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Failure of the components with too low load

Hi,

I'm trying to simulate a compression test to evaluate the buckling load, I use a CLOAD card to impose my load but the A-Arm fails also if I apply 1 N as soon as the simulation starts.

I think that the problem is the mesh but there aren't problems if I try to do this simulation with Optistruct. Could someone help me?

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Radioss

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Simon Križnik

Hi,

the problem is indeed with the mesh- there are some elements with small characteristic length penalizing the natural timestep and the AMS timestep imposed is too high for this mesh. Remeshing is recommended while paying attention to the smallest length. Use check elements panel (F10)>time to check timestep of elements. Use 2d>elem cleanup and/or qualityindex panel or remesh with automesh>QI optimize with minimum element size criteria.

antonio.dortona95

Thank you @Hyperman,

I tried to remesh with QI optimize with the minimum size criteria (0.6), I checked the quality index and I used cleanup tools too but the same problem occurs, but if I decrease the time step of /DT/NODA it works but the time of simulation is too high. Is there another solution?

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Simon Križnik

For explicit analysis time step is the time required for a shock wave to propagate across the smallest distance of the element. Time step of a model is directly proportional to the length of an element and is inversely proportional to the speed of sound. So, if the material is stiffer (higher modulus of elasticity), the speed of sound increases resulting in a lower solution time step, as both material modulus and density control the speed of sound. Using the same mesh from implicit will usually result in long solving time in explicit due to small element length.

III_Considerations about Time-Step - Altair University

You can simulate a quasi-static loading more computationally efficient by time scaling, which means applying the load more quickly in order to reduce the simulation time. Usually the load is ramped up in 0.3 seconds. However, the kinetic energy should be low (5-10%) compared to the internal energy for most of the 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). You can plot kinetic, internal, contact, hourglass and total energy in Hypergraph. Those variables are output by default and can be loaded in HG (file: runname_T01)>Global Variables>Y requests>MAG to plot.

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This topic might help:

https://community.altair.com/community?id=community_question&sys_id=d8a6c47a1b2bd0908017dc61ec4bcb38

From Radioss help:

Any targeted model for AMS application should run first in /DT/NODA/CST with a reasonable energy error (ERROR < +2%) and acceptable added mass (MAS.ER < 0.02) along its simulation time. A model unable to run with /DT/NODA/CST will not run with AMS either.

Too much mass added when using /DT/NODA/CST can cause result differences when compared to an AMS result where no mass is added. If necessary, rerun the /DT/NODA/CST model with a lower time step to reduce the amount of mass added and then compare the AMS results.

An AMS job may either fail or diverge, most likely with an error message. However, once an AMS run goes through, several aspects remain to be carefully checked in order to insure a maximized elapsed time saving, as well as a reliable result quality.

These aspects are:
Time step value evolution and comparison with the targeted time step:
The time step should follow the targeted time step without time step drop (occasionally)

If the time step is at any time smaller than the targeted one, the target time step should be reduced

Entity controlling the time step at cycle (SOLID, SHELL, SPRING, BEAM, NODE, INTER, and so on.)

Energy Error evolution (should decrease slowly negatively - not increase)

Output the number of AMS iterations per cycle via /DT/AMS/Iflag - I_flag =2 may help monitoring convergence quality at no extra CPU cost.
Maximum allowed iterations before the divergence stops is 1000. 75 to 100 iteration per cycle is a sign of a poor convergence 50 still may provide some speedup

30 iterations or less is considered a good convergence.

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