What is the CRASH Energy Correction Factor (ECF)?

'What Is' type questions related to highway safety, accident reconstruction and vehicle simulation
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brian
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What is the CRASH Energy Correction Factor (ECF)?

Post by brian » Fri Oct 16, 2009 11:12 am

Q: What is the Energy Correction Factor (ECF) used in Damage Analysis?
A: Crush properties of vehicles are measured and fitted for crush directions that are perpendicular to the involved end or side of a vehicle. However, in an oblique collision, a component of the tangential friction force acts to increase the effective crush resistance in the direction of crushing and thereby, the absorbed energy. Therefore, an Energy Correction Factor (ECF) is needed for applications of crush coefficients to oblique collisions.
For further information, please see the section on Energy Correction Factor(ECF) from our book McHenry Accident Reconstruction.
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MSI
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Re: What is the CRASH Energy Correction Factor (ECF)?

Post by MSI » Wed Mar 10, 2010 10:27 am

Q: I want to ask you a question about CRASH3 algorithm. There is a correction factor in CRASH3 ( 1+ tan^2. alpha). Is the alpha angle PDOF angle here? It is limited to 45 degrees to make the factor maximum 2. If it is PDOF angle, can we say that PDOF angle is limited to 45 degrees? Because i saw it can take higher values in planar impact mechanics applications of Brach.
A: Alpha is "the angle of crushing relative to a perpendicular to the involved end or side of the vehicle". It may be the same as PDOF, but for example, in side impacts Alpha is adjusted by 90 degrees. The limitations of 2 is for the correction factor based on Alpha and not on the angle Alpha.
From one of our recent publications on ECF
  • "On the basis of a recognition of the limitations on energy absorption that are imposed by realistic levels of tangential friction, a revised form of the ECF was proposed in 1986. Applications of the ECF since that time have generally been restricted to the angular range of ± 45 degrees so that only a limited evaluation of the effects of the revised ECF was possible"
Also see our related discussions on With respect to Brach do you mean Brach values for PDOF? Or some suggestion for correction factor that Brach suggests? And could you please provide an example as in what reference and on what page?
Please also see Some Momentum Misconceptions for limitation of momentum and planar impact mechanics which include simplifying assumptions such as instantaneous exchange of momentum, etc.
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Re: What is the CRASH Energy Correction Factor (ECF)?

Post by MSI » Wed Mar 10, 2010 2:03 pm

Q: In planar impact mechanics spreadsheet in the Brach book, there are PDOF angles, not correction factors. For example, in that example PDOF 1= -1.5 degrees and PDOF 2 = 65.5 degrees. Thats why i asked if PDOF is limited to 45 degrees in CRASH 3. Because when we take that alpha angle as PDOF and use 65.5 degrees for ECF, it is higher than 2.

