WO1990010816A1

WO1990010816A1 - Tubular coupling and method of forming same

Info

Publication number: WO1990010816A1
Application number: PCT/US1990/001169
Authority: WO
Inventors: Robert William Schmitz; Robert Dale Farris; William George Gottenberg; Edwin Siu-Man Chim; King Him Lo
Original assignee: Shell Oil Co
Current assignee: Shell USA Inc
Priority date: 1989-03-16
Filing date: 1990-03-12
Publication date: 1990-09-20
Anticipated expiration: 1991-09-16
Also published as: CA2012113A1

Abstract

A tubular coupling (10) adapted for connection by interference fit and adhesive bonding to a tubular member (12) is disclosed. The tubular coupling (10) includes a tapered section (30) adjacent the adhesive bonding area (18) to reduce the stress concentration in the layer at the point of contact with the composite tube (12). The interference fit (20, 14) is used to hold the composite tube (12) rigidly on the tubular coupling (10) while the adhesive cures. This allows the joint to be assembled quickly. The interference fit design provides means of adjustment to compensate for manufacturing tolerances.

Description

DESCRIPTION TUBULAR COUPLING AND METHOD OF FORMING SAME

Technical Field

This invention relates to mechanically joining a hollow tube, (preferably a hollow composite tube of filamentary material composed of glass, graphite, aramide, boron or other such filaments in a polymer matrix) , to an end connection fitting hereinafter referred to as a tubular coupling. The coupling may be used where it is desired to transfer torsional, bending and/or axial forces to a structure from a (composite) tube containing the tubular coupling, such as in a rocket motor casing, a fluid conveyance pipe, a pressurized fluid containment vessel, a torque transmitting shaft, and a load-transmitting structural strut all of which employ a (composite) tube. It is well recognized that the tube may be formed from other suitable material such as steel or aluminum, and the tubular coupling may also be formed from other non-metallic materials. Background of the Invention

As described in U.S. Patent 4,530,379 entitled "Filament Wound Interlaminate Tubular Attachment", in the field of metal to composite joints, the connection of a fitting to a filamentary composite tube has not been made in a manner as to take full advantage of the multiplicity of load transfer planes available within the tube. Such cases of prior art are those where the fitting is bonded through a single load-transfer plane to the composite tube, the single load-transfer plane being either internal or external to the fitting.

In other cases of prior art connections have been made by radially pinning or bolting through the composite tube into the fitting. Note for example Figure 19 of U.S. Patent

4,649,960, (same inventor as the '379 patent). In this manner, the load is transferred through bearing stresses in composite tube layers which may have low bearing strength compared with that of the metal.

Note also, for example, U.S. Patent 4,185,472 entitled "Fiber Reinforced Composite Shaft With Metallic Connector Sleeves Mounted by Radial Pin Interlock", wherein initially a metal sleeve having apertures is positioned upon a segment of a mandrel. Fibrous material bearing a non-solidified resinous material is thereafter applied around the mandrel and around the apertures in the sleeve. Sharpened spikes are pressed through the fibrous material and into the apertures of the sleeve. Additional fibrous material bearing the non-solidified resinous material is applied around the outer ends of the spikes. At the end of this complex process, the resinous material is next solidified to form a tubular composite shaft, unfortunately irreparable without destruction of the bond between the composite and the metal sleeve.

Another fitting design incorporating pin or "bolt-through" connections may be studied in the article "A Torsionally Stiff-Bending Soft Drive Shaft" by A. J. Hannibal and J. A. Avila presented at the 39th Annual Conference Reinforced Plastics Composite Institute, The Society of the Plastics Industry, Inc., January 16-19, 1984.

Directing our attention now to patents related to the coupling of pipe joints, U.S. Patent 4,541,655 discloses a pipe coupling joint which uses an interference fit between two unthreaded pipe sections. The telescoped interference fit coupling formed at one outer end of a pipe is radially compressed about an inner pipe end to form a groove in the inner pipe, so as to securely couple the pipe sections together. Lined pipe joints and methods and apparatus for performing the improved interference fit joint are also disclosed. It should be noted that a bonding agent is used over the entire joint with the interference fit area being common with the bonding agent application area. The combination interference fit/adhesive bonding area yields joints of questionable strength repeatability due to the wiping of an unknown amount of bonding agent from the interference fit area as the pipes are assembled. In any event, a composite tube cannot be crimped as taught in this •655 patent in a radial manner without significant destruction of the composite matrix with subsequent loss of strength.

