Abstract:
A uniform stiffness laminated composite shell assembly includes a plurality of composite shells. The shells are made of layers of laminates of graphite-epoxy material having fibers oriented in various stacking sequences for performing different functions in the assembly. The shells are concentrically assembled in a desired sequence with some of the adjacent ones of the shells being at least equal to or greater in axial length than others thereof and with some of the shells being adapted to perform a structural load bearing function while others of the shells being adapted to perform a load transfer function. The desired sequence of shells of the composite shell assembly provides enhanced thermal insulating properties and efficient load distributing properties for enabling use of the assembly as a tube suspension in a superconductive magnet.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to tube suspension systems for magnets and, more particularly, is concerned with a laminated composite shell assembly with enhanced thermal insulating and efficient load distributing properties for enabling use in magnet applications such as in superconductive magnets operating at cryogenic temperatures. 
     Superconductive magnets include superconductive coils which generate uniform and high strength magnetic fields, such as used, without limitation, in magnetic resonance imaging (MRI) systems employed in the field of medical diagnostics. The superconductive coils of the magnet typically are enclosed in a cryogenic vessel surrounded by a vacuum enclosure and insulated by a thermal shield interposed therebetween. 
     Various designs of tube suspensions are employed to support the cryogenic vessel of a superconductive coil assembly of the magnet from and in spaced apart relation to both the thermal shield and the vacuum enclosure of the magnet. As one example, the tube suspension can include overlapped fiberglass outer and inner support cylinders, such as disclosed in U.S. Pat. No. 5,530,413 to Minas et al. which is assigned to the same assignee as the present invention. In the Minas et al. tube suspension, the outer support cylinder is located within and generally spaced apart from the vacuum enclosure and positioned outside of and generally spaced apart from the thermal shield. A first end of the outer support cylinder is rigidly connected to the vacuum enclosure while a second end of the outer support cylinder is rigidly connected to the thermal shield. The inner support cylinder is located within and generally spaced apart from the thermal shield and is positioned outside of and generally spaced apart from the superconductive coil assembly. The inner support cylinder has a first end rigidly connected to the thermal shield near the second end of the outer support cylinder and has a second end located longitudinally between the first and second ends of the outer support cylinder and rigidly connected to the superconductive coil assembly. 
     Problems can occur, however, with some designs of tube suspension systems at cryogenic temperatures. For instance, tube suspensions of some current superconductive magnet designs in MRI systems employ metal alloys or glass-epoxy materials. Metal alloys as well as glass-epoxy materials do not provide optimal load distributing and thermal insulating characteristics. Further, metal alloys are heavy and glass-epoxy materials deform as they tend to be compliant. 
     Consequently, a need exists for innovation with respect to tube suspensions for supporting superconductive magnets which will provide a solution to the aforementioned problems. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a uniform stiffness laminated composite shell assembly designed to satisfy the aforementioned need. The uniform stiffness composite shell assembly of the present invention has a sequence of composite shells with different laminates having different stacking sequences and employing graphite-epoxy material that provide enhanced thermal insulating and efficient load distributing properties for enabling use in cryogenic applications. The sequence of composite shells provides the combination of properties needed for accommodating extreme environments, such as one whose temperature ranges between 4° K and 300° K. 
     In an embodiment of the present invention, a uniform stiffness laminated composite shell assembly is provided which can be used in a tube suspension for a superconductive magnet. The laminated composite shell assembly includes a plurality of composite shells with substantially cylindrical configurations and predetermined axial directions. Each shell is made of composite layers of laminates having fibers (e.g., fibers wound into layers) with the fibers being oriented in a plurality of stacking sequences with reference to the axial direction of the shell. The fibers of the shells are made of graphite-epoxy material. The shells are concentrically assembled in a desired sequence with some of the shells being adapted to perform a structural load bearing function while others of the shells are adapted to perform a load transfer function. Some of adjacent ones of the shells in the desired sequence thereof are at least equal to or greater in axial length than others of the adjacent ones of the shells. The stacking sequences are selected to provide a ratio of axial-to-radial stiffness for extensional stiffness of approximately unity ensuring that each of the shells has uniform extension stiffness. The stacking sequences also are selected to provide a ratio of axial-to-radial stiffness for flexural stiffness of each of the shells of approximately unity ensuring that each of the shells has uniform flexural stiffness. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross-sectional view through section A—A, as shown in FIG. 4, of a laminated composite shell assembly of the present invention. 
     FIG. 2 is a chart of various laminates in the composite shells of the assembly of FIG.  1 . 
     FIG. 3 is a chart of the laminate stacking sequence and orientation of fibers of the various laminates in each of the composite shells of the assembly of FIG.  1 . 
