Patent Description:
The term "composite", as used herein, may refer to a composite structure comprising fibers such as carbon fibers set in a matrix such as a thermoset resin.

Composite laminate parts may be joined together by adhesive bonding, co-bonding or co-curing. In the case of adhesive bonding and co-bonding, at least one of the parts requires adequate surface preparation and proper adhesive selection in order to achieve the desired bonding quality. The need for surface preparation and elimination of surface contaminants increases manufacturing costs, and the use of bonding adhesives adds undesired weight to part assemblies. Manufacturing costs are also increased by the need to provide tooling for supporting the uncured parts during the joining process. These issues reduce the options available for efficient staging of manufacturing processes. It would therefore be desirable to provide more alternatives for assembly, joining and/or curing, and process sequencing.

<CIT>, in accordance with its abstract, states that from <NUM> to <NUM>/m2, in particular from <NUM> to <NUM>/m2 of a pulverulent reactive hot-melt composition including a prepolymer and a hardener for the latter are applied onto one or two faces of textile reinforcement. The hardener becomes active only at a temperature above that at which the powder softens to become tacky. <CIT> also states, in accordance with its description, "The reinforcement to be treated. is fed in the form of a strip. It runs under an applicator roll. The powder. thus entrained by the roll. is applied over the whole width of the reinforcement. The reinforcement coated with powder in this way then runs under an infrared rack. which softens the powder and fastens it to the surface. of the reinforcement in the form of points. constituting of the prepolymer-hardener system".

<CIT>, in accordance with its abstract, states that a composite structure is fabricated by staging at least a portion of an uncured, first composite component. The first composite component is assembled with a second composite component, and the staged portion of the first composite component is cocured with the second composite component.

Described herein is a method of making a composite structure, the method comprising: applying a resin curing accelerator to selected regions of fiber reinforced, thermoset resin plies; assembling a layup including the thermoset resin plies to which the resin curing accelerator has been applied; arranging the plies of the layup such that other regions of the layup that are left uncured are located along an interfacial region between the partially cured part and a structure to which the partially cured part is to be joined; forming the layup to a desired shape; producing a partially cured part by curing the selected regions of the thermoset plies having the resin curing accelerator applied thereto, while leaving the other regions of the thermoset plies uncured; placing the partially cured part against the structure; and joining the partially cured part to the structure by co-bonding or cocuring.

Also described herein is a composite laminate part layup, comprising: a plurality of fiber reinforced, thermoset resin plies, wherein at least certain of the thermoset resin plies have a resin curing accelerator applied to at least a region thereof; wherein the layup includes an interfacial region configured to be joined to a structure, and wherein the thermoset plies of the layup along the interfacial region are devoid of the resin curing accelerator.

The disclosure relates in general to manufacturing composite parts, and more specifically to joining composite laminate thermoset parts that are partially cured. A resin curing accelerator in the form of a catalyst is applied to portions of part layups that are to be precured. The catalyst accelerates curing of these portions at low cure temperatures, while leaving other portions of the parts uncured and therefore chemically reactive. Uncured portions of the parts can be brought into contact with each other and co-cured, while the cured portions of the parts provide the parts with stability. The catalyst can be strategically applied to certain regions of one or more plies before they are laid up, resulting in a part that has tailored curing.

According to one example, a method is provided of making a composite structure. The method comprises assembling a layup, including laying up a plurality of plies of fiber reinforced thermoset resin, and applying a resin curing reaction accelerator to a first portion of the layup. The method further comprises curing the first portion of the layup using a cure schedule that is sufficient to cure the first portion of the layup while maintaining a second uncured portion of the layup uncured.

According to another example, a method is provided of making a composite structure. The method comprises applying a resin curing accelerator to selected regions of fiber reinforced, thermoset resin plies, and assembling a layup including the thermoset resin plies to which the resin curing accelerator has been applied. The method further includes forming the layup to a desired shape, and producing a partially cured part by curing the selected regions of the thermoset plies having the resin curing accelerator applied thereto, while leaving other regions of the layup uncured. The method also includes placing the partially cured layup against a structure, and joining the partially cured part to the structure.

According to still another example, a composite laminate part layup is provided, comprising a plurality of fiber reinforced, thermoset resin plies, wherein at least certain of the thermoset resin plies have a resin curing accelerator applied to at least a region thereof.

