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  1. Karthic liked a post in a topic by Simon Križnik in How to Apply static loads before impact loads in Radioss   
    Hi Karthic,
    1. You can simulate a static loading more computationaly efficient by time scaling, which means applying the load more quickly than in the quasi-static experiment in order to reduce the simulation time. However, the kinetic energy should be low (5-10%) compared to the internal energy for most of simulation,  to minimize the inertial effects. You can check the energy fraction in Hypergraph by plotting both energies and using Vehicle safety tools (File-Load-Preference File) to divide curves (Math-Two Curves-Divide w/Zero)
    2. You can use State files to transfer the state of elements (stress, strain, etc.) between simulations. More information on State files
  2. Karthic liked a post in a topic by Toan Nguyen in fatigue analysis load time table problems (tabfat)   
    Hi @Vin566
    Please kindly notice that you can not define a time-dependent force with TABFAT if your time history is complicated.
    With TABFAT, you can only import Loads because TABFAT is used for static mode.
    If your time history is complicated, I suggest you use the Fatigue-transient analysis. 
    For the FATLOAD load collector, instead of using a Linear static for LCID field, please choose a Transient analysis loadstep.
    This means you have to perform a transient analysis first.
    if you don't know about Transient analysis, I recommend you to read following tutorial: OS-1310: Direct Transient Dynamic Analysis of a Bracket
    only Transient Analysis allows you to create time-dependent loads with complicated time history
  3. Karthic liked a post in a topic by Toan Nguyen in fatigue analysis load time table problems (tabfat)   
    Hi @Vin566
    I also have a look in your .csv file too and I have to say, your data is not really appropriate even through it true theoretically. I think you may be having some misunderstanding about Fatigue Analysis in OptiStruct.
    Your data, as you said, you have 2000 cycles. This is the first problem. You should not import all cycles to TABFAT card, only one cycle is enough. the OptiStruct will do its job is predicting the life (the number of the cycles) of your structure. Eg: I am having structure and I do a fatigue analysis with 1 cycle in TABFAT card, the life result I get is 2000. It means after 2000 time of cycle, my structure will be failed. I do the fatigue analysis again, this time I import two cycles to TABFAT card, the life I get will be 1000. Nothing changes at all. the number of cycles is a result, not a data need to be imported.
    The second things, please try to avoid zero value in your data. Too many zero values, sometimes, can cause some problem in programing.
    Hope this help
  4. Karthic liked a post in a topic by gopal_rathore in Crash Analysis with Not Relevant Energy Output   
    It is the energy associated with deleted elements and deleted nodes .The name is coming from word erosion, which means removal of something.
    Here in impact elements after failure are getting removed or getting deleted.
    It can be internal energy as well as kinetic energy. 
    @Ashish_MEXPERT Can you also plot eroded kinetic energy as internal energy in elements after deletion will almost remain constant as they do not have any more capacity to absorb more based  as they are failed. This is also seen in you graph where A plot is getting constant
  5. Karthic liked a post in a topic by Andy in Failure /FAIL/JOHNSON and /FAIL/TAB1   
    /FAIL/JOHNSON and /FAIL/TAB1 are very similar and work on the same basic idea where a plastic failure strain versus stress triaxility curve has to be defined.  In /FAIL/JOHNSON the plastic failure strain versus stress triaxility is created by defining the D1, D2 , D3 parameters which are used in the Johnson-Cook  equation.
    In /FAIL/TAB1, the user defines a /FUNCT which represents the plastic failure strain versus stress triaxility curves.   In /FAIL/TAB1 you can define different /FUNCT which represent different plastic failure strain versus stress triaxility curves at different strain rates.  In /FAIL/JOHNSON the strain rate affect is defined via the D4 parameter input. 
    So you should not need to use both /FAIL/TAB1 and /FAIL/JOHNSON in the same model since they do the same thing.  If you have a plastic strain at failure for compresion (trixaility= -1/3) shear ((trixaility= 0) and tension  (trixaility= 1/3) then you can do a curve fit to calculate the D1, D2, and D3 paramters.  You could also look at /FAIL/BIQUAD which uses two quadratic equations to define the plastic failure strain versus stress triaxility curves and has some built in starting values for some different materials. 
  6. Karthic liked a post in a topic by gopal_rathore in Reaction forces on Rb master node   
    The recommended value for energy equation is as follows:
    (Hourglass energy +contact energy)/TE this should be less than 15 %
    In your case this i very high as contact energy is almost same as TE and even higher than it.
    It seems there are penetrations in the model. Check this in hypercrash. Also check whether any incompatible boundary conditions are there in model.
  7. LKZ liked a post in a topic by Karthic in Enforce Displacement   
    Hi LKZ,
    The node which is given the enforced displacement (SPCD) must also be fixed in same DOF using SPC
  8. Karthic liked a post in a topic by Rahul Ponginan in Contact problem in two different sub-cases.   
    Hi Vitor,

    We should not expect the 2nd subcase to start from the end position of the first one.

    CNTNLSUB only carries over the nonlinear evolution data, for example, the gap status or plastic deformation, but not displ/stress, loading, boundary conditions, etc.

