STRUCTURAL HOLLOW ARTICLES FILLED WITH DAMPING MATERIALS

Abstract: The present invention provides a method of improving the vibrational damping characteristics of a structural material and the articles (1, 4) formed therefrom. The method involves: forming a cavity (2, 5) within the structural material at a point where at least one vibrational mode is active; and placing a nonconstrained vibration damping material (3, 6) into the cavity, wherein the vibration damping material comprises a viscoelastic material and preferably an effective amount of a fibrous material.

Description: STRUCTURAL HOLLOW ARTICLES FILLED WITH DAMPING MATERIALS Field of the Invention The present invention relates to a method for damping an article subject to resonant vibrations. More specifically, the present invention relates to a method of improving the damping properties of an article or structure by introducing a viscoelastic material, and preferably a fiber-reinforced viscoelastic material, into cavities or hollow sections of the structure. Background of the Invention Periodic or random vibrations at or near resonance in structural members, such as beams, plates, panels, sheet metal, etc., can be problematic due to the resultant formation of undesirable stresses, displacements, fatigue, and sound radiation, for example, in or from the structural members. Such undesirable vibrations are typically induced by external forces and can be experienced by a variety of articles and under a variety of conditions. For example, resonant vibrations can cause problems in computer hardware and vehicle engine components, which can experience a wide range of temperatures. Such vibrations cannot typically be avoided by isolating or shielding the structure or its component parts, as by an isolator, for example. This is because isolators simply delay energy transfer rather than convert mechanical to thermal energy. Various techniques are used to reduce vibrational amplitudes or damp, i.e. , dissipate mechanical energy as heat, and thereby decrease the resultant stresses, displacements, fatigue, etc. These techniques supplement the inherent damping that exists in structures due to friction, rubbing, etc. Certain of these techniques use viscoelastic materials in surface damping treatments for damping control. Two types of surface damping treatments are commonly used: (1) free layer damping treatment; and (2) constrained layer damping treatment. Both of these damping treatments provide high damping to the structure, i.e., dissipation of undesirable vibrations, without sacrificing the stiffness of the structure. Examples of such damping techniques are described, for example, 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 (issued December 21, 1993). Free layer damping treatment is also referred to as "unconstrained layer" or "extensional damping" treatment. In this technique, damping occurs by applying a layer of viscoelastic damping material to the surface of a structure. The material can be applied to one or both sides of a structure. The mechanism by which this treatment method dissipates undesirable energy, e.g., resonant vibrations, involves deformation. That is, when the structure is subjected to cyclic loading, for example, the damping material is subjected to tension-compression deformation and dissipates the energy through an extensional strain mechanism. Constrained layer damping treatment is also referred to as "shear damping" treatment. For a given weight, this type of damping treatment 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 viscoelastic damping material and one or more layers of stiff constraining material. That is, this damping technique is similar to the free layer damping treatment wherein a viscoelastic material is applied to a surface of a structure; however, the viscoelastic material is constrained by a stiff constraining layer in the constrained layer treatment. The energy dissipates from the viscoelastic damping material through a shear mechanism that results from constraints by the stiff constraining layer and the base structure. Although these surface damping techniques are widely used, the degree of damping is often times limited by thickness and weight restrictions. Furthermore, they are not applicable to all types of structures, all modes of vibration, or all frequency ranges. For example, if the structure itself is to be used in some chemical fluids, such as oil, the above surface damping treatments may not be suitable, at least because the viscoelastic material may be deteriorated. To overcome this limitation, constrained 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 damp vibrational energy without adversely affecting the structural integrity of the article for a wide variety of articles, structural geometries, modes of vibration, environmental conditions, and frequencies of vibration is needed. Summary of the Invention The present invention provides a method of improving the vibrational damping characteristics 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 nonconstrained vibration damping material into the cavity such that the vibration damping material is substantially completely encased, preferably fully encased, within the structural material. Preferably, the nonconstrained vibration damping material is in a flowable state when it is placed in the cavities, which can be accomplished, for example, by pumping or injecting the material. Depending upon the application, one continuous cavity