Should a CRASH type damage analysis include induced damage?

Topics related to collision & Trajectory analysis formerly on our 'Registrants only' area however which we get asked about frequently so believe shoud be in the open forum too
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MSI
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Should a CRASH type damage analysis include induced damage?

Post by MSI » Fri Jun 19, 2009 11:26 am

2 words of wisdom on CRASH type damage analysis (whatever flavor: EDCRASH, WinCRASH, etc):
  • Be Careful!
In 1988, Tumbus and Smith wrote SAE paper 880072:
  • In the paper they presented some ideas for a standardized protocol to measure damaged vehicles which included special cases like bowing, etc. I recommend the paper for any and all who use damage analysis techniques, whatever flavor of CRASH or damage analysis you use. The standardization of technique is so that anyone measuring a damaged vehicle should produce the same crush measurements.
    For example, in cases of bowing, some extra energy was required to bow the vehicle so the damage measurement protocol should include consideration for the extra energy (or a 'did the vehicle bow?' and 'how much' question should be added and considered in the analysis).
Since publication of the paper there have been some efforts to create a standardized protocol for damage measurement but so far there have been no results.

And note: CRASH was originally created as a preprocessor for SMAC.
  • The mathematical formulation for CRASH is that damage measurements were to be of the direct contact damage only. When NHTSA adopted CRASH damage analysis for the NASS statistical studies they found it was underestimating the DeltaV so they changed the protocol to include the induced damage.
Today, the CRASH clones use the same mathematics and simplifying assumptions as CRASH3 but also include customized crush coefficients.
  • Most are based on a single crash test from the NHTSA NCAP program.
    • From a scientific standpoint, a single test point (measurement) with an assumed 'no damage' intercept is a very crude definition of the crush properties of a vehicle.
The cottage industry that has grown out of damage analysis has generally continued the use of induced damage in the measurement 'protocol' however remember this is an indication of the limitations of damage analysis.
Be sure to use other techniques such as momentum analysis, simulation, etc. in addition to damage analysis to check and refine your damage analysis results.
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brian
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Re: Should a CRASH type damage analysis include induced damage?

Post by brian » Tue Sep 01, 2009 2:03 pm

Some additional information related to the inclusion of induced crush in damage analysis:
  • 1) The NASS training manual instructs to include induced damage but indicates ‘most other CRASH users measure only direct damage

    2) The CRASH3 Manual:
    • Section 3, 3-32-3-32 indicates
      a general rule has evolved that specifies the inclusion of induced and direct damage in the profile for CRASH computation
      (p 132-137 of the PDF of the CRASH3 Manual)
    • p21-23 of the PDF about CRASH
      not for individual reconstructions!
      note the underlining is by someone at NHTSA.
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MSI
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Re: Should a CRASH type damage analysis include induced damage?

Post by MSI » Thu Mar 26, 2015 3:41 pm

Received a followup questions today:
  • Q: I did a Google search of "direct or induced damage" and came across your 2009 response to a question about which measurement is appropriate with CRASH. I was hoping you would answer some questions. When the length of the crush in the damaged are is less than the original length, most of the resources I found suggest that you plug in the original length of the crushed area in CRASH, even if the sum of the crushed zones is less than the original undamaged length[attachment=1]induced.jpg[/attachment]
    • I was curious what the crash program is doing with the field length and the original length?
    • Does CRASH simply increase the width of each crushed zone proportional to the difference between the original length and the crush length?
    • For example, if the original length of the front end of the car shown is 25% greater than the sum of the crush zones, does crash multiply the width of each zone by the same factor (1.2) in order to increase the sum of the effective crush zone width to the actual undamaged width?[/b]
A: In answer to your questions:
  • I was curious what the crash program is doing with the field length and the original length?
    • Nothing. For CRASH damage analysis the equations only uses the user entered ‘crush damage width’ or whatever it is called in whatever flavor of the CRASH program is being used.
  • Does CRASH simply increase the width of each crushed zone proportional to the difference between the original length and the crush length?
    • Since CRASH simply uses whatever damage width the user enters, the answer to the question is essentially YES, although CRASH does not check/care about the overall width or length of the vehicle as compared to the damage width.
      So it isn’t increasing or decreasing….it is simply calculating the segment width based on user entered width.
      But I guess that is essentially ‘increasing the width of each crushed zone’ based on crush length with no consideration of original length (the user has done that by entering the overall length rather than the actual crush length)
      Some other flavor (versions) or CRASH damage analysis may do a check of the damage width to the vehicle width or length and some even allow more than 2,4 or 6 'damage measurements'. However the basic calculation procedure is to calculate the area of damage (below i have posted the 2,4 & 6 point equations from the CRASH3 manual
  • For example, if the original length of the front end of the car shown is 25% greater than the sum of the crush zones, does crash multiply the width of each zone by the same factor (1.2) in order to increase the sum of the effective crush zone width to the actual undamaged width?
    • NO. It simply uses the user input damage width and make each damage segment 1/6, 1/4, 1/2 of the user entered damage width
One important item to consider:
  • Some commercial versions of the CRASH/damage analysis have added options to allow for more than 6 damage measurement and different sized 'segments' for damage measurements
    • (which is mainly to try to give the illusion of some aura of additional accuracy however in tests run using measurements with a simple ruler vs survey equipment to make measurements the accuracy and limitations of CRASH are not improved at all. (i will add a reference to these tests when i get a chance, i believe it was documented in an SAE paper) However i guess it makes a good sales pitch for their 'better mousetrap'!)
    Part of their input process may include input of the length between each random damage measurement. Those programs may include a check of the 'overall damage width' and the summation of the individual segment width and either alert the user to the discrepancy OR do some adjustment to the individual segment lengths to handle the difference
    You will need to check with the individual program vendors to see how they are handling that situation.
The 2,4 & 6 point equations from pages 2.29, 2.30 of the CRASH3 manual [attachment=0]crash 246 pt calcs.jpg[/attachment]
Attachments
crash 246 pt calcs.jpg
crash 246 pt calcs.jpg (43.97 KiB) Viewed 1158 times
induced.jpg
induced.jpg (52.6 KiB) Viewed 1158 times
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