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Simon Križnik

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  1. sebmeca liked a post in a topic by Simon Križnik in Foam compression 1m/s , problem at 50% of deformation , foam go throught impactor   
    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
  2. Simon Križnik liked a post in a topic by Maxime Pouliquen in property SOL_ORTH for solid elements   
    Hello Simon,
     
    That did indeed solve the issue!
     
    Thank you very much.
     
    Maxime
  3. Simon Križnik liked a post in a topic by Adriano A. Koga in Linear Buckling Analysis   
    just highlighting a couple enhancements on this, extracted from RElease notes.
    OS 2019.0 brought RIKS method for helping these unstable snap-trhu behavior.
     
    OS 2019.1
    Imperfection An imperfection can be applied to the model. Note: Only supported for Nonlinear Analysis. The IMPERF Bulk Data and Subcase Entries can be used to apply an imperfection. An imperfection can be introduced into the model in the following ways: TYPE=H3DRES on IMPERF Bulk Data: An h3d file is referenced which contains previously completed analysis results. TYPE=GRID on IMPERF Bulk Data: The perturbation of grids can be directly applied.  
     
    OS 2019.0
     
    Snap-thru with Arc-Length method The Arc-Length method has been implemented to solve snap-thru problems in nonlinear analysis. Solution control is available thru the NLPCI Bulk Entry and three methods (Crisfield, Riks, and Modified Riks).  
  4. Crashphys liked a post in a topic by Simon Križnik in Composite thickness increasing stress??   
    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
  5. Crashphys liked a post in a topic by Simon Križnik in Composite thickness increasing stress??   
    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
  6. Brian DO liked a post in a topic by Simon Križnik in Linear Buckling Analysis   
    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
  7. Moawez Zamir Awan liked a post in a topic by Simon Križnik in ERROR # 312 *** In static load case 1 the compliance is negative or large 1.52002e+09. Optimization/buckling analysis cannot be performed. due to possible rigid body model.   
    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)
     
  8. Roshan Shiveshwar liked a post in a topic by Simon Križnik in 2D Mesh Boolean   
    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
  9. Crashphys liked a post in a topic by Simon Križnik in Understanding the TH file   
    The amplitude of vibration is high because there is no damping in the second step. The damping effect is more pronounced at frequencies near the resonance peaks. The worst-case scenario is when the excitation frequency (ramp-up period) is near the resonant frequency and there is no damping. When you are unsure about the damping, perform a sensitivity analysis by varying structural damping in the realistic range and observe how much the responses of interest are affected. The proper amount of damping should also reduce interface force fluctuation.
     
    Unfortunately, I do not have the documentation on how the total interface force is calculated, but it is probably the sum of all contact forces. Your explanation is plausible, but it does not account for the period of vibration.
     
    p.s. I would appreciate being acknowledged in your thesis. Actually, your challenging queries also sparked my interest in the subject therefore I would like to have a look at your thesis. 
  10. Simon Križnik liked a post in a topic by Crashphys in Understanding the TH file   
    Hi Simon,
     
    This is a very interesting and clever observation. I could not have come to this conclusion alone for sure. I suppose this could very well happen in a realistic situation, however the vibration amplitude is crazy high +/- 1.5 cm or so... 
     
    The thing is, if the cargo sticks to the wall, my intuition tells me it should be absorbing the vibration energy, so it shouldn't be possible that this fluctuation is so high. 
     
    Do you know of any documentation dealing with how the total interface force is calculated? Is it some sort of sum of all the loads? I was thinking it is possible that the load may also be fluctuating because as the particles are lifted by lateral acceleration they lose some contact with the wall (for ex the particles near the bottom of the car), but then they make contact with the rest of the SPH further driving them in?
     
    While the acceleration profile makes sense, I just can't see how total interface force can fluctuate as much as 200,000 N which is approximately 10% the weight of the cargo.
     
    Either way, thank you for the observation.
     
    PS, Not able to message you privately, so I am posting here: I do not know if you are comfortable with it or if you are interested, however I am currently writing up my thesis and you have helped me greatly with my work over its course. If you are interested, I would like to add you to my acknowledgements. I can use your name tag here if you like, or I can temporarily put up contact information if you want to associate your name/work with the acknowledgement. 
     
