Patent Description:
Many aircraft thermoplastic composite structures are formed using stiffeners or substructures to reinforce or "stiffen" the structure. These stiffeners typically provide the structure with resistance to compression buckling or bending, making the structure desirably less flexible in response to shearing, tensile, or compressive stress, or the like. These stiffeners may be joined to the thermoplastic structure by induction welding. Induction welding utilizes electromagnetic induction to heat a workpiece. An induction coil is energized with an alternating electric current which generates an electromagnetic field that heats a workpiece and binds it to a thermoplastic structure. As a stiffener is laid up in a thermoplastic structure, induction welding can generate eddy currents which may heat the edges of the stiffener and thermoplastic structure at higher than desirable temperatures, which may damage the thermoplastic structure.

<CIT> discloses induction welding a thermoplastic fibre composite stiffener to an underlying thermoplastic fibre composite skin, the stiffener or skin being provided with an embedded metal mesh having Z-pins at the interface so that it interlocks with the other of the two, the welding being conducted by heating the metal mesh using an induction welding tool.

<CIT> discloses a fuselage structure of an aircraft and a method for manufacturing the same.

<CIT> discloses a method for connecting a fiber composite component to a structural component of an aircraft and spacecraft and a corresponding arrangement.

According to an aspect of the present invention, there is provided a method of joining a first workpiece to a second workpiece, in accordance with claim <NUM>.

Optionally, the thermoplastic composite structure may comprise a skin. In various embodiments, the skin may be configured to weld to the second workpiece.

Optionally, induction welding the thermoplastic composite structure to the second workpiece may further comprise induction welding the first thermoplastic composite structure to a stiffener.

Optionally, induction welding the thermoplastic composite structure to the stiffener further comprises the stiffener comprising a stiffener cap. The stiffener may further comprise a stiffener web. In various embodiments, the stiffener may comprise a stiffener flange. In various embodiments, the stiffener flange may be configured to weld to the first thermoplastic composite structure.

Optionally, induction welding the stiffener flange to the first thermoplastic composite structure further comprises the stiffener flange at least partially overlaying the skin of the first thermoplastic composite structure. In various embodiments, the stiffener flange may be configured to weld to the skin of the first thermoplastic composite structure at the welding region.

Optionally, induction welding the stiffener flange to the skin may further comprise energizing the coil. In various embodiments, the coil may be configured to generate the electromagnetic field. In various embodiments, the electromagnetic field may be configured to induce the electric current which heats the stiffener flange and the skin at the welding region. In various embodiments, the heat generated by the coil may be configured to weld, by the induction welding tool, the stiffener flange to the skin at the welding region.

According to another aspect of the present invention, there is provided an induction welding apparatus in accordance with claim <NUM>.

Optionally, the thermoplastic composite structure may comprise a skin.

Optionally, the second workpiece may be a stiffener. In various embodiments, the stiffener may comprise a stiffener cap. The stiffener may further comprise a stiffener web. In various embodiments, the stiffener may comprise a stiffener flange. In various embodiments, the stiffener flange may at least partially overlay the skin of the thermoplastic composite structure.

Optionally, the heat generated by the coil may be configured to weld, by the induction welding tool, the stiffener flange to the skin of the thermoplastic composite structure.

<FIG> illustrates a nacelle <NUM> for a gas turbine engine according to various embodiments. The nacelle for a gas turbine engine may be suitable for an aircraft. The nacelle <NUM> may comprise a centerline A-A' <NUM>. Nacelle <NUM> may comprise an inlet <NUM>, a fan cowl <NUM>, and a thrust reverser <NUM>. The fan cowl <NUM> may comprise a stiffener <NUM> configured to reinforce the fan cowl <NUM>. Nacelle <NUM> may be coupled to a pylon <NUM>, which may mount the nacelle <NUM> to an aircraft wing or aircraft body. A nozzle <NUM> may surround a center body <NUM>, between which an engine exhaust stream exits to provide additional thrust.

