POLYMER COMPOSITES POSSESSING IMPROVED
VIBRATION DAMPING
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of priority of U.S. Provisional Patent
Application No. 61/496,547, entitled "POLYMER COMPOSITES HAVING IMPROVED
FLEXURAL VIBRATION DAMPING", filed on, June 13, 2011. The entirety of this
application is hereby incorporated by reference and should be considered a part of this
specification.
BACKGROUND
Field
[0002] Embodiments of the present disclosure pertain to vibration damping and, in
particular, to polymer matrix composites that provide improved mechanical damping while
retaining good mechanical and thermal properties.
Description of the Related Art
[0003] Vibration is commonly encountered in moving mechanical systems. In one
example, automobiles experience vibration during operation due to variations in the surface of
the road. In another example, cameras may vibrate when motion takes place around the mount
to which the camera is attached.
[0004] Controlling the degree to which a structure vibrates may be desirable for a
number of reasons. In one aspect, mechanical vibrations may result in perceptible noise and/or
motion. In the context of consumer products, noise or motion associated with vibration may be
distracting and uncomfortable to a customer. This in turn may lead to a perception of poor
quality of the product. For example, vibrations in automobiles are desired to be kept as low as
possible in order to provide a more pleasant driving experience. In another aspect, the
oscillatory deformation associated with mechanical vibrations may lead to mechanical failure in
a vibrating structure due to high cycle fatigue. In another example, vibrations may induce
undesirable acoustic levels in aircraft, rotor craft or marine vessels.
[0005] A variety of approaches have been adopted to increase damping in vibrating
systems. Typically, these approaches apply a vibration damping material to an area of a
vibrating system, where the vibration damping material attenuates the vibrations. For example,
viscoelastic polymers are often employed in vibration damping applications.
[0006] In general, the amount of damping provided by a viscoelastic material at a
given temperature and frequency is dictated by the dynamic modulus and loss factor of the
viscoelastic material. The dynamic modulus is a measure of the stiffness of the viscoelastic
material under vibratory conditions, while the loss factor is a parameter related to the
viscoelastic damping of the material. As a rule of thumb, as the dynamic modulus of the
viscoelastic material increases, more of the mechanical energy of vibration is stored in the
viscoelastic material per vibration cycle, while as the loss modulus of the viscoelastic material
increases, a greater portion of the stored mechanical energy is dissipated. Thus, a high dynamic
modulus and a high loss factor each favor high energy dissipation in viscoelastic materials.
[0007] However, the loss factor of a viscoelastic material is not independent of the
dynamic modulus. For example, when the dynamic modulus of a viscoelastic material is
increased, the loss factor decreases precipitously beyond a certain modulus level. It is further
observed that the reduction in damping due to the reduced loss factor is not compensated for by
the increase in dynamic modulus. As a result, the net effect of increasing the dynamic modulus
is a reduction in the amount of damping provided in a unit volume of the viscoelastic material.
[0008] While the amount of damping material employed in a damping application
may be increased to compensate for the reduction in the damping per unit volume of the
viscoelastic material, this approach is problematic. In one aspect, adding more damping material
to reduce vibrations occurring in a structure increases the cost of the viscoelastic material used.
In another aspect, adding more damping material to reduce vibrations occurring in a structure
increases the total weight or size of the structure. Either cost, weight or size increases may be
prohibitive in certain applications.
[0009] Therefore, there exists an ongoing need for damping materials that provide
improved mechanical vibration damping while remaining relatively light weight and also
retaining acceptable mechanical properties.
[0010] Poisson' s ratio is a natural characteristic of materials. When a homogeneous
material is stretched (tensile strain) in one direction, it tends to contract (compression strain) in
the other two directions perpendicular to the direction of tension. Poisson' s ratio is the ratio of
the negative compression strain divided by the tensile strain, for small values of these changes.
The theoretical maximum Poisson' s ratio for a homogeneous material is 0.5. Materials with
exceptional Poisson's ratios (e.g., greater than 0.5) may have useful properties.
[0011] Thermal expansion is a natural characteristic of materials. Most materials
expand as temperature increases. Different materials have different expansion rates. The
coefficient of linear expansion is a measurement of length increase per degree of temperature
increase.
[0012] Controlling thermal expansion is desirable in many applications. For example,
thermal expansion can alter the deform measurement tools, creating measurement error. In
another example, temperature changes can lead to residual stress and deformation when
dissimilar materials with different coefficients of linear expansion are bonded together. In
another aspect, a designer may exploit this residual stress to induce motion as of an actuator.
[0013] Therefore, there exists an ongoing need for materials with negative
coefficients of linear expansion to counteract and/or contrast the thermal deformation of
conventional materials.
SUMMARY
[0014] Embodiments of the present invention involve several features pertaining to
vibration damping. Without limiting the scope of this invention, its more prominent features
will be discussed briefly. After considering this discussion, and particularly after reading the
Detailed Description section below in combination with this section, one will understand how
the features and aspects of these embodiments provide several advantages over prior damping
systems and methods.
[0015] One aspect of the disclosed embodiments is the combination, linkage and use
of non-obvious parameters. Certain embodiments seek to increase system loss factor with a
strategy that reduces material loss factor (e.g., as part of a tradeoff with dynamic modulus). For
example, when an elastomer matrix with a high loss factor is reinforced with fibers, the resulting
composite may have a lower material loss factor, offset by a higher dynamic shear modulus,
relative to the viscoelastic matrix. In many applications, the resulting fiber-reinforced elastomer
composite structure may be more effective (e.g., structure may have a higher system loss factor)
than if treated with the unreinforced elastomer.
[0016] Another aspect of the disclosed embodiments is the exploitation of Poisson's
ratios higher than 0.5, which is a value that is theoretically impossible with homogeneous
materials. Certain embodiments exhibit exceptionally high Poisson's ratios to increase system
loss factor. A "strain magnification effect" is disclosed in which materials with exceptionally
high Poisson's may possess exceptional damping properties in certain embodiments.
[0017] Another aspect of the disclosed embodiments is the exploitation of Poisson's
ratios greater than 0.5 to create composite materials with a negative coefficient of linear thermal
expansion in certain embodiments.
[0018] Another aspect of the present invention is a composite material that comprises
a plurality of fiber-reinforced elastomer composite layers having an elastomer and a plurality of
fibers positioned within the elastomer. The composite material possesses a maximum shear loss
factor greater than about 0.5 and a real portion of the dynamic shear modulus value G' greater
than about 1 X 104 psi at the maximum shear loss factor. Certain embodiments of this invention
may possess a Poisson's ratio greater than 0.5. Furthermore, certain embodiments of this
invention may possess a negative coefficient of linear expansion.
[0019] Another aspect of the disclosed embodiments is a structure incorporating
fiber-reinforced elastomer composite layers to increase damping, increase compliance and/or
control thermal expansion.
[0020] Another aspect of the disclosed embodiments is a fiber-reinforced composite
material that comprises an elastomer and a first plurality of fibers positioned within the
elastomer. The composite material possesses a maximum Young's loss factor greater than 0.15
and a real portion of the dynamic Young's modulus value E' greater than 2 x 105 psi at the
maximum Young's loss factor.
[0021] Another aspect of the disclosed embodiments is a method for fabricating a
composite. The method comprises infiltrating an elastomer into a plurality of fibers to form a
fiber-reinforced elastomer layer, stacking a plurality of the fiber-reinforced elastomer layers, and
curing the plurality of fiber-reinforced elastomer layers to form a fiber-reinforced elastomer
composite. The composite possesses a maximum shear loss factor greater than about 0.5 and a
real portion of the dynamic shear modulus value G' greater than about 1 x 104 psi at the
maximum shear loss factor.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] Figure 1 is a schematic illustration of an embodiment of a method of making
polymer composites of the present disclosure;
[0023] Figure 2 is a schematic illustration of embodiments of layers of the polymer
composites of the present disclosure;
[0024] Figure 3 is a plot of storage modulus and loss factor as a function of
frequency for embodiments of the polymer composite of Figure 1 and a comparative elastomer
alone;
[0025] Figure 4A-4G are plots of shear modulus, G', and loss factor as a function of
frequency comparing the performance of embodiments of the polymer composite, neat
elastomer, and damping tile;
[0026] Figure 5 is a plot of a system loss factor as a function of frequency for
embodiments of the polymer composite of Figure 1 and comparative materials;
[0027] Figure 6 is a plot of system loss factor as a function of dynamic Young's
modulus for embodiments of the composite of Figure 1 generated by finite element analysis;
[0028] Figures 7A-7C are plots of G' and loss factor as a function of frequency
comparing the measurements from the DMA to modal testing and finite element analysis (FEA)
[0029] Figures 8A-8C are plots of G' and loss factor as a function of frequency
comparing the measurements from the DMA to modal testing and finite element analysis (FEA)
for G23;
[0030] Figures 9A-9D show plots of dynamic Young's modulus (En) and loss factor
as a function of frequency as measured with modal testing and finite element analysis (FEA);
[0031] Figure 10 is a sample plot of system loss factor as a function of dynamic shear
modulus (G13) for a series of discrete modes within a composite sandwich structure with a
carbon fiber reinforced polyurethane composite core between two carbon fiber reinforced epoxy
constraining layers;
[0032] Figure 11 is a modal system loss factor plot as a function of frequency for two
0.5-inch thick aluminum beams coated with a 0.12-inch thick viscoelastic composite layer on
one surface; and
[0033] Figures 12A-12B show modal system loss factor plots as a function of
frequency for 0.25-inch thick aluminum beams sandwiching a 0.12-inch thick viscoelastic
composite core layer. The Gi3 plot represents a +28° viscoelastic composite fiber orientation,
and the G23 plot represents a +62° viscoelastic composite fiber orientation relative to the long
axis of the aluminum.
