Method for applying pressure to composite laminate areas masked by secondary features

A method for applying pressure to the area of a composite part masked by a secondary feature comprises the steps of positioning a pressure augmentation strip in contact with the masked area, securing a pressure transfer wedge on each of two sides of the secondary feature, such that pressure is transferred to the pressure augmentation strip during co-curing/co-bonding, and co-curing/co-bonding the composite part and the secondary feature. When the area between the composite part and the secondary feature is fairly large, the method can comprise the further steps of positioning a sine wave spring between the pressure augmentation strip and the secondary feature, injecting a structural paste adhesive into the voids around the sine wave spring, and curing the structural paste.

BACKGROUND OF THE INVENTION

The present invention generally relates to methods for manufacturing composite laminate articles and, more particularly, to methods for applying pressure to composite laminate areas that are masked by secondary features. Composite laminate areas masked by secondary features include the area of an uncured part, such as a skin preform, that experiences reduced pressure during co-curing/co-bonding processes due to the presence of a secondary feature, such as a precured frame web.

Composite laminate materials are known in the art. They are used extensively in the manufacturing of aircraft and other lightweight structures. They have a high strength-to-weight ratio and high stiffness. As is known in the art, a plurality of plies of uncured fiber reinforced organic matrix prepregs may be placed on a shaping tool. The prepregs may then be cured using known methods, such as vacuum bag processes, to produce the desired cured structure. Dry fiber preforms are also known in the art and have also been used to produce composite structures.

Vacuum bag processes may comprise covering the prepregs with a release material. The lay-up—comprising the prepregs, tool and release layer may then be hermetically sealed within a flexible vacuum bag. The bagged lay-up may then be evacuated to remove as much air and other volatiles as possible. After evacuation, the bagged lay-up may then be subjected to elevated temperature and/or pressure to cure the prepregs. Resin infused preforms may also be cured by known vacuum bag processes.

During curing, air and volatiles within the prepregs may be removed by vacuum. The organic resin may then flow into the areas previously occupied by the air. This process may be used to make composite laminate structures, but the shape and size of the structure is limited to the shape and size of the tool. Although tool shapes have advanced beyond those used to produce flat sheets, even more complex shaped structures are desired.

Two or more composite laminate structures have been joined together to form larger more complex shaped structures. Cured composite laminates have been joined together by mechanical fastening, adhesive bonding, and thermoplastic welding, such as moving coil welding and fixed coil induction welding. Although these joining methods can be used to produce complex shaped structures, they are not useful for some applications.

Other methods for producing complex shaped structures are known in the art. U.S. Pat. No. 5,817,269 describes methods for producing complex composite structures, such as wing boxes. In the described methods, two uncured parts, such as a wing skin lay-up and a rib lay-up, may be cured simultaneously. Although the disclosed methods may be used to make complex shaped articles, they require complicated tooling techniques. The described tooling techniques may not be feasible or desirable for some applications.

Co-curing/co-bonding processes have also been used to produce complex composite laminate structures. Co-curing/co-bonding processes may comprise laying up and precuring a composite part, such as an I-beam. This precured part may then be placed on an uncured lay-up, such as a wing skin lay-up. Bond clips, such as angle clips and pi ties, as known in the art, may be used to reinforce the interface between the uncured part and the precured part. The uncured lay-up together with the precured part and angle clips may then be vacuum bagged and co-cured/co-bonded. Co-curing/co-bonding processes have been used to produce wing boxes and other complex shaped structures. Unfortunately, known co-curing/co-bonding processes result in the buckling of the uncured part into the gap at the interface between the uncured part and the precured part on relatively thin structures. This buckling may cause wrinkles in the outer mold line. Additionally, for some applications, the known processes result in an undesirable amount of un-reinforced resin at the interface or a low fiber volume within the laminate. Un-reinforced resin or high resin volume laminate reduces the structural integrity of the finished product and has a tendency to microcrack.

As can be seen, there is a need for improved methods for producing complex shaped composite structures. Also, there is a need for a method of eliminating the buckling or wrinkling of uncured laminate during co-cure/co-bond processes. Further, there is a need for a method of reducing the volume of un-reinforced resin between two co-cured/co-bonded parts.