A: There is no limit on the Alpha angle in CRASH. However, the maximum ECF is limited to 2.0. So if the angle produces an ECF greater than 2.0, simply use 2.0.
In the example you cited in the Brach book (p 160, 161) the comparison of CRASH3 damage (uncorrected for tangential impules) with his Planar Impact mechanics calculations is rigged! Brach chose to use ‘the uncorrected DeltaV values given directly from the crush energy equations in Fig 8.5’ (p160). In other words, he did not include the ECF portion of CRASH. For this type of impact CRASH3 would severly underestimate the results.
If the Brach book were to properly present the CRASH3 results including the use of the ECF then the results in Figure 8.5 for the CRASH crush energy and therefore deltaV’s would be basically the same as those in Figure 8.6 for Planar Impacts Mechanics. (NOTE: When comparing analytical techniques one should consider comparing the techniques with reality, such as a full scale test and measurements, since otherwise the truth is no known and possibly both can be wrong!).
Brach takes issue with the simplified characterizations and concepts used to CRASH3 (see Brach p143, 158. etc).
CRASH3 was created as a simple and direct approximation technique, mainly as a pre-processor for SMAC, and then later embraced by NHTSA for NASS, their statistical studies of accidents throughout the country.
  • The concept of ECF was a simplification created to compensate for the fact that work against friction is done in angled impacts.
    The concept of PDOF was used so that visually an initial estimate could be obtained: What direction do you think the Force or Impulse came from on each vehicle?
    The concept of PDOF also more correctly instructs the analyst to consider both vehicles in the overall exchange between the vehicle during a collision. It includes the concept of a line of action (PDOF) between the two vehicles. The analyst considers each vehicle at their orientation at impact and maximum engagement while thinking about the PDOF; do the vehicles and PDOFs line up? Does the line of action of each vehicle coincide for the impact location and angles?
(NOTE: Collision interactions are actually produced through Impulses, as in Forces over time, so PDOF perhaps should be called PDOI. However since it is utilized with the assumption of an instantaneous exchange of momentum and with planar impact model applications, there is no 'time variation' so it can also be referred to as a Force!)(For a discussion of PDOF see How does SMAC determine the DeltaV and PDOF.
The Brach book complicates the process and comes up with the additional burden of 3 extra arbitrary and subjective inputs for his Planar impact analysis:
  • 1) Gamma, the ‘crush surface angle’,
    2) Eps, his value for restitution, and
    3) Mu, the impulse ratio.
Some notes on these inputs:
  • 1) Gamma can lead to serious problems when variations in the local stiffness of vehicles produce variations in the ‘angle of the damage surface’. How do you compensate? A whole new slew of instructions will be required to approximate Gamma to handle local variations in the structural stiffnesses.
    2) Mu, The impulse ratio appears to be a subjective input. You start with the 'theoretical value' for MU and apparenntly change it until you get the results you want or need. But what if want to do an objective scientific analysis? What value do you use then? Looking at the Brach book Table 6.2, the listing for the RICSAC staged collision analysis, the value for MU for identical impacts varies arbitrarily:
    • Tests 1,6,7 are 60 degree front to side impacts, MU=.966, .824, & .772),
      Tests 8,9,10 are 90 degree front to side impacts, MU=.413,.486,.590
      Tests 11,12 are 10 degree front to rear impact, MU is .038, .031
      Tests 3,4,5 are 10 degree front to rear and Mu is -.065, -0.50, -0.09
      Then look at:
      Tests 9 above, 90 degree front to side impact, MU used was .486
      Figure 7.14, p 145, RICSAC 9 example, Mu is 0.512
      Figure 7.13, p 142, RICSAC 9 example, MU is 0.806

      SMAC uses the concept of inter-vehicle friction coefficient, Mu. With a typical value of 0.55, the range is 0.35 to 0.90 depending on whether the collision interaction included more or less friction then "typical". It could be added as an input to CRASH and used in the ECF calculation, but that's one more subjective input requirement.
      3) EPS, the restitution factor we have written suggestions for including restitution in CRASH. NOTE: CRASH does not include restitution! (See CRASH and Restitution for additional discussion of why it does not include restitution
)
Whereas for CRASH3 these are all handled internally (less arbitrary adjustments). Only the PDOF is required as input.
KISS the concept (Keep It Simple Stupid!). PDOF and ECF are KISSed concepts!
It is important to note that the same results are obtained between the simplified CRASH concept and the Brach book Planar Impact Mechanices for the example on page 160-161 if CRASH3 is applied correctly with the ECF!
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Re: What is the CRASH Energy Correction Factor (ECF)?

Post by MSI » Wed Mar 10, 2010 2:11 pm

Q: And one more question: how is the alpha angle defined? It seems hard to define it according to the figure in The Algorithms of Crash, page 28 (see below), moment arms of resultant collision force'
A: Alpha is basically the PDOF. Internally in CRASH3 it is based upon the PDOF. The only adjustment is that for side impacts where the perpendicular is other than 0 degrees. The adjustment is that the angle must be normalized such that it is relative to the perpendicular to the end or side of 0 degrees. So for example a side impact with a PDOF of 100 degrees would have an Alpha of 10 degrees (100-90).
crash p28.jpg
crash p28.jpg (40.61 KiB) Viewed 2013 times
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