Patent Description:
Integrated composite structures are recognized as promising materials for reducing aircraft structure weight and manufacturing cost. 3D woven pi-preforms currently provide a baseline method for achieving robust adhesively bonded joints in composite aircraft structures. However, the cost of 3D woven pi-preforms is prohibitive for numerous applications that would benefit from high performance adhesive joints. <CIT> discloses a pi-shaped preform constructed from reinforcing fiber cloths comprising unidirectional continuous fibers.

The systems and methods of the present disclosure provide several technical advantages over previous pi-shaped preform technology, which include (i) reduced manufacturing cost compared to 3D woven pi-preforms; (ii) improved shear strength (e.g., embodiments of the pi-shaped preform provided herein demonstrate ~<NUM>% shear strength of the baseline 3D pi-preforms); and (iii) excellent formability caused, in part, by the discontinuous, aligned fibers in the prepreg that allows for creation of complex shaping of the pi-shaped preforms while maintaining mechanical properties (e.g., improve shear strength and comparative pull-off strength).

According to one embodiment, the present disclosure provides a pi-shaped preform comprising a base component and a pair of axially elongated legs coupled to the base component to define a channel between the axially elongated legs. The pair of axially elongated legs comprising plies of prepreg oriented in a ply stack, and at least a portion of the prepreg comprises discontinuous, aligned fibers, wherein individual plies of prepreg oriented in the ply stack at an angle of <NUM> degrees or <NUM> degrees relative to a first axis comprise continuous, aligned fibers, and individual plies of prepreg oriented in the ply stack at an angle other than <NUM> degrees or <NUM> degrees relative to the first axis comprise discontinuous, aligned fibers.

According to another embodiment, the present disclosure provides a pi-joint assembly comprising the above pi-shaped preform. The pi-joint comprises a first material, e.g. in the form of a skin, coupled to the base component, and a second material, e.g. in the form of a web, coupled to an inner surface of the channel between the axially elongated legs.

According to one embodiment, the present disclosure provides a method of manufacturing a pi-shaped preform. The method comprises laying plies of prepreg for a base component and a pair of axially elongated legs, and thermal forming the plies of prepreg in a shape of the base component and the pair of axially elongated legs, and bonding the pair of axially elongated legs to the base component to form the pi-shaped preform. At least a portion of the prepreg comprises discontinuous, aligned fibers, wherein individual plies of prepreg oriented in the ply stack at an angle of <NUM> degrees or <NUM> degrees relative to a first axis comprise continuous, aligned fibers, and individual plies of prepreg oriented in the ply stack at an angle other than <NUM> degrees or <NUM> degrees relative to the first axis comprise discontinuous, aligned fibers.

Other technical advantages will be readily apparent to one skilled in the art from the following figures, descriptions, and claims. Moreover, while specific advantages have been enumerated above, various embodiments may include all, some, or none of the enumerated advantages.

For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which:.

3D woven pi-preforms are currently used for achieving adhesively bonded joints in composite aircraft structures. The cost of 3D woven pi preforms is prohibitive for a number of applications that would benefit from high performance adhesive joints. To address these and other challenges associated with typical 3D woven pi-preforms, the disclosed embodiments provide a pi-shaped preform that is a low-cost alternative to 3D woven pi-preforms, with minimal to no performance sacrifice or even higher performance. The provided pi-shaped preforms of the present disclosure therefore allow the use of pi-joint configurations on a broader range of applications within composite designs.

To facilitate a better understanding of the present disclosure, the following examples of certain embodiments are given. In no way should the following examples be read to limit, or define, the scope of the disclosure. Embodiments of the present disclosure and its advantages may be best understood by referring to the included FIGURES, where like numbers are used to indicate like and corresponding parts.

<FIG> is a schematic illustration of a pi-shaped preform element or "preform" <NUM> according to an embodiment of the present disclosure. When viewed in cross-section, the preform <NUM> resembles an inverted Greek letter π or "pi" having a base component <NUM> and a pair of axially extending legs <NUM>, <NUM> coupled to the base component <NUM>. The pair of axially extending legs <NUM>, <NUM> may be perpendicular to the base component <NUM> or angled to fit the structure. A channel <NUM> is defined between the axially elongated legs <NUM>, <NUM>. In some embodiments, the channel <NUM> is a clevis joint.

