Note: Descriptions are shown in the official language in which they were submitted.
~wo96/0449121q468~ c~,
STRucTul~Al I~OLT OW ARTICLES FlT T Fn WITH
SDAMPING MA~RIAI,S
l~leld of the Invention
The present invention relates to a method for damping an article subject
10 to resonant vibrations More specifically, the present invention relates to a
method of improving the damping properties of an article or structure by
hlLIudu~.il-~; a viscoelastic material, and preferably a fiber-reinforced viscoelastic
material, into cavities or hollow sections of the structure
15l~a~lA d of the Invention
Periodic or random vibrations at or near resonance in structural
members, such as beams, plates, panels, sheet metal, etc., can be ~JIubl~"ldtic
due to the resultant formation of, r~ stresses"j;~l ...,. m~ fatigue,
and sound radiation, for example, in or from the structural members. Such
20 U~J~ ;-dble vibrations are typically induced by external forces and can be
CAI~JGI ;~ by a variety of articles and under a variety of conditions. For
example, resonant vibrations can cause problems in computer hardware and
vehicle engine ~u ~ , which can experience a wide range of I . G~,
Such vibrations cannot typically be avoided by isolating or shielding the
25 structure or its component parts, as by an isolator, for example. This is
because isolators simply delay energy transfer rather than convert mechanical tothermal energy.
Various techniques are used to reduce vibrational amplitudes or damp,
i.e., dissipate mechanical energy as heat, and thereby decrease the resultant
30 stresses, di~l)la.~."~ , fatigue, etc. These techniques ~ ,l ,l the inherent
damping that exists in structures due to friction, rubbing, etc. Certain of these
wo 96/04491 2 1 ~ 4 6 8 2 r~
techniques use via-,u.,l~Li~, materials in surface damping treatments for damping
control. Two types of surface damping treatments are commonly used: (1)
free layer damping treatment; and (2) . ~ ' layer Aamping tr_atment.
Both of these damping treatments provide high damping to the structure, i.e.,
5 dissipation of I ' ' '- vibrations, without sacrificing the stiffness of the
structure. Examples of such damping techniques are described, for eYample, in
U.S. Patent Nos. 2,819,032 (issued January 7, 1953); 3,071,217 ~Issued
January 1, 1963); 3,078,969 (issued February 26, 1963); 3,159,249 (issued
December 1, 1964); 3,160,549 (issued December 8, 1964); and 5,271,142
10 (issued December 21, 1993).
Free layer damping treatment is also referred to as "~ ;) layer"
or "Pytpnc;~ l damping" treatment. In this technique, damping occurs by
applying a layer of vi~uel~ damping material to the surface of a structure.
The material can be applied to one or both sides of a structure. The
15 ' by which this treatment method dissipates, ' ' '- ene}gy' e.g.,
resonant vibrations, involves ~l. f.,.. -~;.... That is, when the structure is
subjected to cyclic loading, for example, the damping material is subjected to
tension-c~ r~ ir.~ and dissipates the energy through an
eYtPnC~ strain TnPrh~
t'nnch~inPA layer damping treatment is also referred to as "shear
damping" treatment. For a given weight, this type of damping treztment is
generally more efficient than the free layer damping treatment. In this
technique, damping occurs by applying a damper consisting of one or more
layers of vi~ ocl~Li. damping material and one or more layers of stiff
, material. That is, this damping technique is similar to the free
layer damping treatment wherein a viscùcl~lic material is applied to a surface
of a structure; however, the viscoelastic material is cnn~trAinP~ by a stiff
~" ~ ,: ,g layer in the c~nctr~inP~I layer treatment. The energy dissipates
from the vi~oel~lic damping material through a shear mechanism that results
30 from constraints by the stiff .~ u,.;"",g layer and the base structure.
2 1 94682
096/04491 P~
Although these surface damping techniques are widely used, the degree
of damping is often times limited by thickness and weight 11 1;~
Fu~ u.~;, they are not applicable to all types of structures, all modes of
vibration, or all frequency ranges. For e~ample, if the structure itself is to be
5 used in some chemical fluids, such as oil, the above surface damping treatments
may not be suitable, at least because the -v;~ Li. material may be
,' ' To overcome this limitation, cnnctr~ lpA layer dampers have been
embedded in structures. However, this damping technique may adversely affect
the geometry and stiffness of the structures. Thus, an alternative approach to
10 damp vibrational energy without adversely affecting the structural integrity of
the article for a wide variety of articles, structural penm~tri~ modes of
vibration, t,IIV;I~ ' 1 conditions, and r.~~ of vibration is needed.
'- of tbe Invention
The present invention provides a method of improving tbe vibrational
damping . ~ of an article containing a structural material and the
articles produced therefrom. The method involves forming a cavity within the
structural material at a point where at least one vibrational mode, i.e.,
resonance mode, is active, and placing a ' vibration damping
20 material into the cavity such that the vibration damping material is ' ~Iy
completely encased, preferably fully encased, within tbe structural material.
Preferably, the ,.~ n.1; ~A vibration damping material is in a flowable state
when it is placed in the cavities, which can be ~- cu.~ A for example, by
pumping or injecting the material. Depending upon the arFlir~tinn one
25 continuous cavity or a plurality of cavities can be formed. The cavity or
cavities can be partially or substantially completely filled with the
1 vibration damping material. As used herein, a "n.~nc.,..o,,.:,.. A"
vibration damping material means that the vibration damping material does not
include c~ u~ g layers of the type used in cnn~tr~inpA layer dampers, e.g.,
30 thin gauge aluminum or stainless steel.
WO 96/04491 2 t 9 4 6 8 2 F~
The ", .."~.. u~; ~d" vibration damping material includes a vii.~l~Lc
material or c.~ ;."~ i.e., blends or layers, of different v;l~,o.,l~L;~,
materials. Useful ~ ocl,~Li~ materials are those having a storage modulus of
at least about I psi (6.9 x 103 pascals) and a loss factor of at least about 0.01.