SUMMARY OF THE INVENTION

In one aspect of the present invention, a method of making a composite article comprises the steps of: positioning a pressure augmentation strip in contact with an area of the article masked by a precured secondary feature; positioning one pressure transfer wedge on each of two sides of the secondary feature, such that the pressure transfer wedge is capable of transferring a vacuum pressure to the pressure augmentation strip; and co-curing/co-bonding the composite article and the secondary feature.

In another aspect of the present invention, a method for applying pressure to an area of an uncured part masked by a precured part comprises the steps of providing a pressure augmentation strip and a sine wave spring; preparing the pressure augmentation strip and the sine wave spring for bonding; installing the pressure augmentation strip and the sine wave spring, such that the pressure augmentation strip is in contact with the area, and such that the sine wave spring is between the pressure augmentation strip and the precured part; injecting a structural paste adhesive between the pressure augmentation strip and the precured part; curing the structural paste adhesive; and co-curing/co-bonding the uncured part and the precured part.

In another still another aspect of the present invention, a method for applying pressure to an area of an uncured aircraft skin preform masked by a precured frame web or other composite component comprises the steps of: providing a prepared pressure augmentation strip and a prepared sine wave spring; placing the prepared pressure augmentation strip in contact with the area; positioning the prepared sine wave spring between the prepared pressure augmentation strip and the precured frame web, whereby a plurality of wave voids are produced; injecting a structural paste adhesive into the wave voids; curing the structural paste adhesive; and co-curing/co-bonding the uncured aircraft skin preform and the precured frame web.

In yet another aspect of the present invention, a method for applying pressure to an area of a composite laminate masked by a secondary feature that is too thin to implement the use of the sine wave spring, comprises the steps of: providing a pressure augmentation strip; preparing the pressure augmentation strip for bonding; installing the pressure augmentation strip, such that the pressure augmentation strip is in contact with the area; securing a pressure transfer wedge on each of two sides of the secondary feature, such that each pressure transfer wedge is capable of transferring a vacuum pressure to the pressure augmentation strip; and co-curing/co-bonding the composite laminate and the secondary feature.

DETAILED DESCRIPTION OF THE INVENTION

The present invention generally provides a method for applying pressure to composite laminate areas masked by secondary features. The method of the present invention may be used, for example, for applying pressure to the area of an uncured skin preform that is masked by a precured frame web. The method of the present invention may find beneficial use in many industries including aerospace and aircraft manufacturing. The present invention may be useful in the manufacturing of any composite laminate article, particularly those having a complex shape. The present invention may be useful in any manufacturing process where a secondary feature, such as a precured part, masks an area of an uncured part, such as an infused preform or prepreg laminate, during co-cure/co-bond processes.

Unlike the prior art, the method of the present invention applies pressure to composite laminate areas masked by secondary features, such as precured parts. The method of the present invention, also unlike the prior art, eliminates buckling of the masked areas of preforms and prepregs. Low fiber volume areas resulting from such masking are eliminated by the present invention. Unlike the prior art, the present invention reduces the amount of un-reinforced resin that would fill the gap between a precured part, such as a frame web, and an uncured part, such as skin preform, during vacuum bag co-curing/co-bonding. The present invention may reduce the un-reinforced resin by a volume about equal to the volume of the sine wave spring68and the structural paste adhesive70. The reduced volume may vary and may depend on the dimensions of the masked portion76and the dimensions of the masked void67.

InFIG. 1a, a flow chart of the steps according to a method of the present invention is depicted. The method may comprise a step40of providing a pressure augmentation strip60, a step41of preparing the pressure augmentation strip60for bonding, a step42of installing the pressure augmentation strip60, a step43of installing two pressure transfer wedges65a, and a step44of co-curing/co-bonding the uncured part61and the secondary feature62.