U.S. Patent 4,696,499 entitled "Mechanical Pipe Joint and Method of Forming the Same" discloses a mechanical pipe joint in which a pin formed on the end of one pipe section is inserted in a socket formed in the end of another pipe section. The tapered section on the pin has a taper angle less than the taper of a tapered section on a socket, when the pin is inserted to the desired depth, an angular cavity will be formed between the tapered section in which sealant, not adhesive is trapped to form an annular seal for the joint. Such a system depends on the interference fit entirely for the mechanical strength of the coupling. It should be noted that the interference fit area is extremely small and therefore leaves the joint susceptible to weakening after successive thermal cycling and mechanical cyclic loadings. U.S. Patent 3,973,411 entitled "Coupling For Flexible Shaft Elements" discloses a coupling having a generally cup-shaped sleeve for receiving a flexible shaft element and an integral bottom portion with circumferentially spaced apertures disposed near the integral sleeve. This coupling relies solely on an interference fit with subsequent crimping of an outer section down upon the outer surface of the composite tube. No adhesive is used in the connection process. The reusability of the coupling is suspect since after the flexible element is positioned within the tapered annulus, the sleeve is reduced to a smaller external diameter (about 12%) by crimping or the like. The radially inward forces imparted to the sleeve crush the webs 23 and distort the flutes 22 outwardly and apertures inwardly 44 to define an undulating wave 46 structure at the bottom edge of the coupling. The crushing of the webs and distortion of the flutes changes the shape of the apertures as shown in Figures 4 and 6 of this patent.

A general discussion of the stresses involved in the use of tubular lap joints using adhesive, (with no interference fit) , is available in "The Journal of Adhesion" Vol. 9, No. 1. (1977) edited by Louis H. Sharpe of Bell Telephone Laboratories Inc. In this article the stresses in an adhesive-bonded tubular lap joint, subjected to axial and torsional loads, have been analyzed using axisymmetric quadratic isoparametric finite element analysis.

A tubular coupling apparatus therefore needs to be developed that does not transmit forces in a singularily planer manner from the composite tube to the coupling structure. Such a tubular coupling/composite tube system must allow repairable placement of the tube and/or coupling, especially if it is desired to use the system as an automotive drive shaft. Such a system should therefore not include pin connections as taught by several of the patents referenced above. Additionally, such a coupling system should not require the radial crimping of the composite tube structure due to its catastrophic resulting decrease in strength. Summary of the Invention

The tubular coupling of the present invention has two separate load transfer areas. The first area comprises a recessed surface coatable with a layer of adhesive. The second binding surface, segregated from the recessed surface, forms an interference fit with the end of the composite tube. The area of either surface may be varied during design of the tubular coupling to accommodate any anticipated stress levels transmitted from the composite tube to the tubular coupling.

Since the composite tube is guided over the recessed surface during assembly the thickness of the adhesive layer may be regulated exactly due to the wiping action of the end of the coupling as it passes over the layer of adhesive that has previously been applied to a portion of the inner surface of the composite tube. Exact control of the thickness of the layer ensures accurate stress transfer from the tube into the tubular coupling. A before, the area of the binding surface may be varied as required to meet various design objectives.

The tubular coupling also includes a tapered section formed at the tube engagement end of the coupling having a selected angle to reduce the stress concentrations at the initial adhesive/tube boundary.

The tubular coupling in an alternative embodiment may also incorporate crimping means positioned adjacent the binding surface such that actuation of the crimping means downward into contact with the composite tube does not crush the filaments of the tube.

It is therefore a feature of the present invention to include two segregated load transfer areas each having independently variable geometry.

It is another feature of the invention to include a tapered section at the end of the tubular coupling to reduce the stress concentration where the tube first contacts the coupling.

It is another feature of the invention to include crimping means capable of maximizing load transfer from the tube to the coupling, at the same time minimizing crushing of the composite structure. It is an object of the invention to provide a lightweight tubular coupling having improved stress transfer capability.

More specifically, the method of the present invention would comprise forming the tubular coupling, forming or providing the composite tube, coating a selected portion of the inner surface of the end of the tube with the layer of adhesive, and forcing the first end of the coupling into the composite tube until the end of the tube is positioned over a portion of the binding surface, to form a substantially closed annular cavity between the recessed surface and the inner surface of the tube, and also a mechanical interference fit between the inner surface of the composite tube and the binding surface of the coupling.

