I need a little help. I'm working an accident where a single vehicle skidded over a couple of different surface types, impacted a light pole and severed it, then skidded to stop some distance after the pole impact.
In my analysis, I performed an EDCRASH based crush analysis to get a delta V for the pole impact then calculated all the speed losses over the various surfaces.
Since my last speed loss (after pole impact) was 15mph, I knew that the vehicle speed post impact was 15mph because it came to rest (0mph) at the end of this speed loss.
I added the speed loss of 15mph from my final segment to the delta V from the pole impact (9mph) together to get a speed of 24mph at the moment of impact.
I then took that speed loss and combined it with the remaining speed losses that occurred pre-impact.
I believe my methodology is correct and have had others confirm it, however I have not had any luck finding a source to tell me why it is (or isn't) correct - or even someone more math strong than me to explain it.
I had one individual tell me I should combine all of the speed losses, but I don't believe this is correct.
Let me know if my methodology is correct or not and if you have any sources to share it would be greatly appreciated.
Why did you simply add the DV and the post impact speeds, when you seem quite content to do a combined speed formula for the multi surfaces pre DV? Obtaining DV is only one of several segments of speed loss along the vehicle's journey. Unless someone can suggest why it would be appropriate to add the results together, surely the combined speed formula remains a suitable approach whether one uses DV or not.
My understanding is that Delta-V is a true speed change and not based on an energy equation like the other speed changes are.
Since I have a seperation speed of 15mph immediately after the impact - it stands to reason that if there was a 9mph Delta-V from the pole impact, that the impact speed is 15+9 or 24mph. Then you combine the 24mph with the other speed losses that occured pre-impact since they are all energy based.
Since my original post, I have located a paper that seems to suggest my methodology is correct. The paper SAE 940915 "Delta-V: Basic Concepts, Computational Methods, and Misunderstandings" says this in part:
Delta-V represents a velocity change, not an energy value. The Delta-V calculated above is a scalar. When a scalar is combined with directional information, it becomes a vector. The Delta-V is vectorially combined with the seperation speed from impact, by vector addition, to compute the impact speed.
V(imp) = V(sep) + Delta-V
It would be incorrect to combine these by using the square root of the sum of the squares because the Delta-V cannot be interpreted in terms of energy as can the V(sep) (unless the impact brings the vehicle to rest.)
My understanding is that if the vehicle strikes a pole and comes to rest the Delta-V can be interpreted as an energy value and should be combined with the sum of the squares.
Also, this only applies to linear impacts - so if you have an oblique impact, you would need to perform a vector analysis to determine the Delta-V in direction of travel post-impact.
At least, the above is how I interpreted it and how /why I perfomed the math the way I did.
Reread the OP, misread your combining dv+postimpact then putting into a combined formula with adding to the earlier data. So all you would be doing would be calculating speed either in several small steps or as one combined formula. With the sensitivity and nuances of crush programs, do you feel confident in the data that you originaly put into the program.
I'm not an EDCRASH user, but am familiar with this type of program. Generally you can extract the crush energy from the output. If this is the case with EDCRASH, then you should be able to take this energy term and add it to all the other energy losses to calculate the speed at the onset of the tire marks. I'm not sure how EDCRASH would calculate the delta-V as it doesn't know whether the vehicle struck the pole and came to a stop or struck the pole, broke it, and continued on.