  9. Karthic liked a post in a topic by tinh in Nonlinear Buckling   
    I think nonlinear buckling is nonlinear geometry loadstep.
    To find buckling load, increase it step by step, and observe bending stress value which starts to increase once structure reachs critical
  10. Karthic liked a post in a topic by Prashanth Ar in Procedure to Restart Radioss Run   
    Procedure to Restart
    First, while submitting the files for restart, please create a new engine file (may be copy - paste original file) and edit the extension from *_0001.rad to *_0002.rad
    Next, in the newly created Engine file, you have to edit the number in /RUN, ex. /RUN/Drop_test/1/ will become /RUN/Drop_test/2/
    Finally, while submitting the run call new Engine file(*_0002.rad) which you have created use same number of cores what you have used for the first run. 
    Note:  During restart you can also change many parameters (If needed)  in the engine file like Termination time, Animation Frequency, Time History Frequency, Apply BCs, Apply Loads, Delete Interface/Part/Element etc
    Prashanth A.R
  11. Karthic liked a post in a topic by Rahul R in Error named "ISTOP=-2"   


    Please find attached troubleshooting method for Istop=-2 error.



    Rahul R

  12. Karthic liked a post in a topic by Rahul R in Error: Time Step less or equal DTMIN for Solid Elements   


    Please find attached troubleshooting methods for Istop = 2 error.



    Rahul R

  13. Rahul R liked a post in a topic by Karthic in CONTACT- Node to surface   
    Hi Prakash,
    I am using HM v14.0.
    Error solved by using both master and slave entity as sets instead of SSID- set and MSID- contact surface
  14. Rahul R liked a post in a topic by Karthic in NLPARM   
    Hi Rahul Rajan and Prakash,
    Thank you for the support
  15. Karthic liked a post in a topic by Rahul R in NLPARM   
    This is available with latest Hyperworks desktop 2017.2. PFA screenshot of release notes.

  16. Karthic liked a post in a topic by Rahul Ponginan in Linear and Non linear analysis, iteration, increment, step etc explanation   
    The equation of motion for a static analysis is as below:
    [K] {X} = {F} ------------------------------------------ (1)
    [K] --> Global Stiffness Matrix
    {X} --> Unknown Displacement
    {F} ---> External Force Applied.
    For the body to be in static equilibrium, the net force acting at every node must be zero. Therefore, the basic statement of static equilibrium is that the internal forces, I, and the external forces, F, must balance each other:
    [K] {X} is nothing but internal force 'I'
    Equation (1) now becomes,
    ==> I = F or I - F = 0 -----------------------------------(2)
    In Dynamic Analysis, the imbalance between the external and internal forces results in an acceleration:
    F - I = M a.
    F --> External Forces
    I ---> Internal Force
    M*a --> Inertial Forces (mass times acceleration)
    In linear static analysis the stiffness matrix is constant and shall not change/update throughout the analysis. There are many check need to be performed once you have linear static results for well conditioned problems.
    The equation (1) is decomposed one time to find the unknown displacement.
    [K] {X} ={F}
    After decomposition, a singularity may lead to an incorrect solution. In static analysis to obtain {X} (displacements). Using these displacements, One can calculate a “residual” loading vector as follows:
    [K] {X} -{F} =δ F
    This residual vector should theoretically be null (equation 2) but may not be null due to numeric roundoff.
    In Nonlinear static analysis, The stiffness matrix changes in each and every iteration since the stiffness matrix is dependent on the external load. The external load in Nonlinear static analysis is applied in increments and time here has no physical meaning.
    Time is just a convenient way to apply full load in nonlinear static analysis. In Optistruct the incremental load is controlled by 'NINC' field in the NLPARM card for NLSTAT load steps, this is a fixed load increment method.
    If you add the PARAM,EXPERTNL,YES to the deck, the time increment method becomes automatic in which case, the increment (load) is increased or cut back based on the convergence rate.
    NLGEOM loadstep has automatic time step by default. In NLGEOM loadstep the RAMP load curve can be defined using TABLED1 card and then refer this in NLOAD1 card.
    In nonlinear static analysis, OptiStruct uses the Newton-Raphson method to obtain solutions for nonlinear problems to maintain the residuals close to zero (equation 2)
    In a nonlinear analysis the solution usually cannot be calculated by solving a single system of equations, as would be done in a linear problem. Instead, the solution is found by applying the specified loads gradually and incrementally working toward the final solution. Therefore, OptiStruct breaks the simulation into a number of load increments (NINC) and finds the approximate equilibrium configuration at the end of each load increment.
    It is important that you clearly understand the difference between an analysis step (NLSTAT / NLGEOM), a load increment (NINC of NLPARM card), and an iteration (MAXITER of NLPARM card)
    The load history for a simulation consists of one or more steps. Within a step you will have many no of increments (NINC), within increment there can be many no. of iteration (MAXITER). OptiStruct checks the equilibrium equation ( equation 2) for each and every iteration. If the solution from an iteration is not converged, OptiStruct performs another iteration to try to bring the internal and external forces into balance.
    An increment is part of a step. An iteration is an attempt at finding an equilibrium solution in an increment when solving with an implicit method. If the model is not in equilibrium at the end of the iteration, OptiStruct tries another iteration. With every iteration the solution OptiStruct obtains should be closer to equilibrium; sometimes OptiStruct may need many iterations to obtain an equilibrium solution. When an equilibrium solution has been obtained, the increment is complete.
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