I don't know what you mean by "solid 6t".

When you say you used a "stress strain curve", which material model was that in, and did you model compressive stress-stain separately from tensile stress-strain?

What failure criteria are you using?

When bending a concrete beam, the failure always starts on the tensile side because concrete is so much weaker in tension than compression.

The solver might stop because the step size was too large.

Are you using a Force at the center or a Displacement to bend the beam?  If you use a Force, the solver will stop because there is no static equilibrium beyond the current solution. You have reached the ultimate strength.  If you use a Displacement, the solver can continue past the point of ultimate strength and plot the Force Displacement curve on the downward slope in the post-failure portion.

There are many discussions on this website that describe each of these suggestions. You will find them more easily if you use Google to search the site than the site search bar.