One of the advantages of the disclosed examples is that the need for extensive surface preparation and bonding adhesives of parts to be joined may be eliminated. Another advantage is that tooling costs required for maintaining part shapes can be substantially reduced or eliminated. A further advantage is that specialized equipment is not required to achieve differential heating/cooling of different portions of the parts. Partial curing of parts can be performed using conventional equipment. Still another advantage is that partially cured composite laminate parts can be produced in which cured portions of parts support and maintain the shape of the uncured portions without the need for underlying support tooling or fixtures.

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

The novel features believed characteristic of the illustrative examples are set forth in the appended claims. The illustrative examples, 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 example of the present disclosure when read in conjunction with the accompanying drawings, wherein:.

Attention is now directed to <FIG> which diagrammatically illustrate an apparatus and method for strategically applying a resin curing accelerator <NUM> to selected regions of the plies <NUM> that allows partially cured parts <NUM>, <NUM> to be produced having cured and uncured portions <NUM>, <NUM> (<FIG>) respectively. The resin curing accelerator <NUM> comprises a reaction catalyst <NUM> that causes the resin to which it is applied to cure at a temperature lower than the normal cure temperature and/or more quickly, compared to regions of the plies <NUM> to which the catalyst <NUM> has not been applied. In other words, strategic application of the catalyst <NUM> to regions <NUM> of the plies <NUM> allows differential curing of the parts <NUM>, <NUM>.

Sheet material <NUM>, which may comprise either a prepreg or dry fibers, is drawn from a bulk supply reel <NUM> and guided through one or more rollers <NUM> past a catalyst applicator <NUM>. The catalyst applicator <NUM>, which in the illustrated example is an automated applicator operated in accordance with a set of programmed instructions accessed by a programmed controller <NUM>, applies a suitable reaction catalyst <NUM> to preselected regions <NUM> of the sheet material <NUM>, either by spraying, printing or using other techniques. The catalyst <NUM> is strategically applied to preselected regions of the plies <NUM> that are intended to be precured. Optionally, following the application of the catalyst <NUM>, a catalyst stabilizer <NUM> may be applied to the catalyst <NUM> by a stabilizer applicator <NUM> in order to stabilize the catalyst <NUM>. The catalyst stabilizer <NUM> functions to prevent potential degradation of the catalyst <NUM>. Sheet material <NUM> is then cut to the desired ply lengths by a ply cutter <NUM> before being delivered onto a table <NUM> or other support where they may be picked up manually or by a pick-and-place machine (not shown) to form a ply layup.

While a largely automated process for strategic application of the catalyst <NUM> to selected regions of plies <NUM> has been described, a primarily manual process is possible. For example, plies <NUM> can be cut from bulk prepreg material, and the catalyst <NUM> can then be applied by hand to the desired regions of the plies <NUM> using spraying, brushing or other manual techniques, following which the plies <NUM> can be manually stacked according to a desired ply schedule. Alternatively, the plies <NUM> can be manually transported and laid up by hand on a layup tool (not shown) that is used to shape the plies <NUM> prior to precuring in which the regions of the plies <NUM> having the catalyst <NUM> applied thereto are cured at a temperature lower than the normal cure temperature. Alternately, the catalyst <NUM> may be applied, either manually or under automatic control, to the individual plies <NUM> after the ply <NUM> is placed on the layup tool <NUM> (<FIG>).

<FIG> illustrates four typical plies <NUM> showing the catalyst <NUM> having been strategically applied only to certain regions <NUM> of the plies <NUM> that are intended to be precured, while leaving the remaining regions <NUM> intended to remain uncured devoid of the catalyst <NUM>.

Attention is now directed to <FIG> which illustrates how the acceleration catalyst <NUM> can be strategically applied to plies <NUM> of either thermoset prepreg or dry fiber that is later impregnated with a thermoset resin to form a differentially cured part <NUM>, <NUM> having cured and uncured portions <NUM>, <NUM> respectively. The thermoset resin may comprise, for example and without limitation, epoxy resins, cynate ester resins, polyurethane resins, or phenolic resins. The catalyst <NUM> should be suitable for the selected thermoset resin. For example, <NUM>,<NUM><NUM>-methylene-bis <NUM>-chloro-<NUM>,<NUM>-Diethylaniline can be used as the catalyst for cynate ester resins. In this example, the catalyst <NUM> is applied on the left half of the plies <NUM>, following which the plies <NUM> are stacked to form a ply layup <NUM>. At <NUM>, the layup <NUM> is heated according to a cure schedule that is sufficient to substantially cure the regions <NUM> that are to be precured but is insufficient to cure the remaining regions <NUM> that are to remain uncured and therefore chemically active. In the case of dry fiber plies <NUM>, the plies <NUM> are infused with a thermoset resin, and then heated to a temperature that is sufficient to cure those regions <NUM> of the plies <NUM> to which the catalyst <NUM> has been applied.

