STRUCTURAL INTEGRITY ANALYSIS OF ULTRA HIGH STRENGTH STEEL TUBES FOR AIRBAGS

Cold drawn seamless tubes of OD 60.3 x wt 3.55 mm manufactured using low carbon nickel alloyed steel (LCNi) and low carbon chromium-molybdenum alloyed steel (LCCrMo) both with ultra clean steel practice were studied. LCNi steel presented two types of heat treatments, ultra accelerated cooling and quenched and tempered, while the other was only quenched and tempered. These candidate materials were selected for the development of a novel 160 ksi steel tube to be used as airbags inflator. Each material was experimentally characterized; in particular toughness properties were determined at room and -60 degrees C temperature. LCNi steel, having both heat treatments, presented higher resistance to fracture than LCCrMo steel, this feature was observed at both temperatures. Theoretical relationship between burst pressure and defect depth was obtained using a model that combines plastic collapse and fracture mechanics failure modes. Using this model, burst pressure as function of defect depth was determined for a constant defect aspect ratio. Full scale burst tests on tubes with and without artificial defects were carded out to set up the developed methodology. Artificial defects were longitudinally oriented with surface semi-elliptical geometry. The defects were made by electro discharge machining. Critical defect sizes of different materials for 160 ksi grade tubes used for airbags inflators were theoretically and experimentally determined. It was observed that at room temperature the predominant failure mechanism was plastic collapse for defects up to 40% of wall thickness, while at -60 degrees C the failure by fracture is relevant for defects deeper than 20% of wall thickness. Although the LCCrMo steel presented lower fracture toughness properties at room and -60 degrees C temperatures, burst pressure for defects shallower than 20% of wall thickness did not present a noticeable reduction compared to LCNi steel. This level of defects is considered to be easily detected by current NDT techniques, consequently the critical defect sizes are similar for the three materials regardless the fracture properties and for the current NDT cut-off levels.