Patent Description:
High performance composite structures may be fabricated by laying up plies of prepreg or by resin infusion of dry fibers. The fibers may be in unidirectional, woven or braided fabric form. In some applications, in order to reduce layup time, adjacent plies of the fabric may be co-stitched together using stitch material that remains with the completed structure after the structure is cured. Stitching the plies together allows the plies to the laid up on a tool in ply groups, rather than one-by-one, thereby increasing efficiency of the layup process. Co-stitched fabric plies formed from unidirectional reinforcement fibers are relatively formable, making them well-suited for forming highly contoured structures, however structures fabricated using stitched plies may have less than the desired level of strength and crack resistance.

<CIT>, titled "Non-crimp fabrics" in accordance with its abstract states a "method of producing a non-crimp fabric (<NUM>) and a non-crimp fabric comprising a layup of fabric plies (<NUM>) stitched together by a thread (<NUM>), at least part of which is fusible, is provided. The method can include heating the stitched plies (<NUM>) to soften or melt the fusible thread (<NUM>). The use of fusible thread (<NUM>) can act as an in situ binder within the non-crimp fabric (<NUM>) and tension created by the unfused stitching may create channels for matrix resin infusion during manufacture.

Accordingly, there is a need for a method of fabricating composite structures using co-stitched plies of fabric which reduces or eliminates the presence of stitch material in the cured structure. There is also a need for a preform used in the fabrication of such structures that can be assembled using a co-stitched, multi-layer prepreg, or co-stitched fiber layers suitable for resin infusion.

In an aspect there is provided a method of fabricating a composite structure, comprising: stitching (<NUM>,<NUM>,<NUM>), using a thermoplastic resin, prepreg plies (24a,24b,24c) together into a stitched stack (<NUM>) of prepreg plies having varying fiber orientations, the prepreg plies comprising a matrix resin (<NUM>), wherein the matrix resin is a thermoset resin; thermally curing (<NUM>,<NUM>) the stitched stack (<NUM>)of prepreg plies(24a,24b,24c); and melting (<NUM>,<NUM>) the stitching during thermal curing of the stitched stack (<NUM>) of prepreg plies (24a,24b,24c), wherein melting of the stitching during thermal curing occurs after the viscosity of the matrix resin begins to increase and before the stitched stack of prepreg plies is fully cured.

In another aspect there is provided a composite preform (<NUM>), comprising: a stack (<NUM>) of unidirectional prepreg plies(24a,24b,24c) having varying fiber orientations and including a matrix resin (<NUM>), wherein the matrix resin is a thermoset resin; and stitches (<NUM>) passing through all of the prepreg plies (24a,24b,24c) in the stack (<NUM>) and capable of holding the plies (24a,24b,24c) together, wherein the stitches (<NUM>) are formed of a thermoplastic material capable of melting during thermal curing of the prepreg plies (24a,24b,24c) at a temperature above that at which the viscosity of the matrix resin (<NUM>) increases and wherein: the thermoset resin has a cure temperature at which the thermoset resin is fully cured, and the thermoplastic resin has a melt temperature that is below the cure temperature of the thermoset resin.

The disclosure provides a method of fabricating a composite laminate structure using a co-stitched multi-layer.

According to the disclosure, a method is provided of making a composite preform. A stack of prepreg plies is assembled, wherein each of the plies includes reinforcing fibers held in a thermally curable resin matrix. The prepreg plies are stitched together after the stack has been assembled. The stitching is performed using stitching material that melts during thermal curing of the prepreg plies. Assembling the stack of prepreg plies includes maintaining the plies in registration relative to each other by tacking the plies together. Tacking the plies together is performed using tack of the resin matrix in each of the plies. Assembling the stack of prepreg plies includes using the resin matrix in each of the prepreg plies to hold the reinforcing fibers in the plies in spaced relationship to each other during the stitching. The stitching includes placing stitches substantially completely through the thickness of the stack of prepreg plies. The stack may be assembled by laying prepreg tows, and the stitching may be carried out by placing stitches between the prepreg tows that pass substantially completely through the stack of prepreg plies. During assembly of the stack, the plies are oriented such that they have differing fiber orientations.

