METHOD AND APPARATUS FOR VIBRATION DAMPING STRUCTURAL ELEMENTS

Abstract: This invention relates to vibration damping and, more particularly, to reducing the noise produced by, and increase the sonic fatique life of, structural elements. A method and apparatus for vibration damping structural elements, wherein a rigid constraining layer (27) is viscoelastically attached (29) to at least two transverse (normally orthogonal) legs (23, 25a, 25b) of the undamped structural element (21) to be damped. The viscoelastic attachment to a vibrating leg directly damps the vibrations of the leg. In addition, the rigid constraining layer couples the vibrations of the vibrating leg to the viscoelastic attachment to the other leg. As a result, the other leg provides indirect damping. That is, when one of the legs vibrates, the vibration is damped in two ways. First, the viscoelastic material attaching the constraining element to the vibrating leg provides direct damping. Secondly, the constraining element couples the vibrations to the viscoelastic material attached to the other leg, which provides indirect damping. Depending upon the magnitude of the vibration encountered by the structural element, the rigid constraining layer and the viscoelastic material can be segmented (Figures 4 and 5) or extend along the entire length of the structural element, which can be straight or curved. If segmented, the segments can be connected together by rigid links (48a-48d), which may form part of the rigid constraining layer. Also, two rigid constraining elements (57a, 57b) and viscoelastic layers (59a, 59b), one on either side of the structural elements can be included. Further, the rigid constraining element(s) and the viscoelastic material can include apertures (Fig. 5) to allow other structure, brackets or the like to be attached to the damped structural element. While the invention was conceived for use in reducing the noise in the cabin of an aircraft, it is to be understood that the invention can be used in all types of transporation vehicles.

Description: METHOD AND APPARATUS FOR VIBRATION DAMPING STRUCTURAL ELEMENTS Technical Area This invention relates to vibration damping and, more particularly, to methods and apparatus for reducing the noise produced by, and increase the sonic fatigue life of, structural elements. Background of the Invention While the hereinafter described invention was conceived for use in reducing the noise in the cabin of an aircraft, it is to be understood that the invention can be used in other types of reinforced skin structures to reduce interior noise and vibration, including all types of transportation vehicles — uto¬ mobiles, buses, trucks-, ships, submarines, hovercraft and hydrofoils, for examples. The invention can also be used in the exterior and interior walls of buildings and enclosures where noise 'reduction is desired. The invention can also be used in reinforced skin bulkheaks, partitions or walls in any or all of the ' transportation vehicles listed above and others, including aircraft. It also is to be understood that because interior noise is reduced by damping the vibrations of the structural components of reinforced skin structures, coincidental to the reduction of noise is a corresponding improvement in the sonic fatigue life of the structural components and equipment attached to the reinforced skin structure. That is, reducing the vibrations of structural components not only reduces noise, it also improves the sonic fatigue life of the vibrating structural components and attached equipment. Noise and vibration inside of reinforced skin structures, such as the cabin of an aircraft, affects passenger speech communication, comfort and sleep. Noise and vibration can also cause material fatigue and, thus, the malfunction of equipment mounted in regions of high noise and vibration. Since most transportation structures are designed to be as light in weight as possible (commensurate with structural requirements), in order to obtain maximum fuel efficiency, limitations are placed on what designers can do to reduce interior noise and vibration levels. These constraints are particularly severe in the aircraft design field where weight is extremely critical. In general, noise in an aircraft can be segregated into n contributing to the overall sound pressure level (OASPL) and noise contribut to the speech interference level (SIL). The OASPL is essentially determined the low audio frequency content of the noise and the SIL is determined by mid to high audio frequency content of the noise. Since both the OASPL and affect passengers, noise reduction over the entire, or any portion of the au frequency range is desirable. * Presently the cabin noise of an aircraft in the mid and high au frequency range (above 600 Hz) is reduced by applying skin damping tape, l vinyl sheeting and fiberglass insulation to the walls of the aircraft fusela While the use of such items to reduce noise are effective in the mid and h audio frequengy range, they are essentially ineffective in the low au frequency range, defined as frequencies below 300 Hz. Further, they are o moderately effective in the mid-audio frequency range defined as frequen between 300 and 600 Hz. As a result, the reduction of low and mid-au frequency cabin noise has remained a problem in present day commer aircraft. In the past, it was generally believed that cabin noise below ab 600 Hz was controlled by the structural stiffness of the fuselage of the aircr Thus, attempts to reduce low and mid-audio frequency cabin noise were based various methods of increasing fuselage structural stiffness. For example, in attempt, the number of stringers in the fuselage of a modern commer aircraft were doubled to increase the structural stiffness of the fuselage thereby, reduce cabin noise. Tests taken on this aircraft indicated that altho this 100 percent increase in stringer weight was partially effective in reduc cabin noise in the mid-audio frequency range (e.g., 300-600 