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*MODAL DAMPING
Specify damping for modal dynamic analysis.

This option is used to specify damping for mode-based procedures. It is usually used in conjunction with the *SELECT EIGENMODES option for selecting eigenmodes for modal superposition. If the *SELECT EIGENMODES option is not used, all eigenmodes extracted in the prior *FREQUENCY step will be used with the damping values specified under the *MODAL DAMPING option. If the *MODAL DAMPING option is not used, zero damping values are assumed.

Products: Abaqus/Standard  Abaqus/CAE  

Type: History data

Level: Step

Abaqus/CAE: Step module

References:

Optional, mutually exclusive parameters (if no parameter is specified, Abaqus assumes that the modal damping coefficients are provided on the data lines): 

STRUCTURAL

Include this parameter to select structural damping, which means that the damping is proportional to the internal forces but opposite in direction to the velocity. This parameter can be used only with the *STEADY STATE DYNAMICS, *RANDOM RESPONSE, or SIM-based *MODAL DYNAMIC or *COMPLEX FREQUENCY procedures (see “Mode-based steady-state dynamic analysis,” Section 6.3.8 of the Abaqus Analysis User's Guide; “Random response analysis,” Section 6.3.11 of the Abaqus Analysis User's Guide; “Transient modal dynamic analysis,” Section 6.3.7 of the Abaqus Analysis User's Guide; and “Complex eigenvalue extraction,” Section 6.3.6 of the Abaqus Analysis User's Guide). The value of the damping constant, s, that multiplies the internal forces is entered on the data line.

VISCOUS

Set VISCOUS=FRACTION OF CRITICAL DAMPING to select modal damping using the damping coefficients given in this option. The data lines after the keyword line specify the modal damping values to be used in the analysis.

Set VISCOUS=COMPOSITE to select composite modal damping using the damping coefficients that have been calculated in the *FREQUENCY step (“Natural frequency extraction,” Section 6.3.5 of the Abaqus Analysis User's Guide). These coefficients are calculated from the material damping factors given on the *DAMPING material definition option for procedures that use the traditional architecture and from the composite modal damping factors provided on the *COMPOSITE MODAL DAMPING option for SIM-based analyses that use the Lanczos eigensolver (“Material damping,” Section 26.1.1 of the Abaqus Analysis User's Guide). Composite modal damping can be used only with DEFINITION=MODE NUMBERS.

Set VISCOUS=RAYLEIGH to indicate that the damping for a particular mode is defined as , where and are factors defined on the first data line of the option and is the modal mass and is the modal stiffness for mode M.

This parameter supersedes the previously used MODAL and RAYLEIGH parameters.

Optional parameters:

DEFINITION

Set DEFINITION=MODE NUMBERS (default) to indicate that the damping values are given for the specified mode numbers.

Set DEFINITION=FREQUENCY RANGE to indicate that the damping values are given for the specified frequency ranges. Frequency ranges can be discontinuous.

If both the *MODAL DAMPING and *SELECT EIGENMODES options are used in the same step, the DEFINITION parameter must be set equal to the same value in both options.

FIELD

Set FIELD=ALL (default) to indicate that the damping values are to be applied to both structural and acoustic modes.

Set FIELD=MECHANICAL to indicate that the damping values are to be applied only to structural modes.

Set FIELD=ACOUSTIC to indicate that the damping values are to be applied only to acoustic modes.

This option can be used only with VISCOUS=FRACTION OF CRITICAL DAMPING and DEFINITION=FREQUENCY RANGE for uncoupled structural and acoustic modes obtained through AMS eigenextraction.

Data lines to define a fraction of critical damping by specifying mode numbers (if no parameters are specified or if VISCOUS=FRACTION OF CRITICAL DAMPING and DEFINITION=MODE NUMBERS): 

First line:

  1. Mode number of the lowest mode of a range.

  2. Mode number of the highest mode of a range. (If this entry is left blank, it is assumed to be the same as the previous entry so that values are being given for one mode only.)

  3. Fraction of critical damping, .

Repeat this data line as often as necessary to define modal damping for different modes.

Data lines to define Rayleigh damping by specifying mode numbers (VISCOUS=RAYLEIGH and DEFINITION=MODE NUMBERS): 

First line:

  1. Mode number of the lowest mode of a range.

  2. Mode number of the highest mode of a range. (If this entry is left blank, it is assumed to be the same as the previous entry so that values are being given for one mode only.)

  3. Mass proportional damping, .

  4. Stiffness proportional damping, .

Repeat this data line as often as necessary to define modal damping for different modes.

Data lines to define composite modal damping (VISCOUS=COMPOSITE): 

First line:

  1. Mode number of the lowest mode of a range.

  2. Mode number of the highest mode of a range. (If this entry is left blank, it is assumed to be the same as the previous entry so that values are being given for one mode only.)

  3. Scaling factor for the mass weighted fraction of composite critical damping calculated in the frequency analysis. If omitted, the default value is 1.0. If the mass weighted fraction is excluded from the analysis, enter a value of 0.0. Only relevant in SIM-based analyses.

  4. Scaling factor for the stiffness weighted fraction of composite critical damping calculated in the frequency analysis. If omitted, the default value is 1.0. If the stiffness weighted fraction is excluded from the analysis, enter a value of 0.0. Only relevant in SIM-based analyses.

Repeat this data line as often as necessary to define modal damping for different modes.

Data lines to define structural damping by specifying mode numbers (STRUCTURAL and DEFINITION=MODE NUMBERS): 

First line:

  1. Mode number of the lowest mode of a range.

  2. Mode number of the highest mode of a range. (If this entry is left blank, it is assumed to be the same as the previous entry so that values are being given for one mode only.)

  3. Damping factor, s.

Repeat this data line as often as necessary to define modal damping for different modes.

Data lines to define a fraction of critical damping by specifying frequency ranges (VISCOUS=FRACTION OF CRITICAL DAMPING and DEFINITION=FREQUENCY RANGE): 

First line:

  1. Frequency value (in cycles/time).

  2. Fraction of critical damping, .

Repeat this data line as often as necessary to define modal damping for different frequencies. Abaqus will interpolate linearly between frequencies and keep the damping value constant and equal to the closest specified value outside the frequency range.

Data lines to define Rayleigh damping by specifying frequency ranges (VISCOUS=RAYLEIGH and DEFINITION=FREQUENCY RANGE): 

First line:

  1. Frequency value (in cycles/time).

  2. Mass proportional damping, .

  3. Stiffness proportional damping, .

Repeat this data line as often as necessary to define modal damping for different frequencies. Abaqus will interpolate linearly between frequencies and keep the damping value constant and equal to the closest specified value outside the frequency range.

Data lines to define structural damping by specifying frequency ranges (STRUCTURAL and DEFINITION=FREQUENCY RANGE): 

First line:

  1. Frequency value (in cycles/time).

  2. Damping factor, s.

Repeat this data line as often as necessary to define modal damping for different frequencies. Abaqus will interpolate linearly between frequencies and keep the damping value constant and equal to the closest specified value outside the frequency range.