According to the disclosure, a composite preform is provided. The preform comprises a stack of unidirectional prepreg plies having varying fiber orientations. Stitches passing through all of the prepreg plies in the stack hold the plies together. The stitches are formed of a stitching material capable of melting during thermal curing of the prepreg plies. Each of the prepreg plies includes prepreg tows, and the stitches pass between the prepreg tows. The stitches may be distributed generally uniformly across the stack of unidirectional prepreg plies. Each of the prepreg plies includes a resin matrix, and the stitching material is compatible with the resin matrix. The resin matrix may be a thermoset resin, and the stitching material may be a thermoplastic resin. The thermoset resin has a cure temperature at which the thermoset resin is fully cured, and the thermoplastic resin has a melt temperature that is below the cure temperature of the thermoset resin.

In summary, according to the disclosure, there is provided a method of fabricating a composite structure, including stitching prepreg plies together into a stitched stack of prepreg plies having varying fiber orientations; thermally curing the stitched stack of prepreg plies; and melting the stitching during thermal curing of the stitched stack of prepreg plies.

Optionally, the method further includes assembling the prepreg plies into a stack, wherein each of the prepreg plies has resin tack, and assembling the prepreg plies into the stack includes using the resin tack to adhere the plies together and maintain the fiber orientations of the prepreg plies during the stitching.

Optionally, the stack has a thickness, and stitching the prepreg plies together is performed by using stitches that pass substantially through the thickness of the stack.

Optionally, assembling the prepreg plies into a stack includes laying down prepreg tows, and varying the fiber orientations of the tows for each of the plies.

Optionally, the method further includes debulking, consolidating and curing the stitched stack under a vacuum.

Optionally, melting the stitching is performed before the stitched stack of prepreg plies is fully cured.

Optionally, the method further includes forming the stitched stack of prepreg plies into a desired shape corresponding to the shape of the composite structure.

Optionally, forming the stitched stack of prepreg plies is performed before stitching the prepreg plies together.

Optionally, forming the stitched stack of prepreg plies is performed after stitching the prepreg plies together.

According to the disclosure, there is provided a method of making a composite preform, including assembling a stack of prepreg plies, each of the plies including reinforcing fibers held in a thermally curable resin matrix; and, stitching the prepreg plies together after the stack has been assembled, wherein the stitching is performed using stitching material that melts during thermal curing of the prepreg plies.

Optionally, assembling the stack of prepreg plies includes maintaining the plies in registration relative to each other by tacking the plies together.

Optionally, tacking the plies together is performed using tack of the resin matrix in each of the plies.

Optionally, assembling the stack of prepreg plies includes using the resin matrix in each of the prepreg plies to hold the reinforcing fibers in the plies in spaced relationship to each other during the stitching.

Optionally, the stack of prepreg plies has a thickness, and the stitching includes placing stitches substantially completely through the thickness of the stack of prepreg plies.

Optionally, assembling the stack of prepreg plies includes laying prepreg tows, and the stitching includes placing stitches between the prepreg tows substantially completely through the stack of prepreg plies.

Optionally, assembling the stack of prepreg plies includes orienting the plies such that they have differing fiber orientations.

According to the disclosure there is provided a composite preform, including a stack of unidirectional prepreg plies having varying fiber orientations; and stitches passing through all of the prepreg plies in the stack and capable of holding the plies together, wherein the stitches are formed of a stitching material capable of melting during thermal curing of the prepreg plies.

Optionally, each of the prepreg plies includes prepreg tows.

Optionally, the stitches pass between the prepreg tows.

Optionally, the stitches are distributed generally uniformly across the stack of unidirectional prepreg plies.

Optionally each of the prepreg plies includes a resin matrix, and the stitching material is compatible with the resin matrix.

The resin matrix is a thermoset resin, and the stitching material is a thermoplastic resin.

The thermoset resin has a cure temperature at which the thermoset resin is fully cured, and the thermoplastic resin has a melt temperature that is below the cure temperature of the thermoset resin.

The features, functions, and advantages can be achieved independently in various embodiments of the present disclosure or may be combined in yet other embodiments in which further details can be seen with reference to the following description and drawings.

The illustrative embodiments, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment of the present disclosure when read in conjunction with the accompanying drawings, wherein:.

