Non linear springs and gap

[Elisa Monti Di Sopra]

Hello, I am trying to use springs with non linear properties in a simple model to check the results of the NL analysis. I am encountering two problems: the first one is in the definition of spring properties in the Assign menu. When I add the spring property by first defining the name and creating the property, then checking the NL flag and defining the gap property for a given direction, I am not able to uncheck the NL box any more, I can only add new NL properties for other direction but not uncheck them. This happens also if I create a new spring property, it takes the NL checked properties from the previous spring (even if I uncheck them, define the new name and click on Add property) and doesn’t let me uncheck them by clicking “Modify”, so I cannot create different spring properties.
The second problem I encountered is in the analysis. I have created two cases, one linear static and one nonlinear static, with the same forces applied, one uses the elastic stiffness of the spring, the other the closure stiffness, both have the same value so I should get the same results for the two analysis but I don’t, as shown in the attachments. The solver processes the analysis but I get an unstressed state for the non linear analysis. The spring is applied only to one of the four upper nodes and the second node has all DoFs constrained except for Z.

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[NextFEM Admin]

Hello,
to correctly use the Spring properties mask, please follow the subsequent steps:
– choose a spring name and then click “Add property”; you created and empty spring
– then, the spring is selected and active on the left box. Change its properties as you wish
– click on “Modify property” to store changes.
By now, removing NL dofs is not supported. We’ll add this to the check list for next updates.

For the second issues, please share models to let us check. You may upload them there, or load them at the bottom of the Support page.

[Elisa Monti Di Sopra]

Regarding the creation of a new spring property, I applied exactly the same steps you described above, but since a spring property with NL DoFs was already created, it is not possible to add a new one without NL properties. So I have to delete the first one and start over, creating the one without NL properties first and then the second with the NL for one or more chosen direction, but I still cannot create a third one with a NL property for a different direction because I cannot uncheck the choice from the previous one. Maybe it’s because, as you said, removing NL dofs is not supported and this affect the creation of a new property too?
For the second question, here it is the model. Thank you in advance.

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[NextFEM Admin]

Just to clarify the process, you cannot duplicate directly a spring property. You cannot also create a spring with the same name (as appearing in Name textbox).
Also, there’s no need to replicate the same spring without NL properties, in linear analyses only linear properties are considered; the NL properties set act only in non-linear analyses.
To conclude, you’re able to create a new spring with a different name by re-setting the properties.
ps.
If your spring don’t have zero length, maybe a local coordinate system (local CS) is suitable for you.

For the model: you don’t get 0 stress for the NL model – you’re at time 0, select the last time and you’ll see the results.
ps2
if you’re trying to hang the box by an upper node, maybe you’re looking for a hook spring.

[Elisa Monti Di Sopra]

I’m not trying to duplicate directly the spring. What I meant is that when there’s already a spring property that you created before and you enter the assign property window, it shows you the property you already created, and from there you change the name and create the new one, but if you checked the NL box in the previously created spring property it stays checked even with the new one. The only way to avoid interference from the previously created NL spring when creating another for a different direction is just to set to 0 Kclosure for the direction you’re not applying any property. But if you could make NL DoFs removable in the future, that would be much appreciated! Thanks for the help and sorry about the analysis, I used to work with another software that showed the final step right away.
Not trying to hang the box, but thanks for the advice!:)

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