     FIG. 4 is a three dimensional view of the vacuum enclosure depicting a section A—A in the “Y” direction. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings and particularly to FIG. 1, there is illustrated a laminated composite shell assembly, generally designated  10 , provided as a tube suspension for a superconductive magnet such as used in MRI systems. As is well-known, a superconductive MRI magnet typically has a longitudinal central axis and includes a superconductive coil assembly M at cryogenic temperature, a thermal shield T enclosing the coil assembly and a vacuum enclosure E at ambient temperature enclosing the thermal shield. The coil assembly, thermal shield and vacuum enclosure are radially spaced from one another with reference to the longitudinal axis and are coaxially aligned with the longitudinal axis. The coil assembly includes a cryogenic vessel V containing a cryogenic fluid  101  and superconductive coils. The vacuum enclosure, thermal shield and cryogenic vessel are in the form of tubular shells of annularly cylindrical configurations. An example of an open-type MRI magnet is found in U.S. Pat. No. 5,563,566 to Laskaris et al. whereas an example of a closed-type MRI magnet is found in aforecited U.S. Pat. No. 5,530,413. Both of these patents are assigned to the assignee of the present invention. 
     The tube suspension implemented by the composite shell assembly  10 , and identified in FIG. 1 by the same reference numeral, is employed between the cryogenic vessel, thermal shield and vacuum enclosure, respectively, so as to allow both radial and axial movement of the cryogenic vessel and thus the coil assembly relative to the thermal shield and vacuum enclosure as the temperature of the cryogenic vessel changes between from ambient and cryogenic temperatures. As seen in exaggerated schematical form in FIG. 1, the composite shell assembly  10  can include a pair of inner and outer support cylinders  12 ,  14  axially overlapped with each other and substantially concentrically arranged with one another. The inner support cylinder  12  is located within and generally spaced apart from the thermal shield T and is positioned outside of and generally spaced apart from the cryogenic vessel, being represented by a dashed line V in FIG.  1 . The inner support cylinder  12  has a first end  16  rigidly connected to one end of the thermal shield T and a second end  18  rigidly connected to the cryogenic vessel V. The outer support cylinder  14  is located within and generally spaced apart from the vacuum enclosure E in FIG.  1  and is positioned outside of and generally spaced apart from the thermal shield T. The outer support cylinder  14  has a first end  30  rigidly connected to the vacuum enclosure E and a second end  20  rigidly connected to an opposite end of the thermal shield T. As apparent in FIG. 1, the second end  20  of the outer support cylinder  14  is axially displaced from and generally overlapped with the first end  16  of the inner support cylinder  12 . 
     Referring to FIGS. 1 and 2, the present invention provides a set of composite shells C, such as ones made up of graphite-epoxy material, such as T300/N5208, that can be assembled to form a desired sequence of composite shells C of various laminates L and thereby provide the inner and outer support cylinders  12 ,  14  of the composite shell assembly  10 . In the illustrated embodiment of FIG.  1  and as per chart  22  of FIG. 2, there are composite shells C 1  to C 6  assembled in a desired sequence. Each shell C 1  to C 6  contains at least one of the laminates L 1  to L 5 . Each of the laminates L 1  to L 5  and thus each of the composite shells C 1  to C 6  formed thereof is substantially a cylinder which includes a slightly tapering cylinder having a small angle of taper of about 1.6 degrees relative to its central axis. It can be noted in FIG. 1 that certain of adjacent ones of the laminates, such as L 4  and L 5  in the desired sequence of the shells are at least equal in axial length to one another. It can also be noted in FIG. 1 that certain of adjacent ones of the laminates, such as L 3 , L 2  and L 1 , in the desired sequence of the shells are successively greater in axial length than the other. 
     Referring to FIG. 3, in a chart  24  there is illustrated the laminate stacking sequences that can be used in the construction of the various laminates L making up the various composite shells C. The laminate stacking sequences identify the orientation of the fibers in different composite layers with respect to the axial direction of the shell C. The off-axis orientations or +/−45° fibers are with reference to the axial direction of the composite shells C. When a composite layer is created by adjacent windings of a matrix-coated fiber, such as an epoxy-coated graphite-fiber thread (or tow), the laminate stacking sequences identify the winding orientation of the fibers. The 90° fibers are oriented along the hoop (or circumferential) direction (orthogonal to the axial direction) of the composite shells C whereas the 0° fibers are oriented along the axial direction of the composite shells C. The subscript “t” means the number of times the fiber layer stacking sequence is repeated. 
     The resultant laminates are designed to efficiently carry load and distribute the load both axially as well as circumferentially along the composite shells C. The fiber layer stacking sequences of the various laminates L comprising the composite shells C ensure that the ratio (NR) of axial to radial (or hoop) stiffness, both for extension as well as for flexure, as set forth in Table I, are close to unity, that is, approximately one. (The A/R values of the extension stiffness are on the left side of the slash mark and the A/R values of the radial stiffness are on the right side of the slash mark in the last column of Table I.) 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                   
                 Extension 
                   
