How to define material parameters ?

Greetings,

I have a query regarding the Force versus Displacement (pink) curve obtained from a structure after conducting a 3-point bending simulation, as illustrated below. I am interested in creating a material that behaves same under the same conditions.

I have determined the Flexural modulus for the elastic part using the 3-point bending formula. However, for the plastic zone, I am unsure about extrapolating strain and stress values and how to implement them in a material law, such as LAW 36 (Piecewise linear), to generate a Force versus Displacement curve (Red curve) that precisely replicates the depicted one. 

PLease help me !! 

Figure 1: Force vs Displacement (pink is reference curve Red curve is what i'm trying to find to replicate pink)