Damping properties of materials 9

[WIM32]

I've been looking for documentation on damping properties of materials, non-dependend on the dimensions and construction. Until now it seems that damping ratio is not a material property or constant (like Youngsmodulus). Is there anybody who can shed some light on this subject???
There are many sites and publications that state that concrete has better damping properties than steel. This is oc course true, but a value is not given (at least not un-depending on the structure)...
Any info is welcome!
rgrds
Wim

 

[izax1]

I always think there is (at least) two "black holes" in mechanical engineering. One is damping and the other is joint friction. I think the reasion why it is so difficult to find damping data for specific materials, is that it is generally not a very useful parameter. It will be dependent on if the material is dynamically loaded in tension/compression, bending or torsion, the natural frequency, fundamental or higher order mode etc, etc.

The material damping will generally be a very small part of a total damping of the structure because welds, bolted or riveted connections and other interfaces will have much higher contribution to the overall damping.

I have collected some material on the matter over the years. There is an article in Jouirnal of Applied Mechanics (from 1946!!) called "Internal friction in Engineering Materials" which describes the concepts quite good, and give some examples of material damping of engineering materials.

Good luck!
Bernt

 

[tomirvine]

I have some tables of empirical damping ratios that I have collected from various references. The data covers concrete, steel, and other materials.

I will send you the data if you send an Email request to me.

Tom Irvine
Email: tomirvine@aol.com

http://www.vibrationdata.com

 

[WIM32]

Tom,

I also have a list of emperically attained damping ratio's of several structures, both steel and concrete. The problem is that these ratio's are depending on the construction, and they are not valid for a different structure. For example: a damping ratio of steel, obtained via measurements of a one-sided clamped beam, may not be valid for a structure containing welds and bolted items.
I'm looking for actual material constants, valid for any structure. This constant should then be substituted in a formula, where also form and other material constants are present.

 

[GregLocock]

Um, WIM32, if Tom offers you information you should bite his hand off. He KNOWS stuff.

You are asking for the impossible. I have measured the real damping characteristics of steel structures (car bodies) for 20 years (no wonder I'm eccentric). The variation in modal damping seems inexplicable to me, and my friends (loosely speaking) in the FEA world have yet to come up with a good way of deriving damping ratios, even for structures where the material is well know, and the construction techniques are reasonably consistent.

The best way of acquiring knowledge/cynicism on this subject is to grab a two channel analyser and an impact hammer and an accelerometer. Then hit things. Some companies used to hit every component they made. It seemed silly at the time, but the database they built led to a rapid understanding of which modal characteristics were important.

Cheers

Greg Locock

 

[WIM32]

Hitting things is what we're doing now; we hit every product that leaves our plant. But it seems very difficult to believe that there it is not a material constant. Because if I make a beam of steel and the same beam, but now of concrete, there is a clear difference in damping, so damping IS material related. But looking at the responses I've gotten until now I think I should not search further to find these "constants".
I'd rather not bite Tom's hand off, 'cause then typing becomes mre difficult and we'd not get these fast replies
thanks for your advise though

 

[GregLocock]

The experience you gain by hitting your own products will give an excellent understanding of the damping in those products. I am sure there are underlying physical properties there, but the devil is definitely in the details.

A simple example, for me, is the difference in damping between the first torsional mode of a car body-in-white, and the first bending mode. The frequencies are typically at say 27 and 24 Hz, respectively. The material and joints in the car that are involved are roughly the same for both modes. The damping ratio could be 25% different for these two modes.

Cheers

Greg Locock

 

[hacksaw]

I agree with Greg and the guy without the hand (Hi!), the devil is in the damping.

It is just one of those parameters that everyone knows is bogus, but which everyone uses to explain the measurements. It is more than viscous, almost never entirely structural, and usually non-linear, so about all you can do is allow for a certain amount of ignorance. A good starting point is to take advantage of a helping hands to sort out the dead-ends...

