Damping properties of materials

damping
elastic-modulus
#1

SOURCE https://www.eng-tips.com/viewthread.cfm?qid=49887

QUESTION:
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 Youngs modulus). 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 of course true, but a value is not given (at least not un-depending on the structure)…

REPLIES
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 reason 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 Journal 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.

GregLocock
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.

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”.

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.

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.

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.
"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."

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 on rankings of material damping capacity, the following references provide info on the subject:

Lazan,B.J.& Goodman,L.E.(1956), “Effect of Material and Slip Damping on Resonance Behavior” ASME Booklet Shock and Vibration Instrumentation, pp.55-74
Birchak,J.R. (circa 1976), “Damping Capacity of Structural Materials” Shock and Vibration Digest(?), pp.3-11
Schetky,L.M. & Perkins,J. (1978)“The Quiet Alloys”, 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. “Structural Damping” (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 “material damping of vibration” 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.

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!

EnglishMuffin
Well, yes - I wasn’t really thinking about noise. You never hear of racing guys using nodular iron do you? Its always “billet steel” or a forging, or something like that, nitrided or tuftrided etc. Just out of interest, how do you do a “full durability” 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?

GregLocock
There’s two test for engine durability. The quick and dirty is one hundred hours at full power. That is the make or break for a new design generally, once you can pass that the rest is just details. The other is a rather longer procedure at multiple loads and speeds, which probably adds up to 500 hours (3 weeks).

There are no two ways about it, forged steel cranks are the way to go, everything else is a compromise - but production cars are a compromise all the way from one bumper to the other.

EnglishMuffin
That sort of jibes with what I once heard when I was at college from from a lecturer (= professor US) who had worked at Ricardo’s, which was something to the effect that, as a rule of thumb, if an engine could get through one of their 10 hour tests, then it would probably be OK. 10 hours seemed a bit short to me, but it was a long time ago and maybe I misheard. It sounds as though the crankshaft material thing may have to remain one of those unanswered paradoxes.

unclesyd
If we have some Auto buffs out there I remember in Sports Car Magazine or Hot Rod Magazine as to who had what in cranks. I think it was the Japanese that led the way in DI cranks. I know that Ford did use some CI at one time as we melted tons of the in the early fifties. They had been in the weather for years and based on the chemistry they would have had around been Grade 45.

There ought to be some new information around concerning the acoustic properties on metals as all the new ultrasonic techniques come about.

SignalToNoise
A good source for the damping properties of materials can be found in Cremer and Heckl’s book Structurborne Sound, Springer-Verlag, 1988, Tables III-2 and III-4, Mechanical properties of Metals and Building Materials, respectively. As with any design, acoustics is one of many important parameters to be taken into consideration(including static and dynamic loading, stress concentration, fatigue, weight, costs, etc). You may also consider using viscoelastic damping materials to reduce structural vibrations on the steel. This has worked well for me

aarul
WIM32, I have done some work on measuring damping properties of materials and the two techniques that gives you a reasonable estimate of damping is the constrain beam method and the DMTA (Dynamic Mechanical Thermal Analyser). I have used the latter quite extensively to measure damping properties of adhesive materials.

If you are trying to get damping values of similar type materials, perhaps worth testing it as it is pretty hard to find values of handbooks as described by other members so far.

#2

@GregLocock, do you or others have additional comments, references or links to add this discussion?

#3

You might find some helpful material in ESDU 91012.
Sorry, you have to go through IHS to get a copy.
https://www.esdu.com/cgi-bin/ps.pl?sess=unlicensed_1190625031204dkm&t=gen&p=home

Samples for educational purposes:

For lightly damped structures, typical of those used in the aerospace industry, the natural and resonant frequencies are approximately equal… For panels without additional damping treatments the range of values of viscous damping ratio, based on response measurements on aircraft structural elements, is tabulated below.<

…and a table of typical values for various structural configurations is given, for many common aerospace materials and construction types.
Later…

Further tests on riveted structural elements have shown that the structural damping loss factor appears to be independent of the load amplitude at the fastener line. Based on these experimental results it is concluded that the friction damping at the fastener line of stiffened panels is a constant, dependent only on the number of fasteners along the panel periphery.<

These statements are backed up with cross-references and test data.