Patent Description:
Composite materials offer high stiffness and/or strength-to-weight ratios. Composite tube assemblies are formed from composite materials and are used for transferring loads in structures such as aircraft or spacecraft. Other applications include control rods, containers, ducts, panel inserts, torque tubes, etc..

In vehicles such as aircraft, it is beneficial to use composite tube assemblies rather than assemblies primarily composed of metal. They are lighter and stronger than comparable metal tube assemblies. The composite tube assemblies typically incorporate carbon fiber ("CF") tubes and are lighter in weight, more resistant to corrosion, stronger and more inert relative to substantially metallic tube assemblies. Composite tube assemblies may be used in an overhead luggage bin (or stow bin) assemblies in an aircraft to provide structural support both when the bin is in an open configuration and when it is closed. The composite tube assemblies may also be used as structural members in vehicle frames.

To form the end of a CF tube around a fitting or insert (individually and collectively referred to herein for convenience as "fitting"), such as a Hylock insert, the composite tube end is compressed over a smaller diameter of the fitting to form the composite tube assembly. When doing so, areas of the compressed end of the composite tube bunch up and need to be accommodated or removed. This bunching up of the composite tube end can result in cracked CF material after forming. The compressed and bunched composite tube end then requires post processing to clean up the formed area, which adds complexity and risk to the manufacturing process. Other forming issues are seen with tubes formed from composite materials having higher cure temperature resins, as it is more difficult to achieve the glass transition temperature (Tg) range of such composite materials and thus, it is more difficult to compress such tube ends over the fitting.

Related art is disclosed in <CIT>, <CIT>, and <CIT>.

Composite tube assemblies <NUM> including fittings <NUM> are described in <CIT>. In an example embodiment, an end <NUM> of a composite tube or tube <NUM> is fitted over a fitting <NUM> as shown in <FIG>. The composite tube <NUM> may be produced by winding composite fibers in a form of a filament (and/or a tape) having an epoxy resin over a tubular mandrel. Any of a number of suitable machines known to those skilled in the art can be used for this purpose. The composite tube may be a fiber reinforced composite formed with a thermoset or a thermoplastic resin. In other example embodiments, the composite tube may be a liquid molded tube that may include fiber reinforcement. The composite fibers may be wound along a direction that is substantially helical with respect to a longitudinal axis <NUM> of the composite tube. In other words, in a direction along the circumference transverse or perpendicular to the longitudinal axis of the tube. In one embodiment, the composite fibers are also wound at a very small helical angle (or angles) with respect to the longitudinal axis. However, embodiments of the composite tube <NUM> are not limited thereto. That is, the composite tube <NUM> may be produced by winding filaments and/or pre-impregnated composite tapes in any known manner and at any angle. The composite tube may include fibers oriented in multiple directions, as for example in the hoop direction (<NUM>° to the longitudinal axis) as well as fibers along the longitudinal axis as well as other angles relative to the longitudinal axis, as for example along <NUM>°.

The fitting <NUM> includes a tapered outer surface <NUM>, as for example shown in <FIG> ad <NUM>, tapering from a larger diameter section <NUM> to smaller diameter section <NUM> distally relative to the tube <NUM> defining reduced diameter or tapered portion <NUM>. The tapered outer surface <NUM> may have a smooth outer surface as for example shown in <FIG> or may have a textured outer surface, as for example shown in <FIG>, for better adhesion. In an example embodiment, the textured outer surface may be formed by a plurality of adjacent annular grooves. In other example embodiments, the outer surface may have multiple tapering outer surfaces, as for example shown in <FIG>. For example, a first cylindrical outer surface <NUM> may axially taper at a first angle <NUM>. A second cylindrical outer surface <NUM>, extending axially from the first cylindrical outer surface, may taper axially at a second angle <NUM> greater than the first angle <NUM> such that it tapers to a smaller diameter in a direction away from the first cylindrical outer surface. Fittings also typically include a cylindrical flange <NUM> proximate the smaller diameter of the tapering outer surfaces. A neck <NUM> extends axially from the flange in a direction away from the tapering outer surface. An end fitting, such a clevis bolt <NUM>, may be received in the neck and connect to the fitting, as for example shown in <FIG>.