From the foregoing, it can be appreciated that differential curing of the parts <NUM>, <NUM> can be achieved using to different cure schedules. For example, the first cure schedule used to achieve partial curing of the part <NUM>, <NUM> may be performed at a lower temperature and/or shorter duration, compared to a second cure schedule that is used to fully cure the part <NUM>, <NUM>, during co-curing or co-bonding with another structure. In the case of epoxy resin, for example and without invitation, the first cure schedule may include heating the layup of parts <NUM>, <NUM> to a temperature of approximately <NUM>, while second cure schedule may heating the parts <NUM>, <NUM> to a temperature higher than <NUM>. Once the part <NUM>, <NUM> is bought into contact with another part <NUM>, <NUM>, the two parts <NUM>, <NUM> may be co-cured or co-bonded according to a cure schedule, as described above, that is different than the cure schedule used to partially cure the part <NUM>, <NUM>.

Referring first to <FIG>, a composite structure <NUM> comprises two composite laminate parts <NUM>, <NUM> joined together along an overlapping area <NUM> at an interfacial region <NUM> forming a lap joint. The parts <NUM>, <NUM> shown in <FIG> are joined together along an interfacial region <NUM> either by co-bonding or co-curing. As will be discussed below in more detail, prior to being fully cured as shown in <FIG>, at least one of the parts <NUM>, <NUM> is in a partially cured state, and includes an uncured portion <NUM> (<FIG>) along the interfacial region <NUM> that is achieved by differential curing of the part <NUM>, <NUM>, using the strategic application of a resin curing accelerator <NUM> (<FIG>) to the piles of the part <NUM>, <NUM> in the form of a reaction catalyst <NUM> (<FIG>).

<FIG> illustrates one example of the composite structure <NUM> shown in <FIG> in which the two parts <NUM>, <NUM> are joined together in the overlapping area <NUM> to form a lap joint <NUM>, by co-curing the two parts <NUM>, <NUM>. In this example, each the parts <NUM>, <NUM> includes a cured portion <NUM> and an uncured portion <NUM>. As used herein, "cured portion" means that the cured portion has been cured to a degree that maintains the shape and stability of the part <NUM>, <NUM>. In some applications, the cured portion <NUM> may be fully cured, while in other applications, the cured portion <NUM> may be cured to a greater degree than the uncured portion <NUM>, but nevertheless cured to a point that maintains the overall shape and stability of the part <NUM>, <NUM>. Also, as used herein, "uncured portion" means that the uncured portion remains chemically reactive and capable of being co-cured with another uncured part or bonding adhesive. While in the illustrated examples, the parts <NUM>, <NUM> are substantially flat, in other examples, the parts <NUM>, <NUM> may have three dimensional curves or complex contours resulting from at least partial curing of the parts <NUM>, <NUM> in shaped tooling (not shown) suitable for the application. As will be discussed below, precuring regions of the parts <NUM>, <NUM> having the resin curing accelerator <NUM> applied thereto results in a partially cured part <NUM>, <NUM> that maintains the shape imparted to it by the tooling, in spite of the fact that other regions of the part <NUM>, <NUM> remain uncured. The ability of the partially cured part to maintain its shape allows the part to be handled, transported and/or joined to a structure by co-bonding or co-curing, without the need for supporting tooling or fixtures.

In the example shown in <FIG>, the plies <NUM> of the parts <NUM>, <NUM> throughout their thicknesses <NUM>, <NUM> within the overlapping area <NUM> are uncured prior to being joined together and co-cured. The portions <NUM> of the plies <NUM> outside of the overlapping area <NUM> are precured prior to the joining process as a result of a curing accelerator <NUM> (see <FIG>) having been applied thereto which allows these portions <NUM> to be precured at a temperature lower than normal cure temperature, while maintaining other portions <NUM> uncured. The uncured portions <NUM> of the parts <NUM>, <NUM> that overlap each other have been brought into contact with each other along an interfacial region <NUM>. When subjected to a full cure process explained later, the uncured portions <NUM> of the two parts <NUM>, <NUM> are cocured, joining them together along the interfacial region <NUM>.

