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COMPUTER AIDED ACCIDENT RECONSTRUCTION

Vehicle collision analysis and accident reconstruction have become increasingly popular in recent years due to the seemingly ever expanding rate of civil litigation, thereby commanding a need to investigate a collision through analytical methods. The major questions which almost certainly arise are the determination of the vehicle's impact speed and trajectory.

The advent of the personal computer has made it easier for the engineer to solve those many complex equations which require long and tedious iterative methods. Computer programs are now materially enhanced in terms of interactive user interfaces and new data presentation schematics.

This article discusses the use of computer software to reconstruct motor vehicle accidents.

In a reconstruction, we are interested in the three phases of a collision; (1) post impact - the usual starting point of the reconstruction developed from information obtained from the accident scene (2) impact - determination of the vehicle's impact speeds, impact directions and impact location and (3) pre-impact - determination of speeds and trajectories (usually the unknowns).

This paper will concentrate itself exclusively with the modelling of the impact phase.

CRASH (Calspan Reconstruction of Accident Speeds on the Highway) and SMAC (Simulation Model of Automotive Collisions), are main frame programs which were developed in 1974-75 under contract to NHTSA (National Highway Traffic and Safety Administration) by Calspan as a tool to help improve highway safety. Many micro programs have now been developed from them. The same numerical calculations are still utilized but without the awkward input/output methods of the past. CRASH and SMAC were developed as an aid to the government staging of crash tests for statistics on safety, survivability, standards, product improvement, mechanical and structural assessments, and of course for the simulation of the CRASH sequence.

CRASH and SMAC are applicable to the field of accident reconstructions as a means of increasing the accuracy and uniformity in the interpretation of physical evidence relating to motor vehicle collisions, as a final check to the engineer's own assessment and manual calculations, and to assure the reliability of the information obtained.

CRASH AND SMAC work together in that CRASH estimates the changes in linear and angular velocity based on CRASH geometry and SMAC confirms the results subject to a trajectory simulation.

From CRASH, we can determine the conditions of impact, impact speed, change in velocity, force direction at impact, and absorbed energy damage. For this, we need to know; the damage sustained by the vehicles, the vehicle specifications, principal direction of force, the vehicle stiffness; and the post-crash physical evidence found at the accident scene, impact/rest positions and tire road data.

A CRASH program output for a full capability run includes two separate and independent derived estimates of speed changes experienced by the live vehicles; (1) from impact to rest phase we have to work energy relationship for spinout trajectories and the conservation of momentum for the impact (2) from the area and extent of deformation on the two vehicles involved in the collision, the vehicle speed change estimate is based on energy calculations and stiffness factors derived from staged collisions and well documented accident cases.

How does SMAC analyze a collision? Well, it establishes a force balance between the vehicles as they crush. Force vectors act upon the vehicle's centre of gravity until separation occurs. Then, the linear and angular motion existing at the moment after the crushing has ceased, is analyzed. Finally, SMAC performs a trajectory simulation during the impact to rest phase.

As mentioned previously, SMAC employs the results of the CRASH program or the user can substitute his/her own impact estimates. The user can then make adjustments to the initial conditions until the final results match the physical evidence. I.e. driver input, vehicle terrain interaction, vehicle damage, rest positions, actual tire skid and scuff marks, etc.

The output can take the form of tabulated results or a graphic display (continuous or variable time freeze) of the vehicle trajectory. The graphics can also tie in with animation software for a more illustrative presentation.

Vehicle stiffness coefficients and residual crush are related to impact velocity and speed change for numerous accident configurations. Stiffness coefficients for use in CRASH are derived from CRASH barrier test data and assigned on a vehicle class basis. Most vehicles are continually updated as new cars and body structures are manufactured.

In summary, the applicability of CRASH and SMAC computer programs to the field of accident reconstruction have been established. In particular, the programs provide a successful means of checking the accuracy and uniformity in the interpretation of physical evidence relating to motor vehicle accidents. The CRASH program allows the user to select the speed change that utilizes the more reliable items of evidence. The detailed output from SMAC permits iterative runs to establish results which best support the physical evidence.

 
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