 

[strokersix]

1960s vintage General Motors 292 six cylinder engines were introduced with steel cranks and after only a year or two switched to nodular iron cranks. I'm told the steel cranks had breakage problems from torsional harmonics and the nodular cranks had enough internal damping to eliminate the breakage. Can this be true? I would appreciate both the theoretical perspective as well as practical knowledge.

Thank you.

Mike

 

[EnglishMuffin]

I have always been under the impression that nodular iron has very poor fatigue strength compared to billet or forged steel, so it sounds back to front from that point of view, but there could be some truth in it. However, you might find (for example) that they changed the geometry at the same time as the material, or added a crankshaft damper or something at the same time, so it may never be clear what happened. And if the damping theory is true, did they change the material expressly for the purposes of eliminating the breakage, or was it an accidental benefit, I wonder? Knowing automotive companies, the most likely reason they changed would have been to save money. But I expect someone knows the real scoop.

 

[Margaria]

Hi WIN32,

In my opinion damping ratio isn´t a material property or constant. Meanwhile there is a relationship between damping ratio of a structure and Young Modulus of the material.
So when you say:
"Because if I make a beam of steel and the same beam, but now of concrete, there is a clear difference in damping, so damping IS material related",
in fact damping of the structure changes because the Young Modulus of the materials are differents.

Regards

Margaria

 

[MikeyP]

Margaria,
Strokersix did not mention damping ratio. He/She talked about internal damping.

Strokersix,
There will be differences in the inherent levels of damping of the two materials. However the total damping of a crank mechanism will be determined to a much greater extent by the effect of bearings, interference tolerances etc. The damping effects of these will be several orders of magnitude greater than the inherent material damping.

As for the real reason they changed the material? I favour EMs theory, but then I'm a cynic too.

M

--
Dr Michael F Platten

 

[strokersix]

My assumption from the beginning was the material change to nodular was cost driven. I figured GM had a new product introduction schedule and the steel crank was considered a lower risk approach until the nodular crank could be fully tested and developed. I've heard from more than one source that inline six race engine nodular cranks are more durable than the steel ones. Doesn't make sense to me, hence my question.

Thank you to all who responded and look forward to any more thoughts!

 

[unclesyd]

I have an old grey-iron properties handbook that gives the relative vibration damping qualities of Gray Iron, Ductile (Nodular)Iron, and steel as pictorial graph. The decay period of DI is 1/2 that of steel and double that of GI.

It also has another chart that shows "The Percent Decrease in Amplitude per Cycle at varying stress levels.
Steel is almost a flat line from a 1% decrease @5,000 psi to a 2% @ 30,000 psi stress.
Class 50 CI goes from a 3% decrease @ 1,000 psi to a 9% decrease @ 21,000 psi stress.
Class 25 CI goes from a 16% decrease @ <1,000 psi to a 20% decrease @ 2500 psi stress.

A couple of other general statements given.

The damping capacity is little affected by either temperature, vibrational frequency, or stress history.

Direct quote.
&quot;The acoustic properties are closely related to the damping capacity since the entire duration of sound emanating from a vibrating metal only continues for the actual vibration period of the metal part.
The acoustic resistivity increases with the increasing elastic modulus and specific gravity and is expressed:

mu = sqrt (E x rho)

mu = acoustic resistivity in 10^4 cm^-2 sec^-1
E = modulus of elasticity in kg/mm^2
rho = specific gravity in g/cm^3

The acoustic resistivity is listed as 255 and the velocity sound as 12,000 ft/sec in plain grey iron (Critical Tables).
Both the acoustic resistivity and velocity of sound decrease with increasing temperature.&quot;

 

[EnglishMuffin]

But of course, the issue is whether the undoubtedly greater material damping of nodular iron is significant enough to offset it's inferior fatigue properties, also bearing in mind that the oil shear which occurs in the journal bearings is likely to be a major contributor to the overall amount of damping present. Hence my skepticism. But of course, I could be wrong.

