More unrealistic displacements

ajd-brown
ajd-brown Altair Community Member
edited November 2020 in Community Q&A

Hi Guys,

 

I am having a problem with unrealistic displacements. The first image below shows the wheel modelled with pretension in the spokes (different tensions on each side) This is what I expect to see.... The spokes are under tension and the rim mainly under compression with areas of peak stress around the parts where the spokes attatch.

<?xml version="1.0" encoding="UTF-8"?>Pretension.thumb.PNG.23141ee524883500dad

However when an additional force is added to the rim, I get the response in image 2. The forces added at the bottom, they are around 0.75kN in x,y,z. The problem is that the spokes which should increase in tension just stretch without increasing in stress?? (the stress is the same as the first image) . The main error here is the displacement of the spokes even though they are fully constrained.

<?xml version="1.0" encoding="UTF-8"?>56e3917c4a1d6_PretensionForces.thumb.PNG

 

Iteration Subcase     Variable  Grid/Elem ID   Value
---------------------------------------------------------
   0    1 MaxDisp                      209   -391.773

     0    1 MaxStress                1474088    8083.37  

 

Does anybody know how i can stop this displacement from occurring?

 

I'm pretty sure its not a constraint issue this time!

 

Thanks,

 

Anthony

Unable to find an attachment - read this blog

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Answers

  • Altair Forum User
    Altair Forum User
    Altair Employee
    edited March 2016

    Hi Anthony,

     

    The displacement is high because of the force. Can you please check the magnitude for these loads?

     

     

  • ajd-brown
    ajd-brown Altair Community Member
    edited March 2016

    Hi Prakash,

     

    I have checked the loads and I have placed 3 loads on three nodes of roughly 2000N combined , the pretension in the spokes is around 1200N on one side and 860N in the other. I would have expected that the wheel could sustain these loads without much trouble, my instinct could be wrong however!

     

    Thanks,

     

    Anthony

  • ajd-brown
    ajd-brown Altair Community Member
    edited November 2020

    I've created a new model, and even with only 100N load on a full size bicycle wheel there are large displacements. I really don't understand what is going wrong! The results from the pretension are exactly what I would expect, but as soon as a force is added, everything goes crazy!

     

    The displacement should be minimal since the tension in the spokes is so much greater than the applied force.

     

    I have used PBEAM elements for the spokes and rim with cross sections assigned from HYPERBEAM, should I have used a different element type?

     

    I used the equivilence tool to join the spokes to the rim and RBE3's constrainted in 12345 (free in 6) to attach the spokes to the hub. I must stress that the pretension works perfectly but everything falls apart when a force is added.

     

    .broken.PNG.e4afc336f94c28970a2a99d40739b

     

    Regards,

     

    Anthony

    Unable to find an attachment - read this blog

  • Altair Forum User
    Altair Forum User
    Altair Employee
    edited March 2016

    Hi Anthony,

     

    I had a look at your model. 

     

    Here are my suggestions:

    1) Create a loadcase with only Pretension

    2) Create another loadcase with only load and use STATSUB(PRELOAD) and refer the 1st pretension loadcase here.

     

    This way you can have the effect of pretension when you apply load. Having both pretension and load is the not the right way.

    <?xml version="1.0" encoding="UTF-8"?>Capture.JPG

  • ajd-brown
    ajd-brown Altair Community Member
    edited March 2016

    Hi Prakash,

     

    This seems to have solved my problem thank you very much for your help!

     

    Regards,

     

    Anthony

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