Referring now to <FIG>, in this example, only a few of the plies 44a within the thicknesses <NUM>, <NUM> immediately adjacent to the interfacial region <NUM> within the overlapping area <NUM> are uncured. The cured portions <NUM> may be cured at a lower-than-normal cure temperature due to the resin curing accelerator <NUM> having been applied thereto, while those plies 44a along the interfacial region <NUM> which do not have an application of the resin curing accelerator <NUM> remain uncured. From the foregoing, it may be appreciated that strategic application of the resin curing accelerator <NUM> allows precuring of any selected region e.g. overlapping area <NUM> of any ply <NUM> within the thickness <NUM>, <NUM> of the part <NUM>, <NUM>, while the remaining plies 44a within the same region remain uncured. Thus, it is apparent that the two parts <NUM>, <NUM> may be joined together by co-curing only a few of the uncured plies 44a along the interfacial region <NUM>, while the cured portions <NUM> that have been precured using the resin curing accelerator <NUM> maintain the shapes of the parts <NUM>, <NUM> throughout their lengths. The cured portions <NUM>, which are aligned with and positioned above the uncured portions <NUM>, therefore provide stability and support for the uncured portions <NUM>, without the need for supporting tooling.

<FIG> and <FIG> illustrate another example in which the two parts <NUM>, <NUM> are joined together along an adhesive bondline <NUM> by co-bonding. In this example, an uncured portion <NUM> of part <NUM> comprising several uncured plies 44a along the interfacial region <NUM> are bonded to part <NUM> of which all plies <NUM> have been precured. The regions of the plies <NUM> that are stacked above the uncured plies 44a have had the resin curing accelerator <NUM> applied thereto, allowing them to be precured at a temperature that is lower than the normal cure temperature of the thermoset resin. Once the bonding adhesive is applied to one or both of the parts <NUM>, <NUM>, the two parts <NUM>, <NUM> can be assembled and subjected to a full cure schedule, thereby cocuring the uncured plies of part <NUM> with the adhesive along the adhesive bondline <NUM>.

Attention is now directed to <FIG> which illustrate a composite laminate hat stringer <NUM> that is joined to a composite laminate skin <NUM>. The hat stringer <NUM> comprises a top <NUM>, webs <NUM> and outwardly extending flanges <NUM>. The hat stringer <NUM> includes a cured portion <NUM>, and an uncured portion <NUM> comprising uncured plies 44a on the bottoms 62a of the flanges <NUM>. As best seen in <FIG>, those plies <NUM> near the top of flanges <NUM> have been precured at a lower than normal cure temperature according to a first cure schedule as a result of having the resin curing accelerator <NUM> applied thereto, while the plies <NUM> near the interfacial region <NUM> at the bottom 62a that do not have an applied resin curing accelerator <NUM>, remain uncured and are subsequently cured at the higher, normal cure temperature of the resin according to a second cure schedule. The composite skin <NUM> is also a partially cured structure having cured portions <NUM> and uncured portions <NUM>. The uncured portions <NUM> of the skin <NUM> are located along the interfacial region <NUM>, facing the uncured portions <NUM> of the flanges <NUM>. In this example, the bottoms 62a of flanges <NUM> are cocured to the uncured portion <NUM> of the skin <NUM>. In other examples, the skin <NUM> may be fully cured, in which case the uncured plies 44a on the bottoms 62a of the flanges <NUM> may be co-bonded to the skin <NUM>.

Reference is now made to <FIG> which illustrates how the partially cured hat stringer <NUM> of <FIG> can be produced using the process shown in <FIG> to strategically apply the catalyst <NUM> to regions <NUM> of the plies <NUM> that are to be precured, while leaving other regions <NUM> uncured. A top plan view on the left side of <FIG> depicts three plies 44a, 44b, 44c to which the catalyst <NUM> has been strategically applied. Only three plies are shown for sake of simplicity of description however in practical applications a typical hat stringer <NUM> may comprise a greater number of plies <NUM>. In this example, ply 44a is a bottom ply, 44c is a top ply and ply 44b is an intermediate ply positioned between plies 44a and 44c. Catalyst <NUM> is applied only to the central region <NUM> of plies 44a and 44b which correspond to the top <NUM> and webs <NUM> of the stringer <NUM>. The catalyst <NUM> is also applied only to the regions <NUM> at the left and right edges of ply 44c which correspond to the flanges <NUM>.

