Hi folks,
I am currently trying to analyze a non-linear transient problem (see attached input file). First of all, a short description of what I am trying to accomplish. I want to simulate a thin sheet of metal (property PSHELL T=0.4) that shall be rotated 180° about x-axis at one end (node 585). This bending motion shall be achieved through a torque load which is applied at node 585. The value of this torque load (0.7363 Nm) was calculated in advance by using the same model but instead of a torque load an enfoced displacement (DOF4=3.142) was applied at node 585 and the resulting SPCF at that node was determined. The model for determining the SPCF has the same load curve (TABLED1) as the attached model but terminates at 1s (only ramp-up of load). The other end of the sheet (nodes 1-11) is constrained at every DOF.
The torque load shall follow the load curve TABLED1 which defines a ramp-up of the torque load (0-1s), then a sharp ramp-down of the load (1.0-1.1s) and finally no load at all so that the metal sheet is allowed to oscillate until it regains its initial undeformed state.
The model - as mentioned earlier - shall be subject to a non-linear direct transient analysis. Therefore, NLPARM and NLADAPT cards are defined. And this is where I have some complications in the model. I have run the model multiple times first using default values and then trying to adjust the values to reach convergence. When using default values on NLPARM card (and leaving NLADAPT out of the analysis all together) the time increments become very small (ca. 1e-5) and the solution takes hours. When applying NLADAPT with values for DTMIN and DTMAX the solver fails to reach convergence. This for example is the case, when you run the attached input file. It errors out because the minimum increment size is reached. Could somebody give me a tip as to why my model won't converge? Further, I read in different forum posts and OptiStruct help material that for most problems NLPARM default values will work fine. If this is so, why won't they work for my problem?
In addition, I have tried different load curves for the attached analysis problem. Initially, I modelled only the ramp-up of the torque load (0-1s). The solution took some time and used very small increments but it came to a realistic result. Then I defined a load curve where the load is ramped-up and then held at the maximum level for an additional second (1-2s). Here the ramp-up worked out fine but then, during the holding of the torque level, the metal sheet bounced around and oscillated. Why is this so, can somebody give me clue? Shouldn't the metal sheet stay in the same bent position when maximum torque load is applied? With the current load curve (see attached input file) the ramp-up of the load (0-1s) seems to be no problem. But the sharp ramp-down of the load (1.0-1.1s) seems to cause trouble for the solver.
So, after this long description I hope some of you are still reading. In summary, when trying to simulate the bending and release of a thin metal sheet (non-linear direct transient analysis) I have problems achieving convergence. What could cause these convergence issues? How do I define the NLPARM and NLADAPT cards so that convergence can be achieved in a sufficient and acceptable time frame?
I would be really grateful if somebody with some experience with these kind of problems could have a look at my input file and make some suggestions as to what I am doing wrong or what I could change in the input data.
Cheers,
cfuser
PS: the system of units is N, m, s
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