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  1. Rahul R liked a post in a topic by jiaru72300 in About the Hex Mesh   
    You should to switch the SPC to ASET card
    The rigids element needs to be switched to rbe2
  2. Mrt liked a post in a topic by jiaru72300 in stress concentration problem of contact   
    Model description:
    A set of gears
    Non-linear analysis conditions (NLSTAT)
    There have contact between the gears (Slide)

    Problem Description:
    The ideal result should be that the largest stress is at the root of the tooth, but the analysis result is that the largest stress is at one point of the tooth surface.
    This maybe stress concentration.
    Is it a model problem? Or, if not, how to solve the stress concentration problem caused by this analysis.
    When I look at the results at different times,
    stress is still relatively uniform even at t=0.9, but stress concentration occurs at t=1.0.
    Is the load equally divided?
    The following pictures show the stress results
    On the left is t=0.9. On the right is t=1.0.

  3. Rahul R liked a post in a topic by jiaru72300 in nonlinear no converge   
    I calculated the model. use the NLSTAT.
    The result shows that the model can normally converge,
    But the contact has problems, I recommend using Pcont card to control.
    M14 momen 30%_Nol.out

  4. jiaru72300 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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