What is DeltaV? A Discussion

'What Is' type questions related to highway safety, accident reconstruction and vehicle simulation
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brian
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Joined: Tue Jul 14, 2009 10:52 am

What is DeltaV? A Discussion

Post by brian » Tue Aug 04, 2009 1:49 pm

The following is some comments I made on another forum in 2000 in response to the question: What is DeltaV?
Q: What is DeltaV?
A: DeltaV is the time integral of the acceleration during the collision event. Acceleration alone cannot characterize injury.
I recall seeing a list of some very high and very short term accelerations we experience in daily life without incurring injury.
DeltaV can be used to segregate injury and non-injury producing accidents.
  • This is a forum for accident reconstruction.
  • DeltaV as it relates to accident reconstruction is limited to collision events.
  • Collision events that produce injury normally occur in 50 to 150 milliseconds.
  • They are neither instantaneous nor long term events.
  • DeltaV occurs over the duration of the collision.
Because many reconstruction analyses use simplified linear momentum solution procedures that assume an instantaneous exchange of momentum, the users of such procedures perhaps have forgotten that the instantaneous assumption for collision events is an approximation of events that last 50-150 milliseconds (or more!). Yes, in rear enders and/or when there is override or component interactions (snags), the 'collision event' may last for 200 milliseconds or more.
However, the injury producing event is over and done with in the first 50-150 milliseconds. (unless of course a secondary collision occurs (e.g., car into car then car into tree, etc)
Some responses skewered my statements with comments like 'just braking a vehicle is DeltaV', etc.
The following is a summary of 4 things important to the definition of deltaV (Impact Speed Change):
  • [1] The acceleration of the vehicle generally does not provide a reliable indication of occupant injury potential. With automobile-type collisions (as opposed to encapsulated space vehicles) it is the vehicle Delta-V or impact speed change. The occupants of the vehicle are part of a secondary collision which occurs when the occupants load the seat belt and/or airbag and/or vehicle interior.The occupant acceleration is always greater than the acceleration of the vehicle

    [2] 1st, I was speaking of the peak acceleration acting on vulnerable parts of the occupant body. 2nd, DeltaV establishes how much the vehicle slows down during a collision. The occupant develops relative motion which then must be stopped. How quickly the relative motion is stopped establishes the peak acceleration.

    [3] What is DeltaV? It is the time integral of the acceleration during the collision event. To quote from:
    • Roberts, "The relationship between Delta V and Injury", SAE 933111:
      • "Many of the relationships for the biomechanical response for the human body which have been established and predicted based upon in vitro studies of biological tissues and by the study of human cadavers have been found to be present when the injury response of living humans involved in actual collisions is compared using the Delta V as the measure of collision severity"
    And quoting from
    • Knipling & Kurke, NASS Field Techniques
      • "By comparing DeltaV to vehicle damage and occupant injuries, we learn about the crashworthiness of vehicle and the effectiveness of protective devices for occupant" and also "The deltaV in a collision is analogous to a dose of poison. The greater the dose, the more likely death or disability." Acceleration alone cannot characterize injury. I recall seeing a list of some very high and very short term accelerations we experience in daily life without incurring injury.
    DeltaV can help segregate injury and non-injury producing collision accidents

    [4] In response to the statement regarding acceleration versus delta V. As everyone in this forum is aware, delta V is a change in velocity over an assumed collision duration commonly used to gauge both vehicle damage and injury potential . It is essentially a measure of acceleration because it describes the change in velocity over the collision duration. The 18,000 plus mph speed change from the astronaut example is over an extended duration of time, not 100 to 150 ms as we generally see in an automobile collision. By defining acceleration as delta V / delta t you really defined delta V for the assumed collision duration.
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brian
Posts: 500
Joined: Tue Jul 14, 2009 10:52 am

Re: What is DeltaV? A Discussion

Post by brian » Tue Aug 04, 2009 1:53 pm

some additional comments/responses I posted:
  • DeltaV/DeltaT gives you only the average acceleration of a collision event.
  • It doesn't tell you about the severity of the collision.
  • It also assumes a square wave of acceleration whereas impacts are more nearly a quarter sine wave or acceleration.
  • There are those who read unfiltered acceleration traces and erroneously cite the peak value of the high frequency hash as being a measure of collision severity. The frequency of response of the instrumentation, effects of filtering, and occurrence of structural "ringing" tend to make measures of the peak acceleration unreliable.
So how to gauge collision severity?
  • By integrating the acceleration trace to obtain DeltaV The `peak' acceleration then can be characterized from an equivalent quarter sine wave acceleration corresponding to the DeltaV. In this way, we can standardize the characterization of a collision event.
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MSI
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Re: What is DeltaV? A Discussion

Post by MSI » Thu Jan 31, 2013 9:16 am

1-31-13: Q: What would be, in your opinion, the best way to establish the rms acceleration given a specific profile that was not an ideal half-sine?

A: Please read this post. It was somewhat buried in the forum so i post your question to the post.
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