Referring to <FIG>, a portion of an aircraft fan cowl <NUM> is illustrated according to various embodiments. In various embodiments, the fan cowl <NUM> may be comprised of a metal material, a thermoplastic, a thermoplastic composite material, or the like. In various embodiments, the fan cowl <NUM> may be a thermoplastic composite structure. In such embodiments, the fan cowl <NUM> may include at least one stiffener <NUM> formed along an inner surface or skin <NUM> of the fan cowl <NUM>. The stiffener <NUM> may be formed along the skin <NUM> according to any desirable orientation, geometry, or symmetry, including, for example, a fully interconnected grid stiffening symmetry. A stiffener <NUM> in a fan cowl <NUM> is described, however, a stiffener <NUM> may be utilized in any other suitable aircraft structure, including for example, a fuselage, a wing, panels, and substructures thereof, and the like. In various embodiments, stiffeners as disclosed herein may be comprised of a thermoplastic, a thermoplastic composite, a metallic material, or the like. Stiffener <NUM> comprises a thermoplastic composite material.

<FIG> illustrates a cross-section of a first workpiece <NUM> at least partially overlayed by a second workpiece <NUM> along an x-x axis in accordance with various embodiments. In various embodiments, and with additional reference to <FIG>, a method (step <NUM>) of joining the first workpiece <NUM> to the second workpiece <NUM> may comprise induction welding (step <NUM>) the second workpiece <NUM> to the first workpiece <NUM>. T he first workpiece <NUM> may be a thermoplastic composite structure comprising a thermoplastic composite skin <NUM> and the second workpiece <NUM> may be a stiffener <NUM>. In various embodiments, stiffeners as disclosed herein may be comprised of a thermoplastic, a thermoplastic composite, a metallic material, or the like. Stiffener <NUM> comprises a thermoplastic composite material. In various embodiments, the stiffener <NUM> may overlay or at least partially overlay the skin <NUM>. In various embodiments, the stiffener <NUM> may comprise a stiffener cap <NUM>, a stiffener web <NUM>, and a stiffener flange <NUM>. The stiffener web <NUM> may be offset from the skin <NUM>, for example, at a substantially <NUM>-degree angle. In various embodiments, the stiffener web <NUM> may be offset from the skin <NUM> at any desirable angle, or any angle suitable for reinforcing a thermoplastic composite structure. Referring to <FIG>, a cross-section of the stiffener flange <NUM> at least partially overlaying the skin <NUM> is shown according to various embodiments. In various embodiments, the stiffener flange <NUM> may be welded to the skin <NUM> at a welding region <NUM>.

In continued reference to <FIG>, welding may involve induction welding the stiffener <NUM> to the skin <NUM> at the welding region <NUM>. Induction welding may generate heat that enables the skin <NUM> to bond to the stiffener <NUM>. In various embodiments, the skin <NUM> may comprise a thermoplastic material, a thermoplastic composite material, or the like. Examples of thermoplastic composite materials for the stiffener <NUM> and skin <NUM> may include fiberglass, carbon fiber, aramid fiber, fiber-reinforced matrix systems, and the like. In various embodiments, the stiffener may comprise a metallic material. Examples of metallic materials for the stiffener <NUM> may include aluminum, aluminum alloys, iron alloys (e.g., various steel alloys) and the like.

