We have been doing cyclic tests using various UCS values at different max. load and amplitudes. However, as UCS is not a fixed value for even same rock type, some of the rock samples did not fail even after 80,000 cycles (at 1Hz freq). In the literature, there have been strain controlled tests conducted. Once again, even the strain shows large variation for the same rock types. I wonder if anyone has any other recommendations on this problem.
We started with trapezoid loading then switch to square loading. (Instron) Firstly we used 95% of UCS as Max. load and 95% of Amplitude. Failure, as expected, happened within 10 t0 50 cycles. We kept the 95% of UCS constant and changed the amplitude. Lower than and including 85% of amplitude, rock samples did not fail. So, we had to move back to monotonic loading to crash the samples. We found that max failure occurred at a value of 20% higher than UCS of what we measured.
Mohan, we used 42 mm diameter cylindrical sandstone samples (L/D ratio is around 2.5).
I performed a series of increasing cyclic loading on a hard, brittle rock in different modes of sinusoidal amplitudes. But on Brazilian and CCNBD samples.
I suggest that you do tests in Four point bending. You'll have pure bending stresses and no shear stress effects.
Good luck
Thank you very much Mohan. I'll try that. We are also conducting 3D numerical modelling of uniaxial cyclic loading for rock fatigue tests using ABAQUS. We might find some answers from this modelling.
Thanks Morteza. I read your article. We are conducting uniaxial compressive cyclic loading which is different than Brazilian and CCNBD tests.
You are welcomed Murat. Have you got different results compared to the monotonic one, as I've got quite different.
We have some interesting results and will publish them once we finish the remaining tests. Certainly degradation of the rock strength over the long cycles can be realized by the change in the stiffness curve. Some tests also indicate strain hardening.
Dear Mr,Karakus I'd be thankful if you introduce me some good articles about modeling of rock's behaviour under cyclic uniaxial loads in abaqus and which model you think fitts best with that?
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