In some embodiments, the pi-shaped preform <NUM> is a composite material that may be formed by various components. For example, the axially elongated legs <NUM>, <NUM> may be formed by coupling a U-shaped component <NUM> to a first L-shaped component <NUM> and a second L-shaped component <NUM> positioned opposite the first L-shaped component <NUM>. A filler <NUM> is positioned in a first space formed between the U-shaped component <NUM> and the first L-shaped component <NUM>, and the filler <NUM> is positioned a second space formed between the U-shaped component <NUM> and the second L-shaped component <NUM>. In some embodiments, an optional adhesive <NUM> is positioned between the various components to directly or indirectly couple them together. For example, the adhesive <NUM> may be positioned between: (i) the U-shaped component <NUM> and the first L-shaped component <NUM>; (ii) the U-shaped component <NUM> and the second L-shaped component; (iii) the first L-shaped component <NUM> and the base component <NUM>; and (iv) the second L-shaped component and the base component <NUM> to couple the respective components together. The adhesive <NUM> may also be positioned between the U-shaped component <NUM> and the filler <NUM>.

In some embodiments, the pi-shaped preform <NUM> includes plies of prepreg <NUM> oriented in a ply stack. <FIG> is a schematic illustration of an individual ply of prepreg <NUM>, and <FIG> is a schematic illustration of plies of prepreg <NUM> oriented in a ply stack. In some embodiments, the base component <NUM> and the pair of axially elongated legs <NUM>, <NUM> each include plies of prepreg <NUM>. For example, the U-shaped component <NUM>, the first L-shaped component <NUM>, and the second L-shaped component <NUM> in the axially elongated legs <NUM>, <NUM> may each include plies of prepreg <NUM> arranged in a respective ply stack <NUM>.

By enabling the Pi preforms to be composed of discrete plies of aligned fiber composite, the ply orientations and volumes can be easily and cost effectively tailored to meet the mechanical requirements of each bond application. Current 3D woven Pi preforms cannot be quickly or cost effectively altered to meet specific mechanical requirements.

In some embodiments, the plies of prepreg <NUM> comprise a polymeric matrix <NUM>. In some embodiments, the prepreg <NUM> comprises from <NUM>% to <NUM>% (w/w) of a polymeric matrix <NUM>, based on the total weight of the prepreg <NUM>. In some embodiments, the prepreg <NUM> comprises at least <NUM>%, at least <NUM>%, at least <NUM>%, at least <NUM>% to less than <NUM>%, less than <NUM>%, less than <NUM>%, less than <NUM>%, less than <NUM>%, or less than <NUM>% (w/w), based on the total weight of the prepreg <NUM>. In some embodiments, the polymeric matrix <NUM> comprises a thermoplastic resin, a thermoset resin, or combinations thereof.

Suitable thermoplastic resins include, but are not limited to, polyacrylic acid, polyacrylic ester, poly(methyl methacrylate), acrylonitrile butadiene styrene polymer, polyamide, polylactic acid, polybenzimidazole, polycarbonate, polyether sulfone, polyoxymethylene, polyether ether ketone, polyaryletherketone, polyetherimide, polyethylene, polyphenylene oxide, polyphenylene sulfide, polypropylene, polystyrene, polyvinyl chloride, polyvinylidene fluoride, polytetrafluoroethylene, or combinations thereof. In some embodiments, the thermoplastic is a commercial polymer or an oligomer having a lower molecular weight than a commercial polymer.

Suitable thermoset resins include, but are not limited to, polyester, polyurethane, polyurea, vulcanized rubber, phenol-formaldehyde polymers, melamine polymer, bismaleimide polymer (BMI resin), polyepoxide (epoxy resin), polybenzoxazine, polyimide, polycyanurate, polyfuran, polysilicone, polyphenol, polyvinyl ester, polythiolyte, or combinations thereof. In some embodiments, the thermoset is a commercial polymer or an oligomer having a lower molecular weight than a commercial polymer.