Referring to <FIG>, a composite preform <NUM> comprises a stitched stack <NUM> of prepreg plies 24a, 24b, 24c each of which has unidirectional reinforcement in the form of fiber tows <NUM>. The prepreg plies 24a, 24b, 24c in the stack <NUM>, sometimes also referred to herein as "layers", are tacked together by stitches <NUM> that extend through the thickness of the stack <NUM>. Only the top and bottom of the stitches <NUM> are shown, respectively, in <FIG>. The preform <NUM> may be used to fabricate any of a variety of composite structures, particularly those having simple or compound contours. For example, referring to <FIG>, the preform <NUM> may be employed to fabricate a unitary composite frame section <NUM> by forming the stack <NUM> of prepreg plies 24a, 24b, 24c using suitable tooling (not shown), either before or after the prepreg plies 24a, 24b, 24c are stitched together. In this example, the frame section <NUM> is curved along its length and comprises a curved inner chord flange <NUM>, a curved outer chord flange <NUM> and a web <NUM>. The flanges <NUM>, <NUM> transition into the web <NUM> along radius corners <NUM>, <NUM> which have compounded curvatures. The frame section <NUM> is merely illustrative of a wide range of composite laminate structures that may be fabricated using the disclosed preform <NUM>. The frame section <NUM> shown in <FIG> has a Z-shaped cross section, however other cross-sectional shapes are possible.

Referring particularly to <FIG> and <FIG>, although three plies 24a, 24b, 24c are shown in the exemplary embodiment, the stack <NUM> may comprise as few as two or greater than three plies <NUM>, depending upon the application. In the embodiment illustrated in <FIG> and <FIG>, each of the plies 24a, 24b, 24c comprises a plurality of unidirectional prepreg tows <NUM> that may be placed in multiple, side-by-side bandwidths (not shown) by automatic fiber placement equipment (not shown) or by other techniques. However, as will be discussed later, in comparative examples the stitched stack <NUM> may comprise a stitched stack of dry fiber layers 24a, 24b, 24c of unidirectional dry fibers that may be in the form of tows, unidirectional tape, cut patterns of unidirectional reinforcement or other forms.

The prepreg tows <NUM> each comprise a bundle of individual reinforcing fibers (not shown) that is pre-impregnated with a suitable resin which will be discussed later in more detail. Each of the plies 24a, 24b, 24c may have any desired fiber orientation, but in the illustrated example shown in <FIG>, respectively have <NUM>°, <NUM>° and <NUM>° fiber orientations. In one embodiment, the prepreg tows <NUM> may have a generally circular cross-sectional shape (see <FIG>), while in another embodiment, the prepreg tows <NUM> may have a generally flat cross-sectional shape (not shown), sometimes referred to as a "flat tow" or a "spread tow".

The resin used to impregnate the tows <NUM> comprises a thermally curable resin that is suitable for the application and has a desired cure temperature. For example and without limitation, the reinforcing fibers may comprise carbon and the resin used as the matrix may comprise a thermally curable thermoset resin such as epoxy. Other types of reinforcing fibers are possible, such as without limitation, metal, ceramic and/or glass fibers. Other types of resins may be employed as the matrix, depending upon the application, such as, without limitation polyester resins, vinyl ester resins, phenolic resins, polyimide resins, PBI (polybenzimidazole) resins, and BMI (bismaleimide) resins.

The presence of resin impregnated into the tows <NUM> causes the tows <NUM>, and thus the plies 24a, 24b, 24c to have resin tack, and this resin tack causes the plies 24a, 24b, 24c to adhere to each other when they are laid up on top of each other. The adhesion provided by the resin tack holds the plies 24a, 24b, 24c in registration with each other and in their desired ply orientations during subsequent processing discussed below in more detail. The matrix resin also holds the tows <NUM> of the plies <NUM> in spaced relationship to each other through the thickness "t" of the stack <NUM>. In some applications, it may be necessary or desirable to apply a tackifier to the plies 24a, 24b, 24c to increase the adhesion between the plies 24a, 24b, 24c. Similarly, where the tows <NUM> are dry (not impregnated with resin), a tackifier, sometimes referred to as a binder, may be used to adhere the layers 24a. 24b, 24c together and maintain their respective fiber orientations until the stitched stack <NUM> can be formed into a desired shape.

The stitches <NUM> pass between the tows <NUM> and hold the plies 24a, 24b, 24c in their desired ply orientations. The number, density, size, spacing and type of the stitches <NUM> used will depend upon the application. Similarly, the tightness of the stitches <NUM> may vary, depending upon the number of plies <NUM> in the stack <NUM> and the complexity of the composite structure being fabricated. For example, where the composite structure is highly contoured, it may be desirable to employ relatively loose stitches <NUM> in order to allow the plies 24a, 24b, 24c to slip slightly in-plane relative to each other as they are being formed over tooling. Slight in-plane slippage between the plies 24a, 24b, 24c may allow the stack <NUM> to better conform to contoured tool surfaces and avoid ply wrinkling and/bunching.

