Strain Rate Sensitivity....

[NickE]

How sensitive is 17-7ph;C to strain rate?

We're currently making a part (stamped) and forming the shape in a real slow hydraulic press. We're about to gear up for production and want to use a stamper to form the shape. No stretch is being applied to the steel just a light bending. (15-45 degrees or so.

If my press head is moving roughly .167ft/sec.

Will I have to worry much?


Nick

 

[smslab]

What is it you are worrying about?
For whatever it is worth, I did a lot of tensile tests years ago at various strain rates. The reason is that we were having trouble comparing tensile data (done at a low strain rate) with fatigue data (at 100 Hz the strain rate was several orders of magnitude higher).
The equation for the flow stress is
sigma = (d e / dt)*m x e*n, where n is the work hardening exponent and m is the strain rate sensitivity. At room temperature we tend to assume that m is zero, and at high temperature we tend to assume that n is zero.
In fact, m is not zero at room temperature. I got values of aout 0.015 for aluminum and titanium at room temperature. What that means is that if you know the flow stress at one strain rate and then go and change the strain rate by at least a factor of 1000, you will see a difference in flow stress.
Hope this helps.