In some embodiments, the prepreg <NUM> includes a hardener. In some embodiments, the prepreg <NUM> includes from <NUM>% to <NUM>% (w/w) of a hardener or crosslinker agent, based on the total weight of the prepreg <NUM>. In some embodiments, the prepreg <NUM> includes at least <NUM>% of the hardener, or at least <NUM>%, at least <NUM>%, at least <NUM>%, at least <NUM>%, or at least <NUM>% to less than <NUM>%, less than <NUM>%, less than <NUM>%, less than <NUM>%, less than <NUM>%, or less than <NUM>% (w/w) of the hardener, based on the total weight of the prepreg <NUM>.

Suitable hardeners include, but are not limited to, polyethylene tetraamine, dicyandiamide, phenylene diamine (particularly the meta-isomer), bis(<NUM>-amino-<NUM>,<NUM>-dimethylphenyl)-<NUM>,<NUM>-diisopropylbenzene,bis(<NUM>-amino-phenyl)<NUM>,<NUM>-diiospropylbenzene, diethyl toluene diamine, methylene dianiline, mixtures of methylene dianiline and polymethylene polyaniline compounds, diaminodiphenylsulfone, phenolic hardeners, or combinations thereof.

In some embodiments, the prepreg <NUM> includes an additive. In some embodiments, the prepreg <NUM> comprises from <NUM>% to <NUM>% (w/w) of an additive, based on a total weight of the prepreg <NUM>. In some embodiments, the prepreg <NUM> includes at least <NUM>% (w/w) of the additive, or at least <NUM>%, at least <NUM>%, at least <NUM>%, at least <NUM>%, or at least <NUM>% to less than <NUM>%, less than <NUM>%, less than <NUM>%, less than <NUM>%, less than <NUM>%, less than <NUM>%, or less than <NUM>% (w/w) of the additive, based on the total weight of the prepreg <NUM>.

Exemplary additives include fillers, accelerators, or combinations thereof. Suitable fillers include, but are not limited to, calcium carbonate, kaolin, magnesium hydroxide, wollastonite, silica, carbon black, fly ash, nanofillers (e.g., carbon nanotubes, graphene), polymer foam beads, carboxylated rubbers or combinations thereof. Suitable accelerators include, but are not limited to, <NUM>-phenyl-<NUM>,<NUM>-dimethyl urea, <NUM>-(<NUM>-chlorophenyl)-<NUM>,<NUM>,-dimethyl urea, <NUM>-(<NUM>,<NUM>-dichlorophenyl)-<NUM>,<NUM>-dimethyl urea, <NUM>,<NUM>-toluene bisdimethyl urea, <NUM>,<NUM>-toluene bisdimethyl urea, or combinations thereof.

The formation of complex shapes (e.g., L-shaped components and U-shaped components) in pi-shaped preform using continuous fiber composite materials is difficult without excessive defects such as ply wrinkling occurring during the forming process. Such defects lead to unacceptable mechanical performance in the pi-shaped preform <NUM>. Referring to <FIG>, the prepreg <NUM> includes discontinuous, aligned fibers <NUM> embedded therein. As used herein, the term "discontinuous" refers to fibers <NUM> in the prepreg <NUM> that include at least one gap or interval as the fiber <NUM> extends along an axis <NUM> in the prepreg <NUM>. In some embodiments, the fibers <NUM> are aligned at an angle of less than <NUM> degrees from the axis <NUM> in the individual prepreg <NUM>, or at an angle of less than <NUM> degrees, less than <NUM> degrees, less than <NUM> degrees, less than <NUM> degree, or less than <NUM> degrees from the axis <NUM> in the individual prepreg <NUM>.