Referring now particularly to <FIG>, any of various types of stitches <NUM> may be employed to stitch the plies 24a, 24b, 24c together provided that the stitches <NUM> pass through substantially the entire thickness "t" (<FIG>) of the stack <NUM>, between any adjacent tows <NUM> in each of the plies 24a, 24b, 24c. In the illustrated embodiment, the stitches <NUM> are effectively looped around the tows <NUM>, and extend diagonally across the stack <NUM>. However, in other embodiments, the stitches <NUM> may not be looped around all of the tows <NUM>, and may extend in any direction across the stack <NUM>. The stitches <NUM> may be formed and spaced apart from each other in any of a variety of manners, providing that they adequately hold the plies 24a, 24b, 24c together as the stack <NUM> is being formed over tooling (not shown) employed to shape the stack <NUM> into the desired shape of the composite structure. In some embodiments, however, it may be possible to stitch the plies 24a, 24b, 24c together after the stack <NUM> has been formed into a desired shape.

The material from which the stitches <NUM> is formed (hereinafter "stitch material") may comprise any of a variety of polymer resins that is compatible with the matrix resin of the tows <NUM>, and which has a melt temperature that results in melting of the stitches <NUM> during thermal curing of the matrix resin. For example, the stitch material may comprise a thermoplastic resin such as, without limitation, PEI (polyetherimide) PPS (polyphenylene sulphide), PES (polyethersulfone), PEEK (polyetheretherketone), PEKK (polyetheretherketone), and PEKK-FC (polyetherketoneketone-fc grade), which has a relatively low melt temperature that is within the range of temperatures required to cure the matrix resin. For example, where the matrix resin is an epoxy that cures at approximately <NUM>, the stitch material may comprise a thermoplastic resin having a low melt temperature in the range of <NUM>. In this example, the thermoplastic resin melts and combines with the flowable thermoset resin before the thermoset resin begins to substantially cure and harden. In one embodiment, a thermoplastic stitch material is selected which remains intact to provide the necessary support of the plies 24a, 24b, 24c, 24d as the matrix resin melts and initially becomes flowable. According to the invention, the invention melting of the stitching during thermal curing comprises melting and dissolving the stitching into the matrix resin after the viscosity of the matrix resin begins to increase, as the matrix resin begins harden during an initial stage of curing, and before the stitched stack of prepreg plies is fully cured. Consolidation of the composite laminate structure is accomplished under vacuum which is used to debulk the plies 24a, 24b, 24c and hold the plies 24a, 24b, 24c together without movement while the stitches <NUM> melt into the resin and the structure cures-consolidates.

<FIG> illustrates a cross-sectional side view of one of the stitches <NUM> during an early stage of a cure cycle in which the formed composite laminate structure is cured and consolidated by subjecting it to heat and pressure applied by a vacuum bag and/or an autoclave. The combination of applied heat and pressure causes the matrix resin <NUM> to begin to flow, and consolidate the plies 24a, 24b, 24c. The resin flow comes from the matrix resin <NUM> that is impregnated into the tows <NUM>. At this point in the cure cycle, the stitches <NUM> have not yet been heated to their melt temperature, and therefore remain intact. As the temperature is further increased during the cure cycle however, the stitch material begins to melt and flow <NUM> into the surrounding matrix resin <NUM> which is still flowable, until, as shown in <FIG>, the stitch material is fully dissolved within regions <NUM> of the matrix resin <NUM>. The applied pressure aids in causing the stitch material and the matrix resin <NUM> to flow together and mix with each other. Depending upon the particular polymer resin selected for use as stitches <NUM>, the dissolved stitch material may assist in toughening the matrix resin <NUM>, and may increase mechanical properties, such as impact resistance, of the cured composite structure.

