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Can CRASH calculate accurate Delta V ranges for low impact collisions?

Posted: Fri May 03, 2019 2:34 pm
by MSI
Q: Can CRASH or CRASH3 calculate accurate Delta V ranges for low impact collisions (e.g. < 5 mph). I think I read somewhere that CRASH was designed more for cases >20mph with crush at least 2-4’’

A:CRASH is not good for low speed collisions.
Because the crush coefficients for CRASH are based on generally a single NHTSA barrier crash test at 30+ mph (the NCAP crash tests) and an assumed ‘no damage intercept’ of 4-5 mph
Meaning all undamaged cars have a 4-5 mph deltaV sitting in the sales lot!!
Whereas really any car crush "no damage intercept" will vary depending on where and how and what vehicle strikes the car.
So CRASH is not accurate for low speed impacts.

MacInnis did (and do, for a fee) low speed crash tests (less than 10 mph) of vehicle bumpers which includes looking at the underlying structure to try to better determine low speed tests. But limited data and expensive!!

Prasad, as part of Winsmash research and “CRASH Reformulation"
(Note:Winsmash is supposed to be for nhtsa only use)
did multiple crash tests on single vehicles to try to better characterize the ‘residual’ crush model crush coefficients of crash (remember crash coefficients are based on using residual crush and known impact speed to build a "residual" crush model)
His tests are all published and available but only for a limited number of vehicles, etc

Main point is CRASH was created as a simple pre-processor for SMAC.
SMAC models the actual crushing of the vehicle every millisecond but SMAC needs impact speed as an input**. So NHTSA asked Raymond R McHenry & CALSPAN in early 70s to create a pre-processing program and so he invented CRASH
Fast fwd to now and folks try to present crash as a bonfide accurate crash model!

And obviously if only damage data available then damage analysis must be used
And if for a research study looking at trends that is why nhtsa chose crash, the trends are what they are looking for

**NOTE: with our automatic iteration/optimization of SMACITER, part of the msmac3D program, determines the impact speeds.

Re: Can CRASH calculate accurate Delta V ranges for low impact collisions?

Posted: Fri May 03, 2019 5:47 pm
by MSI
For additional information on the application of crush coefficients in collision reconstruction, please See:
  • "SAE paper 97-0950 Effects of Restitutionin the Application of Crush Coefficients”
    Which includes:
    • “At the present time, crush coefficients for vehicle collision analysis are predominantly based on impact speeds and damage measurements from rigid, fixed barrier crash tests. The residual damage is correlated with the impact speed by means of fitted linear relationships. In general, there is no consideration given to the effects of restitution in applications of the fitted crush coefficients. However, the ignored effects of restitution on the total impact speed-change, corresponding to a given amount of residual crush, are compounded by the fact that restitution acts to reduce the amount of residual deformation, for a given maximum dynamic crush, while also acting to increase the total impact speed change. Thus, substantially different vehicles can share nearly equal slopes and intercepts in CRASH-type plots of the approach period speed-change as a function of residual crush. This can occur even though the actual exposure severity for a given residual crush may be significantly different.“