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Strain Rate in Tensile Testing 2

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isoca

Materials
Mar 16, 2008
58
I am looking information on how to calculate strain rate in tensile testing if I only can control crosshead speed.
Thanks
 
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If you can determine the crosshead velocity (v) in/sec, the engineering strain rate for a tensile test is

v/Lo

where Lo is the initial gage length of the specimen.
 
Thanks for your answer but I am looking for a formula that consider the machine stiffness, for example, in order to obtain the actual strain rate over the material
 
If the machine isn't a few orders of magnitude stiffer than the sample then it isn't suitable.
You should be able to ignore it and consider the frame infinitely stiff.

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P.E. Metallurgy, Plymouth Tube
 
What ED is saying is true for strain rate. However, for actual strain measurements an extensometer on the specimen is used to eliminate the errors of machine stretch.
 
EdStainless said:
If the machine isn't a few orders of magnitude stiffer than the sample then it isn't suitable.
You should be able to ignore it and consider the frame infinitely stiff.

What you say may be valid for a standard tensile test, but doesn't apply to all cases. Slow strain rate tensile tests are corrected for machine compliance, as specified in NACE TM0198.
 
In my slow strain rate experience we monitored the sample optically, so we had actual direct reading of both strain rate and elongation. It was more an issue with the fixtures than the frame.
This is also true in tensile work. The frame should be 1000x as stiff as anything else in the system, but the extensions and grips are another issue.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube
 
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