Elastic Strain energy in prestressed eigenfrequency study-inclusion of static prestress?

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Hello,

I am performing an eigenfrequency analysis of a suspended mechanical structure using the Structural Mechanics Module. The model includes gravity, leading to a significant static prestress in some components.

I extract elastic strain energy using the built-in volume integral

I observe the following behavior:

For gravity-loaded (prestressed) eigenfrequency studies, the elastic strain energy differs significantly from the corresponding eigenfrequency study with gravity removed.

The extracted elastic strain energy does not converge monotonically with mesh refinement, even though eigenfrequencies and mode shapes do.

Directional differences (e.g., longitudinal vs transverse motion) appear in the prestressed case but disappear when gravity is removed.

My questions are:

  • In a prestressed eigenfrequency analysis, does the reported elastic strain energy include contributions from the static prestress field, or only from the linearized modal deformation?

  • Is non-monotonic convergence of elastic strain energy under mesh refinement expected when static prestress is present?

  • When gravity is removed, should the elastic strain energy of low-frequency rigid-body-like modes be expected to become isotropic due to loss of a preferred direction?

  • Is elastic strain energy a reliable quantity for evaluating modal energy in strongly prestressed, gravity-dominated modes?

Thank you in advance for your help!


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