or a plurality of cavities can be formed. The cavity or cavities can be partially or substantially completely filled with the nonconstrained vibration damping material. As used herein, a "nonconstrained" vibration damping material means that the vibration damping material does not include constraining layers of the type used in constrained layer dampers, e.g., thin gauge aluminum or stainless steel. The "nonconstrained" vibration damping material includes a viscoelastic material or combination, i.e., blends or layers, of different viscoelastic materials. Useful viscoelastic materials are those having a storage modulus of at least about 1 psi (6.9 x 10 3 pascals) and a loss factor of at least about 0.01. Advantageously 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 least one vibrational mode. Preferably, the viscoelastic material is a thermoplastic polymer at least because thermoplastic polymers are flowable and easily placed in the cavities by pumping, injecting, etc. In certain preferred embodiments, the vibration damping material also includes an effective amount of a fibrous material. The vibration damping material preferably includes an amount of fibrous material effective to improve 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 at least one vibrational mode. Typically, this requires incorporating 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 nonmetallic fibrous material, such as glass. The present invention also provides a damped article comprising a structural material having at least one cavity substantially completely encased or enclosed, preferably fully encased or enclosed, within the structural material at a point where at least one vibrational mode is active and a nonconstrained vibration damping material, as described above, contained therein. Brief Description of the Drawings Figure 1. A schematic of one embodiment of the present invention showing the cross-section of an I-beam having a continuous cavity completely filled with a vibration damping material. Figure 2. A schematic of an alternative embodiment 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 beam (Model 1 in Example 1). Figure 4. Resulting mode shape and natural frequency of the first bending mode for a hollow beam filled with a preferred viscoelastic damping material. Figure 5. Resulting mode shape and natural frequency of the second mode (or first sway mode) for a hollow beam filled with a preferred viscoelastic 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 viscoelastic damping material. Detailed Description The present invention provides a method of improving damping properties of articles, e.g., structures, structural parts, etc., and thereby solving noise and vibration problems in a variety of engineering applications. More 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 amounts of strain energy in various vibrational modes of interest and dissipates this energy, thereby diminishing noise, vibration, and oscillation. The present invention can be applied to damp, i.e., reduce the vibrational amplitude of, a wide variety of vibrational modes, e.g., bending, torsion, sway, and extensional modes, in a wide variety of structural geometries over a wide frequency range. It results in three-dimensional damping, not simply two-dimensional damping. It can be applied to situations in which surface treatments, such as constrained layer treatments, damped struts, fluid dampers, magnetic and piezoelectric devices, etc., are typically used. The present invention is belived to be useful in large structures, e.g., buildings, to reduce the amplitude and acceleration that result from wind and seismic forces. The method of the present invention involves the introduction of an effective "nonconstrained" vibration damping material into one or more cavities, i.e., hollow sections or pockets, of the structural 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 structural material is an isotropic material, at least with respect to its elastic properties. As used herein, an "isotropic material" refers to a material having properties that are independent 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 respect to this invention, isotropic and anisotropic refer at least to the elastic properties of the material. The vibration damping material is incorporated into the structural 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 be created by any method known in the art, such as machining, molding, casting, etc. Preferably, the articles are cast or molded articles with cavities created 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 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 transfer of mechanical energy from the structural material into the vibration damping material for dissipation. This 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 article either as a surface damper or as an embedded damper. Also, this results in a three-dimensionally damped article. In this way, a damped article can be formed that internally converts mechanical energy to thermal energy. It should be understood that this cavity does not have to be symmetrically enclosed in the structure or structural material. Rather, in certain applications, 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 the surface of the structure as by welding, adhesive bonding, or mechanically fastening, for example, or it could be molded into the structure at the surface. This cap has a sidewall portion and a top portion that