A Ballistic Gas Compressor for Automotive Airbag Initiator Research and Automotive Engine Testing and Development
Bridge-wire initiators are extensively used in aerospace, automotive, and defense industries to ignite pyrotechnic, propellant, and explosive materials. The products of combustion of these secondary materials are used to perform an assortment of functions including actuation of mechanisms, explosive separation of components, and deployment of supplemental restraint or other protective systems. In all of these applications, it is important the initiators perform reliably over a wide range of temperatures, often after prolonged exposure to harsh environmental conditions of humidity, thermal shock, and vibration.
The most common method of measuring initiator performance involves discharging these devices into constant volume closed vessels and monitoring the rate and magnitude of pressure increase. Typically, the maximum pressures generated by the discharge of initiators into closed vessels are used as acceptance criteria for lot acceptance tests of these devices (1). Alternatively, the maximum dynamic pressures may be used to infer the energy liberated by the combustion process, and the derived values of energy release may be used as an input for models used to describe the ignition and combustion of additional gas generating materials and reactive gas mixtures (2). Therefore, it is important to be able to measure the maximum dynamic pressure as accurately as possible.
Our earlier work has focused on closed vessel design, fabrication, and testing as well as understanding how the cumulative effects of uncertainties in instrumentation, data acquisition, and signal processing influence the derived values of initiator energy release (3, 4). Since there is no standard concerning test vessel design, four different test vessels were designed and fabricated encompassing the range of internal volumes (1 cm3 to 100 cm3) typically used by manufacturers for initiator performance testing. The transient pressure characteristics obtained from these tests are dependent on many factors including the load and type of pyrotechnic material and the initial density, temperature and composition of gases contained within the vessels. Typically, initiators are tested by discharging the device into an environment containing air at atmospheric pressure. In these cases, maximum pressures may typically reach 7 MPa on the order of one millisecond. However, for characterization of energy release, initiators are discharged into high pressure inert gas environments and maximum internal pressures may exceed 60 MPa in a similar time frame. In certain circumstances, it may be advantageous to discharge initiators into high pressure, reactive gas environments where maximum pressures of up to 200 MPa may be reached in less than 1 millisecond.
The goal of this research is to design, fabricate, and test an apparatus for calibration of dynamic pressure sensors used in closed vessel initiator testing. The system must be versatile enough to replicate the transient pressure characteristics discussed above. In addition, since this apparatus will be used to calibrate pressure sensors used to determine energy release rates and magnitudes, it is also critically important to characterize the inherent uncertainty in pressures produced by the device. The completed system will be useful in calibration of pressure sensors for characterization of all other rapid combustion events including internal combustion engine diagnostic testing, pyrotechnic burn rate analysis, and detonations of high pressure gases and liquid mixtures.