Planar Impact Model assumptions

Topics related to collision & Trajectory analysis formerly on our 'Registrants only' area however which we get asked about frequently so believe shoud be in the open forum too
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Planar Impact Model assumptions

Post by brian » Sun May 23, 2010 11:55 pm

As a part of responding to some questions on planar impact models as compared to CRASH and/or SMAC (note SMAC is an impact simulation model), we are posting the following:
  • (items in quotes and "bold" are taken directly from the paper, most from page 14)
"Summary of assumptions for planar impact mechanics:"
  • 1. "A single dynamic contact, taking place over a short duration."
    • ------- 'short' actually means instantaneous. real world collision take 100 to 200 milliseconds
    2. "Forces other than the contact force and impulses of forces other than the contact force are negligible"
    • ------- Tire forces and other external forces during the collision are ignored which can lead to significant errors.
      -------Please see External Forces Discussion below
    "3. Rotational motion of the masses can be significant"

    "4. Initial velocities are known and final velocities are unknown".
    • ------- Collision reconstruction is used to determine the impact speeds.
      -------Apparently planar impact mechanics must start with the answer?
      That is great for 'validation' but in real world collisions, the answer isn't known
    "5. Deformation is localized and small compared to the size of the bodies"

    "6. During the contact duration, position and orientation changes are negligibly small, velocity changes are instantaneous and accelerations are large"
    • -------This would lead one to believe the 'short duration' mentioned in 1 means 'instantaneous'.
      -------Real world collisions occur over time, 50 to 150 milliseconds or more, and for some impact configurations
      This can mean significant changes in positions and orientations
    "7. The effects of the normal (crush) and tangential (sliding, shearing, entanglement, crush, etc.) contact processes are known (through coefficients)"
    • ------- From the paper "The energy partitioning process is intuitive, and does not follow directly from any principle of mechanics".
      -------So how are they 'known'?
      'known' must mean subjectively assumed
    "8. A point (impact center), C, common to both vehicles and on the line of action of the contact impulse is known"
    • ------- The collision interface between two colliding bodies in an automobile or truck collision it not a simple point.
      -------The vehicles move relative to each othe during the actual collision and the forces act along the damage interface.
      NOT at a simple point!
    "9. A common crush plane defined by an angle is known"
    • ------- The assumed 'known' angle is only ‘known’ through the "energy partitioning process" mentioned in 7 which is “intuitive and not based on and any principle of mechanics
The assumptions of planar impact mechanics reveal that it may not a practical procedure for use in real world collision reconstructions.

External Forces Discussion:
  • Since the 1970’s it has been recognized that tire forces need to be considered in collision analyses.
    • From the 1973 McHenry SAE paper 73-0980 A Computer Program for Reconstruction of Highway Accidents
      • “The general practice in automobile collision analyses is to consider the impact and the trajectory phases of the event separately. This division of the analytical task is based upon an assumption that the effects of tire forces are negligible during the existence of collision forces. While the assumption appears to be a reasonable one, its application has been found to produce significant errors in the case of moderate-speed intersection collisions in which multiple contacts frequently occur-for example, front-side followed by side-side and/or rear-side contact.
        If secondary contacts are neglected, major errors can be produced in predictions of spin-out trajectories. On the other hand, if the tire forces are neglected throughout the time during which the collision contacts occur, significant errors can be introduced in the lateral motions of the vehicles between impacts. Thus, it is essential in a general procedure for reconstruction calculations that both the collision and tire forces be considered simultaneously.”
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