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The validity of the current practice of using tank pressure curves to discriminate airbag inflators is evaluated. Sled test results of two inflators, which have similar tank pressure curves, are first compared. Significant differences in airbag performance have been observed which suggests that the two inflators are not similar. The gas dynamics of airbag inflation are then reviewed to develop theories to explain the phenomenon. The dual-pressure method, which was previously developed for modeling airbag inflators, is found to be useful in this analysis. The analysis clearly shows that the inequality is due to the difference in gas temperature among inflators. We find that the higher the gas temperature the faster the gas venting and leaking will be. This is why different airbag performance is obtained from inflators which have similar tank pressure curves.

A model is presented in which distinction is made between the contributions of the different mechanisms of heat transfer to an air bag fabric during deployment. An experimental setup, designed for simulation and recording of the thermal response of permeable and coated (impermeable) air bag fabrics, is described. Comparisons between the experimental results and numerical predictions show fair agreement. The preliminary results show that the model provides a framework in which the interplay between the three convective heat transfer coefficients (two surface and one volumetric) that affect the fabric temperature (and the heat loss from the upstream bag gas) can be examined. Currently the magnitude of these surface convective heat fluxes are being examined experimentally.

A passenger side airbag model in CAL3D for occupant dynamics analysis is presented. Two new features, an advanced inflator model and a multi contact ellipsoid method for describing bag shapes, are used in this model. The advanced inflator model can be used to estimate both gas temperature in the inflator, which is crucial in simulating the large gas leakage from a passenger airbag, and mass flow rate from the inflator. The multi-contact-ellipsoid method allows more than one contact ellipsoid to be attached to an airbag. These ellipsoids can then approximate a non-ellipsoidal bag shape such that the local contacts between the bag and the occupant/vehicle system may be accurately simulated. The existing computation scheme of the program for evaluating the airbag volume and gas dynamics is preserved. Illustative examples are presented. It shows that the method is easy to use, efficient in computation, and accurate enough to be used as a design tool for passenger side airbag systems.

In this study the side airbag design for a large car was investigated. The assessment of the airbag efficacy was based on the TTI response of the SID. In general, large size cars have low TTI values to begin with due to their higher mass, stronger structure, and more spacing between the occupant and the door. The CALOPT optimization program was used to search the design space. We found that for this particular impact environment an airbag design with a high mass flow rate and a large vent resulted in the lowest TTI for the SID. The high mass flow rate enables the airbag to contact the dummy thorax early, which causes the dummy to begin to move away from the door before contact is established with the door. The large vent is necessary to avoid excessive force from the airbag during the dummy/airbag interaction. For the two inflators considered in this study it was found that the less aggressive inflator achieved a marginal reduction of 10% from the baseline TTI response.

A design strategy to simultaneously address the interaction of two restraint systems (airbag and belt) and two test conditions (FMVSS 208 and NCAP) was investigated. This design strategy was implemented using a math-based methodology for a midsize car passenger side restraint system. A number of airbag and safety belt design variables were examined and optimized resulting in improved NCAP performance for the midsize car used in the simulations. The result of this study shows that this math-based methodology could be used to project the potential performance of restraint systems for future vehicle programs. As is the usual recommended procedure for math-based results of highly complex nonlinear mechanical analyses of the type under consideration herein, test validation should be carried out prior to implementation of specific results in a production program.

In this paper, we address the need to have a consistent and meaningful means to grade airbag inflators. A grading system based on five key characteristics of inflators has been developed. This grading system is designed to include all the important physical aspects of the inflator that affect airbag performance, but to still retain a simple form. We propose to use this, or a grading system similar to this, as an industry-wide grading system for inflators. With a grading system of this kind, airbag engineers will be able to communicate more effectively. This will enhance the efficiency of airbag system design and development effort.

In this paper, we present an experimental and analytic procedure for determining the effective leak and vent areas of a full size airbag. Three different bag materials were tested and analyzed: 420 D (72 X 46), 420 D (49 X 49), and 840 D (32 X 32). An airbag model in CAL3D was used to inversely compute the effective leak area based on experimental data. Regression analysis was then performed to find the best fit of mathematical models for estimating effective leak areas as a function of pressure. Following a similar procedure, the effective vent area was also analyzed. Data are presented for a passenger bag and a driver bag. These results are useful for occupant dynamics simulations.

The permeability of some airbag fabrics is determined, along with the Ergun coefficient signifying departure from purely viscous flow, from gas flow rates and pressure drop measurements. The dependency of these coefficients on the fabric temperature is also examined. Preliminary results are reported on the transient response of these fabrics to temporal changes in the gas flow rate and temperature. The temperature history is measured and compared with the predictions of some simple models. The models make various assumptions regarding the microscale of the fabrics. The preliminary results show that the very fine microscales do not control the time response of the fabric.

A generalized analytical model of an airbag inflation system has been developed and integrated into the CAL3D occupant dynamics simulation program. The new model allows for tabular input of mass flow rate and gas temperature to represent the inflator, and it accounts for airbag material stretch and airbag gas leakage. With this new model, the CAL3D program can be applied to evaluate the occupant protection performance of the majority of airbag inflation systems of practical interest. A separate gas dynamics analysis is also developed for generating the mass flow rate and gas temperature data for representing a pyrotechnic inflator, either when a tank test of the actual inflator is available or when a particular inflation characteristic is desired.

A hybrid-inflator simulation program, H-ISP, has been developed to facilitate the evaluation of hybrid inflators for airbag systems. Based on fundamental principles of species mass and energy conservation and gas dynamics, this program can simulate the complex thermo-chemical processes of a hybrid inflator, including solid propellant combustion, single-phase or two-phase flow, multiple chemical species with temperature- and composition-dependent thermal properties, and heat transfer phenomena. In this paper, we present the theory, computer program structure and usage of H-ISP with an example.

A numerical model is presented that is capable of isolating and quantifying the heat flux from the gas within the bag to the air bag fabric due to internal surface convection during the inflator discharge period of an air bag deployment. The model is also capable of predicting the volume averaged fabric temperatures during the air bag deployment period. Implementation of the model into an air bag deployment code, namely Inflator Simulation Program (ISP), is presented along with the simulation results for typical inflators. The predicted effect of the heat loss from the bag gas to the fabric on the internal bag gas temperature and pressure and the resulting bulk fabric temperature as a function of fabric parameters and the inflator exit gas properties are presented for both permeable and impermeable air bag fabrics.

Aspirating airbag modules are unique from other designs in that the gas entering the airbag is a mixture of inflator-delivered gas and ambient-temperature air entrained from the atmosphere surrounding the module. Today's sophisticated computer simulations of an airbag deployment typically require as input the mass-flow rate, chemical composition and thermal history of the gas exiting the canister and entering the airbag. While the mass-flow rate and temperature of the inflator-delivered gas can be obtained from a standard tank test, information on air entrainment into an aspirated canister is limited. The purpose of this study is to provide quantitative information about the aspirated mass-flow rate during airbag deployment. Pressure and velocity measurements are combined with high-speed photography in order to gain further insight into the relationship between the canister pressure, the rate of cabin-air entrainment and the airbag deployment.