The pressure augmentation strip60, as seen inFIGS. 2aand2b, may be a precured laminate strip. Methods for producing laminates are known in the art. Known methods may comprise vacuum bag curing of laid-up prepregs. Useful methods may include those described in U.S. Pat. Nos. 4,657,717 and 6,017,484, both of which are herein incorporated by reference. The pressure augmentation strip60may be produced by any known method and may comprise any known laminate materials. They may comprise, for example, a 2-4-ply carbon/epoxy laminate. The pressure augmentation strip60may comprise a fiberglass, Kevlar or graphite based laminate. The pressure augmentation strip60may comprise any thermoplastic or thermoset organic resin. The composition of a useful pressure augmentation strip60may depend on the composition of the uncured part61, the composition of the secondary feature62, and the desired application. For example, when the uncured part61is a dry carbon skin preform and the secondary feature62is a precured carbon/epoxy frame web, a useful pressure augmentation strip60may comprise a precured carbon/epoxy laminate. The uncured part61may comprise a preform or a prepreg. The secondary feature62may comprise a precured laminate part.

The dimensions of a useful pressure augmentation strip60may depend on the dimensions of the area masked by the secondary feature62, referred to herein as a masked portion76. The masked portion76, as seen inFIG. 2a, is the portion of the uncured part61that may experience reduced pressure during co-curing/co-bonding processes due to the presence of a secondary feature62. The width of a useful pressure augmentation strip60may be greater than the width of the masked portion76. For example, when the masked portion76has the dimensions of four feet by one inch, the dimensions of a useful pressure augmentation strip60may be about four feet by about 1.5 inches. The pressure augmentation strip60may have a width of about 0.5 inches greater than the width of the masked portion76. Preferred pressure augmentation strips60may have a width between about 0.4 inches and about 2.0 inches. Useful pressure augmentation strips60may have a thickness between about 0.030 inches and about 0.060 inches.

As seen inFIG. 1a, the present invention may comprise a step41of preparing the pressure augmentation strip60for bonding. Methods for preparing a laminate for bonding are known in the art. These methods may comprise abrading the laminate and washing the laminate. The pressure augmentation strip60may be prepared for bonding by any known method, producing a prepared pressure augmentation strip. For example, the pressure augmentation strip60may be abraded by sanding or sandblasting. The pressure augmentation strip60may then be washed in a solvent, such as acetone or Isopropyl alcohol. Any known laminate washing solvents may be useful in the present invention.

The step42of installing the pressure augmentation strip60may comprise placing the pressure augmentation strip60in contact with the masked portion76of the uncured part61. The pressure augmentation strip60may be placed such that it covers the masked portion76and covers an area of the uncured part61that is adjacent to the masked portion76, as shown inFIG. 2a. The pressure augmentation strip60may be placed such that it covers two areas of the uncured part61on opposite sides of the masked portion76. This is possible because the pressure augmentation strip60may be wider than the masked portion76. As seen inFIG. 3, the pressure augmentation strip60may be placed between two bond clips63, such as angle clips. Bond clips63, as referred to herein, include angle clips, frame bond clips, and pi ties. In this figure, a release layer64covers the uncured part61. Release layers64and bond clips63are known in the art, any of which may be useful in the present invention. The bond clips63may comprise uncured preform or prepreg bond clips, such as dry or impregnated graphite fabric. The release layer64may comprise any known release layer, such as Teflon coated fiberglass, nylon, or other standard release ply material. As better seen inFIGS. 2aand2b, the pressure augmentation strip60may be positioned between the uncured part61and the secondary feature62.

The step43of installing two pressure transfer wedges65amay comprise placing the pressure transfer wedges65ain the desired areas and securing the pressure transfer wedges65awith an adhesive tape66, as seen in FIG.4. Pressure transfer wedges65aare known in the art and may comprise the elastomer pressure intensifiers used in vacuum bag processes. As better seen inFIG. 2a, the pressure transfer wedges65amay be placed such that they are capable of transferring pressure to the pressure augmentation strip60during the step44of co-curing/co-bonding. One edge area77aof a pressure augmentation strip60may be between an uncured part61and a pressure transfer wedge65a. Another edge area77bof the pressure augmentation strip60may be between the uncured part61and a second pressure transfer wedge65a, as seen inFIG. 2a. In this figure, one bond clip63is also between each pressure transfer wedge65aand edge area77a, b. The pressure augmentation strip60may be in contact with the uncured part61and two bond clips63. As shown inFIG. 2b, additional pressure transfer wedges65bmay be used during the step44of co-curing/co-bonding. The additional wedges65bmay be used to apply transfer pressure to other areas78of the secondary feature62, as is known in the art.

