Force-Displacement Data to Predict EBS, Damage Analysis, etc, etc

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MSI
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Force-Displacement Data to Predict EBS, Damage Analysis, etc, etc

Post by MSI »

Recently got a 'get a free SAE paper' email from SAE (apparently not knowing I'm a member?)
  • From: Publications from SAE International <saepublications@sae.org>
    Sent: Thursday, August 4, 2022 2:54 PM
    To: mchenry@mchenrysoftware.com
    Subject: Your SAE Complimentary Resource
    • THANK YOU
      Thank you for your interest in SAE International, the global leader for authoritative mobility engineering knowledge and resources. We hope this complimentary content will be helpful to your organization. <-in email was link to the 2016 paper referenced below
      Want to learn more about our subscription options? Talk to our team to find the one that best fits your needs and budget.
      Regards,
      SAE International
The paper i received was
  • Abstract:
    • 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.
We had not previously come across this paper, not sure how we missed it (if part of AR 2016 at SAE) ...we have it now!

COMMENT

At first look at some of the figures we thought what an interesting possible look at loading, unloading, restitution, etc.
Fig 1 & 5 from 2016 SAE paper.jpg
Fig 1 & 5 from 2016 SAE paper.jpg (43.51 KiB) Viewed 109 times
We had published about the importance of addressing the effects of restitution in Damage Analysis in our 1997 paper And note the similarity of the shape of things to our loading/unloading model in the 1997 paper...
fig 1A 97 paper.jpg
fig 1A 97 paper.jpg (9.63 KiB) Viewed 50 times
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."
    NOTE: It is NOT the linear stiffness coefficients, it is the Virtual Linear crush coefficients!
    • 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
OUR PRELIMINARY REVIEW & COMMENTS ON THE PAPER
  • 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.
    They compare the NHTSA reported barrier impact speeds directly with the "EBS" calculated from force-displacement curves and find
    1. an overall 4.+/- 4% overestimation of the EBS and
    2. a 21.+/- 9% overestimation of the "EBS" for the lower speed tests!
    EBS is defined as the impact velocity into a fixed barrier that would result in the same magnitude crush in a subject crash. Obviously that IS the approach velocity as reported by NHTSA for each of the tests. The DeltaV of a barrier collision will generally include effects of restitution making it larger than the approach speed. This effect is greatest at low closing speeds!

    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.
And a final note:
  • Equivalent Barrier Speed (EBS) is the same as Barrier Equivalent Velocity (BEV)

    From our 2008 Book:
    • 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]).
      A barrier impact velocity of X MPH produces an impact speed-change, Delta-V, of X MPH plus the rebound velocity, if any. If the barrier is replaced by a standing, “mirror-image” vehicle, the same extent of damage and the same impact speed-change, Delta-V, as occurred in the barrier impact will be produced by a closing velocity twice as large as the barrier impact velocity (i.e., a closing velocity of 2X). The interpretation of damage in actual collisions in terms of BEV, or Delta-V, requires proper consideration of the mass and stiffness ratios of the collision partners and of the effects of any collision offset (e.g., Reference 3).
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