Planar Impact Model assumptions

General Questions related to the Momentum Based Analysis programs
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brian
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Joined: Tue Jul 14, 2009 10:52 am

Planar Impact Model assumptions

Post by brian »

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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:
  • NOTE: 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 50 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 in FOOTNOTES 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
      -------See 97-0949 and others which demonstrate the movement between point of impact (POI) and point of separation(POS) in collisions.
      • Here is a simple illustration from the 1997 paper which is based on real world tests
      • Also see RICSAC97 SAE 97-0961 and other 'real world' tests which demonstrate vehicle move between POI and POS.
        sample POI to POS movement.png
        sample POI to POS movement.png (33.58 KiB) Viewed 7120 times
      -------This also means DO NOT use planar impact models for side-slap collisions
      Main point: Real world collisions take time and have 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
      Definition of intuitive is "using or based on what one feels to be true even without conscious reasoning; instinctive"...NOT scientific!
The many arbitrary and subjective assumptions of planar impact mechanics reveal that it may not a practical procedure for use in real world collision reconstructions.

__________________________
FOOTNOTES:

**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.”
    • From our 1997 research and SAE paper
      • CRASH-97 - Refinement of the Trajectory Solution Procedure in which we quoted from
      • "Development of a Computer Program to Aid in the Investigation of Highway Accidents" on:
        • TIRE-TO-GROUND "EXTERNAL" FORCES:
          • The effects of tire-ground forces must be considered in a motor vehicle collision reconstruction. During the early development of the SMAC program [19, 20] tests were performed to determine the effects of external tire forces on the collision solution procedure. It was concluded that
            • “The conventional assumptions that the effects of vehicle deformations and of tire forces can be neglected in analytical reconstructions of collisions can lead to significant errors. This is particularly true for intersection-type collisions at low to moderate vehicle speeds, in which prolonged or multiple contacts and significant movements of the involved vehicles occur”
            and that
            • "therefore it is essential in a general procedure for reconstruction calculations that both the collision and tire forces be considered simultaneously."
        and further in the 1997 CRASH-97 paper we stated:
        • "The successful inclusion of effects of external tire forces and of a finite time for the collision momentum exchange in the refined CRASH solution procedure benefited from the outstanding resources afforded by modern day Pentium computers. Rather than being forced to perform iterations of individual runs for comparison tests of the effects of individual input variables, the whole collection of test cases could be iterated by control algorithms to quickly evaluate the effects of the assumptions on all of the validation comparison runs. A "suite" of tests was easily set up and the various variables tested. Some of the many assumptions and approximations tested and refined in this manner included:
          • The approximation technique used for determining the separation positions and orientations.
          • The approximate duration of the time to separation.
          • The magnitude and duration of the forces and moments used to approximate the effects of external tire-forces during the collision.
          • The various error evaluation terms and weighting functions used to guide the iteration and achieve an “acceptable” match."
        References:
        • 19. McHenry, R.R., "Development of a Computer Program to Aid the Investigation of Highway Accidents”, Contract FH-11-7526, December 1971, Calspan Report VJ-2979- V-1,NTIS PB# 208537
        • 20. McHenry, R.R.,"A Computer Program for Reconstruction of Highway Accidents", SAE Paper 73-0980, Proceedings of the 17th Stapp Car Conference, November 1973
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