In some embodiments, the prepreg <NUM> comprises from <NUM>% to <NUM>% (w/w) of discontinuous, aligned fibers <NUM>, based on the total weight of the prepreg <NUM>. In some embodiments, the prepreg <NUM> comprises at least <NUM>% (w/w) of discontinuous, aligned fibers <NUM>, or at least <NUM>%, at least <NUM>%, at least <NUM>%, at least <NUM>%, at least <NUM>% to less than <NUM>%, less than <NUM>%, less than <NUM>%, less than <NUM>%, less than <NUM>%, or less than <NUM>% (w/w) of the discontinuous, aligned fibers <NUM>. Suitable discontinuous, aligned fibers <NUM> include, but are not limited to, carbon fibers, glass fibers, aramid fibers, graphite fibers, boron fibers, or combinations thereof. In some embodiments the discontinuous, aligned fibers <NUM> have a nominal fiber length of at least <NUM>,<NUM> (<NUM> inches), at least <NUM>,<NUM> (<NUM> inches), at least <NUM>,<NUM> (<NUM> inch), at least <NUM>,<NUM> (<NUM> inches), at least <NUM>,<NUM> (<NUM> inches) to less than <NUM>,<NUM> (<NUM> inches), less than <NUM>,<NUM> (<NUM> inches), less than <NUM>,<NUM> (<NUM> inches) or less than <NUM>,<NUM> (<NUM> inches). In some embodiments, the discontinuous, aligned fibers <NUM> have about the same nominal fiber length (e.g., nominal fiber length for each of the discontinuous, aligned is within <NUM>% to <NUM>% of the nominal fiber length).

Referring to <FIG>, the plies of prepreg <NUM> may be arranged in a ply stack <NUM>. The ply stack <NUM> includes a layup of individual plies of prepreg <NUM>. In some embodiments, the plies of prepreg <NUM> may be stacked such that the orientation of the discontinuous, aligned fibers <NUM> differs between adjacent plies of prepreg <NUM> in the ply stack <NUM>. For example, discontinuous, aligned fibers <NUM> in a first prepreg <NUM> may be aligned along a first axis <NUM>, while the discontinuous, aligned fibers <NUM> in a second prepreg <NUM> adjacent to the first prepreg <NUM> may be aligned along a second axis <NUM>. The second axis <NUM> may be positioned at an angle <NUM> to the first axis <NUM>. The angle <NUM> may be any angle, but in some exemplary embodiments, the angle <NUM> is ±<NUM> degrees, ±<NUM> degrees, ±<NUM> degrees, or ±<NUM> degrees.

In some embodiments, the plies of prepreg <NUM> are arranged in a ply sequence within the ply stack <NUM>. As used herein, the term "ply sequence" refers to a stack arrangement for the plies of prepreg <NUM> that is repeated throughout at least a portion of the ply stack <NUM> or throughout the entire ply stack <NUM>. The ply sequence may be selected based on the desired structural properties of the pi-shaped preform <NUM>. In one non-limiting example, the plies of prepreg <NUM> for base component <NUM> or the axially elongated legs <NUM>, <NUM> may be arranged in a ply sequence selected from [<NUM>/<NUM>/-<NUM>/<NUM>]s or [<NUM>/<NUM>/-<NUM>/<NUM>/-<NUM>/<NUM>/<NUM>/<NUM>/-<NUM>/<NUM>/-<NUM>/<NUM>/<NUM>]t, where <NUM> refers to discontinuous, aligned fibers <NUM> that are oriented to align with the first axis <NUM>. The ability to tailor the ply sequence, in this way, to achieve desired anisotropic performance in the overall pi-shaped preform <NUM> provides superior mechanical performance as compared to current 3D woven pi-preforms.

As discussed above, the discontinuous, aligned fibers <NUM> offer a combination of formability and performance to achieve complex pi-shaped geometries. In addition, ply stacks <NUM> and specific layups can be tailored to drive pi-shaped preform <NUM> performance, while reducing the cost compared to conventional technologies. In some embodiments, each of the individual plies of prepreg <NUM> in the ply stack <NUM> comprise discontinuous, aligned fibers <NUM>. However, for some applications, it may be desirable to increase the strength of the pi-shaped preform <NUM>. Accordingly, in some embodiments, at least a portion of the individual plies of prepreg <NUM> in the ply stack <NUM> may include continuous, aligned fibers. As used herein, the term "continuous" refers to a fiber <NUM> that extends the entire length of the prepreg <NUM> without gaps or intervals, or fibers <NUM> that extend at least <NUM>%, or at least <NUM>%, or at least <NUM>% the length of the prepreg <NUM> without gaps or intervals. In some embodiments, individual plies of prepreg <NUM> oriented in the ply stack <NUM> at an angle of <NUM> degrees, <NUM> degrees, or <NUM> degrees relative to the first axis <NUM> comprise continuous, aligned fibers, while individual plies of prepreg <NUM> oriented in the ply stack at an angle other than <NUM> degrees, <NUM> degrees, or <NUM> degrees relative to the first axis <NUM> comprise discontinuous, aligned fibers <NUM>.