Hz), it ineffective in the low audio frequency range (e.g., below 300 Hz). Thus, altho this change improved the subjective impression of the noise" level in the cabi the aircraft, the overall sound pressure level (OASPL) was virtually unaffect As a result, attempts have been made to overcome the cabin noise problem us other techniques. One recently developed method and apparatus for significa reducing the noise produced by stringer response is described in United Sta patent application Serial No. 029,705 entitled METHOD AND APPARATUS F REDUCING LOW TO MID-FREQUENCY INTERIOR NOISE, filed April 11, 19 by Gautam SenGupta and Byron R. Spain. This patent application descri reducing the vibration response of the legs of U-shaped stringers (e.g., strin response) to vibration disturbances by applying rigid strips across the stringer flanges, the ends of the rigid strips being attached to the flanges by thin viscoelastic layers. This method of damping the vibration of the legs of the U-shaped stringers has been found to reduce structural vibration and cabin noise during cruise in the low frequency range. However, the method and apparatus described in this patent application is limited to structural elements— such as U- shaped stringers — wherein a rigid strip can be connected across vibrating legs. As a result, this method is generally inapplicable when the structural element has some other structural shape, or vibrates in a different way, such as by twisting, for example. In this regard, aircraft frames are often formed of structural elements that have a Z--shaped cross sectional configuration and vibrate in two generally different ways. First, frame structural elements bend in response to disturbances, which is a vibration of the flanges (as opposed to the web) of the frame structural elements. In addition, frame structural elements twist in response to disturbances, which is a vibration of the web connecting the flanges together. (Obviously, these elements— web or flange— are the predominate vibrating elements since the web may vibrate slightly as a result of frame bending and the flanges may vibrate slightly as a result of frame twisting.) This invention is directed to a method and apparatus for cooperatively damping the vibration of the flanges and webs of such structural elements in order to reduce the noise created by such vibration and improve the sonic fatigue life of the vibrating elements. Therefore, it is an object of this invention to provide a new and improved method and apparatus for damping the vibrational response of structural elements. It is another object of this invention to provide a cabin noise and vibration reduction method and apparatus for structural elements that is particularly effective in the low and mid-audio frequency ranges. It is a still further object of this invention to provide a method and apparatus for damping the twisting and bending vibrational response of structural elements. Summary of the Invention In accordance with this invention a method and apparatus for vibration damping structural elements are provided. The method generally comprises the step of viscoelastically attaching a rigid constraining element to at least two transverse (normally orthogonal) legs of the structural element to be damped. The viscoelastic attachment material is coupled together via the rigid constraining layer such that the viscoelastic attachment to one of the legs assists in the vibration damping of the other leg and vice versa. More specifically, viscoelastic attachment to the vibrating leg directly damps the vibrations of t leg. In addition, the constraining layer couples the vibrations of the vibrating to the viscoelastic attachment to the other leg, which provides indirect dampi Thus, when a leg vibrates, the vibration is damped in two ways. First, viscoelastic medium attaching the rigid constraining element to the vibrating provides direct damping. Secondly, the constraining element couples vibrations to the viscoelastic medium attaching the constraining element to other leg, which provides indirect damping. Even though the mode of vibrat (e.g., twist, as opposed to bending or vice versa) of the two legs is differe indirect or supplemental damping is still provided. Vibration damping apparatus formed in accordance with invention comprises a rigid constraining layer and a viscoelastic attach material for attaching the constraining layer to at least two transverse legs the structural element to be damped. Preferably, the rigid constraining layer a configuration that mates with the configuration of the legs of the structu element to be damped. For example, if, as is conventional, the at least two l lie orthogonal to one another, the constraining element defines a mating ri angle. -Further, if more than two legs of the structural element are to damped, the constraining element will be formed so as to mate with the legs structural element to be damped. Thus, if the structural element is Z-shap the rigid constraining layer will be Z-shaped. Still further, the rigid constrain •layer and the viscoelastic attachment medium may be segmented or may ext along the entire length of the structural element. If desired, the segments be linked together by links forming part of the rigid constraining layer. Or, constraining layer can include apertures through which items can be attached the structural member. The segmented and apertured versions are particula valuable where weight is " at a premium. Still further, the rigid constraining layer can be formed of t layers, each of which mates with, and is viscoelastically attached to, one of two surfaces of the structural element. Or multiple layers can be attached one or both surfaces of the structural element. As will be appreciated by those skilled in the vibration damping and others from the foregoing brief description, the invention provides grea increased vibration damping without significantly increasing the weight of damped structural element over the weight added when