Plastic deformation of a guard 2

[WardHollowayPE]

I am designing a guard for a material handling system. Low velocity, high mass. During impact I want the guard to deform to absorb the energy like a crumple zone in a car. Can someone point me to the proper analysis or method of calculating the size of the guard required. The construction will use round tube as a cantilever to absorb impact.

Thanks

Ward Holloway, Jr, P.E.

 

[rb1957]

so this isn't a basketball guard we're talking about ... slow day ...
the guard is to prevent things getting in, and the impact is from stuff trying to get out ... what if you designed the guard to be sprung down against stops (so it could deflect outwards but not inwards ?

 

[rstupplebeen]

Time to break out your rigid body mechanics text book. Figure out how far you have to catch the mass. Then you can size the guard. Depending on your guard design cantilever beam or spring equations may be appropriate. More complex geometries will require FEA. I hope this helps.

Rob Stupplebeen

 

[FeX32]

Since you mention it is plastic deformation you want to utilize to absorb the energy it may be difficult of calculate an accurate number. Plastic stress-strain curves are valid under static conditions only.
I suggest a dynamic experiment.
However, you mention "low velocity" thus the quasi-static analysis may still hold given a few assumptions.


Fe

 

[WardHollowayPE]

Here is some history on this project.
The design is similar in function to a guard rail on a road. My original design was large enough to keep the strain in the elastic limit of the steel. This design is too expensive for management. So I figured we can permanently deform the guard during impact, and replace after. This isn't a physical design question, more of a energy calculation question. The new design is lighter and may generate some spare parts business also.

Ward Holloway, Jr, P.E.

 

[drawoh]

WardHolloway,

You want to convert kinetic energy to some other form of energy. Read up on strain energy.

Does the thing have to deform plastically? What happens of the material bounces back upon impact?

JHG

 

[FeX32]

If it bounces back it's not plastic deformation


Fe

 

[rb1957]

ok, i thought you were designing a guard for a machine.

i think you're in for a heck of a calc ...
1) strain energy of the barrier, elastic and plastic,
2) deformation of the barrier supports,
3) deformation of the ground around the supports (soft sand, hard 'crete ?)
4) deformation of the impacter (look a a bird strike video)
5) height of the impactor (if we're talking concrete mixers, will they ride over the barrier with a "bump, bump" ?)

you know what the old "armco" barriers looked like ?

 

[bvanhiel]

Ward,

As rb1957 suggests, you will find this difficult to accurately calculate even with FEA. It's a large displacement dynamic problem with a non-linear material that involves buckling.

I would do a quick back of the envelope beam calculation to get in the ballpark and then build and test until you have something satisfactory.

-b

 

[GregLocock]

Before FEA cars met crash requirements.

Probably the easiest way to do it is to draw a mechanism for a plastically deformed structure with (for example) 3 hinges. The total rotation is 4 theta, the moment of plasticity of the section is Mp, the first moment of area * length*yield stress.

Hence KE=4*theta*Mp

That was used to design steel frame buildings, crush tubes in cars, and probably Armco.







Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[WardHollowayPE]

Greg
This is what I was looking for. Can you post titles for books or reference material so I can read up on this?

Thanks.

Ward Holloway, Jr, P.E.