Thermal Stresses Due to Laser Welding in Bridge-Wire Initiators

In ongoing research at the University of Idaho, potential failure mechanisms of airbag initiators are being investigated. Cracking of the cylindrical glass-to-metal seal (GTMS) present in these devices has been observed. These cracks could be a path for moist gas to diffuse into the initiator, potentially leading to bridge-wire degradation and late-fire or no-fire initiator failure. Previous research has shown that cracking may be caused by thermal stresses induced by the GTMS formation process. The goal of this research was to determine if welding of the output-can onto the initiator header could produce stresses in the glass great enough to cause cracking. A finite element analysis solution was chosen to model the transient heat transfer and temperature distribution in the initiator assembly during the welding process. The thermal stresses were calculated with a mechanical analysis once the temperature distribution was determined. Compressive stresses induced by pressing the header assembly into the output-can as part of the manufacturing process were also investigated with a closed-form mechanics of materials solution. The welding thermal stress model initially predicted radial stresses greater than the tangential stresses. This conflicts with observed radial cracks, which would be induced by tangential stresses. Subsequent investigations with an interface region stiffness model showed that when the stiffness of the bond at the pin-glass and glass-header interfaces is decreased, the maximum radial stress is greatly reduced and that the maximum tangential stress stays relatively constant. These predicted stresses were still in excess of the range of glass strengths reported in literature. However, superposition of the compressive stresses due to the press-fit and residual stresses created when the GTMS is formed with these thermal stresses results in the total radial and tangential stresses being on the same order as the reported strengths. It was determined that when initiators are overheated during welding, radial stresses due to thermal expansion cause the bond to fail and separation to occur over a portion of the pin-glass interface. Tangential stresses developed for the same reason are sufficient enough to cause radial cracking, where the bond is still intact.