Abstract:
A method of fabricating an isolator mount is presented. The method includes plasticating a polymeric material, forcing the polymeric material through a shaping die, forming a continuous extrusion so that shear flow aligns a plurality of energy damping particulates within the polymeric material along a preferred direction, and cutting the continuous extrusion after the forming step. In yet another embodiment, the fabrication process includes extruding a polymeric material having a plurality of energy damping inclusions therein, inducing a magnetic flux density field so as to align the energy damping inclusions in a preferred direction before cooling of the polymeric material, and inducing a magnetic bias across the polymeric material so as to form an RL equivalent circuit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a divisional application of co-pending application Ser. No. 10/188,446, filed on Jul. 2, 2002, and claims benefit of U.S. Provisional Application No. 60/302,579, filed on Jul. 2, 2001. The subject matters of the prior applications are incorporated in their entirety herein by reference thereto. 
     
    
     FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     None.  
       BACKGROUND OF THE INVENTION  
       [0003]     1. Field of the Invention  
         [0004]     The present invention generally relates to a method of fabricating an isolator mount having alloys and other inclusions oriented in a preferred direction within a plastic, composite, or metal matrix.  
         [0005]     2. Related Arts  
         [0006]     Naval ships employ a wide variety of isolator mounts to impede acoustic transmissions and to protect sensitive equipment from shocks and vibrations. Presently, isolator mounts are specifically designed for a limited range of shocks and vibrations. As such, a variety of mounts are required to satisfy a wide range of mechanical load conditions.  
         [0007]     Energy dissipation mechanisms employed within presently known devices quickly degrade with use thereby requiring frequent replacement. For example, passive mounts comprised of rubber and metal rapidly lose their damping capacity. Consequently, isolator mounts are often used well beyond their effective lifetime thereby compromising the integrity and performance of shipboard systems.  
         [0008]     Active mounts with integrated electronics increase the range of shocks and vibrations effectively isolated. However, active mounts are generally less durable and sensitive to environmental conditions.  
         [0009]     Low-frequency shocks, typically from 3 to 10 Hz, and vibrations, typically from 5 to 30 Hz, exclude many passive and active damping devices. For example, the effectiveness of viscoelastic damping increases with frequency and is therefore of limited utility at low frequencies. Passive damping by piezoelectric or electrostrictive devices, also referred to as direct effect damping devices, is not particularly useful at low bandwidths since damping is dependent upon hysteresis loops and elastic mechanical-to-electrical energy coupling. Coupling coefficients are generally poor and total loss is insignificant at the lower dynamic range.  
         [0010]     Piezopolymers are better direct coupling materials than piezoceramics and electrostrictors, therefore applicable to piezo-passive damping devices. In a passive-mode device, a generalized matched impedance circuit is coupled to the active ferroelectric materials so as to transfer elastic energy to heat. In a semi-active mode, the circuit is variably tunable. However, strength and stiffness characteristics preclude the use of ferroelectric polymers, examples including PVDF and urethane, as active devices.  
         [0011]     Therefore, what is required is a method of fabricating an isolator mount capable of coupling elastic energy from shocks and vibrations into one or more other energies within a damping element.  
       SUMMARY OF THE INVENTION  
       [0012]     An object of the present invention is to provide a method of fabricating an isolator mount capable of coupling elastic energy associated with shocks and vibrations into one or more other energies within a damping material.  
         [0013]     An isolator mount is composed of one or more materials, often referred to as lossy, capable of absorbing and dissipating energy. For example, lossy materials may passively damp shocks and vibrations when composed of magneto-mechanical alloys so as to couple mechanical energy to magnetism. Alloys may be combined with highly durable fiber-reinforced elastomeric materials to further enhance the isolation of shocks and vibrations. Isolators may be composed of rare earth coatings, laminated materials, and ferrous treated rare earth particulates.  
         [0014]     Isolator mounts are fabricated via a non-conventional method of extrusion enabling the netshape production of thermoplastic damping elements, one example being Hytrel® sold by the DuPont Company, about an energy dissipating material. Damping alloys are introduced as particulates or fibers during the pre-mixing stage. Although the present invention may be similarly applied to injection molding techniques, extrusion allows ferromagnetoelastic materials to be aligned into virtual chains to increase damping effectiveness.  
