New definitions of deformation index for the measurement of bus survival space in crash

Vehicle crashworthiness can be described as the capability of the vehicle to protect the occupant during a crash. The space containing the occupants should remain intact and should not allow any potential intrusion or crushing to injure the occupants. For rollover crash test of a bus, this safety space is well defined in a standard, that is, the United Nations Regulation No. 66 (UN R66), where it gives pass/fail criteria. However, the standard does not measure and quantify the bus structural deformation. The measurement of such deformation has been systematically quantified by the angular deformation index (DI_α), in which the deformation of bus structure can be generalized for comparative study giving more descriptive assessment of the bus structure deformation level. DI_α is suitable for rollover test that is done strictly in a controlled environment, where the deformation is typically not severe. In real crashes, impact load varies widely and can cause severe structural damages, especially among the aged buses with poor structural strength, which are still broadly used in less developed countries. If the deformed bus structures penetrate deeply into the residual space, DI_α index becomes irrelevant because the angular values in the equation give exponential results. It is therefore suggested that a new deformation index, termed as the area deformation index (DI_A), is used for real rollover crashes involving severe damages to the bus structure. This is to quantify the damages in two dimensions once the structures penetrate into the residual space, up to where the structure is totally collapsed. By having this new index, a suitable structural deformation severity rating for the bus with respect to real rollover crashes can be proposed. This severity rating can be correlated to the injury severity (e.g. number and level) of the bus occupants, and can be used to evaluate some specific parameters influencing crashworthiness performance (e.g. aging effects). Case studies are discussed to show the practicality of the proposed DI_A concept. Volume deformation index (DI_V) is also proposed to quantify the deformation in three dimensions.