Positive value means displacement in the +X direction whole negative value means displacement in the -X direction.
To be honest, I’m not sure how it works here because I’ve never had to use initial displacement in CalculiX and Abaqus doesn’t even have it.
I would run a simple test externally first:
*NODE,NSET=NALL
1,0.,0.,0.
2,10.,0.,0.
*ELEMENT,TYPE=SPRING2,ELSET=EALL
1,1,2
*BOUNDARY
1,1,3
2,2,3
*SPRING,ELSET=EALL
1,1
10.
*INITIAL CONDITIONS, TYPE=DISPLACEMENT
2,1,1
*STEP,NLGEOM
*STATIC
*BOUNDARY
2,1,1,2.
*NODE FILE
U
*NODE PRINT,NSET=NALL
U
*END STEP
This is a model with just one SPRING2 element acting in the X axis. Node 1 is fixed and node 2 is pre-stretched, then pulled. For me, there’s no difference with the pre-stretch, IMO due to the last point here. For the same reason, you can’t pre-stretch it in two steps. Thus, special connector elements are used in Abaqus instead.
You can’t use thermal expansion either because springs don’t have material assignments. For me, amplitude within the same step would be better. Assuming that the DOF is not constrained by attachment of the spring to the model.
Do you understand the whole syntax of this short input deck or would you like me to explain something there ?
To sum up, I would use beam elements instead.