separate damp treatment is applied to each of the at least two transverse legs. Grea increased vibration damping is provided because of the indirect damping that added to the direct damping provided by the viscoelastϊcal attachment to the vibrating legs. The indirect or supplemental damping decreases the total treatment required to achieve a particular level of damping, whereby the additional weight required to achieve a particular level of damping is reduced. Brief Description of the Drawings The foregoing objects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description when taken in conjunction with the accompanying drawings wherein: FIGURE 1 is a perspective view of a Z-shaped structural element wherein the web and both flanges are damped in accordance with the invention; FIGURE 2 is a series of exaggerated cross-sectional views showing twist vibration of a Z-shaped structural element of the type illustrated in FIGURE 1; FIGURE 3 is a series of exaggerated side views showing the bend vibration of a Z-shaped structural element of the type illustrated in FIGURE 1; FIGURE 4 is a partial perspective view of a Z-shaped structural element damped in accordance with the invention wherein the damping treatment layer is segmented; FIGURE 5 is a partial perspective view of a Z-shaped structural element wherein the constraining layer is segmented and the segments are linked together; FIGURE 6 is a partial perspective view of a Z-shaped structural element wherein a constraining layer is viscoelastically attached to both surfaces of the Z-shaped structural element; FIGURE 7 is a partial pictorial view of a Z-shaped structural element wherein an L-shaped constraining layer is viscoelastically attached to the inner surface defined by one flange and the web of the Z-shaped structural element; FIGURE 8 is a partial pictorial view of a Z-shaped structural element wherein an L-shaped constraining layer is viscoelastically attached to the exterior surface defined by one flange and the web of the Z-shaped structural element; FIGURE 9 is a partial pictorial view of an l-shaped structural element having constraining layers viscoelastically attached to all of its surfaces; FIGURE 10 is a partial pictorial view of an L-shaped structural element constraining layers viscoelastically attached to both the inner and outer surfaces defined by the legs of the element; FIGURE 11 is a partial pictorial view of a C-shaped structu element having constraining layers viscoelastically attached to both surfaces both flanges and to the web of the element; 5 . FIGURE 12 is a partial pictorial view of a T-shaped structu element having constraining layers viscoelastically attached to both surfaces the web and flanges of the element; and, FIGURE 13 is a pictorial view of a Z-shaped structural elem illustrating that the region of the constraining layer connecting together • 10 regions of the constraining element viscoelastically attached to the web flanges of the Z-shaped structural elements can be a series of links, rather t continuous. Description of the Preferred Embodiments FIGURE 1 illustrates a Z-shaped structural element 21 damped 15 accordance with the invention. As usual, the Z-shaped structural element 2 elongate and includes a web 23 and a pair of flanges 25a and 25b. The flan 25a and 25b project orthogonally outwardly from opposite longitudinal edges the web 23. The outer edges of the flanges 25a and 25b include a reverse bend In accordance with the invention a rigid constraining layer 27 20 attached by a layer of viscoelastic material 29 to one of the surfaces defined the web 23 and the flanges 25a and 25b of the Z-shaped structural element The cross sectional configuration of the rigid constraining layer 27 is Z-sha and mates with the surface of the structural component 21 to which it attached by the layer- of viscoelastic material 29. In FIGURE. 1, the ri 25 constraining layer 27 is shown as continuous and unapertured. Preferably rigid constraining layer is formed of the same material as the Z-sha structural element 21 and is of the same thickness. Alternately, the constrain layer can also be made from fiber reinforced composite materials of h stiffness to weight ratio. 30 Prior to describing the operation of the damping treatm illustrated in FIGURE 1 (which comprises the rigid constraining element 27 the attaching viscoelastic layer 29) a brief discussion of the types of vibrat that occurs in a Z-shaped structural element when it is used to form the frame an aircraft is described. In this regard, attention is directed to FIGURES 2 FIGURE 2 is a series " of three cross-sectional views of a Z-sha structural element. The left-most view illustrates an untwisted Z-sha structural element oriented such that the web is vertical, the upper fla extends outwardly to the left and the lower flange extends outwardly to the right. When untwisted, the flanges lie orthogonal (e.g., 90°) to the web of the Z- shaped structural element. The middle view of FIGURE 2 illustrates in an exaggerated manner what happens to a Z-shaped structural element when a counterclockwise twist force is applied to it. In this case, the angle between the flanges and the web changes from orthogonal to obtuse. The increased size of the angle, of course, is related to the magnitude of the twist force. The right¬ most view of FIGURE 2 illustrates in an exaggerated manner what happens to the Z-shaped structural element when a clockwise twist is applied to it. In this case, the angle between the flanges and the web change from orthogonal to acute. Again, the reduced size of the angle is related to the magnitude of the twist force. As will be readily appreciated by those skilled in the art, when a Z-shaped structural element executes torsϊonal (twisting) oscillations, it continuously moves between the two configurations illustrated in the middle and the right of FIGURE 2, passing through the untwisted configuration illustrated on the left side of FIGURE 2 during each change. That is, each