Damping

Damping is energy dissipation in a structure. There are two types of damping:

• Viscous damping is proportional to velocity and is caused by viscous effects, such as in a shock absorber.

• Hysteretic (or structural) damping is proportional to displacement and is caused by external friction, such as from slippage in structural joints, or by the internal friction related to the properties of the material.

The type you should use depends on the physics of the mechanism that is causing the damping and is sometimes dictated by regulatory standards.

The effect of damping on your analysis depends on the type and duration of the loading. You can usually ignore damping for short-duration loadings, such as loads resulting from a crash impulse or shock blast, because the structure reaches its peak response before significant energy has had time to dissipate. Damping is more important for longer loadings, such as earthquakes, and it is critical for rotating force loads that continuously add energy to the structure.

You can apply physical damping and/or user-defined modal damping.

When you use the Simcenter 3D Motion Solver, the default value for viscous damping is 5% if the .op2 file contains no damping values, or if it contains viscous and hysteric damping values of 0.0.

Simcenter Nastran and MSC Nastran physical damping

For frequency and transient events, you can include in your response evaluations the physical viscous and/or hysteretic damping calculated by Simcenter Nastran.

Physical damping is based on physical and material properties in the Simcenter Nastran or MSC Nastran model. Simcenter Nastran and MSC Nastran calculate this damping as part of the normal modes solve. This damping is useful if you know that specific elements or materials in the model have known damping values. For example, you can apply physical damping as viscous damping in a bushing element or structural damping in a material.

To use physical damping, you must first define it in your model before solving for the normal modes.

Viscous damping:

• CDAMPi element—Define the Viscous Damping value in the PDAMP physical property table.

• CDAMP2 element—Define the Viscous Damping value in the mesh associated data for the element.

• CVISC element—Define the Viscous Damping values in the PVISC physical property table.

• CBUSH element—Define the Viscous values in the PBUSH physical property table.

Hysteretic (structural) damping:

• CELAS and CELAS2 elements—Define the Damping Coefficient in the PELAS physical property table.

• CBUSH element—Define the Structural value in the PBUSH physical property table.

• G parameter—Create a Solver Parameters modeling object in the Parameters tab in the Edit Solution dialog box.

• Material damping—Define the Structural Damping Coefficient (GE) in the materials record.

After you solve for the normal modes and create the Response Dynamics solution process, use the Physical Damping Settings dialog box to enable the physical damping in your response evaluations and scale it as necessary. The ability to scale the physical damping lets you experiment with the damping level without having to re-solve your modal solution.

During the analysis, physical damping is converted to modal damping. For example, suppose you have a CELAS element that represents a rubber mount. You may want to assign a Damping Coefficient (GE) to the physical property table for the CELAS so that the physical damping is converted to modal damping only for the modes in which the spring has strain energy.

You can view the percentage of the damping ratio for each mode in the Response Dynamics Details View in the %Phys Visc and %Phys Hyst columns. The values displayed are the diagonal terms of the viscous damping matrix or hysteretic damping matrix multiplied by any Scale factor you defined.

For steps, see Define physical damping. For information about the equations the software uses, see Physical damping.

User-defined modal damping

Often, you may not know how damping is associated with the model's physical properties. In this case, you can assign a modal damping ratio to the computed modes.

You can adjust the damping of normal modes by defining viscous and hysteretic damping ratios in the Edit Damping Factor dialog box.

As a starting point, you can try using these sample values:

Component type	Typical damping value
Cast components	0.25% – 1%
Bolted/riveted/welded systems	2% – 5%
Bearing/gear couplers/contact (such as a geared driveshaft system)	8% – 15%

You can view the percentage of the damping ratio for each mode in the Response Dynamics Details View in the %Viscous and %Hysteretic columns.

For steps, see Define modal damping. For information about the equations the software uses, see Modal damping.

Rayleigh's damping

Rayleigh's damping is a software calculation of the viscous modal damping ratios for all the normal modes. The calculated damping is proportional to the stiffness and mass of the model. To use Rayleigh's damping, you can use either of the following methods:

• Define constant values for stiffness and mass in the damping formula.

• Define a viscous damping factor to be applied across a frequency range of your normal modes.

For steps to calculate Rayleigh's damping, see Calculate viscous damping using Rayleigh's method. For information about the equations the software uses, see Modal damping.

Damping is additive

When you run an analysis, physical and user-defined modal damping is additive; the damping matrix is comprised of all of the input damping sources. For example, physical viscous damping due to CDAMPi elements adds to damping due to the G solver parameter and any material damping, and adds to any user-defined modal viscous damping.

For example, you may want to use both physical and modal damping when modeling a car suspension. The shock absorbers have highly localized damping that can be modeled with viscous damper elements. The resulting physical damping significantly damps the modes involving motion at the shock absorbers, but will cause very little damping elsewhere on the model. To distribute a damping over the rest of the system, you could add a 3% modal viscous damping to all of the modes.

You can view the effects of the total, combined damping in the Response Dynamics Details View in the Damped Frequency column.

How do I

Define modal damping

Define physical damping

Calculate viscous damping using Rayleigh's method

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Damping, Simcenter 3D 2021.1 Series

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