Tube flattening equation

[MTUMetEng99]

I am trying to calculate the amount of stress imposed at the OD surface of 0.625" dia. tube during a flattening test (ASTM A1016), and then compare it with the stress produced by expanding the tube diameter ~0.015 - 0.030".

Any assistance with either or both of these calculations is appreciated.

 

[metengr]

MTUMetEng99;
Why not evaluate the % strain versus stress? Comparing stress required for producing the above deformation in flattening and roll expanding does not seem to be relevant.

 

[MTUMetEng99]

Here is some more background...

Subject material is exhibiting shallow ductile tearing following the second step of the ASTM A1016 flattening test.

Subject material will not be flattened during fabrication but rather expanded 0.015" in diameter into tube sheet for heat exchanger application.

Customer is concerned that the shallow ruptures indicate a deficiency with the material.

I am trying to show that the minor bit of ductile tearing is simply the result of stress at the OD surface exceeding the tensile strength of the material, and that said stress is of greater magnitude than that produced when the tube diameter is expanded.

I suppose strain calculations would work also.

 

[EdStainless]

Is this an austenitic stainless? If so then localized ruptures on flattening are not a good sign.
Either this material has surface texture issues or it is under annealed (or both).
When you roll expand you do yield the OD surface until you contact the tubesheet hole. Any local failures can result in longitudinal defects that will potential leak paths once the unit is in service. When tubes are rolled there is a lot of apparent wall thinning, but most of this is from the expansion not actual compressive thinning.

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Plymouth Tube

 

[metengr]

Now, I have one question, you realize in A/SA 1016 there is a provision for low D-to-t tubular products where if the D-to-t ratio is less than 10, ductility cracks are permitted in certain locations. Read the specification!

 

[MTUMetEng99]

Yeah, I am aware of what the spec allows. That isn't good enough for the customer though...

 

[metengr]

Well then, scrap the heat if the customer had a typical specification of no cracks (like our internal specification) for purchasing tubes.

 

[EdStainless]

You flatten in two stages?
First to the partial using the formula in the spec, and then full flat.
There should be no pucker, fissure, or cracking in the first step. There often will be when you go dead flat, but that isn't required.
I have some 5/8" x 0.109" wall here that flattened just fine (304N).

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Plymouth Tube