Modal basis in vibration load events definition

While specifying a mode set to define a random or harmonic data source, you can choose the free-free or fixed-fixed boundary conditions to compute the modes contained in the mode set, or set the solver to extract the modal participation factors from a source file, instead of computing them.

Free-Free boundary condition

For the free-free boundary condition, the solver uses the fixed vibration inputs, which you define for the random or harmonic data source, to constrain the model and re-compute the modes during the specialist durability solution process. Choose the free-free boundary condition when:

• The mode shapes are computed for a model that is unconstrained at the excitation degrees of freedom (DOFs).

• You define fixed vibration inputs on the DOFs where the model is to be constrained during durability solution.

• You define excitation loads, such as vibration inputs using either enforced motion or nodal force.

When using Free-Free boundary condition, the mode set must fulfill the following requirements:

• Stress results are available at all the DOFs that are required for durability analysis.

• Displacement results are available at the exited DOFs.

• Rigid body modes are activated in the mode set.

• If modal damping is retrieved from tests, the selected boundary condition, free-free or fixed-fixed, reflects the test, such that the mode shapes and frequencies are the same or highly correlated. For more information, see Simcenter 3D FE Model Correlation.

It is recommended that the mode set adhere to the following guidelines:

• Modes are normalized to modal masses. Otherwise, you must check that the modal masses in the selected mode set are correct.

• Additional static modes reflecting the excitation are included for improving the solution accuracy. The recommended way to include additional static modes is to create the mode set from a flexible body solution, as used in multi body analysis. The unit displacement modes are added, and the mode set is automatically transformed into a modal basis. For more information, see Flexible body analysis and Fixed and free boundary degrees of freedom (Nastran, ANSYS, and Abaqus).

Fixed-Fixed boundary condition

For the fixed-fixed boundary condition, the solver excludes the fixed vibration inputs, which you define for the random or harmonic data source, during the specialist durability solution process. Choose the fixed-fixed boundary condition when:

• The mode shapes are computed for a model that is constrained at the excitation DOFs.

• You do not define fixed vibration inputs.

• You define excitation loads such as vibration inputs using enforced motion.

When using Fixed-Fixed boundary condition, the mode set must fulfill the following requirements:

• Stress results are available at all the DOFs that are required for durability analysis.

• Reaction force results are available at the excited DOFs.

• Rigid body modes do not exist in the mode set.

• If modal damping is retrieved from tests, the selected boundary condition, free-free or fixed-fixed, reflects the test, such that the mode shapes and frequencies are the same or highly correlated. For more information, see Simcenter 3D FE Model Correlation.

It is recommended that modes are normalized to modal masses. Otherwise, you must check that the modal masses in the selected mode set are correct.

Read Modal Participation Factors

Instead of computing the modal participation factors, the solver reads them from the mode set or an external forced vibration solution results file that you specify. When using the read modal participation factors option, the mode set must fulfill the following requirements:

• The mode set for the Specialist Durability solution process is the same as the one used in the analysis to compute the modal participation factors. The mode set for the Specialist Durability solution process may be achieved on a refined mesh only if the modes shapes, frequencies and modal masses are the same or highly correlated.

• Stress results are available at all the DOFs that are required for durability analysis.

• The frequencies of the modal participation factors are equidistant, and all the modal participation factors are defined for the same frequencies.

• The frequency range of the modal participation factors is the same or include the frequency range used in the power spectral densities (PSD).

How do I

Create a temperature source load for part of the model

Create a temperature source load for the entire model

Automatically create a duty cycle load event

Create a random data source

Create a vibration input

Create a random load event

Create a harmonic data source

Create a harmonic load event

Learn more

Supported solution types for load events

Creating a temperature source load

Creating a block load event

Creating a superposition event

Creating a transient event

Creating a flexible body event

Creating a duty cycle load event

Automatically creating a duty cycle load event

Creating a random load event

Creating a harmonic load event

Managing specialist durability load events

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Modal basis in vibration load events definition, Simcenter 3D 2021.1 Series

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