    Thanks again
  11. Moawez Zamir Awan liked a post in a topic by Simon Križnik in How to have multiple components in hypermesh?   
    There are differences in available options as you are using version 13.0 and I'm using version 2019.1- upgrading to a newer version is suggested. 
     
    I see the split component by body/part option is available at your end, but they are grayed out. This can be expected when importing a part (sldpart file), but not assembly (sldasm file).
  12. Crashphys liked a post in a topic by Simon Križnik in Understanding the TH file   
    Hi,
     
    it would be useful to have the animation (my workstation is too weak for this model)- use the HVtrans tool to reduce the file size to upload limits (only stress, contact forces and displacements; reduce frames if necessary). 
     
    Verify the AMS run against a CST (or without mass scaling) run with a reasonable energy error (ERROR < +2%) and acceptable added mass (MAS.ER < 0.02) along its simulation time. 
  13. Crashphys liked a post in a topic by Simon Križnik in Understanding the TH file   
    The animation file is not available for some reason. Can you share it through Google Drive?
     
    Compare the interface forces between AMS and CST runs on the same plot. 
     
    It is strange that T01 and T02 plots not matching in trend nor the magnitude.
     
  14. Crashphys liked a post in a topic by Simon Križnik in Understanding the TH file   
    Since the AMS run matches other runs without time scaling, the conjugate gradient is below 30 and energy error stays low throughout we can conclude the observed behavior is not an artifact of AMS and the run can be considered verified. 
     
    Because I haven't researched sloshing I am not able to comment on the validity (matching physical behavior), but I can provide my explanation of the simulation results. 

    There is not a single peak, because the lateral acceleration is ramped up, is held for .8 s then ramped down. The cargo "sticks" to the wall as long as there is lateral loading- instead of bouncing right off. A single peak would be expected if the impulse was a lot shorter (without hold period) or there was an initial velocity imposed on the model. However, I think the acceleration pulse used is consistent with the railcar cornering.
    The lateral load is ramped up over a period of .3s which excites the bending eigenmode in the sidewall. In the graph below, the X displacement in the region of the highest amplitude of vibration is plotted. The period of vibration of the sidewall coincides with the period of vibration of the contact force; the hypothesis is as the sidewall vibrates against the cargo the contact forces exhibit pulsating forces in phase with sidewall vibration. 

    Therefore the T01 seems consistent with the expected behavior (given the boundary conditions). But I have no idea why T02 does not overlap the T01 trend.

  15. Moawez Zamir Awan liked a post in a topic by Simon Križnik in Free size optimization and some questions relating to upper and lower limits and design variable parameter chosen in particular what mindim is and how to choose an appropriate one?   
    hi
    1. mindim is a minimum member size manufacturing constraint. It penalizes the formation of small members and reduces checkerboarding effect. It is recommended that MINDIM be at least 3 times the average element size for all elements referenced by that DSIZE (or all designable elements when defined on DOPTPRM). The average element size for 2D elements is calculated as the average of the square root of the area of the elements, and for 3D elements, as the average of the cubic root of the volume of the elements.
    2. Volfrac is a fraction of design space, expressed in the range between 0-1. Volfrac as the optimization constraint with an upper limit as 0.2 means the optimizer will utilize only 20% volume of the design space.
    3. Use LOADADD and SPCADD load collectors to combine multiple loads and spc load collectors, respectively then reference them properly in the loadstep. 
    Free-size Optimization Manufacturability.pdf
  16. Moawez Zamir Awan liked a post in a topic by Simon Križnik in ERROR # 312 *** In static load case 1 the compliance is negative or large 1.52002e+09. Optimization/buckling analysis cannot be performed. due to possible rigid body model.   
    Hi,
    This error occurs when a model is under constrained or incorrect material property definition or contact definitions are incorrect or missing.

