques for the inflator. The polyacrylonitrile (PAN)-derived BASF carbon fibers have a 550-kpsi tensile strength, a 34-Mpsi tensile modulus, and a density of 0.063 lb/|in.sup.3~. The Dow Chemical Co. vinyl ester resin has a low viscosity and short cure time suitable for resin transfer molding, and a tensile strength, tensile modulus, and density of 12 kpsi, 490 kpsi, and 0.04 lb/|in.sup.3~, respectively.
Braiding was selected for construction of the body because it is a high-output textile manufacturing method that can be automated and is suitable for automotive production volumes. To fabricate the inflator, individual fiber strands were braided to form a tubular module, or preform, containing:
* the grains of gas-generant;
* the filter, which removes particulates from generated gas;
* the plenum screen, which directs gas to the nozzles and ensures proper gas flow throughout the inflator;
* the burst foil, which slightly increases back pressure during ignition;
* a silica-paper layer, which protects the composite body from excessive heat; and
* the end caps.
The braiding technique is very flexible, enabling structural details such as nozzles and end caps to be formed directly. When braiding is used to form the filter and the plenum screen, strength is added uniformly to the entire device.
Resin transfer molding was selected because it is a high-rate process that produces repeatable, high-quality structures. To fabricate the composite inflator, the braided dry-fiber preform was placed inside a closed, sealed mold, and the low-viscosity resin was then injected into the preform under low pressure. The resin flows around the preformed nozzles and end caps, hermetically sealing the inflator. After the resin displaced all of the air in the mold, injection was halted and the part was cured.
The manufacturing process for the composite inflator thus differs from that of metal inflators only in the order of assembly. Instead of the separate insertion of each component into a steel inflator body, the composite body is formed around a module that already contains all the components. This eliminates stresses encountered in stuffing each component into the body and avoids the damage potential inherent in such operations.
To prevent resin from penetrating into the filter and gas generant, a seal is placed just beneath the composite body. The layer of low-cost insulating silica paper reduces post-deployment smoke by preventing heat soak-out temperatures from exceeding the degradation temperatures of the resin matrix.
As in the current steel inflator design, a layer of metal burst foil is required to increase back pressure. The foil also prevents the silica insulation paper from entering and blocking the plenum screen. In the final composite inflator design, the foil might have raised dimples at the nozzle locations instead of actual perforations. These dimples would allow the carbon-fiber body to form nozzles during molding, thus eliminating a post-drilling operation.
Both the composite and metal inflators use plenum screens made of woven steel. Studies were conducted on using plenums of braided and woven steel in both types of inflators. Replacement of the woven-steel plenum with a braided, heat-resistant polymer monofilament is now being considered for the composite inflator. This may improve assembly tolerances and reduce plenum cost.
The composite inflator's end caps were made of a heat-resistant, fiber-filled phenolic. A metallic bushing was used for the threading for the ignitor cap, but fabrication of polymer threading may be feasible. Conventional steel end caps were also tested for comparison. All other components of the composite inflator were exactly the same as those of the steel inflator, allowing comparison of baseline data from the two devices.
To comply with government regulations, automotive vehicle manufacturers require several prequalifying tests for all vehicle components. The most challenging test for the composite inflator was bonfire testing because the resin burned off during heat-induced deployments, allowing the end caps and ignitor to separate from the assembly. Steel end caps significantly improved the integrity of the inflator during bonfire testing.
In laboratory temperature tests, the composite inflator performed successfully from -40|degrees~F to 150|degrees~F with no observable problems. Analysis of post-deployment gases showed few or no problems; only small amounts of toxic gases (carbon monoxide and styrene) were present. No compatibility problems were observed when the gas generant was exposed to the resin and the polymerization catalysts.
Ballistic tank tests of generated-gas output showed that gases produced by the composite inflator are at slightly higher pressures and temperatures than gases produced by steel and aluminum inflators, possibly because less heat is transferred from the gases to the inflator wails. Computer analysis supports this hypothesis.
Burst testing of the composite inflator showed a benign failure mode, in which the resinated fiber matrix tore apart harmlessly without emitting body fragments. The inflator can withstand enormous amounts of energy without fragmenting or failing catastrophically because the energy is dissipated as the fibers are pulled through the resin matrix. Drop testing of the composite inflator revealed no structural defects. In long-term field use, industrial composites have shown negligible environmental effects.
The post-deployment temperature of composite inflators was substantially lower than that of steel inflators in instrumented heat-soak tests. The composite device was cool enough to be handled immediately after deployment, and surface temperatures during soak-out did not exceed the polymer degradation temperature.
Replacing the steel body of the passenger-side air-bag inflator with a carbon-fiber/resin composite body resulted in a body weight saving of at least 65%; the composite body would also be substantially lighter than an aluminum body. The composite inflator also lends itself to modular fabrication with low-cost tooling, making the device faster and easier to manufacture. Fewer manufacturing steps are required, and more components can be built in-house, leading to improved quality control and lower overall cost. The nozzles and end caps are sealed with resin during fabrication, eliminating an expensive
post-sealing operation. Assembly tolerance problems are also eliminated because the composite inflator is layered from the inside-out rather than outside-in.
The low-weight composite inflator could evolve into an all-composite module fabricated with adhesive bonding and other nonmetallic attachment techniques, Replacement of the inflator's metallic components--the end caps, plenum, and filter--with composite components could result in even greater weight savings. If polymeric end caps are used, a wrap-and-tie method for securing the caps to the composite body could be devised. In addition, the filter component could be formed over the generant package, improving the inflator's heat-sink characteristics, and thus reducing the amount of gas generant required.
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|Author:||Faigle, Ernst M.; Lee, Chienhom; Semchena, John H.; Saccone, Paul; Thompson, Richard|
|Date:||Nov 1, 1992|
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