DETAILED DESCRIPTION
[0034] Embodiments of the present disclosure provide fiber-reinforced polymer
composites possessing improved damping ability. In one aspect, the fibers may be configured to
provide the composite with a relatively high dynamic modulus over a broad range of frequencies
for a given temperature. In another aspect, the polymer may comprise a viscoelastic polymer
configured to possess a relatively high loss factor for a given frequency and temperature. The
polymer may be further tailored to control the center frequency at which the maximum loss
factor of the polymer is achieved. The composite formed from the fiber and modulus so
configured exhibits a relatively small reduction in loss factor with significant increase in
dynamic modulus over a broad range of frequencies for a given temperature. As a result, when
employed to damp vibrations in a structure, the damped system (composite and structure)
exhibits a relatively high loss factor that is substantially constant, as compared to conventional
damping materials. Thus, embodiments of the disclosed composites dissipate significantly more
energy during each vibration cycle than conventional damping materials. To better appreciate
the advantages and benefits of the disclosed composites, energy dissipation in viscoelastic
materials is discussed in brief below.
[0035] In general, the degree of damping provided by a viscoelastic material may be
strongly influenced by the dynamic modulus of the material and the loss factor of the material.
In a material undergoing vibration (i.e., oscillatory motion), the ratio of stress to strain is given
by the dynamic modulus. Using a hysteretic model of viscoelasticity, where damping is assumed
to be proportional to strain and independent of rate, the dynamic modulus of a viscoelastic
material may be represented mathematically by Equation 1:
E(T, f) = Ei + iE2 = Ei(l + ί h) (1)
where E(T,f) is the dynamic modulus, as a function of temperature and frequency, Ei is the
storage modulus of the structure, E2 is the loss modulus of the structure, i is the imaginary unit,
and h is the loss factor, which may also be expressed as E2/E;.
[0036] As illustrated in Equation 1, the dynamic modulus is expressed as a complex
equation, where the storage modulus is the real portion and the loss modulus is the imaginary
portion. The storage modulus relates to the elastic behavior of the material and characterizes its
stiffness. The loss modulus relates to the viscous behavior of the material and characterizes its
energy dissipation ability. Notably, in general, the dynamic modulus of a material increases as
its storage modulus (i.e., stiffness) increases.
[0037] The dynamic modulus and loss factor may be influenced not only by the
viscoelastic material but also environmental and loading conditions. For example, changes in
temperature (T) and/or frequency (f) significantly influence dynamic modulus and loss factor.
Thus, the damping behavior of embodiments of the disclosed composites may be discussed in
the context of temperature and/or frequency. However, it may be understood that the scope of
the disclosed embodiments are not limited to temperature and frequency but may include any
parameter that exerts an influence upon the dynamic modulus and loss factor.
[0038] The amount of energy dissipated by the viscoelastic material on unloading
during a vibrational cycle, D, is proportional to both the loss factor h and the total amount of
mechanical energy stored by the viscoelastic material during loading, U (Equation 2).
Ό ,u (2)
[0039] In general, the energy U increases with increasing dynamic modulus of the
material. The dynamic modulus also increases with the storage modulus of the material. Thus,
the energy U increases with increasing storage modulus. In contrast, the loss factor decreases
with increasing storage modulus. Thus, the amount of energy D dissipated by the viscoelastic
material as the dynamic modulus of the material depends upon competition between the decrease
in the loss factor and the increase in the stored energy. If the amount by which the stored energy
increases with a given change in dynamic modulus is greater than the amount by which the loss
factor decreases, the net effect is an increase in dissipated energy. Conversely, if the amount by
which the stored energy increases with a given change in dynamic modulus is less than the
amount by which the loss factor decreases, the net effect is a decrease in dissipated energy.
[0040] As discussed in greater detail below, embodiments of the disclosed
composites comprise one or more layers of a viscoelastic polymer matrix (e.g., an elastomer)
reinforced with a plurality of fibers. The resulting composite, possessing a high loss factor and
high modulus, may provide an improvement in mechanical damping while maintaining desired
mechanical properties (e.g., modulus, strength, etc.). For example, when coupled with a
vibrating structure, the composite provides a significant increase in the loss factor of the
composite as compared to conventional matrix materials, such as epoxies and vinyl ester resins.
These and other advantages of the disclosed embodiments are described in detail below.
[0041] In the description that follows, the damping properties of the composite (e.g.,
loss factor, dynamic modulus, etc.) may be referred to as material or composite properties.
Properties of the system formed by attachment of the composite to an underlying structure may
be referred to as system properties.
[0042] Embodiments of the damping composite may be applied to a single surface of
a structure (e.g., a beam). When the composite is attached to a single side of the beam structure,
deformation of the beam structure may be referred to as extensional. When the beam is bent
under these conditions, the composite will undergo extension and compression generally along
the direction of the longitudinal axis of the beam structure (e.g., En) owing to the asymmetry of
the composite- structure system. As a result, the damping properties of the composite are
dominated by the dynamic Young's modulus (e.g., tensile storage modulus and tensile loss
modulus) of the composite.
[0043] In contrast, when the composite is attached between two beams, deformation
of the beam structure may be referred to as constrained. When the beam is bent under these
conditions, the composite will undergo shear owing to the symmetry of the composite-structure
system. As a result, the damping properties of the composite are dominated by the dynamic
shear modulus, G13, of the composite (e.g., shear storage modulus and shear loss modulus).
[0044] The fibers of the composite may be configured so as to provide a relatively
high dynamic modulus, in either tension or shear. In certain embodiments, one or more
parameters of the fibers may be selected in order to adjust the material dynamic modulus,
including, but not limited to, the fiber composition, the fiber architecture, the fiber orientation,
and the relative concentration of the fibers to the matrix. In certain embodiments, the fibers may
include carbon, graphite, glass, E-glass, S-glass, polymer (such as nomex, aramid, polyethylene,
ultra high molecular weight polyethylene (UHMW), polypropylene, polyester, nylon, polyamide,
poly-p-phenylene-benzobisoxazole (PBO), Innegra®, Kevlar®, Spectra®, thermosets,
thermoplastics, or combinations thereof), composite, fiberglass, oxide, ceramic, silicon carbide,
Nextel®, quartz, metal (such as boron, steel or stainless steel) fibers. However, it may be
understood that other fibers may be employed without limit.
[0045] In other embodiments, the composition of the elastomer matrix may be
configured such that the elastomer exhibits a relatively high loss factor. In further embodiments,
the composition of the elastomer may be tailored so that the maximum loss factor for a given
temperature occurs at a selected center frequency. In this manner, the center frequency of the
composite may be controlled. As discussed in greater detail below, in certain embodiments, the
elastomer may comprise polyurethanes. However, other elastomers may be employed without
limitation.
[0046] In additional embodiments, ranges of the dynamic modulus and loss factor of
the polymer composite have been identified over which the damping behavior (e.g., loss factor)
of a system having an embodiment of the composite applied to a structure undergoing vibration
are significantly improved over conventional damping materials. For example, improved
damping performance is found in constrained loadings when the composite material possesses a
maximum shear loss factor greater than 0.5 and a real portion of the dynamic shear modulus
value G' (e.g., G13) greater than about 1 X 104 psi at the maximum shear loss factor. In another
example improved damping performance is found in extensional loadings when the composite
material possesses a maximum Young's loss factor greater than 0.15 and a real portion of the
dynamic Young's modulus value E' (e.g., En) greater than about 2 X 105 psi at the maximum
Young's loss factor (Figure 9).
[0047] For example, as illustrated in the examples below, an embodiment of the
disclosed composites, a carbon-fiber reinforced-polyurethane composite having a dynamic shear
modulus and loss factor within the ranges disclosed above was attached to an aluminum beam in
a constrained configuration. The system loss factor of the composite-aluminum beam was
approximately four times greater than a comparable aluminum beam damped with state-of-theart
damping tile. Furthermore, this improvement in vibration damping was observed for
vibration modes ranging between about 10 Hz to at least about 3000 Hz. In further
embodiments, vibration damping of vibration modes less than about 10 Hz and/or greater than
about 3000 Hz may be obtained. As the Young's modulus of the composite was observed to
increase with frequency, this result indicates that the decrease in the energy damping capability
due to reduction in the material loss factor were offset by a corresponding increase in the amount
of stored energy.
