Convergence issue (general tips)

[Anonymous]

Hello,

I am having difficulty getting a model to converge. It is a model which involves very large deformations (but no contact). It runs very stable up to a certain point where it basically "runs into a wall". I am guessing that up to this point the behaviour is smooth, but then when the material at this point starts to stretch more and more, it gets much harder to find a solution. The problem can be generally described as pressurizing a bag.

This is what the convergence history looks like:

Time        Percent     DT       L Iter. Residual
   0.000000   0.00     0.010000  1   0   0.0000E+00
   0.010000   0.33     0.010000  1   1   7.9477E-01
   0.010000   0.33     0.010000  1   2   1.0709E-02
   0.010000   0.33     0.010000  1   3   8.5464E-04c
   0.020000   0.67     0.010000  1   1   6.6037E-01
   0.020000   0.67     0.010000  1   2   7.5254E-03c
   0.030000   1.00     0.010000  1   1   4.7514E-01
   0.030000   1.00     0.010000  1   2   5.8254E-03c
   0.040000   1.33     0.010000  1   1   3.4853E-01
   0.040000   1.33     0.010000  1   2   4.7869E-03c
   0.050000   1.67     0.010000  1   1   2.7449E-01
   0.050000   1.67     0.010000  1   2   4.3138E-03c
   0.060000   2.00     0.010000  1   1   2.2872E-01
   0.060000   2.00     0.010000  1   2   4.1690E-03c
   0.070000   2.33     0.010000  1   1   1.9860E-01
   0.070000   2.33     0.010000  1   2   4.1572E-03c
   0.080000   2.67     0.010000  1   1   1.7709E-01
   0.080000   2.67     0.010000  1   2   4.1921E-03c
   0.090000   3.00     0.010000  1   1   1.6029E-01
   0.090000   3.00     0.010000  1   2   4.2438E-03c
   0.100000   3.33     0.010000  1   1   1.4614E-01
   0.100000   3.33     0.010000  1   2   4.3069E-03c
   0.110000   3.67     0.010000  1   1   1.3364E-01
   0.110000   3.67     0.010000  1   2   4.3929E-03c
   0.120000   4.00     0.010000  1   1   1.2231E-01
   0.120000   4.00     0.010000  1   2   4.5402E-03c
   0.130000   4.33     0.010000  1   1   1.1191E-01
   0.130000   4.33     0.010000  1   2   4.8458E-03c
   0.140000   4.67     0.010000  1   1   1.0232E-01
   0.140000   4.67     0.010000  1   2   5.5194E-03c
   0.150000   5.00     0.010000  1   1   9.3600E-02
   0.150000   5.00     0.010000  1   2   6.9283E-03c
   0.160000   5.33     0.010000  1   1   8.6519E-02
   0.160000   5.33     0.010000  1   2   9.4035E-03c
   0.170000   5.67     0.010000  1   1   8.5322E-02
   0.170000   5.67     0.010000  1   2   1.1362E-02
   0.170000   5.67     0.010000  1   3   7.8842E-03c
   0.180000   6.00     0.010000  1   1   1.1110E-01
   0.180000   6.00     0.010000  1   2   1.1999E-02
   0.180000   6.00     0.010000  1   3   5.7486E-03c
   0.190000   6.33     0.010000  1   1   1.8934E-01
   0.190000   6.33     0.010000  1   2   1.9690E-01
   0.190000   6.33     0.010000  1   3   1.0105E-02
   0.190000   6.33     0.010000  1   4   7.1759E-03c
   0.200000   6.67     0.010000  1   1   2.7777E-01
   0.200000   6.67     0.010000  1   2   2.5949E-01
   0.200000   6.67     0.010000  1   3   9.0875E-03c
   0.210000   7.00     0.010000  1   1   3.0643E-01
   0.210000   7.00     0.010000  1   2   2.4690E-01
   0.210000   7.00     0.010000  1   3   7.2738E-03c
   0.220000   7.33     0.010000  1   1   2.5610E-01
   0.220000   7.33     0.010000  1   2   1.4178E-01
   0.220000   7.33     0.010000  1   3   6.5146E-03c
   0.230000   7.67     0.010000  1   1   1.8876E-01
   0.230000   7.67     0.010000  1   2   5.5400E-02
   0.230000   7.67     0.010000  1   3   4.3195E-03c
   0.240000   8.00     0.010000  1   1   1.4351E-01
   0.240000   8.00     0.010000  1   2   1.9818E-02
   0.240000   8.00     0.010000  1   3   3.0770E-03c
   0.250000   8.33     0.010000  1   1   1.1547E-01
   0.250000   8.33     0.010000  1   2   7.4187E-03c
   0.260000   8.67     0.010000  1   1   1.0545E-01
   0.260000   8.67     0.010000  1   2   4.9805E-03c
   0.270000   9.00     0.010000  1   1   9.0345E-02
   0.270000   9.00     0.010000  1   2   3.8216E-03c
   0.280000   9.33     0.010000  1   1   7.9443E-02
   0.280000   9.33     0.010000  1   2   3.7428E-03c
   0.290000   9.67     0.010000  1   1   7.1928E-02
   0.290000   9.67     0.010000  1   2   3.6637E-03c
   0.300000  10.00     0.010000  1   1   6.6100E-02
   0.300000  10.00     0.010000  1   2   3.5460E-03c
   0.310000  10.33     0.010000  1   1   6.1110E-02
   0.310000  10.33     0.010000  1   2   3.4560E-03c
   0.320000  10.67     0.010000  1   1   5.6546E-02
   0.320000  10.67     0.010000  1   2   3.4840E-03c
   0.330000  11.00     0.010000  1   1   5.2184E-02
   0.330000  11.00     0.010000  1   2   3.5791E-03c
   0.340000  11.33     0.010000  1   1   4.7930E-02
   0.340000  11.33     0.010000  1   2   3.7271E-03c
   0.350000  11.67     0.010000  1   1   4.3783E-02
   0.350000  11.67     0.010000  1   2   3.8994E-03c
   0.360000  12.00     0.010000  1   1   3.9827E-02
   0.360000  12.00     0.010000  1   2   4.0531E-03c
   0.370000  12.33     0.010000  1   1   3.6279E-02
   0.370000  12.33     0.010000  1   2   4.4362E-03c
   0.380000  12.67     0.010000  1   1   3.3807E-02
   0.380000  12.67     0.010000  1   2   8.1883E-03c
   0.390000  13.00     0.010000  1   1   3.1914E-02
   0.390000  13.00     0.010000  1   2   9.8297E-03c
   0.400000  13.33     0.010000  1   1   2.8432E-02
   0.400000  13.33     0.010000  1   2   7.0273E-03c
   0.410000  13.67     0.010000  1   1   2.4682E-02
   0.410000  13.67     0.010000  1   2   5.3665E-03c
   0.420000  14.00     0.010000  1   1   2.1676E-02
   0.420000  14.00     0.010000  1   2   4.9013E-03c
   0.430000  14.33     0.010000  1   1   1.9327E-02
   0.430000  14.33     0.010000  1   2   4.8288E-03c
   0.440000  14.67     0.010000  1   1   1.7582E-02
   0.440000  14.67     0.010000  1   2   4.8893E-03c
   0.450000  15.00     0.010000  1   1   1.6444E-02
   0.450000  15.00     0.010000  1   2   5.0106E-03c
   0.460000  15.33     0.010000  1   1   1.5903E-02
   0.460000  15.33     0.010000  1   2   5.1718E-03c
   0.470000  15.67     0.010000  1   1   1.6018E-02
   0.470000  15.67     0.010000  1   2   5.3588E-03c
   0.480000  16.00     0.010000  1   1   1.6664E-02
   0.480000  16.00     0.010000  1   2   5.5736E-03c
   0.490000  16.33     0.010000  1   1   1.7780E-02
   0.490000  16.33     0.010000  1   2   5.8154E-03c
   0.500000  16.67     0.010000  1   1   1.9282E-02
   0.500000  16.67     0.010000  1   2   6.0829E-03c
   0.510000  17.00     0.010000  1   1   2.1075E-02
   0.510000  17.00     0.010000  1   2   6.3763E-03c
   0.520000  17.33     0.010000  1   1   2.3076E-02
   0.520000  17.33     0.010000  1   2   6.7005E-03c
   0.530000  17.67     0.010000  1   1   2.5264E-02
   0.530000  17.67     0.010000  1   2   7.0504E-03c
   0.540000  18.00     0.010000  1   1   2.7584E-02
   0.540000  18.00     0.010000  1   2   7.4320E-03c
   0.550000  18.33     0.010000  1   1   2.9998E-02
   0.550000  18.33     0.010000  1   2   7.8440E-03c
   0.560000  18.67     0.010000  1   1   3.2505E-02
   0.560000  18.67     0.010000  1   2   8.2869E-03c
   0.570000  19.00     0.010000  1   1   3.5086E-02
   0.570000  19.00     0.010000  1   2   8.7623E-03c
   0.580000  19.33     0.010000  1   1   3.7757E-02
   0.580000  19.33     0.010000  1   2   9.2859E-03c
   0.590000  19.67     0.010000  1   1   4.1369E-02
   0.585000  19.50     0.005000  2   1   6.6893E-02
   0.582500  19.42     0.002500  3   1   8.1757E-02
   0.582500  19.42     0.002500  3   2   1.6046E-02
   0.581250  19.38     0.001250  4   1   9.2190E-02
   0.581250  19.38     0.001250  4   2   1.2774E-02
   0.581250  19.38     0.001250  4   3   8.9851E-03c
   0.582500  19.42     0.001250  4   1   1.9329E-01
   0.581875  19.40     0.000625  5   1   1.7487E-01
   0.581563  19.39     0.000313  6   1   1.6868E-01
   0.581406  19.38     0.000156  7   1   1.6703E-01
   0.581328  19.38     0.000078  8   1   1.6668E-01
   0.581289  19.38     0.000039  9   1   1.6664E-01
   0.581270  19.38     0.000020 10   1   1.6666E-01
   0.581260  19.38     0.000010 11   1   1.6668E-01
   0.581255  19.38     0.000005 12   1   1.6669E-01
   0.581252  19.38     0.000002 13   1   1.6670E-01
   0.581251  19.38     0.000001 14   1   1.6670E-01
 5.8125E-01  19.38   6.1035E-07 15   1   1.6670E-01
 5.8125E-01  19.38   3.0518E-07 16   1   1.6670E-01
 5.8125E-01  19.38   1.5259E-07 17   1   1.6670E-01
 5.8125E-01  19.38   7.6294E-08 18   1   1.6670E-01
 5.8125E-01  19.38   3.8147E-08 19   1   1.6670E-01

So as you can see something drastic happens at about 19%, but I'm unsure exactly what it is. Neither the stresses, nor the displacements seems to make a jump" when I plot it against time. Maybe it has something to do with the elements starting to experience an abrupt change in stiffness (previously it was more or less pure bending, but as the bag tightens, it becomes almost pure membrane stress)? Could it be some kind of locking?

[S.LI]

Hi, Björn:

Do you still have the problem for restarting Riks from nonlinear static?

I think Riks is allowed to restart from Nonlinear static, instead of MES.

If your problem is between Riks and Nonlinear static, could you share your model here?

Thanks.

[Anonymous]

Hi,

I have only tried restarting from an MES, but I am now trying to run the entire analysis as a Riks analysis instead. It runs fine up to a certain level, then I get convergence issues again. There's really only one model that has been able to capture the snap-through behaviour (the load curve actually switches sign) but after that it grew unstable again.

I can share the model with Autodesk support, but feel uncomfortable doing it on a public forum. I have shared it with Shakeel Mirza and Michael Smell, if you have access to their files?

[S.LI]

Hi, Björn,

I got the model from Mike.

Where do you want the model to go?

you know, sometimes, the force load is just a way to make the model moving, but nobody knows if the model/material could hold the load.

How large displacement is what you expect?

[Anonymous]

Hi,

Well, all I want is to model the pressurizing of the bag. We have done that with a prototype that we got from our client, and it could take the load that I have applied without bursting or anything like that. What happens though, is that the edges bend upon themselves; they become "warped" so to say.

I have managed to simulate this somewhat in an early model I made, using only shell elements, and a half-model (I abandoned the idea because this symmetry would not be valid for the type of buckling behaviour I saw).

I expect a displacement of about 60 mm out from the centerline of the bag.

I have had another thought; may it be that the mesh is too coarse that causes these problems? In the one (although only shell elements, and symmetry) model I did manage to run, I saw today that the mesh was finer. I'm trying one or two models with this size instead now, but it takes a very long time when I use solid elements also, of course.

[xli]

Just another related model setting you may need to make sure in trying this idea of restarting between Riks method model and general MES model. There are two slightly different kinds of restart features in MES or Riks under setting analysis parameter::advanced:time-step::restart. You should use "restart from another analysis" selection. Basically it means that in restart only maintains models starting displacements, velocities, and accelerations are continued from the ending state of the previous model. And it will be a new job that you may change solution methods, material setting, etc. It will have new outputs, ie. not merging results with previous.

The other restart kind has more serious conditions imposed on the restart job, many things is not allowed to change in restated one because it is for completing or extending same analysis job of previous analysis.

-xli

[Anonymous]

Hi, Xli

I have done this, but when I attempt a restart from an MES or Non-linear analysis to Riks, I get an error saying

"Error reading file"

I will see if I can find one of my older attempts and copy the log file here.

[xli]

OK, we will get your model from Mike and try. Thanks.

-xli

[S.LI]

Hi, Björn:

After changing several settings, I got the model running to a Z-displacement of -24~43mm.

At this point, the maximum effective strain reaches to 87%.

As you know, this strain is so high that isotropic elastic material model is not suitable any more.

I'm sending the updated model and results for Mike, please contact him.

Of course, you can continue the model without updating the material model.

All of the simulation is MES, not Riks.

[Anonymous]

Hi,

That's about the same amount of displacement that I can achieve also. The reason I'm only running an elastic isotropic material model is because I haven't been able to get a hold of any real plastic material properties from the manufacturer yet, so I'm taking one step at a time.

While working on this separately, I tried removing the "mid" part, and instead setting all those nodes to zero Z-displacement. This shouldn't affect the results overly, and I'm not interested in that region anyway. Then I managed to get it running to up to 60mm z-displacement before it started cutting the time steps immensely. I will see if I can do something with this also.

Thanks,

Björn

[S.LI]

In general, material properties are not constants, which vary with deformation.

For small deformation, the change is not big so that we can all them "constants".

The simulation results become unreliable if material properties are not accurate.

[Anonymous]

Yes, I know.But as I said, I only care about getting the model to run somewhat properly at this point. It may even be that the particular material they have used is much stronger than what I have deduced (by online resources and some experience). I will try to get some answers tomorrow, or at least try with differen material properties.

Best regards

Björn