Is damping coefficient of 1-DOF system (simple spring-mass-damper system) dependent on the shape of the mass? Is the usage of 2% damping ratio for the first mode of all metallic structures justified? Should it not be shape-dependent?
Hello,
The damping coefficient depends on the shape of the body when the effect of the fluid on the solid is considered. For instance, the viscous effect of the air/water the mass vibrates in.
Unless you are considering the fluid-structure interaction, the damping coefficient is more likely dependent on the mechancal frictions in your system, especially if connections between different parts are involved.
I guess the 2% you mention is the intrinsic damping of a generic continuous metallic structure, which takes into account the internal frictions of the material.
Best,
G.
Hello,
The damping coefficient depends on the shape of the body when the effect of the fluid on the solid is considered. For instance, the viscous effect of the air/water the mass vibrates in.
Unless you are considering the fluid-structure interaction, the damping coefficient is more likely dependent on the mechancal frictions in your system, especially if connections between different parts are involved.
I guess the 2% you mention is the intrinsic damping of a generic continuous metallic structure, which takes into account the internal frictions of the material.
Best,
G.
Dear all
Damping can be many things. I find that it usually boils down to it being damping or the dam-d thing. Some ramblings of mine on the subject of damping can be found here
http://qringtech.com/2014/06/22/designed-damping-types-mechanisms-application-limitation/
Take a look at Table 1 in the above link which is the result from Experimental Modal Analysis made on four piping systems running, more or less, in parallel. Table 1 is C/Ccrit, i.e critical damping ratio and thus, C/Ccrit 1% is 2% structural damping - the value Mr Pedrazzi correctly informs us that the standard instructs us to use and, consequently, is the most often used damping value in piping analysis.
My conclusion is a simple one - if you do not specifically design damping to be part of your system - you simply get something and should not be surprised at the variation.
To be a bit provocative - the design process for damping when building things is the equivalent action of telling people just to cut pieces and then make something from the parts while the standard instructs people to cut all parts into 2 m segments and then build something.
There is room for improvement.
To answer your specific question.
A sdof or 2-dof system is comprised from lumped mass(es) and a mass-less springs, i.e. from so called lumped elements. A lumped element exists at a point, i.e the mass has zero area and the spring zero length. It therefore follows that a true lumped system cannot have any shape dependency on damping as such elements lack the property shape.
Lumped systems are a mathematical abstraction and, therefore, can have any damping value.
Sincerely
Claes
Hi Claudio
If I did not know anything, I would guess 2% as well as this is the usual ballpark value for damping in built up structures that are bolted together.
I understand that the gist of the matter is to err and to err on the safe side of things. This is the design methodology also in aerospace.
The crux of the matter, as you can see from Table 1 is that the 2% is not a conservatively low value for the fundamental piping modes but rather a stab at landing somewhere in the middle for most built up structures that are bolted together.
Material damping for steel depends on the material type and usually varies between 5E-4 and 5E-5 for the structural loss factor, i.e. pipe damping does not originate within the system but very much from its coupling to other systems. The damping perceived on the pipe system is more of a radiation type of damping and friction damping at its supports.
Using the 2% value therefore becomes more than optimistic when dealing with some smallbore cases, i.e. local vibration on part of the system. A smallbore resonance that vibrates in torsion of pumps in/out of the pipe shell only loads the pipe systems locally and, hence, has a loss factor closer to that of the material damping.
Nonlinear effects likely add damping as friction then may come into play.
What I am driving at is that pipe systems usually exist for a great number of years - it is not a big thing to vibrate these using bump test or make FRF measurement to estimate its damping value. If it falls below the design criteria, then it is not a big thing to add damping to the system using, e.g. Gerb viscodampers.
I guess the dust settles on the observation that damping is overlooked, i.e. that it should be a design parameter rather than a value on a wish list.
Just my 2 cents
Claes
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