elongation results in ASTM E8 samples are scalable? 1

[kevlar49]

Can someone confirm for me that since ASTM E8 holds constant the Length/Diameter of the tensile coupon that the elongation at 1" gage should be the same as 2" gage?

 

[EdStainless]

If you are using round bars, and the diameter to gage length ratio is kept constant then you should see similar elongations for all sample sizes.
However, little things like grain size and strain rate can have a large impact depending on the alloy and test method.

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[redpicker]

If you are dealing with round-bar specimens with a L/D ratio of 4, you are correct.

Some testing specificaitons require testing of full section or strip samples, which specify a 2" gauge length (or, sometimes, an 8" gauge length). With these specimens, you will have to look in the product specification as to how to deal with the elongation requriement when testing alternate specimens.

rp

 

[TVP]

For flat specimens, elongation is not the same with a 1" gage length as it is for 2". In my experience, the amount of elongation from a 1" specimen will be larger than that for 2" specimen, usually by several % points. For example, one test on cold rolled steel with a tensile strength of 700-730 MPa showed elongation of 3-4% in a 2” gage length, but 8-10% for 1”. Yield and tensile strength does not show the same sensitivity.

 

[EdStainless]

The more ductile a metal is the less impact the gage length will have on apparent elongation reading.
In annealed 304L stainless you will get >50% elongation, and it is spread out over a lot of the sample length, so measuring over l/d=8 will only give a value slightly lower than using l/d=4.
We also do some high strength SS (cold worked to 150ksi UTS) that for l/d=4 we get 30% but l/d=8 the value is 20%.
In high strength materials the deformation will tend to be concentrated near the fracture so the longer the gage length the lower the apparent elongation.

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