The step44of co-curing/co-bonding may comprise vacuum bag processes and autoclave processes. A portion of a vacuum bag72is shown inFIG. 2b. As can be seen, portions of the bond clips63may be pulled into a masked void67and may fill the masked void67during the step44of co-curing/co-bonding. The masked void67is the gap between the secondary feature62and the masked portion76.

Vacuum processes used in co-curing/co-bonding may cause voids, such as masked voids67, to be filled by adjacent uncured composites, such as uncured parts61and bond clips63. During co-curing/co-bonding processes using prior art methods, portions of the uncured part61are pulled into the masked void67, resulting in skin buckling. When using the method depicted inFIG. 1a, portions of the bond clip63may fill the masked void67. Using the method of the present invention, the uncured part61may be prevented from buckling into the masked void67during co-curing/co-bonding. This prevention may be due to the presence of the pressure augmentation strip60and the pressure transfer wedges65a. During co-curing/co-bonding, a vacuum pressure may be transferred from the pressure transfer wedge65ato the pressure augmentation strip60. This vacuum pressure may then be transferred to the uncured part61, thereby preventing the masked portion76from buckling. Using the known process of filling the gap between the cured part and the uncured skin with a “noodle” of uncured or dry fiber can further reduce the occurrence of voids but is extremely tedious when the precured part is thin. With the pressure augmentation strip60in place, the “noodle can more easily be inserted.

For some applications, the method of the present invention can optionally comprise the additional steps depicted inFIG. 1b. These applications may include, for example, situations wherein the secondary feature62has a width between about 0.15 inches and about 0.5 inches. The steps depicted inFIG. 1bmay be useful for any application wherein the secondary feature62has a width of at least about 0.150 inches.

A method of the present invention may further comprise a step50of providing a sine wave spring68; a step51of preparing the sine wave spring68for bonding; a step52of installing the sine wave spring68, whereby a plurality of wave voids69are produced; a step53of injecting a structural paste adhesive70into the wave voids69; and a step54of curing the structural paste adhesive70.

The sine wave spring68of step50is shown in FIG.5. The sine wave spring68may comprise a precured laminate. The sine wave spring68may be produced by known laminate forming processes. Laminate prepregs may be laid-up on a sine wave shaped tool and cured to form a sine wave spring68. As with the pressure augmentation strip60, the composition of the sine wave spring68may vary depending on the application. The sine wave spring68may comprise a compatible composite. For example, when the secondary feature62comprises carbon/epoxy, the sine wave spring68may comprise carbon/epoxy. The dimensions of the sine wave spring68may depend on the dimensions of the masked portion76and the dimensions of the masked void67. The width of the sine wave spring68may be about equal to the width of the masked portion76. Useful sine wave springs68may have a width between about 0.150 inches and about 0.5 inches. The sine wave spring68may fit within the masked void67.

The step51of preparing the sine wave spring68for bonding may comprise the same methods of abrading and washing as used in step41for preparing the pressure augmentation strip60for bonding. The step51may produce a prepared sine wave spring. The sine wave spring68may be installed after it has been prepared for bonding.

The step52of installing the sine wave spring68may comprise positioning the sine wave spring68between the pressure augmentation strip60and the precured secondary feature62, as shown inFIGS. 5 and 6. The sine wave spring68may be installed such that it is within the masked void67. The sine wave spring68may be in contact with the secondary feature62and the pressure augmentation strip60, as shown in FIG.6. In this figure, the bond clip63has been folded to reveal the sine wave spring68.

The step53of injecting a structural paste adhesive70into the wave voids69is shown in FIG.7. The wave voids69may be the voids defined by the sine wave spring68, as shown in FIG.6. As seen inFIG. 5, the structural paste adhesive70may surround the sine wave spring68. Methods for injecting a structural paste adhesive70are known in the art, all of which may be useful in the present invention. As shown inFIG. 7, an injector device71may be useful. The composition of the structural paste adhesive70may vary depending on the application. All known structural paste adhesives70may be useful. Useful structural pastes adhesives70may include structural epoxy adhesive and acrylic adhesive.