The tubular coupling in a preferred embodiment will also include mechanical connection means located opposite the end connected with the composite tube, the mechanical connection means being used for transmission of the forces from the composite tubes/coupling to other devices. These and other features, objects and advantages of the present invention will become apparent from the following Detailed Description, wherein reference is made to the Figures in the accompanying drawings. In the Drawings

Figure 1 show a schematic representation in cross-section of the tubular coupling mated with the composite tube.

Figure 2 shows a schematic representation inn cross-section of the tubular coupling of an alternative embodiment including crimping means positioned adjacent the end of the composite tube.

Figure 3 shows an alternative embodiment of the second tapered section shown in Figure 1. Description of the Preferred Embodiment

Referring now to Figure 1 a tubular coupling 10 is shown connected with a composite tube 12 the tube having an inner surface 14 and an outer surface 16. The tubular coupling includes a binding surface 18 defined in a circular manner about the outer periphery of a portion of a tubular coupling 10, the binding surface 18 having an outer diameter 20 sized to form a predetermined interference fit with the inner surface 14 of the composite tube. The tubular coupling 10 also includes a recessed surface 22 defined in a circular manner about the outer periphery of the coupling 10, having an outer diameter 24 less than the binding surface outer diameter 20. The recessed surface 22 is located between the binding surface 18 and a first end 26 of the tubular coupling and is capable of being coated with a layer of adhesive 28 of a selected thickness, when the end of the coupling is inserted into the end of the composite tube.

A first tapered section 30 in a preferred embodiment having a chamfer over at least a portion of the section is located between the binding surface 18 and the recessed surface 22. The chamfer eases the end 32 of the composite tube outwardly as it is force-fit over the binding surface 18. The chamfer in a preferred embodiment would have an angle of 45 degrees, though it should be well understood that the chamfer may have other angles as well.

A central opening 34 is defined through a portion of the coupling with a second tapered section 36 being tapered inward from the first end 26 of the coupling. The second tapered section reduces the stresses at the first end of the coupling and therefore ensures that the adhesive stresses do not affect the integrity of the adhesive layer, the angle 38 of the second tapered section 36 is preferably selected after stress analysis calculations have been performed. It should be well understood that the tapered section may also taper outward, as shown in an alternative embodiment by tapered section 36A (Figure 3) , and still accomplish the same mechanical effect.

Mechanical connection means 39 such as a flange or a screw connection or other mounting means well known to the art are shown located at the second end 40 of the coupling and are used to transmit and/or connect the tubular coupling and/or composite to another mechanical device.

When the tube is assembled to the coupling, a substantially closed annular cavity 42 containing the layer of adhesive 28 is formed between the recessed surface and the inner surface of the tube.

The angle 38 of the second tapered section 36 (measured relative to the central longitudinal axis 44 of the coupling) in a typical embodiment will be from about 3° to about 30°, to reduce the stress concentration at the first end of the coupling. In a typical embodiment also, the layer of adhesive coated on the recessed surface 22 may be selected from the group consisting of Lord Fusor 310, Scotch 3M 3569, Scotch 3M 3448, Dextor Hysol 9394, or Dextor Hysol 9432, though it should be well understood that other structural adhesives with acceptable performance characteristics in hot, wet, operating environments may also be used. It is an advantage of the invention that the binding surface and recessed surface carrying the adhesive layer remain segregated from one another, such that they would not interfere with each other's load transfer capability and mechanism. The mechanical response of each surface 22, 18 may therefore easily be anticipated through calculations.

Additionally, segregation of the areas allows each area's dimension to be altered without affecting the dimensions of the adjacent area. The length 46 of the binding surface and the length 47 of the recessed surface are adjusted^'to maximize performance and minimize weight and cost of each coupling/composite tube structure. In a preferred embodiment, it has been found that the mechanical strength of the coupling/composite tube is maximized when the length 46 of the binding surface comprises from about 1.0 to 1.5 times the length 47 of the recessed surface. Actual test specimens were fabricated with both lengths 46, 47 equal to 2 inches.

Additionally, in a preferred embodiment it has been found that the mechanical strength is maximized wherein the thickness 48 (Figure 2) of the recessed section comprises from 3 to 10 times the thickness 50 of the adhesive layer. Actual test specimens had a recessed section thickness 48 to 0.083 - 0.100 inches and an adhesive thickness 50 of 0.015 - 0.020 inches. Referring now to Figure 2 a tubular coupling may also, in an alternative embodiment include crimping means 52 preferably formed in an annular manner and positioned a spaced distance at least equal to the thickness 53 of the composite tube away from the binding surface, the outer diameter 54 of the composite tube sized to fit beneath the crimping means 52. The crimping means 52 ends before extending over the first tapered section 30, so that the composite tube is not catastrophically crushed downwards into the adhesive layer 28 due to lack of sufficient mechanical backing.