[0048] Optionally, a constraining layer may also be applied to at least a portion of at
least one surface of the polymer composite system. In an embodiment, the constraining layer
may comprise one or more layers of a carbon fiber-reinforced epoxy. In certain embodiments,
one or more of the fiber material, the fiber architecture, the fiber orientation, and the relative
concentration of the fibers to the polymer matrix within the constraining layer may be adjusted
to increase or decrease the modulus and loss factor of the polymer composite, as desired.
[0049] Figure 1 illustrates one embodiment of a process 100 of manufacturing a
polymer composite that provides improved vibration damping. It may be understood that the
process 100 may include greater or fewer operations and the operations may be performed in an
order different than that illustrated in Figure 1, without limit.
[0050] The process 100 includes forming a plurality of layers of the polymer
composite system in blocks 102-106. In block 102, the components of a selected polymer may
be mixed together. In block 104, the polymer may be infiltrated into a plurality of fibers. In
block 106, the polymer and the plurality of fibers may be at least partially cured to form a layer
(e.g., a lamina) of the polymer composite. Optionally, one or more constraining layers may also
be applied to at least a portion of the surface of the exterior layers of composite in block 110 to
impart additional stiffness to the composite.
[0051] Embodiments of the polymer may include elastomers. Examples of the
elastomers may include, but are not limited to, polyurethanes, polyureas, rubbers, thermoplastic
elastomers, plasticized polymers (e.g., plasticized epoxies), silicones, and polyvinyl chlorides.
In further embodiments, the elastomer may be selected so as to possess a processing temperature
range between about room temperature to less than or equal to about 250°F.
[0052] In another embodiment, a vacuum infusion process (e.g., VARTM) may be
employed if the viscosity of the elastomer is low enough (e.g., less than or equal to about 300 cp
at about the processing temperature). In an alternative embodiment, the elastomer may be
selected such that the viscosity of the elastomer is within the range between about 100 cp to
about 300 cp at about the processing temperature. However, other viscosities may be employed
without limit.
[0053] Embodiments of the polyurethane may be formed through reaction of a
diisocyanate with a polyol. The diisocyanate and polyol may be provided in a stoichiometric
ratio according to equivalent weights of the respective components. In further embodiments, an
excess of the diisocyanate may be provided to allow for substantially complete reaction of the
polyol. The diisocyanate may include aliphatic and aromatic compounds. Examples of the
diisocyanate may include, but are not limited to, toluene diisocyanate (TDI), methylene diphenyl
4,4' -diisocyanate (MDI), hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI),
and the like. Examples of the polyol may include, but are not limited to, diols, triols,
polypropylene glycol (PPG), polytetramethyl ether glycol (PTMEG), polycarbonate diols,
polyester polyols, hydroxy-terminated butadienes, and the like.
[0054] Embodiments of the polyurea may be formed through reaction of an
isocyanate with an amine. The isocyanate may react with the amine in a stoichiometric ratio
according to equivalent weights of the respective components. In further embodiments, an
excess of the isocyanate may be provided to allow for substantially complete reaction of the
amine. The isocyanate may include aromatic and aliphatic isocyanates. Examples of the
isocyanate may include, but are not limited to, toluene diisocyanate (TDI), methylene diphenyl
4,4' -diisocyanate (MDI), hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI),
and the like. The polyamines may be aliphatic and aromatic. Examples of the polyamines may
include, but are not limited to, ethylene diamine, 1,3 diaminopropane, amine terminated polyols
and hexamethylenediamine.
[0055] In certain embodiments, the maximum loss factor of the elastomer may be
adjusted to change the loss factor of the composite. For example, the loss factor of the elastomer
may be configured to be as high as possible at a selected temperature and frequency of interest so
as to increase the loss factor of the composite. In other embodiments, the maximum loss factor
of the elastomer may be configured to fall within a selected range at a selected temperature and
frequency of interest. In one example, in one embodiment, the shear loss factor of the elastomer
may be configured to be greater than or equal to about 0.5 (e.g., about 1). The frequency range
of interest may be selected between about 1 Hz to about 100,000 Hz, about 10 Hz to about
10,000 Hz, about 50 Hz to about 10,000 Hz. The temperature range of interest may be between
about -50°C to about 250°C, about -50 to about 100°C, about -5°C to about 40°C, about 5°C to
about 30°C.
[0056] In further embodiments, the stiffness and orientation of the fibers may be
varied in order to change the dynamic modulus and loss factor of the composite. For example, in
general, changes in the composition, configuration (e.g., continuous or discontinuous, 1-, 2-, 3-
dimensional, type of weave, etc.), and/or angle of orientation of the fibers which result in an
increase in the stiffness of the composite may reduce the composite loss factor. Accordingly, by
varying one or more of the composition and/or configuration of the fibers, the composite loss
factor may be adjusted. The impact of fiber architecture is shown in Figure 3.
[0057] In operation 104, the elastomer may be impregnated into a dry preform of the
selected plurality of fibers, using either vacuum only or pressure and vacuum to form a
composite layer. In an embodiment, the elastomer may be present in a concentration varying
within the range between about 30 vol. % to about 70 vol. % (e.g., about 40 vol. % to about 70
vol. %), on the basis of the volume of the composite layer to be formed. In certain
embodiments, the fiber parameters may be selected such that the composite exhibits a selected
dynamic modulus value that improves the damping properties of the composite. Embodiments
of the fiber parameters may include, but are not limited to, fiber composition, fiber orientation,
fiber weave, and the relative concentration of fibers within the composite (e.g., fiber volume
fraction). For example, fiber parameters may be selected to provide a maximum shear loss
factor greater than about 0.5 and a real portion of the dynamic shear modulus value G' (e.g., G13)
greater than about 1 x 104 psi at the frequency-of-maximum-shear- loss-factor.
[0058] In other embodiments, the fiber parameters may be selected such that the
composite material possesses a maximum Young's (e.g., En) loss factor greater than about 0.15
and a real portion of the dynamic Young's modulus value E' greater than about 2 X 105 psi at the
frequency of maximum Young's loss factor.
[0059] In one embodiment, the fibers may comprise substantially continuous fibers.
Examples of these continuous fibers may include, but are not limited to, fiber fabrics and fiber
braids. In alternative embodiments, the fibers may include discontinuous fibers. Examples of
discontinuous fibers may include, but are not limited to, fiber mats, carbon, graphite, glass, Eglass,
S-glass, nomex, aramid, polymers, thermoplastics, polyethylenes, ultra high molecular
weight, polypropylenes, polyesters, poly-p-phenylene-benzobisoxazole (PBO), boron,
polyamide, Innegra®, Kevlar®, nylons, ceramics, metals, fiberglass or composites. However, it
may be understood that other fibers may be employed without limit. The concentration of the
fibers may vary within the range between about 30 vol. % to about 70 vol. % on the basis of the
total volume of the composite layer to be formed.
[0060] The orientation of the fiber reinforcement may also be varied. For example,
the fibers may adopt a ± (plus/minus) orientation with respect to a selected direction (e.g., the
principle axis of the composite). For example, the orientation of the fibers may vary within the
range between about 0° to about 90°, 10° to about 35°, etc. Examples of fiber orientations may
include, but are not limited to, about 0°/90°, quasi-isotropic, + architectures between about 15°
and about 45° or greater (e.g., about +15°, about +20°, about +25°, about +30°, about +45°, etc.).
For example, varying fiber architecture can create highly orthotropic materials in which the
dynamic Young's modulus in one direction (e.g. the E direction of a +25° fiber architecture)
may be more than 20 times higher than in another direction (e.g. E22) . In this same embodiment,
the loss factor in one direction (e.g. the E22 direction of a composite with +25° fiber architecture)
may be more than two times higher than in another direction (e.g. the E direction). The impact
of fiber architecture is shown in Figure 3.
[0061] In certain embodiments, the principle axis of the viscoelastic composite
material may be aligned with a key vibration mode of the structure. In certain embodiments, the
long axis of a beam may be selected as the principle axis because it aligns with the fundamental
bending mode. The fundamental bending mode is the vibration mode with the lowest frequency.
Fundamental modes tend to align with the longest unsupported span of a structure, which is a
logical alignment feature when selecting a principle axis. Aligning the principle axis of the beam
with the principle axis of the composite may provide effective damping and may ensure a
symmetric vibration response.
[0062] In other embodiments, an alternative principle axis selection may be
desirable. In certain embodiments, the short axis of a beam may be selected as a principle axis to
align the "soft" direction of the viscoelastic composite with the fundamental vibration mode
because the soft direction of the viscoelastic composite may have a higher material loss factor.
The tradeoff between modulus and loss factor is complex, and may be evaluated on a case by
case basis.
[0063] In other embodiments, the vibration modes of a structure may be modeled
with finite element software. Many applications have certain critical vibration modes that are
targeted for damping, such as rotating-harmonic-frequency- vibration modes. A principle axis
may be selected such that the stiff axis (e.g. the principle axis) of the viscoelastic composite is
aligned with the principle axis of the structure to maximize system loss factor for the targeted
vibration mode(s).