In an example embodiment, a tube end portion <NUM>, to be fitted over (i.e., mated with) the tapered outer surface <NUM> of the fitting, is slotted with a slot <NUM>, in at least one location but, preferably in at least at two different locations to form at least two cantilevered arm sections or arms <NUM> as for example shown in <FIG>. For illustrative purposes, only the tapered portion <NUM> of the fitting is schematically depicted next to the tube end portion <NUM> in <FIG>. Each slot <NUM> extends to the distal end <NUM> of the tube so as to form the cantilevered arms <NUM>. In an example embodiment, more than two slots are formed as for example three, four, five, six, seven, eight, nine, ten, or more than ten to form multiple arms <NUM>. These arms are bent and molded over the tapered surface of the fitting for coupling the tube to the fitting, as for example shown in <FIG> which depicts the end portion of the tube fitted over the tapered outer surface of the fitting without showing the fitting.

A tapered portion <NUM> of the composite tube <NUM> is formed by heating the end portion <NUM> arms <NUM> of the composite tube <NUM>, into which the fitting <NUM> was inserted, to a temperature sufficient for the composite tube end portion arms to become thermoplastic or moldable, as for example by softening the resin forming the composite tube. In one example embodiment, the tube is heated to a temperature within the glass transition temperature (Tg) range of the composite material forming the composite tube. A heated die or any other suitable instrument may be used for this purpose. Once the end portion <NUM> arms <NUM> of the composite tube <NUM> are in a moldable state, they are then pressed and deformed to engage the tapered portion <NUM> of the fitting <NUM>. That is, the end portion <NUM> arms <NUM> of the composite tube <NUM> are deformed to collectively have a shape and dimension substantially conforming to a shape and dimension, respectively, of the tapered portion <NUM> of the fitting <NUM>. The tapered portion <NUM> of the composite tube <NUM> is thereby formed. As the tapered portion <NUM> of the composite tube <NUM> cools, it compresses radially on to the tapered portion <NUM> of the fitting <NUM>. The tapered portion <NUM> of the composite tube <NUM> is thereby secured to the tapered portion <NUM> of the fitting <NUM> to form a mechanical lock. The tapered portion <NUM> of the composite tube <NUM> may also bond to the tapered portion of the fitting <NUM> as the softened resin cools and bonds onto the tapered portion of the fitting. An adhesive may also be used to help adhere the composite tube reduced diameter portion <NUM> to the fitting tapered portion <NUM>. If an adhesive is used between the composite tube and the fitting, the adhesive by itself or in combination with the resin creates a bond between the composite tube and the fitting.

Applicants discovered multiple benefits by slotting (i.e. forming slots <NUM>) or slitting the tube end portion when forming a composite tube assembly. Because the arms <NUM> provide flexibility (i.e., they are more flexible then the composite tube end portion when not slotted), heat, although desirable, is not necessary for bending each arm and attaching and coupling it to the tapered outer surface of the fitting. Heat is, however, desirable for accelerating the cure of the structure adhesive used on the composite tube and thus the arms. The heating temperature can range anywhere from ambient temperate to the upper end of the composite tube material Tg range. Since the slotted tube end portion configuration defining multiple arms also allows for ambient temperature forming or bending of the arms onto the tapered outer surface <NUM> of the fitting, a collar <NUM> is installed over the arms to provide appropriate force on the arms against the tube to provide a forming force in the tube without having to heat the arms (<FIG>, <FIG>). Another benefit provided by lower temperature forming is the ability to maintain tolerances caused coefficient of the thermal expansion differences in the materials used for the tube and the fitting. Another benefit is the ability to use tooling made for materials that would otherwise degrade at the higher forming temperatures. Being able to use lower temperature forming, further provides a great benefit, in that it allows composite tubes made from materials with higher Tg to be used as the higher temperatures required when using traditional forming methods with many materials are not necessary.

Since the hoop structure is cut to form the slots and arms, composite materials may be used to form the composite tube that typically do not form very well. This include standard, or intermediate, or high or ultra-modE fibers and resins systems with high Tg, even with Tg significantly over <NUM>° F. These include thermoplastic or thermoset matrix materials.

In the prior art embodiments, where slots were not formed, the bunching up of the material after the end of the tube was compressed over the tapered surface, and required machining away of the excess material. Material then had added to the outer diameter of the tube. Such machining and addition of material is not required by slotting and forming arms on the composite tube end portion.

In an example embodiment, the slotted ends of the tube may be formed in the prepreg material prior to rolling to form the tube or may be forming to the other tube after the tube is formed.

In an example embodiment, each slot has a length <NUM> such that when the arms are mounted to the tapered outer surface of the fitting, each slot extends along the entirety, or substantially the entirety, of the length <NUM> of the fitting annular tapered outer surface <NUM>. In other example embodiments, the length of each slot may be less than the entire length of the tapered outer surface <NUM>.