         [0015]     The alignment of particulates for pseudo-fiber construction is described in “ Magnetostriction, Elastic Moduli and Coupling Factors of Composite Terfenol - D Composites ,” Journal of Applied Physics, 1999. In the present invention, an FMSA, MSM, cobalt ferrite, or Terfenol alloy is mixed with a low-viscosity resin. After sieving, the mixture is degassed and prepared for particle alignment. Particles are aligned along the magnetic flux lines produced by a large magnetic field. The assembly is cured after particulates are aligned during thermoset and/or extrusion.  
         [0016]     In preferred embodiments, the present method includes extrusion between two large magnetic field devices, one example being permanent magnets. The magnetic field and direction of extension aligns the embedded magnetic particulates into so-called pseudo chains oriented in a preferred direction to enhance damping. In some alloys, it may be desirable to add ferrite so as to facilitate the alignment process.  
         [0017]     The present invention exploits the passive capability of rare earths and metallic alloy composite materials. Terbium, Dysprosium, and ferromagnetic particulates become increasingly magnetic with decreasing temperature. At a sufficiently low temperature, pseudo-fiber alignment of rare earth particulates within a resin is achieved in the absence of any additional ferromagnetic particle fraction. However, this behavior eliminates many useful thermoplastics that do not set at temperatures required for alignment. More exotic matrix materials may be required, adding to the cost of manufacture. Thus, the isolator may include randomly distributed Terfenol, FMSA, or other damping alloys.  
         [0018]     The preferred manufacture process may also include super-elastic materials, either ribbon or short fiber inclusions, within a resin mix. Size, volume fraction, and preloading of alloy, one example being NiTi, is related to the pre-stress exerted by the matrix as it shrinks about the particulates, just as with rare earth or rare earth-ferromagnetic inclusions. Greater pre-stress will generally improve both passive magneto-elastic damping by Terfenol and rare earth inclusions and shock isolation by super-elastic inclusions. The relative softness of such materials allows for an embodiment having NiTi, in a suitably chosen super-elastic phase, laminated onto a mount. The laminate is further coated or laminated with a durable material, one example being urethane.  
         [0019]     Extrusion is a low-cost manufacturing method for C-mounts. The described method utilizes an extruder to plasticate a polymeric material so as to have an aligned molecular orientation as it exits the shaping die. A traveling saw may be used to cut the continuous extrusion to provide C-mounts of defined length.  
         [0020]     The present method facilitates the manufacture of durable isolator mounts for a wide variety of shock and vibration applications. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which:  
         [0022]      FIG. 1  is a perspective view of an exemplary C-mount.  
         [0023]      FIG. 2  is a perspective view of an alternate embodiment of the C-mount.  
         [0024]      FIG. 3  is a graph showing damping capacity versus stress amplitude for several magneto-mechanical alloys.  
         [0025]      FIG. 4  is a perspective view of an exemplary extrusion machine for the oriented manufacture of isolator mounts.  
         [0026]      FIG. 5  is a schematic diagram showing an extruded C-mount with machine-directional (MD) molecular orientation.  
         [0027]      FIG. 6  is a schematic diagram showing an extruded C-mount with cross-directional (CD) molecular orientation. 
     
    
     REFERENCE NUMERALS  
       [0000]    
       
           1  First layer  
           2  Second layer  
           3  Third layer  
           4  First insert  
           5  Second insert  
           6  Fastener  
           7  Composite shell  
           8  Rigid element  
           9  Damping element  
           10  C-mount isolator  
           11   a - 11   b  Alignment device  
           12  Machine  
           13  Composite  
           14  Damping material  
           15   a - 15   b  Alignment device  
           16  Extrusion direction  
           17  Extruder die  
           18  Extruded C-mount  
           19  Molecular orientation  
       
     
       DESCRIPTION OF THE INVENTION  
       [0047]     Referring now to  FIG. 1 , an exemplary laminate embodiment of a C-mount isolator  10  is shown having a first insert  4  and second insert  5  embedded within a third layer  3  and thereafter sandwiched between a first layer  1  and a second layer  2 . While c-shaped mounts are shown and described, other shapes are possible.  
         [0048]     First layer  1  and second layer  2  provide structural rigidity during normal loading conditions. First layer  1  and second layer  2  are composed of an energy absorbing material capable of withstanding repeated deflections and large strains. Preferred materials include spring steel and thermoplastics.  