[0064] In one embodiment, fiber architecture can be designed to create composite
materials with exceptionally high Poisson's ratios (e.g. greater than about 0.5). When a
homogeneous material is stretched (tensile strain) in one direction, it tends to contract
(compression strain) in the other two directions perpendicular to the direction of tension.
Poisson's ratio may be defined as the ratio of the negative compression strain divided by the
tensile strain, for small values of these changes. The theoretical maximum Poisson's ratio for a
homogeneous material is about 0.5.
[0065] In certain embodiments, an exceptionally high Poisson's ratio (e.g. greater
than 0.5) may produce exceptionally high damping (e.g., loss factors) for a given stiffness. For
example, a certain embodiment (e.g. fiber-reinforced polyurethane viscoelastic composite with a
biaxial fiber orientation) with a Poisson's ratio (e.g. v12) of 3.0 may demonstrate exceptional
damping properties. In response to a given input tensile strain in the 11-direction (e.g,. e =
+100 microstrain), this viscoelastic composite response will be three times the input strain in a
compressive 22-direction (e.g., e22 = -300 microstrain), which produces a highly distorted
element that must compensate with expansion in the 33-direction (e.g., e33 = approximately
+200 microstrain, assuming an incompressible material). These strain components (e.g., e and
e22 and e33) are components of the total strain energy available to be converted into heat (i.e.,
damping) by the viscoelastic material. For example, if one assumes a constant strain input of
100 microstrain, a material with a Poisson's ratio of 3.0 may generate a total strain of 600
microstrain by summing e and e22 and e33 strain magnitudes (i.e., 100 + 200 + 300 = 600
microstrain assuming constant volume). In contrast, a 100 microstrain input into a material with
a Poisson's ratio of 0.5 (i.e., the theoretical maximum for homogeneous materials) may generate
66% less total strain in response to (i.e., 100 + 50 + 50 = 200 microstrain assuming constant
volume). Based on these simplified assumptions, a high-Pois son-ratio material may generate
about three times more total strain in response to a given input strain. This "strain magnification
effect" may result in a composite with exceptional damping performance (e.g. an effective
combination of dynamic storage modulus and loss factor).
[0066] In certain embodiments, the present invention may increase system damping
loss factor more than 100% (e.g., a treated aluminum beam). Figure 5 shows the effectiveness of
a strain magnification effect within the present invention bonded to an aluminum beam. The
present invention is 100% to 300% more effective (e.g. the system loss factor is 2 times to 4
times higher) compared to a state of the art damping tile bonded to an equivalent aluminum
beam. It should be noted that these highly simplified assumptions are intended to illustrate the
mechanics of an exceptionally high Poisson's ratio, and are not intended to represent a precise or
rigorous examination of the physics underlying the damping process. It is understood that the
interactions are complex, and that increasing stiffness tends to decrease total strain magnitude.
[0067] In another embodiment, certain biaxial orientations (e.g., a fabric layer with
fibers aligned in two different directions) may possess Poisson's ratios above 0.5 (e.g., certain
embodiments of the present invention with a biaxial fiber orientation range from about +1° to
about +55° relative to the measurement axis), in which relatively small linear expansion in one
direction results in relatively large contraction in another direction. This may be visualized as a
"scissor-type motion" of the fibers, which tends to increase shear strain energy loss in response
to a given input strain. Increasing Poisson's ratio may increase the strain magnification effect,
which may improve composite damping performance (e.g. an effective combination of dynamic
storage modulus and loss factor). The impact of fiber architecture is shown in Figure 3.
[0068] In another embodiment, engineering calculations show Poisson's ratios
greater than about 6.0 (i.e., 12 times higher than the theoretical maximum for homogeneous
materials) may be practical with biaxial fiber orientations with a biaxial fiber orientation range
from about +8° to about +25° relative to the measurement axis. Of course, Poisson's ratio is just
one of many factors influencing system value, and it is understood that the optimum design may
or may not necessarily maximize Poisson's ratio. It is simply noted that certain embodiments
with Poisson's ratios above about 0.5 exhibit useful damping properties.
[0069] In another embodiment, certain unidirectional orientations may possess
Poisson's ratios above about 0.5 (e.g., a certain embodiment of the present invention with a fiber
orientation range from about 1° to about 55° relative to the measurement axis in which relatively
small linear expansion in one direction results in relatively large contraction in another direction.
Increasing Poisson's ratio may increase the strain magnification effect, which may improve
composite damping performance.
[0070] In certain embodiments, the fiber architecture may create a shear coupling
effect, in which a pure shear input may induce normal (i.e., longitudinal) strains. This shear
coupling effect enables in-plane fiber architecture (e.g., fibers oriented in the 1-2-plane) to
influence out-of-plane shear properties (e.g., G13 ) . For example, Figure 4A shows a polyurethane
matrix with the real portion of the dynamic shear modulus measuring about 1000 psi at the
frequency of maximum loss factor. Certain embodiments (e.g., the embodiments shown in
Figures 4C through 4G) demonstrate shear moduli more than 10 times higher than the matrix
shear modulus. This shear modulus magnification factor (e.g., increasing matrix shear modulus
more than a factor of 10) is much higher than would be expected by a simple rule of mixtures.
Factors that may enhance shear coupling and strain magnification effects include the elastomer
matrix, fiber composition and fiber architecture (e.g., ply orientation and/or fabric texture). The
present invention demonstrates many embodiments that demonstrate exceptional combinations
of dynamic moduli and loss factors.
[0071] It should be noted that these descriptions of "Poisson' s ratio", "tensors"
"strain magnification effects" and shear coupling effects may serve as useful design tools, and
are not intended to represent a precise or rigorous examination of the physics underlying the
damping process. These descriptions in no way limit the scope of the invention.
[0072] In certain embodiments, the type and orientation of the fiber reinforcement
may be designed to create a negative coefficient of linear expansion in one direction (e.g., along
the principle axis of a biaxial carbon fiber reinforced elastomer). This negative coefficient of
linear expansion property is unusual because most engineering materials possess a positive
coefficient of linear expansion.
[0073] In certain embodiments, a large Poisson' s ratio (e.g., greater than about 0.5)
may be exploited to generate negative expansion in one direction because the natural thermal
expansion in one direction creates a Poisson-effect contraction in another direction. If the
Poisson' s ratio is high enough, the Poisson' s effect from matrix expansion in one direction may
overcome the natural expansion of the matrix to induce negative expansion in the stiff direction
of the composite. This may be visualized as a "scissor-type motion" of the fibers.
[0074] In a further embodiment, the fiber selection may induce a more extreme
negative coefficient of linear expansion. For example, carbon fibers have a slightly negative
coefficient of linear expansion (e.g., negative-1 microstrain per °F). The examples show a
carbon fiber reinforced polyurethane elastomer with a +28° fiber orientation had a coefficient of
linear expansion in the principle axis measuring about negative-5 microstrain per °F over a broad
temperature range from 0°F to 100°F, which is several times more negative than typical carbon
fibers.
[0075] In certain embodiments, the elastomer-infiltrated fiber layers may be cured in
block 106 so as to allow handling of the layer. Curing may be performed at a temperature and
time which causes the elastomer to adopt at least a selected viscosity that allows the layer so
cured to be handled without substantially damaging the layers. A selected number of fiberreinforced
elastomer layers 202 may be assembled without limit to form the composite 200, as
illustrated in Figure 2. For example, the number of layers may be selected in order to provide a
composite 200 having a selected thickness and/or weight.
[0076] Optionally, the composite 200 may include a constraining layer 204 applied to
one or more surfaces of the fiber-reinforced elastomer layers 202 in block 106. The constraining
layer 204 may further increase the system loss factor of the composite 200 in a selected direction
(e.g., about the longitudinal direction, En). For example, as illustrated in Figure 2, the
constraining layer 204 may be applied to at least a portion of the exterior surface of the polymer
composite 200. For example, Figures 11 and 12A-12B show how adding an aluminum
constraining layer increases the system loss factor.
[0077] In certain embodiments, the constraining layer 204 may comprise a metal
layer (e.g. foils, sheets, plates, beams, rods, tubes or rebar). One or more of the metal layer
parameters of the constraining layer 204 may be varied with respect to the fiber-reinforced
elastomer layers 202. Example variable parameters include alloys, thickness, length, width,
diameter, shape and form. However, it may be understood that other variable parameters may be
employed without limit. Example alloys include alloys of aluminum, steel, stainless steel,
nickel, copper, titanium, magnesium and bronze. However, it may be understood that other
materials and forms may be employed without limit.
[0078] In certain embodiments, the constraining layer material may include
alternative materials, such as polymers, ceramics, concrete, rebar, glass, plastics, thermoplastics,
thermosets and fiber-reinforced composite materials (e.g., fiberglass/epoxy composites,
fiberglass/polyester composites, carbon/polymer composites, and carbon/epoxy composites). In
certain embodiments, the structure itself (i.e., the structure to be treated to improve damping)
may function as a constraining layer.
