Patent Description:
It is sometimes necessary to reinforce composite structures, such as those used in aerospace industry in order to meet strength and/or stiffness requirements. These structures may be reinforced by adding stiffeners to the structure which may provide the structure with additional strength and rigidity. In the past, individual stiffeners have been attached to primary structural members using any of several secondary assembly processes, including but not limited to the use of discrete fasteners, bonding, co-bonding and co-curing. Each of these processes may have disadvantages, such as, without limitation, the additional time and labor to complete the assembly process and/or the need for relatively expensive capital equipment such as autoclaves used to co-cure parts. Additionally, in some cases, the stiffeners may comprise multiple parts which may add undesirable weight and/or part count to a vehicle and/or increase the time and labor required to complete the assembly process. In some applications where the stiffeners are integrated into the structural member, the ends of the stiffeners, referred to as "runouts", may not blend or transition smoothly into the member, which may result in undesirable stress concentrations in the structural member. Existing runout designs have discontinuous fibers at trimmed edges that may only moderately reduce stress concentrations in the surrounding structure.

Accordingly, there is a need for a simple, cost effective method of making stiffened composite structures in which stiffeners are integrated into structural members to form a unitized structure. There is also a need for a stiffener design having runouts that blend smoothly into the structural member and reduce or eliminate stress concentrations at the ends of the stiffeners.

<CIT> states, in its abstract, "Composite sections for aircraft fuselages and methods and systems for manufacturing such sections are disclosed herein. A composite section configured in accordance with one embodiment includes a skin and at least first and second stiffeners. The skin can include a plurality of unidirectional fibers forming a continuous surface extending <NUM> degrees about an axis. The first stiffener can include a first flange portion bonded to an interior surface of the skin and a first raised portion projecting inwardly and away from the interior surface of the skin. The second stiffener can include a second flange portion bonded to the interior surface of the skin and a second raised portion projecting inwardly and away from the interior surface of the skin. A method for manufacturing a section of a fuselage in accordance with one embodiment includes positioning a plurality of uncured stiffeners on a mandrel assembly. The method can further include applying a plurality of fiber tows around the plurality of uncured stiffeners on the mandrel assembly.

Paragraphs [<NUM>-<NUM>] of <CIT>, state:.

14A-14C are cross-sectional end views illustrating various stages of a method for bonding a stiffener <NUM> to a laminate <NUM> in accordance with an embodiment of the invention. Referring first to FIG. 14A, the uncured stiffener <NUM> can be positioned in a tool <NUM>. The stiffener <NUM> can be a hat section stiffener (e.g., a hat section stiffener that is at least generally similar in structure and function to the stiffeners <NUM> and <NUM> discussed above with reference to FIGS. 2A-2B and FIGS. 7A-7B, respectively). In addition, the tool <NUM> can be at least generally similar in structure and function to the tool segment <NUM> described above with reference to FIGS. After the stiffener <NUM> is positioned in the tool <NUM>, a tubular bladder <NUM> supporting a portion of fabric <NUM> (or tape, etc.) is positioned inside the stiffener <NUM> so that the fabric <NUM> contacts an interior surface <NUM> of the stiffener <NUM> between opposing flange portions 1431a and 1431b.

Referring next to FIG. 14B, once the bladder <NUM> and the fabric <NUM> are positioned inside the stiffener <NUM>, composite materials are laminated over the tooling segment <NUM> to form a skin <NUM> that contacts the flange portions <NUM> and the fabric <NUM>. In one aspect of this embodiment, the skin <NUM> can be at least generally similar in structure and function to the skin <NUM> and the laminate <NUM> described above with reference to FIGS. 2A-2B and <FIG>, respectively.

Referring next to FIG. 14C, a compressible pad or caul sheet <NUM> is positioned over the skin <NUM>. Next, a vacuum bag <NUM> is positioned around the caul sheet <NUM> and the tooling segment <NUM>. The space between the vacuum bag <NUM> and the bladder <NUM> is then evacuated to apply an even pressure against the composite parts (i.e., the stiffener <NUM>, the skin <NUM>, and the fabric <NUM>). The composite parts are then cocured at an elevated temperature while under vacuum. After curing, the stiffener/laminate combination is debagged and removed from the tooling segment <NUM>.

<CIT> states, in its abstract, "A composite structure is provided including a first fabric and a second fabric. A substantially elongate and substantially rigid first member is spaced apart from and coupled with the first fabric via the second fabric. A resin substantially is infused into the first fabric and the second fabric, and substantially encapsulates the first member to form a unitary structure.