5 Ad~ ~ Iy and preferably, an amount of the vibration damping material is
placed into the cavity or cavities formed within the structural material to
improve the vibrational damping of the article or the structural material of
which it is made by at least about 10%, and more preferably by at least about
20%, in at le,st one vibrational mode. Preferably, the v;~ material is a
10 Ih- ~ "" ~ polymer at le,~st because ~h ~ polymers are flowable and
e,sily placed in the cavities by pumping, injecting, etc.
In certcun preferred . .,,holl;~ the vibration damping materi,l also
includes an effective amount of a fibrous material. The vibration damping
material preferably includes an amount of fibrous material effective to improve
15 vibrational damping of the article or the structural material of which the article
is made by a factor of at least about two in strain energy ratio of a~ Ieast onevibrational mode. Typically, this requires i ~ about 3-60 wt-% of the
fibrous material into the vibration damping material, based on the total weight
of the vibration damping material. Preferably, the fibrous material is a
20 n~lnTn~-T~ fibrous material, such asglass.
The present invention also provides a damped article comprising a
structural material having at least one cavity substantially completely enc,sed or
enclosed, preferably fully encased or enclosed, within the structural material at
a point where at least one vibrational mode is active and a
25 vibration damping material, as described above, contained therein.
Brief DescriTotion of the l)rawinp~
Figure 1. A schematic of one ~ ho.l;".. .,1 of the present invention
showing the cross-section of an I-beam having a continuous cavity completely
30 filled with a vibration damping material.
~ wo96/04491 219~682 r l,. c~,
Figure 2. A schematic of an alternative elnl,~ ~ of the present
invention showing a cross-section of a portion of an article having several
cavities, each of which is partially filled with a vibration damping material.
Figure 3. A typical finite element model for a solid steel cantilever
S beam (Model I in Example 1).
Figure 4. Resulting mode shape and natural frequency of the first
bending mode for a hollow beam filled with a preferred vi~w~,lc.aii~, damping
material.
Figure 5. Resulting mode shape and natural frequency of the second
10 mode (or first sway mode) for a hollow beam filled with a preferred
vi~L~lic damping material.
Figure 6. Resulting mode shape and natural frequency of the third mode
(or second bending mode) for a hollow beam filled with a preferred vi,.,~l~lic
damping material.
I!~tail~d Description
The present invention provides a method of improving damping
properlies of articles, e.g., structures, structural parts, etc., and thereby solving
noise and vibration problems in a variety of ~ i" ~ e ~ . More
20 specifically, the present invention provides a damping technique that uses a
highly dissipative damping material, with a high loss factor, i.e., at least about
0.01, preferably at least about 0.1. This material generates significant amountsof strain energy in various vibrational modes of interest and dissipates this
energy, thereby ~ ;t6 ~, noise, vibration, and oscillation.
The present invention can be applied to damp, i.e., rcduce the
vibrational amplitude of, a wide variety of vibrational modes, e.g., bending,
torsion, sway, and l~YtPn~ modes, in a wide variety of structural geometries
over a wide frequency range. It results in three~ "~:-",;ll damping, not
simply tWo-~ Pno~ damping. It can be applied to situations in which
~ 30 surface treatments, such as c~ncfr~inpl1 layer treatments, damped struts, fluid
dampers, magnetic and ~ devices, etc., are typically used. The
W0 96/04491 2 1 9 4 6 8 ~ ~ ,/.,~ r ~ ~,
.
present invention is belived to be useful in large structures, e.g., buildings, to
reduce the amplitude and ~ 1. ,a;..~ that result from wind and seismic forces.
The method of the present invention involves the illLlvd~-Liun of an
5 effective ", - ~ "' vibration damping material into one or more
cavities, i.e., hoUow sections or pockets, of the st}uctural material of which an
article is made. As used herein, the phrase "structural material" refers to the
material of which the article is made in which unwanted vibrational modes are
active, e.g., steel, aluminum, structural-grade plastics. Preferably, the
10 structural material is an isotropic material, at least with respect to ieS elastic
properties. As used herein, an "isotropic material" refers to a material having
properties that are ', ' of the direction in which the material is
measured for that property. That is, an isotropic material is one in which the
properties are generally the same throughout, i.e., in all directions. With
15 respect to this invention, isotropic and anisotropic refer at least to the elastic
properties of the material.
The vibration damping material is i..~ull ' into the structur 1
material, e.g., a nonferrous casting, in cavities, e.g., pockets, within the
structural material that forms the article. The articles and cavities therein can
20 be created by any method known in the art, such as machining, molding,
casting, etc. Preferably, the articles are cast or molded articles with cavitiescreated therein. Such cavities can be in any shape, i.e., oval, cylindrical, etc.
In this way the vibration damping material is substantially completely
surrounded by, i.e., encased or enclosed within, the structural material. That
25 is, the vibration damping material is encased or enclosed within the structural
material itself of which the article is made in such a manner that there is
intimate contact between the vibration damping material and the structural
material. This contact allows for the tr~msfer of mechanical energy from the
structural material into the vibration damping material for ~liccip~ n This
30 results in creating an inherently damped article such that the article is self-
damped, as opposed to using a separate and isolated damper that is added to the
-6-
~ WO 96104491 2 1 9 4 6 8 2 P~ JJI
article either as a surface damper or as an embedded damper. Also, tbis results
in a three~ ,1Ally damped article. In this way, a damped article can be
formed that internally converts ' 1 energy to thermal energy.
It should be understood that this cavity does not have to be
5 ~y lly enclosed in the structure or structural material. Rather, in
certain ~ , the cavity could be created at the surface of the structure
by a cap attached to the surface of the structure. This cap would be integrally
attached to tbe surface of the structure as by welding, adhesive bonding, or
r~-r~h~nir:llly fastening, for example, or it could be molded into the structure at
10 the surface. This cap has a sidewall portion and a top portion that ~"1.~1 '''~;Any
completely enclose (along with the surface of the structure) the vibration
damping material. Preferably, the sidewall portion of this cap is thinner than
the top portion, i.e., less than about 80% of the thickness of the top portion,
more preferably less than about 65% of the thickness of the top portion. This
15 cap could be circular, oblong, square, or ~ ,uku in shape.
