EES vs. DeltaV

General Questions related to the CRASH Program and clones
Damage Analysis & Momentum Based Analysis programs
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EES vs. DeltaV

Post by MSI »

Q: Email question: What is the difference between EES (Energy Equivalent Speed) and the DeltaV (aka Impact Speed Change?)

A: The "energy equivalent speed (EES),i.e. the vehicle speed equivalent to the energy consumed to cause the vehicle deformation" (from Uncertainty of EES) or "The EES is determined as a measure for the kinetic energy dissipated by the vehicle during the contact phase, i.e. the energy converted to thermal energy through deformation."
  • The EES is a Damage Analysis method like CRASH or other damage based analysis methods.
  • For damage based analysis you measure the damage, apply the A & B (or other) coefficients to calculate an ‘impact speed to the point of common velocity'
  • Damage based analysis methods do NOT include restitution, and only calculate the Impact Speed Change to the point of a common velocity (so in a barrier collision it is the impact speed, NOT the total impact speed change)
  • An example,
    • in a 35 MPH barrier impact
      • The EES should be 35 MPH
      • The actual DeltaV (Impact Speed Change) would be somewhat greater since it includes restitution of 5%-10% so it would be approx 37 to 39 MPH
Damage-analysis techniques such as the CRASH computer program produce an approximation of the approach-period portion of the impact speed-change, rather than the total impact speed-change.

The SMAC Simulation model,
  • Is essentially a mathematical full scale test, calculates the Total Impact Speed change, or DeltaV,
  • The SMAC based DeltaV is calculated based on the accelerations produced during the collisions and includes restitution.
  • If you are using a simulation model like SMAC and wish to determine the EES for a given collision:
    • you need to reduce the Total DeltaV from SMAC by the approximate amount of restitution (EES and damage analysis procedures are to the 'point of common velocity')
    • Restitution is approx 0.3-0.4 for DeltaVs 10 MPH or less and near 0.0 at appro 50 mph DeltaV.
The use of EES or other damage based analysis techniques like CRASH, is for convenience:
  • CRASH was initially created to assist SMAC users in determining first estimates of the impact speeds.
    These techniques were created and used for statistical studies of crashes in recognition that most investigative agencies don’t have the time, or personnel, to deploy to investigate the crash scenes and can more conveniently simply merely look at and measure the damaged vehicles (the crush), at a later date. They then apply a damage based analysis like CRASH or EES.
Here are some quotes from related article found in a quick search:
  • “...the delta-v values are somewhat higher, delta-v > EES and delta-v > VK. The reason for this is the elasticity in the deformed structure which leads to a rebound at the end of the collision. Depending on the nonlinear relationship between speed and energy, the difference between delta-v and EES is clearly greater than the difference between EES and VK. lf the structures were completely plastic, EES would always· be equal to delta-v and equal to VK in a collision without glance-off.”
    • Note: VK= translational velocity at the beginning of collision
    • In a simplified approach it can be stated that the velocity change is greater than the energy equivalent speed EES if a stiff vehicle structure and a full overlap are involved, the impact takes place at a low velocity change and no glance-off takes place.
      In an analog manner it can be concluded that the velocity change of a vehicle is smaller than the EES if soft vehicle structures and only a small overlap as well as high velocity changes are involved and glance-off occurs.
      As a general statement it has to stressed that important misjudgements take place if in an accident sample with different collision circumstances such as overlap, glance-off and structure stiffness the EES is erroneously thought to be equal than the velocity change delta-v.
Also from Uncertainty of EES:
  • 5. Recapitulation and conclusions
    Based on a series of example calculations and on actual deformation measurements carried out on post-crash vehicles, a statement may be made that the energy equivalent speed (EES) cannot be accurately estimated if the data available are limited to the permanent bodywork deformation values. The energy equivalent speed depends on many factors, especially on the measuring method, measuring instrument, EES value determining method, etc. All these factors cause the EES value to be burdened with an error, the range of which is defined by the estimation uncertainty discussed herein
    • Based on an analysis of the results presented, the following conclusions have been
      • – The highest nominal EES values are usually obtained when the simplified method is used; for the other methods, these values reach a similar level.
      • – The lowest uncertainty values, amounting to (2÷4) % in all the cases under analysis, were achieved when the CRASH3 method was used; somewhat higher values, of (9÷14) %, were obtained in the case of using the simplified method.
      • – The uncertainty reaches the highest values, at a level of about 20 %, when the method employed in the PC-CRASH program is used.
      • – A relative uncertainty level of 30 % was not reached for any of the EES values determined in this work
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