<CIT> states, in its abstract, "Sheet-like component of an aircraft made of fiber-reinforced composite material, which forms a surface with a material thickness, which further comprises webs with a web height, the partial surfaces of the planar component, wherein at least one partial surface is designed with at least one reinforcing bead of a predetermined bead extension extending over the partial surface between the webs, wherein the one bead extension is at most <NUM> and thus at most <NUM>% of the web height of the partial surfaces surrounding webs, and has a round, oval or semicircular cross-section, wherein the at least one reinforcing bead has a locally thickened area of the planar component, so that the material thickness of the partial surface is thickened only by the at least one reinforcing bead.

In summary there is provided, according to claim <NUM>, a method of making a stiffened composite structure, comprising: fabricating a first fiber preform; placing the first fiber preform in a mold recess having the geometry of a stiffener; placing a second fiber preform over the mold recess covering the first preform; co-infusing the first and second fiber preforms with a polymer resin; and curing the resin-infused preforms, wherein: fabricating the first fiber preform includes braiding fibers into a flexible tubular shell, and filling the shell with continuous unidirectional fibers, and placing the first fiber preform in the mold recess includes conforming the tubular shell to the geometry of the recess.

The disclosed embodiments provide a unitized composite structure having integrated stiffeners with smoothly transitioned runouts at their ends which reduce or substantially eliminate stress concentrations. The stiffeners are produced using fiber preforms that are shaped to blend the ends of the stiffeners into the surrounding structure. This smooth blending avoids abrupt terminations or discontinuous transitions between the stiffener and the surrounding structure, and may reduce or eliminate the need for trimming operations at the ends of the stiffeners. The reduction in trimming operations may reduce fabrication time, process steps and/or labor costs. The runout transitions the stiffener cross section from a tall narrow profile to a wide flat profile, while maintaining a constant perimeter and cross <NUM>;JAS;MMG1 sectional area. A variety of stiffener architectures and physical characteristics may be realized using various preform fabrication processes. The runout design allows fabrication of stiffeners having complex geometries, resulting in greater design flexibility and process optimization.

According to one disclosed embodiment, a unitized composite structure comprises a composite member and at least one composite stiffener formed integral with the composite member for stiffening the member. At least one end of the stiffener includes a runout forming a substantially smooth transition into the composite member. The composite member includes a first resin infused fiber reinforcement, and the stiffener includes a second resin infused reinforcement, wherein the infused resin is substantially continuous and homogeneous throughout the first and second fiber reinforcements. The runout has a cross section that varies in shape but remains substantially constant in area along its length.

According to another embodiment, an integrally stiffened composite structure comprises a cured polymer resin matrix, a structural member portion including a first fiber reinforcement held in the matrix, and a stiffener portion for stiffening the structural member portion. The stiffener portion includes a second fiber reinforcement held in the matrix. The stiffener portion includes at least one end having a runout forming a substantially smooth transition into the structural member portion. The matrix is substantially continuous and homogeneous throughout the first and second portions.

There is also described a unitized composite structure, including.

There is also described a unitized composite structure wherein.

There is also described a unitized composite structure wherein
the composite member is one of:.

There is also described a unitized composite structure wherein the runout is substantially smoothly contoured along its length.

There is also described a unitized composite structure wherein the runout has a cross section that varies in shape but remains substantially constant in perimeter along its length.

There is also described a unitized composite structure wherein the runout has a cross section that varies in shape but remains substantially constant in area along its length.

There is also described an integrally stiffened composite structure, including.

There is also described a stiffened composite structure wherein the matrix is substantially continuous and homogeneous throughout the first and second portions.

There is also described a stiffened composite structure wherein the structural member portion is one of:.

There is also described a stiffened composite structure wherein the stiffener portion includes an outer shell and an inner core.

There is also described a stiffened composite structure wherein the core includes reinforcement fibers extending longitudinally through the stiffener portion.

There is also described a stiffened composite structure wherein the core includes a filler in which the reinforcing fibers are held, and
the shell includes at least one ply of interconnected fibers.

There is also described a stiffened composite structure wherein the runout is contoured along its length.

There is also described a stiffened composite structure wherein the stiffener has a cross section that varies in shape but remains substantially constant in area along the length of the runout.

There is also described a stiffened composite structure wherein the stiffener has a cross section that varies in shape but remains substantially constant in perimeter along the length of the runout.

There is also described a stiffened composite structure wherein the stiffener includes a crown having a width, sides each having a height and a base having a width, wherein.

There is also described a stiffened composite structure wherein the stiffener decreases in height and increases in width along the length of the runout.

Referring first to <FIG>, a unitized composite structure <NUM> comprises a structural member <NUM> having a plurality of integrally formed stiffeners <NUM> which may provide the structural member <NUM> with additional strength and rigidity. In the illustrated example, the structural member <NUM> is a substantially flat panel 32a, and the stiffeners <NUM> are arranged to extend substantially parallel to each other on one side of the panel 32a. Each of the stiffeners <NUM> includes a runout <NUM> on each end thereof which blends the stiffener <NUM> substantially smoothly into the panel 32a in order to reduce peak stress concentrations in the panel 32a. As will be discussed later, the structural member <NUM> may have other shapes and geometries, depending on the application, including but not limited to channels, beams, webs, flanges, skins and the like.