<FIG> illustrates a pi-joint assembly <NUM> according to some embodiments of the present disclosure. The pi-joint assembly <NUM> includes a pi-shaped preform <NUM> having the same components as described in <FIG>. The pi-shaped preform <NUM> may be used to join a first material <NUM> to a second material <NUM>. In some embodiments, the first material <NUM> is a skin of an aircraft and the second material <NUM> is a web of an aircraft. In some embodiments, the first and second material <NUM>, <NUM> may be metallic or composite materials. The first material <NUM> may be coupled to the base component <NUM> of the pi-shaped preform <NUM>, and the second material <NUM> may be coupled to an inner surface <NUM> of the channel <NUM> between the axially elongated legs <NUM>, <NUM>. In some embodiments, an optional adhesive <NUM> (not shown in <FIG>) couples the first material <NUM> to the base component <NUM> and the second material <NUM> to the axially elongated legs <NUM>, <NUM>.

In some embodiments, the first material <NUM> is a thermoset composite material, where the thermoset composite material may be uncured, partially cured or fully cured. In some embodiments, the second material <NUM> is a thermoset composite material, where the thermoset composite material may be uncured, partially cured or fully cured. In some embodiments, the first material <NUM> is a thermoset composite material and the second material <NUM> is a thermoset composite material comprising the same or different resin and the same or different fiber from those of the first material, whereas one or both the thermoset composite materials may be uncured, partially cured or fully cured. At the fully cured state the thermoset composite materials may reach maximum glass transition temperatures.

<FIG> illustrates a method <NUM> for manufacturing a pi-shaped preform <NUM>. The method <NUM> may begin at operation <NUM> which includes laying plies of prepreg <NUM> in a ply stack <NUM> for the base component <NUM> and the pair of axially elongated legs <NUM>, <NUM>. In some embodiments, operation <NUM> may also include laying plies of prepreg <NUM> in a ply stack for the U-shaped component <NUM>, the first L-shaped component <NUM>, and the second L-shaped component <NUM>. Materials used to form the filler <NUM> are cut to size.

At operation <NUM>, the method <NUM> includes consolidation and thermal forming the plies of prepreg <NUM> in a shape of the base component <NUM> and the pair of axially elongated legs <NUM>, <NUM>. For example, operation <NUM> may include thermal forming the plies of prepreg <NUM> in a shape of the U-shaped component <NUM>, the first L-shaped component <NUM>, and the second L-shaped component <NUM>. Thermal forming may include heating the plies of prepreg <NUM> to a thermal profile suitable for shearing the polymeric matrix <NUM> to create structural shapes and may advance chemical bonds that integrally link the plies of prepreg <NUM> together. Boundary tooling (e.g., molded shapes of silicone rubber, metal, or other materials) may be used as templates to shape the ply stack <NUM> into the desired shape of the base component <NUM> and the pair of axially elongated legs <NUM>, <NUM>. Operation <NUM> may further include placing the plies of prepreg <NUM> under pressure or vacuum to conform the plies of prepreg <NUM> to the shape of the boundary tooling. Specialized tooling is used to consolidate the ply stack and achieve sharp corners without the formation of wrinkles in the final preform. Operation <NUM> may include forming of the filler <NUM> in a shape as shown in <FIG> using thermal forming and tooling.

At operation <NUM>, the method <NUM> includes bonding the pair of axially elongated legs <NUM>, <NUM> to the base component <NUM> to form the pi-shaped preform <NUM>. For example, operation <NUM> may include positioning a filler <NUM> in a first space between the pair of axially elongated legs <NUM>, <NUM> and the base component. In some embodiments, operation <NUM> may include positioning the filler <NUM> in a first space between the U-shaped component <NUM> and the first L-shaped component <NUM>, and a filler <NUM> in a second space between the U-shaped component <NUM> and the second L-shaped component <NUM>. Operation <NUM> may further include bonding the first L-shaped component <NUM>, the U-shaped component <NUM>, the second L-shaped component <NUM>, and the base component <NUM> to form the pi-shaped preform <NUM>. In some embodiments, the method <NUM> further includes manufacturing a pi-joint assembly <NUM> by bonding the first material <NUM> to the base component <NUM>, and the second material <NUM> to an inner surface <NUM> of the channel <NUM>. Bonding may include curing composite or specifically an adhesive that is positioned between the aforementioned components. Curing the composite or the adhesive may be performed through any curing method, such as light curing or heat curing.

