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Timestep Calculation


Attached is a simple Excel sheet that can be used to quickly evaluate the timestep calculated by LS-DYNA for solids, beams and shells.
It must be noted that for solid elements, Q is based on the C1 (default = 0.06) only.

timestep.xls

Update
For solid elements using MAT_SPOTWELD, LS-DYNA uses a slightly different method of computing the characteristic length. Rather than using the traditional method (ElementVolume/LargestSurfaceArea), MAT_SPOTWELD based solid elements use LargestArea/DiagonalLength. This is done to overcome a small timestep due to bad spotweld geometry.

  • Morteza says:

    Thanks Suri for the Excel sheet. I think “Dt” for solid element should change to:
    0.9*(F5/(I14 SQRT(I14^2 H14^2)))
    There’s no big difference for this example though.

    Thanks again,
    Morteza

  • Morteza says:

    Also, I think ‘Dt’ for shell element should be as follow:
    0.9*(F6/H16)

  • adam says:

    I have a question

    I want to simulate a proces which in real takes 10 seconds

    Schould I run simulation for 10 sec too

    In LS DYNA manual in example time was 0,01 sec and the velocity of punch was 5000 mm / s

    I have no idea reading about timestep and mass articless

    Can Someone help me ??

  • Suri Bala says:

    Adam,

    If you are using Implicit Static, time has no meaning. If you are using Implicit Dynamics, it may be feasible to perform slow-speed events in actual time. If you are using Explicit, it is infeasible to simulation long-duration events as you lose the advantage of small number of inexpensive timesteps. This leads to methods such as higher-rate-of-loading or mass-scaling with the goal of reducing the number of computational cycles without significantly increasing the inertia forces.

    Suri Bala

  • santu says:

    Can anyone tell me how this time step formula for solid element is came?

  • syazwan says:

    dear sir, can you give some explanation what is a iteration count in CONTROL_IMPLICIT_AUTO keyword.

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