In operation, a tubular coupling is joined to the composite tube in the following manner. The inner surface of the end of the composite tube which is adjacent the final assembled position of the recessed section is coated with an adhesive. The recessed surface of the coupling may also be coated with a layer of adhesive. The first end of the coupling is forced into the composite tube until the end of the tube is positioned over a portion of the binding surface.

By design the end of the tube will substantially cover all of the binding surface, though in anticipation of possible stress conditions and length tolerances it is entirely possible that the end of the composite tube may not cover the entire binding surface in a particular application. Additionally, the inner diameter of the composite tube may be sized to obtain a predetermined interference fit between the tube and the binding surface of the coupling.

As the end of the coupling is passed through the adhesive layer that has been applied to the inner surface of the tube the coupling's recessed surface effectively levels the surface of the adhesive.

Once the composite tube is positioned at the desired location on the tubular coupling if crimping means are included on the tubular coupling the crimping means 52 are thereafter crimped downward into contact with the outer surface of the tube. Such crimping means may comprise a plurality of "fingers" positioned parallel to the central longitudinal axis of the coupling, slots having been cut or machined in between each finger to allow each finger to move radially independent of one another. As mentioned before the crimping means 52 fingers would not contact the composite tube over the adhesive layer/recessed surface area due to possible crushing of the composite tube. The geometry of the crimping means is selected to provide an additional load transfer capability without inducing stress concentration on the composite tube and the adhesive layer in the recessed area in the tube coupling.

In an illustrative embodiment of the present invention, a previously formed composite tube having an inner diameter 20 of 3.84 inches is formed from a selected arrangement of 10 plys of glass and carbon fibers, wetted and thereafter dimensionally stabilized by a vinylester resin. The thickness 53 of the composite tube is .1425 inches, the adhesive layer has a thickness of .018 inches and the interference mismatch is .006 inches. The inner diameter 60 of the coupling is 3.638 inches. The binding surface has a length of 2 inches, and the recessed surface has a length of 2 inches. The minimum torque to failure with this design is a measured 38,000 to 40,000 in/lbs at ambient temperature and humidity, with the torque to failure at 200°F and 100% humidity having the same magnitude.

The interference fit is also used to hold the composite tube rigidly on the coupling while the adhesive cures. This allows the joint to be assembled quickly. The interference fit design provides a means of adjustment to compensate for manufacturing tolerances. The tubular coupling design, having two segregated load transfer areas, allows shear stress to be minimized for given adhesive, composite tube, and tubular coupling configuration. The mechanical interference interlock is designed in such a way that it will not disturb the thickness of the adhesive bond. Rather, it helps to control and maintain the appropriate design thickness for the bond line to ensure optimum load transfer effectiveness. In addition, it provides a redundant load transfer path to insure the joint integrity from any accidental bond line failure. The optional mechanical crimping at the interference interlocking region provides additional degrees of safety and reduces the surface area needed for interference interlocking. It should be well noted that whereas this joint is capable of use for connecting a composite tube to a metal connector, this design may also be used for metal to metal or composite to composite connections. The repairability of the coupling/tube system is improved over the pinned end systems in that, with enough axial force, the tube may be removed from the coupling, new adhesive applied, and a replacement tube refitted over the original coupling.

Many other variations or modifications may be made in the apparatus and techniques hereinbefore described, both by those having experience in this technology, without departing from the concept of the present invention.

Accordingly, it should be clearly understood that the apparatus and methods depicted in the accompanying drawings and referred to in the foregoing description are illustrative only and are not intended as limitations on the scope of the invention.

Claims

CLAIMS 1. A method of joining a tubular coupling to a composite tube having an inner and an outer surface, said method comprising the steps of: - forming the tubular coupling, said tubular coupling including:

- a binding surface having an outer diameter,

- a recessed surface having an outer diameter less than the binding surface outer diameter, said recessed surface located between the binding surface and a first end of the coupling,

- a first tapered section located between said binding surface and said recessed surface,

- a central opening defined through a portion of said coupling with a second tapered section tapered at an angle relative to the central longitudinal axis of said coupling from the first end of said coupling, and

- mechanical connection means located at the second end of said coupling,

- providing the composite tube including sizing the inner diameter of the composite tube to obtain a predetermined interference fit between the inner surface of the composite tube and the binding surface of the coupling,

- coating the inner surface of the end of the composite tube which will be adjacent the final assembled position of the recessed section with an adhesive, and

- forcing the first end of the coupling into the composite tube until the end of the tube is positioned over a portion of the binding surface to form;

- a substantially closed annular cavity between the recessed surface and the inner surface of the tube, said cavity containing the adhesive, and

- a mechanical interference fit between the inner surface of the composite tube and the binding surface of the coupling.

