Q: In planar impact mechanics spreadsheet in the Brach book "Vehicle Accident Analysis and Reconstruction Methods"
, 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 not known and if comparing two techniques with each other who know which is correct? Both might be wrong!
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!)
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, defined as their ‘crush surface angle’,
- 2) Mu, defined as their impulse ratio, and
- 3) Eps, defined as their value for restitution
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.
- Using a simple PDOF concept makes it much more easy to explain and train: Main concept is from what clock direction do you think the main force came from?
(and of course in a collision it is not a single force but an impulse (changing force over time of the collision)
since CRASH and Planar Impact models and Momentum solution procedures ALL assume an INSTANTANEOUS EXCHANGE OF MOMENTUM you are approximating the MAIN FORCE DIRECTION!)
- Look at the vehicles, or drawings of the vehicles with damage displayed, fit them together like at MAXIMUM ENGAGEMENT and voila! Very easy to approximate the PDOF clock direction for each vehicle!
- To model the actual Forces time history you need to use a simulation model like mSMAC3D
- 2) Mu, The impulse ratio appears to be a subjective input. You start with the 'theoretical value' for MU and apparently 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:
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".
- 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
- WHY??? Is their ANY objective scientific reason for this variation on the RICSAC 9 test? And any justification/explanation for the variations between identical impacts?
It could be added as an input to CRASH and used in the ECF calculation, but that's one more 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 will be obtained between the simplified CRASH damage analysis concept and the Brach book "Vehicle Accident Analysis and Reconstruction Methods" for the examples on page 160-161 if CRASH3 is applied correctly with the ECF!