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Strain rate and temperature dependent compression stress relaxation with TNM in Abaqus

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Dear Dr.Jorgen,

Thanks for replying to my previous inquiry, I really appreciate the time taken to help me. For some reason, the reply button did not appear for me so I could not thank you there. I have another question regarding TNM.

When I export my TNM model to my abaqus compression stress relaxation model to verify the TNM model predictions, I apply the same initial stress force (10 MPa) with a general static displacement step with 180 s duration (same as my experiments) then a visco step for 8 hours. The model predicts the relaxation accurately compared to my experiments for (23,40,60,and 80 C). However, things get different when I apply the force (10 MPa) using a general static displacement step with shorter duration than my experiments (30 and 60 s) i.e. higher strain rates. For higher temps like 80 C, when I shorten the displacement duration, the stress relaxation happens faster and the final stress after 8 hours is about 15 to 25% percent less. However, for lower temps such as 23 and 40 C, using a shorter displacement step slightly increases the stress drop or does not even change it. I cannot quite understand why is this occurring, is it the TNM model or Abaqus model or HDPE actually should behave like this in the real world. 

Thank you very much for your time and I will be waiting for your reply.