In various embodiments, and with additional reference to <FIG>, a thermoplastic composite skin <NUM> may tend to degrade as the heat generated by induction welding reaches the edges <NUM> of a stiffener <NUM>. This may be caused by inconsistent eddy currents generated along the skin <NUM>, as well as inconsistent heat generation at the welding region <NUM>. For example, in various embodiments, the skin <NUM> and stiffener <NUM> may comprise at least one thermoplastic composite ply. In various embodiments, the thickness of the skin <NUM> and stiffener <NUM> may vary, for example, between <NUM> inches (<NUM> millimeters) to <NUM> inches (<NUM> millimeters) in various embodiments, <NUM> inches (<NUM> millimeters) to <NUM> inches (<NUM> millimeters) in various embodiments, <NUM> inches (<NUM> millimeters) to <NUM> inches (<NUM> millimeters) in various embodiments, and <NUM> inches (<NUM> millimeters) to <NUM> inches (<NUM> millimeters) in various embodiments. Stiffeners as disclosed herein may be comprised of a thermoplastic, a thermoplastic composite, a metallic material, or the like. Stiffener <NUM> comprises a thermoplastic composite material.

The composite skin <NUM> and stiffener <NUM>, and associated thickness, may determine the amount of heat associated with joining the skin <NUM> to the stiffener <NUM> using induction welding. Accordingly, every skin <NUM>/stiffener <NUM> configuration requires careful selection of an applied induction welding current, as well as adjustments to welding speed. Thus, the lack of uniform skin <NUM>/stiffener <NUM> thickness and undesirable eddy current generation may result in inconsistent heat generation, increased processing time to modify the induction welding process, and an increased risk of skin <NUM> and/or stiffener <NUM> degradation.

Accordingly, in various embodiments, and in further reference to <FIG>, the method of joining a first workpiece <NUM> to a second workpiece <NUM> includes a second workpiece <NUM> having an embedded metallic strip <NUM> at the welding region <NUM>. In various embodiments, the metallic strip <NUM> may be configured to conduct (step <NUM>) heat uniformly across the welding region <NUM> during induction welding. Accordingly, the metallic strip <NUM> may enable faster and more controlled processing time for a given induction welding process. In various embodiments, the metallic strip <NUM> may prevent eddy current generation in the thermoplastic composite skin <NUM>. The temperature of the induction weld may be governed by the properties of the metallic strip <NUM>, such as, for example, the density of the metallic strip <NUM>. The metallic strip <NUM> is a mesh (i.e., grating). In various embodiments, the mesh <NUM> may interfere with electromagnetic fields across the mesh and prevent a magnetic flux across its surface. This may be a function of the size of the mesh pores, the wavelength of the electromagnetic wave, and the conductivity of the mesh.

In various embodiments where the second workpiece <NUM> is a thermoplastic composite stiffener <NUM> and the first workpiece <NUM> is a thermoplastic composite skin <NUM>, the metallic strip <NUM> may be configured to conduct heat uniformly along the z-z axis of the stiffener flange <NUM> during the induction welding process. Unlike the skin <NUM>/stiffener <NUM> combination, which may have varied thickness and electrical conductivity properties that complicate the induction welding process and increases processing time, the mesh <NUM> may comprise a thickness tailored to the specific needs of an induction welding process. In various embodiments, the mesh <NUM> mesh may be tailored for a specific temperature delivery at the welding region <NUM>.

Moreover, in various embodiments, the mesh <NUM> may be tailored based on conductivity and pore size. Pore geometry in a mesh <NUM> may be of any suitable geometry, such as, for example, a square, rectangle, circle, parallelogram, rhombus, and the like. The pore size may be tailored to be, at least, smaller than the wavelength of a given electromagnetic wave used in the induction welding process. A benefit of using the mesh <NUM> to join the stiffener <NUM> to the skin <NUM> using induction welding is that a weld temperature will depend on the electrical conductivity properties, such as the thickness and pore size of the mesh <NUM>, rather than the properties of the thermoplastic composite skin <NUM> or stiffener <NUM>. Accordingly, the mesh <NUM> may be configured to shield the skin <NUM> from electromagnetic waves and prevent eddy current formation along the skin <NUM>. In various embodiments, the mesh <NUM> may comprise any conductive metallic material. In various embodiments, the mesh <NUM> may comprise copper, aluminum, gold, or silver.

