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Among the forces acting on an occupant during the deployment of an airbag, the impact force due to gas jets coming out of the inflator may be important. Consequently, the semi-analytical theory of high-velocity gas jets including velocity spreading due to turbulent diffusion has been incorporated into the explicit finite element computer program PAM-SAFE for occupant safety. The jet model, described in this paper, takes into account the geometry and thermodynamics of the inflator, the transfer of jet momentum to the moving airbag and the distribution of momentum due to diffusion. Since the intersection of every jet with the airbag is computed each timestep, the algorithm is consistent with numerical airbag unfolding. Experimental validation of the jet model has been performed at Automotive Systems Laboratory Inc. by firing actual inflators on rigid flat plates at several distances.

With the advent of widespread use of airbags as restraint systems for both driver and passenger, a new challenge has appeared for the design engineer. Numerical models of various degrees of complexity have been developed to model the interaction between airbags and occupants, ranging from simplified models such as the airbags included in rigid body dynamic models, to more sophisticated, fully three-dimensional ones based on Finite Elements (FE). However, some key questions of airbag design such as interaction of the unfolding bag with out-of-position occupants and effects of different airbag folding patterns remained unanswered, and their design still requires extensive test series. In this paper, the approach developed at Engineering Systems International over the past two years to solve the problem of numerical simulation of fully folded bags is presented in the larger context of the PAM-SAFE program for analysis of occupant safety.

In the last several years the Finite-Element Method has been successfully used in assessing and optimizing vehicle crash performance. Occupant modelling however, has remained the domain of gross motion simulators which make use of rigid body dynamics. New passive restraint systems like the airbag, have raised demands to mathematical modelling which cannot be satisfied with the existing models. The simulation of ‘out-of-position’ vehicle occupants is a prime example. A new approach to occupant simulation using the Finite Element Method is presented. The three-dimensional modelling of an airbag using the modified explicit dynamic FE-Code PAM-CRASH is the first step in this direction. An airbag inflation model has been added to the PAM-CRASH code. It incorporates tabular input of the mass flow, calculation of the airbag's thermodynamics and gas flow out of the bag via vents and leakages. The airbag fabric is modelled with a newly implemented membrane element.