Referring now to <FIG>, each of the stiffeners <NUM> is modular in design and comprises an inner core <NUM> surrounded by an outer shell <NUM> having a bottom cap <NUM> joined to the panel 32a along a butt joint <NUM>. An optional layer of adhesive <NUM> may be used to assist in joining the stiffener <NUM> to the panel 32a at the butt joint <NUM>. As will be discussed later in more detail, the shell <NUM> comprises one or more layers (not shown in <FIG>) of a resin infused composite member fiber reinforcement or preform <NUM> (<FIG>) that is braided. The reinforcing fibers may comprise carbon, glass or a variety of polymers or other suitable reinforcements. In this example, the shell <NUM> is continuous and includes a radius top or crown <NUM> and sidewalls <NUM> that are integrally connected with radius sections <NUM> overlying the panel 32a.

The inner core <NUM> may be partially or completely filled with a structural or nonstructural material, depending upon the application. In the case of the example shown in <FIG>, the inner core <NUM> is filled with a suitable unidirectional carbon fiber reinforcement <NUM>. <FIG> illustrates an alternate embodiment of the modular stiffener <NUM> wherein the shell <NUM> comprises multiple layers 36a of braided fibers held in a resin matrix, and the core <NUM> is filled with one or more plies <NUM> of fiber reinforced resin which may be in the form of unidirectional tape, tows or a fabric.

As mentioned above, the stiffener <NUM> may have numerous variations in geometry and/or construction details. <FIG> illustrates a stiffener <NUM> similar to that shown in <FIG>, but wherein through thickness reinforcements, such as but not limited to Z-Pins, <NUM> are optionally used to aid in joining the stiffener <NUM> to the panel 32a and to provide additional reinforcement of the structure <NUM>. The Z-pins <NUM> extend through the panel 32a and bottom cap <NUM> of the stiffener <NUM> into the core <NUM>. The stiffener <NUM> shown in <FIG> is generally trapezoidal in cross section however, other cross sectional shapes may be possible, including but not limited to a "T", a "J", a "C", an "I", a "Z" or a hat. In other embodiments, the stiffener <NUM> may comprise a solid laminate (not shown), or a core with solid laminate facesheets (not shown).

<FIG> illustrates another variation of the stiffener <NUM> which includes an inner shell <NUM> that divides the core <NUM> into a hollow core section <NUM> separating two core sections 38a. 38b that may or may not be filled with structural reinforcement <NUM> or other filler. In this example, the base cap <NUM> is joined directly to the panel 32a along a butt joint <NUM>, and adhesive <NUM> is used along the outer margins <NUM> of the butt joint <NUM>.

<FIG> illustrates another version of the stiffener <NUM>, similar to that shown in <FIG>, but wherein the core <NUM> is hollow.

Still another variation of the stiffener <NUM> is shown in <FIG> having an inner core <NUM> filled with reinforcement <NUM> and lower side wall edges <NUM> that are radiused.

<FIG> illustrates another embodiment of the stiffener <NUM> wherein the outer shell <NUM> has laterally extending flanges <NUM> overlying the base cap <NUM>. The flanges <NUM> increase the area of the butt joint <NUM> between the stiffener <NUM> and the panel 32a, while also providing a smooth transition between the shell <NUM> and the panel 32a that assists in minimizing peak stress concentrations on the panel 32a, as mentioned previously.

Still another example of the stiffener <NUM> is shown in <FIG>. This embodiment of the stiffener <NUM> is similar to the embodiment shown in <FIG> except that one or more additional plies <NUM> are wrapped over the outer shell <NUM> and extend laterally to form flanges <NUM>. The ply wraps <NUM> both strengthen the stiffener <NUM> and increase the area of contact between the panel 32a and the shell <NUM>/flanges <NUM>, while the flanges <NUM> form part of the stiffener runouts <NUM> which assist in minimizing peak stress concentrations on the panel 32a.

Another embodiment of the stiffener <NUM> is shown in <FIG> in which an outer shell <NUM> comprises a flat cap <NUM> and inclined sidewalls <NUM> that are joined to a base <NUM> having laterally extending flanges <NUM>. As in the case of the embodiments shown in <FIG> and <FIG>, the laterally extending flanges <NUM> increase the area of the butt joint between the stiffener <NUM> and the panel 32a, while also providing a smooth transition between the shell <NUM> and the panel 32a that assists in minimizing peak stress concentrations on the panel 32a.

Another variation of the stiffener <NUM> is shown in <FIG> in which the outer shell <NUM> includes a radiused crown <NUM>, and sidewalls <NUM>. The sidewalls <NUM> transition through radius sections <NUM> into integral flanges <NUM> which are attached to the panel 32a.

