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Nonlinear study elastoplastic - reaction force to high, plasticizes to early

Question asked by Juan Rodríguez on Nov 8, 2018
Latest reply on Jan 23, 2019 by Bill McEachern

Hello Community,


I am having troubles with the following (spring) geometry:

As you can see it´s basically a spring element which is locked in position at the bottom surface and gets loaded from the top. My goal is to calculate the stiffness (N/mm) of this structure in the elastic region and the elastic limit force before permanent deformation.

My results show way too high reaction forces when compressing the spring compared to my experimental data.

I am conducting a non linear simulation (Plasticity von Mises Model) which shows the following Force <-> Deflection behaviour.


Simulation ResultExperimental Data


Time 1s equals 0.1 mm (edited) of Displacement which gives me a stiffness k of approx. 30000 N/mm in the elastic region.

The experiment shows the same degressive trend of the force - but with a much lower stiffness in the elastic region and a higher maximum force.
(Stiffness 10x lower, maxForce 3x higher)

Big Deformations
Big Displacement
Initial Time Step: 0,01
Intel Direct Sparse

My thoughts:
Reason for that behaviour could be for a too coarse mesh, an overconstrained model or wrong material parameters (EMod; ETan; YielStrength).
I doublechecked all those parameters and tried different mesh refinement (AspectRatio & Jacobian seem to be ok) but i never came close to the experimental data. I dont think shear locking is an issue here ...

- so are there any other reasons which could increase the stiffness and decrease the maximum Force?