The following examples are provided to illustrate the invention but are not intended to limited the scope thereof.

Torayca Prepreg T800/<NUM>-<NUM> in discontinuous ET-<NUM> form was used to make a base component, a first L-shaped component, the second L-shaped component, and a U-shaped component. The base component included thirteen plies of prepreg arranged in a [<NUM>/<NUM>/-<NUM>/<NUM>/-<NUM>/<NUM>/<NUM>/<NUM>/-<NUM>/<NUM>/-<NUM>/<NUM>/<NUM>]t ply sequence. The first and second L-shaped component included eight plies of prepreg arranged in a [<NUM>/<NUM>/- <NUM>/<NUM>]s ply sequence. The U shaped component included eight plies of prepreg arranged in a [<NUM>/<NUM>/-<NUM>/<NUM>]s. Sub-component stack plies widths were reduced every two plies to create a tapered edge. The filler is composed of an axially aligned ply of T800/<NUM>-<NUM> prepreg, cut to width to form the proper volume, rolled, and formed to the shape of filler with round tools on a flat plate at approximately <NUM> degrees centigrade. The first space positioned between the first L-shaped component and the U-shaped component includes the filler, and the second space positioned between the second L-shaped component and the U-shaped component includes the filler. The sub-assembly is centered on a base, tooling is inserted into the channel and outside the L-shaped components, bagged and de-bulked and final formed into the pi-joint assembly.

A pi-joint assembly was assembled using the pi-shaped preform along with a composite skin and web laminate composed of Solvay IM7-<NUM>-<NUM> epoxy resin prepreg that was assembled and cured. The <NUM>,<NUM> (<NUM> inch) thick, fabric reinforced web panel and the <NUM>,<NUM> (<NUM> inch) thick, uni-tape with fabric outer ply reinforced skin utilized a quasi-isotropic prepreg stack sequence with outer peel plies of glass fabric/ <NUM>-<NUM> epoxy. Cured panels were cut to <NUM>,<NUM> (<NUM>") wide and <NUM>,<NUM> (<NUM> inch) length (web) and <NUM>,<NUM> (<NUM> inch) length (skin). The web edge to be bonded was rounded slightly to better match the clevis geometry of the pi-shaped preform. <NUM>,<NUM> (<NUM> inch) by <NUM>,<NUM> (<NUM> inch) strips of <NUM> AF191 <NUM>,<NUM> Pascal (<NUM> pound/square foot) adhesive with a knitted carrier were cut to cover the interior clevis and bottom base surfaces with excess width to supply excess adhesive to the joint end of part areas. The pi-shaped preform is built up with the skin panel centered on the flat cure tool, the peel ply is removed from the center <NUM>,<NUM> (<NUM> inch) width to expose highly bondable epoxy surface and one strip of AF191 is applied to the center of peeled section. The pi-joint assembly is centered on the AF191 and pressed to adhere. The first <NUM>,<NUM> (<NUM> inch) of the web panel to be bonded into the pi-joint is peeled on both faces for adhesive application. The second strip of AF191 adhesive is folded in half and wrapped around the peeled web before insertion into the clevis of the pi-shaped preform. The webs are attached to vertical, articulating tool bars which maintain <NUM> degree orientation to the skin panel and has force application threaded fasteners to pull the web into the tight clevis. Composite shims equal to the estimated stack-up height of the skin, adhesives and joint stacks of the base and legs of the pi-shaped preform are inserted under the excess web length and the tool pull fasteners used to pull them tight establishing proper joint geometry and per ply thickness. The assembled "T" panel joint component is bagged using industry standard materials and silicone over-presses to maintain formed geometry during cure. Teflon film inserts are used to prevent bonding in "T" panel areas which will be removed for shear coupon machining. The joint was cured for <NUM> hrs. at <NUM> degrees centigrade and <NUM> Pa (<NUM> psi).