[0079] In certain embodiments, the constraining layer 204 may comprise a fiberreinforced
polymer layer. One or more of the fiber material, fiber orientation, and relative
concentration of the constraining layer 204 may be varied with respect to the fiber-reinforced
elastomer layers 202.
[0080] In certain embodiments, the polymer matrix of the constraining layer 204 may
comprise epoxies, polyesters, vinyl esters, cyanate esters, polyurethanes, and other engineering
polymers known in the art.
[0081] In further embodiments, the fibers of the constraining layer 204 may include,
but are not limited to, substantially continuous fibers, such as fiber fabrics and fiber braids and
discontinuous fibers, such as fiber mats. The fibers may be carbon, graphite, glass, E-glass, Sglass,
nomex, aramid, polymers, thermoplastics, polyethylenes, ultra high molecular weight
polyethylenes (UHMW), polypropylenes, polyesters, poly-p-phenylene-benzobisoxazole (PBO),
boron, polyamide, Innegra®, Kevlar®, nylons, ceramics, metals, fiberglass or composites.
However, it may be understood that other fibers may be employed without limit. The
concentration of the constraining fibers may be selected within the range between about 20 vol.
% to about 70 vol. % on the basis of the total volume of the composite layer to be formed.
[0082] In further embodiments, the orientation of the fibers in the constraining layers
may also be varied. For example, the constraining fibers may adopt a ± orientation with respect
to a selected direction (e.g., the principle axis of the composite). For example, the orientation of
the constraining fiber may vary within the range between about 0° to about 90°, 10° to about 35°,
10° to about 35°, etc. Examples of fiber orientations may include, but are not limited to, about
0 90°, quasi-isotropic (e.g., [0/+45/90/-45]s), all 20°, about +15°, about +20°, about +25°, about
+30, about +45°, etc.
[0083] In certain embodiments, the constraining layer 204 may be cured to allow
handling (e.g., partially cured). Curing may be performed at a temperature and time which
causes the constraining polymer composite to adopt a selected viscosity that allows the
constraining layer 204 to be handled without substantially damaging the layer 204. In further
embodiments, the fiber reinforced elastomer layers 202 and the constraining layers 204 may be
affixed to one another. In one embodiment, the layers 202 and 204 may be assembled in a
partially cured state and fully cured together. In another embodiment, one or more partially
cured constraining layers 204 may be applied to cured layers of the fiber reinforced elastomer
202 and then fully cured. In additional embodiments, the fiber reinforced elastomer layers 202
and constraining layer 204 may be attached to one another using an adhesive, as is known in the
art. In certain embodiments, the adhesive may be a curable adhesive.
[0084] In certain embodiments the method of fabricating the viscoelastic composite
may include vacuum infusion, resin transfer molding (RTM), vacuum assisted resin transfer
molding (VARTM), resin film infusion (RFI), compression molding, pultrusion, extrusion,
prepreg layup, adhesive bonding, co-molding and/or autoclave curing. However, it may be
understood that other manufacturing methods may be employed without limit.
[0085] In certain embodiments, the viscoelastic composite material may be joined to
a structure, constraining layers, and/or other viscoelastic composite layers via joining methods
such as bonding, mechanical fastening (e.g., riveting or bolting), mechanical locking, pressfitting
and/or other methods as is known in the art. However, it may be understood that other
joining methods may be employed without limit.
[0086] In further embodiments, the viscoelastic composite material may be joined
directly onto at least a portion a surface of a structure. For example, in certain embodiments, a
viscoelastic composite material may be molded or bonded to an interior surface of a structure
(e.g., a hull plate) to damp vibrations. In further embodiments, a viscoelastic composite material
may be molded or bonded within a bonded assembly to form an integrated structure (e.g., a rotor
blade) to damp vibrations. In further embodiments, a viscoelastic composite material may be
molded or bonded within a cavity of a structure (e.g., a rotor blade) to damp vibrations.
However, it may be understood that other methods may be employed without limit.
[0087] In further embodiment, aforementioned methods (e.g., VARTM and/or
autoclave curing) may be applied in situ to mold the viscoelastic composite directly into the
structure itself. However, it may be understood that other bonding or manufacturing methods
may be employed without limit.
[0088] In certain embodiments, the viscoelastic composite may be incorporated as an
integral part of a structure that damps vibrations in rotating component applications such as
rotors (e.g., helicopter rotors), rotor blades (e.g., marine rotor blades), impellers (e.g., pump
impellers), drive shafts, fans, motors, engines, propulsion systems, mechanical linkages, torque
transmission interfaces, flexible couplings and rotor heads. In further embodiments, the
viscoelastic composite may be applied to at least a portion of the surfaces (e.g., between the
leading edge and the rotor structure) of leading edges and/or trailing edges of rotor blades to
damp vibrations within these edges and within the rotor itself. However, it may be understood
that other applications may be treated without limit. It is readily understood that damping
vibrations at the source (e.g., the rotating component) may reduce vibrations and noise
throughout the surrounding structure.
[0089] In certain embodiments, the viscoelastic composite may be incorporated into a
structure to create an integrally damped structure for vibrating structure applications such as
nacelles, aerostructures, hulls, fairings (e.g. marine fairings), panels (e.g. rotorcraft panels),
housings (e.g., fan housings), foundations (e.g. motor foundations), rotorcraft structures, aircraft
structures, aircraft panels, vehicle suspensions (e.g. integrated leaf springs), mechanical linkages,
struts, control surfaces, tail cones, empennages, stabilizers, acoustic structures, shaving products,
landscaping equipment, tools, and/or computer housings. However, it may be understood that
other applications may be treated without limit.
[0090] In certain embodiments, the viscoelastic composite may be incorporated
within a leaf spring structure to make a lightweight integrally damped low-profile suspension
system. In certain applications (e.g., race cars), the integrally damped composite may eliminate
the need for relatively heavy shock absorbers.
[0091] In certain embodiments, the flexibility of the fiber-reinforced elastomer
composite may be useful as a compliant structure in applications such as deformable structures,
flexible couplings, rotor heads, mechanical joints, mechanical linkages, flexible linkages,
alignment-compensation joints, torque transmission interfaces, rotor blade attachments, rotor
attachments, drive shafts, shaft linkages, and axle joints. However, it may be understood that the
present invention may be incorporated into compliant structures without limit.
[0092] In certain embodiments, the negative coefficient of linear thermal expansion
of the fiber-reinforced elastomer composite may be used to reduce deformation of a system in
response to thermal stimulus. In further embodiments, this composite material may be bonded to
surface to negate the thermal expansion of certain structures. In further embodiments, this
composite material may be applied in a symmetric configuration (e.g. a sandwich structure) to
create a thermally stable structure. However, it may be understood that the present invention
may be incorporated into thermally stable structures without limit.
[0093] In certain embodiments, the negative coefficient of linear thermal expansion
of the fiber-reinforced elastomer composite may be used to increase deformation of a system in
response to thermal stimulus. In further embodiments, this composite material may be bonded to
a surface to of certain structures to impart motion (e.g., actuation) in response to thermal
stimulus. In further embodiments, this composite material may be applied in a non-symmetric
configuration (e.g. onto one side of an aluminum beam) to impart a bending moment to improve
actuation effectiveness. However, it may be understood that the present invention may be
incorporated into thermally-actuated structures without limit.
[0094] In certain embodiments, the present invention may be applied within a wide
variety of applications, such as vehicles, land vehicles, aircraft, rotorcraft, marine vessels,
rockets, space vehicles, offshore platforms, civil engineering structures, buildings, bridges,
towers, power plants, engines, motors, pumps, computers, fans, propulsion, HVAC systems,
tools, jackhammers, landscaping equipment, shaving equipment, measurement equipment, test
equipment and consumer products. However, it may be understood that the present invention
may be incorporated into other applications without limit.
Examples
[0095] The vibration damping performance (e.g., loss factor) of embodiments of the
fiber-reinforced polymer composites discussed above will now be illustrated. The damping
behavior of the composites was evaluated using modal testing and dynamic mechanical analysis
to evaluate dynamic modulus and loss factor as a function of frequency. In the tests, beams of
the fiber-reinforced elastomer composites, both alone and attached to vibrating structures were
examined. Embodiments of the fiber-reinforced elastomer composites were further examined
with and without constraining layers. The performance of the composites was compared with
control beams in order to illustrate damping improvements provided by the composites. Finite
element simulations were further performed to examine the loss factor of systems damped by the
composite as a function of modulus.
[0096] As discussed in detail below, embodiments of the fiber-reinforced elastomers
were found to exhibit a dynamic shear modulus that was between about 3.5 to about 45 times
greater than a comparable state-of-art damping tile, as a function of the frequency of vibration.
Furthermore, the loss factor of the carbon fiber-reinforced polymer was found to only decrease
by about 35 to 50% as compared to the damping tile over the same frequency range. As a result
of this favorable trade-off between modulus and loss factor, attaching an embodiment of the
composite to a vibrating beam provides a system loss factor that is approximately three to four
times that of a comparable vibrating beam with the control damping tile materials. It may be
understood that these examples are discussed for illustrative purposes and should not be
construed to limit the scope of the disclosed embodiments.
