Dodd-Restrepo material

[seiche]

Hi, I'm having some problems with using the Dodd-Restrepo material. I'm modelling a simple spring and giving it a push-pull cyclic load. I get good results with steel02 and Ramberg-Osgood material, however when I use the Dodd-Restrepo I get negative deformations even if I'm applying a positive force. I'm using the material like this:

#uniaxialMaterial Dodd_Restrepo $tag $Fy $Fsu $ESH $ESU $Youngs $ESHI $FSHI <$OmegaFac>
uniaxialMaterial Dodd_Restrepo 1 350. 500. 0.011 0.075 200000. 0.022 391.0

The units are in mm and N (therefore MPa).
Somewhere along the analysis the NewtonRaphson algorithm fails to solve.

In the commands manual there is a note that says "Stresses and strains are defined in engineering terms, as they are reported in a tensile test" although I don't know what this means. I tried using strain values in % but this didn't work also.

Thanks in advance for your help!