*SUPERELASTIC nitinol built-in definitions

[ryankb]

I am working on transitioning over to the built-in *Superelastic definitions of nitinol in Abaqus, and at the same time working to include plasticity (new model requires very high strains). I'm having a rough time including the Material Hardening and Material Hardening Modifications.

I built a simple unit-cell mesh and pull on it with various amounts of strain. Here's the output:

There's obviously a few things wrong here, so let's tackle them one at a time (they may actually be linked, but I'll discuss them separately):

Upper plateau stress superelastic window:
The end of my superelastic window should be around 6.5% for this material model. Note, however, when going beyond the martensitic elastic region that with each subsequent loop (12%, 14%, 16%) that the original length of the upper plateau is preserved, so I seemingly have a material which exhibits elasticity almost to 10% after a 15% pull. This is obviously nowhere near correct.

Degradation of plateau stresses with subsequent loops:
I've attempted to use the Super Elastic Hardening Modifications table provided, but the documentation is woefully inadequate (as I've found typical of Abaqus) to actually help a user correctly use the features rather than just tell what is is. I'm wondering if I were able to use this correctly then the length of the upper plateau might also fall in line?

Any thoughts?

 

[Dave442]

hard to clarify the issue without the material definition. Can you share that?

If I were you I would try the following:
superelastic material model - apply tension to 6%, unload, reapply tension to 20%. verify output
superelastic plastic material model - apply tension to 6%, unload, reapply tension to 20%, verify output
add plateau stress adjustment - apply tension to 6%, unload, reapply tension to 20%. verify output

there is additional info on the superelastic material models on the Simulia Learning community.

 

[ryankb]

Dave-

I've completed those steps, and it all works just fine. The problem arrises, however, when doing the following:

tension to 6%, unload, reapply tension to 12%, unload, reapply tension to 16%, unload, reapply tension to 20%

When tensioning to the 16% and 20% loops, the upper plateau remains as long as it did during the 6% loop, but due to plasticity it started several percent later, thereby making it appear as though the upper plateau can go out to 8, 9, 10% after initial plasticity has begun. Using the hardening suboption I was able to modify this a little, but it isn't great.

With as common as NiTi is used in the med device industry, it baffles me why Simulia hasn't created a better/more friendly method than users writing/using complex user subroutines to adequately capture the stress/strain behavior of the material.

 

[Dave442]

why does your stress remain positive. Are you plotting max principal stress?

Plot the directional components of stress/strain. if you loaded the element in the X-direction plot LE11 vs. S11.

if you plot the directional components i think you can see the material model is behaving correctly.

 

[IceBreakerSours]

With as common as NiTi is used in the med device industry, it baffles me why Simulia hasn't created a better/more friendly method than users writing/using complex user subroutines to adequately capture the stress/strain behavior of the material.

There is no user friendly method to capture stress/strain behavior of NiTi simply because the material itself isn't user friendly

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