A further embodiment of the stiffener <NUM> shown in <FIG> which is similar to that shown in <FIG> except that through the thickness reinforcements <NUM> such as Z-pins <NUM> extend from within the core <NUM> into the panel 32a. The reinforcements <NUM> aid in joining the stiffener <NUM> to the panel 32a and provide additional reinforcement of the structure <NUM>.

<FIG> illustrates another example of the stiffener <NUM> which is similar to that shown in <FIG> except that one or more additional plies <NUM> are wrapped over the outer shell <NUM> and are used to form laterally extending flanges <NUM>.

From <FIG>, it may be appreciated that the stiffener <NUM> may have a wide range of geometries, features, core fillers and reinforcements which may add strength and/or stiffness to the stiffener <NUM> and/or increase the strength and/or damage tolerance of the joint <NUM> between the stiffener <NUM> and the panel 32a. It may also be appreciated from the forgoing description, that the stiffened composite structure <NUM> comprises a substantially continuous and homogeneous polymer resin matrix that functions to hold both a structural member portion <NUM> and a stiffener portion <NUM>. The structure <NUM> is unitized by virtue of the structural member and stiffener portions <NUM>, <NUM> respectively being integrated by the matrix material.

Attention is now directed to <FIG> which illustrates several basic steps of a method of making a unitized composite structure <NUM> having one or more integrally formed stiffeners <NUM> (<FIG>). As shown at <NUM>, a single-piece, simple tool <NUM> has a tool face 56a that defines the inner mold line (IML) of the finished composite structure <NUM>. The tool face 56a may be substantially flat, as shown in <FIG>, or may have one or more curves or features matching the IML of the finished structure <NUM>. One or more grooves <NUM> are formed in the tool face 56a that correspond to the geometry of the stiffeners <NUM> that are to be integrated into the finished structure <NUM>. The depth D of the grooves <NUM> substantially correspond to the height H of the stiffeners <NUM> (see <FIG>). The tool face 56a may also include additional, cavity-like grooves (not shown) into which nodal connectors (not shown) may be placed in order to form network-like interconnections between the stiffeners <NUM>, as will be discussed below in more detail.

As shown at <NUM>, dry, or substantially dry fiber stiffener preforms <NUM> are placed in the grooves <NUM> either manually, or using an automated placement equipment <NUM>. Depending on the shape and construction of the stiffener preforms <NUM>, portions of the stiffener preforms <NUM> may be tacked together with tackifiers or binders to assist in holding the preform <NUM> together and/or to maintain their shapes until they are infused with resin. Prior to being infused with resin and cured, the stiffener preforms <NUM> may be cord-like and continuous in length, allowing them to be stored in roll form, dispensed and cut to length as needed. Alternatively, the preforms <NUM> may be stiff and formed substantially to the required length, size and shape and stored flat, or any variation between continuous/flexible and discrete/stiff. When automated placement equipment <NUM> is used, the preforms <NUM> may be placed on the tool <NUM> at relatively high rates. Because the grooves <NUM> in the tool <NUM> are pre-aligned, the location and orientation of the stiffeners <NUM> relative to the composite member <NUM> can be precisely controlled. In other words, the fixed position of the grooves <NUM> in the tool face 56a automatically indexes the preforms <NUM> relative to each other, and relative to the fiber reinforcement <NUM>. The preforms <NUM> are substantially identical to the stiffeners <NUM> previously described except that they have not yet been infused with a resin and are therefore relatively flexible.

The grooves <NUM> may have a cross sectional profile (not shown) that substantially matches that of the preforms <NUM>, so that when placed in the grooves <NUM>, the preforms <NUM> substantially completely fill the grooves <NUM>, resulting in a substantially smooth IML profile. Flexible preforms <NUM> readily conform to the cross sectional profile and curvature (if any) of the grooves <NUM>. Discrete/stiff preforms may be pre-formed to at least substantially match the cross sectional profile and curvature (if any) of the grooves. The grooves <NUM> essentially recess the stiffener preforms <NUM> in the tool <NUM> relative to a fiber reinforcement <NUM> so that the top of the preforms <NUM> lie generally flush with the tool face 56a. Optionally, a film adhesive (not shown) may be placed in the grooves <NUM>, overlying the stiffener preforms <NUM>, in those applications where it is desired to adhesively bond the stiffener caps <NUM> to the composite member <NUM> along the butt joint <NUM>, as shown in <FIG>.