Example 1 - Effects of fiber reinforcement on composite damping performance
[0097] The influence of the fiber configuration of the damping performance of the
fiber-reinforced elastomer composites was investigated. Figure 3 illustrates a plot of storage
modulus and loss factor as a function of frequency for embodiments of the fiber-reinforced
elastomer composites having carbon-fiber mats, 0°/90° Innegra fibers, +65° carbon-fiber plain
weave, +45° carbon-fiber plain weave, and +28° carbon-fiber braid (with respect to the long axis
of the composite), where the elastomer was polyurethane. A control elastomer, without
reinforcement, was also tested for comparison.
[0098] The samples each possessed dimensions of about 3 mm thickness, about 10
mm width, and about 10 mm length. The fibers of the composites were oriented with respect to
the long axis of the composite. The fiber volume fraction was about 50%.
[0099] As illustrated in Figure 3, each of the composites exhibited a maximum loss
factor greater than about 0.5. Furthermore, each of the composites exhibited a loss factor less
than that of the elastomer and a storage modulus greater than that of the elastomer. This result
indicates that the presence of the fibers within the composite increases the composite stiffness
but decreases the loss factor.
[0100] These results indicate that improvements in the storage modulus of the
elastomer may be achieved by incorporating reinforcing fibers into the elastomer. Notably,
though, the improvement in stiffness results in a drop in the peak loss factor of the composite.
Therefore, the choice of reinforcement may be determined by the lowest stiffness that is
acceptable in order to provide the highest loss factor for that stiffness.
Example 2 - Dynamic mechanical analysis (DMA) of composite and damping tile
[0101] Samples having dimensions of about 3.0 mm thickness, 10.0 mm width, and
9.5 mm length were examined using a Dynamic Mechanical Analyzer (DMA). A neat
polyurethane elastomer (Figure 4A) with no reinforcements was examined. A state-of-the-art
damping tile (Figure 4B) was examined. A composite with carbon fiber having a plain-weave
configuration and a fiber orientation about +65° in a volume fraction of about 50% (Figure 4C)
was examined. A composite with carbon fiber having a plain-weave configuration and a fiber
orientation about +45° in a volume fraction of about 50% (Figure 4D) was examined. A
composite with carbon fiber having a braid configuration and a fiber orientation about +45° in a
volume fraction of about 50% (Figure 4E) was examined. A composite with carbon fiber having
a plain-weave configuration and a fiber orientation about +25° in a volume fraction of about 50%
(Figure 4F) was examined. A composite with carbon fiber having a braid configuration and a
fiber orientation about +20° in a volume fraction of about 50% (Figure 4G) was examined.
[0102] Samples were tested in a DMA using a single cantilever clamp. A sinusoidal
strain was applied at 0.2 and the resulting real and imaginary impedance was measured. The
stiffness was converted to a modulus using a K-value calculated from the energy theory. The
measurements were converted to G' using a Poisson's ratio calculated from micromechanics
engineering simulations. The G' and loss factor values were shifted using time-temperature
superposition to evaluate the measurement against frequency. For example, the maximum loss
factor is clearly identifiable in Figures 4A through 4G, in which G' and loss factor are plotted
versus frequency at a given reference temperature. In this case, the maximum shear loss factor
aligns with a "center frequency". An alternative DMA plot may plot G' and loss factor versus
temperature at a given reference frequency, and the maximum shear loss factor will align with
the "glass transition temperature". In each case, G' at the maximum loss factor is quantifiable.
[0103] The real portion of the dynamic shear modulus (G') as a function of frequency
for each of the tested beams is illustrated in Figures 4A-4G. It may be observed that, over the
frequency range tested, each of the test samples exhibited a center frequency value at which both
the dynamic shear modulus and the loss factor significantly increased. In the case of the
dynamic shear modulus, the modulus continued to increase as the frequency increased beyond
the center frequency. In contrast, the loss factor decreased with increasing frequency beyond the
center frequency.
[0104] The dynamic shear modulus was observed to be significantly larger in the
composite than in the damping tile. For example, at about 1 Hz, the G' for the softest composite
(carbon fiber having a plain-weave configuration and a fiber orientation about ±65°), was about
5 times greater than the damping tile (e.g., about 2.91 x 10 psi as compared to about 5.65 x
10 psi). At the maximum loss factor, the G' for the softest composite was about 3.5 times
greater than the damping tile (e.g., about 1.22 x 104 psi as compared to about 3.56 x 103 psi). In
contrast, the maximum loss factor of the softest composite was about 35% lower than the
damping tile (e.g., about 0.72 as compared to about 1.1). In another example, at about 1 Hz, the
G' for the stiffest composite (carbon fiber having a braid configuration and a fiber orientation
about +20°) was about 45.5 times greater than the damping tile (e.g., about 2.57 x 104 psi as
compared to about 5.65 x 10 psi). At the maximum loss factor, the G' for the stiffest composite
was about 19.8 times greater than the damping tile (e.g., about 7.07 x 104 psi as compared to
about 3.56 x 10 psi). In contrast, the maximum loss factor of the stiffest composite was about
52% lower than the damping tile (e.g., about 0.53 as compared to about 1.1).
Example 3 - Modal testing and finite element analysis validation of dynamic mechanical
analyzer evaluation
[0105] Modal testing of the composite attached to aluminum beams was evaluated at
frequencies from about 100 Hz to about 3000 Hz. One composite included 11 plies (about 0.1"
total thickness) of carbon fiber laminates, each having fibers oriented at about + 25° relative to
the long axis of the composite. The viscoelastic composite was bonded between two aluminum
beams having dimensions of about 17.75" x 2.75" x 0.25" to provide shear deformation. To
extend the measurement range, a second sandwich beam was constructed wherein the
viscoelastic composite was bonded between two aluminum beams having dimensions about
3.75" x 2.75" x 0.25". The temperature was maintained at about 23°.
[0106] The modal testing was evaluated using finite element analysis to calculate the
resulting shear modulus G' and loss factor. Finite element analysis was completed for both G13
and G23. The G13 calculations are equivalent to the DMA testing of the composite with fibers
oriented at about + 25° (Figure 7). The G23 calculations are equivalent to the DMA testing of the
composite with fibers oriented at about + 65° (Figure 8).
[0107] The Gi3 calculation validates the DMA results for the composite with fibers
oriented at + 25°. The G13 shear modulus at the maximum loss factor calculated from the modal
testing and finite element analysis is about 28% lower than the DMA results (e.g., about 2.50 x
104 psi as compared to about 3.46 x 104 psi). The modal testing and finite element analysis
maximum G13 loss factor is about 4% lower than the DMA results (e.g., about 0.64 as compared
to about 0.67). The G23 calculation validates the DMA results for the composite with fibers
oriented at + 65°. The G23 shear modulus at the maximum loss factor calculated from the modal
testing and finite element analysis is about 7% greater than the DMA results (e.g., about 1.30 x
104 psi as compared to about 1.22 x 104 psi). The modal testing and finite element analysis
maximum G23 loss factor is about 4% greater than the DMA results (e.g., about 0.75 as
compared to about 0.72).
Example 4 - Modal testing of composite and control beams
[0108] Modal testing of composite beams and the control beams attached to
aluminum beams is illustrated in Figure 5. The composite included 11 plies (about 0.1" total
thickness) of carbon fiber laminates, each having fibers oriented at about + 26° relative to the
long axis of the composite. The composite was bonded to one side of an aluminum beam having
dimensions of about 40" x 4" x 0.25" to provide an extensional damping configuration. The
temperature was maintained at about 23°F. The fiber was a carbon fiber having a plain-weave
configuration in a volume fraction of about 50%.
[0109] Control beams were prepared for two thermoplastic elastomers and uncoated
aluminum. Thermoplastic elastomer state of the art damping tile sheets having an equivalent
thickness of 0.1" were bonded on one side of an aluminum beam having dimensions of 40" x 4"
x 0.25" (extensional damping configuration). The control beams further included uncoated
aluminum beams having dimensions of 40" x 4" x 0.25".
[0110] Figure 5 plots the system loss factor (damping material + aluminum beam) as
a function of the natural frequency. As illustrated in Figure 5, the polymer composite beam of
the present disclosure exhibits a loss factor that is about 3 to 4 times greater than that of the TPE
damping tile control beams and about 15 times greater than that of the uncoated aluminum beam.
Furthermore, this result held approximately constant over the range of frequencies examined,
about 10 to about 1100 Hz.
[0111] From these results, it may be concluded that embodiments of the composite
system provide significantly improved damping performance over conventional elastomers. For
example, by employing a composite having a maximum Young's loss factor greater than 0.15
and a real portion of the dynamic Young's modulus value E' greater than about 2 x 105 psi
located at the frequency-of-maximum-Young's-loss-factor, the aluminum beam system loss
factor with the composite was significantly higher than with the control thermoplastic elastomer
damping tiles.
