Material Loss Factor

[xydrex]

Hi, I am using the modal dynamics step (linear perturbation)to model the response of a structure with & without damping layers applied to it. I specify the 'material loss factor' by entering the value in the damping, composite section of the material definition. Is this the right way? I assume this 'composite' damping is the 'material loss factor' but I'm not sure. Has anyone used this before? Problem is the documentation has no hits for "loss factor" search, so they must not call it by this name. The only other options (for material definition)are alpha & beta but these are valid only for direct integration steps while composite is used for the modal dynamics.
Thanks.

 

[xydrex]

Correction, it is a Steady State Dynamics, Modal step; although issue would still probably be the same.
Thanks.

 

[PamCrash]

Seems youre talking about the modal damping (*MODALDAMPING....). The correct value to be used is not the loss factor, it is the share of the critical damping. As far asI rememeber this damping value is half of the loss factor.

 

[Drej]

Pam - as you rightly say, the damping ratio ("xi") is half the loss factor ("neta"). The loss factor is sometimes called the "Structural damping factor". If you're entering the damping as a function of material properties it could be many things. It can be the "stiffness multiplier" or sometimes the "constant material damping coefficient". What is it that you want to specify for your damping? It sounds as though you need some form of general damping such as xi, but is it:

- the loss factor?
- Rayleigh stiffness/mass multipliers (alpha/beta damping)?
- quality factor?
- amplification factor?
- etc.

If you want some general form or damping, you should be looking at the damping ratio (xi). Recall the damping ratio is the ratio of a system's actual damping to its 'critical' damping value. If you need to tune (or select) the damping to specific frequencies - or a range - you should be looking at Rayleigh damping and the two constants. You will still need a damping ratio to work to to use this as well. How have you obtained this 'material loss factor' and what are the units?


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[xydrex]

Thanks for your ideas;
The symbol for this material loss factor is the greek 'h' Im not sure the name for it, it looks like an english 'n' with the right side extending down. I am currently working from methods I read about in conference papers which state this material loss factor as being 0.4 to 0.7 for different viscoelastic materials, a unitless quantity. It can vary with frequency, although w/in a certain range they used it as constant in the simulation. Another paper called it simply 'damping factor' in the material properties table. It must be an inherent property of the material, not the system since both works are aimed at maximizing the loss factor of the system by re-distributing the damping material across the surface of steel structures, a kind of optimization. The simulation I have been trying requires you enter the damping property of the material in the Property module (of CAE) leading to:

*Material, name=Material-1
*Damping, composite=0.4

then to activate these in the Step module:

*Step......
*Steady State Dynamics
........
*Modal Damping, modal=COMPOSITE

The only other option in the material definition is to use alpha&beta which I read in ABQ manuals applies to direct integration steps (eg general dynamic implicit) while I am using the Steady State Dynamics which uses the composite.

Other options exist at the step definition including direct modal (for critical damping fraction), structural (damping constant having units)and also the Rayleigh. However I must use something I defined in the material definition because the aim is to see how the different distributions of the damping material across the structure affect the overall system loss.

Thanks..