 

[vanstoja]

Getting back to WIM32's original question from 3/19/03 on rankings of material damping capacity, the following references provide info on the subject:
Lazan,B.J.& Goodman,L.E.(1956), &quot;Effect of Material and Slip Damping on Resonance Behavior&quot; ASME Booklet Shock and Vibration Instrumentation, pp.55-74
Birchak,J.R. (circa 1976), &quot;Damping Capacity of Structural Materials&quot; Shock and Vibration Digest(?), pp.3-11
Schetky,L.M. & Perkins,J. (1978)&quot;The Quiet Alloys&quot;, Machine Design, 4/6/78, pp.202-206
Lazan (who contributed to a book on the subject)provides tabled and plotted data for gray iron, magnesium, aluminum, 1020 steel, 403SS, and some superalloys using a damping parameter called specific damping energy in in-lb
per cu-in per cycle.
Birchak's literature review of 34 papers plots specific damping capacity vs stress for 2 plastics and 17 metals or metal alloys including cast iron, steels, brasses, magnesium alloys, stellite and a titanium alloy. He identifies Lazan's book input as:
Lazan,B.J. &quot;Structural Damping&quot; (J.F.Ruzika,ed.), ASME, 1959
Schetsky's article provides Specific Damping Capacity(SDC) values for 20 metals or alloys with SDC's ranging from 49%(Magnesium) to less than 0.2%(aluminum/nickel/titanium alloys and brasses). High-damping alloys are assigned SDCs of 20% or higher. SDC is defined as the percent of strain energy that is dissipated per cycle from decay of free oscillation after initial deflection of torsional, bending or axial deflection of a sample bar, assuming strain energy is proportional to amplitude.
An internet search of &quot;material damping of vibration&quot; indicated that most of the current work on development of high damping materials is related to composites rather than metals or metal alloys.

 

[GregLocock]

Re crankshafts - our SG cast iron crank will run a full durability without a Torsional Vibration damper. I have worked with engines that would break forged steel cranks if the TV damper failed.

I know this is not a direct comparison, but it is indicative that the damping of CI could more than compensate for its lousy fatigue life.

TVs are relatively poorly damped in an engine, so the basic material is one of the few ways of introducing damping into the system unless you add explicit components like a TV damper.

This is in contrast to many resonances on an engine which are damped by gaskets or joints, rather than the innate material of the component.

Cheers

Greg Locock

 

[EnglishMuffin]

Interesting. If that is true, it must be one of the few instances in engineering, not just specifically engines, where material damping alone really counts for something. Are you absolutely sure that there are no geometry aspects that might be a contributory factor in this particular case? (Presumably not, since as you say, it is not a direct comparison).

 

[GregLocock]

I wish I knew. We do have a steel crankshaft (machined from solid), but I can't imagine anyone will let me break it just to find out!

Oh, if you want examples where intrinsic damping is important then there are many applications in the noise and vibration world - diff housings (CI not aluminium) - intake manifolds (plastic not aluminium) - engine covers (plastic not aluminium again).

If you want examples where the innate damping is useful against structural failure of the same component, yes I agree, that is rare.

My /guess/ is that the internal discontinuities required to generate damping also compromise the molecular structure of the material, so what you gain in damping you lose in yield strength. Also, these localised discontinuities are a good way of pumping energy into the lattice, which will speed the growth of dislocations.

Well that's a damn fine theory, completely unprovable!





Cheers

Greg Locock

 

[EnglishMuffin]

Well, yes - I wasn't really thinking about noise. You never hear of racing guys using nodular iron do you? Its always &quot;billet steel&quot; or a forging, or something like that, nitrided or tuftrided etc. Just out of interest, how do you do a &quot;full durability&quot; test of a crankshaft ? Is it done using extreme stress levels ? At normal levels, it would take an awfully long time to failure wouldn't it ?