The step54of curing the structural paste adhesive70may comprise any known method for curing a structural paste adhesive70. A useful method may depend on the composition of the structural paste adhesive70. For example, when the structural paste adhesive70is a structural epoxy adhesive, the step54may comprise curing at room temperature until fully gelled. The structural paste adhesive70may be cured prior to the step44of co-curing/co-bonding. When the uncured part61is a preform, preform resin infusion may occur after the structural paste adhesive70has been cured. Preform resin infusion is known in the art and comprises infusing the preform with a polymer resin.

After the structural paste adhesive70has been cured, the method may comprise a step44of co-curing/co-bonding. Vacuum processes used in co-curing/co-bonding may cause voids, such as masked voids67, to be filled by un-reinforced resin from the adjacent uncured composites, such as uncured parts61and bond clips63. During co-curing/co-bonding processes of prior art methods, this may result in structures with an unacceptable volume of un-reinforced resin at the interface between the two co-cured/co-bonded parts. Using the method of the present, the un-reinforced resin volume may be reduced by 80-90%. The sine wave spring68and the structural paste adhesive70may be within the masked void67. The presence of the sine wave spring68and structural paste adhesive70may reduce the volume of the masked void67that is capable of receiving laminate resin during co-curing/co-bonding processes. When the volume of the masked void67is reduced, the volume of un-reinforced resin in the finished product may be equally reduced. Cured structural paste adhesive70may be stronger and less prone to microcracking than un-reinforced resin. The method of the present invention can be used to produce improved structures.

An alternative embodiment of the present invention may comprise steps40-42, a step44, and steps50-54. A step43of installing two pressure transfer wedges may not be necessary in some applications. The presence of the sine wave spring68and structural paste adhesive70may provide sufficient pressure to the pressure augmentation strip60during co-curing/co-bonding processes, thereby making the pressure transfer wedges65aunnecessary in some applications. A further embodiment of the present invention may comprise the use of the pressure augmentation strip60without the sine wave spring68. The sine wave spring68may be used only where the secondary features62, such as frames, meet or exceed 0.15 inches thick in the web area. The majority of the fuselage frames may be much thinner so only the pressure augmentation strips60may be used.

A rotary wing fuselage structure73, as shown inFIG. 8, was constructed using the method of the present invention. Each of twenty-six pressure augmentation strips60(thirteen to a side, split at the middle for ease of installation) was placed between two frame bond clips and in contact with a fuselage skin75. The fuselage skin75was comprised of dry fabrics stitched together as a preform. The frame bond clips were comprised of dry fabrics stitched together and stitched to the fuselage skin75. The pressure augmentation strips60were comprised of 2-4 ply carbon/epoxy precured laminate and had dimensions varying from 0.5″ wide to 0.8″ wide. Each of six sine wave springs68was positioned between one pressure augmentation strip60and one thick fuselage frame member74. The fuselage frame members74were comprised of precured carbon/epoxy laminates. The thin fuselage frame members74and thin areas on the heavy fuselage frame members74did not require sine wave springs68nor could they be used due to the thin (less than 0.150″ thick) frame laminate. The use of the sine wave spring68within the rotary wing fuselage structure73was limited to six locations, about nine inches long each in thick areas of three frame members74. The balance of the frame members74used only the pressure augmentation strips60and the elastomer wedges. Structural paste adhesive70was not used where the frame members74were less than 0.150″ thick and no sine wave spring68was installed. The sine wave springs were comprised of 2-4 ply carbon/epoxy precured laminate and had the dimensions of 0.25″ to 0.35″ wide by 8 inches to 9 inches long. An epoxy structural adhesive was injected into the areas around the sine wave springs68and then cured at room temperature per manufacturer's Instructions. The fuselage skin75and the fuselage frame members74were then co-cured/co-bonded by placing in an oven and running a cure per resin manufacturer's recommendations. The rotary wing fuselage structure73was fourteen feet long, four feet wide, and four feet deep. The fuselage skin75did not buckle during co-curing/co-bonding.

As can be appreciated by those skilled in the art, the present invention provides improved methods for composite manufacturing. A method for applying pressure to an area masked by a secondary feature is also provided. Additionally, methods are provided for eliminating the buckling of aircraft skin composites. Further, a method for reducing the amount of un-reinforced resin in a composite structure is provided.