In various embodiments, and with further reference to <FIG>, the mesh <NUM> may be configured to embed in the stiffener flange <NUM> at a surface of the flange <NUM> that is distal to the welding region <NUM>. Accordingly, and in various embodiments, the stiffener flange <NUM> may be prefabricated with the mesh <NUM> prior to induction welding. Moreover, embedding the mesh <NUM> at a surface of the flange <NUM> that is distal to the welding region <NUM> may prevent the mesh <NUM> from becoming a structural component of the bonded materials.

With reference to <FIG>, a cross-section of an induction welding apparatus <NUM> is shown. In various embodiments, the induction welding apparatus <NUM> may comprise an induction welding tool <NUM>. The induction welding tool <NUM> may have an induction welding tool surface <NUM> configured to operate (step <NUM>) on at least one workpiece. As shown, the apparatus <NUM> may further comprise a first workpiece <NUM> and a second workpiece <NUM>. The first workpiece 513is a thermoplastic composite structure. In various embodiments, the second workpiece <NUM> may be a thermoplastic composite stiffener. In various embodiments, the stiffener may comprise an embedded metallic strip <NUM>. The metallic strip <NUM> is a mesh. In various embodiments, the apparatus <NUM> may comprise a coil <NUM> located proximate the thermoplastic composite structure <NUM>.

In various embodiments, the coil <NUM> may be configured to energize (step <NUM>) with an electric current, for example, an alternating electric current. The energized coil <NUM> may be configured to generate (step <NUM>) an electromagnetic field that induces a current which heats (step <NUM>) the apparatus <NUM>. In continued reference to <FIG> and with additional reference to <FIG>, in various embodiments, the heat generated by the coil <NUM> may be configured to enable the induction welding tool <NUM> to weld (step <NUM>) the stiffener <NUM> to the thermoplastic structure <NUM> along the z-z axis at a welding region <NUM>.

In various embodiments, with continued reference to <FIG> and additional reference to <FIG>, the thermoplastic composite structure <NUM> may comprise a skin <NUM>. In various embodiments, the stiffener <NUM> may comprise a stiffener cap <NUM>, a stiffener web <NUM>, and, as shown in <FIG>, a stiffener flange <NUM>. Stiffeners as disclosed herein may be comprised of a thermoplastic, a thermoplastic composite, a metallic material, or the like. Stiffener <NUM> comprises a thermoplastic composite material. As shown in <FIG>, the stiffener flange <NUM> may overlay or at least partially overlay the skin <NUM> of the thermoplastic composite structure <NUM>. In various embodiments, the stiffener flange <NUM> may comprise an embedded mesh <NUM>.

In various embodiments, the heat generated by the coil <NUM> may be configured to weld the stiffener flange <NUM> to the skin <NUM> of the thermoplastic structure <NUM>. In various embodiments, the mesh <NUM> embedded in the stiffener flange <NUM> may be configured to conduct heat uniformly across the welding region <NUM>.

In various embodiments, and with reference to <FIG> and <FIG> which fall outside the wording of the claims, a method (<NUM>) of joining a first workpiece (<NUM> and <NUM>) to a second workpiece (<NUM> and <NUM>) is disclosed herein. In various embodiments, the method of joining the first workpiece (<NUM> and <NUM>) to the second workpiece (<NUM> and <NUM>) may comprise induction welding (step <NUM>) the second workpiece (<NUM> and <NUM>) to the first workpiece (<NUM> and <NUM>). In various embodiments, the first workpiece may be a thermoplastic composite structure (<NUM> and <NUM>) comprising a thermoplastic composite skin (<NUM> and <NUM>) and the second workpiece may be a stiffener (<NUM> and <NUM>). Stiffeners as disclosed herein may be comprised of a thermoplastic, a thermoplastic composite, a metallic material, or the like. Stiffener (<NUM> and <NUM>) comprises a thermoplastic composite material. In various embodiments, the stiffener (<NUM> and <NUM>) may overlay or at least partially overlay the thermoplastic structure <NUM> and skin (<NUM> and <NUM>). In various embodiments, the stiffener (<NUM> and <NUM>) may comprise a stiffener cap <NUM>, a stiffener web <NUM>, and a stiffener flange (<NUM> and <NUM>). In various embodiments, the stiffener flange (<NUM> and <NUM>) may overlay or at least partially overlay the skin (<NUM> and <NUM>).