2. A method of joining a tubular coupling to a composite tube having an inner and an outer surface, said method comprising the steps of:

- forming the tubular coupling, said tubular coupling including:

- a binding surface having an outer diameter,

- mechanical connection means located at the second end of said coupling,

- providing the composite tube including sizing the inner diameter of the composite tube to obtain a predetermined interference fit between the inner surface of the composite tube and the binding surface of the coupling, - coating the recessed surface of the coupling with a layer of adhesive, and

3. The method of claim 1, wherein the step of forming the tubular coupling further includes the step of selecting the angle of the second tapered section relative to the central longitudinal axis of the coupling from about 3 degrees to about 30 degrees, to reduce the stress concentration at the first end of the coupling.

4. The method of claim 1 including prior to the step of coating the inner surface of the composite tube with a layer of adhesive, the step of selecting the adhesive from the group consisting of Lord Fusor 310, Scotch 3M 3569, Scotch 3M 3448, Dextor Hysol 9394, or Dextor Hysol 9432.

5. The method of claim 1, wherein the step of forming the tubular coupling further includes the step of selecting the length of the recessed surface.

6. The method of claim 1, wherein the step of forming the tubular coupling further includes the step of selecting the thickness of the recessed section from 3 to 10 times the thickness of the adhesive layer.

7. A method of joining a tubular coupling to a composite tube having an inner and an outer surface, said method comprising the steps of:

- forming the tubular coupling, said tubular coupling including:

- a binding surface having an outer diameter, - a recessed surface having an outer diameter less than the binding surface outer diameter, said recessed surface located between the binding surface and a first end of the coupling, - a first tapered section located between said binding surface and said recessed surface,

- mechanical connection means located at the second end of said coupling, - providing the composite tube including sizing the inner diameter of the composite tube to obtain a predetermined interference fit between the inner surface of the composite tube and the binding surface of the coupling, - coating the inner surface of the end of the composite tube which will be adjacent the final assembled position of the recessed section with an adhesive, and

- a mechanical interference fit between the inner surface of the composite tube and the binding surface of the coupling, and - crimping the crimping means downward into contact with the outer diameter of the composite tube.

8. A tubular coupling adapted to be connected with a composite tube, said tue having an inner surface, said tubular coupling comprising:

- a binding surface having an outer diameter sized to form a predetermined interference fit between the inner surface of the composite tube and the binding surface of the coupling,

- a recessed surface having an outer diameter less than the binding surface outer diameter, said recessed surface located between the binding surface and a first end of the tubular coupling and capable of being coated with a layer of adhesive,

- a first tapered section located between said binding surface and said recessed surface, - a central opening defined through a portion of said coupling with a second tapered section tapered at an angle relative to the central longitudinal axis of said coupling from the first end of said coupling, and - mechanical connection means located at the second end of said coupling, said tubing coupling and composite tube when assembled by forcing the first end of the coupling into the composite tube until the end of the tube is positioned over a portion of the binding surface, forming,

- a substantially closed annular cavity between the recessed surface and the inner surface of the tube, said cavity containing the layer of adhesive, and

9. The tubular coupling of claim 8, wherein the angle of the second tapered section relative to the central longitudinal axis of the coupling comprises from about 3 degrees to about 30 degrees, to reduce the stress concentration at the first end of the coupling.

10. The tubular coupling of claim 8, wherein the layer of adhesive coated on the recessed surface is selected from the group consisting of Lord Fusor 310, Scotch 3M 3569, Scotch 3M 3448, Dextor Hysol 9394, or Dextor Hysol 9432.

11. The tubular coupling of claim 8, wherein the length of the binding surface comprises from 1.0 to 1.5 times the length of the recessed surface.

12. The tubular coupling of claim 8, wherein the thickness of the recessed section comprises from 3 to 10 times the thickness of the adhesive layer.

13. The tubular coupling of claim 8 further including crimping means positioned a spaced distance at least equal to the thickness of the composite tube away from said binding surface, the outer diameter of the composite tube sized to fit beneath the comprising means.