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One element of primary interest in the analysis and reconstruction of vehicle collisions is an evaluation of impact severity. The severity of an impact is commonly quantified using vehicle closing speeds and/or velocity change (delta-V). One fundamental methodology available to determine the closing speed and corresponding velocity change is an analysis of the collision based on a combination of the principles of Conservation of Momentum and Conservation of Energy. A critical element of this method is an assessment of the amount of kinetic energy that is dissipated during plastic structural deformation (crush) of the involved vehicles. This crush energy assessment is typically based on an interpolation or an extrapolation of data collected during National Highway Traffic Safety Administration (NHTSA) sponsored crash testing at nominal speeds of 30 or 35 mph.

Crush energy assessment methods rely on the characterization of a vehicle’s structure, through a comparison with crash tests of a similar vehicle. For frontal impacts, the vast majority of these tests involve a flat rigid barrier. When the reconstructionist is presented with a frontal underride/override crash, however, the structural load pattern and the deformation mode suggest that the comparison with flat barrier tests may not be valid. This has been confirmed by prior studies. With few exceptions, for any given vehicle, there are no crash data in an underride/override mode that are useful for analysis purposes. The purpose of this research was to bridge the gap so that flat barrier data, specific to the vehicle in question, could be applied to underride/override cases. This entailed the development of a measurement protocol, a structural model for such crashes, and a procedure for analyzing the load cell data that exist for many barrier crash tests.

Residual damage caused during a collision has been related through the use of crush energy models and impact mechanics directly to the collision energy loss and vehicle velocity changes, ΔV1 and ΔV2. The simplest and most popular form of this crush energy relationship is a linear one and has been exploited for the purpose of accident reconstruction in the well known CRASH3 crush energy algorithm. Nonlinear forms of the relationship between residual crush and collision energy also have been developed. Speed reconstruction models that use the CRASH3 algorithm use point mass impact mechanics, a concept of equivalent mass, visual estimation of the Principle Direction of Force (PDOF) and a tangential correction factor to relate total crush energy to the collision ΔV values. Most algorithms also are based on an assumption of a common velocity at the contact area between the vehicles.

In frontal crashes, lateral deformations can occur as a result of various mechanisms. Unfortunately, the crush energy associated with such deformations cannot be assessed as long as the structural properties are unknown. That has been the situation to date, due to the lack of appropriate crash test data. The present research attempts to address this deficit. A passenger car was crash-tested in a mode designed to induce lateral deformations that are significant compared to longitudinal crush. This was done via a series of three repeated impacts on the same vehicle so as to obtain, in a cost-effective manner, structural characterization data at increasing crash severities. Various cause-and-effect relationships (structural characterization models) were considered with an eye to selecting the one that best predicts the crush energy. Insights obtained from analyzing the behavior of the front structure are presented.

Due to the upgrade of FMVSS 214 and the emergence of side NCAP tests, there is a growing body of crash test data on vehicle side structures. Such data would be very useful to reconstructionists, except that the struck vehicle behavior is masked, in part, by the use of a deformable moving barrier in the test. The post-impact dynamics and the energy absorption by the barrier itself must be accounted for if the desired vehicle structural characterization is to be extracted. Attempts prior to this paper to achieve a side structure characterization have dealt with these issues by invoking various simplifying assumptions. Unfortunately, these have not been supported by a foundation in either physics or measurement. Questions have also been raised whether prior characterizations of the barrier face are appropriate, in view of the prior crash modes being so unlike the FMVSS 214 test. To address these issues, crash tests of the barrier itself, in an appropriate crash mode, have been conducted.

A key aspect of accident reconstruction is the calculation of how much kinetic energy is dissipated as crush. By far the most widely used methods are derivatives of Campbell’s work, in which a linear relationship between residual crush and closing speed is shown to imply an underlying linearity between force and crush. “Consant-stiffness model” is the term used for such a representation of structural behavior. Difficulties arise, however, when significant non-uniformities are present in the crush pattern (as in narrow-object and/or side impacts, for example). The term “residual crush” becomes more ambiguous. Do we mean maximum crush, area-weighted average crush, or some other measure of residual deformation? And is it sufficient to represent the non-uniform crush pattern by a single parameter? Such considerations led to a re-development of the fundamental structural models, with an eye to determining whether the classical constant-stiffness model is the most appropriate.