Can You Use Maximum Crush For CRASH A&B Crush Coefficients?

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MSI
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Can You Use Maximum Crush For CRASH A&B Crush Coefficients?

Post by MSI »

Q: Can you use Maximum Crush for the creation of CRASH Crush Coefficients?

Got a note requesting additional clarification on HOWTO Calculate CRASH3 A & B Coefficients and here is some additional information:
  • This was in response to a posting on another forum where a vendor stated A&B may change We also found that they sometimes use maximum crush to calculate the A&B coefficients and when we asked them 'on what scientific basis'
    • THEY claimed:
      • "You get conservative stiffness values".
      • "You will likely underestimate the actual speed (this is a GOOD THING from the Law Enforcement perspective)".
      • "If you do it regularly and compare to AVERAGE crush when AVERAGE crush can be calculated, you can start to say how much you are likely underestimating the speed by in the instances where the only crush value you get from the test is the MAXIMUM crush".

      Our response:
      • Simple answer…you are creating something that while it may look and sound through twisted logic as ok to you,
        Does it pass the smell test?
        • If you are using it to approximate the structural restitution then great!
          And comparing deltaV v impact speed for energy feedback, great!
        However to use it to create the virtual A&B crush coefficients
        • it violates the CRASH simplifying assumptions,
        • it has NO scientific basis, and
        • it creates meaningless A & B Coefficients.
        Any inference that is somehow created 'conservative' A&B coefficients is gibberish
        it has NO scientific basis and will get your clients, and you, eviscerated in depo or trial.
Now onto the details of WHY you should not use Maximum crush for A&B Coefficients:


Quick response:
  • Q: Can you use Maximum Crush for the creation of CRASH Crush Coefficients?
  • A:NO! its use in creation of crush coefficients is erroneous and meaningless.
Now for the long answer:

From Review of CRASH Damage analysis and the NHTSA "reformulation" (and all other publications on CRASH)

CRASH is a
  • simple linear relationships between the impact speed-change and the extent of residual crush

The use of some arbitrary value for maximum crush to create A & B Crush Coefficients for Damage Analysis is RANDOM and WRONG! By arbitrarily fitting impact speed (Or do they use total DeltaV?) to the "maximum crush" is meaningless.
  • It has NO relationship to the scientific basis of damage analysis and CRASH
The simplifying beauty of using CRASH damage analysis,
is the ability to measure a fully restituted vehicle at any time after the crash
and then determine an approximate impact speed in a uniform manner.

