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  1. Hello Rahul, thanks a lot for this quick reply! Ok, so I'll just use ALPHA1 and ALPHA2 then. Cheers, cfuser
  2. Dear all, I am currently working on a transient analysis (not modal analysis!) and want to introduce damping to the model. I figured that damping can be added through PARAMs. Using PARAM, ALPHA1 and ALPHA2 will add Rayleigh-damping is that correct? If so, is Rayleigh-damping by itself sufficient when one wants to add damping to a model? Further, there are PARAM, W3, W4 and G that can be used for adding damping. Could I alternatively use these parameters to successfully introduce damping to the model? Or is it necessary to use all the parameters simultaneously (ALPHA1, ALPHA2, G, W3, W4) to achieve sufficient damping? As always I'd be really grateful for any help! Cheers, cfuser PS: I have looked into the OptiStruct online help and found it a bit confusing. Can someone give me a quick and simple explanation of the parameters involved (ALPHA1, ALPHA2, G, W3, W4)?
  3. Hi Young, could you also share your NLPARM entry, please?
  4. Hi Folks, are the output parameters CSTRESS and CSTRAIN supported for nonlinear analysis of composites (nonlinear quasi-static, nonlinear transient...)? I was trying to analyse a composite component in two subcases. First small displacement with linear static analysis and then large displacements with nonlinear quasi-static analysis. For the linear static analysis results for ply related stresses and strains are obtainable. For the nonlinear quasi-static analysis no results for ply strains and stresses are given (only N/A is shown as result). This leads me to believe that CSTRESS and CSTRAIN is only supported for linear static cases. I had a look in the help material but couldn't find any conclusive answer to my question. Can someone give me an answer? Are the parameters CSTRESS and CSTRAIN only applicable to linear static analysis? Cheers, cfuser
  5. Dear Support Forum, I want to define to load steps in one model. First load step: nonlinear quasi-static (NLSTAT) bending of a thin shell structure (large displacements involved). Second load step: nonlinear transient (DTRAN) release and spring back of the formerly loaded bent structure. For the second step CNTNLSUB is activated. Is it possible to continue the bending load of the first load step into the second load step? In other words: can a load curve be defined streching over several load steps? Additionally, can one define a load step in which no load is applied. Let's say, you clamp a beam at one end and bend it at the other. Then you just "let go" of the free end and let it spring back and oscillate with out any applied loads? Cheers, cfuser
  6. Hello Rahul, first of all I'd like to thank you for the swift reply. In which way are the points three and four, made by you, different from each other? Isn't checking for unconstrained rigid body modes and checking if the structure is properly constrained the same? Further, I'd like to ask, what you mean by: "No solution available for set of assumed PDE (buckling and collapse, plasticity)". As the analysed model involves lockal buckling (flattening of the blade in the middle section), does that mean I should not be using OptiStruct for this category of problem? If that should be the case, what is your suggested solver? Kind regards, cfuser
  7. Hi all, I read in the Altair ebook "Introduction to Nonlinear FEA using Optistruct" (free ebook for students) that EXPERTNL is currently not supported with large displacement nonlinear analysis. As this ebook is from 2018 I wanted to ask, if things have changed since? Is EXPERTNL supported with large displacement analysis in the 2019 Version of OptiStruct? I am working on a nonlinear analysis with large displacements at the moment and am running into convergence issues and thought setting PARAM, EXPERTNL to YES might do the trick. I had a look in the online help of the 2019 Version but it only states that this parameter "activates nonlinear expert system to aid in the convergence of small displacement nonlinear problems. Can somebody help me with this question? Kind regards, cfuser
  8. Dear all, I am working on an nonlinear quasi-static (NLSTAT) analysis of a curved blade (see attached Input-Files) in 2017.2 Student Edition. CQUAD4 Elements and PSHELL property are used. I want to study the bending of this curved blade through 180°. It is of interest to me how high the SPC-Forces are at the two end nodes (nodes at which RBE2-Elements come together). I am aware that this is quite a complex problem with nonlinear geometry and flattening of the curved blade as it gets bent so I am looking for help with the following issue. The attached models are basically the same in all respects except the boundary conditions. "Blade_BC1" is clamped at one end node (Node 3; all DOF=0), while the other end node has DOF1,4,5,6 constrained (Node 4; DOF...