Crush Energy Considerations in Override/Underride Impacts

In automobile accident reconstruction it is often necessary to quantify the energy dissipated through plastic deformation of vehicle structures. For collisions involving the front structures of accident vehicles, data from Federal Motor Vehicle Safety Standard (FMVSS) 208 and New Car Assessment Program (NCAP) frontal barrier impact tests have been used to derive stiffness coefficients for use in crush energy calculations. These coefficients are commonly applied to the residual crush profile of the front bumper in real-world traffic accidents. This has been accepted as a reasonable approach, especially if there has been significant involvement of the front bumper and its supporting structures. For impacts where the structures above the bumper level are deformed more than the bumper itself, this approach may not be so readily applied. These types of impacts are called override/underride, and are encountered quite often in truck-to-car accidents where there is a vertical difference in bumper heights and also in accidents where bumper height mismatches are created through vehicle brake dive. In this paper we examine the crush-energy considerations of override/underride impacts. The limited available literature and test data are reviewed. Two test programs that involved impacts over a wide range of severities are analyzed in detail.