Generating Impact Load

[Enhineyero]

Hi all, I am currently given a problem that is involving impact or vehicle loads. Say I have a truck who has a total mass of 4000kg moving at a speed of 80km/h that will hit a barrier/wall. what will be the value of the force that must be resisted by the wall.

I use some basic physics to come up with a value. But I need to verify if other engineers will take the same approach as I did. your replies would be appreciated.

 

[jhardy1]

Among other factors, it depends on the "effective stopping distance" (F = ma), which depends on the rigidity of the barrier and the "frangibility" of the truck body.

You will get a very different answer for a World-War-II-vintage army truck (with a rigid ladder-chassis made of steel RSJs) crashing into a massive concrete wall, compared to a 2012-build European truck (with deformable crumple zones) crashing into an Armco deformable barrier.

(Hint: "crumple zones" significantly increase the "stopping distance", which gives significantly smaller peak deceleration, which gives better survivability for the occupants!)

http://julianh72.blogspot.com

 

[GregLocock]

High guess 20g, so F=200*4000=800 kN

Low estimate - crush length =s is 2m, v=80/3.6 m/s, v^2=2*a*s

Well that's damn close to the high guess!



Cheers

Greg Locock


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[rb1957]

this is a rerun of an earlier thread, yes?

this depends on how quickly the vehicle's velocity will be reduced to zero ... this and other motherhood.

greg shows an estimate of 20g is similar to a stopping distance of 2m.

the basic physics is newtonian (we don't need relativity here), any high schooler should be able to write out the equations. the problem is filling in the variables ... how long a collision time ? how stiff (or how long to allow for the momentum transfer) ? personally 20g isn't the high guess (that's be 200g, which you might get in a "hardened" WW2 truck).

have you searched car collision tests ? crash barrier design ??

 

[MiketheEngineer]

Will this question EVER go away??

Check the threads - it has been discussed ad nauseam!!

How fast will it stop??? - this will give you the de-acceleration. Then go to F=ma

Now call Ford, Chevy and BMW and ask why they crash millions of dollars worth of cars per year - BECAUSE even their best engineers can't answer the above!!!

IT DEPENDS on way too many things....

 

[GregLocock]

Here's some real world data, emphasising that there is no simple answer

http://www.nhtsa.gov/cars/rules/crashworthy/offrt.htg/img3.gif

A good non linear FEA model of a car can predict that sort of pulse fairly well, but there is a lot of correlation data required. It may seem redundant to model the event you have physical data for, but the model can then be used to develop the airbag strategy etc.



Cheers

Greg Locock


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