Stiffness of specimen under tension

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SUMMARY

The forum discussion centers on the finite element (FE) modeling of a steel specimen surrounded by concrete, where the user encounters issues with the initial stiffness of the force-displacement curve. The user reports that while the stiffness aligns with experimental results, the initial displacement values are negative due to preloading. To correct this, the user attempted to shift the curve by subtracting the initial displacement from all values, but the resulting curve remains problematic. A suggestion is made to consider the preload force in the adjustments to achieve accurate results.

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Sadeq
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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
 

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Not my field but...

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

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