time the frame oscillates from one configuration to the other configuration it passes through the untwisted configuration. Consequently, during a full cycle of oscillation the untwisted configuration is passed through twice. FIGURE 3 is a series of three longitudinal views illustrating the second type of vibration, commonly occurring in Z-shaped structural elements forming an aircraft frame. While the illustrated Z-shaped structural element is shown as straight, as will be readily appreciated by those skilled in the aircraft frame art, in actuality aircraft frame elements are longitudinally curved. (They may also have a longitudinally changing cross-sectional size.) The illustrated frame element is shown as straight to better illustrate bending vibration, which occurs in a similar (but. not as easily seen manner) in curved frame elements. The top view of FIGURE 3 shows the Z-shaped structural element as unbent, i.e., straight; the middle view of FIGURE 3 illustrates a concave curvature (viewed from above) in the Z-shaped structural element; and the bottom view illustrates a convex curvature (viewed from above) in the Z-shaped structural element 21. When the Z-shaped structural element bend vibrates, it continuously moves between the convex and concave curature configurations illustrated in the center and bottom views of FIGURE 3. During each complete cycle of bending, of course, two passes are made through the unbent state illustrated in the top view of FIGURE 3. In addition to bending in the manner illustrated in FIGURE 3, which is in a plane lying parallel to the plane in which the web 23 lies, of course, Z-shaped structural elements can also bend in a plane lying parallel to the pl of the flanges 25a and 25b. As a practical matter, Z-shaped structural eleme can bend in any longitudinal plane, which bends can, of course, be mat matically resolved into planes lying parallel to the web 23 and the flanges and 25b, using conventional vector analysis techniques. Further, studies h shown that aircraft frame bend and twist vibration frequencies lying in the l (below 300 Hz) audio frequency range control the fuselage structural vibrati and sound transmission into the aircraft cabin. As will be readily appreciated by those familiar with viscoelas materials, such materials damp vibration by converting vibration energy i heat energy. Preferably, the viscoelastic materials used in actual embodime of the invention will have a vibration to internal heat energy dissipation p that lies at or near the temperature of the environment in which the structu elements are to be used. More specifically, as will be readily appreciated those familiar with viscoelastic materials, viscoelastic materials damp vibrati by dissipating vibration energy as heat. As will also be appreciated by th familiar with viscoelastic materials, the magnitude of the vibration energy t can be converted to heat by a particular material peaks " at a cert temperature. As a result, in order to obtain maximum energy dissipation, i desirable to choose a viscoelastic material that peaks at the temperature of environment in which the structural elements are to be used. Alternatively, i viscoelastic material cannot be chosen that peaks in this temperature range, chosen viscoleastic material should have a peak as near to the temperature ra as possible. Various types of well known viscoelastic materials, some of wh are described in U.S. Patent Application Serial No. 029,705 filed April 11, 19 noted above, can be used by the invention. The layer of viscoelastic material 29 located between the ri constraining layer 27 and the Z-shaped structural element 21 illustrated FIGURE 1 damps vibration in two different manners. When the vibration sou is twisting of the web 23, the region of the viscoelastic material layer ly between the web 23 and the adjacent region of the rigid constraining layer provides direct damping. In addition, because the rigid constraining layer coup web vibration to the region of the viscoelastic material layer 29 lying betw the flanges 25a and 25b and the adjacent -region of the rigid constraining la 27, these regions of the viscoelastic material layer provide indirect damping. When the Z-shaped structural element 21 bends, as illustrated FIGURE 3, the major source of vibration is movement of the flanges 25a and 2 In this instance, direct damping is provided by the region of the viscoelastic material layer 29 attached to the flanges 25a and 25b. Indirect damping is provided by the region of the viscoelastic material layer 29 attached to the web 23 of the Z-shaped structural component 21 because that region is coupled to the vibrating flanges 25a and 25b by the rigid constraining layer 27. As will be readily appreciated by those skilled in the vibration damping art and others, because of the indirect damping assistance to direct damping, the total weight of the damping treatment provided in accordance with the invention is substantially less than the weight of the damping treatment that would be. required if separate damping treatment in the form of a separate constraining layer viscoelastically attached to each of the webs and the two flanges of the Z-shaped structural element 21 illustrated in FIGURE 1. As a result, in many instances, adequate vibration damping can be provided by segmenting and/or aperturing the rigid constraining layer and the layer of viscoelastic material attaching the rigid constraining layer to the structural , element rather than using continuous layers, as illustrated in FIGURE 1. In this regard, attention is directed to FIGURE 4 which illustrates a Z-shaped structural element 31 comprising a web 33 and a pair of orthogonal outwardly projecting flanges 35a and 35b. FIGURE 4 illustrates spaced-apart rigid constraining layers 37a and 37b, each of which is formed to mate with the same surface of the web and flanges of the Z-shaped structural element 31. Layers of,, viscoelastic material 29a and 29b attach the spaced apart rigid constraining