What Speed should I Use to Calculate CRASH3 A&B Coeff?

General Questions related to the CRASH Program and clones
Damage Analysis & Momentum Based Analysis programs
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MSI
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What Speed should I Use to Calculate CRASH3 A&B Coeff?

Post by MSI »

Sept 2013: NOTE: This post from April 2011 has been copied from thread Re: How can I calculate CRASH A & B Coefficients? to make this separate posting.
Q: In calculating A & B Coefficients from crash tests which speed is correct to use, the test impact speed or the delta-v speed? Were the A & B formulas developed from using test impact speeds or delta-v speeds? This forum post indicates “The Impact Speed”. Some other references mention test impact speed and some instructors teach that the proper method is to use the actual delta-v from the crash test.
Which is correct?

A: The confusion may comes from a couple factors:
  • 1) CRASH damage analysis is based on simple linear relationships which equate the Impact speed to the extent of residual crush.
    2) Some refer to it as the Impact Speed Change, but the Impact Speed change must be ONLY to the point of common velocity.
    2) There is no provision in CRASH for inclusion of restitution. 3) Most CRASH A&B Coefficients are calculated based upon full scale barrier impact tests which are run with a known Impact Speed.
    4) For other collision types, because the A&B coefficient fitting procedure requires the impact speed, some have misinterpreted that to mean 'Total impact speed 'change' which included restitution.
    • That is incorrect!
    5) Since generally CRASH underestimates the impact speed change, particularly in low speed collisions (because it does not include restitution) some have also erroneously suggested using the Total Impact Speed change as the value for 'Impact Speed' in the A&B fits to try to increase the impact speed calcuations from CRASH.
    • Again, that is another fundamental error!
In summary, for CRASH A&B fits, use the
  • IMPACT SPEED
    Or the
    IMPACT SPEED CHANGE TO THE POINT OF COMMON VELOCITY
Any use of TOTAL Impact Speed change in a CRASH type fit is a fundamental error and erroneous*.

*except in the case of a "perfectly-plastic" collision!
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