fake Stress in Hyperview

Korosh_20927
Korosh_20927 Altair Community Member
edited October 2020 in Community Q&A

hi my Dear

I simulate impact (Intrusion test) with front Door and Pole impactor.

after define interface,velocity,spotweld and .... 

then in Hyperview ,show the result.

my problem is, Before impact between door and impactor, i see some Stress on door at result.

please guide me, why I see stress in Door Mesh before impact?

Bestregard

Unable to find an attachment - read this blog

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Answers

  • Simon Križnik
    Simon Križnik Altair Community Member
    edited February 2020

    Hi,

     

    the stress on the door before the impact is because too high mass scaling is used; the imposed timestep increases the total mass by 633% (check the DM/M or mass error in engine out file), while the recommended is less than 2%.

    There are several other issues in your model:
    -the begin card declares unit system of kg m s, but material and model dimensions are consistent with tonne, mm, s.
    -the initial velocity of the pole is 200- if the units are m/s it means 720 km/h! The pole impact is performed at 32 km/h. 
    -the pole is deformable with added mass applied on it. The computationally more efficient approach would be to put a rigid body on the pole and add mass to this rigid body. 
    -there are initial contact penetrations. Remove them in Hypercrash: Quality>Check all Solver Contact Interfaces or in Hypermesh: Tool>penetration check. If penetrations are less then 1/3 of the gap the Inacti=5 or 6 can be used. 
    -the pole could be moved closer so the impact happens sooner

     

    Unable to find an attachment - read this blog

  • Korosh_20927
    Korosh_20927 Altair Community Member
    edited February 2020

    thank you very much

  • Korosh_20927
    Korosh_20927 Altair Community Member
    edited February 2020

    my Dear,

    I have 2 question. 

    1) in my Model , mean Mesh size 7mm , Density : 9.85E-9 ton/l , E=210000 Mpa, i want to calculate Nodal time step by this equation: tn =L/(E/Density)^0.5

    But i don't know , How to choose the right unit?

    2) if after check Out file, and see increase %ERR= - 99% , is that simulation is good or bad?

    I attached two different Out file,Please checked and guide me.

    best regards

  • Korosh_20927
    Korosh_20927 Altair Community Member
    edited February 2020

    sorry, I forgot to send the files

    Unable to find an attachment - read this blog

  • Simon Križnik
    Simon Križnik Altair Community Member
    edited February 2020

    Glad to help.

     

    1. Actually the formula for nodal timestep is:

    Δtnodal=2mk

    Where m is Nodal mass and k is Equivalent nodal stiffness

    Regarding units, just follow a consistent set:

    https://www.altairuniversity.com/wp-content/uploads/2012/04/Student_Guide_55-57.pdf

    For timestep calculation refer to:

    8f463b80afef8461af97eb889aee5f6736edc29d.jpg?image_play_button_size=2x&image_crop_resized=960x540&image_play_button=1&image_play_button_color=7b796ae0

    III_Considerations about Time-Step - Altair University


    You can review the nodal timestep in the starter out file. Initial Added mass estimation is also printed in starter out file:

    image.png.c50c57c50854c4513ad0ec5dcf06b5e4.png

    2. %ERR= - 99% is not acceptable and means the simulation is diverging and the negative energy means it is dissipated. Energy error debugging:

    ·         Check the last animation file (written before the computation stops with option /STOP in the engine file).

    ·         Usual issue :

    ·         Imposed time step too high

    ·         Material badly defined

    ·         Elastic material with large deformation

    ·         Contact interface (stiffness, gap, contact penetrations and intersections) not correctly defined

    In your case the imposed timestep is too high again: see last column mass error- it should be below 2e-2 (2%), but you have 1 (100%) and 194 (19400%) mass added

     

    You may find helpful Free eBook: Introduction to Explicit Analysis with Altair RADIOSS. I have recently revised the third edition which should be available in the near future.

     

  • Korosh_20927
    Korosh_20927 Altair Community Member
    edited February 2020

    You explained so easily.

    thank you very much

  • Korosh_20927
    Korosh_20927 Altair Community Member
    edited February 2020

    Hi my friend

    With your good help I was able to solve Mass Err.

    But there is still a fault with the energy error(ERR= -99%).I changed some parameter in my model(Contact,Material , ...).However, as soon as a collision occurs, the amount of energy increases.If possible, please guide me to fix this issue.

    Is there an educational film about this?

    Best regards and I hope to meet my good friends in the future.

  • Korosh_20927
    Korosh_20927 Altair Community Member
    edited February 2020

    Model with Impactor(Rigid wall)

    Unable to find an attachment - read this blog

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