The ". -. ~ " vibration damping material of the present
invention excludes ~ 1,;, ,g layers, e.g., layers of aluminum or stainless
steel, used in c~ I layer dampers to "constrain" the v;~l~lic material.
It should be llnrlrr~rlorl however, that the present invention does not exclude
20 the use of short fibers intermixed with the vibration damping material, whichcan act to "constrain" the ~ .,oelc~, material. The ~ ~ ~g~ .1 of the present
invention is also 1' ~ ' ' from a laminated C~ IIu~,Liull wherein the
vibration damping material is exposed at all edges. Thus, the ~ ,. ,,. ~ of
the present invention suhct~n~i~lly protects the vibration damping material from2~ attack by moisture, lubricants, chemicals, and oxygen.
Preferably, the structural cavities or hollow sections are cnhc~n~i~lly
completely filled with a damping material, although it is not a 1~l of
the invention that they be completely filled. Typically, an amount of the
damping material is placed in the cavities to improve the damping
30 ~ Ir~ ;r c of the article. Preferably, a sufficient amount of the vibration
damping material is used such that the damping is improved by at least about
wo s6/044sl 2 ~ 9 4 ~ 8 ~ JI
10% (preferably at least about 20%) in at least one vibrational mode. As a
result of this technique, high mechanical strains are introduced into the damping
material when the structure is excited at one or more of its natural r,~.l.,.....
The resulting mechanical strain energy in the damping material is then
5 dissipated in the form of thermal energy, i.e., heat. The higher the strain
energy in the damping material, the more vibration energy is dissipated from
the structure.
The placement of the cavities, i.e., pockets or hollow sections, and
therefore the vibration damping material within the article, e.g., structural
10 member, depends on the geometry of the article and the vibrational modes thatare to be diminished in amplitude. That is, the cavities, and therefore the
vibration damping material, are placed in the article where one or more
vibrational modes are active. By such placement, the amount of strain energy
that is generated in the damping material can be ' The ' ~
15 of these locations can be determined by one of skill in the art using modal
analysis or finite element analysis.
The articles damped by the method of the present invention can be made
of any "structural~ material, although it is preferably an isotropic material.
This includes, for example, metals, epoxy resins, plastics, concrete, and the
20 like. Typical materials are those that are used in structural members, such as
common castings or moldings of plastic, aluminum, titanium, iron, or steel.
This structural material is not a v;~u~l~tic material. In castings or moldings
the cavities are built into the part during the casting or molding process. It is
to be l~n~l~rr~oo~l however, that the damping concept of the present invention
25 can be used in nUIIi~lllUIJiC~ i.e., anisotropic, composite materials, e.g., plastic
reinforced with carbon fiber, as well.
r..lLhc;~ c, it is to be understood that the structural material can
include more than one type of material. For example, two parts each made
from a different type of material can be combined through mechanical
30 fastening, adhesive bonding, or welding, for example, to form a cavity. This
cavity can then be filled with the vibration damping material. In such a
-8-
~ WO 96/04491 2 1 9 4 6 8 2 . ~
situation, the two materials should have a mismatch in modulus, e.g., shear
modulus or Young's modulus, of no more than about 10%, preferably, no more
than about 5%. In the most preferred ~ ;""c, there is no mismatch. If
there is a significant mismatch in modulus, i.e., greater than about 10%, there
5 is an inefficient transfer of ' ' energy into the vibration damping
material for ~liccip~ n Such a situation would occur, for example, in an
article in which a cavity is formed by an el ~1~~ ;r material such as
pol~. ' having a flexural modulus of 1,000-50,000 psi (6.9 x 106 - 3.5 x
108 pascals) and a metal, which typically has a flexural modulus of greater thanI0 1,000,000 psi (6.9 x 109 pascals). See, for example, U.S. Patent No.
5,290,036 (issued March 1, 1994).
The ,~ l";" rl" vibration damping material can include any
material that is viscoelastic. A v;~ , material is one that is viscous, and
therefore capable of dissipating energy, yet exhibits certain elastic properties,
15 and therefore capable of storing energy. That is, a v;~cocl~,Lic material is an
, ' material typically containing long-chain molecules that can convert
l energy into heat when they are deformed. Such a material typically
can be deformed, e.g., stretched, by an applied load and gradually regain its
original shape, e.g., contract, sometime after the load has been removed.
Suitable viscoelastic materials for use in the vibration damping materials
of the present invention have a storage modulus, i.e., measure of the energy
stored during d~fv~ .Liull~ of at least about I psi (6.9 x 103 pascals). The
storage modulus of useful vi~.,vel.,~lic materials can be as high as 500,000 psi(3.45 x 109 pascals); however, typically it is about 10-2000 psi (6.9 x 104 -
25 1.4 x 10~ pascals). P~u~i~,ukuly preferred viscoeLI~ materials provide the
structure with a strain energy ratio, i.e., fraction of strain energy stored in the
damping material relative to the total strain energy stored in the structure, of at
least about 2 % .
Suitable viscoelastic materials for use in the vibration damping materials
30 of the present invention have a loss factor, i.e., the ratio of energy loss to
energy stored, of at least about 0.01. Preferably the loss factor is at least about
2 1 94682
~vo s6/044sl P~ 5
0.1, morc preferably about 0.5-10, and most preferably about 1-10, regardless
of the frecluency and ~ ., r, ~ C; by the material. This loss factor
represents a measure of the energy dissipation of the material and depends on
the frecluency and i . c ~ i by the material. For example, for a
S ~ '- k~l acrylic polymer, at a frecluency of 100 Hz, the loss factor at 20~C
is about 1.0, while at 70~C the loss factor is about 0.7.