Next, as shown at <NUM>, the dry or substantially dry composite member fiber reinforcement <NUM> is placed on the tool face 56a, overlying and contacting the stiffener preforms <NUM> and the tool face 56a. The composite member fiber reinforcement <NUM> as well as the fiber preforms <NUM> may be tackified with a binder (not shown). The composite member fiber reinforcement <NUM> may comprise, for example and without limitation, a preform that may include multiple plies of woven or knitted fabric that are laid up ply-by-ply on the tool face 56a, or which are stacked and then placed as a single pre-assembled lay-up on the tool face 56a. In the illustrated example, the composite member fiber reinforcement <NUM> is substantially flat however, in other embodiments, it is possible that the composite member fiber reinforcement <NUM> may be a preform that is shaped before the composite member fiber reinforcement <NUM> is placed on the tool face 56a. At <NUM>, a caul sheet <NUM> is placed over the composite member fiber member reinforcement <NUM>. The caul sheet <NUM> aids in controlling the OML (outer mold line) surface finish and skin mark-off adjacent the stiffener <NUM>. Then, at <NUM>, preform <NUM> and composite member fiber reinforcement <NUM> are co-infused with a suitable thermoset resin using any of various well known resin infusion techniques, including, for example and without limitation, vacuum assisted resin infusion molding (VARIM). As will be discussed below, the preform <NUM> and fiber reinforcement <NUM> may be compacted and consolidated prior to resin infusion. The infused preform <NUM> and composite member fiber reinforcement <NUM> are then cured by the application of heat though any suitable means such as an oven <NUM>.

Attention is now directed to <FIG> which shows additional details of a VARIM layup assembly <NUM> that may be used to carry out the steps of the method previously discussed in connection with <FIG>. The stiffener preforms <NUM> are placed in the grooves <NUM> in the tool <NUM>, following which the composite member reinforcement <NUM> is placed on the tool face <NUM>, overlying and in contact with the stiffener preform <NUM>. A peel ply <NUM> is placed over the composite member fiber reinforcement <NUM> and a suitable resin distribution media <NUM> is placed over the peel ply <NUM> to aid in moving and evenly distributing flowing resin. A peel ply <NUM> may also be placed under the outer edges of the composite member fiber <NUM>.

A rigid or semi-rigid caul sheet <NUM> is placed over the resin distribution media <NUM>, following which a vacuum bag <NUM> is placed over the layup and is sealed to the tool <NUM> by means of a sealant tape <NUM> or by similar means. In other embodiments, a double vacuum bag technique may be used in which a second vacuum bag (not shown) is placed over the first vacuum bag <NUM> in order to protect the preform <NUM> from leaks in the first vacuum bag <NUM> during the resin infusion and curing processes. The use of the caul sheet <NUM> and resin distribution media <NUM> is illustrative of one typical arrangement for resin infusion, but may not be required when other resin infusion techniques are employed. A variety of other resin infusion techniques are possible. A supply reservoir of thermoset resin <NUM> is coupled by a resin inlet tube <NUM> to an inlet channel tube <NUM> within the vacuum bag <NUM>. An outlet vacuum reservoir <NUM> is coupled by a resin outlet tube <NUM> to an outlet channel tube <NUM> inside the vacuum bag <NUM>.

A vacuum within the bag <NUM> generated by the outlet vacuum reservoir <NUM> evacuates the bag <NUM> of air, creating a pressure less than atmospheric pressure within the bag <NUM> that draws resin from the supply reservoir <NUM> into the bag <NUM> through the inlet channel tube <NUM>. Prior to resin infusion, the bag <NUM> may be used to compact and consolidate the preform <NUM> and fiber reinforcement <NUM>. Resin flows from the inlet channel tube <NUM> and exits the bag <NUM> through the outlet channel tube <NUM> where it is collected in the vacuum reservoir <NUM>. As the resin travels from the inlet channel <NUM> to the outlet channel <NUM>, preform <NUM> and composite member fiber reinforcement <NUM> are co-infused with a single shot of the resin while atmospheric pressure forces the bag <NUM> down onto the caul sheet <NUM>. As mentioned earlier, <FIG> illustrates merely one of a number of resin infusion techniques that may be used to make the stiffened composite structure <NUM>.

The caul sheet <NUM> applies substantially even pressure over its area to the infused preform <NUM> and composite member fiber reinforcement <NUM>, causing the preform <NUM> and composite member fiber reinforcement <NUM> to be compacted and forced against each other during the resin infusion process. Heat may be applied to the infused preform <NUM> and composite member fiber reinforcement <NUM> both during and after the resin infusion process in order to encourage the resin flow, and then cure the resin to produce a unitized composite structure <NUM> in which the stiffeners <NUM> are essentially integrated into the composite member <NUM>. The co-infusion of the preform <NUM> and composite member fiber reinforcement <NUM> with resin results in a substantially continuous and homogeneous resin matrix which holds and integrates the structural member and stiffener portions <NUM>, <NUM> respectively.

