Current Status of SMAC
In the early 1970’s, NHTSA sponsored a research project to develop a computer program that would achieve improved uniformity, as well as improvements in accuracy and detail, in the interpretation of physical evidence in highway accidents. The resulting prototype computer program was the Simulation Model of Automobile Collisions (SMAC) [12-15]. At the completion of the NHTSA sponsored research at Calspan in 1974, a preliminary version of the SMAC program was delivered to the NHTSA and it has subsequently been distributed as the NHTSA SMAC computer program.
Subsequent follow-up contracts for research and development of the SMAC program sponsored by NHTSA went to other organizations [16-18]. Further research and development on the SMAC program was also continued independently at Calspan [19] and additional corporate-sponsored research to support criticism of the SMAC program [20,21] was also performed. There were no significant changes by NHTSA into the 1974 NHTSA SMAC at the completion of the NHTSA follow-up contracts.
In 1986, Day and Hargens created EDSMAC [22], a PC version of the 1974 NHTSA SMAC program converted to the BASIC programming language. Subsequent reports related to the EDSMAC program [23-25] reveal that except for very minor modifications, the EDSMAC program is essentially the same as the original 1974 NHTSA SMAC program. Related development efforts by the distributors of the EDSMAC program have been directed towards a mini-computer based high-end graphics environment [25-29].
In 1988, a number of suggestions for further refinement and extensions of the SMAC program were presented [30]. In 1989, some suggestions for avoiding misapplication of computer programs [31] were presented.
In 1997, further suggestions for extension and refinement of the SMAC computer program were presented [32] and suggestions for refinement of the restitution portion of the collision model of SMAC were presented [33].
In 1999, EDSMAC4 [34] claims to include a "more realistic modeling of actual vehicle structural behavior" based on "an A, B stiffness model". Please note that the "A,B stiffness model" is a "virtual" model equating residual crush to dissipated kinetic energy. The "A,B stiffness model" was not intended to be a dynamic model. With the “A,B stiffness model” a very stiff near plastic vehicle can share the same “A,B stiffness” as a very soft near elastic vehicle. The residual crush on a vehicle does not tell you anything about the vehicle stiffness EXCEPT in the case of a plastic vehicle. Motor vehicles crushing in collisions are not plastic. They have restitution.
Restitution consists of two separate aspects: (1) a partial dimensional recovery and (2) a partial restoration of kinetic energy. In our 1997 SAE paper on restitution [33], we pointed out that the current implementation of collision modeling in the SMAC computer program includes identical loading and unloading load-deflection rates and that the unloading (restitution) phase of the collision was at near peak levels. The effect of partial restoration of the energy at near peak levels is that the SMAC program acts to return part of the absorbed energy but it does so with a less-than-actual dimensional recovery.
EDSMAC4 also has included in the “A,B stiffness model” a “threshold force to be applied before deformation begins”. What mechanism exists in the real world which provides a force without a deflection? Are they assuming that the bumper is a pre-loaded spring? You have to do work to absorb energy in the vehicle structure. Work equals Force * Displacement. The effect of the changes in EDSMAC4 is that the acceleration peaks are higher and the duration of the impact impulse is shorter.
EDSMAC4 has chosen to artificially stiffen the vehicle to better match the residual crush. In effect they have created a “residual crush” stiffness model which reduces the validity of the modeling of the loading phase of the collision. The most important aspect of a collision is the loading phase.
The current PC versions of the SMAC program available commercially are EDSMAC[35], WinSMAC [36], m-smac [37] and smacFX [38]. Before considering the purchase and use of any accident reconstruction software you should obtain complete information on any and all programs available. For that reason you will find our web sites and technical reports include many links and references to other accident reconstruction programs. Your choice should be an informed decision based on knowledge of the state-of-the-art of accident reconstruction software
Analytical Approach: Collision
Routine