Preferred vi,~u~ ic materials typically remain functional after
., _ a wide range of ~Il~ ,l.ltUlC,~, e.g., 40~F (40~C) to 300~F
(149~C). That is, they are capable of surviving a wide range of i
10 without a significant decrease in their loss factors.
Useful vi,~o.,]~Lic damping materials can be isotropic as well as
anisotropic materials, p~i ~ 'y with respect to its elastic properties. As used
herein, an ''a.d~vLIu~h; material" or "IlO...~ullu~J;c materia]" is one in which the
properties are dependent upon the direction of l.l~ulc.,l~..L Suitable
15 v;~- u. I~LiC materials include urethane rubbers, silicone rubbers, nitrile rubbers,
butyl rubbers, acrylic rubbers, natural rubbers, styrene-butadiene rubbers, and
the like. Other useful damping vi~u~l~ui., materials include polyesters,
pul.~ ' poly ' ethylene-vinyl acetate 1uyulyl~ polyvinyl
butyral, polyvinyl butyral-polyvinyl acetate cul,olyl..~ , and the like. Specific
20 examples of useful materials are disclosed or referenced in U.S. Patent Nos.
5,183,863 (issued February 2, 1993) and 5,308,887 (issued May 3, 1994).
U. S. Patent No. 5,183,863 discloses a useful viscoelastic resin
for vibration-damping material which comprises (A) at least one
amorphous polyester resin of low specific gravity in which more than 40 mol %
25 of the dibasic acid moiety is of aromatic type, (B) at least one amorphous
polyester resin of high specific gravity in which more than 80 mol % of the
dibasic acid moiety is of aromatic type, and (C) at least one hardener selected
from the group consisting of ~uly;~uc~ Lc ru ~i-u~ epoxy group-
containing r,.",l.,.",~l~ and acid anhydrides, said C.~ (A) and (B) being
30 in the ratio of from 90:10 to 3û:70 by weight and differing from each other in
specific gravity (at 30~ C.) by 0.06 to 0.15 and also in molecular ~veight by
-10-
WO 96/04491 2 1 9 4 ~ ~ 2 P~,lr-. ~ Q55~/
10000 or more, with that of eithe} of them being higher than 5000. This resin
gives a vibration-damping material which exhibits improved
vibration-damping properties, adhesive strength formability, amd heat resistanceafter forming.
According to U.S. Patent No. 5,183,863, the polyester resins are
formed from dibasic acids and glycols. The dibasic acids include aromatic
di~bu/~ylic acids (such as t~ hlllalic acid, isophthalic acid, c,-i~' ' ' -
acid, 1,5 -~ 'bUAyliC acid, 2,6 . ~ h l, ~1 - bll~ylic acid, 4,4'-
' .' yhl;~1Ju~ylic acid, 2,2-' ,' .ylL~bu~ylic acid, and 5-sodium
~ -~ acid), alicyclic di~bu~ylic acids (such as 1,4-~ loll~Aalle
diw.l)o~-ylic acid, 1~3-cy~' ' " bo;~ , acid, and 1,2-c~,lull~,~.e
di~bu~ylic acid), and aliphatic d;~lJu~ylic acids (such as succinic acid,
adipic acid, azelaic acid, sebacic acid, d~ ,..i;. - buAylic acid, and dimer
acid). These dibasic acids may be used in . ' with tribasic acids (such
15 as trimellitic acid and pyromellitic acid) in amounts harmless to the resin
properties.
The glycols are ~ ' by aliphatic glycols (such as ethylene
glycol, propylene glycol, 1,4-butanediol, 1,3-butanediol, I ~S-I~
neopentyl glycol, 3-u.~ yll..-~A....I:,.I, 1,6 h .~n..l ~~'; ,Y1J!, ~ 1,
20 1,~ ' 1, 2-methyl-1,8-octanediol, 2,2-diethyl-1,3-p.,. ' 1, 2-ethyl-2-
butyl-1,3-1,., . ' 1, diethylene glycol, and triethylene glycol), alicyclic diols
(such as 1,4-~ 1ul,.,,.a.le dimethanol), and aromatic ring-containing diols (such
as adduct of bisphenol A or bisphenol S with ethylene oxide or propylene
ocide). These glycols may be used in ~ nl with I i' 1 or
25 multifunctional c~ un~ "~ such as ilhlldll~lulplup~c~ glycerin, and
p~ ylllli~ul in amounts harmless to the resin properties.
According to U. S. Patent No. 5,183,863, it is essential that the two
polyester resins differ from each other in number-average molecular weight by
at least 10000, preferably by more than 12000, and also in specific gravity (at
30 30~C.) by 0.06-0.15, preferably by 0.08-0.125.
WO 96/04491 2 1 9 4 6 8 2 ~ "~
.
U.S. Patent No. 5,308,887 discloscs a silicone/acrylic basesl
u~ n . ., . The . comprises:
(a) from about 5 parts to about 95 parts by weight of acrylic monomer
wherein the acrylic monomer comprises:
(i) from about 50 to about 100 parts by weight of alkyl
acrylate monomer, the alikyl groups of which have an average of 4 to 12 carbon
atoms; and
(ii) . , " ~ly from about 50 parts to about 0 parts by
weight of a lliul~ocLlil.~L,.Ii~ dlly ~ ' cul~olyl~ modifier monomer;
wherein the amounts of (i) and (ii) are selected such that the total
amount of (i) plus ~li) equa'is 100 j~arts by weight of the acrylic monomer;
(b) c Ull~ a~ùild~ ly from about 95 parts to about 5 parts by weight of
silicone pressurc sensitive adhesive wherein the amounts of (a) and (b) are
selected such that the tot~i amount of (a) plus (b) equa'is 100 parts by weight; (c) about 0 part to about 5 parts by weight of a 1 ' based
upon 100 parts by weight of the acrylic monomer; and
(d) about 0 to about 5 part by weight of a crosslinker basesl upon 100
parts by weight of (a) plus (b).
