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Simon Križnik

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About Simon Križnik

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    compliant mechanisms, crashworthiness, origami engineering, structural optimization, pneumatic structures
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  1. This material law initially follows the Maxwell-Kelvin-Voight viscoelastic model until it intersects the defined yield curve, which limits the visco-elastic stress in tension and compression. For more details refer to the attached documentation (from Radioss help): Visco-elasto Materials for Foams (LAW33).pdf _MAT_LAW33 (FOAM_PLAS).pdf
  2. Hi, this modelchecker error indicates badly shaped 3D element (meshing issue). The solid is decomposed into sub-volumes associated to each integration point. If the element is badly warped, one sub-volume could be negative. Check your element quality. Tool > check elems > 3D > aspect, jacobian, warpage, tet collapse
  3. Hi, the issue is because the interface "impacteur vs foam" had dtmin=0.6 If the time step of a slave node in this contact becomes less than dtmin, the slave node is deleted from the contact and a warning message is printed in the output file. This dtmin value takes precedence over any model interface minimum time step entered in /DT/INTER/DEL. Because dtmin is quite high, a lot of nodes are deativated from the interface, therefore penetrating the plate. The recommended solid property parameters for material law 70: If there is hourglassing, use Isolid=17, however your model runs fine with the above. model16_edit_0000.rad
  4. Hi, There are initial penetrations; check and resolve using: -in Hypercrash: Quality>Check all Solver Contact Interfaces -in Hypermesh: Tool>penetration check The issue is due to the yield stress vs. volumetric strain curve. Because the curve has a negative last slope Radioss extrapolates negative stress, which is not valid (The input stress should be positive for both tension and compression). As a workaround, define an additional curve segment with a zero or positive slope. IA_Setup_edit2.hm Yield stress vs. volumetric strain curve should look like:
  5. Hi, the minimum timestep warning in the interface is due to the kinematic time step of the interface. It is because the node is penetrating too far into the gap. You can try increasing the Gapmin in your TYPE7 interface which will allow the contact to work sooner and prevent node from penetrating so far. Or you could try making the interface stiffer, Istf=3 which uses the maximum stiffness of the slave and master. First check for initial penetrations and intersections: -in Hypercrash: Quality>Check all Solver Contact Interfaces -in Hypermesh: Tool>penetration check Check the gap that is used for this interface.The value must be physically realistic and based on shell thicknesses in case of contact between shells. In case of constant gap (Igap =0) with no input gap (Gapmin=0), RADIOSS determines a default value for Gapmin and you can check this value in the starter out file. Gapmin=1mm usually works in most simulations. If some elements fail on the master side or on the slave side of the interface, it is important to set Idel =2 (or Idel =1) for this interface. This will prevent nodes connected to deleted elements whose stresses are released to impact with a possible high velocity.
  6. Hi, Ishell/Isolid=QEPH/HEPH=24 is not recommended for orthotropic materials because the physical hourglass stabilization forces are computed based on isotropic assumptions. Instead, use fully integrated elements Isolid=14 or 17. one_element_0000.rad
  7. Make sure to set TYPE=PLASTIC and TYPSTRN=1 on the MATS1 card.
  8. Hi, the error is because the first two X coordinate entries are the same (not ascending). The initial coordinates in the plasticity curve (TYPSTRN=1) should satisfy zero,non-zero requirement (the yield stress can not be zero). So your curve should start with 0,360 instead of 0,0. https://altairuniversity.com/wp-content/uploads/2016/10/1189_MATS1-definition-with-TYPSTRN-in-Optistruct1.pdf tensile_lg_str.fem
  9. I haven't done breakout model optimization yet so take this with a grain of salt: One way coupling is a limitation of breakout modeling: the boundary conditions transferred from the global model may not be valid in a local model, if the local model's stiffness is changed significantly (in a way that alters global behavior). Check how the global response changes at upper when compared to lower bound stiffness (thickness). Afterward, perform optimization with a narrower stiffness range for which boundary conditions are still valid. Also try including a more extended transition region. You may find the following article useful: https://www.digitalengineering247.com/article/global-local-modeling-fea/
  10. While there are forces and moments in the FSI_LOADS load collector, there are also imposed displacements on the model boundary as a result of breakout modeling. Running without any loads results in non-zero displacements. ThickComp.rar
  11. Hi, These counterintuitive results can be explained if the displacements are imposed: the thicker structure is stiffer and develops more stress under the same imposed displacements. For similar reasons when optimizing for stiffness: -imposed loads= minimize compliance -imposed displacements= maximize compliance
  12. Glad to help. Hypermesh is unitless and it is the user's responsibility to follow a consistent set of units. https://www.altairuniversity.com/wp-content/uploads/2012/04/Student_Guide_55-57.pdf Since the material properties are consistent with the tonne, MPA, mm unit system, the model dimensions and loads should follow the same units. So yes, the moment should be Nmm. You are getting a compliance error because of the unrealistically high pressure load of 24000 MPa. Applying only the moment load in the model you shared, the displacement is actually minimal (0.0068 mm). The pressure load should be calculated according to unit system (1 MPa = 1 N/mm2 = 1000000 Pa)
  13. Hi, While nonlinear buckling could be done in Optistruct, it is very is likely the implicit solver will experience convergence difficulties resulting in long run times or even fail due to nonconvergence. Alternatively use Radioss integration to solve your model with the explicit method in Optistruct. Therefore I suggest using Radioss explicit solver instead following this procedure: 1.perform modal analysis in Optistruct 2.in postprocessing create a derived load case>linear superposition>use small scale factor (1e-2 to 1e-3) 3.export the deformed shaped. 4. import the deformed shape into Hypermesh Radioss user profile and set up non-linear buckling analysis. By using the deformed shape obtained from the modal analysis the structure will have geometry imperfection triggering a buckling pattern consistent with modal and linear buckling analysis. Nonlinear buckling analysis is recommended to be performed within Radioss. Post buckling can be solved using nonlinear geometry (Implicit) loadcase. Use any of the Arc-Length methods to solve post-buckling analysis. Buckling.pdf1.46 MB · 130 downloads 2_2_snap_roof___implicit.pdf663.71 kB · 105 downloads There are two tutorials and one example on NL buckling: RD-T: 3030 Buckling of a Tube Using Half Tube Mesh (Hypercrash) RD-T: 3530 Buckling of a Tube Using Half Tube Mesh (Hypermesh) RD-E: 0300 S-Beam Crash RD-T_ 3030 Buckling of a Tube Using Half Tube Mesh.pdfUnavailable RD-T_ 3530 Buckling of a Tube Using Half Tube Mesh.pdfUnavailable RD-E_ 0300 S-Beam Crash.pdfUnavailable
  14. Hi, perhaps mesh imprint functionality can be utilized. https://altairuniversity.com/wp-content/uploads/2017/10/Mesh_Imprint.pdf https://connect.altair.com/CP/SA/training/self_paced/aero_v13/PDF/chapter5_demonstration.pdf Mesh Edit Panel.pdf
  15. The most convenient way to get the average size of elements is with TCL: Note: in case mindim specified is smaller than the actual average element size, it will be set as the average size. Yes, setting 0.2 as the upper limit volfrac means 20% volume should be retained. Setting volfrac as 1 is a trivial case wherein all of the available design space is utilized and the optimized configuration is the same as the initial.
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