Help with the unit of displacment in modal analysis

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Help with the unit of displacment in modal analysis

Rodrigo J. Silva
Hi guys! I'm a beginner in ABAQUs.

I'm realizing a modal analysis (free vibration) on a thin plate to obatin
the first 10 mode
shapes. I need, specifically, the displacments in Z direction (U3). I'm
entering with the inpunt data to obtain results with units em meters
(Elastic Modulus in Pa; dimensions in meters and density in Kg/m^3).

The plate have dimensions of 270E-3 x 360E-3 meters (270x360 millimeters).

The problem is the values of displacments em Z. They are absurds! Look to
the node 1, for example... can this one have a displacment of 3.5 meters in
a plate of 270x360 millimeters? Definitely there is a problem here, right?

Someone can help me?

<http://abaqus-users.1086179.n5.nabble.com/file/t8918/tabela.jpg>
<http://abaqus-users.1086179.n5.nabble.com/file/t8918/tor%C3%A7%C3%A3o.jpg>



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RE: Help with the unit of displacment in modal analysis

David Lindeman
Modal analyses do not give you displacements, they give you eigenvectors (which are normalized to one, or normalized using the mass matrix).  Ask yourself one question:  what is my loading?  Without a meaningful load you can’t have meaningful displacements, strains, or stresses.  A simple frequency analysis will only give you mode shapes.

Regards,

Dave Lindeman
Staff Scientist
Corporate Research Systems Laboratory
3M Center 235-3G-08
St. Paul, MN 55144
651-733-6383

From: [hidden email] <[hidden email]>
Sent: Wednesday, August 14, 2019 6:46 AM
To: [hidden email]
Subject: [EXTERNAL] [Abaqus] Help with the unit of displacment in modal analysis



Hi guys! I'm a beginner in ABAQUs.

I'm realizing a modal analysis (free vibration) on a thin plate to obatin
the first 10 mode
shapes. I need, specifically, the displacments in Z direction (U3). I'm
entering with the inpunt data to obtain results with units em meters
(Elastic Modulus in Pa; dimensions in meters and density in Kg/m^3).

The plate have dimensions of 270E-3 x 360E-3 meters (270x360 millimeters).

The problem is the values of displacments em Z. They are absurds! Look to
the node 1, for example... can this one have a displacment of 3.5 meters in
a plate of 270x360 millimeters? Definitely there is a problem here, right?

Someone can help me?

<http://abaqus-users.1086179.n5.nabble.com/file/t8918/tabela.jpg>
<http://abaqus-users.1086179.n5.nabble.com/file/t8918/tor%C3%A7%C3%A3o.jpg>

--
Sent from: http://abaqus-users.1086179.n5.nabble.com/





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RE: Help with the unit of displacment in modal analysis

Rodrigo J. Silva
Yes, I understand that.
But , the modal shapes have "fictive displacements" that represent the shape
of the plate in a specifc modal shape. Right?

I need these displacments in Z direction for numerics aplications.

Softwares like "ANSYS" presents this displacments with consistent values
(43E-3 meters, for example). Theses values aways work out in the method I
intend use.

But, in ABAQUS, these valores are so strangers (3.45 meters). That values,
includ,  seemsing out of scale when we look at the image of the modal shape
(second imagem that I posted).



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RE: Help with the unit of displacment in modal analysis

David Lindeman
Yes, the “displacements” represent the mode shapes, but they do not have units (in Abaqus or ANSYS).  You can apply any arbitrary scale factor to the “displacements” and still have the same mode shape.  So, if the values reported by Abaqus don’t work with your “method”, then you’ll need to figure out what scale factor is appropriate.  By default, ANSYS MAPDL normalizes the mode shapes to the mass matrix (see documentation for MODOPT command).  The default normalization method used by Abaqus depends on the solver options selected, but will generally default to the mass matrix when the AMS eigensolver is used, and to unity when the Lanczos solver is used.  So, the differences you are seeing may very likely be due to the normalization method.

Regards,

Dave Lindeman
Staff Scientist
Corporate Research Systems Laboratory
3M Center 235-3G-08
St. Paul, MN 55144
651-733-6383

From: [hidden email] <[hidden email]>
Sent: Thursday, August 15, 2019 7:06 PM
To: [hidden email]
Subject: [EXTERNAL] [Abaqus] RE: Help with the unit of displacment in modal analysis



Yes, I understand that.
But , the modal shapes have "fictive displacements" that represent the shape
of the plate in a specifc modal shape. Right?

I need these displacments in Z direction for numerics aplications.

Softwares like "ANSYS" presents this displacments with consistent values
(43E-3 meters, for example). Theses values aways work out in the method I
intend use.

But, in ABAQUS, these valores are so strangers (3.45 meters). That values,
includ, seemsing out of scale when we look at the image of the modal shape
(second imagem that I posted).

--
Sent from: http://abaqus-users.1086179.n5.nabble.com/





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