According to U.S. Patent No. 5,308,887, the term " yll l.;~lly
20 1 ~;u,uol~..-. .;~le modifier monomer", also referred to as the
~modifier monomer" refers to a monomer that is capable of increasing the Tg
(glass transition t~ dl~ ) of a copolymer fonned from the acrylic
monomer, i.e., the alkyl acrylate and the modifier monomer, so that the Tg of
the copolymer would be higher than that of a hullluuuly~ of the alkyl acrylateby itself. The modifier monomer is selccted from the ~. ono~ Ll.~l~ .,;.,dlly
' cu; oly.ll~ dl,lc monomers wherein the hu~u~ly of the
modifier monomer has a higher Tg than the l.u...ùj oly...~. of the alkyl acrylate.
Preferred v;scolld"ic materials for use in the vibration damping material
are flowable materials that are ' , 'y hardened, either by a catalyst,
water, hcat, cooling, etc. One type of such material is a ~.lllu,uLI~ic
polymer. A II....,.u~ polymer softens when exposed to elevated
2 1 94682
WO96/044sl l~l/~) j,l
and generally returns to its original physical state when cooled to
ambient i , c~. During the ~ process, the fh. ."".~ "~
polymer is heated above its softening i . , and often above its melting
Lm~.a~ul~, to enable it to be i..w.l ' into the cavities of the article, as by
5 injecting or pumping. After the cavities are filled, e.g., partially (e.g.,
typically at least about 10% and preferably at least about 50% by volume) or
! ' ' .1~, fully filled (i.e., greater than 90% filled), the ll,...,.,~
polymer is cooled and solidified. Thus, with a ~ ;r polymer,
techniques such as injection molding can be used to prepare the damped
10 articles, e.g., structural members. Thus, the via~,u~ iu material can be
hl.,ullJuldt~l into the structural cavities in a very fluid (low viscosity) flowable
form when uncured, even under ambient conditions. Herein, the phrase
"ambient . ' " and variants thereof refer to room i l , which can
be about 15-30~C, but is generally about 20-25~C, and which can be about
15 30-50% relative humidity, but is generally about 35~5% relative humidity.
Preferred Ih ,"",~ ;, polymers, i.e., ~ u~ materials, ofthe
invention are those having a high melting; , c; and/or good heat
resistant properties. That is, preferred 1~ ul materials have a softening
point of at least about 100~C, preferably at least about 150~C. Additionally,
20 the softening point of a preferred Lh~ u~ , materials is s~lff 'y lower,
i.e., at least about 50~C lower, than the melting t~ .alu~ of the fibrous
material described below (if such a material is used). In this way, the fibrous
material is not adversely effected during the melting process of the
Ih "",l,~ r material. Examples of Ih .",..~ l;r materials suitable for use as
25 the vibration damping material in articles according to the present inventioninclude, but are not limited to, those selected from the group consisting of
pulyal,ly' , pOly~aulJ~ , poly ll .i ~ , polyesters, puly~ulru~
poly~ly-~ , acrylonitrile-butadiene-styrene block cùl)Olylllc;la~ poly~,lul,yl~
acetal polymers, pulyalll; l~s, polyvinyl chlorides, puly.,lhyl~ s, pulyuldi~ s,30 and. ' thereof.
WO96/04491 2 ~ q46g2 I~ J~I
Useful vis~u.L~,l;c materials can also be ~,lu~ lh~l lc to enhance their
strength. Such vi~ Lic~ are classified as Ih., ~ 1;,." resins. If the
\,;~I~Lic material is a 11,. ~ l;,,g resin, prior to the r ' of the
structural romr~npnt~ the i' ~ resin is in a Ih . .~ state.
5 During the, -- r- U..;"g process, the !~ , resin is cured and/or
crosslinked typically to a solid state, although it could be a gel upon curing as
Iong as the cured material possesses the ~ o~ Li~ properties described above.
Depending upon the particular Ih ~ .Il..~.~u;,~ resin employed, the Ll.~. gresin can use a curing agent, e.g., catalyst. When this curing agent is exposed
10 to an ~ lu energy source (such as thermal energy or radiation energy)
th~e curing agent will initiate the pol~....,li~Liul. of the i' - ~ resim
Particularly preferred Ih.,.l ~ v;~,ù~l~Li~, damping materials are based on
partially crosslinked acrylates.
In general, any suitable viscoelastic material can be used. The choice of
15 v;i,.,~l~Lic material for a particular set of conditions, e~g~ l..Lu-t and
frequency of vibration, etc., is within the knowledge of one of skill in the art of
viscoelastic damping. It is to be understood that blends or layers of any of theforegoing v;scocl~Lic materials can also be used.
In addition to the viscoelastic material, the "r '" vibration
20 damping material of certain preferred i~bo 1;~ of the present invention
includes an effective amount of a fibrous material. Herein, an "effective
amount" of a fibrous material is a sufficient amount to impart at least
illllJlU._Iil~,.l~ in desirable 1 ~ to the \';~.,O~,Id~LiC material, but not so
much as to give rise to any significant number of voids and detrimentally effect25 the structural integrity of the articles in which the viscoelastic material is
incul~ u-dL~d. Generally, the fibrous material is used in an amount effective toincrease the strain energy ratiû of a component containing the same amount and
type of viscoelastic material ~-vithout the fibrous material. Generally, an
increase in the strain energy ratio of a factor of at least about two in at least
30 one vibrational mode is desired. Typically, the amount of the fibrous material
in the vi~l~Lic material is within a range of about 3-60 wt-%, preferably
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wo96/04491 2 ~ 9 4 6 8 2 I~
about I0-SO wt-%, more preferably about 15-45 wt-%, and most preferably
about 30-35 wt-%, based on the tot~l weight of the vibration damping material.
