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Preloaded modal analysis

Hello everyone,

I would like to ask for an advice in solving a modal problem on a preloaded structure. I know that this is easily attainable with linear statics using STATSUB card, but I would like to find normal modes of a structure after non-linear static analysis (with contacts). Is it possible to run this sequence of analyses in OptiStruct: nonlinear statics --> normal modes? If not, would you have any suggestions how to do this?

Best regards,

Jakub

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Altair Forum User

Hi Jakub,

Could you please let me know what your objective is?

What is the type of contact you are using in your model?

Rahul Rajan_21763

Hi,

You need to create two load cases in a single job.

The first load case will be nonlinear Quasi-Static (NLSTAT) and the second load case will be Normal Modes

In the normal mode step, pick the first subcase using STATSUB(PRELOAD) option.

See attached thumbnail.

Regards

Rahul R

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Altair Forum User

Thanks for the fast reply.

I am trying to do a modal analysis of a highly elastic system which is preloaded before it enters the operational conditions. So the preloading force deflects the system to high extent and after that it goes under harmonic excitation. I would like to find modal response of this system, which is nonlinearly deflected.

Unfortunately, when NLSTAT is chosen for the STATSUB(PRELOAD) in modal analysis, the solver gives an error:

*** ERROR # 1999 *** in the input data:
In SUBCASE 2: card STATSUB(PRELOAD) references wrong type SUBCASE 1.

where SUBCASE 1 refers to 'geom non-linear (impl statics)' with a NLPARAM parameter and SUBCASE 2 is simply 'normal modes' analysis.

Regards,

Jakub

Update: sorry, I forgot to mention what contact model I am using: its interface type7.

hiranwale_bharat

Hello,

I tried the preloaded modal analysis using two subcases. First with static load and spc and second with spc and the initial static load as the preload to find normal modes. But I am getting same frequency values for both the cases.

In hyperworks help I found the following comment.(found in topic 'loads and boundary conditions')

All results that are supported for regular structural analyses are also available in the corresponding prestressed analyses. It is important to note that, while the prestressed analysis includes the effects of preloading as a weakening or a stiffening of the structure, the results from the prestressed analysis do not include the preloading results. For example, the displacements from prestressed static analysis do not include the preloading displacements. In order to get the overall deflection/stresses of the structure, the displacements/stresses from the prestressed analyses have to be carefully superposed with the preloading displacements/stresses while post-processing. Particularly, while post-processing prestressed direct FRF complex results, the valid approach is to first obtain the complex results for a given phase and then superpose the appropriate preloading result. Any other approach of superposing would lead to wrong results.

It says that results are to be superposed to get the final values. So do I have to perform this superposition in hyperview to see the prestressed modal frequencies. If yes how?

Regards,

Bharat