Stress Distribution 1

[harrytos23]

Hi, this is a structure that we designed to decrease the force as the absorber breaks (thin plates that is designed to break upon impact).

My question now is, is there a way to calculate the force after the first contact with the first plate? because I know the 1st plate will be destroyed and as well as the second but wondering how much less the impact force would be on the 2nd plate and for the rectangular tube as well. Is there any resource/book/formula that can show how forces are distributed after breakage of the material? Thank you.

 

[FEA way]

More advanced FEA software offers material damage criteria and element deletion upon failure so you can predict destruction of the structure if you have the right input data.

 

[harrytos23]

More advanced FEA software offers material damage criteria and element deletion upon failure so you can predict destruction of the structure if you have the right input data.

sorry for the confusion, I don't want to use the Simulation software for it since I only have Solidworks and there's no way it can do it. I was wondering if there's a way calculate it by hand? formula to follow? so I will know how many percent less is the force after the first impact and apply it to the Solidworks Simulation. Thank you by the way

 

[rb1957]

no. The impact force is all about the time taken for the structure to deflect, which you can't get (with any reasonable accuracy) from a linear (static) FEM.

There are always "hand-waving" assumptions like "assume the impact takes 0.1 sec" so the vehicle decelerates over this time, and therefore you get a force.
Is it a linear deceleration ? almost certainly not, but you can hand wave as you like.

The whole point of your design is that the structure crumples and extends the impact event.

 

[goutam_freelance]

A simplified method could be the the use of strain energy.

It assumes that you know the kinetic energy of the impacting body.

The kinetic energy will be reduced by consumption in the strain energy U.

The stress could be the breaking stress of the material in question.

 

[3DDave]

A lot depends on the speed of the collision. At a low enough speed one can integrate the energy absorbed in elastic deformation of the structure. If that isn't enough to take all the energy then one makes continuing increments of plastic deformation to see how much is absorbed in irreversible changes to the structure - buckling and thinning - for example. If that still isn't enough then it passes into conditions of rupture, such as tearing of metal or welds breaking. The farther along one goes in the analysis tree, the more sensitive the results are to initial conditions. Small changes in thickness, yield strength, weld size, the effect of heat on the base metal, and so on, can give very different results.

If using a linear analysis, one that is fully elastic, it will tend to under report deflections.

For example, in a simple tensile bar, the elastic load would continue to increase past the typical yield point ramping up the energy that theoretically would be stored as the square of the displacement, but the real condition sees the load plateau while the deformation increases for a linear amount of energy absorbed.

However, if the speed of collision is very high compared to the speed of sound in the material then those assumptions are out the window.

 

[GregLocock]

I can't help thinking a drawing of what is going on and some dimensions and other parameters might speed resolution.

 

[dtimberlake]

If the upper and lower edges of two spaced tablets are welded to the jaws of the fixed Stillson wrenches, AND the purple forces are applied as shown I'm thinking it may be more of a weld shear tester.

If the purple forces will actually be applied more towards the center of the tablets it might be more of a Charpy test. If this is an outdoor installation in New England the results may vary a lot with temperature.

As drawn, I think the wrench jaws will rotate horizontally away from each other as the centers of the "front" faces of the large square columns cave in.

Mostly I'm thinking the region of the as-drawn base plate near the LS column attachments will not be "fixed," and the bulk of the energy to catch the runaway trolley will be dissipated in mauling the relatively thin base plate.

https://www.aisc.org/globalassets/modern-steel/steelwise/steelwise-february-2013.pdf

 

[Snickster]

So what I understand is that you have basically an object moving at a horizontal velocity on wheels that you are trying to stop. The object hits the first plates, shearing the first plates or the weld of the first plates to the structure causing an absorption of some of the kinetic energy and reducing the velocity of the object. Then it strikes the second plate absorbing some more of the energy and reducing the velocity. Then it strikes the columns of the structure. Is this correct? If so do you have specifics such as mass and velocity of the object and details of plates and structure?

 

[goutam_freelance]

The stress could be the breaking stress of the material in question.

In my earlier post, I considered linear region only.

A simplified model should include the plastic region also till rupture.

A simple method will be to take the stress-strain diagram to the breaking point and numerically integrate it for the area beneath the curve. Multiplication of this with the volume of the specimen (after suitable adjustment for units) will give the energy absorbed by the specimen before rupture,

 

[Snickster]

OK I will take a shot at it. Assuming an object on wheels moving horizontally is stopped by the structure by first hitting plate 1, then plate 2 then column. Using method of considering kinetic energy is converted into work of bending column and shearing weld that attaches plates to the structure, and simplified with assumptions indicated in attached calculation. Also attached is similar method from "Design of Welded Structures" by Blodgett that considers kinetic energy of object falling onto a beam.

 

[Snickster]

I over simplified the energy absorbed by failure of the plates. Need a better approximation of the energy absorbed by the plates during failure.