In continued reference to <FIG> and <FIG>, welding may involve induction welding the stiffener flange <NUM> to the skin <NUM> at the welding region <NUM> using an induction welding tool <NUM> located proximate the stiffener flange <NUM>. The induction welding tool <NUM> may be configured to operate (step <NUM>) on at least one workpiece. The induction welding tool <NUM> may have an induction welding tool surface <NUM>.

Induction welding may generate heat that enables the skin <NUM> to weld to the stiffener flange <NUM>. In various embodiments, a coil <NUM> located proximate the thermoplastic structure <NUM> may be configured to energize (step <NUM>) with an electric current. The energized coil <NUM> may be configured to generate (step <NUM>) an electromagnetic field and magnetic flux across the surface of the thermoplastic structure <NUM> and stiffener <NUM>, inducing a current which heats (step <NUM>) the thermoplastic structure <NUM> and the stiffener <NUM>. In various embodiments, the heat generated by the coil <NUM> may be configured to enable the induction welding tool <NUM> to weld (step <NUM>) the stiffener <NUM> to the thermoplastic structure <NUM> at a welding region <NUM>.

In various embodiments, the method of joining the first workpiece (<NUM> and <NUM>) to the second workpiece (<NUM> and <NUM>) may further comprise a polymer tape (<NUM> and <NUM>) at least partially overlaying the second workpiece (<NUM> and <NUM>). In various embodiments, the polymer tape <NUM> may be disposed between the second workpiece <NUM> and a metallic strip <NUM>. In various embodiments, the metallic strip <NUM> may be a mesh. In various embodiments, the mesh <NUM> may be fixed or otherwise coupled to the induction welding tool surface <NUM>.

In various embodiments, the second workpiece is the stiffener (<NUM> and <NUM>) and the first workpiece is the thermoplastic structure (<NUM> and <NUM>) with skin (<NUM> and <NUM>). The polymer tape (<NUM> and <NUM>) may overlay or at least partially overlay the stiffener flange (<NUM> and <NUM>). Accordingly, the polymer tape <NUM> may be disposed between the stiffener flange <NUM> and the mesh <NUM>. In various embodiments, the polymer tape (<NUM> and <NUM>) may be removable. In various embodiments, the method of joining the first workpiece (<NUM> and <NUM>) to the second workpiece (<NUM> and <NUM>) may further comprise removing (step <NUM>) the polymer tape (<NUM> and <NUM>) from the welding region (<NUM> and <NUM>) after induction welding. In various embodiments, the polymer tape (<NUM> and <NUM>) may comprise any suitable polymer material that resists sticking upon heating, such as, for example, polytetrafluoroethylene (PTFE), polyoxymethylene, polychlorotrifluoroethylene (PCTFE), perfluoroalkoxy (PFA), tetrafluoroethylene-perfluoropropylene (FEP), and the like.

In various embodiments, the mesh <NUM> may be configured to conduct (step <NUM>) heat uniformly across the welding region <NUM> during induction welding. Accordingly, the mesh <NUM> may enable faster and more controlled processing time for a given induction welding process. In various embodiments, the mesh <NUM> may prevent eddy current generation in the thermoplastic composite skin <NUM>. Accordingly, the temperature of the induction weld may be governed by the properties of the mesh <NUM>, such as, for example, the density and pore size of the mesh <NUM>.

