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Why my mode shapes are purely imaginary ?

Experimenter
Experimenter

Hello,

 

   Some hammer hits were given on a low frequency structure. After using the LSCE method to estimate the modal parameters, I found out that the mode shapes had no real part, they were purely imaginary.

 

ComplexModeShape_TestLab.jpg

 

Can somebody explain why the mode shapes are purely imaginary ? What is the meaning ? Am I in the wrong tab ?

(These are normally mass normalized mode shapes by using tab "Modal Validation" -> "Unity Modal Mass" -> Normalize)

ScreenShot001.jpg

 

Also their amplitude seems very low. How would it be possible to retrieve the mass matrix and stiffness matrix ?

 

ScreenShot002.jpg

 

Thanks for your answers,

1 REPLY

Re: Why my mode shapes are purely imaginary ?

Valued Contributor
Valued Contributor

Think this has to do with when you calculate the shapes in the Time MDOF or Polymax worksheets.

 

If you pick "Complex" the shapes have real and imaginary.  If you pick "Real" the shapes are imaginary only.  Picture below:

 

mode_shape.png

 

 

This is a selection you make before hitting the "Calculate" button (middle left of screen) in the curvefitting worksheet.

 

If you think that your structure should have all normal modes (like a simple metallic structure) then use Real.  This will force every shape to be a normal mode shape, where all the vectors are either in or out of phase with each other.

 

If you think your structure has complex shapes (perhaps a rubber bushing or something like this) and that all the modes would not be normal modes, than use Complex.  The shapes can have vectors with an arbitrary phase relationship to each other, rather than just being in and out of phase.