Attention is now directed to <FIG> which broadly illustrates the overall steps of a method of fabricating a composite laminate structure using a stitched prepreg. Beginning at step <NUM>, a stack <NUM> of unidirectional prepreg plies is assembled wherein plies may have varying fiber orientations. Then at step <NUM>, after the prepreg plies having been assembled into a stack <NUM>, the stack <NUM> may be formed into a desired shape using tooling or other forming techniques. In some embodiments, however it may be possible to layup the preform <NUM> in a particular stack shape and then stitch the plies of the preform <NUM> together. In other words, the stitching of step <NUM> may be carried out after the forming of step <NUM>. At step <NUM>, the stitched and formed stack <NUM> is thermally cured, as by placing the stack <NUM> into an oven or an autoclave. At <NUM>, during thermal curing of the stitched stack <NUM>, the stitching material that melts, causing the stitches to dissolve into the surrounding matrix resin <NUM> undergoing curing.

<FIG> broadly illustrates the overall steps of a method of making a prepreg preform <NUM> using prepreg plies that are stitched together with stitching material that melts during subsequent curing of the prepreg. At step <NUM>, a stack <NUM> of prepreg plies is assembled. Each of the plies includes reinforcing fibers held in a thermally curable matrix resin <NUM>. At step <NUM>, the prepreg plies are stitched together after the stack has been assembled, using a stitching material that melts and dissolves during thermal curing of the prepreg plies.

Referring again to <FIG>, as previously mentioned, in embodiment comparative example, the preform <NUM> may be a dry fiber preform suitable for use in any of various types of resin infusion processes in which the preform serves as a reinforcement that is infused with resin. In this example, the preform <NUM> comprises a stitched stack <NUM> of the layers 24a, 24b, 24c, each of which is formed by a unidirectional dry fiber reinforcement such as fiber tows <NUM> (<FIG>) or unidirectional dry fiber tape.

The layers 24a, 24b, 24c have varying fiber orientations relative to each other. The fiber tows <NUM> used in the dry fiber preform <NUM> may comprise one or more materials similar to the materials discussed above that may be used to produce the fiber tows <NUM> of the prepreg embodiment of the preform <NUM>. The dry fiber layers 24a, 24b, 24c are temporarily stitched together by stitches <NUM> (<FIG>) that pass completely through the thickness "t" (<FIG>). The stitches <NUM> hold the layers 24a, 24b, 24c together as a preform, but may be lose enough to allow the layers 24a, 24b, 24c to slip slightly relative to each other when the preform <NUM> is formed down onto contoured surfaces of a tool (not shown) used in a resin infusion process. As previously mentioned, in some comparative examples, the dry fiber layers 24a, 24b, 24c may be formed into a desired shape before the dry fiber layers 24a, 24b, 24c are stitched together into a preform <NUM>.

The stitches <NUM> assist in holding the layers 24a, 24b, 24c in their desired orientations and in spaced apart relationship to each other as the preform <NUM> is debulked, consolidated and infused with resin. By maintaining the dry fiber layers 24a, 24b, 24c in their desired orientations and spatial relationships until the matrix resin begins to harden with the onset of curing, the reinforcement of the cured composite structure may be more uniformly distributed and therefore contribute to improving the mechanical performance of the composite structure.

As in the previous prepreg preform <NUM> example, the material from which the stitches <NUM> are formed may comprise any of a variety of polymer resins that is compatible with the matrix resin used to resin infuse the preform <NUM> after it has been placed on a tool. The stitch material has a melt temperature that results in melting of the stitches <NUM> during thermal curing of the matrix resin following resin infusion of the dry fiber preform <NUM>. For example, the stitch material used to stitch the layers 24a, 24b, 24c together as a dry fiber preform <NUM> may comprise a thermoplastic resin such as, without limitation, PEI (polyetherimide) PPS (polyphenylene sulphide), PES (polyethersulfone), PEEK (polyetheretherketone), PEKK (polyetheretherketone), and PEKK-FC (polyetherketoneketone-fc grade), which has a relatively low melt temperature that is within the range of temperatures required to cure the matrix resin used in a resin infusion process.

<FIG> broadly illustrates the steps of a method of fabricating a composite structure using resin infusion of a dry fiber preform <NUM>. Beginning at step <NUM>, a stack of unidirectional dry fiber layers 24a, 24b, 24c is assembled, in which the layers have varying fiber orientations. At step <NUM>, optionally, a tackifier may be applied to the layers 24a, <NUM>, 24c in order to assist in maintaining their respective fiber orientations. At <NUM>, the dry fiber layers 24a, 24b, 24c are stitched together into a stitched stack <NUM>, after the stack <NUM> has been assembled. The stitches <NUM> hold the layers 24a 24b, 24c of the stack <NUM> together. At <NUM>, the stitched stack <NUM> of dry fiber layers may be formed into a desired preform shape.

