
Re: Extremely long solution time on nonlinear static shell model
Bill McEachern Jan 22, 2014 2:02 PM (in response to Jesse Blake)It sounds like you are using force control. If there is are not any no penetration contacts in the model you might have more success with displacement control. However, it might be bad elements in the mesh. Lower mesh densities on some problems tend to let the solution to progress more readily.

Re: Extremely long solution time on nonlinear static shell model
Shaun Densberger Jan 22, 2014 4:09 PM (in response to Jesse Blake)As Bill pointed out, you might want to try an enforced displacement (if possible) instead of an applied load. Nonlinear analyses usually use some form of Newton's Method, which can have stability/convergence issues with applied loads (especially if the slope of the forcedeflection curve becomes negative).
Regarding the discrepancy between your linear and nonlinear trials, there is most likely some improvements you can make to the mesh and load step to obtain an accurate solution in less time. However, depending on what type of nonlinear effects you're trying to capture, there can be a very, very large difference between a linear and nonlinear analysis.

Re: Extremely long solution time on nonlinear static shell model
Jared Conway Jan 22, 2014 5:04 PM (in response to Jesse Blake)it meshes and solves in 2 mins in linear
and then you only switched to nonlinear and now it takes 15+ hours to solve just to 0.03s?
have you tried just running the same problem as linear in nonlinear just to get an idea of what effect that has only

Re: Extremely long solution time on nonlinear static shell model
Jesse Blake Jan 22, 2014 6:12 PM (in response to Jared Conway)Thanks for the suggestions!
@Bill & Shaun I tried switching to displacement control and got the "...displacements are too big or too small..." error about 30min in. I am not sure that I applied everything correctly so I'll keep looking into the proper use of that control method.
@Jared Correct. Linear static runs clean and fast. Using the same model and mesh info with timed loads in nonlinear static takes forever. When you say run nonlinear as linear do you mean apply 100% load at t=0 or to ramp all the loads evenly from t=0 to t=1?
Thanks.

Re: Extremely long solution time on nonlinear static shell model
Jared Conway Jan 22, 2014 6:15 PM (in response to Jesse Blake)just ramp the load from t=0 to t=1s in nonlinear to reduce the number of variables you're playing with.
i'd also post how many elements and DOF we're tlaking about here

Re: Extremely long solution time on nonlinear static shell model
Jesse Blake Jan 22, 2014 7:36 PM (in response to Jared Conway)I am running it now with a simplified load linearly applied (rather than using the 'curve' definition) but it seems to be following the same behavior. The DOF count reported by the solver does seem odd though over 25DOF per node:
783,318 DOF
30,859 Nodes
62,861 Elements
If I kill the solver and look at "mesh details" it reports:
130,859 Nodes
62,861 Elements
Same mesh.

Re: Extremely long solution time on nonlinear static shell model
Jared Conway Jan 22, 2014 8:38 PM (in response to Jesse Blake)what does linear report?

Re: Extremely long solution time on nonlinear static shell model
Jesse Blake Jan 23, 2014 10:53 AM (in response to Jared Conway)The linear study solver reports:
1042,734 DOF
174,095 Nodes
135,020 Elements
The linear "mesh details" reports:
130,859 Nodes
62,861 Elements
Both studies are high quality curvature based meshes, 150mm max, 50mm min. The entire model is surfaces meshed as shells, with 9 tension only springs added into the study.

Re: Extremely long solution time on nonlinear static shell model
Jared Conway Jan 23, 2014 11:42 AM (in response to Jesse Blake)are you sure the problems are setup exactly the same between linear and nonlinear.
probably at a point where i would need to see this to try and figure out wahts going on

Re: Extremely long solution time on nonlinear static shell model
Jesse Blake Jan 23, 2014 1:18 PM (in response to Jared Conway)Jared,
I think the node/element count may just be a display glitch. I have revised the model with a finer mesh running arclength control and the solver is now reporting:
"Degrees of Freedom: 1322,388
Number of Nodes: 21,192
Number of Elements: 08,938"
I would guess its dropping the leading number in the display for node and element count.
Jesse








Re: Extremely long solution time on nonlinear static shell model
Jerry Steiger Jan 22, 2014 7:08 PM (in response to Jesse Blake)Jesse,
I'm not very familiar with SolidWorks Simulation. When you say you run a nonlinear analysis, are you just talking about allowing for large deflections, or are you using a nonlinear material model, or both?
Jerry S.

