Low Speed Case with information only on one vehicle

General Questions related to the CRASH Program and clones
Damage Analysis & Momentum Based Analysis programs
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MSI
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Low Speed Case with information only on one vehicle

Post by MSI »

Q: I’m looking at a low speed case in which there is only information on the target vehicle (photos & repair estimate). As far as the bullet vehicle, there are only a couple of bad photographs available and the extent of the damage is difficult to determine.
The other expert in the case used the information available on the target vehicle to get a bumper test and from that test, extrapolated other information using momentum to come up with a Delta V. I have concerns as to the accuracy of the results given the limited data on the bullet vehicle.
In examining the case, I’ve looked at numerous SAE papers; all of which seem to indicate that damage information from both vehicles is required to get to a reasonable solution. My questions are:
  • Is it an accepted practice in low speed analysis to use data from only one vehicle?
  • Are there guidelines (proper practices) when performing a low speed analysis? If so, can you point to a reference?
  • The SAE Papers I’ve reviewed are: 2001-01-0500, 2003-01-0492, 850437, 940915, and 960887.
A: in low speed collisions, particularly when one vehicle data not available...be careful!
In low speed crashes there is a wide range of issues which can skew the results
main point: Be sure to bracket and make assumptions at ranges of approximation.

Some of the issues:
Brakes for each vehicle on or off? that can change the resulting impact speed change. You say no data or little data which can also be an issue.
  • How to determine how much energy absorbed by the partner vehicle? and do force balancing, etc
In low speed collision Restitution needs to be considered
  • is there low speed crash test data for either of the vehicles?
  • on the vehicle available can you look up under the bumper to look for damage (per MacInnis papers etc on low/no damage vehicle inspections and reconstructions)
Damage analysis DOES NOT INCLUDE RESTITUTION
  • It also includes an assumption of a somewhat arbitrary 'no damage intercept' (4-5 MPH for most vehicles)
    It is based on a 'virtual crush resistance' relationship.
    • The term "virtual" is used to emphasize the fact that the crush energy is dissipated during the dynamic crushing of the vehicles and that equating the residual (restituted or static) crush (the resulting crush on vehicle after maxlimum crush and restitution) to the energy dissipated is a "virtual" relationship, i.e., they do not occur simultaneously).
    In creating A,B crush coefficients used in CRASH/CRASH clones the residual damage is equated with the Impact Velocity
    There is no consideration of
  • Structural Restitution which acts to increase the DeltaV Impact Speed Change to greater than the Impact velocity while also reducing the residual crush from the maximum dynamic crush
    • A vehicle with higher restitution and low stiffness can share the same 'virtual' CRASH crush resistance as a very stiff vehicle with little or no restitution:
      The dynamic crush resistance (aka stiffness) of the two vehicles can be very different YET their 'virtual crush resistance and CRASH coefficients can be the same/similar.
      The Impact velocity and residual crush for the two vehicles for a given crash test(s) on which the coefficients are based is all that matters to a damage based analysis
      See SAE paper The Effects of Restitution on Crush Coefficients for more information
Also see our post What technique should be used to reconstruct low speed collisions?
and in the event someone used or referenced the OLDMISS routine of CRASH, please see What is CRASH OLDMISS, the Missing vehicle Algorithm?

main point: Be sure to bracket and make assumptions at ranges of approximation.
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