The following example demonstrates some hypothetical inputs for a Snag impulsive restraint. The Snag impulsive constraint may be required when two vehicles collide head-on with a narrow overlap. With the original version of SMAC, simulations of narrow overlap head-on collisions would produce results wherein the vehicles 'wedged' apart. In many instances for a narrow overlap head-on collision, the wheels or structure of the vehicles become momentarily interlocked during the collision which produces a rotational spinout subsequent to impact.
In our hypothetical example, ADVSAMP4, the interlock of the LF wheels is produced by the use of a Snag impulsive restraint. The input cards are from Advanced sample project ADVSAMP4.prj, dataset ADVSAMP4.si5.
Care should be exercised in the use of the Snag impulsive restraint. The inputs for the Snag impulsive restraint should be determined on an individual basis from an examination of the damage to the vehicles and extent of rotation. The specification of the location of the Snag interlock position on V1 and timing of start and end of the snag should be carefully selected. The initial location on vehicle 1 should correspond with evidence (e.g., wheel interlock location, etc.) and the timing of the start of the snag should produce a location of the Snag interlock position on V2 which also corresponds to evidence. Several iterative runs may be required to achieve a match of evidence. You should check the output datasets for the SNAG. In our hypothetical example, ADVSAMP4, the inputs were chosen based on the following criteria:
• The initial contact overlap was determined by the extent of damage along the front of each vehicle.
• The LF wheel of V1 interlocked with the LF wheel of V2 which required Snag option choice 2, and a location on V1 10" forward of the LF wheel centerline. (X1=60", Y1=-25").
• The start time for the snag was determined by iterative adjustment until the start time produced a snag location in V2 that coincided with a location 10" forward of the LF wheel of vehicle 2. (X2=60", Y2=-25").
• The Snag impulsive restraint was limited to 10 g's peak. [(Vehicle weight * 10) / (Linear load-deflection rate of constraint)].
• The onset of the Snag was adjusted to limit the extent of damage along the vehicle length.
The following are the input cards for the Snag impulsive restraint:
0.28 0.075 2. 60. -25. 10000. 5. 16
An echo of the inputs for the snag impulsive constraint option is as follows:
TENSILE AND/OR IMPULSIVE FORCE
TIME,BEGIN (T,I)FORCE = .280 SEC.
TIME (T,I)FORCE DURATION= .075 SEC.
(T,I)OPTION,REACTION LOC= 2.(=0 NONE,=1 SPACE FIXED,=2 VEH2)
XSP1,X IN VEH 1,FORCE PT= 60.000 INCHES
YSP1,Y IN VEH 1,FORCE PT= -25.000 INCHES
AKSP,LINEAR LOAD-DEFLEC.= 10000.0 LB/IN
DELSPM,SATURATION DEFLEC= 5.000 INCHES