Re: Extremely long solution time on nonlinear static shell model
Jesse Blake Jan 22, 2014 7:45 PM (in response to Jerry Steiger)Jerry,
I am using time dependent loads to try to buckle a structure from the induced changes in geometry. At the moment I am using a linear material model. Thanks,
Jesse

Re: Extremely long solution time on nonlinear static shell model
Bill McEachern Jan 23, 2014 9:14 AM (in response to Jesse Blake)Post buckling can be tricky and the fact that is solves in linear is not something that should provide a lot of faith in being menaingful. Consider that you have a strucutre that has, as it is loaded, a small insifginifcant peice of strucutre that buckles at almost no load but does not really compromise the integrity of the structure as a whole  some little bracket some where trips at a small fraction of the over all structures capacity. This can cause your solution to hit the ditch lickety split. Displacement and arc length controls allow you to avoid the solution stoping at some inflection such as this, the reasons for which you can do some research on. Now notwithstanding what Gerry (post edit: sorry I meant Shaun and not Jerry  sorry about the sp as well) said, in SWX sim you can not apply a fixed displacement as a load in a displacement controlled solution  you need to apply a force. What happens is the the solution process sort of gets reversed. In the displacement control dialog you need to pick a node  a vertex in SWX Sim  and a single degree of freedom  displacement in the direction your are concerned about or mroe loosely the direction the load is applied in. Pick a spot whose deflection and direction would be meaninful in assessinghte buckling response of the structure. Then you need to specify how far this DOF needs to go to resolve (buckle in your case). then what happens is the solver advanced the dof and calculate the load required to get it to whatever increment it is attempting and converge the force to achieve the displacment. This avoids ttying to iterate to convergence onto a peak of hte force dispalcement trajectory where the force tangent has a slope of zero. Force control will always have some difficulty overcoming buckling if the force slope goes to zero.

Re: Extremely long solution time on nonlinear static shell model
Jesse Blake Jan 23, 2014 6:54 PM (in response to Bill McEachern)Bill,
Thanks for the explanation! It is very possible that's what was going on. I switched to arc length control and am able to run the model in a reasonable length of time.
Do you know of a good online resource describing how to properly set up timed loads and time steps for arc length controlled studies? I'm still a little confused on the correlation between the time stepping options, the applied load vs time curves, and what "load factor" is when viewing the results.
Thanks!
Jesse

Re: Extremely long solution time on nonlinear static shell model
Bill McEachern Jan 24, 2014 3:08 AM (in response to Jesse Blake)Time in a static simulation is not really time. It is more a way to control the sequence of things or process steps if you will. So a loading going from 0 to 1 can equally be modeled as 0 to 0.1 if auto stepping is enabled in say a force control. I just did a pre stress and then reload force controlled simulation that went like this: at time 0 all loads are off, at time 1.5 I went to load factor 1.5 on a centrifugal load, at time 2 the centrifugal load went from LF=1.5 to 0. And then from time 2 to time 3 the centrifugal went back up to LF=1 as well as a pressure load went from 0 to LF=1 and the simulation stopped at time 3. This revealed the drop in stress due to compressively induced stress from yeliding that occured from the high centrifugal run on the normal operating conditions. Ther stess dropped from 40 ksi to about 23 ksi vastly reducing fatigue concerns.
In a displacemnt controlled solution I would typically put auto steping on and go from 0 to 1. I arc length you loose control of hte step size and SWX Sim doesn't auto increase the step size as it used to, at least in my recent expereicne it appears that way, so it can take a while to run. The perecent complete in a dispalcement solution is the fraction fo the displacement achieved by the control node. In an arc length solution it is the fraction of the max number of steps allowed. To find out more about the arc length method you could look up more on Crisfield or Riks methods.
To clarify things on load factor set up some small little fast running experiments and explore the subtelties.
Hope that helps.

Re: Extremely long solution time on nonlinear static shell model
Qinghai Jin Jan 5, 2018 5:57 PM (in response to Bill McEachern)Thanks, I got some. however, It would be wonderful if some fea exercise /applications with the time steps setting up is uploaded here.