                 Flexural 
                   
                   
               
               
                 Composite 
                 Stiffness 
                   
                 Stiffness 
                   
                 Ratio 
               
             
          
           
               
                 Shell 
                 Axial 
                 Radial 
                 Axial 
                 Radial 
                 A/R 
               
               
                   
               
             
          
           
               
                 C1 
                 25.26 
                 21.51 
                 9.20 
                 8.56 
                 1.17/1.07 
               
               
                 C2 
                 11.62 
                 9.11 
                 0.71 
                 0.62 
                 1.27/1.14 
               
               
                 C3 
                 5.81 
                 4.55 
                 0.06 
                 0.08 
                 1.27/0.75 
               
               
                 C4 
                 19.69 
                 17.19 
                 4.97 
                 5.00 
                 1.14/0.99 
               
               
                 C5 
                 13.64 
                 12.39 
                 1.19 
                 1.87 
                 1.10/0.64 
               
               
                 C6 
                 19.69 
                 17.19 
                 4.97 
                 5.00 
                 1.14/0.99 
               
               
                   
               
             
          
         
       
     
     Extension stiffness is terminology used to identify behavior of the shell which all takes place in-plane, that is, in the curvature of the shell. Flexural stiffness involves the bending behavior of the shell. The two behaviors, extensional and flexural, differ in that extensional behavior is “in-plane deformation” while flexural behavior is “out-of-plane” deformation. The “plane” referred to is the “plane of curvature” of the shell. Any behavior that acts along axes, such as a lengthwise axis and a directional through-the-thickness axis, coincident with the plane of curvature, that is, at any point on the curvature of the shell, involves the extensional behavior and relates to the extensional stiffness of the shell. By contrast any behavior that acts out of the plane of curvature of the shell, such as bending behavior, is flexural behavior and relates to the flexural stiffness of the shell. Each type of behavior (extensional and flexural) has an axial component which acts along the length of the shell and a hoop component which acts along the curvature of the shell. The goal is for the axial-to-radial ratio (A/R) for each type of behavior to equal one which means uniform stiffness behavior. 
     The ratio being close to unity or to one ensures that the effective laminate designs have uniform stiffness for all composite shells C. The effective response measure for stiffness is an estimate of the frequency of the assembled structure. The modal frequency of the first critical mode (without attached masses) is 207.73 Hz. The modal frequency of the first critical mode (with attached masses) is 31.81 Hz. 
     It will be observed that the laminate L 1  has a stacking sequence identical to that of laminate L 5 , and laminate L 2  has a stacking sequence identical to that of laminate L 3 , but the identical laminates are found at different locations in the composite shells C made up of the laminate L 1  to L 5 . Thus, even though the laminates L are identical with respect to their stacking sequences, they are identified as different laminates, have different L numbers, because their functionality is different. For example, with respect to composite shell C 2 , being made up of identical laminates L 2  and L 3 , laminate L 3  provides or forms a structural component performing a structural function while laminate L 2  provides or forms a transitional component performing a load transition function into the structure laminate L 3 . So laminate L 2  is there to provide compatibility between shells C 1  and C 2 . So the functional behavior of certain shells are different even though their laminates are identical. The same is true for laminates L 1  and L 5  although their state of stress and the joint functionality are different. With respect to laminate L 1 , it is behaving as an expansion component for the cryogenic vessel which is at around 4° K. With respect to laminate L 5  (having the same stacking sequence as laminate L 1 ), it is behaving as a continuity component at the one end of the thermal shield T which is at around 300° K. So in one location it (the same laminate with a particular stacking sequence) functions as a continuity component while in the other case it functions as an expansion component. The laminate L 4  contains the adhesive bond between laminates L 5  of shells C 4  and C 5  and the thermal shield T. 
     Composite shells C 1  and C 2  are joined only at certain locations, that being, at laminates L 2  and L 3  which are common between shells C 1  and C 2 . Laminates L 1  and L 2  must satisfy certain conditions of continuity between shells C 1  and C 2 . These conditions of continuity that must be satisfied are displacement compatibility, strain compatibility and load transfer between the two shells. The same holds true between shells C 2  and C 3 , between shells C 3  and C 4 , between shells C 4  and C 5 , and shells C 5  and C 6 . So the provision of a sequence of shells C having various combinations of laminates L is basically a means by which one can ensure these conditions of displacement, strain and load continuities are satisfied between any two adjacent shells. 
     Composite shell C 1  is associated with a flange of the cryogenic vessel V. The behavior of shell C 1  is very different from the behavior of shell C 2 . The behavior of shell C 1  is to act to transfer the load from the cryogenic vessel V to the composite shell assembly  10 . That particular flange, shell C 1 , has to move radially and axially. Allowance for the shell C 1  to expand is also a function of shell C 2  which has to accommodate that expansion. Shell C 2  has to give the appropriate stiffness relaxation to allow the movement of shell C 1 . Shell C 1  creates stress, which has to be transferred into shell C 3  via shell C 2 . That is why the laminate L 2  is provided to perform the function of transferring that stress into shell C 3  from shell C 1 . 
     In conclusion, the composite shell assembly  10  can be the main suspension for the superconductive magnet that mounts the cryogenic vessel V to the vacuum enclosure E. At the middle is the re-entrant cylinder with the thermal shield T to insulate the colder inside environment of the cryogenic vessel V from the warmer outside environment of the vacuum enclosure E. This composite shell assembly  10  acts as both a thermal insulator and a load transfer mechanism acting to transfer the load from the cryogenic vessel V to the vacuum enclosure E. 
     It is thought that the present invention and its advantages will be understood from the foregoing description and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the above-described embodiment(s) being merely exemplary thereof.