The fibrous material can be in the form of fibrous strands or in the form
of a fiber mat or web, although fibrous strands are preferred. The fibrous
strands can be in the form of threads, cords, yarns, rovings, filaments, etc., as
long as the vi~.,u.,L~ can wet the surface of the material. They can be
dispersed randomly or uniformly in a specified order. Preferably, the fibrous
strands, i.e., fibers or fine threadlike pieces, have an aspect ratio of at least
about 2:1, and more preferably an aspect ratio within a range of about 2:1 to
10 about 10:1. The aspect ratio of a fiber is the ratio of the longer dimension of
the fiber to the shorter dimension.
The fibrous material can be composed of any material that increases the
damping capability of the vi~,u~,la~ ic material. Examples of useful fibrous
materials in ~l,l.l,. -~;.,..~ of the present invention include metallic fibrous15 materials, such as aluminum oxide" ~.O ' ~; ~, or steel fibers, as well as
llir fibrous materials, such as fiberglass. Generally, high Young's
modulus fibrous materials, i.e., those having a Young's modulus of at least
about 100,000 psi (6.9 x 108 pascals), are preferred. More preferably, useful
fibrous materials have a Young's modulus of at least about 500,000 psi (3.45 x
109 pascals), and most preferably at least about 1,000,000 psi (6.9 x 109
pascals). Most preferably, the fibrous material is rl~mmrf71iir The n~mm~ot7nit
fibrous materials can be a variety of materials, including, but not limited to,
those selected from the group consisting of glass, carbon, minerals, synthetic or
natural heat resistant organic materials, and ceramic materials. Preferred
fibrous materials for ~ ;." c of the present invention are organic materials,
glass, and ceramic fibrous material.
By "heat resistant" organic fibrous material, it is meant that useable
organic materials should be ~urrlu;~,..ly resistant to melting, or otherwise
softening or bre~ing down, under the conditions of IlldulurduLLllc and use of the
structures of the present invention. Useful natural organic fibrous materials
include, but are not limited to, those selected from the group consisting of
WO 96/04491 2 1 9 4 ~ 8 2 r~ J~/ ~
wool, silk, cotton, and cellulose. Examples of useful synthetic organic fibrous
materials include, but are not limited to, those selected from the grDup
consisting of polyvinyl alcohol, nylon, polyester, rayon, polyamide, acrylic,
polyolefin, aramid, and phenolic. The preferred organic fibrous material for
5 ~ of the present invention is aramid fibrous material. Such a
material is ~ ly available from Dupont Co., ~ ;tu~l, DE under
the trade names of "Kevlar" and "Nomex."
Generally, any ceramic fibrous material is useful in _1,1.l;. -l;.~..- of the
present invention. An example of a ceramic fibrous material suitable for the
10 present invention is NEXPEL~ which is ~i~lly available from
Minnesota Mining and r~ r ~ co.~ St. Paul, MN. Fxamples of useful,
~,u111111~ ,;ally available, glass fibrous material are those available from PPGIndustries, Inc. Pittsburgh, PA, under the prDduct name E-glass bobbin yam;
Owens Coming, Toledo, OH, under the product name "Fiberglass" continuous
15 filament yam; and Manville Corporation, Toledo, OH, under the product name
"Star Rov 502" fiberglass roving.
Advantages can be obtained through use of fibrous materials of a length
as short as about 100 1~ u111~h1~. Generally, however, fibers shcrter than this
do not provide sufficient ~il.ru~ . Depending on the ~pFli~ n the
20 length of the fibers will be dictated by the amount of shearing surfaces between
fibers. The thickness, i.e., degree of fineness, of typical fibrous rnaterial is at
least about S n.i~,1u1l1~t~ . The finer the fiber, the higher the surface area of
the fibrous material. Thus, preferred fibrous materials are very fine. It is
understood that the thickness is strongly influenced by the particular type of
25 fibrous material employed.
In addition to fibers, the vibration damping material of the present
invention can include additives such as fillers (e.g., silica), toughening agents,
fire retardants, ~ntil~irh~f~, and the like. Sufficient amounts of each of thesematerials can be used to effect the desired result.
The damped hollow structures utilize the damping of viscoelastic
materials without adversely affecting the structural geometry and stiffness.
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WO96/04491 ~19~632 P~ c~g~
Thus, the damped hollow structures of the present invention are good
candidates for new products that require weight reduction, as the damping
material is generally lighter than the structural material. A significant feature
of the current invention is that cavities introduced into a structure do not
5 adversely affect the stiffness of the article. Such a finding should be very
useful to engineers dealing with design of structural parts such as in
' 1P, aircraft, and Co~ U~LiUll industries.
Articles that can use this damping concept include closed-section tubular
structural members as well as op~.. s_,lio,~ structural members such as beams
10 and channels used in machinery supports. Additionally, articles such as golf
clubs, tennis rackets, fan blades, as well as auto, aircraft, or marine
c~ etc., can use the damping concept of the present invention.
Generally, the articles that can hl~ this damping concept the most
readily are cast or molded articles, at least because cavities can be readily
15 ' r ' ~ into these parts during casting or molding. It is to be understood
that more than one type of vibration damping material, e.g., ' of
oel~c material(s) and fibrous material(s), can be used in any one article,
thereby taking advantage of the different sound-deadening and vibration-
damping ~ "~ of different materials.
Figure I is a schematic of one l l~o~l;. .,l of the present invention
showing an I-beam (1) having a continuous cavity (2) completely filled with a
vibrational damping material (3). This cavity can be only partially filled with a
vibrational damping material if desired. Figure 2 is a schematic of an
altemative . ..,I-o~ of the present invention showing a portion of a
25 structural article (4) having several cavities (5), each of which is partially filled
with a vibrational damping material (6). As described above, the vibrational
damping material can include a viscoelastic material or a ~ l.",-~;.... of
viscoelastic material with a fibrous material. It is to be understood that the
vibration damping material can include a blend of viscoelastic materials as well30 as a variety of different fibrous materials. There are no particular limitations
-
2 1 946PJ2
~ . . . . .
