I am having Convergence issues for a non-linear problem
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For context the simulation is the expansion of a coronary stent via an expanding balloon, please see the attached images of the geometries as well as the mesh.
A displacement is applied to the balloon in a 2 step configuration which expands the stent from 3mm to 4.5mm. The stent is modelled as a PLLA (polymer) bilinear isotropic hardening and the balloon as a mooney-rivlin 2 parameter.
In the Solution Output section under the Displacement/Force etc Convergence, the simulation will reach around 17% solved and then struggle to converge. After the maximum number of substeps it stops the solver but adding more wouldn't reach convergence I don't think. I have tried some of the troubleshooting suggestions but no luck.
I will add that a previous simulation with a different geometry, which has much less elements in both the balloon and stent solves in ~40 mins. All the settings from the previous simulation were kept the same for this new simulation so I'd expect it to solve whilst taking longer, but it doesn't solve at all.
I will attach as much information as possible in the contact, analysis and displacement settings. Any help would be greatly appreciated!
I will attach some screenshots of the solver output. Yes I closely followed that course but with a different geometry, mentioned briefly in initial post, I will attach a photo of the mesh. The simulation ran fine for that simulation which is a steel stent and a simpler geometry. It also ran fine when I changed the material to a polymer. The problem arises when I change the geometry to that of the initial post, the set-up is all the same just a different geometry and thus mesh.
I was thinking that potentially the mesh in the initial post might have some more irregular element shaping/sizing. I'm not sure if that would drastically hinder the simulation. I tried adding a face meshing to the stent and got triangular element shapes instead of quadrilaterals. However, the simulation gets stuck at almost the exact same point which is around Time = 0.36, please see the force convergence for the new mesh.
I have only had the simulation end once, and I let it run for a very long time (~10-12hrs) but I didn't get a screenshot of the errors. I'll try to let this one run and get the errors for you. I recall the main one being that the solution didn't converge in the maximum number of sub-steps which leads to warnings about not using the data as it would be inaccurate. I will let this one run and post the final messages post simulation in a new comment. I have attached screenshots of the solver above for the new geometry. I should add I get the same messages for the initial geometry and simulation which solved in 40mins.
Thank you for your help!
I ran the simulation with a coarser quad mesh and received the following errors after the simulation ended. Do you think that the mesh isn't fine enough to run properly? Additionally, some of the elements are fairly skewed, although this is more noticeable in the initial finer mesh. I also changed the augmented lagrange to normal lagrange.
I will copy and paste all of the error messages below in the order of the image above:
You have specified result time(or frequency) = 2 for a result file which has maximum time(or frequency) = 1.If you specify 0(ZERO), the solution for final time will be displayed.
You have specified result time(or frequency) = 2 for a result file which has maximum time(or frequency) = 1.If you specify 0(ZERO), the solution for final time will be displayed.
You have specified result time(or frequency) = 2 for a result file which has maximum time(or frequency) = 1.If you specify 0(ZERO), the solution for final time will be displayed.
The solver engine was unable to converge on a solution for the nonlinear problem as constrained.Please see the Troubleshooting section of the Help System for more information.
The solution failed to solve completely at all time points. Restart points are available to continue the analysis.
Large deformation effects are active which may have invalidated some of your applied supports such as displacement, cylindrical, frictionless, or compression only.Refer to Troubleshooting in the Help System for more details.
Although the solution failed to solve completely at all time points, partial results at some points have been able to be solved.Refer to Troubleshooting in the Help System for more details.
The unconverged solution (identified as Substep 999999) is output for analysis debug purposes. Results at this time should not be used for any other purpose.
Element 2553 located in Body "Stent" (and maybe other elements) has become highly distorted.You may select the offending object and/or geometry via RMB on this warning in the Messages window.Excessive distortion of elements is usually a symptom indicating the need for corrective action elsewhere.Try incrementing the load more slowly (increase the number of substeps or decrease the time step size).You may need to improve your mesh to obtain elements with better aspect ratios.Also consider the behavior of materials, contact pairs, and/or constraint equations.If this message appears in the first iteration of first substep, be sure to perform element shape checking. Named Selections for the offending element can be created via the Identify Element Violations property on the Solution Information Object.
One or more MPC contact regions or remote boundary conditions may have conflicts with other applied boundary conditions or other contact or symmetry regions. This may reduce solution accuracy. Tip: You may graphically display FE Connections from the Solution Information Object for non-cyclic analysis. Refer to Troubleshooting in the Help System for more details.
One or more objects may have lost some scoping attachments during the geometry update. You can identify these tree objects by filtering the tree using the Scoping option set to Partial.
Many thanks for all your help thus far!
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