[0112] Furthermore, this increase was relatively constant over a relatively broad
frequency range. This result demonstrates the importance of a relatively high modulus because
the system loss factor remains relatively constant even as we observe an approximate 50% drop
observed in the maximum loss factor when evaluated with a Dynamic Mechanical Analyzer.
Rather, the Young's modulus of the composite increased enough to compensate for this effect. It
is anticipated that similar performance benefits would also be observed in constrained
configurations as well.
Example 5 - Effects of constraining layer on system loss factor
[0113] Finite element studies were further performed to simulate the damping
performance of embodiments of the disclosed composites that included relatively thin
constraining layers interspersed between viscoelastic composite layers, and all these layers
between two 2-inch thick carbon/epoxy constraining layers on the external surfaces. The
simulated damping composites possessed the constitutive properties of a carbon-fiber reinforced
polyurethane having a constraining layer of carbon-fiber reinforced epoxy attached to each face.
The polyurethane composites had a thickness between about 1/16" to about ¼" and the epoxy
composites had a thickness ranging between about 0.03" to about 0.13". The loss factor of the
polyurethane composites was assumed to be about 1 and the loss factor of the epoxy composite
was assumed to be about 0. The simulated structure damped by the composite described above
was assigned the constitutive properties of a carbon-fiber reinforced epoxy having a thickness of
about 2".
[0114] The simulated system loss factor as a function of the Young's modulus of the
damping composite is illustrated in Figure 6. In one aspect, it may be observed that the system
loss factor exhibits a peak in the range between about 1 x 105 psi to about 1.2 x 105 psi for each
of the composites, irrespective of the thickness of the polyurethane and epoxy composites.
Notably, this system loss peak lies within the ranges of composite modulus and loss factor
identified to give significant improvement in the system loss factor.
[0115] The effect of the thickness of the fiber-reinforced elastomer composite on the
system loss factor may be observed by comparison of composites B and C, which each have a
constraining layer of about 0.03 inches and different thickness. It is observed that varying the
thickness of the fiber-reinforced elastomer composite from about 1/16" to about 1/4" does not
significantly influence the system loss factor. However, a modest shift in the modulus at which
the loss factor peak occurs, from about 1.55 x 105 to about 1 x 105 is observed.
[0116] The effect of constraining layer thickness on the system loss factor may be
further observed by comparison of composite A with composites B and C. Composite A has a
constraining layer that is the same thickness as the underlying polyurethane composite. In
contrast, the constraining layers of composites B and C, are relatively thinner than the underlying
polyurethane composite. It is observed that the system loss factor of composite A, about 0.047,
is modestly less than that of composites B and C, about 0.049. This result indicates that the
constraining layer should be relatively thinner than the underlying fiber-reinforced elastomer
composite.
Example 6 - Optimum dynamic shear modulus
[0117] Finite element studies were further performed to simulate the system damping
performance of embodiments of the disclosed composites. The simulated damping composites
possessed the constitutive properties of a carbon-fiber reinforced polyurethane. The loss factor
of the polyurethane composite was assumed to be about 0.5 and the loss factor of the epoxy
composite was assumed to be about 0. The simulated sandwich structure damped by the
viscoelastic composite described above was assigned the constitutive properties of a carbon-fiber
reinforced epoxy skins having a thickness of about 2".
[0118] The simulated system loss factor for selected vibration modes are plotted as a
function of the dynamic shear modulus (e.g., G13) of the polyurethane composite is illustrated in
Figure 10. In one aspect, it may be observed that each vibration mode has a different optimum
dynamic shear modulus. In another aspect, it may be observed that the optimum shear modulus
is greater than 1 x 104 psi for all selected modes (assuming constant material loss factor).
Furthermore, it may be observed that the optimum shear modulus for these modes is beyond the
range of state of the art homogeneous materials. In another aspect, it may be observed that
certain applications demand exceptionally stiff viscoelastic materials to maximize system loss
factor. Notably, these optimum shear modulus values lie within the ranges of viscoelastic
composite shear modulus identified to give significant improvement in the system loss factor.
Example 7- Fiber reinforced laminates with Poisson's ratio above 0.5
[0119] Poisson's ratio studies were further performed to simulate the Poisson's ratio
of the disclosed composite materials that include various fiber orientations, based on the carbonfiber-
reinforced polyurethane composites manufactured for example 2. Figure 4A shows the
dynamic shear modulus of the polyurethane matrix. These properties were input into a
micromechanics simulation to calculate the dynamic Poisson's ratio of various configurations.
Engineering calculations show this embodiment may have a Poisson's ratio greater than 0.5 for
unidirectional fiber orientations ranging from about 1° to about 55° relative to a principle axis
(e.g., the fiber axis).
[0120] Figures 4C through 4G show embodiments that possess a maximum shear
loss factor greater than 0.5 and a real portion of the dynamic shear modulus value G' (e.g., G13)
greater than about 1 x 104 psi at the frequency-of-maximum-shear-loss-factor. Figure 7 shows
how the fiber architecture magnifies the real portion of the dynamic shear modulus by about
2500% relative to the matrix polymer (Figure 4A) with a relatively small (e.g. about 60%)
reduction in maximum loss factor relative to the matrix polymer (Figure 4A). Figure 10
illustrates the importance of a large dynamic shear modulus for certain system damping
applications.
Example 8 - Biaxial fiber reinforced laminates with Poisson's ratio above 0.5
[0121] Poisson's ratio studies were further performed to simulate the Poisson's ratio
of the disclosed composites that included various fiber orientations. Engineering calculations
show carbon fiber reinforced polyurethane elastomer composites have a Poisson's ratio greater
than 0.5 in the principle axis for biaxial fiber orientations (e.g., in the 1-2-plane) ranging from
about +1° to about +55° relative to the principle axis (e.g. the 11-direction). Exceptional
Poisson's ratios above about 2.5 may be achieved for biaxial fiber orientations angles ranging
from about +5° to about +35° relative to the principle axis. A carbon fiber reinforced
polyurethane elastomer with a fiber orientation about +28° relative to the principle axis was
manufactured and the dynamic Poisson's ratio (e.g., when measured at the center frequency)
measured about 3.2 at the frequency of maximum shear loss factor. Furthermore, this orientation
(i.e., +28° fibers in 11-direction) that possesses the high Poisson's ratio has the higher out-ofplane
shear modulus (i.e., G13). In contrast, the 22-direction has a lower Poisson's ratio and a
lower (i.e., about 50% lower) out-of-plane shear modulus (i.e., G23). This example shows how
in-plane fiber architectures can influence out-of-plane properties. These examples demonstrate
embodiments with exceptionally high Poisson's ratios relative to conventional materials.
[0122] These exceptionally high Poisson's ratios may induce a "strain magnification
effect", which may result in a composite with exceptional damping performance (e.g. an
effective combination of dynamic Young's modulus and Young's loss factor). The performances
of certain embodiments are illustrated in Figure 9 and Figure 5, which is described in Example 3.
Example 9 - Fiber reinforced laminate with negative coefficient of linear expansion
[0123] A carbon fiber reinforced polyurethane elastomer with a fiber orientation
about +25° relative to the principle axis was manufactured and tested. The coefficient of linear
expansion in the principle axis measured about negative 5 microstrain per °F over a broad
temperature range from 0°F to 100°F, which is more negative than conventional materials. This
example demonstrates unusual thermal expansion relative to conventional materials.
[0124] The terms "approximately," "about," and "substantially" as used herein
represent an amount close to the stated amount that still performs a desired function or achieves
a desired result. For example, the terms "approximately," "about," and "substantially" may refer
to an amount that is within less than 10% of, within less than 5% of, within less than 1% of,
within less than 0.1% of, and within less than 0.01% of the stated amount. Furthermore, ranges
stated in terms of "about x to y" may be understood to include ranges of "about x to about y."
[0125] The term "room temperature" as used herein has its ordinary meaning as
known to those skilled in the art and may include temperatures within the range of about 16°C
(60°F) to about 32°C (90°F).
[0126] The term "elastomer" as used herein has its ordinary meaning as known to
those skilled in the art and embodiments may include, but are not limited to, polyurethanes,
polyureas, rubbers, thermoplastic elastomers, plasticized polymers, plasticized epoxies,
elastomeric epoxies, silicones, polyvinyl chlorides, and combinations thereof. Elastomers may be
considered as polymers existing above their glass transition temperature.
[0127] The term "fiber" as used herein has its ordinary meaning as known to those
skilled in the art and may include one or more fibrous materials adapted for the reinforcement of
composites. Fibers may take the form of whiskers, short fibers, continuous fibers, filaments,
tows, bundles, sheets, plies, and combinations thereof. Fibers may further include any of wires,
cables, rebar and rods, which fulfill the role of reinforcing fibers on a large scale. Continuous
fibers may further adopt any of random, unidirectional, multi-dimensional (e.g., two-or threedimensional),
non-woven, woven, knitted fabrics, stitched, wound, and braided configurations,
as well as swirl mat, felt mat, and chopped mat structures. Woven fiber structures may comprise
a plurality of woven tows having less than about 1,000 filaments, less than about 3,000
filaments, less than about 6,000 filaments, less than about 12,000 filaments, less than about
24,000 filaments, less than about 48,000 filaments, less than about 56,000 filaments, and less
than about 125,000 filaments. In further embodiments, the tows may be held in position by
cross-tow stitches, weft-insertion knitting stitches, or a small amount of resin, such as a
thermoplastic resin.
