Modeling and analysis of an inflatable lap belt airbag restraint system for frontal crash protection of mass transit bus operators
Mass transit bus is one of the key modes of transportation, especially in highly populated areas of the United States (US). Review of literature has shown that there is very little research, which focuses on the safety and comfort of a bus operator. National Highway Traffic Safety Administration (NHTSA) has a Fatal Accident Reporting System (FARS). FARS shows that there were 300 fatal bus accidents during the period from 1995 to 1999. Statistical data also shows that 16 bus operators suffered fatal injuries during the period from 1991 to 2000. The operators end up driving for long duration of time, which makes the three-point type of belt restraint system uncomfortable. This may also lead to stress and fatigue. The use of a two-point seat belt restraint system may be more comfortable for the operator since his hands and torso will be unrestrained. However, the physical sled testing of this seat belt configuration revealed severe life threatening risk for the operator. It was observed that the Head Injury Criteria (HIC15) value was above the threshold limit of 700 and the femur load value was well over their allowable threshold limit of 10,000 N. The Neck Injury Criteria (Nij) value was very close to the allowable threshold limit of 1. Thus, safety and comfort of the transit bus operator appear to be contradictory to each other. In this research, an effort had been made to develop a methodology to design an airbag restraint system for a mass transit bus operator, which can satisfy the two contradictory objectives. An Inflatable lap belt type of airbag restraint system has been previously implemented in aircraft safety. This system is comprised of a two-point restraint type seat belt system with an airbag embedded in it. This type of airbag safety system was tested in this study for a mass transit bus operator with an aim to keep the HIC and the Nij values within allowable limits. Additional knee-padding was designed to minimize the femur loads. All of the research was conducted for frontal impact of a bus with the rigid-wall crash condition. A model development of a two-point restraint type seat belt system was carried out with the occupant simulation code, MADYMO. This model is validated with the help of physical sled test data. The baseline simulation model was used to develop the numerical model of an inflatable lap belt airbag restraint system. With the help of numerical models, the prototypes of airbag and inflators were manufactured. A physical sled test was also carried out for this system under frontal impact condition. This test data was utilized to validate the simulation results obtained from the MADYMO model. The physical sled testing as well as numerical models were developed for the three-point belt restraint system. Except the femur injury, all of the injuries were within the allowable safe limit. A physical sled test with the airbag and the knee-padding was also conducted to test the performance of an inflatable lap belt airbag restraint system. A knee-padding played a vital role to minimize the femur injury. The Results from this study can be utilized in the design and development of a safety belt system for optimal protection of mass transit bus operators in the event of a frontal impact crashes. This study can be utilized further for designing the safety restraint system for other crash conditions.