layers to the same surface of the web and flanges of the Z-shaped structural element 31. While attached to the same surface of the web and flanges, the rigid constraining layers are independent of one another. Further, while two rigid constraining layer segments are illustrated in FIGURE 4 obviously this by way of example only. In an actual embodiment of the invention various numbers of segments of the same or different lengths can be used. FIGURE 5 illustrates an embodiment of the- invention similar to FIGURE 4 except that the rigid constraining layer segments are linked together. More specifically, FIGURE 5 illustrates a Z-shaped structural element 41 comprising a web 43 and a pair 4 of orthogonal outwardly projecting flanges 45a and 45b. FIGURE 5 also illustrates a segmented rigid constraining layer formed to a plurality of segments 47a and 47b having a cross-sectional configuration that mates with one of the surfaces of the web and flanges of the Z-shaped structural element 41. The segments are attached to the mating surface of the web 43 and flanges 45a and 45b of the Z-shaped structural element 41 by layers of viscoelastic material 49a and 49b. The segments 47a and 47b are joined by a plurality of connecting links 48a, 48b, 48c and 48d. While the FIGUR embodiment of the invention is slightly heavier than the embodiment illustra in FIGURE 4 it produces substantially the same amount of damping as embodiment of the invention illustrated in FIGURE 1 because of the links 4 48b, 48c and 48d between the segments 47a and 47b. It should be noted that apertures between the links 48a, 48b, 48c and 48d provide space for various ite to be attached to the web and flanges of the structural element 41. In t regard, the links can take on a wide variety of shapes other than that illustra in FIGURE 5. The links can be longer, narrower, wider, shorter,- etc. Furth obviously the number of linked segments can be other than two, as illustrat And, of course, other apertures can be included to provide access to structural element at non-uniform positions. FIGURE 6 illustrates a -further embodiment of the invent wherein a Z-shaped structural element 51 has damping treatment applied to b surfaces of its web 53 and the flanges 55a and 55b. More specifically, first second rigid constraining layers 57a and 57b mate with, and are viscoelastic attached by first and second viscoelastic layers 59a and 59b to, opposed surfa of the web 53 and flanges 55a and 55b of the Z-shaped structural element 53. a result, direct and indirect damping of the web and flanges of the Z-sha structural element 51 is provided. More specifically, with respect to the web direct damping is provided by the regions of the first and second viscoelas layers attaching the first -and second rigid constraining layers to the web. addition, indirect damping is provided by the regions of the first and sec viscoelastic layers attaching the first and second constraining layers 57a and 5 to the flanges 55a and 55b. If desired, as illustrated in FIGURE 4, the ri constraining layers 57 and 57b can be segmented; and, the segments can be lin together as illustrated in FIGURE 5. FIGURE 7 illustrates an embodiment of the invention where damping treatment is applied to the web 63 and only one flange 65a of Z-shaped structural element 61. The other flange 65b does not receive dampi treatment. More specifically, FIGURE 7 illustrates a rigid constraining layer that mates with one side of the web 63 and the outer surface of one of flanges 65a of the Z-shaped structural element 61 and is attached to this surfa by a layer of viscoelastic material 69. As with the previously describ embodiments of 'the invention, direct and indirect vibration damping of w vibrations is provided by this embodiment of the invention. However, indir damping is provided only by the viscoelastic attachment to one flange, rat than by viscoelastic attachment to two flanges. And, of course only one flange \j directly and indirectly vibration damped. The other flange is undamped. Further, if desired, the rigid constraining layer 67 can be segmented (FIGURE 4) and the segments can be linked together (FIGURE 5), if desired. FIGURE 8 also illustrates an embodiment of the invention wherein damping treatment is applied to the web 73 and only one flange 75a of a Z-shaped structural element 71. The other flange 75b is undamped. The difference between FIGURE 7 and FIGURE 8 is that the damping treatment is attached to the inside surface of the angle defined by the damped flange and the web, as opposed to being attached to the outside surface. More specifically, FIGURE 8 illustrates a rigid constraining layer 77 that mates with one surface of the web 73 and the inner surface of one of ' the flanges 75a of the Z-shaped structural element 71 and is attached to this composite surface by a layer of viscoelastic material 79. As with other embodiments of the invention, while not illustrated herein, the rigid constraining layer 77 can be segmented with or without the segments being linked together, if desired. As will be readily appreciated from viewing FIGURES 1 and 4-8, in accordance with the invention, a wide variety of damping treatments can be applied to * Z-shaped structural elements. The nature of the damping treatment chosen for an actual embodiment of the invention will depend upon the magnitude of the vibration that occurs in the structural element to be damped and acceptable weight additions. That is, whether the chosen damping treatment will cover both sides of the structural component as illustrated in FIGURE 6, or only the web and one of the flanges as illustrated in FIGURE 7 and 8, or lie somewhere ϊnbetween these two extremes, as illustrated in the other FIGURES, will depend upon two factors. The first factor is the magnitude of the vibration that occurs in the structural element and the second factor is the- amount of acceptable weight that can be added to the structural element in order to reduce the noise created by the