~' on the Lhlckness or amount of the dampmg matenal wlthtn the cavltles, the
thickness or amount in general being determined by the particular application.
The articles of the present invention can be made by any suitable
technique for creating cavities wiLhin a structure, introducing a vibration
5 damping material into the cavities, and then optionally sealing the cavities such
that they are isola[ed from the environment. These techniques are generally
known to those of skill in the art. ~or example, a structural part made out of
steel and having one or more cavities therein at points of stress can be prepared
using standard casting techniques; a vibration damping material containing a
10 viscoelastic material in flowable form is then injected, for example, into the
cavities and allowed to solidify or cure; and the cavities are then sealed to fully
enclose the vibration damping material, although sealing is not necessarily a
requirement as long as the vibration damping material is cnbs~n~ially
completely encased or enclosed within the structural material. Thus, as used
15 herein, "substantially completely enclosed" makes allowances for situations in
which Ihe small entry ports, i.e., injection ports, are not sealed. These
openings, however, constitutc a very small portion of the total area of the
stmcture's external surfacc, i.e., less Lhan about 10%. Prefcrably, howcvcr,
these ports arc sealed to fully cnclose or encase Lhc vibratioll damping material
20 and completely protect it from the environment.
Examoles
The invention has been described with reference to various specific and
preferred embodiments and will be further described by reference to the
25 following detailed examples. It,~ understood, /h~wever, that ,~ere are m~ny
extensions, v,~'riations, and mo,p~fications on t~7/ basic theme~6f the pres~nt
invention b~yond that show,r~/in the exampl~ and detailed~escription,/which
are wit~ the spirit and s~bpe of the pre~ént invention.
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, ~ .
2~ 94682
~ WO 96/04491 P~
F. ' I
In order to study the built-in \ ;~uel~L. damping effects (or damped
structural cavity effects) in a structure, a finite element analysis was used. The
associated finite element model was generated by dividing the continuous
5 structure into small "finite elements" that are joined at a discrete number of nodal points along their ~ ' ~ The method has been widely used for
structural analysis, and has been proved to be a powerful, accurate and fast tool
in dealing with complex, ~ I structures an-d loading situations. Tbe
finite element program used in this work was the "COSMOS/M" code
10 developed by Structural Research & Analysis Corporation, Santa Monica, CA.
DPcrription of MOIIPIC
For the purpose of d .. ~n~l;.,g the invention, two types of damping
materials were modeled: (1) a low ~ ' ' soft damping material of a 90:10
15 isooctyl ~lyl.,t~ /a. lylic acid polymer as described in U.S. Patent No. Re.
24,906 (herein referred to as 90:10 IOA/AA); and (2) a i ~' ' ' damping
material prepared from a composite of a short fiber (i.e., a Young's modulus of
10.5 x 106 psi (72 x 104 pascals) reinforced E-glass and the 90:10 IOA/AA
polymer. The composite damping material model consisted of randomly
20 oriented E-glass short fibers having a volume fraction of 35~0 and the (90:10 IOA/AA) damping material having a volume fraction of 65%.
The basis of the model was a solid cantilever beam made of stainless
steel. Hollow sections were introduced into the solid beam model followed by
filling with the desired damping materials to be modeled. A free vibration
25 analysis using the three-.l;. ..~ l finite element method was carried out to
predict natural fi~L~.Ic;cs and mode shapes. The modal damping property,
i.e., damping of each mode of vibration, was then computed by using the
resulting mode shapes on the basis of the modal strain energy method.
Five models were used: (1) Model 1, a solid steel cantilever beam with
30 1l;,,....~:.1..~ of 10.25 inches (26 cm, length-z) x 1.375 inches (3.5 cm, width-x)
x 0.625 inches (1.6 cm, thickness-y); (2) Model 2, a cavity section with
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2 1 q468~
WO 96/04491 P~ J.~I
dimensions of 10 inches (25 cm) x 1.125 inches (2.85 cm) x 0.6 inch (1.5 cm)
was introduced into the center of Model 1, i.e., a hollow beam; (3) Model 3,
the cavity section of Model 2 was partially filled with 90:10 IOA/AA polymer,
i.e., only half of the cavity section in the thickness direction was filled with5 90:10 IOA/AA polymer; (4) Model 4, the cavity section of Model 2 was fully
filled with 90:10 IOA/AA damping material; and (5) Model 5, the cavity
section of Model 2 was fully filled with a mixture of E-glass and 90:10
IOA/AA polymer.
10 ~ain Ener~v-Damyino Analysis
A free vibration/finite element analysis was performed to predict
natural r.~ . and mode shapes for each model. There are three key
parameters in a modal analysis. They are natural frequency, mode shape and
damping for each mode of vibration. The finite element analysis has provided
15 the two modal parameters as natural frequency and mode shape, I~
The remaining damping parameter was determined by using a "Modal Strain
Energy Method" or "Strain Energy Method".
The use of strain energy in the treatment of damping was introduced by
Ungar and Kerwin in 1962 (J. Acoustical Society of America. 34, 954 (1962)).
20 The application of strain energy to the analysis of damping has been well
d..~...,.. ~t. J in a variety of IJ,,I.li~ u.."~ The basis of the strain energy method
is that damping of a material can be . l, - .- ~ ., .l by the ratio of the energy
dissipated in the material to the energy stored in the material. Thus, for a
structural system consisting of a number of different materials, the total system
25 damping can be expressed in terms of the material damping and the fraction ofthe elastic strain energy stored in each of the constituent materials, as shown in
Equation (1).
