Spring Stiffness Recovery after a pause in cyclic loading

[bsg001]

I am cycling the (wave)spring between its min and max operating length and after a certain no. of cycles(10k cycles) I pause it and perform a quasi-static load vs deflection test. Load vs deflection data is collected by fully unloading the spring and compressing to the minimum operating height (twice, ~10minutes). After that, the cyclic test is resumed. But when I resume I see a higher stiffness(load) at the minimum operating height. What could be the cause? does the spring recover some of the stiffness in that short break?

 

[GregLocock]

What's it made of? What are the end constraints?

 

[EdStainless]

What is the loading frequency?
How are you keeping the temperature of the spring constant?
We used to fatigue test wire is tanks of oil to minimize temperature fluctuations.

 

[desertfox]

Hi

Is it possible the spring wire has work hardened ?