=0) and an SPCD applied as a rotation of 180° about the x-axis (DOF4=3.142). "Blade_BC2" has one end node (Node 3) that is constrained in all DOFs and has an SPCD applied as a rotation of -90° about the x-axis (DOF4=-1.571). The other end node (Node4) is constrained in all DOFs but DOF2 (translation in y-direction is possible) and has an SPCD applied as a rotation of 90° about the x-axis (DOF4=1.571). Analysing "Blade_BC1" leads to nonconvergence (see first screenshot and out-file), analysing "Blade_BC2" is no problem at all with full convergence (see second screenshot). From a physical/mechanical perspective both blades (Blade_BC1 and Blade_BC2) are experiencing pure bending and it should not make a difference if one uses boundary conditions BC1 or boundary conditions BC2. That is why I am wondering how one model fails to converge while the other converges without troubles. Yet, as the only thing that differentiates the two models, the boundary conditions must be the deciding factor over convergence. Can someone give me advice as to which paramaters to change in the Blade_BC1-Model so that it also converges? For more detail please have a look at the attached input-files. To study non-convergence I have also added the out-file of Blade_BC1. As always, I am very grateful for any help. Cheers, cfuser PS: Don't be confused by the linear static subcase that comes before the nonlinear subcase. This was created for verification purposes. Blade_BC1.fem Blade_BC1.out Blade_BC2.fem
  9. Dear Support Community, I am currently working on the analysis of a highly deformed composite laminate. A very thin composite laminate sheet (ca. 0,4 mm) is "clamped" (all DOFs constrained) at one end bent around 180° to form a "c-shape" at the other. I am using the non-linear quasi-static solution option in OptiStruct, the composite part is made up of HEX8-elements with PCOMPLS property. When running the analysis the results (e.g bending moment (SPCF)) are by far stiffer than the predicted results. That got me wondering: Does OptiStruct regard the nonlinear behaviour of composite materials at high levels of strain (and therefore large displacements). I mean, does it take into account, that the moduli of the material behave nonlinear for very thin composites and very high levels of strain? I had a look into the OptiStruct help. It says there that "classical lamination theory is used to calculate effective stiffness and mass density of the composite shell". This is some information, but doesn't really answer my questions. As always I would be very grateful for any helpful advice given. Kind regards, cfuser
  10. Hi, I am currently working on a model for a non-linear transient analysis. While doing so, I wanted to change the MREF value in the TSTEP Bulk Data Entry from its default value (MREF=1) to 0. First I tried this by using the Card Edit Panel on the HyperMesh GUI. But on this panel I can only change entries regarding SID, N, DT, NO, W3, W4. Next I had a look in the input file of the model. I then tried typing in the requested value into the appropriate field. As I am using the Altair HyperWorks Student Edition 2017.2 I then had to load the solver deck into Hypermesh, go to the Analysis panel and before solving it save the changed input file again. After the solver had done its job I had a look in the input file, because I had the feeling that my MREF=0 entry was ignored by the solver. Looking into the input file I found out that HyperWorks indeed did not save the changed value I had typed into the input file before loading the solver deck into HyperWorks. I checked if I put the wrong number (integer/real) into the field. The number seems to be correct. I have the feeling, that the saving process before clicking the "OptiStruct" button on the Analysis panel to run the analysis overwrites my entries I have typed in "by hand". Is running input files altered "by hand" not supported in my Student Edition of HyperWorks? That would seem very odd... Does anyone have some advice? Cheers!
  11. Hi, the shell thickness is supposed to be this thin (0.0004m). And yes, the unit system is N, m, s. I am aware of the warning (thanks for pointing it out anyways). As it does not effect similar analysis I have run with same shell thickness, I don't suppose this will be the source of the oscillations. Cheers
  12. Hi Hyperman, I am using Altiar HyperWorks 2017.2 Student Edition. Do you think it could be a HyperWorks Version issue? I ran with solver run options: -nt 4. Thank you, by the way, for your shared files. Really helping me out here PS: I had a look at the h3d-file in HyperView. I'm glad that the you were able to solve this analysis setup, but I find the oscillation after unloading of the metal sheet quite extreme. In comparable models (same parameters for damping etc.) I observed considerably smaller oscillations. I checked some possible error sources e.g. the display settings concerning the deformation shape. But they are all set to reasonable values. Have you got any tips?
  13. Thank you for your tip, Hyperman. I'll have a look into the link and also the example OS-E 0125.
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