Does CRASH/EDCRASH include restitution?

Post by **MSI** » Fri Jun 19, 2009 10:23 am

Q: Does CRASH/EDCRASH include restitution?
A: NO!

Restitution is not included in the fundamental equations of CRASH.
Restitution is NOT included in any commercial version of CRASH.

Restitution consists of two separate aspects:

A partial dimensional recovery and
A partial restoration of kinetic energy.

During a motor vehicle collision, the maximum dynamic deformation generally exceeds the residual deformation.
Subsequent to the peak dynamic deformation, the collision partners begin a restitution phase as the deformed structures restore kinetic energy, or "spring" back.
The restitution force level and duration determine the impulse that acts on the collision partners during the restitution phase.
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.

NOTE: Why such simplifications? See Why was the CRASH program created?, which includes: As a pre-processor for SMAC! NHTSA liked it and since ONLY for statistical studies, didn't mind the possible large errors. Fast forward 40+ years and there is a cottage industry built around CRASH.

Our 1997 SAE paper, 97-0960:

Effects of Restitution in the Application of Crush Coefficients",
which initially encountered misguided erroneous criticism apparently to try to thwart publication

Has now taken its rightful place as one of the "significant steps forward within the field of crash reconstruction"

in a recent SAE book Crash Reconstruction Research: 20 Years of Progress (1988-2007).

We have not created a commercial version of CRASH with our restitution enhancements partially because of the joyous reception the paper received these past 10+ years. ,
and secondly because we have numerous other research projects that are keeping us busy.

One last note, don't think that the lack of consideration of restitution in CRASH will stop some folks (`experts') from saying that it DOES consider restitution.
Yep. Seen it in testimony wherein they relayed

"that since the measurements to determine the crush stiffness values are the restituted crush measurements then obviously CRASH includes restitution!"

That is misleading and incorrect!

____________________________
FOOTNOTE
Restitution is NOT included in any commercial version of CRASH.
_____________________________________________
For additional information on damage analysis & CRASH, please see:

The Algorithms of CRASH, paper presented at 2001 SECCC, Aug 2001
NHTSA CRASH3 Technical Manual - 1986 NHTSA collection/summary of CRASH papers
From the McHenry Forum Topic on CRASH: