
Re: Nonlinear modeling of plastics in SW
Jared Conway Jan 15, 2014 5:00 PM (in response to Dermott McHugh)did you follow all the recommendations from the dialog?
do you need a nonlinear material? generally recommend starting with linear to see if you can get the problem to run then you can add complexity as you go.

Re: Nonlinear modeling of plastics in SW
Dermott McHugh Jan 15, 2014 5:17 PM (in response to Jared Conway)Yes I ran it as a nonlinear static analysis with a linear elastic material to test out boundary conditions. The stress state resulting from that analysis is above.
I tried reducing the singularity elimination factor, but it didn't seem to have an effect. My Etan is > Ex/100, and changing it changes the material properties, so that doesn't seem like a great option either.

Re: Nonlinear modeling of plastics in SW
Jared Conway Jan 15, 2014 7:14 PM (in response to Dermott McHugh)what about just linear with linear material

Re: Nonlinear modeling of plastics in SW
Dermott McHugh Jan 15, 2014 8:03 PM (in response to Jared Conway)I have run the study in linear static with a linear elastic material model, but I used a shrink fit contact condition. For the study using displacements, it gives very unrealistic results because the large displacement option is not available in 2D simplifications.
It seems that the failure may be due to the material right around the contact buckling for some reason, possibly because the tangent modulus is so low. Does that make sense? If so, how could I go about limiting that behavior?

Re: Nonlinear modeling of plastics in SW
Jared Conway Jan 15, 2014 8:22 PM (in response to Dermott McHugh)if your material is correct, you will need to try changing the solver parameters. changing and restarting can help.
if shrink works in linear, why not use shrink in nonlinear?

Re: Nonlinear modeling of plastics in SW
Dermott McHugh Jan 15, 2014 8:27 PM (in response to Jared Conway)Interesting. Which solver parameters would you recommend altering? I have tried quite a few of them without luck so far.
Here are some pictures showing the contact right before the solver fails. The first image is the second to last step and the second is the final step before failure. Sorry I'm having trouble using the insert picture button right now so they're attached.

Before Solution Failure.png 214.1 KB

At Solution Failure.png 180.4 KB

Re: Nonlinear modeling of plastics in SW
Jared Conway Jan 15, 2014 8:44 PM (in response to Dermott McHugh)there is no optimal combination of them, you end up having to try modifying within their parameters. once you have exceeded your options, i would recommend contacting your reseller and having them pass this to the developers. it may be a limit of what the solver can do or require significant digging to determine if a setup change can allow it to complete.

Re: Nonlinear modeling of plastics in SW
Dermott McHugh Jan 16, 2014 10:40 AM (in response to Jared Conway)Have you ever had success running simulations with contact and soft, nonlinear materials? What material model did you use?
Since modifying Etan helps the solution, and a linear elastic model works well, I suspect that the material model is what is affecting the solver. Has anyone had experience running nonlinear simulations with contact using soft materials which soften considerably after yield, or at least have similar stress v strain curves?
Currently I am trying to run the simulation with a maximum time step of .001, which seems to make it past the critical point at which the solutions with a large maximum time step failed. We'll see if it makes it to completion.

Re: Nonlinear modeling of plastics in SW
Bill McEachern Jan 16, 2014 10:56 AM (in response to Dermott McHugh)Nonlinear elastic or VM Plasticity would be my recommendations.
Further I think you ought to try lowering your mesh density on the target bit  make it coarser.
I have to say I don't get your BC's. Should they ot be out near the boundarys on this one as it is axis symmetric  could be I just don't have a grasp on what you are after here but it strikes me as odd where they are.
In my expereince the shrink fit in NL studies can be problematic.
You should be using surface to surface contact and not node to surface contact if you wan the thing to get very far.
Hope this helps.

Re: Nonlinear modeling of plastics in SW
Dermott McHugh Jan 16, 2014 11:32 AM (in response to Bill McEachern)Hi Bill,
I'd be happy to clarify, thanks for your help here.
The appearance of the fixtures is a little bit of a graphics glitch. See the attached figure for the full idea of it. Basically the outside edges of each body are completely constrained, so that all the parts except the plastic ring are rigid (the option to make a part rigid is not available in axisymmetric). I'm also using surface to surface contact, defined explicitly through 3 contact sets. The nonlinear simulation with a linear elastic material had no trouble solving, so I feel pretty good about my contact and boundary conditions.
I will try lowering the mesh density and see how that goes. Just curious: any idea why that would help the solver? I had made it coarser because the failures I was getting earlier seemed to be due to buckling of the edges of individual elements near the contact.

Study Setup.png 41.7 KB


Re: Nonlinear modeling of plastics in SW
Dermott McHugh Jan 16, 2014 12:29 PM (in response to Bill McEachern)It seems like coarsening the mesh did help quite a bit. I have finally gotten a study to solve. Unfortunately, the contact conditions seem to have some issues, because at the final solution, the plastic part was interfering with the other metal parts significantly. The contact pressure plot does not look unrealistic, but the final position of the plastic part clearly has some issues. I am pretty baffled by this result... any ideas?
Also, Jared, I have gotten in touch with goengineer as well to see where that takes me.

Completed Study.png 152.5 KB

Completed Study CP.png 120.0 KB

Re: Nonlinear modeling of plastics in SW
Bill McEachern Jan 16, 2014 1:37 PM (in response to Dermott McHugh)Hi Dermott,
I would just make the other bits steel or unobtainium (use whatever finite stiffness you would like) and put the BC's not at the contacts as it seems to me to be very unrealistic and as you probably know with NL analysis unrealistic doesn't really bode well for success. Steel seems pretty stiff relative to UHMWPE. However, it may not make any difference  your call.
I doubt that buckling is an issue. I would suspect the solution is failing due to contact convergence issues. I would coasren the mesh further on the target bit and try to get as uniform a mesh as you can get on both bits in contact:Try the standard mesher it will be more uniform it will make the solution faster and may make it more robust.
I would turn off friction: friction is nonconservative and Cosmos's friction handling is not exactly world renowned. If it work without friction then try it with friction (start small and work your way up).
It does seem odd that with contact failure as eveidence by the plot you still get a decent contact pressure plot. Is the default plot maginifaction set to 1? The other end looks as if it has contact issues as well so maybe the plot mag is not set to 1?



Re: Nonlinear modeling of plastics in SW
Jared Conway Jan 16, 2014 11:26 AM (in response to Dermott McHugh)we have performed nonlinear simulations with contact on rubber seals for example. i don't have the details of the rubber off the top of my head but i think they are probably softer than what you are using. in some situations we were not get them to complelely fold over on themselves due to numerical instabilities but we usually got pretty close. some required a lot of work on time step/solver parameters, coarsening the mesh like bill said and sometimes only ran with draft. we've also had to modify the setup of the problem to get it to solve. usualy under the guidance of the developers.

Re: Nonlinear modeling of plastics in SW
Dermott McHugh Jan 16, 2014 11:40 AM (in response to Jared Conway)Okay that's good to know that other people have been successful with solidworks at least. I am sorely missing my old seat of abaqus right now.
Am I correct in thinking that having no friction should simplify the solution?
Just to clarify a little further, which solver parameters are you referring to, Jared? I understand you can't tell me exactly how to change which one, but I'm just looking for some examples.
Thanks again, everyone.

Re: Nonlinear modeling of plastics in SW
Jared Conway Jan 16, 2014 11:46 AM (in response to Dermott McHugh)all of them
singularity elimination factor
min time step
...etc
removing friction can help sometimes
but also adding friction can help sometimes
its a delicate process when it comes to including all nonlinearities
i should also note that sometimes i've had more success with 3d than 2d
if you haven't engaged your VAR yet, I would definitely do that to make sure you aren't going down a rabbit hole. this assumes you've confirmed that your setup is valid overall. (bills comments)





Re: Nonlinear modeling of plastics in SW
Jaspreet Hothi Jan 16, 2014 11:40 AM (in response to Dermott McHugh)There are couple of things I do when I suspect buckling. First, change the solver to Arc length, however you cannot give diplacement BC for arc length, you will have to apply force. You can also loosen the convergence tolerance. I generally start the solution with the default tolerance and when the solver kicks the bucket I lower the convergence tolerance a little and restart the solution. In my case it is enough to take me through the problamatic areas. Then I stop and continue the solution at default convergence tolerance.
Looking at your before and after pictures it appears that the contacts are doing their job properly before failure but at failure the contacts are behaving erratically. The highest stress apears to be at a location barely in contact. Another way to find if it is a contact issue is to isolate part 4 and look at the deformations for part 4 at higher scaling factor. You may see some unrealistic edge deformation at failure. Playing with mesh densities may help.







Re: Nonlinear modeling of plastics in SW
Dermott McHugh Jan 15, 2014 5:20 PM (in response to Dermott McHugh)Here is the state at failure with an ETan of 500 MPa (5x of actual).

Higher Etan Failed.png 170.2 KB

Re: Nonlinear modeling of plastics in SW
Paul Gessler Oct 20, 2014 9:17 AM (in response to Dermott McHugh)Hello everybody,
I´m currently having the same problem. Does anyone of you know how to finally fix this problem?
Thanks in advance!
Paul

Re: Nonlinear modeling of plastics in SW
Jared Conway Oct 21, 2014 1:56 AM (in response to Paul Gessler)Isolate what element of your analysis causes the issue and work on it. There is no silver bullet. Involve your reseller if you have narrowed down the factors and are still. It may required a developer to investigate. Or it may be beyond the softwares capability if nothing helps.

