CRASH analysis applied with no evidence?!!

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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CRASH analysis applied with no evidence?!!

Post by MSI » Thu Dec 20, 2018 12:17 pm

Q: I have an an expert who reported using EDCRASH to calculate delta v and closing speeds on several impacts where there were no crush measurements, and poor photos of the vehicles, and no scene measurements, and no idea of impact or final rest locations. Any idea how to evaluate?

A: Some ‘experts’ out there are of the ‘you pay, we say’ variety and love it when no good definition of the evidence (crush measurements, scene photos, etc). Easier for them to be flexible and ‘say’ whatever their client wants!

Most CRASH residual crush coefficients are based on a single test at 35 MPH and an assumption of 4 mph ‘no-damage’ intercept. Think about that: every car out there sitting in the showroom has 4 mph damage!
Definitely a ‘first approximation’. Also CRASH does not include any consideration for restitution which could be a factor in low speed collisions. See Effects of Restitution in the Application of Crush Coefficients

CRASH was created for NHTSA to aid in their NASS studies as a simple pre-processor for initial approximations for speeds to then run the more sophisticated SMAC program:
  • Raymond R. McHenry combined Damage analysis and simple momentum equations to provide a first approximation. Because NHTSA liked its simplicity and small cost per run* they accepted the possible errors because of their large sample size for their NASS study and expected that the errors would wash out (they were mainly looking for trends)
Somehow now folks run around using it as the basis for an scientific opinion!
It can be a good first approximation, it can be bad, it is what it is.

*NOTE:CRASH was created in the early 1970s on time-share Mainframe computers and was a lot cheaper to run than the SMAC program)
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