NONLINEAR SPRING modeling 1

[lfmarini]

Hello guys,


I'm working with FEMAP V10, using NX Nastran, and I have a problem in modeling a nonlinear spring.
Using a DOF element element, I've chosen a Function Dependence stiffness and then I entered a function of Force Vs Displament, using a Dimensionless function.
The type of solution I used was 601
Then the spring work as if it was a Linear spring...

I've made an simple model and tested diffent types of solutions (106, 101), differents types of functions , and different types of elements (Spring/damper) nothing changed..

Anyone can help?

Thanks in advance, lfmarini

 

[BlasMolero]

Dear lfmarini,
Without the model in hand is difficult to assess anything. Please note unlike the spring/damper element which acts along the line between the elemental end points, the DOF SPRING element you choosed connects two nodal degrees of freedom - independent of their orientation relative to each other. Like the spring however, you can specify both stiffness and damping. The "Force vs Displacement" function allows you to specify a nonlinear behavior for the spring as it extends.

Best regards,
Blas.

 

[lfmarini]

Dear Blas,

Thanks for your answer, particularly it's my first post at this forum.

My problem is in specifying this nonlinear behavior in the analysis. I've tried even DOF elements and Spring/damper elements, both didn't follow the "Force vs Displacement" curve I've specified in element property.

Do u have a simple example showing this nonlinear behavior? It's a quite simple problem, I guess.

Thanks

lfmarini

 

[BlasMolero]

Dear lfmarini,
First you must solve the problem using a constant spring stiffness for both linear static with small displacement (SOL101) and nonlinear static with large displacement (SOL106 and "PARAM,LGDISP,1"), then request the output of all grid displacements and all element forces, and compare results. You will realize that nonlinear displacements are smaller than linear ones.

Let's suppose you use a spring stiffness k=1000 lb/in, and you have a linear spring resultant displacement of say 30 in with a spring axial force of 30e3 lb, and solving the problem as nonlinear the spring grid displacement is only 10 in and spring axial force is 10e3 lb.

Well, you see where I go: depending the point values you enter for the Spring force vs. displacement curve you may run all the time in the same segment for both linear & nonlinear solution. If your first point is (0,0) and second point of the force vs. displacement point is say (30e3,30) the rest of the points doesn’t matter because the spring will never reach that values.

Best regards,
Blas.

 

[lfmarini]

Dear Blas, thank you very much for your help, it worked and finally I could finish this job.

Thanks