ASTM E8 Help 3

[ME231]

Greetings to all...
We are trying to set up a tensile tester (United) in order to perform some tests per ASTM E8. As I read thru E8, I am having some trouble understanding the differences between constant strain rate and constant cross head speed. In my mind the two are the same since "strain on specimen" should be equal to "distance moved by cross head". Am I missing something?

Also, how do I determine a constant stress rate for an unknown material? E8 seems to suggest that the constant stress rate control method is more prevalent. However, in my mind the constant cross head speed method seems more convenience - please comment.

As you can probably tell, I am a novice at this am and trying to learn. Any info would be much appreciated!
Best Regards and thanks in advance.

 

[Guest102023]

I think the distinction may be between true strain and engineering strain. The latter is proportional to crosshead speed. The former is based on the instantaneous cross-sectional area, a bit tricky to obtain during a test.

Look here

http://en.wikipedia.org/wiki/True_strain#Engineering_strain

 

[EdStainless]

I think that in practical terms the bigger issue is trying to control the load rate in a hydraulic machine.
Our older ones are not closed loop control. This a real trick.
There are a number of material specifications that call out required ranges for strain rate.

Constant stress rate just means that the load increases in a roughly linear fashion.
The real issue is which one are you controlling, crosshead location or actual load?

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