Pyrotechnic initiators are widely used in aerospace and automotive systems. In automotive applications, they are used as the ignition mechanism for all supplemental occupant systems (i.e., airbags) including frontal, passenger, side, curtain, knee bolster, seat belt pretensioning, and hood lifting systems. Aerospace applications include ejection seats, inflatable restraints, cartridge actuated and pressure actuated devices (CAD/PADs), ordnance and ordnance delivery systems, and spacecraft launch, reentry, separation, and deployment functions. In all these applications, it is critical that these electro-explosive devices operate reliably without failure or diminished performance for the intended service life of the vehicle or system.
Recent research at the University of Idaho indicates that initiators returned from field service occasionally display cracks and flaws in critical glass-to-metal seal (GTMS) components. This is of particular concern when considering the reliability of these devices as cracked GTMSs may provide leak paths through which moisture may penetrate into the bridge-wire region of these devices and possibly corrosion, degradation, and diminished performance.
Work in this research project is focused in two general areas. First, a FEA model will be developed to understand the thermal stresses introduced into the GTMS during the manufacturing process. While earlier work focused on understanding the thermal stresses imparted to the GTMS during its fabrication, it did not consider the role of subsequent initiator manufacturing processes on the integrity of the GTMS. In particular, this work will address the effect of welding the output can onto the initiator header and associated GTMS. Although experimental evidence suggests that cracks in the GTMS may be induced in this process, there are no published analytical results to support this hypothesis. Second, a model of diffusion of moist gases through cracked GTMSs will be developed.
Experimental work is in progress using 85Kr radioisotope methodology to estimate the effective flow area through a small population of cracked GTMSs. The data used from this work will then be analyzed using various flow, permeation, and diffusion models to arrive at a prediction of the rate moisture transport a cracked GTMS. This work will be the first of its kind and immensely important in predicting the possible rate and extent of corrosion of bridge-wires.
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 analyzed carefully to understand and differentiate the possible contribution of many coupled failure mode effects. These coupled failure mode effects are currently not well understood ands represent a significant liability in the aerospace and automotive industries.