Laminated composite shell assembly with joint bonds

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 having joint bonds.

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. Also, adhesive bonded joints are oftentimes supported by bolts or rivets. Such joints tend to act as a local weak link in the assembled structure. In the presence of cryogenic environments and combined thermo-mechanical loads, adhesive bonds become stiffer and can result in regions of localized failure leading to a compromise in structural integrity.

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 laminated composite shell assembly of the present invention employs one or more laminated pinch rings reinforcing one or more joint bonds of the assembly. The laminated composite shell assembly per se has a sequence of laminated composite shells which comprises the invention of patent application Ser. No. 09/196,423 assigned to the assignee of the present invention. The laminated composite shells are made of composite layers having fibers (such as graphite-epoxy material) and assembled together to form the assembly. The graphite-epoxy material is stiffer than glass-epoxy material and tends to deform elastically rather than plastically. The use of bolts or rivets to support joint bonds connecting the assembled composite shells to external structures would cause the holes in the laminates to act as stress concentrators and weaken the assembly. The introduction of laminated pinch rings adjacent the locations of the joint bonds in place of bolts or rivets eliminates the need for holes in the laminates and so avoids the problem of stress concentration. The laminated pinch rings reinforce the joint bonds by providing additional stiffness and simultaneously acting as load redistribution members.

In an embodiment of the present invention, a laminated composite shell assembly is provided which can be used in a tube suspension for a magnet. The laminated composite shell assembly includes a plurality of laminated composite shells assembled to one another and having one or more joint bonds at which the composite shells are connected to an external structure. The assembly further includes one or more laminated pinch rings assembled to the composite shells adjacent to the joint bonds so as to reinforce the joint bonds.

The composite shells and pinch rings are substantially cylindrical in configuration and have a predetermined axial direction. Each shell and pinch ring is made of composite layers having fibers with the fibers being oriented in a plurality of stacking sequences with reference to the axial direction of the shell and pinch ring. An example of the fibers of the shells and pinch rings are graphite fibers of a 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. The laminated pinch rings are concentrically assembled to the shells so as to perform additional stiffening as well as load redistributing and transfer functions in the regions of the joint bonds between the assembled shells and the external structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1

is an exaggerated schematic cross-sectional view of the laminated composite shell assembly comprising the invention of above-cited patent application, the assembly being provided without any laminated pinch rings.

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 an exaggerated schematic cross-sectional view of the laminated composite shell assembly of the present invention having joint bonds reinforced by laminated pinch rings.

FIG. 5

is a chart of various laminates in the composite shells and pinch rings of the assembly of FIG.

4

.

FIG. 6

is a chart of the laminate stacking sequence and orientation of fibers of the laminates in the composite shells and pinch rings of the assembly of FIG.

4

.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings and particularly to FIG.

1

and

FIG. 4

, there is illustrated in exaggerated form a laminated composite shell assembly, generally designated

10

and

100

in FIG.

1

and

FIG. 4

respectively, 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 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, and identified in FIG.

1

and

FIG. 4

by reference numeral

10

and

100

respectively, 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

and

FIG. 4

, the composite shell assembly

10

and composite shell assembly

100

, respectively 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

and FIG.

4

. 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, being represented by a dashed line E in FIG.

1

and FIG.

4

and is positioned outside of and generally spaced apart from the thermal shield T. The outer support cylinder

14

has a first end

22

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

and

FIG. 4

, 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 cylindrical

12

.

The construction of the composite shell assembly

10

of

FIG. 1

per se constitutes the invention of copending patent application Ser. No. (General Electric Docket No. RD-26475) assigned to the assignee of the present invention. The composite shell assembly

10

of

FIG. 1

, which does not have reinforced joint bonds, is substantially equivalent to composite shell assembly

100

of

FIG. 4

if its reinforced joint bonds are excluded. Therefore, the detailed description of the composite shell assembly

10

set forth in the copending patent application may be repeated and used herein to provide an enabling disclosure of the composite shell assembly

100

.

The composite shell assembly

10

as shown in

FIG. 1

, 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 of the assembly

10

. The laminate stacking sequences identify the orientation of the fibers in different composite layers with respect to the axial direction of the shell 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 off-axis orientations or +/−45°, +/−30° and +/−10° fibers are with reference to the axial direction of the composite shells C of the assembly

10

. 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 (A/R) 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

Composite

Extension Stiffness

Flexural 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 critcal 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 compatability 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 non-reinforced adhesive bonds

110

between laminates L

4

and 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

.

Referring to

FIG. 4

, there is illustrated the composite shell assembly

100

of the present invention having reinforced joint bonds

102

connecting the assembly

100

with the thermal shield T. The same reference numbers are used to identify the respective components of the assemblies

10

and

100

of

FIGS. 1 and 4

that are substantially the same. Although the laminate stacking sequences of the laminates L

1

′ to L

5

′ of the composite shell assembly

100

differ somewhat from the laminate stacking sequences of laminates L

1

to L

5

of the composite shell assembly

10

described above, their functions in the various shells C, C′ of the respective assemblies

10

,

100

remain substantially the same and so there is no need to describe in detail the stacking sequences and orientations of the fibers of the shells C′ of the composite shell assembly

100

of

FIG. 4

as set forth in charts

104

and

106

in

FIGS. 5 and 6

, respectively. Only the reinforced joint bonds

102

of the laminated composite shell assembly

100

will be described hereinafter.

