Truss elements and precondition 1

[ardalansv]

I want to simulate a case in abaqus that my truss element does not get activate in the very beginning. As a clear-cut example, assume its first length is 2 mm. I want it to become active when it gets to let say 6mm. So I want to define a condition for it, that it gets to a certain point of length and then become active.

I would be happy for any input or comment you may have.

Regards,
Ardalan

 

[IceBreakerSours]

What are trying to model? It sounds like you might want to assume a hyperelastic material model for the truss. This method is the preferred one.

If, for whatever reason, you really want linear material models, then you could have two trusses with their nodes equivalenced (i.e., nodes on top of each other in the exact same geometric location), with two different materials specified to each truss. Alternatively, you could use a field variable to change the stiffness as the field variable changes. While the latter approach is simple, either of these methods will create a bit of a headache for the solver because the stiffness is not going to be smooth.

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[ardalansv]

Dear Icebreakersours, Actually yes I want to assign hyper elastic material properties to the truss elements. I want to simulate two different blocks connected by a truss elements (such as tether or rope that gets lax). after making the blocks close to each other, the trusses are also getting shorter in length (however in the reality the rope gets lax). From now on, if I pull the blocks the truss elements would be tensioned. but I want them to react as they were in their initial position, since the rope would get to the first position and then becomes active.

Regards,
Ardalan

 

[Mustaine3]

I'm not sure if your approach would work. Maybe it would, but then you have to write at least a UAMP subroutine to change the values of a field variable to change the material properties.


Maybe this is easier:
Split the rope/beams in the middle and add a connector element in between. Use connector stop to indicate that a specific movement is not allowed. With that, the connector can be compressed without resistance, but lengthening is not allowed.

 

[ardalansv]

Dear Mustaine3, In that way I would most probably get solver problem errors. since I have many of these truss elements in my model. But I would give it a try.

Regards,
Ardalan

 

[Mustaine3]

Also check if the material option
*No Compression
could be helpful.

 

[IceBreakerSours]

I do not think I understand. Can you explain the application? It seems you are trying to simulate a surgery. What surgery is it?

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[IceBreakerSours]

*No compression, as suggested by Mustaine3, is the most straightforward option. However, at the transition from tension to compression, the solver will hit a bump. Robust commercial solvers can handle such discontinuities to a (small) degree, especially if there are few nonlinearities. If the model contains several nonlinearities (contact being the worst), then the solver will likely run in to trouble.

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[ardalansv]

IceBreakerSours I am simulating disc degeneration in my model. The vertebrae are connected using ligaments (truss elements). I want to change the length of the disc and do range of motion tests. So I need the ligaments to lax. I want to change the behavior of the ligaments to become active when they get to their initial lengths.

Ardalan