Pyrotechnic initiators are commonly used in automotive inflatable restraints systems (i.e., airbags) for supplemental occupant protection. The use of these devices continues to expand as world wide production of driver, passenger, side impact, and curtain inflation systems exceeds 100 million units yearly. In this potentially life-saving application, it is critical that these electro-explosive devices operate reliably without failure or diminished performance for the intended service life of the vehicle – typically specified as 15 years. Recent research at the University of Idaho indicates that these devices are often not able to meet the stringent quality control requirements specified for other electrical elements of the ignition train.
Work in this research project is focused in two general areas: First, particular attention is placed on the hermetic and non-hermetic behavior of actual devices with associated leak rate determination, modeling, and analysis. The goal of this work is to understand and quantify the failure modes related to lack of hermetic behavior so that leak rates may be properly specified and initiator design may be improved. The second area of work is focused on the characterization and modeling of the ballistic response of pyrotechnic initiators under different environmental conditions of temperature, pressure, and composition. Although the exact specification of an initiator may vary depending on the type of gas generator used (solid, hybrid, or cold gas) and the specific airbag application (driver, passenger, or side impact), it is critical that the initiators perform in a repeatable manner over the anticipated range of environmental conditions. The ballistic test facility under development at the University of Idaho will permit confined and vented initiator tests at temperatures from -40°C to 110°C at static pressures in excess of 30 MPa for inert and reactive gas mixtures.
Research has shown that hermetic behavior and ballistic response are often intricately coupled such that careful study and analysis are required to understand their separate affects in initiator performance. For example, moisture ingestion may cause deleterious effects on pyrotechnic performance that ultimately lead to ignition delay. At the same time, moisture ingestion may cause corrosion of the initiator bridgewire that ultimately leads to ignition delay through reduction of the heat transfer rate from the wire to the pyrotechnic. (And, of course, combined failure mode effects are possible as well.) But ignition delay can also be caused by inherent problems with stoichiometry and mixture properties of the pyrotechnic. Due to these (and other) complexities, experimental data must be carefully analyzed to understand and differentiate the possible contribution of many coupled failure mode effects. These coupled failure mode effects are currently not well understood and represent a significant liability in the industry.
Leak rate determinations and moisture ingestion specifications for initiators are currently performed using theoretical flow descriptions in conjunction with assumed design guidelines. Yet no supporting publications or documentation can be found in the literature to validate these flows models as applied to the small cavities and torturous passages typical of initiators. Therefore, a set of calibrated, small cavity standards has been designed and manufactured. The standards are made of high quality stainless steel, and feature small laser-drilled orifices. These standards will be bombarded with radioisotope mixtures so that the flow of gas from the components can be monitored as a function of time. Then the experimentally derived flow rates can then be compared to theoretically predicted values. Results will be used to valid flow models used to aid in initiator design, and better understand the rate of moisture ingestion into small cavities.