Non-linear modal analysis 1

[Alakin]

hi do you know if it's possible to run a non-linear modal analysis? I have a structure with gap elements and i want to find the first ten eigenvectors. I think that if i used a 103 nastran would convert the gaps in springs

 

[BlasMolero]

Hello!,
Yes, With FEMAP & NX NASTRAN you can perform NON LINEAR MODAL ANALYSIS. You can request prestressed normal modes at the end of each subcase in SOL 106 by adding the following items to the input file.
[ul]
[li]Add METHOD=SID in the subcase of interest. A prestressed normal mode analysis will then be performed at the end of this subcase. The METHOD command points to an EIGRL or EIGR Bulk Data entry which then selects an eigenvalue method.[/li]
[li]For multiple normal mode analyses in SOL 106, the METHOD command may appear in more than one subcase or above all subcases. In the latter case, a normal mode analysis is performed at the end of each subcase. The stiffness used for modal analysis corresponds to the last step of the subcase. Modal analysis cannot be performed at intermediate solution step.[/li]
[li]A PARAM,NMLOOP,loopid command must appear in the Case Control or Bulk Data Section to request a normal mode analysis at the end of those subcases with a METHOD command. The actual value of loopid is unimportant as long as it is a positive integer. This alleviates the cumbersome task of figuring out the exact loopid.[/li]
[li]Add the appropriate EIGRL or EIGR entry in the Bulk Data Section of the NX Nastran Quick Reference Guide[/li]
[/ul]

You can also run a modal analysis SOL103 with contact elements, not only node-to-node CGAP elements but also surface-to-surface no penetration contact elements, take a look to this link:

http://iberisa.wordpress.com/2011/1...-ensamblaje-con-contactos-surface-to-surface/

NX Nastran provides a contact capability for SOL 101 linear static analysis, and also in consecutive SOLs 103, 105, 111 and 112. Contact for the SOLs 601 and 701 is also available in the Advanced Nonlinear Module. Contact conditions allow the solution to search and detect when element faces come into contact. The software then creates contact elements, thus preventing the faces from penetrating and allowing finite sliding with optional friction effects.

Contact Conditions in Dynamic Solutions (SOLs 103, 111 and 112)
==================================================================
A contact condition can be included in a normal mode solution (SOL 103), and in an optional dynamic response calculation (SOLs 111 and 112). In the normal mode solution, contact stiffness result is added from the end of the converged linear statics contact solution. The contact stiffness values in the normal mode solution represents the final contact condition of the structure around the contact interface. Thus, it will appear that the resulting contact surfaces are attached during the normal mode analysis. Since the calculated normal modes include the final contact interface conditions, the response calculation (SOLs 111 and 112) which use these normal modes automatically include the same conditions.

The inputs for the normal mode solution are consistent with differential stiffness solutions which require a linear statics subcase. The difference is that the linear statics subcase should include the BCSET case control command. When defining the normal modes subcase, a STATSUB case control command must be included to reference the subcase id containing the contact definition. The contact solution in the linear statics subcase must fully converge before moving to the normal mode portion of the run.

Contact conditions can be used with the element iterative solver. However, differential stiffness conditions cannot be generated with the element iterative solver. Therefore, the default sparse solver will always be used, even when the element iterative solver is requested.

Hope it helps.
Best regards,
Blas.

~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director

IBERISA
48011 BILBAO (SPAIN)
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http://www.iberisa.com

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[Alakin]

Thanks for your help, you've been very helpful