As applied to embodiments where the second workpiece (<NUM> and <NUM>) includes a stiffener flange (<NUM> and <NUM>), and the first workpiece (<NUM> and <NUM>) is a thermoplastic composite skin (<NUM> and <NUM>), the metallic strip <NUM> may be configured to conduct heat uniformly along the z-z axis of the stiffener flange (<NUM> and <NUM>) during the induction welding process. In various embodiments, the mesh <NUM> may be tailored based on conductivity. In various embodiments, the mesh <NUM> may comprise any conductive metallic material. In various embodiments, the mesh <NUM> may comprise copper. In various embodiments, the mesh <NUM> may be tailored for a specific temperature delivery at the welding region (<NUM> and <NUM>).

A benefit of using the mesh <NUM> to join the stiffener flange (<NUM> and <NUM>) to the skin (<NUM> and <NUM>) using induction welding is that a weld temperature will depend on the electrical conductivity properties, such as the thickness and pore size, of the mesh <NUM>, rather than the properties of the thermoplastic composite skin (<NUM> and <NUM>) or stiffener flange (<NUM> and <NUM>). Accordingly, the mesh <NUM> may be configured to shield the skin (<NUM> and <NUM>) from electromagnetic waves and prevent eddy current formation along the skin <NUM>.

Claim 1:
A method of joining a first workpiece (<NUM>, <NUM>, <NUM>) to a second workpiece (<NUM>, <NUM>, <NUM>), the method comprising:
induction welding the first workpiece (<NUM>, <NUM>, <NUM>) to the second workpiece (<NUM>, <NUM>, <NUM>), wherein the second workpiece (<NUM>, <NUM>, <NUM>) at least partially overlays the first workpiece (<NUM>, <NUM>, <NUM>), wherein the first workpiece (<NUM>, <NUM>, <NUM>) is a first thermoplastic composite structure and the second workpiece (<NUM>, <NUM>, <NUM>) is a second thermoplastic composite structure, wherein the second workpiece (<NUM>, <NUM>, <NUM>) comprises an embedded metallic strip (<NUM>, <NUM>, <NUM>), wherein the metallic strip (<NUM>, <NUM>, <NUM>) is configured to conduct heat uniformly at a welding region (<NUM>, <NUM>, <NUM>) between the second workpiece (<NUM>, <NUM>, <NUM>) and the first workpiece (<NUM>, <NUM>, <NUM>), wherein the metallic strip (<NUM>, <NUM>, <NUM>) comprises at least one of copper, aluminum, gold, or silver, wherein the metallic strip (<NUM>, <NUM>, <NUM>) is a mesh, and wherein the metallic strip (<NUM>, <NUM>, <NUM>) is embedded at a surface of the second workpiece (<NUM>, <NUM>, <NUM>) that is distal to the welding region (<NUM>, <NUM>, <NUM>),
wherein the induction welding comprises:
locating an induction welding tool (<NUM>) proximate the second workpiece (<NUM>, <NUM>, <NUM>); and
locating a coil (<NUM>) proximate a second side of the first workpiece (<NUM>, <NUM>, <NUM>),
wherein the induction welding is at the welding region (<NUM>, <NUM>, <NUM>), and wherein the induction welding further comprises:
operating the induction welding tool (<NUM>) at the welding region (<NUM>, <NUM>, <NUM>), wherein the induction welding tool (<NUM>) comprises an induction welding tool surface (<NUM>); and
energizing the coil (<NUM>) located proximate the first workpiece (<NUM>, <NUM>, <NUM>), wherein the coil (<NUM>) is configured to energize with an electric current, wherein the energized coil (<NUM>) is configured to generate an electromagnetic field that induces the electric current which heats the first workpiece (<NUM>, <NUM>, <NUM>) and the second workpiece (<NUM>, <NUM>, <NUM>) at the welding region (<NUM>, <NUM>, <NUM>), and wherein the energized coil (<NUM>) is configured to weld, by the induction welding tool (<NUM>), the second workpiece (<NUM>, <NUM>, <NUM>) to the first workpiece (<NUM>, <NUM>, <NUM>) at the welding region (<NUM>, <NUM>, <NUM>).