         [0049]     The third layer  3  is composed of a composite, polymer, or elastomer, preferably a fiber-reinforced elastomer. The primary function of the third layer  3  is to provide sufficient stiffness so as to transfer strain into the first insert  4  and second insert  5  while ensuring a level of elastomeric damping at higher frequencies.  
         [0050]     First insert  4  and second insert  5  are composed of various materials. For example, inserts may be composed of either a magnetostrictive alloy or a magnetostrictive composite composed of Terfenol, cobalt ferrite, FMSA, MSM, or Metglas. It is likewise possible for the first insert  4  and second insert  5  to be composed of a magneto-memory material, preferably constrained within a third layer  3  composed of a short-fiber, reinforced elastomer.  
         [0051]     First insert  4  and second insert  5  may also be composed of different damping alloys. For example, the first insert  4  may be a magneto-mechanical alloy and the second insert  5  a shape memory alloy both embedded within a third layer  3  composed of a fiber-reinforced elastomer. First layer  1 , second layer  2 , and third layer  3  may be molded to shape and machined, via techniques understood in the art, so as to enable attachment at either end via fasteners  6 . Preferably, fasteners  6  should allow for the passage of a bolt to secure the C-mount isolator  10  between a mounting surface and a shipboard component. Thickness and relative modulus of the laminate materials are design dependent and chosen to maximize the coupling of elastic energy within shocks and vibrations into heat and magnetic energies within the damping materials. The invention may employ a variety of inserts integrated within the C-mount isolator  10 , either internally, as shown in  FIG. 1 , or externally, as add-on laminates.  
         [0052]     As is understood in the art, magneto-mechanical alloys and composites dissipate mechanical energy as magnetic energy, whereas super-elastic alloys and elastomers dissipate energy as heat. Magnetostrictive composites are formed by mixing one or more powdered magnetic materials, examples including but not limited to Terfenol-D, SmPd, SmFe 2 , and CbFe. Application dependent properties are tailored by elastomer type, volume fraction of ferromagnetic powder and insulated magnetic binders, and orientation of magnetostrictive particles. Preferred embodiments are solidified having pseudo-chains therein induced via a magnetic field during extrusion. The heating process may be performed in the presence of a magnetic field with powder ground in an inert environment. Feedstocks are chosen based upon their magnetic and electrical properties to optimize eddy current losses and maximize magnetic hysteresis attributes.  
         [0053]     The damping device may also include an external laminate construction composed of magneto-mechanical alloys, Terfenol, super-elastic alloys, and constrained layer viscoplastic laminates with a loss factor optimized for room temperature.  
         [0054]     Referring now to  FIG. 3 , several Terfenol samples are shown having a damping capacity well above unity, thereby indicating applicability to high-stress applications. In general, magneto-elastic and ferromagnetic materials are more dissipative at bias stress. A third layer  3  composed of fiber-reinforced silicon rubber may be used to pre-stress the alloy and to provide a protective anti-corrosion cover.  
         [0055]     Referring now to  FIG. 2 , the C-mount isolator  10  may be composed of a rigid element  8  onto which is attached damping elements  9 , either magneto-mechanical or super-elastic alloys, thereafter encased within a composite shell  7 . The composite shell  7  is composed of an extruded, cast, or molded fiber-reinforced plastic. The composite shell  7  may be confined between a first layer  1  and a second layer  2 , as represented in  FIG. 1 . Damping elements  9  may include one or more continuous layers or segmented elements along the length of the C-mount isolator  10 . Damping elements  9  may be composed of one or more magneto-mechanical and super-elastic alloys. Fasteners  6  are also provided as described in  FIG. 1 .  
         [0056]     Referring now to  FIG. 4 , one possible orientation of the preferred manufacture process is shown whereby solidification of a ferromagnetic powder into pseudo-chains is enabled by a pair of alignment devices  11   a ,  11   b . The alignment devices  11   a ,  11   b  are aligned and flush mounted at the extrusion exit so as to induce magnetic flux lines in a desired orientation within the first insert  4  and second insert  5  or dampening element  9  during cool down. As the isolator materials travel in the extrusion direction  16 , the alignment devices  11   a ,  11   b  induce the desired solidification alignment. A second set of alignment devices  15   a ,  15   b  may reside within the machine  12  so as to induce additional pre-alignment during the heating stage. Example machines  12  include injection molding and extrusion equipment, both understood within the art.  
