## Is CRASH damage analysis a virtual relationship?

General Questions related to the CRASH Program and clones
Damage Analysis & Momentum Based Analysis programs
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### Is CRASH damage analysis a virtual relationship?

Q: I have heard the term 'virtual' used with a CRASH damage analysis, why?

A: In response to your question we repeat the following which is taken from other threads and we noted this as a part of our participation in a 1987 SAE subcommittee**(see below for footnote).
The following are important issues when applying ANY damage based analyses like CRASH and CRASH clones:
• The basic premise of damage ­based only reconstruction techniques is an assumption of a linear delta-V vs. residual crush "virtual" relationship
• The term "virtual" is used to emphasize the fact that the crush energy is dissipated during the dynamic crushing of the vehicles and that equating the residual (restituted or static) crush to the energy dissipated is a "virtual" relationship, i.e., they do not occur simultaneously).
• In creating A,B crush coefficients used in CRASH/CRASH clones the residual damage is equated with the Impact Velocity
• There is no consideration of restitution which acts to increase the DeltaV Impact Speed Change to greater than the Impact velocity while also reducing the residual crush from the maximum dynamic crush
• A vehicle with higher restitution and low stiffness can share the same 'virtual' CRASH crush resistance as a very stiff vehicle with little or no restitution:
• The dynamic crush resistance (aka stiffness) of the two vehicles can be very different YET their 'virtual crush resistance and CRASH coefficients can be the same/similar.
• The Impact velocity and residual crush for the two vehicles for a given crash test(s) on which the coefficients are based is all that matters to a damage based analysis
• See SAE paper The Effects of Restitution on Crush Coefficients for more information
• Many collision reconstructionists use or buy a single full-scale crash test data point for a given vehicle, combined with an assumption regarding a "no­ damage" intercept, to calculate custom-fitted coefficients for use in individual case reconstructions.
• This must be recognized as a crude "first-approximation" procedure.
• Any suggestion that the resulting coefficients constitute a reliable definition of the structural crush responses of a given vehicle is misleading and not in keeping with sound engineering practices and principles.
• Problems associated with the repeatability of full-scale crash test results which produce scatter, limit the reliability and accuracy of a single data point.
• Minimum scientific principles would require an absolute minimum of at least three data points.
• In response, some researchers are using techniques involving multiple crash tests on an individual vehicle to create multiple data points for a given vehicle.
The problems associated with the reliability of the crash test procedure, the differences in the crush response characteristics of a pre-damaged vehicle vs. a "fresh" undamaged vehicle and the underlying assumption of the linearity of the "virtual" relationship of delta-V vs. residual crush still classify the damage-based only reconstruction as a crude "first-approximation" technique
Footnote:
• **In 1987 a CRASH coefficient protocol subgroup committee was formed at SAE.
• At the first meeting a company annouced that they had quickly created a commercial 'crush coefficient database' product and so the interest in the subgroup quickly faded.
• Here was my 1987 SAE Committee Correspondence to the group after that meeting to reiterate important and essential points I made during the meeting about the calculation of Crush Coefficients.
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