Damping during modal analysis? 6

[9865395996]

Why we do not applying damping factor while performing modal analysis?
will it not affect the resulting natural frequency values??

 

[PicoStruc]

The damping is included in the Acceleration Response Design spectra.

Each acceleration response for each frequency is dependant of damping used for each time history analysis used to create the spectrum.

If you use CQC method of combination, you should specify damping too.

 

[ESPcomposites]

Modal analysis is an eigenvalue solution that will yield the undamped natural frequency...is is analogous to sqrt(k/m). An eigenvalue solution does not consider damping.

Brian

http://www.espcomposites.com

 

[PicoStruc]

ESPcomposites, you are right, I thought that the OP was refering to Spectrum Response Superposition !

For modal analysis (only), it is possible to account for damping but the solution will be in complex numbers form and the calculation of that is a lot more complex... Check for advance dynamic articles !

 

[9865395996]

thanks picostruct,espcomp.. from your replies i understood that we go for eigen value solution for modal analysis to make the calculation simple and easy.

But now my doubt is, Does the modal frequency values will differ with damping and without damping or will it remain same for both cases ??

if it varies means , how much will be the difference in approximate??

 

[BlasMolero]

Hello!,
Proper specification of damping is probably the most difficult modeling input in Advanced Dynamic Analysis with any fea code, as for example NX NASTRAN. The easiest way to specify damping is to use modal damping, which is often specified as the percentage of critical damping.

Two types of damping are generally used for linear-elastic materials: viscous and structural. The viscous damping force is proportional to velocity, and the structural damping force is proportional to displacement.

In NX NASTRAN the definition is via the structural damping coefficient GE. An alternate method for defining structural damping is through PARAM,G,r where r is the structural damping coefficient. This parameter multiplies the stiffness matrix to obtain the structural damping matrix. The default value for PARAM,G is 0.0. The default value causes this source of structural damping to be ignored.

Please note that Modal damping can be used only in modal frequency response (SEMFREQ SOL111) and modal transient response (SEMTRAN SOL112). Other forms of damping have to be used for the direct methods of response. For frequency response analysis, GE (field 9 of the MAT1 entry) and PARAM,G define structural damping. These variables are also used to specify structural damping for transient response analysis but are not activated unless PARAM,W3 and PARAM,W4 are set to nonzero values. A common mistake is to forget to set these values.

In many cases damping is not an important consideration. For example, a structure's peak response due to an impulsive load is relatively unaffected by damping since the peak response occurs during the first cycle of response. Damping in a long duration transient excitation, such as an earthquake, can make a difference in the peak response on the order of 10 to 20% or so, but this difference is small when compared to the other modeling uncertainties. Therefore, it is often conservative to ignore damping in a transient response analysis.

Best regards,
Blas.

~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director

IBERISA
48011 BILBAO (SPAIN)
WEB:

http://www.iberisa.com

Blog de FEMAP & NX Nastran:

http://iberisa.wordpress.com/

 

[hacksaw]

modal analysis is for excitation free, linearized response conditions.

damping does not alter the natural frequencies in this limit.

If you are dealing with non-linear deformations under forced excitation, then resonance condition is influenced by damping, but then the damping models them selves are no longer linear.

 

[BlasMolero]

Hello!,
A few notes about the fourth type of damping, in addition to viscous, modal & structural damping -- nonlinear damping!!:
• Frequency response and complex eigenvalue solutions are not available in nonlinear analysis.
• A modal formulation (and therefore modal damping) is not available in a nonlinear solution.
• Plastic yield in the nonlinear materials automatically absorbs energy when the structure follows a loading and unloading cycle. This is an actual hysteresis effect that produces an accurate form of damping. However, note that strain rate effects are not calculated directly. Strain rate effects must be modeled with structural damping parameters, which are converted internally to viscous damping.
• The actual damping on nonlinear elements is unpredictable and can change answers for different runs on the same problem—depending on the convergence rate and iteration strategy. It is recommended that the matrix update strategy forces an update on the tangent matrix at every time step.

Best regards,
Blas.

~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director

IBERISA
48011 BILBAO (SPAIN)
WEB:

http://www.iberisa.com

Blog de FEMAP & NX Nastran:

http://iberisa.wordpress.com/

 

[Charlemagne2012]

If you include an arbitrary damping matrix in your model, the modes will no longer be orthogonal with respect to the eigenproblem.

Simply speaking, the “normal” modal solution doesn’t exist. (In addition to this your FE package would have to solve a polynomial eigenproblem, but that could be fixed)

The standard way to overcome this problem is to assume no damping, solve the problem and then add damping as an ad hoc to the solution.

Since damping usually is small and notoriously difficult to model this is viewed as an accepted method for most problems. If you have a large known damping that’s unevenly spread across your structure you should use a direct solver for each frequency instead.

 

[spongebob007]

For most structures of practical interest the error introduced to the natural frequency calculation by ignoring damping is small.

For damped free vibration:

Wd=sqrt(1-R)*Wn

where Wd is the damped natural frequency, R is the damping ration, and Wn is the undamped natural frequency.

The damped natural frequency is always less than the undamped natural frequency.

If there is no damping Wd=Wn

For small values of R Wd~Wn

 

[spongebob007]

Correction:

Wd=sqrt(1-R^2)*Wn

So for R=.05, Wd=.999Wn