DISCUSSION: Side-Slaps & Secondary Collisions

A unique feature of the original SMAC program (and therefore the m-smac program) is the ability to simulate and therefore reconstruct vehicle collisions that include sideslaps and/or secondary impacts. In the SMAC program, each vehicle is represented by a rectangular box with the length and width dimensions of the actual vehicle. Collision detection is accomplished by continually checking the corners of each vehicle "box" to determine if it is within the periphery of the other vehicle "box".

Once a corner point is found to be in contact, the program begins calling the collision routine to use the collision model to scan for interference and contacts and to calculate the associated collision forces. The program also changes to the integration time increment to the user specified collision integration interval ( DTCOLL, normally 1 millisecond (0.001), set on m-smac input card 1, field 4).

The end of the collision event is assumed when a fixed time increment has passed wherein the accelerations for each vehicle are both below 1 g-unit. The program then changes into a separation mode and utilizes the separation time increment (DTCOLT, normally 5 milliseconds (0.005), set on m-smac input card 1, field 5).

If a fixed number of separation time increments are passed without any accelerations greater than 1 g-unit, then the program shifts to the trajectory time increment (DTTRAJ, normally 10 milliseconds (0.010), set on m-smac input card 1, field 3).

Much of the logic associated with the changing of the integration time increment was created to reduce the computer overhead and costs associated with running a SMAC simulation on time-share mainframe computer systems. With the use of SMAC on PC computers, there is no cost associated with the use of one millisecond time increments for all the time increments at least for final checkout runs to insure that secondary sideslap collisions are not missed or inadequately modeled. Problems have been found and reported in our recent SAE paper, SMAC-97 (Ref. 10), with applications of the EDSMAC program where the program may miss the accelerations associated with "side-slap" impacts. We have implemented a number of changes/options in the m-smac program to help avoid this problem from occurring. The following is a brief description of the changes/options implemented in m-smac related to side-slaps and/or secondary impacts:

m-smac inputs, Card 90, Field 7

0.0     Some refinements have been made to the logic associated with the internal testing for side-slap impacts in the m-smac, the default is to use the general option improvements..

2.0     An additional option has been added per the SMAC-97 paper to check for a fixed time period of 0.25 seconds after initial impact. To test this logic, set the option to 2.0. If you have a simulation with a sideslap, you should test it with this options. Please report any problems you encounter. At some point in the future this may be made the default logic for m-smac.

1.0     You can also set this option=1.0 to use the logic of the original NHTSA smac program (Caution: the original NHTSA smac logic may produce lesser side-slaps than actual and/or miss a sideslap impact dependent on the DTTRAJ and DTCOLT time increment sizes chosen).

DamageOutFull option of m-smac:

As a part of the development and refinement of the m-smac program for the PC environment, options have been added to the program to reduce the redundant amount of damage records written to the graphics output. This is accomplished with flags set in various routines to reduce the amount of output. If you have a simulation which graphically appears to miss a sideslap (but there are accelerations and impact speed changes in the m-smac output) , it may be due to a problem with the graphics, not the m-smac program. To test, a DamageOutFull option is available. To set to output all Damage Records, do the following:

From m-edit, Select the Tools menu

Select the Options menu item

Select the m-smac tab, and the DOSX32 m-smac tab (note that this tab sets options for all versions)

There are three 'groups', General, Graphics & Animation. In the General group section, in the box adjacent to the line entitled Options, add DF.

Click OK and then run either version of m-smac

If you run with and without this option set, you will find that the size of the output file increase dramatically (by a factor of 7 to 10). You should only use this option for checkout runs and/or if a problem occurs. Reset the Option field to blank after testing/use)


Impact Configuration SetIND Option