I am using sand hypoplastic model in my research and I have encountered some problem when looking into cyclic simple shear test in the range of small strains.
Firstly, I have tried to simulate cyclic simple shear test from Shahnazari & Towhata (2002), Fig. 5, on Toyoura Sand. It is in the range of medium strains, up to 3%. The simulation looks really good (see attached pdf file). The sample contracts systematically with cycles thus the stiffness increases as per the experiment. Note contraction is with negative sign, Abaqus standard notation.
Secondly, I have subjected the same sample to cyclic loading but with significantly reduced strain amplitude, i.e. 0.1%. As expected, also here the sample contracts, however, the stiffness decreases with cycles which to me does not seem physical.
The model is calibrated as shown in Herle & Gudehus (1999) Table VII for Toyoura Sand. Intergranular strain parameters are set to also rather common values: R=0.0001, m_R=5.0, m_T=2.0, Beta=0.5, ksi=6.0. I have tried using both umats, from prof. Masin and prof. Fellin, both give the same answers.
I am wondering where is it I can be wrong here. Theoretically, I think I should be able to model sand behaviour for both strain levels with a single set of calibrated parameters.
Thank you all in advance for any comments and tips on how to approach my problem.