Please be sure to also read Why was the CRASH damage analysis procedure created

To randomly use some characterization of the "maximum crush", which depending on structural restitution properties, may be a little or a lot over the residual crush, to create A & B crush coefficients is erroneous and meaningless.
  • And since they use it when there is no or questionable residual crush measurements, how do you know how it might compare to the actual residual crush?
Maximum ‘measurements’ aren’t really ‘measurements’
  • Depending on the testing facility there is a variety of possibilities of what might be reported as 'Maximum crush':
    • Double integrated accelerometer results to produce velocity and then displacement are subject to ringing, position in vehicle issues, etc etc
    • Or are they based on GPS measurement devices which have their own set of issues.
    • Or are they calculated by film analysis and monitoring targets or a single target on the vehicle? (resolution issues)
    • Or are they simply reporting the Maximum Static Crush?
    And of course, this is a single number for a single point...NOT the MAXIMUM crush across the entire vehicle contact area!
    You need to check each and every report to determine!
    • 2023 NOTE: Recently had a need to revisit this topic as an 'expert' was using derived crush factor based on maximum crush in a low speed litigation matter.
      In rereading one of the reports utilizing 'maximum crush' it appears they may be simply using the reported 'maximum residual crush'. Unfortunately that number may be a variety of things:
      • Maximum measured crush?
      • Maximum crush at elevation?
      • Maximum crush at any elevation?
      • Maximum dynamic crush?
      What is reported as 'maximum crush' in the NCAP reports? (if reported)
      The maximum crush does not define an area so what is to be used for the width? The overall width?
      MAXIMUM Residual crush means NOTHING when you're trying to calculate the actual area of residual damage and relate it to the impact speed.
  • Unless you have a comparison of valid residual crush measurements there is NO way to know how any reported maximum crush relates to the residual crush.
  • And if you have valid residual crush measurements, WHY would you need to use the maximum crush EXCEPT to try to characterize the structural restitution properties of the vehicle?
DO NOT USE MAXIMUM CRUSH TO CREATE A & B CRUSH COEFFICIENTS!!!
  • It violates the CRASH simplifying assumptions, it has NO scientific basis, and it creates meaningless A & B Coefficients.
    • The argument that since the Maximum may (or may not) be larger than residual so then it is 'conservative'
      • if that is true do a research study of the reported maximum v residual crush areas to quantify that assertion.
    • The determination of what is reported as "the maximum crush" may vary widely and wildly between testing organizations (see above)
    Only through a properly prepared research project can you quantify and clarify exactly what any reported 'maximum crush" might represent in the NHTSA reports and based on what testing organization did the crash test and reported the measurements
Taking a step back, lets rehash some things that have been published and posted about CRASH:
  • From Is CRASH damage analysis a virtual relationship?:
    • The original CRASH program, which utilized both piecewise-linear trajectory solution procedures and a damage analysis procedure, was created to provide an initial estimate for the SMAC computer program for use in the NHTSA NASS investigations.
    • NHTSA adopted mainly the damage analysis portion of the CRASH program for their NASS statistical studies since they considered the average error in severity determination as more important than any individual errors.
    • NHTSA understood the importance in statistical research for the uniform interpretation of crash evidence
    • Now fast forward 40+ years and many software vendors and crash reconstructionists use CRASH damage analysis (or clones of it) in individual crash reconstructions.
    • Crash Reconstructionists use CRASH without ANY modifications (see notes on CRASH accuracy)
    • NHTSA distanced themselves from CRASH by creating WinSmash (with minor cosmetic changes)
      • This was to get away from litigation related subpoenas related to CRASH! (since folks use it for litigation related individual case reconstructions)
    • In 1997 we published research Effects of Restitution in the Application of Crush Coefficients which showed a pathway to updating/improving the original CRASH formulation and accuracy.
      • We got a lot of misguided and erroneous pushback from the litigation fueled cottage industry built around the CRASH damage analysis procedure. Some 'reviewers' had financial interests in being the self proclaimed ‘crash coefficient experts’ and so we were strapped with incompetent and inaccurate ''peer review' garbage attached the paper as part of the flawed SAE paper ‘peer review’ process.
  • Another thing folks are trying to sell to try to imply 'improved accuracy' by adding more than 6 points of measurements:
    • This is silly since the accuracy of damage analysis does not merit any more than a few measurement…just need the general area of crush
      Some companies sell using a cloud scanner to measure in detail the 3 dimensional damage!! silly! NO improvement in accuracy!
      • the raw data, the actual scientific basis for damage analysis, is based on the crash test database and the 2,4 or 6 CRUDE MEASUREMENTS OF THE RESIDUAL CRUSH made at a single elevation. The vehicle is crashed into a barrier as part of the NCAP certification testing
      Tests have been done using electron microscope type scanner measurements vs simple tape measure
      • the results are similar/same/no significant difference!
    • CRASH damage analysis is limited in accuracy

      See our October 26, 1987 SAE Committee Correspondence
      • Since 1987 and before we’ve been trying to reel in the claims of accuracy related to damage analysis which mostly is based on a single crash test of questionable veracity and an assumption of a ‘no damage’ intercept.
      But folks still love it and use it and the best we can do is try to temper the claims of accuracy.
    SUMMARY: Randomly basing A&B crush coefficients on questionable "maximum crush" reporting because there is no residual crush available, or simply to compare as a ‘what if’, is of questionable scientific value.
    • No damaged car from a crash or barrier test is at ‘maximum crush’ (unless the vehicle has plastic crush properties!) and so coefficients based on it are meaningless and have no scientific basis.
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For additional information on damage analysis & CRASH, please see: