ABAQUS Keywords Reference Manual  

*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.

Product: ABAQUS/Standard  

Type: History data

Level: Step  

References:

Optional, mutually exclusive parameters: 

MODAL

Set MODAL=DIRECT 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. If the *MODAL DAMPING option is used without parameters, MODAL=DIRECT is assumed.

Set MODAL=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 Manual) from the material damping factors given on the *DAMPING material definition option (“Material damping,” Section 12.1.1 of the ABAQUS Analysis User's Manual). Composite modal damping can be used only with DEFINITION=MODE NUMBERS.

RAYLEIGH

Include this parameter to select Rayleigh damping. The damping term 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 .

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 option can be used only with the *STEADY STATE DYNAMICS (“Mode-based steady-state dynamic analysis,” Section 6.3.8 of the ABAQUS Analysis User's Manual) or *RANDOM RESPONSE procedures (“Random response analysis,” Section 6.3.11 of the ABAQUS Analysis User's Manual). The value of the damping constant, , that multiplies the internal forces is entered on the data line.

Optional parameter:

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.

Data lines to define a fraction of critical damping by specifying mode numbers (MODAL=DIRECT 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 (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 (MODAL=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.)

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, .

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 (MODAL=DIRECT 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 (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, .

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.