         [0057]     The emergent composite  13  may also include one or more pre-aligned damping materials  14 . Damping materials  14  are integrated into the composite  13 , for example a short fiber-reinforced elastomer, during extrusion. The composite  13  is cleaved after exiting the machine  12  to the desired length. Alignment devices  11   a - 11   b  and  15   a - 15   b  may include permanent magnets, magnetic field effect devices, EMP (electromagnetic pulse) devices, and cool magnets.  
         [0058]     Referring again to  FIG. 4 , the fabrication process uses a stationary high magnetic flux density arrangement causing alignment as the material extrudes and before cooling is initiated. Particulates within the damping alloy are aligned at high temperature, since they become less ferromagnetic at lower temperatures. This behavior is advantageous to semi-passive mount isolator designs. The disappearance of magnetic response by the magnetostrictive particulates may allow the introduction of a second set of magnetic particles, one example being AlNiCa, which may be used to “internally” tune the mounts damping parameters. A static magnet or coil may be used to induce the necessary magnetic bias across the mount. The system becomes an RL equivalent circuit as the static value is changed or an external resistance is modified through the dial-up rotation of NdCo bias magnets.  
         [0059]     As the polymer emerges from the die at an exit velocity, it is pulled by take-up equipment through a cooling medium, one example being a water bath. A key process variable is the take-up ratio (TUR) of line velocity to exit velocity. The line velocity, established by the take-up equipment, is generally higher than the die exit velocity.  
         [0060]     The main challenge to extrusion manufacture lies in the nature of molecular alignment during extrusion. Due to the parabolic nature of the velocity profile, there is a high tendency for alignment in the machine direction. That is, for a typical L-to-D ratio of 10, the alignment of molecules or fiber whiskers is in the direction of the flow field at the outlet.  
         [0061]     The orientation of molecules within the polymer is an important characteristic influencing the final mechanical properties of the mount. Referring now to  FIG. 5 , an extruder die  17  and extruded C-mount  18  are graphically represented. In conventional extrusion processes, the predominant molecular orientation  19  is in the direction along which the polymer emerges from the extruder die  17 , also called the machine direction (MD). As such, the extruded C-mount  18  is stronger along its MD axis than its cross direction (CD) axis.  
         [0062]     Two mechanisms contribute to MD orientation. Shear stresses cause the chain-like molecules to orient along the flow direction, as the polymer flows through the die. A TUR greater than one stretches the extruded C-mount  18  to favor MD orientation that is frozen or fixed as the polymer solidifies.  
         [0063]     When a product has a predominant direction of orientation, it is said to have anisotropic properties. In terms of tensile strength, anisotropy results when a product is stronger along a first axis and weaker along a second axis perpendicular to the first axis. It is advantageous to extrude mount material in a manner that favors CD orientation and tensile strength.  
         [0064]     Referring now to  FIG. 6 , an extruded C-mount  18  is shown with molecular orientation  19  along the CD axis. CD orientation may be achieved by a shear flow that aligns molecules in the CD and a TUR that minimizes MD orientation.  
         [0065]     Magnetic particulates, one example being commercial grade AlNiCo, are introduced directly into the composite  13  at a Curie temperature below the magnetization temperature of the ferromagnetic particulates to create an internal, and moreover tunable, RL equivalent impedance.  
         [0066]     Preferred embodiments of the C-mount devices are composed of MSM alloys wherein the magnetic field moves microscopic regions of the material, referred to twins, thereby creating a netshape change to the material. This mechanism enables more complicated shape changes than conventional linear strain. FMSA powder/polymer micro-composites are made with a layer of soft magnetic material, one example being Fe-Co, to enhance the response to magnetic fields by exchanging coupling for reduced dc hysteresis, lower eddy-current loss, and lower actuation field. A low actuation field is particularly advantageous to enable quasi-static tuning of mounts for variable load applications.  
         [0067]     Referring again to  FIG. 4 , short fibers may be added to the composite  13  during the manufacturing process to form a polymeric treatment so as to become an integral part of the exterior lamination. The spacing between and/or length of the fibers may be adjusted to optimize the damping characteristics of the element either during or after manufacturing. The resulting device provides increased vibration damping without a constraining layer. Fiber orientation is critical to the effective attenuation of vibration.  
         [0068]     The description above indicates that a great degree of flexibility is offered in terms of the invention. Although the present invention has been described in considerable detail with reference to certain preferred methods, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.