<FIG> illustrate stiffened composite structures <NUM> having various layout patterns of the stiffeners <NUM>. <FIG> illustrates a composite panel 32a stiffened with a plurality of integrally formed, generally parallel stiffeners <NUM>, similar to the embodiments shown in <FIG>. <FIG> illustrates a stiffened composite panel 32a in which the stiffeners <NUM> are arranged in a crossing-like grid pattern. <FIG> shows another variation in which the stiffeners <NUM> are arranged side-by-side but collectively taper along the length of the panel 32a. <FIG> illustrates an embodiment in which the stiffeners <NUM> are arranged in an iso-grid pattern, wherein the ends of the stiffeners <NUM> are interconnected at connecting nodes <NUM>. <FIG> shows the use of generally concentric, oval stiffeners <NUM> surrounding an opening <NUM> in a panel 32a in order to reinforce the area of the panel 32a around the opening <NUM>.

<FIG> illustrates another example of a nodal grid stiffened panel 32b in which the stiffeners <NUM> are interconnected by connecting nodes <NUM> which may be recessed into the tool face 56a (<FIG> and <FIG>) during forming so that the connecting nodes <NUM> and the stiffeners <NUM> are integrally formed with each other and with the panel 32a during the fabrication process. In this example, the panel <NUM> is curved in a single direction, and thus, at least a certain number of the stiffeners <NUM> are also curved in the direction of the panel curvature. The connecting nodes <NUM> may comprise, for example and without limitation, a preformed rigid member, such as a metal member, a pre-cured composite member, or a dry or substantially dry fiber preform that is co-infused with resin with the composite member fiber reinforcement <NUM>. In those embodiments where the connecting node <NUM> is a preformed rigid member, it may be co-bonded with the stiffener <NUM> and panel 32a, or it may be secondarily bonded with the stiffener <NUM> and the panel 32a using a layer (not shown) of adhesive placed between the connecting node <NUM>, the stiffener <NUM> and the panel 32a.

<FIG> illustrates a panel <NUM> having a variation <NUM> in thickness. This variation in thickness <NUM> may be accommodated by forming an appropriate depth contour in the tool face <NUM>. The flexibility of the stiffener preform <NUM> allows the preform <NUM> to conform to the thickness contour <NUM> of the underlying panel 32b.

<FIG> illustrates another unitized, stiffened composite structure <NUM> in the form of a leading edge <NUM> of an aircraft wing. This example illustrates the ability of the stiffeners <NUM> to conform to relatively severe curvatures, including compound curvatures. Of the composite members <NUM> that they are intended to stiffen.

<FIG> illustrates the use of a stiffener <NUM> to reinforce a panel <NUM> curved in one direction. The curvature of the stiffener <NUM> matches that of the panel <NUM>.

<FIG> illustrates a unitized, stiffened composite structure <NUM> in the form of a C-shaped channel beam 32c that is reinforced by integrally formed rib-like stiffeners <NUM> matching the cross section of the beam 32c and which are spaced along the length of the beam 32c The rib-like stiffeners <NUM> may be employed in composite structures <NUM> having other cross sectional shapes.

Attention is now directed to <FIG> which broadly illustrates the steps of a method of making a unitized composite structure <NUM> having the disclosed integrally formed stiffeners <NUM>. Beginning at step <NUM>, grooves <NUM> having the appropriate depth and geometry are formed in the tool face 56a by any suitable fabrication technique, such as milling the grooves <NUM> in a hard material such as steel. At <NUM>, the stiffener preforms <NUM> are formed which may include laying up multiple plies of dry fiber material, which as previously noted, comprise braided material. The stiffener preforms <NUM> may or may not be filled with a filler of the types previously discussed.

At <NUM>, the stiffener preforms <NUM> are placed in the grooves <NUM> in the tool face 56a, following which at <NUM> the composite member fiber reinforcement <NUM> is placed on the tool face 56a, overlying and contacting the stiffener preforms <NUM>, as previously described in connection with <FIG>. At <NUM>, the remaining components of the layup <NUM> are assembled, including placing the vacuum bag <NUM> over the preform <NUM> and composite member fiber reinforcement <NUM> and sealing it to the tool <NUM>. Next, at <NUM>, a vacuum is drawn in the bag <NUM>, following which at <NUM>, the preform <NUM> and composite member fiber reinforcement <NUM> are infused substantially simultaneously (i.e. co-infused) with a thermoset resin in a one-shot resin infusion process. The vacuum within the bag <NUM> may aid in drawing the resin into and through the preform <NUM> and the composite member fiber reinforcement <NUM>. Although not shown in <FIG>, a vacuum can be drawn in the bag <NUM> prior to the resin infusion step <NUM> in order to compact and consolidate the stiffener preform <NUM> and the fiber reinforcement <NUM> in order to reduce their volume so that a composite structure is produced having the lowest volume of resin. Alternatively, the compaction and consolidation process may be achieved during the resin infusion step <NUM>. Finally, at step <NUM>, the resin infused structure is cured by heating the structure according to a desired cure schedule.