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-
wo 96/04491 2 1 9 4 6 8 2 r~ 5 l
n n
W~ W; (1)
k=l ~ k=l Wt
5 where
k = material number
n = total number of materials in the structure
71 = total structural loss f~tor (a measure of damping) of the
system
71~ = loss f~tor of the kth material
W,~ = strain energy stored in the kth material
Wt = total strain energy stored in the structure = ~W~
The strain energy stored in each material is calculated on the basis of
15 mode shape from finite element results. In other words, the resulting mode
shape was used to calculate strain energy of the ~~-",. ~;~,.. 1; ~g mode of
vibration using the linear elastic C~ Llulivt~ law. Loss f~tor data for each
material, however, were determined from ~ j",~"l,l ' A
typical value of loss factor for the 90:10 IOA/AA v;~,oCl~LtC materials
20 modeled is about one over a wide ~ lt range, e.g., about -20~C to
about 100~C, and a frequency range of about 10-1000 Hz. On the other hand,
loss factors of metallic materials such as steel and aluminum are very small (inthe range of 0.001 - 0.0001). Thus, the ~u~ 1) of energy dissipation by
the steel material (determined by the product of loss factor and strain energy of
25 steel material) is trivial to total structural loss factor. t'~ I 'y, since the
models consist of steel and damping material, Equation (I) can be simplified to
the following form without ~;m~ d, ~ the energy dissipated from the steel
material.
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WO96/04491 2 1 q 46~2 P~ J~
i7 = 71v Wv (2)
W~
5 where
~1~ = loss factor of v;~las~i~ damping material
W~ = sh~in energy stored in the \,;~ol~i.tic damping material
As indicated in Equation (2), the total shuchural loss factor is a funchion
10 of shain energy raho (frachon of shain energy in damping material relahve to
the total sh-ain energy in the sh~cture) and loss factor of damping material.
Now, if we choose a damping material according to the operahng i
and frequency ranges to ophmize the peak damping property or loss factor of
one ('1v = 1), Equahon (2) can be reduced to Equahon (3).
1 = ~.. for 11v = 1 (3)
W,
Thus, the sh~in energy rahio of viscoelashc damping material can be
used as a measure of structural loss factor.
Results
A finite element mesh pattern for a canhlever beam made of steel
(Model 1) is shown in Figure 3. Figures 4-6 show the resulhng mode shapes
and nahural ~ ~u~llc;es of the first three vibrahon modes of Model 4. The first
mode is a bending mode with a nahural frequency of 267 Hz (Figure 4), the
second mode is a sway mode vibrating along width (x direchon) with a natural
frequency of 444 Hz (Figure 5), and the third mode is the second bending mode
with a natural frequency of 1535 Hz (Figure 6).
Table 1 presents the resulhng data of natural frequency and shain energy
raho for the five models. The strain energy ratio is defined as the percentage
of shain energy stored in the damping material. Thus, the resulhng sh~in
energy raho shown in Table 1 can be used as an indication of loss factor of the
~-~".~1,..,..1, ~, models.
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2 1 94682
wo 96104491
In order to investigate the effects of higher modes, the finite element
analysis was performed on the basis of the first ten modes for the first four
models. On the whole, we can conclude from Table 1 that Model 4 provided
higher strain energy ratios than that of Model 3. The strain energy ratio
5 increased with increasing the number of mode. For example, from the
resulting three modes of Model 4, strain energy ratio increased from 0.012%
for the first mode up to 1.431 % for the third mode. This is because a more
complex state of strain is generated at higher modes, and that the effects of
shear are more profound at higher modes.
Structural damping was improved more ~ by filling cavities
with the E-glass/(90: 10 IOA/AA) composite damping material described above.
This composite material, not only increases the stiffness over the pure 90:10
IOA/AA material, but preserves the high damping property offered by the
90:10 IOA/AA material. In other words, the E-glass fibers provide high
15 stiffness, while the 90:10 IOA/AA polymer offers high damping.
The damped composite model used here was developed based on the
same geometry of Model 2 as described above with the cavity section fully
filled with the E-glass/(90:10 IOA/AA) composite damping material. The E-
glass/(90:10 IOA/AA) composite model is denoted as Model 5 shown in Table
1, and the results are compared with those of the previous four models. On the
basis of the first three modes, the strain energy ratio of Model 5 increased from
1.346 % for the first mode to 3.127 % for the third mode. Comparing this with
Model 4, wherein the strain energy ratio ranged from 0.012% to 1.431 %, the
damped composite material (Model 5) offers much higher damping to hollow
structures.
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2 1 94682
Wo 96/04491
Table 1
Model Model Model Model 4 Model 5
2 3 (100% filled (100% filled
(Solid) (Hollow) (50% filled with 90:10 with E-glass
with 90: 10 IOA/AA) and 90: 10
IOA/AA) IOA/AA)
Mode Freq. Freq. Frequency Frequency Frequency
No. * (Hz) (Hz) (Hz)/Strain (Hz)/Strain (EIz)/Strain
Energy Ratio Energy Ratio Energy Ratio
(%) (%) (%)
I (b) 235 291 278/0.007 267/0.012 262/1.346
2 (s) 426 486 463/0.30C 444/0.134 439/1.919
3 (b) 1440 1680 1605/0.300 1535/1.431 1528/3.127
4 (t) 2340 2400 2343/9.634 2324/8.177
5 (s) 2470 2720 2653/7.621 2390/65.520
6 (b) 3890 4230 3990/9.159 3772/69.987
7 (e) 4980 4910 4794/19.171 4693/7.777
8 (s) 6300 6600 6456/2.159 6351/7.726
9 (t) 7030 6660 6563/7.642 6428/22.911
10(b) 7300 7200 8257/65.225 8646173.872
*Alphabet in ~uc,.~h~,;, denotes the type of vibration mode: b =
bending, s = sway, e = extension, t = torsion.
The foregoing detailed description and examples have been given for
clarity of ~ g only. No I y limitations are to be understood
therefrom. The invention is not limited to the exact details shown and
described, for variations obvious to one skilled in the art will be included
5 within the invention defined by the claims.
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