The laminated composite shell assembly

100

includes the plurality of laminated composite shells C′ (C

1

′ to C

8

′ in

FIG. 4

) assembled to one another and having one or more of the reinforced joint bonds

102

(see

FIG. 4

) at which the composite shells C′ are connected to an external structure, such as the thermal shield T. The composite shell assembly

100

further includes at least one and preferably both of inner and outer laminated pinch rings IPR and OPR (see

FIG. 4

) assembled to the composite shells C′ adjacent the inner support cylinder first end

16

(IPR) and the outer support cylinder second end

20

(OPR), respectively so as to establish the reinforced joint bonds

102

. The composite shells C′ and inner and outer pinch rings IPR, OPR are substantially cylindrical in configuration and have an axial direction. Similar to each shell C′, each pinch ring IPR, OPR is made of composite layers of fibers (such as graphite fibers of a graphite-epoxy material), with the fibers being oriented in the plurality of stacking sequences of

FIG. 6

with reference to the axial direction of the shell C′ and pinch ring IPR, OPR.

As explained earlier, the shells C′ are concentrically assembled in a desired sequence wherein some of the shells C′ are adapted to perform a structural load bearing function while others of the shells C′ are adapted to perform a load transfer function. The laminated pinch rings IPR, OPR are concentrically assembled to the shells C′ so that the pinch rings are adjacent and parallel to laminate L

3

′ and to laminate L

4

′ which includes the adhesive bonds

110

and perform additional stiffening as well as load redistributing and transfer functions in the regions of the reinforced joint bonds

102

between the assembled shells C′ and the thermal shield T external to the shells C′.

Therefore, the reinforcing pinch rings IPR, OPR are designed for stiffening at the regions of the reinforced joint bonds

102

as ensuring alternate load transfer in such regions. The pinch rings IPR, OPR provide additional strength and stiffness to the structure of assembly

100

and simultaneously change the direction of the load from axial to a shear load. The adhesive bonds

110

are highly effective in shear and provide further efficient load transfer between laminates L′ when the load is transferred as a shear load. The laminated inner pinch ring IPR and outer pinch ring OPR having the laminate stacking sequences of

FIG. 6

provide efficient load redistribution. The outer pinch ring OPR can provide compressive force while the inner pinch ring IPR can provide tensile force by tailoring the laminate stacking sequence under combined loads. A 50% axial, a 29% hoop and a 35% shear stress reduction is achieved when the reinforcement combination, pinch rings IPR, OPR, is used instead of the laminate comprising the suspension tube or when there is no reinforcement applied. The effect of the reinforcement enables efficient load transfer as well as efficient stress redistribution. The reinforcement pinch rings also minimize bending moments at the edge of the reinforced joint bond

102

regions.

In Table II below, the properties of the inner and outer pinch rings IPR, OPR are set forth. Ex and Ey are Young′ s modulus values in x and y axes; Gxy is the shear modulus; &agr;x, &agr;y and &agr;xy are the thermal coefficients in x and y axes and shear thermal coefficient. The &agr;x for inner pinch ring IPR has a positive (+) sign, while the &agr;x for the outer pinch ring OPR has a negative (−) sign. Also, their magnitudes are completely different. The &agr;y is very different for the inner and outer pinch rings in terms of magnitude. That means that substantial strain can be generated in either compression or tension at the respective locations. So pinch rings IPR and OPR have particular functions at a particular location. Thus, their stacking sequences are therefore different to serve their different functions.

TABLE II

&agr;x

&agr;y

&agr;xy

Ex

Ey

Gxy

Vxy

ppm/

ppm/

ppm/

kpsi

kpsi

kpsi

—

deg F.

deg F.

deg F.

IPR

8.61

7.54

4.59

0.46

2.29

3.34

−0.001

OPR

11.63

4.55

4.40

0.70

−.09

8.45

0.014

In summary, the composite shell assembly

10

of FIG.

1

and also the composite shell assembly

100

of

FIG. 4

can serve as the main suspension for the superconductive coil assembly M that mounts the cryogenic vessel V to the vacuum enclosure E. The thermal shield T insulates the colder inside environment of the cryogenic vessel V from the warmer outside environment of the vacuum enclosure E. The composite shell assemblies

10

,

100

each can perform 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. The composite shells C, C′ of the assemblies

10

,

100

include different laminates L, L′ 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 which involve extreme environments, such as one whose temperature ranges between 4° K. and 300° K. The laminated pinch rings IPR, OPR made of graphite-epoxy material the same as the shells C′ and having different stacking sequences are provided to establish the reinforced joint bonds

102

between the shells C′ and the thermal shield T.

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.