Forming the stack <NUM> may be performed by forming the stack <NUM> onto tooling, either before or after the stack <NUM> has been stitched. Where the stack <NUM> is stitched before it is formed to a desired shape, and the tooling has one or more contours, the stitching <NUM> may allow the dry fiber layers 24a 24b, 24c to slip slightly relative to each other in order to better allow the layers to conform to contoured surfaces of the tool. Depending upon the type of resin infusion process being used, the dry fiber preform <NUM> may be transferred to a resin infusion tool at step <NUM>. In some embodiments, the tool on which the dry fiber layers 24a, 24b, 24c are formed into the shape of the preform <NUM> may be the tool that is used during the resin infusion process. At step <NUM>, the dry fiber preform <NUM> is infused with resin, and at <NUM>, the resin is thermally cured. The stitches <NUM> assist in holding the layers 24a, 24b, 24c in their desired orientations and in spaced apart relationship to each other as the preform <NUM> is debulked, consolidated and infused with resin. At step <NUM>, the stitching <NUM> that is used to hold the layers of the preform <NUM> together, melts and dissolve into the resin used to infuse the preform <NUM>.

Embodiments of the disclosure may find use in a variety of potential applications, particularly in the transportation industry, including for example, aerospace, marine, automotive applications and other application where composite laminate structures, particularly those that are contoured and are fabricated in relatively high volume. Thus, referring now to <FIG>, embodiments of the disclosure may be used in the context of an aircraft manufacturing and service method <NUM> as shown in <FIG> and an aircraft <NUM> as shown in <FIG>. Aircraft applications of the disclosed embodiments may include, for example, without limitation, composite laminate frame sections, spars, stringers and beams, to name only a few. During pre-production, exemplary method <NUM> may include specification and design <NUM> of the aircraft <NUM> and material procurement <NUM>. During production, component and subassembly manufacturing <NUM> and system integration <NUM> of the aircraft <NUM> takes place. Thereafter, the aircraft <NUM> may go through certification and delivery <NUM> in order to be placed in service <NUM>. While in service by a customer, the aircraft <NUM> is scheduled for routine maintenance and service <NUM>, which may also include modification, reconfiguration, refurbishment, and so on.

As shown in <FIG>, the aircraft <NUM> produced by exemplary method <NUM> may include an airframe <NUM> with a plurality of systems <NUM> and an interior <NUM>. Examples of high-level systems <NUM> include one or more of a propulsion system <NUM>, an electrical system <NUM>, a hydraulic system <NUM> and an environmental system <NUM>. Any number of other systems may be included. Although an aerospace example is shown, the principles of the disclosure may be applied to other industries, such as the marine and automotive industries.

Systems and methods embodied herein may be employed during any one or more of the stages of the production and service method <NUM>. For example, components or subassemblies corresponding to production process <NUM> may be fabricated or manufactured in a manner similar to components or subassemblies produced while the aircraft <NUM> is in service. Also, one or more apparatus embodiments, method embodiments, or a combination thereof may be utilized during the production stages <NUM> and <NUM> , for example, by substantially expediting assembly of or reducing the cost of an aircraft <NUM>. Similarly, one or more of apparatus embodiments, method embodiments, or a combination thereof may be utilized while the aircraft <NUM> is in service, for example and without limitation, to maintenance and service <NUM>.

Claim 1:
A method of fabricating a composite structure, comprising:
stitching (<NUM>,<NUM>,<NUM>), using a thermoplastic resin, prepreg plies (24a,24b,24c) together into a stitched stack (<NUM>) of prepreg plies having varying fiber orientations, the prepreg plies comprising a matrix resin (<NUM>), wherein the matrix resin is a thermoset resin;
thermally curing (<NUM>,<NUM>) the stitched stack (<NUM>) of prepreg plies(24a,24b,24c); and
melting (<NUM>,<NUM>) the stitching during thermal curing of the stitched stack (<NUM>) of prepreg plies (24a,24b,24c), wherein melting of the stitching during thermal curing comprises melting and dissolving the stitching into the matrix resin after the viscosity of the matrix resin begins to increase, as the matrix resin begins to harden during an initial stage of curing, and before the stitched stack of prepreg plies is fully cured.