The Major Shortcoming of Simplified Momentum Simulation and Analysis

General Questions related to the CRASH Program and clones
Damage Analysis & Momentum Based Analysis programs
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MSI
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The Major Shortcoming of Simplified Momentum Simulation and Analysis

Post by MSI »

Jan 13, 2021: This message I repeated in a few posts so decided to split it off as a separate topic:
These comments apply to any crash analysis in general and simplified momentum analysis in particular which include a major simplifying assumption when applied to motor vehicle crashes.
  • And please note this is not to dismiss simplified momentum analysis, it is to help all who use it to understand the assumptions, to encourage that they test a range of assumed input numbers to reveal any sensitivities, and then that they try to test and refine any conclusions.
This applies to programs like PC-CRASH, Virtual Crash, Planar Impact Models, and others as well as hand calculation and spreadsheet techniques:

A summary of the major shortcoming when applying some simplified momentum solutions in equations or simulations:
There are many programs out there with good options & graphics.
  • This is just a word of warning to be very careful as momentum and some simplified momentum crash simulation programs include the simplifying assumption of an 'instantaneous exchange of momentum' which requires the user select a subjective 'point and angle' for that assumed instantaneous exchange (begs question: if so easy, why isn't it automatic?)
    These programs are useful however they can be very sensitive in many impact configurations.

ALWAYS CHECK FOR SENSITIVITIES OF ANY RESULTS!


Simplified momentum equations and programs which include this simplifying assumption require the user to pick a point and angle/direction to instantaneously exchange the momentum of a vehicular crash.
  • Any vehicular crash takes 50-150 milliseconds (or more) for the collision interaction.
  • These programs/equations which include the simplifying assumption consider a crash a SINGLE 'instant' (less than 0.001 second!).
    • See SAE 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 and rotate between POI and POS.
        sample POI to POS movement.png
        sample POI to POS movement.png (33.58 KiB) Viewed 22 times
        This also means DO NOT use these simplified momentum solution procedures for side-slap collisions
        Main point: Real world collisions take time (50-150 ms or more) and have changes in positions and orientations
  • When applying a momentum solution with the simplifying assumption it is very subjective and random HOW and WHERE a user places/sets the LOCATION and the ANGLE/DIRECTION for the INSTANT of the momentum to be "instantaneously exchanged"
How do you best approximate the reality of 50 to 150 or more milliseconds aka "instants" during a collision interaction as a SINGLE INSTANT with a SINGLE angle/direction?
  • Simplified momentum program vendors need to prepare OBJECTIVE instructions on these subjective random inputs for placing and directing the "point of momentum exchange"
    • Their current instructions like "place at point of maximum engagement" and/or "fit the damage areas together like a puzzle" are extremely subjective and random.
      • To be more honest about it they should add to their instructions "You should randomly and subjectively adjust the point and angle for the 'instantaneous exchange' until you get the answer you want and need!!'
        THAT would be more honest!
    • The validations for these programs use the BEST approximation for the random and subjective momentum exchange POINT and ANGLE/DIRECTION to get the best possible MATCH of the KNOWN results.
      • They have done 'sensitivity studies' on these programs which interestingly use the 'point and angle' from the 'validation' and leave THAT out of the sensitivity study...In other words they don't vary the 'point and angle' for the instantaneous exchange which would demonstrate the major issue which can occur with a BAD point and angle for the 'point of instantaneous exchange'.
    • The instructions ignore the fact that vehicles have different stiffness and restitution properties and so the residual damage in a crash MAY NOT BE a good indication of the maximum damage and engagement of the vehicles during the crash
    Also how do you approximate a SINGLE angle/direction for that SINGLE point/instant to approximate all 50-150 or more milliseconds/instants of changing positions and directions (see this video for an example!)?
IF it is so SIMPLE...

WHY NOT...

make it AUTOMATIC!
    For additional information please also see Some Momentum Misconceptions
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