Hi, as a first approximation, I would recommend using a multi-linear hardening model (MISO) in order to take into account the non-linear stress-strain response of ECC during tensile load. Here you can define a piece-wise linear stress-total strain curve according to your experimental data. I hope this will be helpful for you.

https://www.sharcnet.ca/Software/Ansys/17.0/en-us/help/ans_mat/amp8sq21dldm.html#miso