I performed uniaxial tensile tests on notched concrete cubes and now want to model them in ATENA. In the simulation, I apply a displacement and measure the resulting support reactions on the cube as well as the force at the point of applied displacement. These forces are equal in magnitude.

Unfortunately, after reaching the tensile strength, the force-displacement diagram remains constant at the tensile strength level and does not show the typical softening behavior expected for concrete.
What could be the reason for this?

Thanks for your help