The plies 44a, 44b and 44c are laid up on a suitable tool <NUM> having a cross-sectional shape corresponding to a completed stringer <NUM>. In one example, the plies 44a, 44b, 44c are laid up flat as a stack, and the stack is transferred onto the tool <NUM> where it is formed down onto, heated and consolidated using autoclave or out of autoclave processing. In another example, the plies 44a, 44b, 44c and the individually laid up and formed down onto the tool <NUM>. The ply layup on the tool <NUM> is then heated and consolidated in an autoclave, pressclave or oven, according to a first cure schedule that is sufficient to cure those regions <NUM> that are to be precured, i.e., those portions of the plies 44a, 44b, 44c that are to be precured. Following this initial curing process, the formed and stiffened shape of the partially cured hat stringer <NUM> is maintained as a result of the stiffness of the precured regions <NUM>, while the uncured regions <NUM> of the bottom and intermediate plies 44a, 44b remain uncured and therefore chemically reactive.

Following the forming and pre-curing process described above, the stringer <NUM> is removed from the tool <NUM> and transferred to the skin <NUM> (<FIG>) where the bottoms 62a of flanges <NUM> are brought into contact and placed against the skin <NUM>. The bottoms 62a of the flanges <NUM> can be co-cured or co-bonded with the composite skin <NUM> or other composite structure, using a second cure schedule. Curing according the second cure schedule results in full curing of the bottoms 62a of the flanges <NUM> as well as adhesive that may be used to cobond the stringer <NUM> and the skin <NUM>. Although not shown in the Figures, it may be possible to tack certain of the plies <NUM> together by applying the catalyst <NUM> to regions of the plies <NUM> that are to be tacked together. Using this technique, certain of the plies <NUM> may be tacked together, while other uncured plies <NUM> are allowed to slip relative to the other plies <NUM> during the forming.

Referring now to <FIG> and <FIG>, a blade stringer <NUM> comprises a blade <NUM> and a pair of flanges <NUM> joined to a composite skin <NUM>. The blade stringer <NUM> includes a pair of L-shaped members <NUM> joined together to form a preassembled part before being joined to the skin <NUM>. Each of the L-shaped members <NUM> includes a flange section <NUM> and a blade section <NUM>. Each of the blade sections <NUM> includes uncured portions <NUM> along an interfacial region <NUM> between the two blade sections <NUM>. More particularly, the uncured portions <NUM> of the two blade sections <NUM> are joined together by co-curing. Referring particularly to <FIG>, the uncured portions 42a of the flange sections <NUM> comprise uncured plies <NUM> along the interfacial region <NUM> that are co-bonded to the fully cured plies <NUM> of the skin <NUM> along an adhesive bondline <NUM>.

Thus, from the foregoing, it can be appreciated that the L-shaped members <NUM> each comprise cured and uncured portions <NUM>, <NUM> and are joined together by co-curing, while the flanges sections <NUM> are joined to the skin <NUM> by co-bonding. As in previous examples discussed above, cured and uncured portions <NUM>, <NUM> are achieved by strategic application of a resin curing accelerator <NUM> to only selected regions of selected plies <NUM> that are to be precured. A two-step curing process is then used in which regions of the plies <NUM> having the resin curing accelerator <NUM> are cured at a lower-than-normal cure temperature, and the remaining uncured regions of the plies <NUM> without the curing accelerator <NUM> are subsequently cured at the normal, full cure temperature.

The plies 44a, 44b and 44c are laid up on a suitable tool <NUM> having a cross-sectional shape corresponding to a completed stringer <NUM>. The ply layup is then heated to a temperature that is sufficient to cure those regions <NUM> that are to be precured, i.e., those portions of the plies 44a, 44b, 44c that are to be precured. Following this initial curing process, the formed shape of the partially cured hat stringer <NUM> is maintained as a result of the stiffness and rigidity of the precured regions <NUM>, while the uncured regions <NUM> of the bottom and intermediate plies 44a, 44b remain uncured and chemically reactive. Consequently the bottoms 62a of the flanges <NUM> can be co-cured or co-bonded with an underlying composite part, such as a composite skin <NUM> (not shown in <FIG>).

