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R
reignofratch1-Visitor
1-Visitor
December 7, 2017
Question

Inverting a matrix does not invert the order of magnitude

  • December 7, 2017
  • 3 replies
  • 5120 views

Hello, I am trying to invert a matrix to solve F=K*U in an FEA problem where F and K are known.

On paper this is solved by K^-1*F=U

 

All the units in K keep their same order of magnitude, which I believe is incorrect.

Multiple online calculators for inversing matrices give several very different answers so I no longer know what to believe.

 

My final answer for U is around 10^20 orders of magnitude higher that the expected value as shown below.

U values should be a 10^-3*m and 10^-1*rad or less.

 

I have attached the files as both pdf and mathcad prime express 4.0.

 

Please let me know what I am doing wrong.

1.PNG

 

3 replies

L
LucMeekes23-Emerald IV
23-Emerald IV
December 7, 2017

Please attach your worksheet.

{You have to ZIP it first, there's a bug on this forum that often causes a failure to attach a worksheet.

To zip, in Windows explorer select the worksheet file, then with the right-mous button select 'Send to'=> 'compressed (zipped) folder' the resulting .zip file can be attached.}

 

Luc

R
reignofratch1-VisitorAuthor
1-Visitor
December 7, 2017

Thanks for the info, I have attached the zippped file.

F
Fred_Kohlhepp23-Emerald I
23-Emerald I
December 7, 2017

Working in Prime 3.0:

minv.PNG

    R
    reignofratch1-VisitorAuthor
    1-Visitor
    December 7, 2017

    Could you explain what is different in your solution?

    It seems like you used the same function on slightly rounded numbers and got a different  very answer.

    Is my issue perhaps intrisic to 4.0?

    L
    LucMeekes23-Emerald IV
    23-Emerald IV
    December 7, 2017

    Your Kb matrix has zero values in the top-right and bottom-left corners. Fred's matrix has all non-zero values.

    If I change the 1,5 and 5.1 elements in your K-matrix (the source) from 0 to 0.01 then all elements in the Kb matrix become non-zero and the results kb-1*Fb get into the range of 10^6 instead of 10^17...

    I guess it's your data...

     

    Success!

    Luc

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