substantially 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 cap could be circular, oblong, square, or rectangular in shape. The "nonconstrained" vibration damping material of the present invention excludes constraining layers, e.g., layers of aluminum or stainless steel, used in constrained layer dampers to "constrain" the viscoelastic material. It should be understood, however, that the present invention does not exclude the use of short fibers intermixed with the vibration damping material, which can act to "constrain" the viscoelastic material. The arrangement of the present invention is also distinguished from a laminated construction wherein the vibration damping material is exposed at all edges. Thus, the arrangement of the present invention substantially protects the vibration damping material from attack by moisture, lubricants, chemicals, and oxygen. Preferably, the structural cavities or hollow sections are substantially completely filled with a damping material, although it is not a requirement of the invention that they be completely filled. Typically, an amount of the damping material is placed in the cavities to improve the damping characteristics of the article. Preferably, a sufficient amount of the vibration damping material is used such that the damping is improved by at least about 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 frequencies. The resulting mechanical strain energy in the damping material is then 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 member, depends on the geometry of the article and the vibrational modes that are 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 maximized. The identification 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 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 viscoelastic material. In castings or moldings the cavities are built into the part during the casting or molding process. It is to be understood, however, that the damping concept of the present invention can be used in nonisotropic, i.e., anisotropic, composite materials, e.g., plastic reinforced with carbon fiber, as well. Furthermore, 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 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 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 applications, there is no mismatch. If there is a significant mismatch in modulus, i.e., greater than about 10%, there is an inefficient transfer of mechanical energy into the vibration damping material for dissipation. Such a situation would occur, for example, in an article in which a cavity is formed by an elastomeric material such as polyurethane having a flexural modulus of 1 ,000-50,000 psi (6.9 x 10 6 - 3.5 x 10 8 pascals) and a metal, which typically has a flexural modulus of greater than 1,000,000 psi (6.9 x 10 9 pascals). See, for example, U.S. Patent No. 5,290,036 (issued March 1, 1994). The "nonconstrained" vibration damping material can include any material that is viscoelastic. A viscoelastic material is one that is viscous, and therefore capable of dissipating energy, yet exhibits certain elastic properties, and therefore capable of storing energy. That is, a viscoelastic material is an elastomeric material typically containing long-chain molecules that can convert mechanical 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 deformation, of at least about 1 psi (6.9 x 10 3 pascals). The storage modulus of useful viscoelastic materials can be as high as 500,000 psi (3.45 x 10 9 pascals); however, typically it is about 10-2000 psi (6.9 x 10 4 - 1.4 x 10 7 pascals). Particularly preferred viscoelastic 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 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 0.1, more preferably about 0.5-10, and most preferably about 1-10, regardless of the frequency and temperature experienced by the material. This loss factor represents a measure of the energy dissipation of the material and depends on the frequency and temperature experienced by the material. For example, for a crosslinked acrylic polymer, at a frequency of 100 Hz, the loss factor at 20 °C is about 1.0, while at 70 C C the loss factor is about 0.7. Preferred viscoelastic materials typically remain functional after experiencing a wide range of temperatures, e.g., -40°F (-40°C) to 300°F (149°C). That is, they are capable of surviving a wide range of temperatures without a significant decrease in their loss factors. Useful viscoelastic damping materials can be isotropic as well as anisotropic materials, particularly with respect to its elastic properties. As used herein, an "anisotropic material" or "nonisotropic material" is one in which the properties are dependent upon the direction of measurement. Suitable viscoelastic materials include urethane rubbers, silicone rubbers, nitrile rubbers, butyl rubbers, acrylic rubbers, natural rubbers, styrene-butadiene rubbers, and the like. Other useful damping viscoelastic materials include polyesters, polyurethanes, polyamides, ethylene-vinyl acetate copolymers, polyvinyl butyral, polyvinyl butyral-polyvinyl acetate copolymers, and the like. Specific 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 composition for vibration-damping material which comprises (A) at least one amorphous polyester resin of low specific gravity in which more than 40 mol % 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 polyisocyanate compounds, epoxy group- containing compounds, and acid anhydrides, said constituents (A) and (B) being in the ratio of from 90: 10 to 30:70 by weight and differing from each other in specific gravity (at 30° C.) by 0.06 to 0.15 and also in molecular weight by 10000 or more, with that of either of them being higher than 5000. This resin composition gives a vibration-damping material which exhibits improved vibration-damping properties, adhesive strength formability, and heat resistance after 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 dicarboxylic acids (such as terephthalic acid, isophthalic acid, orthophthalic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 4,4'- biphenyldicarboxylic acid, 2,2-biphenyldicarboxylic acid, and 5-sodium sulfoisophthalic acid), alicyclic dicarboxylic acids (such as 1 ,4-cyclohexane- dicarboxylic acid, 1 ,3-cyclohexanedicarboxylic acid, and 1,2-cyclohexane- dicarboxylic acid), and aliphatic dicarboxylic acids (such as succinic acid, adipic acid, azelaic acid, sebacic acid, dodecanedicarboxylic acid, and dimer acid). These dibasic acids may be used in combination with tribasic acids (such as trimellitic acid and pyromellitic acid) in amounts harmless to the resin properties. The glycols are exemplified by aliphatic glycols (such as ethylene glycol, propylene glycol, 1,4-butanediol, 1,3-butanediol, 1,5-pentanediol, neopentyl glycol, 3-methylpentanediol, 1,6-hexanediol, trimethylpentanediol, 1 ,9-nonanediol, 2-methyl-l ,8-octanediol, 2,2-diethyl-l,3-propanediol, 2-ethyl-2- butyl-l,3-propanediol, diethylene glycol, and triethylene glycol), alicyclic diols (such as 1 ,4-cyclohexane dimethanol), and aromatic ring-containing diols (such as adduct of bisphenol A or bisphenol S with ethylene oxide or propylene oxide). These glycols may be used in combination with trifunctional or multifunctional components such as trimethylolpropane, glycerin, and pentaerythritol 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°C.) by 0.06-0.15, preferably by 0.08-0.125. U.S. Patent No. 5,308,887 discloses a silicone/acrylic based composition. The composition 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 alkyl groups of which have an average of 4 to 12 carbon atoms; and (ii) correspondingly from about 50 parts to about 0 parts by weight of a monoethylenically unsaturated copolymerizable modifier monomer; wherein the amounts of (i) and (ii) are selected such that the total amount of (i) plus (ii) equals 100 parts by weight of the acrylic monomer; (b) correspondingly from about 95 parts to about 5 parts by weight of silicone pressure-sensitive adhesive wherein the amounts of (a) and (b) are selected such that the total amount of (a) plus (b) equals 100 parts by weight; (c) about 0 part to about 5 parts by weight of a photoinitiator based upon 100 parts by weight of the acrylic monomer; and (d) about 0 to about 5 part by weight of a crosslinker based upon 100 parts by weight of (a) plus (b). According to U.S. Patent No. 5,308,887, the term "monoethylenically unsaturated copolymerizable modifier monomer", also referred to as the "modifier monomer" refers to a monomer that is capable of increasing the Tg (glass transition temperature) of a copolymer formed 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 homopolymer of the alkyl acrylate by itself. The modifier monomer is selected from the monoethylenically unsaturated copolymerizable monomers wherein the homopolymer of the modifier monomer has a higher Tg than the homopolymer of the alkyl acrylate. Preferred viscoelastic materials for use in the vibration damping material are flowable materials that are subsequently hardened, either by a catalyst, water, heat, cooling, etc. One type of such material is a thermoplastic polymer. A thermoplastic polymer softens when exposed to elevated temperatures and generally returns to its original physical state when cooled to ambient temperatures. During the manufacturing process, the thermoplastic polymer is heated above its softening temperature, and often above its melting temperature, to enable it to be incorporated into the cavities of the article, as by 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 substantially fully filled (i.e., greater than 90% filled), the thermoplastic polymer is cooled and solidified. Thus, with a thermoplastic polymer, techniques such as injection molding can be used to prepare the damped articles, e.g., structural members. Thus, the viscoelastic material can be incorporated into the structural cavities in a very fluid (low viscosity) flowable form when uncured, even under ambient conditions. Herein, the phrase "ambient conditions" and variants thereof refer to room temperature, which can be about 15-30°C, but is generally about 20-25 °C, and which can be about 30-50% relative humidity, but is generally about 35-45% relative humidity. Preferred thermoplastic polymers, i.e., thermoplastic materials, of the invention are those having a high melting temperature and/or good heat resistant properties. That is, preferred thermoplastic materials have a softening point of at least about 100°C, preferably at least about 150°C. Additionally, the softening point of a preferred thermoplastic materials is sufficiently lower, i.e., at least about 50°C lower, than the melting temperature 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 thermoplastic material. Examples of thermoplastic materials suitable for use as the vibration damping material in articles according to the present invention include, but are not limited to, those selected from