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- Using Force-Displacement Data to Predict the EBS of Car into Barrier Impacts, SAE paper 2016-01-1483
- The objective of this study was to assess the accuracy of using high-speed frontal barrier crash tests to predict the impact speed, i.e. equivalent barrier speed (EBS), of a lower-speed frontal barrier crash. Force-displacement (F-D) curves were produced by synchronizing the load cell barrier (LCB) data with the accelerometer data. Our analysis revealed that the F-D curves, including the rebound phase, for the same vehicle model at the same impact speed were generally similar. The test vehicle crush at the time of barrier separation, determined from the F-D curves, was on average 17±16% (N = 150) greater than the reported maximum hand-measured residual crush to the bumper cover. The EBS calculated from the F-D curves was on average 4±4% (N=158) greater than the reported EBS, indicating that using F-D curves derived from LCB data is a reliable method for calculating vehicle approach energy in a crash test. Our method of using F-D curves from high-speed tests to predict the EBS of a lower-speed barrier crash overestimated the EBS of actual lower-speed tests by an average of 21±9% (N = 129). Further work in developing and refining our method is needed to improve the accuracy of predicting a lower-speed EBS.
COMMENT
At first look at some of the figures we thought what an interesting possible look at loading, unloading, restitution, etc.
We had published about the importance of addressing the effects of restitution in Damage Analysis in our 1997 paper
- "Effects of Restitution in the Application of Crush Coefficients", SAE 970960
- From that paper
- "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."
- "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.
and we have a forum topic (and others) on Restitution - From that paper
However, the paper makes no mention or recognition of restitution?
From their conclusions/discssion:
- Our overall goal was to evaluate whether force-displacement (F-D) curves generated from the load cell barrier (LCB) data and accelerometer data acquired during NHTSA front-into-barrier crash tests could be used to predict the collision severity of a less severe crash.
And they speculate on effects of bumper covers, an approximations of airgap,
and they include:- "The residual crush values in the NHTSA crash test reports are used by collision reconstructionists to determine the linear stiffness coefficients of a vehicle, and these linear stiffness coefficients are then used to calculate the EBS of a vehicle that has sustained a different amount of residual damage."
- Equating the residual (restituted or static) crush to the energy dissipated based on the impact speed to calculate a 'crush coefficient' for a vehicle and then apply that relationship to damage measured in a crash...
That is a "virtual" relationship, i.e., they do not occur simultaneously
- This paper misuses the term 'EBS" (the Barrier Equivalent Velocity), sometimes also referred to as the Barrier Equivalent Velocity (BEV).
- NOTE: Speed is a scalar quantity defining magnitude only, whereas velocity is a vector quantity having direction as well as
magnitude. For full frontal NHTSA NCAP barrier collisions like evaluated for the paper the Speed and Velocity are the same.
- an overall 4.+/- 4% overestimation of the EBS and
- a 21.+/- 9% overestimation of the "EBS" for the lower speed tests!
The concept of equivalent Barrier Speed (EBS) whereby the severity of a collision is estimated on the basis of a damage comparison with the results of a rigid barrier collision is of very limited value in the general case where the collision partners have dissimilar crush stiffness's and weight characteristics. - NOTE: Speed is a scalar quantity defining magnitude only, whereas velocity is a vector quantity having direction as well as
- Equivalent Barrier Speed (EBS) is the same as Barrier Equivalent Velocity (BEV)
- See our writeup What is BEV?
- The concept of assessing impact severity by relating the damage sustained in a collision to that which the same vehicle sustains in an experimental barrier impact test (e.g., the Barrier Equivalent Velocity (BEV) concept) was first described by Mackay in 1968 (Ref. [5]).
- When the two vehicles involved in a car-to-car collision have essentially similar deformation and weight characteristics, such a BEV method of assessing impact severity can be reasonably reliable for use in statistical studies.
- However, if the vehicles have dissimilar characteristics, the BEV ratings can be considerably in error as a measure of impact severity. For example, if a heavy soft car collides with a light stiff car, it is possible for the stiffer car to experience a substantial change in velocity with no appreciable damage (Ref. [5], [6]).
- Ref 3 Knipling, R.R., Kurke, D.S., “NASS Field Techniques – Volume IV – CRASH MEASUREMENTS”, US Department of Transportation, National Accident Sampling System,
(NOTE: link was to NHTSA site, no longer valid, will post up full report soon) - Ref 5. Ashton, S.J., Hardy, J.L.G., Mackay, G.M., “The Use of the Vehicle Deformation Index and Collision Speed Assessments”, Proceedings of the International Accident Investigation Workshop, Pilot Study on Road Safety, NATO, Brussels, Belgium, June 28-29, 1973.
- Ref 6. Hight, P.V., Lent-Koop, D.B., Hight, R.A., “Barrier Equivalent Velocity, Delta-V, and CRASH3 Stiffness in Automobile Collisions”, Society of Automotive Engineers Paper No. 850437.