Jumps in Shear Stress after a Step

[RShah7]

Hi I am fairly new to Abaqus. I am modeling the interaction of a steel probe
with a hyperelastic material. Both are 2-d shells. The hyperelastic material
is a square. The tool touches the square at the center of one edge. There
are 2 Static, General steps. In step-1, the tool pushes into the square
material with a displacement inwards. (Ramped). In step-2, nothing new is
added (the tool remains at the penetrated depth of about 0.5% of the
material length). The tool is constrained to move only into the
hyper-material and not along the edge, and the opposite edge of the
hyper-material is fixed.

After meshing and submitting the job, when I view the plot of the shear
stress of the hyper-material center node (where the tool touches it), it
gives me a sharp drop after step-1, as if the tool were lifting off the
material and then rises again, as if the tool had been pushed in again. (I
am not sure this is the possible case, though. It was the best way I could
interpret it)

I don't understand this effect- could someone please explain it to me?

Is it possible that Abaqus deletes the BCs of step-1, and then applies those
of step-2? Other quantities (like normal stresses and reaction force on tool
become zero after step-1).

Sorry for the long message.

 

[IceBreakerSours]

Open the boundary condition manager and check if the displacement boundary condition gets promoted to step 2 or not. If not, correct it appropriately.

http://www.eng-tips.com/faqs.cfm?fid=376

http://www.eng-tips.com/faqs.cfm?fid=1083

 

[RShah7]

Thank you for your reply. The boundary condition does get promoted to the next step. I also checked the contact pressure, and that is non-zero throughout the process, so I guess that loss of contact is not the problem. Could there be another reason for such a sharp change in shear? (It goes from 45 to -45 instantaneously). Is this kind of shear change noticed in any other situations, so that perhaps I could draw a parallel?

 

[IceBreakerSours]

What are you doing in step 2?

http://www.eng-tips.com/faqs.cfm?fid=376

http://www.eng-tips.com/faqs.cfm?fid=1083

 

[RShah7]

Nothing. That is, nothing new. The probe remains at the displacement that it reached at the end of step-1 (it deforms the tissue accordingly).

 

[IceBreakerSours]

Plot the stress with respect to time. I think its the jump in increment size that makes you think there is a jump in stress.

http://www.eng-tips.com/faqs.cfm?fid=376

http://www.eng-tips.com/faqs.cfm?fid=1083

 

[RShah7]

I am plotting it against time- I've plotted the odb field output (unique nodal, S12) for the node of the material in contact with the probe.

 

[IceBreakerSours]

I guess I should have said it differently. Import the contents of the .sta file in to EXCEL and extract the stress data (simply copy the data in the table) and put it in EXCEL. Plot the stress with respect to a few time columns (I can't remember which one off the top of my head).

http://www.eng-tips.com/faqs.cfm?fid=376

http://www.eng-tips.com/faqs.cfm?fid=1083

 

[RShah7]

I'll try that. Thanks.