Hi

Just to comment on the Claudio Pedrazzi

The material loss factor for steel typically lies in the ballpark 5e-4 to 5e-5, i.e. it is transmission to connecting systems that largely determines the damping for systems such as piping. Local modes, e.g. for small bore (appurtenance) may come close to that of the material damping when the mode shape does not load supports which further increases response variation.

When relying on the damping that, depending on your belief system, god or luck chooses to hand out - it is hardly surprising that the damping value both various and can be hard to assess - after all, it is an uncontrolled variable.

The remedy is to actively design the damping and to make it much stronger than naturally occurring damping phenomena.

Another comment is that earthquake design can goof things up. Protecting equipment from earthquake loads often entails vibration isolation. This may cause a situation where response during normal operation may exceed that of the earthquake response by several orders of magnitude when the source is on the isolated ('protected') side , i.e. we may risk using up the equipment fatigue life well before the earthquake.

Figure 7 demonstrates this conundrum. A SEM (a sensitive machine in a nanolab) is protected from external vibration. The protection level is excellent, until the machine itself is turned on and internal sources start to add vibration. In this case, the situation could be left as is as it did not affect the machine operation or quality. http://qringtech.com/2014/01/28/need-to-know-about-vibration-isolation/

Again, actively adding damping is a way to resolve this catch 22 situation.

Just my 2 cents

Claes

Because it's difficult to analytically calculate

To determine the damping ratio, you have to measure the system displacement for different natural frequencies. This is not an easy task.

Hi

I agree with Dr Pany - our understanding of damping is poor. The rest follows from this. I usually say that it is damping or the dam-d thing...

Some ramblings of mine on the matter of damping can be found here

http://qringtech.com/2014/06/22/designed-damping-types-mechanisms-application-limitation/

Sincerely

Claes

" Any reason why so difficult to determine damping ratio?"

Well, think about how to do it.

Mass/inertia: easy, can physically measure or use solid modeling tools to obtain fairly accurately without much trouble

Stiffness: easy, can use FEA simulation or devise static F = kx experiment to measure

Damping: hard, need to experimentally measure (dynamically)

If you can do dynamic experiments across the active frequency range, then you have damping, but it can be a tough or time-consuming experiment.

Its a complex thing to understand. More because to calculate it we have to measure experimentally the displacement in consecutive cycles of vibrations and see how long it takes to reduce to equilibrium. Now this displacement in each cycle depends on so many factors, the material, stiffness, friction, aerodynamic drag etc. So its quite difficult to analytically calculate correctly....almost impossible.

Damping ratio can be calculated from the hysteresis loop obtained from Cyclic Triaxial Test in case of soil.

Area under hysteresis loop indicates energy dissipation during cyclic loading.

Damping ratio depends on how densely the particles are arranged in a material and how linearly the material behaves to the external loading.

It changes with cycles and decreases if denseness increases.If denseness increases the behavior become more linear.

The main reason for me is that damping is dominated not by the intrinsic loss of the material, which can be determined very accurately versus temperature and frequency on a dedicated machine, but is dominated by the losses in the junctions and assemblies, which cannot be easily measured. Still, properly executed, dynamic tests with an instrumented hammer or even better a vibration shaker, and processed by a proper modal analysis software, will provide precise results as long as we remain in a linear domain (small amplitudes)

It depends on the system you are considering. Are you talking about soil, structures, machines, etc? In the case of structural damping, it isn't difficult to determine the damping ratio for a single degree of freedom system.

Nice query@ Jack Chong

Following

Interesting question.

Interested

Hi

Damping can be internal or external (i.e. radiated, convected).

Internal damping implies conversion of vibration energy into heat. Some IR cameras have the ability to measure vibration using heat.

Vibratory heat can be measured as amplitude/phase (real,imaginary) when measuring heat with a reference. Depending on the reference, the real part would be the loss and the imaginary part energy storage or, vice versa.

Here are some interesting papers on this take.

Article Damping heat coefficient – Theoretical and experimental rese...

Conference Paper Damping characterization by infrared thermography

Sincerely

Claes

Hi

Just to comment on the Claudio Pedrazzi

The material loss factor for steel typically lies in the ballpark 5e-4 to 5e-5, i.e. it is transmission to connecting systems that largely determines the damping for systems such as piping. Local modes, e.g. for small bore (appurtenance) may come close to that of the material damping when the mode shape does not load supports which further increases response variation.

When relying on the damping that, depending on your belief system, god or luck chooses to hand out - it is hardly surprising that the damping value both various and can be hard to assess - after all, it is an uncontrolled variable.

The remedy is to actively design the damping and to make it much stronger than naturally occurring damping phenomena.

Another comment is that earthquake design can goof things up. Protecting equipment from earthquake loads often entails vibration isolation. This may cause a situation where response during normal operation may exceed that of the earthquake response by several orders of magnitude when the source is on the isolated ('protected') side , i.e. we may risk using up the equipment fatigue life well before the earthquake.

Figure 7 demonstrates this conundrum. A SEM (a sensitive machine in a nanolab) is protected from external vibration. The protection level is excellent, until the machine itself is turned on and internal sources start to add vibration. In this case, the situation could be left as is as it did not affect the machine operation or quality. http://qringtech.com/2014/01/28/need-to-know-about-vibration-isolation/

Again, actively adding damping is a way to resolve this catch 22 situation.

Just my 2 cents

Claes

There is no doubt. The damping, as already well explained here, depends upon the interaction of the system with the medium. Differently of the mass and the stiffness, the damping is the primary source of uncertainty in vibrating phenomena. Maybe the most accurate form to estimating the damping be frequency domain experiments. The ultimate thing to do is apply the Rayleigh rule of proportional damping, that is a reasonable estimative when every other attempt to fail.

It does depend on what we are talking about. There are several models to simulate damping behaviour, and the relevant testing procedures about it. If we are talking about material damping, standards like ASTM_E756, ISO_16940 could work quite efficiently and reliably, especially with plastics or composites where damping is higher. But when we are talking about large parts or even complex assemblies, like an aeroplane or a car, then the only way to measure anything is (as mentioned by Bernard Garnier above) is a properly designed and executed dynamic test using impact hammer or electrodynamic shaker.

To try to answer your question, factors like interphase (composites) between the fibres and the matrix, bolts and adhesives in assemblies, method of excitation, amplitude of excitation, material properties' dependence on humidity and temperature, material inconsistency in micro-level etc. affect damping. And since we are talking about a small magnitude, which is really close to measurement error, quantifying damping is rather difficult.