vibration to an acceptable leveL Whether or not the damping treatment formed by the rigid constraining and the viscoelastic material layers is continuous, apertured or segmented will depend upon similar factors, plus other factors, such as nature, amount and type " of -items to be attached to - the. structural element. - Preferably, the rigid constraining layer attached to the structural element by the layer of viscoelastic material is of the same material as, and has a thickness generally " equal to the thickness of the web or flanges of, the structural element. While this is the preferred material and thickness, obviously, other materials and thickness can be utlized, if desired, keeping in mind that the important factor to be met is that the constraining layer be rigid so that it can provϊde the coupling effect previously described. As illustrated in FIGURES 9-12 and hereinafter described, . invention is not limited to use with Z-shaped structural elements. Rather, invention can be utilized with other types of structural elements, for exa the I, L, C and T-shaped structural elements illustrated in FIGURES 9-12 hereinafter described. Prior to describing these FIGURES, it is pointed out while these FIGURES illustrate damping treatment applied to all of the surf of the illustrated structural components, less than all of the surfaces can b treated if desired. Further, while the damping treatment formed by r constraining layers and viscoelastic attachment material is illustrated continuous, it can be apertured or segmented (with or without the segments b linked) if desired. Thus, as with the Z-shaped structural elements descri above, the extent of damping treatment can vary and in an actual embodimen the invention will depend upon the amount of vibration reduction desi commensurate with weight restrictions. FIGURE 9 illustrates an elongate l-shaped structural element which includes a web 83 and upper and lower flanges 85a and 85b. As usual, flanges 85a and 85b are longitudinally centered along the edges of the web 8 that the cross sectional configuration of the structural element 81 has the sh of an I, as previously noted. C-sh'aped rigid constraining layers 87a and 87b viscoelastically attached by layers of viscoelastic material 89a and respectively, to the interior surfaces defined by the web 83 and the flanges and 85b of the l-shaped structural element 81. In addition, planar r constraining layers 91a and 91b are viscoelastically attached by layers viscoelastic material 93a and 93b to the outer surfaces of the flanges 85a 85b of the l-shaped structural element 81. Thus, all of the potentially vibra surfaces (e.g., the web and the flanges 85a and 85b) of the l-shaped struct element 81 are covered by two rigid constraining layers and their attac layers of viscoelastic material. The C-shaped constraining layers 87a and couple the vibration region of the structural element (e.g., web 83 or flanges and 85b) to the other regions so that indirect as well as direct dampin provided, as heretofore described. FIGURE 10 illustrates an L-shaped structural element 101 ha first and second legs 103 and 105. First and second L-shaped rigid constrai - ' layers 107a and 107b are attached by first and second layers of viscoela material 109a and 109b to the inner and outer surfaces of the legs 103 and 10 the L-shaped structural element 101. More specifically, the first L-shaped r constraining layer 107a is attached by the first layer of viscoelastic mate ' 109a to the inner surface of the legs 103 and 105. The second L-shaped rigid constraining layer 107b is attached by the second layer of viscoelastic material 109b to the outer surface of the legs 103 and 105. As with the previously described embodiments of the invention, vibration of one of the legs of the L- shaped structural element is directly damped by the regions of the layers of viscoelastic material attached to the vibrating leg and indirectly damped by the regions of the layers of viscoelastic material attached to the other leg as a result of the coupling provided by the L-shaped rigid constraining layers 107a and 107b. FIGURE 11 illustrates an elongate C-shaped structural element 111, comprising a web 113 and first and second flanges 115a and 115b projecting orthogonally outwardly on the same side, but from opposite longitudinal edges of, the web 113. A first C-shaped rigid constraining layer 117a is attached by a first layer of viscoelastic -material 119 a to the interior surface of the C-shaped structural element 111. A second C-shaped rigid constraining layer 117b is attached by a second layer of viscoelastic material 119b to the outer surface of the C-shaped structural element 111. As previously discussed, vibration of the web or the flanges of the C-shaped structural element is directly damped by the layer of viscoelastic material attached to the vibrating region and indirectly damped by the layer of viscoelastic material attached to the other regions of the C-shaped structural element due to the coupling provided by the L-shaped rigid constraining elements 117a and 117b. FIGURE 12 illustrates an elongate T-shaped structural element 121 comprising a web 123 and a cross member flange 125. The cross member flange 125 is centered along one longitudinal edge of the web 123 so that the cross- sectional configuration of the combination is in the shape of a T. A first L- shaped rigid constraining layer 127a is attached by a first layer of viscoelastic material 129a to one surface defined by the web 123 and the cross member flange 125 of the T-shaped structural element 121. A second L-shaped rigid constraining layer 127b is viscoelastically attached by a second layer of viscoelastic material 129b to the other surface defined by the web 123 and the cross member flange 125 of the T-shaped structural element 121. In addition, a planar rigid constraining layer 131 is attached by a third layer of viscoelastic material 133 to the top or outer surface of the cross member flange 125 of the