Fix nodes after application of a loading

Hi everyone,
I am doing an analysis made of two parts on the elasticity of a rectangular plate of length L_0:
in the first part I stretch a plate with a fixed force, the plate is deformed and reaches a length L_1
in the second part I apply a force at the center of the plate.

I would like the length of the plate in the second part to be fixed and equal to L_1,
how can I do that?

Thanks