# m-smac

The reconstruction of highway accidents, which serves a number of purposes (e.g., law enforcement, statistical research, and litigation), has been accomplished to some extent since the early 1970's by the use of computer programs. Prior to that time, the analytical techniques (i.e., "hand" calculations) that were traditionally applied were predominantly "closed-form" calculations based on piecewise linear solutions of the equations of motion. The accuracy of such calculations varied widely with the level of sophistication of the specific selected relationships and with the subjective interpretations of evidence included in the reconstruction.

In 1970, 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, Ref 1 Ref 58, Ref 59 ).

SMAC is a time-domain mathematical model in which the vehicles are represented by differential equations derived from Newtonian mechanics combined with empirical relationships for some components (e.g., crush properties, tires) that are solved for successive time increments by digital integration. Each vehicle is limited to the three degrees of freedom associated with plane motion (i.e., 2 translation, 1 rotation). The tire forces are modeled by a non-dimensional side force function and the "friction circle" concept is included for the interaction between side and circumferential tire forces. The collision force simulation is achieved by means of the modeling of each vehicle as a rigid mass surrounded by an isotropic, homogeneous periphery that exhibits elastic plastic behavior.

The SMAC computer model is an "open-form" of reconstruction procedure wherein the user specifies the dimensional, inertial, crush and tire properties of the vehicles, the initial speeds, angles and driver-control inputs. The program, through step-wise integration of the equations of motion, produces detailed time-histories of the vehicle trajectories including the collision responses. The user compares the SMAC-predicted trajectories and collision deformations with the physical evidence to determine the degree of correlation. Iterative runs can then be performed, varying initial speeds, heading angles and control inputs until an acceptable match of the physical evidence is achieved.

McHenry Software, Inc. (MSI) has ported the m-smac program to run on a PC in native 32-bit code. MSI has also integrated the management of m-smac projects including creation, editing, submission of runs and evaluation of output, including animation, into the m-edit environment which runs in the Microsoft Windows environment.

MSI has also extended and refined the original SMAC program to include:

• Input and outputs in either Original, Metric or EDSMAC compatible. Capability to easily convert between the input/output units.
• The capability to easily add detailed scene and vehicle graphical objects to the m-smac graphics display to permit an easy comparison of the physical evidence in individual accident reconstructions with the m-smac predicted responses. Please see our Demo page for sample of the graphics capabilities.
• The ability to view and print the m-smac reconstruction in either an animated or static display including predicted damage. The predicted damage is displayed in animation while it occurs. Also, the m-smac animated view size, location and orientation may be changed interactively while the animation is being displayed and individual frames printed. Please see our Demo page for sample of the graphics capabilities.
• Optional creation of CSV (comma delimited ASCII) datasets of the time-history of positions, velocities and accelerations from m-smac simulations for easy importation into spreadsheet programs and/or graphical programs to permit further analysis and/or the creation of graphical displays.
• Increased collision modeling capabilities to permit 1 degree increments of radial vectors and an increase in the number of iterations permitted.
• Optional specification of accelerometer location other than the center of gravity and the monitoring of the speed change (delta-V), acceleration and velocity at the optional location in addition to the center of gravity
• Option to permit up to 2 Polygon Friction zones to permit the inclusion of irregularly shaped friction zones
• Option to include the use of Impulsive Constraints to permit the modeling of vehicle interactions independent of crush forces (i.e., momentary interlock of vehicle structures)
• Option to allow the specification of a Damage Interface origin independent of the vehicle center of gravity. This option avoids problems with other versions of SMAC (which have the damage interface only at the center of gravity), when the proximity of the damage is near the CG or very far from the CG.
• Optional individual tire friction variation to permit simulation of the differences between peak friction coefficients between trucks and automobiles.
• Steer degree-of-freedom option
• Articulated vehicle(s) option
• Option to simulate the interaction of the vehicle structure with barriers
• Option to simulate the interaction of the vehicle structure with poles
• Option to permit the simulation of rear wheel steer whether due to damage or 4-WD steering

For a comparison of SMAC program features, please see the SMAC features comparison table.

Please also read SMAC-type computer programs and Information on obtaining a copy of m-smac/m-edit

These and other options/enhancements are explained and discussed in more detail in our SAE papers: "SMAC-87" (88-0227), "SMAC-97" (97-0947) and "Effects of Restitution in the Application of Crush Coefficients" (97-0960).

| List of References | McHenry Software Homepage main site |