A 3D woven pi-preform was fabricated using a IM7 3D woven preform (LMdrwg #LMA-MB0031, style A1111). The 3D woven pi-preform was infiltrated with Solvay <NUM>-<NUM> epoxy resin. A pi-joint assembly was assembled using the 3D woven pi-preform along with a composite skin and web laminate composed of Solvay IM7-<NUM>-<NUM> epoxy resin prepreg that was assembled and cured. The <NUM>,<NUM> (<NUM> inch) thick, fabric reinforced web panel and the <NUM>,<NUM> (<NUM> inch) thick, uni-tape with fabric outer ply reinforced skin utilized a quasi-isotropic prepreg stack sequence with outer peel plies of glass fabric/ <NUM>-<NUM> epoxy. Cured panels were cut to <NUM>,<NUM> ( <NUM>") wide and <NUM>,<NUM> (<NUM> inch) length (web) and <NUM>,<NUM> (<NUM> inch) length (skin). The web edge to be bonded was rounded slightly to better match the clevis geometry. <NUM>,<NUM> (<NUM> inch) by <NUM>,<NUM> (<NUM> inch) strips of <NUM> AF191 <NUM>,<NUM> Pa ( <NUM> pound/square foot ) adhesive with a knitted carrier were cut to cover the interior clevis and bottom base surfaces with excess width to supply excess adhesive to the joint end of part areas. The 3D woven pi-preform is built up with the skin panel centered on the flat cure tool, the peel ply is removed from the center <NUM>,<NUM> (<NUM> inch) width to expose highly bondable epoxy surface and one strip of AF191 is applied to the center of peeled section. The pi-joint assembly is centered on the AF191 and pressed to adhere. The first <NUM>,<NUM> (<NUM> inch) of the web panel to be bonded into the pi-joint is peeled on both faces for adhesive application. The second strip of AF191 adhesive is folded in half and wrapped around the peeled web before insertion into the clevis of the 3D woven pi-preform. The webs are attached to vertical, articulating tool bars which maintain <NUM> degree orientation to the skin panel and has force application threaded fasteners to pull the web into the tight clevis. Composite shims equal to the estimated stack-up height of the skin, adhesives and joint stacks of the base and legs of the 3D woven pi-preform are inserted under the excess web length and the tool pull fasteners used to pull them tight establishing proper j oint geometry and per ply thickness. The assembled "T" panel joint component is bagged using industry standard materials and silicone over-presses to maintain formed geometry during cure. Teflon film inserts are used to prevent bonding in "T" panel areas which will be removed for shear coupon machining. The joint was cured for <NUM> hrs. at <NUM> degrees centigrade and <NUM> Pa (<NUM> psi).

Shear strength testing is performed with Example <NUM> and Comparative Sample A. Shear strength testing is performed by pulling the web panel out of the bonded pi-joint along the axial length of the joint. The test fixture maintains alignment, coupling the test coupon to the test frame grips. Ultimate strength and load vs strength are recorded. Example <NUM> exhibited exceedingly high shear strength. Specifically, Example <NUM> exhibited ~<NUM>% shear strength compared to Comparative Sample A.

Pull-off strength testing is performed with Example <NUM> and Comparative Sample A. Pull-off strength testing is performed by pulling the web normally out of the joint with test frame gripping the web and the skin supported by rods. Ultimate strength and load vs strength are recorded. Example <NUM> exhibited comparable pull-off strength compared to Comparative Sample A. Specifically, Example <NUM> exhibited ~<NUM>% pull-off strength compared to Comparative Sample A.

Despite being a low cost altemative compared to Comparative Sample A, Example <NUM> provides improved shear strength and comparable pull-off performance.

Claim 1:
A pi-shaped preform comprising:
a base component; and
a pair of axially elongated legs coupled to the base component to define a channel between the axially elongated legs, the pair of axially elongated legs comprising plies of prepreg oriented in a ply stack,
characterized in that at least a portion of the prepreg comprises discontinuous, aligned fibers, wherein individual plies of prepreg oriented in the ply stack at an angle of <NUM> degrees or <NUM> degrees relative to a first axis comprise continuous, aligned fibers, and individual plies of prepreg oriented in the ply stack at an angle other than <NUM> degrees or <NUM> degrees relative to the first axis comprise discontinuous, aligned fibers.