the group consisting of polyacrylates, polycarbonates, polyetherimides, polyesters, polysulfones, polystyrenes, acrylonitrile-butadiene-styrene block copolymers, polypropylenes, acetal polymers, polyamides, polyvinyl chlorides, polyethylenes, polyurethanes, and combinations thereof. Useful viscoelastic materials can also be crosslinkable to enhance their strength. Such viscoelastics are classified as thermosetting resins. If the viscoelastic material is a thermosetting resin, prior to the manufacture of the structural component, the thermosetting resin is in a thermoplastic state. During the manufacturing process, the thermosetting resin is cured and/or crosslinked typically to a solid state, although it could be a gel upon curing as long as the cured material possesses the viscoelastic properties described above. Depending upon the particular thermosetting resin employed, the thermosetting resin can use a curing agent, e.g., catalyst. When this curing agent is exposed to an appropriate energy source (such as thermal energy or radiation energy) the curing agent will initiate the polymerization of the thermosetting resin. Particularly preferred thermosetting viscoelastic damping materials are based on partially crosslinked acrylates. In general, any suitable viscoelastic material can be used. The choice of viscoelastic material for a particular set of conditions, e.g., temperature 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 the foregoing viscoelastic materials can also be used. In addition to the viscoelastic material, the "nonconstrained" vibration damping material of certain preferred embodiments 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 improvement in desirable characteristics to the viscoelastic material, but not so much as to give rise to any significant number of voids and detrimentally effect the structural integrity of the articles in which the viscoelastic material is incorporated. Generally, the fibrous material is used in an amount effective to increase the strain energy ratio of a component containing the same amount and type of viscoelastic material without the fibrous material. Generally, an increase in the strain energy ratio of a factor of at least about two in at least one vibrational mode is desired. Typically, the amount of the fibrous material in the viscoelastic material is within a range of about 3-60 wt-%, preferably about 10-50 wt-%, more preferably about 15-45 wt-%, and most preferably about 30-35 wt-%, based on the total 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 viscoelastic 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 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 viscoelastic material. Examples of useful fibrous materials in applications of the present invention include metallic fibrous materials, such as aluminum oxide, magnesium, or steel fibers, as well as nonmetallic 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 10 8 pascals), are preferred. More preferably, useful fibrous materials have a Young's modulus of at least about 500,000 psi (3.45 x 10 9 pascals), and most preferably at least about 1 ,000,000 psi (6.9 x 10 9 pascals). Most preferably, the fibrous material is nonmetallic. The nonmetallic 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 applications 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 sufficiently resistant to melting, or otherwise softening or breaking down, under the conditions of manufacture 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 wool, silk, cotton, and cellulose. Examples of useful synthetic organic fibrous materials include, but are not limited to, those selected from the group consisting of polyvinyl alcohol, nylon, polyester, rayon, polyamide, acrylic, polyolefin, aramid, and phenolic. The preferred organic fibrous material for applications of the present invention is aramid fibrous material. Such a material is commercially available from Dupont Co., Wilmington, DE under the trade names of "Kevlar" and "Nomex." Generally, any ceramic fibrous material is useful in applications of the present invention. An example of a ceramic fibrous material suitable for the present invention is NEXTEL™ which is commercially available from Minnesota Mining and Manufacturing Co., St. Paul, MN. Examples of useful, commercially available, glass fibrous material are those available from PPG Industries, Inc. Pittsburgh, PA, under the product name E-glass bobbin ya ; Owens Coming, Toledo, OH, under the product name "Fiberglass" continuous 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 micrometers. Generally, however, fibers shorter than this do not provide sufficient reinforcement. Depending on the application, the 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 material is at least about 5 micrometers. 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 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, antioxidants, and the like. Sufficient amounts of each of these materials 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. 