    In your case (the units are tonne, MPA, mm) the pressure loading of 24000 MPA is causing excessive deformations.  You should review the unit consistency and/or pressure load magnitude.
  17. antonio.dortona95 liked a post in a topic by Simon Križnik in Non Linear Contact In 3 Point Bending   
    There is a tool for material system review: Tools>orientation review>material system

    Orientation Review - Altair University
  18. Moawez Zamir Awan liked a post in a topic by Simon Križnik in How to have multiple components in hypermesh?   
    Hi,
     
    import geometry>solidworks>split components by>part (default is body)

    Try other formats and/or import options if the issue is not resolved.
  19. Simon Križnik liked a post in a topic by antonio.dortona95 in Non Linear Contact In 3 Point Bending   
    So isn't there a way to verify with vectors on the elements, the x-axis (for example) direction after the user assignment?
     
    Thank you Simon you are the best!!
  20. antonio.dortona95 liked a post in a topic by Simon Križnik in Non Linear Contact In 3 Point Bending   
    Stresses are calculated in the material coordinate system. The material coordinate system may be defined as the basic coordinate system (CORDM=0), a user defined system (CORDM = Integer > 0), or the element coordinate system (CORDM=-1).
     
    Edit the PSOLID property and change the CORDM to USER and select the coordinate system which will be the material orientation.
     
    Alternatively, material orientation can be assigned to elements directly with analysis>systems>material orientation. In the model you shared the CHEXA elements are oriented with THETA option. The element angle option for Hexa solid (Theta) is very useful if the material system is changing from element to element.
     
    Make sure to define only one of these options to avoid confusion. Perform a simple 1 element tensile test to verify material orientation.
     
    The like button is a nice way of showing appreciation for the help 
  21. antonio.dortona95 liked a post in a topic by Simon Križnik in Non Linear Contact In 3 Point Bending   
    Hi,

    The laminates were connected to the core with freeze contact. Since both components have matching nodes on the surface the connection can be established with node-to-node connectivity (equivalence in edges panel- F3). 

    The supports were penetrating the plate and are tied to it with freeze contact (equivalent to gluing the supports to the plate). This is the possible reason for discrepancy compared to test- sliding contact should better approximate the test boundary conditions. Some friction should be able to prevent the plate sliding in Z direction.

    Quarter or half symmetry can be used for better computational efficiency and also constrain the plate from sliding. 
    3PB_NLS_edit2.fem 3PB_NLS_edit2.hm
     
    Refer to example OS-E: 0180 3-Point Bending using RBODY
    3Point_Bending_RBODY.fem OS-E_ 0180 3-Point Bending using RBODY.pdf
  22. antonio.dortona95 liked a post in a topic by Simon Križnik in Non Linear Contact In 3 Point Bending   
    For shell elements, the contact and tie search considers the shell thicknesses. This implies that the defined search distance is expected to be the true distance between the shell surfaces facing each other. For example, in the case of shells without offset, if the geometric distance between the two shell surfaces facing each other is 5.0, and the shell thicknesses are 2.0 each, then the actual distance between the shell surfaces facing each other is 3.0. If the Search distance field is now set to 3.0, then the contact is generated as expected (in this scenario, if SRCHDIS is set lower than 3.0, then there is no contact generated).
    For more details, please refer to the attached document.
     
    To constrain the plate from sliding:
    -use symmetry (refer to 10.3)
    -constrain some nodes in the Z DOF
    Contact.pdf
  23. Moawez Zamir Awan liked a post in a topic by Simon Križnik in *** ERROR # 1000 *** in the input data: Incorrect data in field # 3.   
    Hi,
     
    The errors are due to:
    -the shell property has no thickness defined (T field)
    -the moment load has no grid/gridset definition
     
    If you are new to finite element analysis I recommend you go through: Free eBook: Practical Aspects of Finite Element Analysis
  24. Simon Križnik liked a post in a topic by Ingeniorator in Shape optimization error 339: The dependent d.o.f. is constrained by grid or spc data   
    Coming from other FEA/CFD packages and having used Hypermesh previously only as a preprocessor, I taught myself Optistruct on the fly. But I see that I have a lot of catching up to do. Thank you very much for the explanation and the links.
  25. ve3D liked a post in a topic by Simon Križnik in How to clean zero length beam element   
    Hi,
     
    go to check elements panel (F10)>1D and specify "length" as 0 or some very small number and "save failed". Open delete panel (F2), click "elems" to access extended entity selection, then "retrieve" and finally "delete entity".
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