Reference is now made to <FIG> which illustrate additional details of the runout <NUM> on each end of each of the stiffeners <NUM>, previously mentioned in connection with <FIG>. The runouts <NUM> form a substantially smooth and continuous transition of the ends <NUM> of the stiffener <NUM> into the surrounding composite member <NUM>, which in this case, is a panel or skin 32a. Intermediate the ends <NUM> of the stiffener <NUM>, the cross sectional geometry of the stiffener <NUM> is defined by the shell <NUM> and the base cap <NUM>. Intermediate the ends <NUM>, the top or crown <NUM> of the shell <NUM> is relatively narrow, and the sides <NUM> are relatively steep, while the base cap <NUM> has a substantially constant width. Along the runout <NUM>, however, the width "W" of the crown <NUM> constantly increases, while the height "H" of the side <NUM> constantly decreases, and the base cap <NUM> splays outwardly as shown at 35a. The rate of change in the width "W" the crown <NUM>, the height "H" of the sides <NUM> and width "w" of the base cap <NUM> will depend upon the particular application, and the geometry of the skin 32a.

Attention is now particularly directed to <FIG> which show the change in cross sectional profile of the stiffener <NUM>. Although not drawn to scale, these figures illustrate that as the height "H" decreases along the runout <NUM>, the width "W" increases. However, the total cross sectional area "A" of the stiffener <NUM> remains substantially constant along the length of the runout <NUM>. The constant cross section of the runout <NUM> results in the internal structure of the stiffener <NUM> continuing to the outer extremities of the stiffener <NUM>. Maintaining constant cross sectional area "A" allows the core <NUM> to be continuous throughout the runout <NUM>, and may therefore not require any material such as carbon tows to terminate (drop-off) at an intermediate point along the runout <NUM> which may otherwise create production complexity, potential resin rich zones and stress concentrations in the cured structure <NUM>.

<FIG> illustrates the perimeter "P" of the cross sections shown in <FIG>. Like the total cross sectional area "A" of the stiffener <NUM>, the total perimeter "P" of the cross section of the stiffener <NUM> remains substantially constant along the length of the runout <NUM>. Thus, the perimeter P<NUM> of the stiffener <NUM> at section line <NUM>-<NUM> in <FIG> is equal to the perimeter P<NUM> at section line <NUM>-<NUM> and is equal to perimeter P<NUM> at section line <NUM>-<NUM> in <FIG>. The provision of a constant perimeter in the runout <NUM> allows a constant fiber orientation within the outer shell <NUM> to be maintained throughout the length of the runout <NUM> which aids in minimizing fiber distortion that could otherwise lead to resin rich zones and a reduction in mechanical properties.

<FIG> illustrates one embodiment of a fiber preform <NUM> that may be resin infused to form the stiffener <NUM>, including the runout <NUM> shown in <FIG>. The preform <NUM> comprises a braided fiber tubular shell <NUM> having a core <NUM> filled with loose, unidirectional fibers <NUM>. As shown in <FIG>, the tubular fiber preform <NUM> is fabricated in continuous lengths (not shown) by an assembly process <NUM> that involves braiding unidirectional yarn <NUM> into a tubular braided shell <NUM>. The shell <NUM> is then filled with unidirectional fiber tows <NUM>. Use of the tubular preform <NUM> is desirable because it may be efficiently cut to length from a continuous supply and is readily conformable both in cross section and along its length to tool recesses, such as the tool recess <NUM> shown in <FIG>. An alternative method of fabricating the preform <NUM> is illustrated in <FIG>, wherein a single braiding process <NUM> may be employed to form the shell <NUM> around a group <NUM> of unidirectional fibers <NUM>.

<FIG> illustrates an alternate embodiment of a rigid or semi-rigid fiber preform <NUM> for forming the stiffener <NUM> with a runout <NUM>. The preform <NUM> comprises a shaped, braided shell <NUM> which may include lateral flanges <NUM> and a core <NUM> filled with unidirectional reinforcement fibers <NUM>. The preform <NUM> shown in <FIG> may be fabricated by a process illustrated in <FIG>, wherein a single braiding and forming process <NUM> may be employed to form the shell <NUM> around a group <NUM> of unidirectional fibers <NUM> in which the cross sectional shape of the preform <NUM> substantially matches that of a mold cavity <NUM> (<FIG>). In this example, the shell <NUM> and the group <NUM> of fibers unidirectional fibers <NUM> are braided together and formed to shape substantially simultaneously. The formed preform <NUM> may be fabricated in continuous lengths (not shown).