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 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 automobile, aircraft, and construction industries. Articles that can use this damping concept include closed-section tubular structural members as well as open-section structural members such as beams and channels used in machinery supports. Additionally, articles such as golf clubs, tennis rackets, fan blades, as well as auto, aircraft, or marine components, etc. , can use the damping concept of the present invention. Generally, the articles that can incorporate this damping concept the most readily are cast or molded articles, at least because cavities can be readily incorporated into these parts during casting or molding. It is to be understood that more than one type of vibration damping material, e.g., combination of viscoelastic material(s) and fibrous material(s), can be used in any one article, thereby taking advantage of the different sound-deadening and vibration- damping characteristics of different materials. Figure 1 is a schematic of one embodiment 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 alternative embodiment of the present invention showing a portion of a 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 combination 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 well as a variety of different fibrous materials. There are no particular limitations on the thickness or amount of the damping material within the cavities, 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 within a structure, introducing a vibration damping material into the cavities, and then optionally sealing the cavities such that they are isolated from the environment. These techniques are generally known to those of skill in the art. For 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 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 substantially completely encased or enclosed within the structural material. Thus, as used herein, "substantially completely enclosed" makes allowances for situations in which the small entry ports, i.e., injection ports, are not sealed. These openings, however, constitute a very small portion of the total area of the structure's external surface, i.e., less than about 10%. Preferably, however, these ports are sealed to fully enclose or encase the vibration damping material and completely protect it from the environment. Examples The invention has been described with reference to various specific and preferred embodiments and will be further described by reference to the following detailed examples. It is understood, however, that there are many extensions, variations, and modifications on the basic theme of the present invention beyond that shown in the examples and detailed description, which are within the spirit and scope of the present invention. Example 1 In order to study the built-in viscoelastic 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 structure into small "finite elements" that are joined at a discrete number of nodal points along their boundaries. 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 geometrical structures and loading situations. The finite element program used in this work was the "COSMOS/M" code developed by Structural Research & Analysis Corporation, Santa Monica, CA. Description of Models For the purpose of demonstrating the invention, two types of damping materials were modeled: (1) a low-modulus, soft damping material of a 90: 10 isooctyl acrylate/acrylic acid polymer as described in U.S. Patent No. Re. 24,906 (herein referred to as 90: 10 IOA/AA); and (2) a high-modulus damping material prepared from a composite of a short fiber (i.e., a Young's modulus of 10.5 x 10 6 psi (72 x 10 9 pascals) reinforced E-glass and the 90: 10 IOA/AA polymer. The composite damping material model consisted of randomly oriented E-glass short fibers having a volume fraction of 35% 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 analysis using the three-dimensional finite element method was carried out to predict natural frequencies 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 dimensions 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 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 with 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. Strain Energy-Damping Analysis A free vibration/finite element analysis was performed to predict natural frequencies 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 the two modal parameters as natural frequency and mode shape, respectively. 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. , 954 (1962)). The application of strain energy to the analysis of damping has been well documented in a variety of publications. The basis of the strain energy method is that damping of a material can be characterized 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 damping can be expressed in terms of the material damping and the fraction of the elastic strain energy stored in each of the constituent materials, as shown in Equation (1). n n = Σ η k W,, = Σ η k W-- (1) k= l ΣW k k= l W t where k = material number n = total number of materials in the structure η = total structural loss factor (a measure of damping) of the system η k = loss factor of the kth material W k = strain energy stored in the kth material W, = total strain energy stored in the structure = ΣW k The strain energy stored in each material is calculated on the basis of mode shape from finite element results. In other words, the resulting mode shape was used to calculate strain energy of the corresponding mode of vibration using the linear elastic constitutive law. Loss factor data for each material, however, were determined from experimental measurements. A typical value of loss factor for the 90: 10 IOA/AA viscoelastic materials modeled is about one over a wide temperature 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 (in the range