# Divergence Issue in Simulation of Flexible Membrane

Hi there,

I am currently simulating the deformation of a flexible membrane (200mm diameter, 2mm thickness). Unfortunately, the calculation consistently diverges at a certain time step, with the following error message:

`````` iteration 1

Using up to 1 cpu(s) for the stress calculation.

Using up to 1 cpu(s) for the symmetric stiffness/mass contributions.

Factoring the system of equations using the symmetric spooles solver
Using 1 cpu for spooles.

Using up to 1 cpu(s) for the stress calculation.

average force= 2928585056663572563823814321527470231126016.000000
time avg. forc= 1008465928603158625915133441517266403328.000000
largest residual force= 11529784025529486236246453018676178542187773952.000000 in node 407 and dof 3
largest increment of disp= 2.488520e+00
largest correction to disp= 2.488520e+00 in node 472 and dof 2

no convergence

iteration 2

Using up to 1 cpu(s) for the symmetric stiffness/mass contributions.

Factoring the system of equations using the symmetric spooles solver
Using 1 cpu for spooles.

Using up to 1 cpu(s) for the stress calculation.

average force= nan
time avg. forc= 0.775881
largest residual force= 1136001680829127150529799508419411582124032.000000 in node 445 and dof 1
largest increment of disp= 6.542111e+00
largest correction to disp= 6.542111e+00 in node 524 and dof 3

divergence; the increment size is decreased to 2.500000e-04
the increment is reattempted

Adjusting time step for transient step
precice_dt dtheta = 0.000017, dtheta = 0.000004, solver_dt = 0.000250
Adapter reading coupling data...
No suitable eForces fuction!!! Please check WEC type!!!
Reading FORCES coupling data with ID '2'.
increment 2904 attempt 3
increment size= 2.500000e-04
sum of previous increments=2.872000e+00
actual step time=2.872250e+00
actual total time=4.872250e+00

iteration 1

Using up to 1 cpu(s) for the stress calculation.

Using up to 1 cpu(s) for the symmetric stiffness/mass contributions.

Factoring the system of equations using the symmetric spooles solver
Using 1 cpu for spooles.

Using up to 1 cpu(s) for the stress calculation.

average force= 317284113003602816.000000
time avg. forc= 109257614670662.890625
largest residual force= 1203916751776381927424.000000 in node 1207 and dof 3
largest increment of disp= 4.040671e-02
largest correction to disp= 4.040671e-02 in node 1253 and dof 2

no convergence

iteration 2

Using up to 1 cpu(s) for the symmetric stiffness/mass contributions.

Factoring the system of equations using the symmetric spooles solver
Using 1 cpu for spooles.

Using up to 1 cpu(s) for the stress calculation.

average force= nan
time avg. forc= 0.775881
largest residual force= 2218609082492506966326937449053849005494108160.000000 in node 1307 and dof 2
largest increment of disp= 1.893813e+00
largest correction to disp= 1.893813e+00 in node 1355 and dof 3

``````

I have tried several approaches to resolve the issue, including:

• Using smaller time steps
• Refining the mesh
• Testing different element types (C3D10, C3D8, C3D20, etc.)

Despite these efforts, the problem persists. I would greatly appreciate any insights into potential causes for this divergence. How can I make the simulation more stable?

Thank you in advance for your help!

CalculiX version: 2.15

can you share the .inp and a picture of the mesh?

Why arenâ€™t you using shell or membrane elements for this model ? How is the part supported and loaded ? What material model is used ?