Multistep Analysis for Ramped Stress Relaxation

[nyelaura]

Hello,

I would like to fit data for a ramped stress relaxation experiment. We apply a strain and allow the tissue to relax for a period of time and then apply another strain. There are 5 total consecutive strains.

1) Is it possible, in preview, to set multiple prescribed displacements and allow the material to relax for a period of time? I seem to be able to get a second prescribed displacement but then can't get the material to relax afterwards.

2) I was also wondering why the first pull takes ten time steps while the second is instantaneous?

3) Lastly, after my second pull, I see the stress increases rather than relaxes like after the first pull. Is there a reason for this?

bladder_26.feb

Thank you!
Laura

[maas]

Hi Laura,

If I understand what you are trying to do, then I think it is definitely possible, and looking at the file you are on the right track. I hope the following will help you get the model working.

1. You can do this using multiple steps and only defining the boundary condition in the steps you want to displace. In your current model you have conflicting BCs since you are defining the first displacement as a permanent BC, which will persist through all the time steps. You will have to move that one to the first step. If you want to follow this step with a relaxation step, then don't define any displacements or loads in this step.

Also, both your load curves go from time 0 to 1, but your first step goes from 0 to 5 and your second step from 5 to 6. You have to make sure that your load curves work in the time range of the step in which they are used.

2. This last point explains why the second displacement appears instantaneous: The second step starts at time 5, which is already past the peak of the corresponding load curve. If you wish to add to the displacement a rigid body has you should make it a relative displacement:

<prescribed bc="x" lc="2" [B]type="relative"[/B]>0.5</prescribed>

3. I'm not sure about this one, but maybe after the BCs are setup correctly we can revisit this if it persists.

I hope this helps. Let us know what you find.

Cheers,

Steve

[nyelaura]

Hi Steve,

I ended up setting the rigid constraint to be in step one and then just varied the load curve by adding additional points to it and it works pretty well! Not sure what was causing my problem from question 3 but it's working now as well.

Thank you,
Laura