Questions on CRASH

Topics related to collision & Trajectory analysis formerly on our 'Registrants only' area however which we get asked about frequently so believe shoud be in the open forum too
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MSI
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Posts: 1289
Joined: Thu Jun 18, 2009 12:37 pm

Questions on CRASH

Post by MSI » Mon Jun 18, 2018 11:44 am

From April 2018 email:
Q: Could you answer my following questions? These questions concern car accident.
Pictures are enclosed to this e-mail. I'll be grateful for your answer.
1. Program Crash3 assumes, that the profile of deformation (the width and depth of deformation) of car body has to be uniform in vertical direction. Does Picture 1 depict this properly?
Pic. 1.
Picture 1.jpeg
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In accident which interests me, large truck (weight about 43 mT = 43 000 kg) struck at passenger car.
Picture 2 shows the real appearance of damages of passenger car. Sill and floor were deformed partially (they slipped under the truck). Wheels were separated.
Pic. 2.
Picture 2.jpeg
Picture 2.jpeg (50.21 KiB) Viewed 968 times
Is following equation (1) valid in this case (this is equation 2.48 from “Crash3 Technical Manual”)?
Can we use damage profile, as shown in Picture 3 (hatched area)?
Are the coefficients of stiffness (A [lb/in] and B [lb/in2]) still valid?
Equation (1)
Equation (1).jpg
Equation (1).jpg (19.15 KiB) Viewed 968 times
Pic. 3.
Picture 3.jpeg
Picture 3.jpeg (59.35 KiB) Viewed 968 times
2. To which Vehicle Class Category of passenger cars belongs AUDI 80 B3 (according to SAE 960897)?
3. In “Crash3 Technical Manual” you use unit lb. Is this the unit of mass (pound), or is this unit of force (pound multiplied by gravitational acceleration)?

A: First realize that CRASH3 was developed as a pre-processor for the SMAC program and so there are a lot of simplifying assumptions (as with any mathematical model)
When Ray McHenry/Calspan delivered SMAC to NHTSA they couldn’t come up with an initial speed, so Ray developed CRASH which NHTSA embraced as adequate for their statistical studies where the possible errors wash out in the sample size.
Fast forward 40 years and today folks consider damage analysis as a bonified crash reconstruction tool and make erroneous statements about it’s accuracy.
The original crush tables (A,B) were based on a small sample of crash tests.
Today generally folks try to find actual tests of a vehicle or a ‘clone’ (similar) vehicle to base their A&B crush coefficients on.
We of course caution that A& B coefficients based on a single test do not constitute a scientifically accurate technique and so should be used only as a first approximation method and then other techniques (combining trajectory AND damage like with SMAC) provide a better technique.
Here are a few forum topics on the subject So in response to your questions we caution on the limitations of the CRASH3 mathematical model.
With that said, the following are answers to your specific questions:

1. Program Crash3 assumes, that the profile of deformation (the width and depth of deformation) of car body has to be uniform in vertical direction. Does Picture 1 depict this properly?
  • Yes your picture is approximate. Of course you are showing a side impact where some of the limitations of CRASH3 are most apparent due to the wide variation of the approximate crush stiffness if the sill or the wheels are included in the impact.
In accident which interests me, large truck (weight about 43 mT = 43 000 kg) struck at passenger car.
Picture 2 shows the real appearance of damages of passenger car. Sill and floor were deformed partially (they slipped under the truck). Wheels were separated. Is following equation (1) valid in this case (this is equation 2.48 from “Crash3 Technical Manual”)? Can we use damage profile, as shown in Picture 3 (hatched area)?
Are the coefficients of stiffness (A [lb/in] and B [lb/in2]) still valid?
2. To which Vehicle Class Category of passenger cars belongs AUDI 80 B3 (according to SAE 960897)?
  • See answer above. Tables based on wheelbase so compare in table.


3. In “Crash3 Technical Manual” you use unit lb. Is this the unit of mass (pound), or is this unit of force (pound multiplied by gravitational acceleration)?
  • lb = pounds is weight which INCLUDES the gravitations acceleration.
    If Mass used it does not include gravitational acceleration.
Question? Comment? Please email forum@mchenrysoftware.com. Also see the McHenry Forum Index
Visit McHenrySoftware.com for technical information & software.
(c) McHenry Software, Inc ALL Rights Reserved.

MSI
Site Admin
Posts: 1289
Joined: Thu Jun 18, 2009 12:37 pm

Re: Questions on CRASH

Post by MSI » Mon Jun 18, 2018 12:22 pm

From June 16, email:
Could you answer me next two questions?
1. In "Crash3 Technical Manual” was written, that collisions with moving trans/ large trucks are one of non-applicable accident types. Why shouldn't we use program in these cases? What does cause difficulty (or can produce errors) in these accident types - damage or trajectory analysis?
  • A: I believe that limitation is:
    For damage analysis:
    • because there is little information on large truck crush resistance and/or crash tests so no data for the creation of a A&B characterization for these types of vehicle and the structure is so much different from passenger vehicles it goes beyond the initial simplifying assumptions.

    For trajectory analysis:
    • The CRASH trajectory option is a single step application of the conservation of Linear momentum and so for very large differences in weight the equations can be very sensitive:
      • for example, think of a small vehicle crossing in front of a large vehicle. If the large vehicle swerves even a small amount pre-impact and that is not considered, the speed of the smaller vehicle can calculate as extremely large (in forward or reverse). Sensitivities due to large mass differences can cause problems in single step applications of the conservation of linear momemtum

2. The figures below I send quotation from “Crash3 Technical Manual”, which concerns assumption of uniform profile of deformation.
What, if:
  • a) sill and floor are overridden (they slid under the truck);
  • b) only side parts and roof are deformed;
  • c) profile is based on the side parts and roof deformation?
If calculated energy of deformation will be underestimated or overestimated?
Figure 2.4.jpg
Figure 2.4.jpg (36.4 KiB) Viewed 967 times
Quotation.jpg
Quotation.jpg (42.06 KiB) Viewed 967 times
Question? Comment? Please email forum@mchenrysoftware.com. Also see the McHenry Forum Index
Visit McHenrySoftware.com for technical information & software.
(c) McHenry Software, Inc ALL Rights Reserved.

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