T-shaped structural element 121. As with the other embodiments of the invention, the vibration of either the web or the< cross member flange of the T- shaped structural element 121 is directly damped by the layer of viscoelastic material attached to the vibrating region and indirectly damped by the layer of viscoelastic material attached to other regions, as a result of the coupling provided by the first and second L-shaped rigid constraining layers 127a a 127b. Previously described FIGURE 5 illustrates that the rigid constrai ing layer and the layer of viscoelastic material attaching the rigid constraini layer to the structural element can be longitudinally segmented, with t segments coupled together by links. FIGURE 13 also illustrates a linked ri constraining layer; however, the links connect together the regions of the ri constraining layer attached to the various areas of the structural element. Mo specifically, FIGURE 13 illustrates an elongate Z-shaped structural element 1 comprising a web 143 and a pair of flanges 145a and 145b that proje orthogonally outwardly from opposite longitudinal edges of the web 143 and opposite sides thereof. A mating Z-shaped rigid constraining layer 147 viscoelastically attached to one of the surfaces of the Z-shaped structur element 141. The Z-shaped rigid constraining layer 147 has three distin regions— a web 149 and two flanges 151a and 151b. A series of links 153a, 153 153c . . . 1531 formed unϊtarϊly with the web and the two flanges connect the w to the flanges of the Z-shaped rigid constraining layer 147. Thus, the links 15 . . . 1531 lie at the bends in the Z-shaped rigid constraining layer. A first layer viscoelastic material 155 attaches the web 149 of the Z-shaped rigid constraini layer 147 to the web 143 of the Z-shaped structural element. Second and thi layers of viscoelastic material 157a and 157b attach the flanges 151a and 151b the Z-shaped rigid constraining layer 147 to the flanges 145a and 145b of the shaped structural element 141. Since the links 153a . . . 1531, couple t vibration of one region (web or flanges) of the Z-shaped structural element to t other region or regions, the embodiment of the invention provides indirect well as direct damping similar to that provided by the previously describ embodiments of the invention. In this regard, it is pointed out that a rig constraining layer link configuration of the type illustrated in FIGURE 13 can used in other embodiments of the invention such as those illustrated in FIGUR 1 and 4-12, and variations thereof. A linked rigid constraining layer has, course, a weight advantage over a continuous type rigid constraining layer. As will be readily appreciated from the foregoing description preferred embodiments of the invention, the invention provides a method a apparatus for vibration damping structural elements having at least t transverse .legs. The method generally comprises viscoelastically attaching rigid contrainϊng layer to the at least two transverse legs of the undamp structural element to be damped. As a result, the vibration of one of the legs directly damped by the viscoelastic layer attached thereto. Additional indire \J damping of the vibrating leg is provided by the viscoelastic layer attached to the other leg as a result of the link created by the rigid constraining layer. Depending upon the magnitude of leg vibration, such vibration damping treatment can be applied to one or both sides of the two legs, if desired. Also, multiple layers of damping treatment can be applied to one or both sides of the legs. Further, if the structural component includes more than two legs, similar damping treatment can be applied to the other leg or legs. Moreover, the constraining element can run the entire length of the structural element or can be apertured or segmented in various ways. If desired, the segments can be linked together. Consequently, the invention can be practiced in a wide variety of manners all of which are not specifically illustrated and described herein. Thus, it is to be understood that the invention can be practiced otherwise than as specifically described herein.

Claims: The embodiments of the invention in which an exclusive proper privilege is claimed are defined as follows: 1. A method of vibration damping a structural element ha at least two transversely oriented legs, said method comprising the ste viscoelastically attaching a rigid constraining element to said at least two le that the vibration of one of said legs is directly damped by the viscoel attachment between said vibrating leg and said rigid constraining element indirectly damped by the viscoelastic attachment between said other leg and rigid constraining element. 2. The method claimed in Claim 1 wherein said constraining element is a rigid constraining layer configured to mate with surface of said at least two transversely oriented legs and wherein viscoelastic attachment is a layer of viscoelastic material mounted between rigid constraining layer and said one surface of said at least two transve oriented legs. 3. The method claimed in Claim 2 wherein said at least transversely oriented " legs are orthogonally oriented and define a web a flange and wherein said structural element also includes a second fl orthogonally oriented with respect to said web. 4. The method claimed in Claim 3 wherein said constraining layer is configured to mate with one surface of said web and two flanges and wherein said viscoelastic layer attaches said rigid constrai element to said one surface of said web and said two flanges. 