Q: In calculating A & B Coefficients from crash tests which speed is correct to use, the test impact speed or the delta-v speed? Were the A & B formulas developed from using test impact speeds or delta-v speeds? This forum post indicates “The Impact Speed”. Some other references mention test impact speed and some instructors teach that the proper method is to use the actual delta-v from the crash test.
Which is correct?
A: The confusion may comes from a couple factors:
- CRASH damage analysis is based on simple linear relationships which equate the Impact speed to the extent of residual crush.
- Some refer to it as the Impact Speed Change, but the Impact Speed change must be ONLY to the point of common velocity.
- There is no provision in CRASH for inclusion of restitution.
- See forum topic CRASH does not include restitution
- Most CRASH A&B Coefficients are calculated based upon full scale barrier impact tests which are run with a known Impact Speed.
- For other collision types, because the A&B coefficient fitting procedure requires the impact speed, some have misinterpreted that to mean 'Total impact speed 'change' which included restitution.
- That is incorrect!
Impact Speed change to the point of common velocity!
- That is incorrect!
- For other collision types, because the A&B coefficient fitting procedure requires the impact speed, some have misinterpreted that to mean 'Total impact speed 'change' which included restitution.
- Since generally CRASH underestimates the impact speed change, particularly in low speed collisions (because it does not include restitution) some have also erroneously suggested using the Total Impact Speed change as the value for 'Impact Speed' in the A&B fits to try to increase the impact speed calcuations from CRASH.
- Again, that is another fundamental error!
- IMPACT SPEED
Or the
IMPACT SPEED CHANGE TO THE POINT OF COMMON VELOCITY
*except in the case of a "perfectly-plastic" collision!
________________________________________________________________
March 2024 For convenience added:
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:
- Why was the CRASH program created?
- What Speed Should I Use to Calculate CRASH3 A&B Coeff?
- Is CRASH damage analysis a virtual relationship?
- Should a CRASH type damage analysis include induced damage?
- What is the "No Damage Intercept" of CRASH
- Can You Use Maximum Crush For CRASH A&B Crush Coefficients?
- Effects of Restitution in the Application of Crush Coefficients
- Crush Coefficients Conversions: CRASH (McHenry), Campbell, WinSMASH (Prasad)
..and more, see the McHenry Forum and specifically the McHenry Forum topic CRASH and Damage Analysis