Are metals and metal alloys practically incompressible? 1

[ccw]

I am thinking this is an important concept when it comes to cold forming.

If I bend a piece of sheet stock on a small radius it seems to flow out along the bend axis.

If I do a hardness test on a piece of metal, the ball penetrator forms a crator with some metal flowing up and forming a crater rim. If I do several hardness tests on a small strip, the strip seems to change length and width.

What is the hive consensus on this question?

 

[kingnero]

Is this really a question?
Think about it this way: can you change the density of the metal workpiece, no matter what you do with it (temp. changes not included)?

 

[mrfailure]

Total volume must be conserved on the workpiece. Compression in one direction will cause expansion in others.

Aaron Tanzer

http://www.lehightesting.com

 

[BMKR]

Solids are solid for a reason:

Atomic-scale: the atoms in a lattice arrange ideally* in a close packed orientation, and in a best case will occupy a FCC or CCP formation at an efficiency ~74%(occupying as much space as possible)

*single crystal with no defects(ha)

Macro-scale: If you compare metals to other "solids" like a ceramic column or some other porous solid there are clear gaps between the agglomerates making up this solid (ie water on a sidewalk), this would allow for compression, you are not compressing the solid, you are simply pushing the air out.

 

[btrueblood]

Well, you could say that ideal plastic behavior of metals includes incompressibility.

But the metal still has elastic compressibility...otherwise certain nuclular weapons would not work the way they do.

 

[Metalhead97]

ccw,

Solids are compressible to the extent of their Bulk Modulus.

Metalhead

 

[btrueblood]

Doh, can't even mis-spell properly. That shoulda been "nucular".

 

[ccw]

About the nuclear thing. I think you can compress a hollow sphere to a smaller sphere and still not compress material, just wind up with thicker walls. Is this correct?

Anyway, thanks for the tips. You guys, as usual, very helpful.

 

[KENAT]

Also depends what state the metal is in. I'm guessing you can compress aluminum foam for example - though arguably you're actually squeezing the air out or similar.

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[mrfailure]

To clarify, dimensions would still alter while in the elastic range of the solid material to conserve volume. There would be no evidence because the material did not permanently deform and will go back to its original shape when unloaded.

Aaron Tanzer

http://www.lehightesting.com

 

[cloa]

Under isotopic compression, even full dense metals will compress by altering the electronic state of the atoms. Anyway the original poster has forgotten thickness which could have a quite minor change and there is plastic tensile deformation also possible.

 

[btrueblood]

What cloa said, which is what I said, which is what metalhead said.

Plastic material is generally modeled as incompressible fluid; for incompressibility, Poisson's ratio v = 0.5

But for most metals in the elastic stress range, v = 0.3 or so, indicating that the elastic deformations do not conserve volume.

Not a big deal, but helps account for some funny behavior (like springback, maybe, but we digress, and I've been told that it's a forbidden subject especially if you are just a student) - when you realize that plastic deformations leave residual stresses, which cause the material to be in an elastically deformed state from that residual stress field.

Interestingly, rubber materials generally do behave incompressibly, ie. they are best modelled with v=0.5.

 

[Maui]

An elastic deformation will be accompanied by a corresponding change in volume for metal alloys whose Poisson's ratio does not equal 0.5. Since most of the commonly used metal alloys have a Poisson's ratio between about 1/4 and 1/3, technically speaking they will not conserve volume when stressed within the elastic range. But from a practical standpoint, the change in volume is so slight that in most cases it can effectively be ignored.

In terms of plastic deformation, unless a change in microstructural phase is induced by that deformation (which can happen with some grades of austenitic stainless steels, for example) the concept of constant volume can be applied in most cases as a good approximation.

Maui

http://www.EngineeringMetallurgy.com