of 0.001 - 0.0001). Thus, the contribution of energy dissipation by the steel material (determined by the product of loss factor and strain energy of steel material) is trivial to total structural loss factor. Consequently, since the models consist of steel and damping material, Equation (1) can be simplified to the following form without considering the energy dissipated from the steel material. V = η v _W V _ (2) W, where rj v = loss factor of viscoelastic damping material W v -= strain energy stored in the viscoelastic damping material As indicated in Equation (2), the total structural loss factor is a function of strain energy ratio (fraction of strain energy in damping material relative to the total strain energy in the structure) and loss factor of damping material. Now, if we choose a damping material according to the operating temperature and frequency ranges to optimize the peak damping property or loss factor of one (τj v = 1), Equation (2) can be reduced to Equation (3). 7 = W v for τj v = 1 (3) W t Thus, the strain energy ratio of viscoelastic damping material can be used as a measure of structural loss factor. Results A finite element mesh pattern for a cantilever beam made of steel (Model 1) is shown in Figure 3. Figures 4-6 show the resulting mode shapes and natural frequencies of the first three vibration modes of Model 4. The first mode is a bending mode with a natural frequency of 267 Hz (Figure 4), the second mode is a sway mode vibrating along width (x direction) 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 resulting data of natural frequency and strain energy ratio for the five models. The strain energy ratio is defined as the percentage of strain energy stored in the damping material. Thus, the resulting strain energy ratio shown in Table 1 can be used as an indication of loss factor of the coπesponding models. 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 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 significantly 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 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. Table 1 Model Model Model Model 4 Model 5 1 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 (Hz)/Strain Energy Ratio Energy Ratio Energy Ratio (%) (%) (%) 1 (b) 235 291 278/0.007 267/0.012 262/1.346 2 (s) 426 486 463/0.300 444/0.134 439/1.919 3 (b) 1440 1680 1605/0.300 1535/1.431 1528/3.127 4 (f) 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 (f) 7030 6660 6563/7.642 6428/22.911 10(b) 7300 7200 8257/65.225 8646/73.872 ♦ Alphabet in parenthesis 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 understanding only. No unnecessary 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 within the invention defined by the claims.

Claims: WHAT IS CLAIMED IS: 1. A method of improving the vibrational damping characteristics of an article containing a structural material comprising: (a) forming a cavity within the structural material at a point where at least one vibrational mode is active; and (b) placing a nonconstrained vibration damping material into the cavity such that the vibration damping material is substantially completely enclosed within the structural material; wherein the nonconstrained vibration damping material comprises a viscoelastic material. 2. The method of claim 1 wherein the vibration damping material further includes an effective amount of a fibrous material. 3. The method of claim 2 wherein the vibration damping material includes an amount of fibrous material effective to improve vibrational damping of the structural material by a factor of at least about two in strain energy ratio of at least one vibrational mode. 4. The method of claim 1 wherein the vibrational damping is improved by at least about 10% in at least one vibrational mode. 5. A method of improving the vibrational damping characteristics of an article made of a structural material comprising: (a) forming at least one cavity within the structural material at a point where at least one vibrational mode is active; and (b) placing a nonconstrained vibration damping material into the at least one cavity formed, wherein the vibration damping material comprises a flowable viscoelastic material that subsequently hardens to a material that has a storage modulus of at least about 1 psi and a loss factor of at least about 0.01. 6. A damped article comprising a structural material having at least one cavity substantially completely enclosed within the structural material at a point where at least one vibrational mode is active and a nonconstrained vibration damping material contained therein, wherein the nonconstrained vibration damping material comprises a viscoelastic material. 7. The article of claim 6 wherein the vibration damping material further includes an amount of fibrous material effective to improve vibrational damping of the article by a factor of at least about two in strain energy ratio of at least one vibrational mode. 8. The article of claim 6 wherein the cavity is completely filled with the vibration damping material. 9. The article of claim 6 wherein the cavity is created by a cap integrally attached to the surface of the structure. 10. The article of claim 6 wherein the article is a golf club or a steel beam or a nonferrous casting.

Inventors: HWANG SHWILONG, NELSON ALFRED DWAYNE, HWANG, SHWILONG, NELSON, ALFRED, DWAYNE
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1995
Main classification: F16F13
IPC codes: A63B59, F16F9, F16F13, F16F15