In one example embodiment, each slot is rectangular shaped in that each slot includes generally parallel, or parallel, opposite edges <NUM> extending along the entire length, or substantially the entire length, of each slot, as for example shown in <FIG>. When fitted and mated with the tapered outer surface of the fitting, the arms <NUM> shown in <FIG> define a shape as shown in <FIG>. In another example embodiment as shown in <FIG>, each slot <NUM> is triangularly shaped, such that the vertex <NUM> of the triangle is proximal and the base <NUM> of the triangle is distal of the tube <NUM> defining the maximum spacing of the slot, or the maximum width of the slot, at the distal end <NUM> of the tube such that the largest width of the slot is closest to the smallest diameter section <NUM> of the tapered surface <NUM> of the fitting when the fitting if fitted in the tube end. In other example embodiments, the slots may have nonlinear or varying widths to accommodate different geometries of tapering of the surface of the fitting, as for example shown in <FIG>. For example, the slots may have varying widths such that the slot width varies along two different angles so that the arms <NUM> formed can mate with a fitting that has a first tapered surface <NUM> tapering at a first angle <NUM> and a second tapered surface <NUM> extending axially from for the first tapered surface and extending at a second angle <NUM> that is greater than the first angle, as for example shown in <FIG>. In an example embodiment, the edges of each slot are curving or continuously curving. In example embodiments, the opposite edges <NUM> of the slots are such that when the arms are mated over the fitting outer tapered surface, such opposite edges <NUM> are parallel or almost parallel to each other and are close to each other or abut each other, as for example shown in <FIG> and <FIG>. This configuration is expected to provide better results.

Three example embodiments are provided herewith.

The tube end (or hoop area) portion includes constant width slots as for example shown in <FIG>. The width of each slot is based on the reduced circumference at the distal end of the tube when the tube is mated to the fitting end and the number of slots used. The proximal ends of the slots may incorporate a radius at each corner to reduce stress concentrations.

In this example embodiment, each slot width is defined by the difference of the circumference along the fitting taper divided by the number of slots. The length of the slot may vary based on material properties and design requirements, though in most cases the proximal end of the slot will be close to, or at, the point at which the proximal end of the taper on the fitting is located. The open volume left after forming between the opposite edges <NUM> of each slot may be filled with a filler material such as structural adhesive, or left open.

The width of the each slot is <MAT> where,.

In an example embodiment, the width of each slot can be a value within <NUM>% of W derived by Formula <NUM>. In another example embodiment, the width of each slot can be within <NUM>% of W derived by Formula <NUM>. In yet another example embodiment, the width of each slot can be within <NUM>% of W derived by Formula <NUM>. In another example embodiment, the width of each slot can be a value within <NUM>% of W derived by Formula <NUM>. In a further example embodiment, the width of each slot can be within <NUM>% of W derived by Formula <NUM>. As is well known in the art, circumference is calculated by multiplying the diameter by π.

The tube end portion is formed with linearly varying width slots, as for example shown in <FIG>. The slots are shaped such that when mated over the single angle tapered outer surface section of the fitting, the two opposite edges of each slot generally parallel or parallel to each other and meet or abut each other along the entire length of the slot.

The slots are triangular in shape in plan view, with the point of the "V" or point of the triangle pointing towards the proximal end of the tube, and the distal end of the slot, which is the widest end of the slot, being at the distal end of the tube.

The length of the slot may vary based on material properties and design requirements, though in example embodiments the proximal end of the slot will be close to the point at which the proximal end of the tapered outer surface of the fitting is located.

In this example embodiment, each slot is approximately an isosceles triangle with the base defined as the slot max width (W) at the distal end <NUM> of the tube and the angle bisector from the vertex <NUM> (i.e. the "V") of the triangle to the base <NUM> defined as the slot length (L). In this regard, when the arms are bent and fitted over the fitting tapered outer surface, the opposite edges of each slot are very close together or they abut each other.

In example embodiments where the opposite edges <NUM> of each of the slots do not abut each other after being mated to the fitting outer surface, the open volume left after forming may be filled with a filler material such as structural adhesive, or left open.

The proximal end of the slot is the theoretic point at which the two angled opposite edges defining each slot meet. In example embodiments, the physical end of the slot may not be precisely at this point due to manufacturing limitations, and/or to reduce stress concentrations.

The slot width WX varies linearly as a function of a distance "X" from the proximal apex and is <MAT>where,.

