Energy absorbing property of metals..... 2

[mohanbabu2013]

Hello Group,
Can anyone help me how to calculate the energy absorbing property of a metal?? Because I am working on designing a steel structure and the customer requirements are

“…… structure shall be constructed of nickel-copper-chromium-molybdenum-columbium steel per ASTM Specification A-710. If a steel other than nickel-copper-columbium steel is used for the structure the design must have an energy absorbing performance equivalent to nickel-copper-chromium-molybdenum-columbium steel per ASTM-A710…”

And I only need 4’X4’ piece of steel to make the structure. As A-710 is not a very common alloy, the minimum I have to buy is 8’X20’ for almost 13 grand. I would like to use A572-50 grade steel instead which we carry in stock all the time.

So, how would I calculate the energy absorption performance of A-710 and compare it to A572-50??

Any suggestion is greatly appreciated. Thank you in advance.
--MB

 

[KENAT]

What kind of energy, thermal, mechanical...

I'd assume it's talking about spring energy?

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[dhengr]

Mohanbabu2013:
Don’t keep so many secrets. What kind of structure is it, and how does it absorb the energy? But, I suspect that what they want is a particular Charpy test value (notch toughness), or a projectile energy absorbing ability from a mil. spec.

 

[mohanbabu2013]

Sorry my bad, should have made it clear.
It is a collision post for railroad maintenance of way equipment. Two posts are built into front of the car body so posts absorb most of the energy due to collision with other rail vehicles. The energy is the mechanical energy due to collision.

Thank you KENAT and dhengr.

--MB

 

[GregLocock]

From a practical perspective I'd use typical mild steel. It has the best energy absorbing characteristics. In collisions what you are after is low yield point and great ductility and low notch sensitivity.

Cheers

Greg Locock


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[racookpe1978]

How thick?

I had a 4 x 6 foot plate bent for me three days ago of 1/8 thick steel and it was less $200.00.

You can buy a LOT of pounds of A36 carbon steel for 13,000.00

 

[3DDave]

Energy absorption is yield-strength times yield amount, roughly. This is on top of elastic energy storage.

How do the yield and elongation of the two materials compare?

It is an odd request as collision resistance is usually so shape controlled, much more than material.

 

[Tmoose]

Most (~ both ?) of the guys I knew driving VW buses swore the spare tire mounted the nose save their lives when they smashed into another vehicle.

 

[mohanbabu2013]

@racookpe1978 – Customer specs is ASTM A-710 material, which is not a very common structural steel so has to be a special order from the mill and is approx..$82 psf and min order is 8’X20’.

@3DDave – for A710 yield is 65ksi and elongation is 20%; for A572-50 yield strength is 50ksi and elongation is 21% in 2”. Do you have a reference for “Energy absorption is yield-strength times yield amount, roughly.” I would like read up on it. Whatever shape I design the collision post, I cannot use A710 as it’s very expensive. To take an exception from using A710 I have to show them that the material I choose either meet or exceed the energy absorption performance of A710.

I am still not sure how to compare the energy absorption performance of A710 and A572-50 materials.

thanx guys...

 

[3DDave]

Energy absorbed is the integral of the force applied and the distance it is applied to.

It looks like a fair match between the materials, with only a 23% reduction from the required material to the alternate. To make up, one could change the dimensions of the alternate, but again, it's highly dependent on the geometry it is used in.

This is based on the assumption that it is plastic deformation that is the important characteristic.

 

[CoryPad]

For steels, there is that pesky ductile-to-brittle transition temperature. Charpy impact testing is the common method to evaluate the material performance, and you should compare these two choices. By the way, ASTM A710 lists six different options, so it is not just one material.

 

[3DDave]

Went searching - found this:

http://www.steeluniversity.org/content/html/eng/default.asp?catid=151&pageid=2081271949

Surprisingly good.

 

[tbuelna]

If the shape/thickness of the post can be altered, then it would seem to be possible to redesign the component using a different (less costly) material while being able to demonstrate by analysis and validate by test that it meets the performance spec requirements.

Unfortunately, what you may find is that the cost of performing the analysis and test work your customer requires for the material substitution might be greater than the cost savings provided by the alternate material. In reality, $13K does not cover a whole lot of analysis and testing work.

 

[Tmoose]

"Two posts are built into front of the car body so posts absorb most of the energy due to collision with other rail vehicles."

Couldn't find any pictures with Google, but I'm guessing the posts must be sculptured and scalloped if they are to "absorb" much energy.

 

[mohanbabu2013]

@ 3DDave & tbuelna– I agree, it depends on the geometry. If we have any thickness or size limitations then we could justify it by making it bigger or thicker and use less grade material, but we do not.
@CoryPad – I am aware of different options of A710 and for any option 65ksi is min. I don’t think any option is cheap.