is it possible because of Mullin's effect?

Hello,

I am working on testing of rubber couplings. I have a problem, that while doing torsional test of the coupling, we are getting different values of stiffness with change in time. Even if we do the testing and then take a small break of 10-20 minutes, then also after 20 minutes I am getting different values.

My question is, that is it possible because of Mullin effect? If so, How can we check that the rubber couplings that I have are filled or not( one piece rubber or filler with some other elastomer)? 

I will be very thankful with any kind of help on this topic.

Thanks

Prahel Bhandari