Stiffness of specimen under tension

[Sadeq]

Hello,

I am conducting FE modelling on specimen of steel surrounded by concrete,
and i obtain curve which have stiffness similar to experiment(all assume linear behaviour for all materials)
but the start of curve is not convenient
displacement
-0.0071033
0.0123265
0.0310888
0.059264
0.101501
0.164798
0.259773
0.402099
0.615813
0.93855
1.42313
1.87749

force
-5.13E-10
7006.94
8918.8
11488.5
15339.3
21095.9
29776.8
43126.5
63036.4
90018.3
129831
169167

the negative displacement at the beginning is anticipated due to the fact the sample was preloaded(shrink in length)

So what i did is shift the curve to the right by subtracting all displacement value from the initial one
and maintain the same values of force

AND I got the following curve which is not convenient in the beginning
so did i do maths error when i shift the curve or what?

ANY HELP IS APPRECIATED

please consider the blue line

 

[CWatters]

Not my field but...

Don't you need to know or work out the preload force and subtract/add that as well?