<FIG> and <FIG> illustrate the use of the acceleration catalyst <NUM> to produce a partially cured composite laminate patch <NUM> that may be used in the field to rework a composite skin <NUM> of an aircraft. The composite skin <NUM> has a scarfed depression within the area requiring rework. The patch <NUM> comprises a plurality of composite plies <NUM>, at least certain of which are partially precured. In the illustrated example, the outer extremities of a number of the plies <NUM> comprise uncured portions <NUM> while the remainder of those plies comprise cured portions <NUM>. The bottom ply 44a of the patch <NUM> remains entirely uncured. In this example, it can be appreciated that the entire area of the patch <NUM> that is to be joined to the skin <NUM> is uncured. The uncured portions <NUM> of the patch <NUM> are co-bonded to the cured skin <NUM> using a suitable bonding adhesive. Following placement of the patch <NUM> on the skin <NUM>, the patch <NUM> is heated to a temperature sufficient to fully cure the uncured portions <NUM>. The uncured portions <NUM> are supported by the rigid cured portions <NUM>.

<FIG> illustrates a partially cured rework patch <NUM> that can be applied to the surface of the skin <NUM> that is not scarfed. In this example, the partially cured patch <NUM> includes two bottom plies 44a that are entirely uncured, two intermediate plies 44b that are partially cured, and a top ply 44c that is fully cured. The fully cured and partially cured plies 44a, 44b respectively maintain the shape and stability of the rework patch <NUM> even though the patch <NUM> includes uncured portions <NUM>.

Attention is now directed to <FIG> which broadly illustrates the steps of a method of fabricating a composite structure using partially cured parts. At <NUM>, a composite ply layup is assembled comprising either dry fiber plies which are later infused with a thermoset resin, or by laying up a plurality of prepreg plies. The method includes ordering the plies <NUM> such that regions of plies <NUM> that remain uncured are located along an interfacial region <NUM>. At <NUM>, a catalyst <NUM> which functions as a resin curing accelerator <NUM> is applied to a first portion of the layup. At <NUM>, optionally, the layup is formed to a desired shape. At <NUM>, the first portion of the layup is cured using a cure schedule that is sufficient to cure the first portion of the layup while maintaining a second portion of the layup uncured, i.e. in a substantially reaction-active state. The cured portions of the layup maintain the shape of the layup without the need for tooling. In those applications where the layup has been formed against tooling during precuring, the precured part is removed from the tooling and transported to a structure to which it is to be joined. At <NUM>, the layup is brought into contact with a structure to which it is to be joined, in which the uncured second portions of the layup are placed in face-to-face contact with a structure which may be cured or uncured. At <NUM>, the layup is joined to the structure by curing the second portion of the layup.

Examples 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 pressurized fluid tubes, such as fuel systems and hydraulic systems in aircraft, may be used. Thus, referring now to <FIG>, examples 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 examples may include, for example, without limitation, a wide array of composite laminate parts that are joined together to form a structure. 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. The disclosed examples may be utilized in any one or more of steps <NUM>, <NUM>, <NUM>, <NUM> and <NUM>.

For the purposes of this description, a system integrator may include without limitation any number of aircraft manufacturers and majorsystem subcontractors; a third party may include without limitation any number of vendors, subcontractors, and suppliers; and an operator may be an airline, leasing company, military entity, service organization, 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.

Claim 1:
A method of making a composite structure (<NUM>), the method comprising:
applying a resin curing accelerator (<NUM>, <NUM>) to selected regions (<NUM>) of fiber reinforced, thermoset resin plies (<NUM>);
assembling a layup (<NUM>) including the thermoset resin plies (<NUM>) to which the resin curing accelerator (<NUM>, <NUM>) has been applied;
arranging the plies (<NUM>) of the layup (<NUM>) such that other regions (<NUM>) of the layup (<NUM>) that are left uncured are located along an interfacial region (<NUM>) between the partially cured part (<NUM>, <NUM>) and a structure (<NUM>, <NUM>) to which the partially cured part (<NUM>, <NUM>) is to be joined;
forming the layup (<NUM>) to a desired shape;
producing a partially cured part (<NUM>, <NUM>) by curing the selected regions (<NUM>) of the thermoset plies (<NUM>) having the resin curing accelerator (<NUM>, <NUM>) applied thereto, while leaving the other regions (<NUM>) of the thermoset plies (<NUM>) uncured;
placing the partially cured part (<NUM>, <NUM>) against the structure (<NUM>, <NUM>); and
joining the partially cured part (<NUM>, <NUM>) to the structure (<NUM>, <NUM>) by co-bonding or cocuring.