<FIG> illustrates a further embodiment, not within the scope of the claims, of a preform <NUM> comprising a shell <NUM> formed by fabric overlap plies which includes lateral flanges <NUM>, and a fabric base cap <NUM>. The preform <NUM> further includes a core <NUM> filled with unidirectional fibers <NUM>. The preform <NUM> may be formed in discrete lengths (not shown) and may be substantially rigid both in cross section, and along its length. <FIG> illustrates one process, not within the scope of the claims, for fabricating the preform <NUM> shown in <FIG>. Beginning at <NUM>, a recess <NUM> having a suitable geometry is formed in a tool <NUM>, and an overlap fabric ply <NUM> is placed on the tool <NUM> overlying the recess <NUM>. At <NUM>, a forming tool <NUM> is used to form the ply <NUM> into the recess <NUM> and conform it to the contours of the recess <NUM>, using force F applied to the forming tool <NUM>. At <NUM>, the tool <NUM> is removed, the formed overlap ply <NUM> is filled with unidirectional reinforcing fibers <NUM>, and a second overlap ply <NUM> is placed over the first ply <NUM>. At <NUM>, a suitable tool <NUM> may be used to compact the plies <NUM>, <NUM>.

Reference is now made to <FIG> which broadly illustrates the steps of a method of fabricating a unitized, stiffened composite structure <NUM> such as the stiffened panel <NUM> shown in <FIG>. Beginning at <NUM>, a mold <NUM> (<FIG>) is fabricated having recesses that include runout sections <NUM> on opposite ends <NUM> (<FIG>) thereof. In one embodiment shown at <NUM>, a first fiber preform or reinforcement <NUM>, <NUM> may be fabricated in continuous lengths. At <NUM>, fibers are braided into a continuous tubular outer shell <NUM> having a conformable or semiconformable cross section. At <NUM>, the tubular shell is filled with continuous reinforcing fibers and at <NUM> the first preform <NUM>, <NUM> is cut to suitable lengths. At <NUM>, the first preform is placed in a mold recess <NUM> and conformed to the geometry of the recess <NUM>.

Another embodiment for making the first fiber preform is shown at <NUM>. At <NUM>, a single braiding process is used to form the shell <NUM> around a group <NUM> of unidirectional fibers <NUM>, and at <NUM> the first preform <NUM> is cut to suitable lengths. At <NUM> the flexible preform <NUM> is placed in the mold recess <NUM> and conformed to the geometry of the recess <NUM>.

In a further embodiment for making the first fiber preform shown at <NUM>, a first fiber ply <NUM> is placed on a mold <NUM> overlying a mold recess <NUM>, as shown at <NUM>. Next at <NUM>, the first fiber ply <NUM> is formed into the mold recess <NUM> and conformed to the contours of the recess <NUM>. At <NUM>, the formed first ply <NUM> is filled with continuous reinforcement fibers <NUM>, following which, at <NUM>, a second fiber ply <NUM> is placed on the mold <NUM> overlying the fiber filled first ply <NUM>.

Still another embodiment for making the first preform <NUM> is shown at <NUM>. At <NUM>, shell and core fibers are braided together and simultaneously formed into a semi-rigid preform <NUM> that is pre-shaped to substantially match the geometry of the mold recess <NUM>. The pre-shaped and semi-rigid preform <NUM> is then placed into the mold recess <NUM> at <NUM>.

After the first preform <NUM>, <NUM> is made as described above, a second preform or fiber reinforcement <NUM> (<FIG>) is placed on the mold <NUM> overlying the first preform, as shown at <NUM>. At <NUM>, the first and second preforms are co-infused with resin, following which the composite structure <NUM> is cured at <NUM>.

Embodiments of the disclosure may find use in a variety of potential applications, particularly in the transportation industry, including for example, aerospace, marine and automotive applications. Thus, referring now to <FIG> and <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 a wide variety of structural composite parts and components, including for example and without limitation, control surface skins, wing and empennage skins, stiffened access doors and panels, and stiffened ribs and spar webs, 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. The disclosed method may be employed to fabricate stiffened parts, structures and components used in the interior <NUM> and in the airframe <NUM>. 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, parts, structures and components corresponding to production process <NUM> may be fabricated or manufactured in a manner similar to parts, structures and components produced while the aircraft <NUM> is in service. Also the disclosed method embodiments 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 making a stiffened composite structure (<NUM>), comprising:
fabricating a first fiber preform (<NUM>);
placing the first fiber preform (<NUM>) in a mold recess (<NUM>) having the geometry of a stiffener (<NUM>);
placing a second fiber preform (<NUM>) over the mold recess (<NUM>) covering the first preform (<NUM>);
co-infusing the first and second fiber preforms (<NUM>,<NUM>) with a polymer resin; and
curing the resin-infused preforms, wherein
fabricating the first fiber preform (<NUM>) includes braiding fibers into a flexible tubular shell (<NUM>),
and filling the shell with continuous unidirectional fibers, and
placing the first fiber preform (<NUM>) in the mold recess (<NUM>) includes conforming the tubular shell to the geometry of the recess (<NUM>).