Search Results

The CRASH3 computer program models a vehicle structure as a homogeneous body with linear force-deflection characteristics. Crush stiffness coefficients determined from full-overlap crash tests, when used in this computer program, allow for an accurate reconstruction of collisions where the accident damage profiles are full-overlap. In the past, partial-overlap frontal crash tests were not performed. The lack of partial-overlap frontal crash tests meant that a reconstructionist only had crush stiffness coefficients available that were determined from full-overlap frontal crash tests. In a reconstruction, the assignment of stiffness coefficients to a partial-overlap damage profile required engineering judgement. Often the basis of such judgement was questioned because of the lack of supporting partial-overlap test data. Recently partial-overlap crash tests have been performed and the test data has been made available to the public.

CRASH3 based computer programs model a vehicle structure as a homogeneous body. Crush stiffness coefficients determined from full-overlap crash tests, when used in these computer programs allow for an accurate reconstruction of collisions where the accident damage profiles are full-overlap. The structures of vehicles, however, might not be purely homogeneous in their crush response. How accurately do crush stiffness coefficients that were determined from full-overlap crash tests represent the crush response of that same vehicle in a partial-overlap/offset frontal collision? Before this question can be answered a method needs to be developed for determining crush stiffness coefficients from partial-overlap/offset frontal test collisions. These crush stiffness coefficients then could be used in a comparative analysis of the crush response of vehicles tested in both full-overlap and partial-overlap/offset frontal collisions.

A revision of the modeling of restitution properties in the SMAC computer program and the introduction of restitution properties in the CRASH3 program have been proposed by McHenry [1], the author of M-CRASH and M-SMAC [2]. The accuracy of an accident reconstruction, which uses the proposed restitution model, is directly related to the accuracy of the crush stiffness coefficients employed. The ideal condition for determining crush stiffness coefficients for these programs requires crash tests of a vehicle structure through a wide range of impact speeds with a rigid barrier. Reports from these crash tests should contain the usual data found in NHTSA crash test reports plus rebound velocity and maximum dynamic crush. Unfortunately, such comprehensive tests are very rare. As a result, certain vehicle properties need to be generalized from the existing available crash tests. These generalized properties can then be used to calculate crush stiffness coefficients for these programs.

There are many collisions in which the “standard” analysis methods are not sufficient to complete an analysis. Many times the points of rest for the vehicles are not documented or the vehicles were “driven” to the points of rest. There are also cases in which one of the vehicles is repaired prior to being documented. In these cases, there is a method that can be used to establish the approximated speed change of the vehicles. This method involves using the crush profile of one of the vehicles and balancing the opposing forces across the crush profile to determine an equivalent crush depth on the undocumented vehicle. Using this “balanced forces” method requires a detailed crush profile of one of the vehicles and good stiffness data for both vehicles. The method is not as accurate as standard methods because of the unknowns, but does yield reasonable results for the speed change severity for the vehicles involved.

This paper provides an overview of a vehicle property database that can be accessed directly by the HVE Vehicle Editor. Vehicles are selected from the database according to their type (Passenger Car, Pickup Truck, etc.), year, make, model, body style, engine size and trim package. Each vehicle, when selected from the database, is predefined by its exterior dimensions, wheel locations, CG location, vehicle mass, inertial properties, drive train properties, suspension properties, tire properties and crush stiffness coefficients (A, B, & KV). A selected vehicle then may be exported into a ‘physics package’ (EDCRASH, EDSMAC, etc.) for use in a reconstruction or simulation. This paper also provides an overview of how the vehicle property database was created. The sources of the data used to define each of the vehicles are specified in the paper. Obtaining a complete data set for a vehicle often was not possible.

The accuracy of an accident reconstruction is dependent upon the availability and quality of relevant data. Often by the time an engineer is asked to reconstruct a traffic accident the only source of scene data is a police report. Important scene data, such as the point of impact and the points of rest of the vehicles, usually are set forth in the police report. Occasionally, however, the points of rest are not indicated in the police report and they can not be determined by other means. In a typical scenario a left turning “target” vehicle is struck on its right side by the front end of a “bullet” vehicle. The vehicles are available for inspection and the configuration of the roadway at the scene has not been altered since the time of the accident. The point of impact is set forth in the police report, however, the points of rest of the vehicles are not specified. No other sources of information regarding the points of rest of the vehicles are available.

An automobile equipped with a conventional brake system often will produce four skid marks on a roadway surface during maximum braking. This condition often occurs immediately prior to a collision in a traffic accident. Knowing the length of the skid marks, SS, and using the dynamic coefficient of friction for the roadway surface, μ, a reconstructing engineer can determine the amount of kinetic energy converted to work during the skidding process on a level roadway. The equation used in this process states that, the portion of the kinetic energy of the vehicle that was used to perform the work of slowing the vehicle is equal to the braking force applied to the vehicle through the skidding distance. Solving the equation for the speed of a vehicle that skids to a stop yields, the traditional speed from skids equation. Problems exist with the traditional speed equation that limit its practical use in traffic accident reconstruction.

The CRASH3 computer program increasingly is being used by engineers as a tool to reconstruct automobile accidents. The damage analysis portion of CRASH3 provides a useful means for quantifying the change of velocity, ΔV, that was experienced by a vehicle during the collision phase of a traffic accident. The degree of usefulness of the damage analysis portion of the program, however, is dependent upon the availability of valid crush stiffness coefficients. Published crush stiffness coefficients are available for a large number of vehicles *[1] & [2]. These publications, however, contain only a limited number of coefficients that describe the stiffness characteristics of the side structure of vehicles. Engineers are often asked to perform an accident reconstruction when there are neither published stiffness coefficients for the side structure of an involved vehicle nor crash test data from which to determine the stiffness.

The accuracy of an accident reconstruction, which employs the damage analysis feature of the CRASH3 computer program, is directly related to the accuracy of the crush stiffness coefficients employed. Crush stiffness coefficients, however, are available only through a limited number of publications and for a limited number of vehicles. In addition, assumptions made in the determination of these published stiffness coefficients bring their accuracy into question and, as a result, limit their value to a reconstructing engineer. It is concluded, therefore, that an engineer must use a critical eye when viewing the results of a CRASH3 reconstruction in which these stiffness coefficients were employed. A method is set forth for quantifying stiffness coefficients from crash test data available in a database which can be obtained from the National Highway Traffic Safety Administration (NHTSA).