In an example embodiment, the width Wx at a distance X of each slot can be within <NUM>% of Wx derived by Formula <NUM>. In another example embodiment can be within <NUM>% of Wx derived by Formula <NUM>. In yet another example embodiment it can be within <NUM>% of Wx derived by Formula <NUM>. In another example embodiment, it can be within <NUM>% of Wx derived by Formula <NUM>. In a further example embodiment it can within <NUM>% of Wx derived by Formula <NUM>.

The end portion area has non-linearly varying width slots as for example shown in <FIG>. The slots are shaped such that when formed over a varying angle tapered outer surface of a fitting, the two sides of the slot nearly meet or abut each other along the entire length of the slot. In example embodiments where the opposite edges <NUM> of each of the slots do not abut each other after being mated to the fitting outer surface, the open volume left after forming may be filled with a filler material such as structural adhesive, or left open.

After machining and prior to forming, the slots are shaped based on the radial projection of the varying taper angle of the tapered outer surface of the fitting, which may result in a polygonal or curved slot shape.

The slot width WX varies as a function of a distance "X" from the proximal apex of the slot and is <MAT> where,.

In an example embodiment, the width Wx at a distance X of each slot is within <NUM>% of the Wx derived by Formula <NUM>. In another example embodiment it is within <NUM>% of Wx derived by Formula <NUM>. In yet another example embodiment it is within <NUM>% of Wx derived by Formula <NUM>. In another example embodiment, it is within <NUM>% of Wx derived by Formula <NUM>. In a further example embodiment it is within <NUM>% of Wx derived by Formula <NUM>.

Applicant had discovered that after mating the tube end <NUM> to the fitting tapered outer surface there may be micro-buckling of the tube formed in a direction radially away from the fitting at in location <NUM> proximate the end of the fitting opposite the flange <NUM> of the fitting, when the tube end is cured over the fitting. Such micro-buckling may also occur during use of the composite tube assembly. Applicants discovered they can overcome this problem by forming an annular a step, or steps, <NUM> on the outer surface of the fitting at the end opposite the flange, as for example shown in <FIG>. In an example embodiment, a first step 78a is provided by reducing the outer diameter of the fitting. In another example embodiment, multiple steps <NUM> are provided by having a first diameter reduction then followed by a second diameter reduction distally towards the end opposite the flange to create another step, followed by a third diameter reduction and so forth, as for example shown in <FIG>. Although one diameter reduction is necessary, multiple diameter reductions may be provided as described herein.

An annular step <NUM> is provided on the inner surface of the tube end portion prior proximate the tube end. This annular step is formed by increasing the inner surface diameter of the tube by removing tube material from the inner surface of the tube or by forming the tube to have such step. The annular step <NUM> formed on the tube is such that when the tube mates with the outer surface of the fitting, the annular step <NUM> of the fitting mates with the step <NUM> of the tube. In an example embodiment, by providing the fitting with multiple steps, the same fitting can be used to accommodate tubes having different tube wall thicknesses. For example, in the embodiment shown in <FIG>, three different thicknesses of tubes may be accommodated with a single fitting. In other example embodiments, the tube inner surface may have multiple annular steps complementary to any plurality of annular steps <NUM> defined on the fitting.

Claim 1:
A method for forming a composite tube assembly comprising:
obtaining a composite tube (<NUM>) comprising an end (<NUM>), wherein said composite tube is formed from fibers and resin, said tube comprising an end portion (<NUM>) extending to the end comprising a plurality of axial slots (<NUM>), wherein each of the axial slots comprises a two edges (<NUM>) at the outer surface of the said tube end extending along a length (<NUM>) each slot, wherein said slots define a plurality of arms (<NUM>) on said end portion;
obtaining a fitting (<NUM>) comprising a first end opposite a second end, said fitting comprising an annular tapered outer surface (<NUM>) tapering from a first diameter at, or proximate, the first end to a second diameter, wherein the first diameter is greater than the second diameter; and
placing the tapered outer surface of the fitting in the tube end portion such that the second diameter of the fitting outer surface is at, or proximate, the end of the tube and the first diameter of the fitting tapered outer surface is further into the tube than the second diameter of the fitting,
characterized in that the method further comprises:
radially compressing each of the arms over the fitting annular tapered outer surface;
connecting or adhering each of the arms to said fitting tapered outer surface; and
placing a bolstering structure or a collar (<NUM>) over said arms for retaining said arms against said fitting tapered outer surface, wherein, after placing said bolstering structure or said collar, the two edges of each slot do not overlap each other.