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# Equivalent diameter in tensile test

## Equivalent diameter in tensile test

(OP)
A client has sent us a specification to manufacture 316L tubes (12.7 mm OD) wich indicates the tensile test shall be carried out using a gauge length of 5D, where D is the equivalent diameter. My question is, what does it mean by "equivalent diameter"?
The specimen shall be tested in full cross section.
Thanks

### RE: Equivalent diameter in tensile test

Equivalent diameter usually refers to the flow area of a tube. For a circular tube, the equivalent diameter will be equal to the ID.

### RE: Equivalent diameter in tensile test

(OP)
Thanks for the answer, but I have a question.
If the equivalent diameter is related to flow area (cross section?), where does it come from that it is equal to the ID? As far as I understand, the ASTM E8M indicates that the gauge length must be equal to 5D where D is the external diameter.

### RE: Equivalent diameter in tensile test

I think your customer used the wrong terminology and this should be clarified with the customer. Equivalent and external seem like easy enough words to swap if English isn't a first language.

### RE: Equivalent diameter in tensile test

In the US people use 2" or 4D (outside) for tube GL.
In Europe it is more common to use 5D.
I have seen both OD and mean D used in the GL calculation.
At the end of the day it doesn't matter.
Annealed 316L will be up over 50% elong however they measure it.
Strain rate matters more.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

### RE: Equivalent diameter in tensile test

WHAT 316L material-tube specification, WHAT strain hardness and WHAT wall thickness?

Is this a structural tube or a fluid-flow ['hydraulic' quality] tube?

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

### RE: Equivalent diameter in tensile test

(OP)
The tube is an ASTM A213. We have a lot of experience in manufacturing this type of tubes, however in this case the client requests that for the tensile test the elongation shall be taken using a gauge length equal to 5D where D is the equivalent diameter. What is not clear to me is the meaning of "equivalent diameter", that's all.
Thank you

### RE: Equivalent diameter in tensile test

The client doesn't know themselves.
I would use the OD.
And you need to also report the value with a 2" GL in order to comply with A213

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

### RE: Equivalent diameter in tensile test

Theoretically, equivalent diameter = Squad (OD^2-ID^2)

Practically, you can use equivalent diameter = (OD+ID)/2. Or, simply use equivalent diameter = OD

Intrinsically, all probably donot matter much, as Ed pointed out, since 316 material would have a large elongation value (unless the deformation rate >95% during manufacturing).

The key is to clarify with the customer.

### RE: Equivalent diameter in tensile test

(OP)
Ok thanks for the information. Obviously it is something that I asked the client, but sometimes the answers tend to come too late.

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