[0128] The composition of the fibers may be varied, as necessary. Embodiments of
the fiber composition may include, but are not limited to polymers, metals, and ceramics. For
example, carbon, graphite, glass, E-glass, S-glass, aramid, quartz, polyethylene, polyester,
fiberglass, thermoset, thermoplastic (polyethylene, ultra high molecular weight polyethylene
(UHMW), polypropylene, nylon, etc.), polypropylenes, polyesters, Innegra®, Kevlar®, nylons,
poly-p-phenylene-benzobisoxazole (PBO), boron, polyamide, silicon carbide, silicon nitride,
Astroquartz®, Tyranno®, Nextel®, and Nicalon®, and combinations thereof.
[0129] The term "impregnate" as used herein has its ordinary meaning as known to
those skilled in the art and may include the introduction of a matrix or resin material between or
adjacent to one or more fibers. The matrix or resin may take the form of films, powders, liquids,
and combinations thereof. Impregnation may be facilitated by the application of one or more of
heat, pressure, and solvents.
[0130] The terms "cure" and "curing" as used herein have their ordinary meaning as
known to those skilled in the art and may include polymerizing and/or cross-linking processes.
Curing may be performed by processes that include, but are not limited to, heating, exposure to
ultraviolet light, chemical reaction, and exposure to radiation. In certain embodiments, curing
may take place within a polymer matrix or resin. Prior to curing, the matrix or resin may further
comprise one or more compounds that are, at about room temperature, liquid, semi-solid,
crystalline solids, and combinations thereof. In further embodiments, the matrix or resin may be
partially cured in order to exhibit a selected stickiness or tack. In certain embodiments,
consolidation and curing may be performed in a single process.
[0131] The term "damping" as used herein has its ordinary meaning as known to
those skilled in the art and may include reduction in the amplitude of resonant vibrations by
conversion of a portion of the mechanical energy of the vibration into thermal energy.
[0132] Although the foregoing description has shown, described, and pointed out the
fundamental novel features of the present teachings, it will be understood that various omissions,
substitutions, changes, and/or additions in the form of the detail of the apparatus as illustrated, as
well as the uses thereof, may be made by those skilled in the art, without departing from the
scope of the present teachings. Consequently, the scope of the present teachings should not be
limited to the foregoing discussion but by a fair reading of the claims which follow.

WHAT IS CLAIMED IS:
1. A fiber-reinforced composite material comprising:
an elastomer; and
a first plurality of fibers positioned within the elastomer, wherein the composite
material possesses a maximum shear loss factor greater than 0.5 and a real portion of the
dynamic shear modulus value G' greater than 1 x 104 psi at the maximum shear loss
factor.
2. The composite material of Claim 1, wherein the composite material possesses a
Poisson's ratio greater than 0.5 in at least one direction.
3. The composite material of Claim 1, wherein the elastomer is selected from the
group consisting of polyurethanes, polyureas, rubbers, thermoplastic elastomers, plasticized
polymers, plasticized epoxies, elastomeric epoxies, silicones, and polyvinyl chlorides.
4. The composite material of Claim 1, wherein the elastomer is a polyurethane.
5. The composite material of Claim 1, wherein the first plurality of fibers is one of
carbon or glass or polymer.
6. The composite material of Claim 1, wherein the first plurality of fibers has an
architecture selected from the group consisting of tows, yarns, filaments, fabrics, woven fabrics,
braided fabrics, multi-axial braided fabrics, unidirectional, three-dimensional, random, knitted
fabrics, whiskers, chopped fibers, nanotubes, submicron fibers, wires, cables, rods, and rebar.
7. The composite material of Claim 1, wherein the first plurality of fibers has an
architecture that is a braided fabric or a woven fabric.
8. The composite material of Claim 1, wherein at least a first portion of the first
plurality of fibers is oriented in a first direction and a second portion of the first plurality of
fibers is oriented in a second direction, and wherein the first and second directions are about 90°
with respect to each other.
9. The composite material of Claim 1, wherein at least a first portion of the first
plurality of fibers is oriented in a first direction and a second portion of the first plurality of
fibers is oriented in a second direction, and wherein the first and second directions are within a
range of about 30° to about 70° with respect to each other.
10. A damping structure, comprising:
the fiber-reinforced composite of Claim 1; and
at least one constraining layer possessing a static Young's modulus at least 1 x
105 psi.
11. The damping structure of Claim 10, wherein the at least one constraining layer is
positioned on at least a portion of an exterior surface of the fiber-reinforced composite.
12. The damping structure of Claim 10, wherein the at least one constraining layer is
formed from a material selected from a group consisting of metals, aluminum alloys, steel alloys,
nickel alloys, polymers, ceramics, concrete, rebar, glass, plastics, thermoplastics, thermosets,
fiber-reinforced composite materials, fiberglass/epoxy composites, fiberglass/polyester
composites, carbon/polymer composites, and carbon/epoxy composites.
13. The damping structure of Claim 10, wherein the fiber-reinforced composite
material is placed between two constraining layers.
14. The damping structure of Claim 10, wherein the at least one constraining layer
comprises:
a matrix; and
a second plurality of fibers positioned within the matrix.
15. The damping structure of Claim 14, wherein the matrix is selected from the group
consisting of polymers, thermosets, thermoplastics, epoxies, polyesters, cyanate esters, vinyl
esters, polyurethanes, polyvinyl chlorides, metals, aluminum alloys, ceramics, and concretes.
16. A fiber-reinforced composite material comprising:
an elastomer; and
a first plurality of fibers positioned within the elastomer, wherein the composite
material possesses a maximum Young's loss factor greater than 0.15 and a real portion of
the dynamic Young's modulus value E' greater than 2 x 105 psi at the maximum Young's
loss factor.
17. The composite material of Claim 16, wherein the composite material possesses a
Poisson's ratio greater than 0.5 in at least one direction.
18. The composite material of Claim 16, wherein the elastomer is selected from the
group consisting of polyurethanes, polyureas, rubbers, thermoplastic elastomers, plasticized
polymers, plasticized epoxies, elastomeric epoxies, silicones, and polyvinyl chlorides.
19. The composite material of Claim 16, wherein the elastomer is a polyurethane.
20. The composite material of Claim 16, wherein the first plurality of fibers is one of
carbon or glass or polymer.
21. The composite material of Claim 16, wherein the first plurality of fibers has an
architecture selected from the group consisting of tows, yarns, filaments, fabrics, woven fabrics,
braided fabrics, multi-axial braided fabrics, unidirectional, three-dimensional, random, knitted
fabrics, whiskers, chopped fibers, nanotubes, submicron fibers, wires, cables, rods, and rebar.
22. The composite material of Claim 6, wherein the first plurality of fibers has an
architecture that is a braided fabric or a woven fabric.
23. The composite material of Claim 16, wherein at least a first portion of the first
plurality of fibers is oriented in a first direction and a second portion of the first plurality of
fibers is oriented in a second direction, and wherein the first and second directions are about 90°
with respect to each other.
24. A damping structure, comprising:
the fiber-reinforced composite material of Claim 16; and
at least one constraining layer possessing a static Young's modulus of at least 1 x
105 psi.
25. The damping structure of Claim 24, wherein the at least one constraining layer is
formed from a material selected from the group consisting of metals, aluminum alloys, steel
alloys, nickel alloys, polymers, ceramics, concrete, rebar, glass, plastics, thermoplastics,
thermosets, fiber-reinforced composite materials, fiberglass/epoxy composites,
fiberglass/polyester composites, carbon/polymer composites, and carbon/epoxy composites.
26. The damping structure of Claim 24, wherein the fiber reinforced composite
material is placed between two constraining layers.
27. The damping structure of Claim 24, wherein the at least one constraining layer
comprises:
a matrix; and
a second plurality of fibers positioned within the matrix.
28. The damping structure of Claim 27, wherein the matrix is selected from the group
consisting of polymers, thermosets, thermoplastics, epoxies, polyesters, cyanate esters, vinyl
esters, polyurethanes, polyvinyl chlorides, metals, aluminum alloys, ceramics, and concretes.
29. A method for fabricating a composite comprising:
infiltrating an elastomer into a plurality of fibers to form a fiber-reinforced
elastomer layer;
stacking a plurality of said fiber-reinforced elastomer layers; and
curing the plurality of fiber-reinforced elastomer layers to form a fiber-reinforced
elastomer composite, wherein the composite material possesses a maximum shear loss
factor greater than 0.5 and a real portion of the dynamic shear modulus value G' greater
than about 1 x 104 psi at the maximum shear loss factor.
30. The method of Claim 29, wherein the composite material possesses a Poisson's
ratio greater than 0.5 in at least one direction.