5. The method claimed in Claim 2 wherein said constraining element includes a second rigid constraining layer configure mate with the other surface of said at least two transversely oriented legs wherein said viscoelastic attachment includes a second layer of viscoel material mounted between said second rigid constraining layer and said o surface of said at least two transversely oriented legs. 6. Apparatus for vibration damping a structural element ha at least two transversely oriented legs, said apparatus comprising: a rigid constraining element; and, " viscoelastic means for attaching said rigid constraining element to said at least two transversely oriented legs so that the vibration of one of said legs is directly damped by the viscoelastic attachment between said vibrating leg and said rigid constraining element and said rigid constraining element couples 5 said vibration of said one leg to said viscoelastic attachment between said rigid constraining element and said other leg whereby said viscoelastic attachment between said rigid constraining element and said other leg indirectly damps said vibrations of said one leg. 10 7. Apparatus as claimed in Claim 6 wherein said rigid constraining element is a rigid constraining layer configured so as to mate with one surface of said at least two transversely oriented legs and wherein said viscoelastic means is a layer of viscoelastic material mounted between said rigid constraining layer and said one surface of said at least two transversely oriented 15 legs. 8. Apparatus as claimed in Claim 7 wherein said rigid constraining layer is segmented. 20 9. Apparatus as claimed in Claim 8 wherein said segments of said segmented rigid constraining layer are linked together by link& 10. Apparatus as claimed in Claim 9 wherein said links form part of said rigid constraining layer. 25 11. Apparatus as claimed in Claim 7 including a second rigid constraining layer configured so as to mate with the other surface of said at least two transversely oriented legs and a second layer of viscoelastic material, said second layer of viscoelastic material attaching said second rigid constraining 30 layer to said other surface of said at least two transversely oriented legs. 12. Apparatus as claimed in Claim 11 wherein said rigid constraining layers are segmented. '35 13. Apparatus as claimed in Claim 12 wherein said segments of said segmented rigid constraining layers are linked together by links. 14. Apparatus as claimed in Claim 13 wherein said links form " U REA part of said rigid constraining layers. 15. Apparatus for vibration damping a structural elem comprising a web and two transversely oriented flanges, such as Z, C, T, H an shaped structural elements, said apparatus comprising: a rigid constraining element; and, viscoelastic means for attaching said rigid constraining elemen said web and at least one of said transversely oriented flanges so that vibration of one element of said web and said at least one of said transver oriented flanges is directly damped by the viscoelastic attachment between vibrating element and said rigid constraining element and said rigid constrai element couples said vibration of said one element to said viscoela attachment between said rigid constraining element and said other elem whereby said viscoelastic attachment between said rigid constraining elem and said other element indirectly damps said vibrations of said one element. 16. Apparatus as claimed in Claim 15 wherein said r constraining element is a rigid constraining layer configured so as to mate one surface of said web and said at least one of said transversely oriented l and wherein said viscoelastic means is a layer of viscoelastic material moun between said rigid constraining layer and said one surface of said web and one of said transversely oriented legs. 17. Apparatus as claimed in Claim 16 wherein said r constraining layer is segmented. 18. Apparatus as claimed in Claim 17 wherein said segment said segmented rigid constraining layer are linked together by links. 19. Apparatus as claimed in Claim 18 wherein said links f part of said rigid constraining layer. 20. Apparatus as claimed in Claim 16 including a second r constraining layer configured so as to mate v/ith the other surface of said and said at least one of said transversely oriented legs and a second laye viscoelastic material, said second layer of viscoelastic material attaching second rigid constraining layer to said other surface of said web and said at l one of said two transversely oriented legs. 21. Apparatus as claimed in Claim 20 wherein said rigid constraining layers are segmented. 22. Apparatus as claimed in Claim 21 wherein said segments of said segmented rigid constraining layers are linked together by links. 23. Apparatus as claimed in Claim 22 wherein said links form part of said rigid constraining layers. 24. Apparatus as claimed in Claim 15 wherein said . rigid constraining element is a rigid constraining layer configured so as to mate with one surface of web and said two transversely oriented legs and wherein said viscoelastic means is a layer of viscoelastic material mounted between said rigid constraining layer and said one surface of said web and said two transversely oriented legs. 25. Apparatus as claimed in Claim 24 wherein said rigid constraining layer is segmented. 26. Apparatus as claimed in Claim 25 wherein said segments of said segmented rigid constraining layer are linked together by links* 27. Apparatus as claimed in Claim 26 wherein said links form part of said rigid constraining layer. 28. Apparatus as claimed in Claim 24 including a second rigid constraining layer configured so as to mate with the other surface of said web and said two transversely oriented legs and a second layer of viscoelastic material, said second layer of viscoelastic material attaching said second rigid constraining layer to said other surface of said web and said two transversely oriented legs. 29. Apparatus as claimed in Claim 28 wherein said rigid constraining layers are segmented. , 30. Apparatus as claimed in Claim 29 wherein said segments of said segmented rigid constraining layers are linked together by links. 31. Apparatus as claimed in Claim 30 wherein said links f part of said rigid constraining layers.

Inventors: SENGUPTA G
Applicants: BOEING CO
Application year: 1981
Main classification: B64C1
IPC codes: B60R13, B62D25, B64C1, E04B1, F16F9, G10K11