Patent Description:
Various processes and equipment may be used to form a flat composite charge into a stiffener such as a stringer having a desired cross-sectional shape. For example, stringers may be made by forming the composite charge into a die cavity using a punch or inflatable bladder. In those applications where the stringer is contoured long its length as part of the forming process, stresses are locally created within the composite charge during forming that may result in buckling or wrinkling in the areas of the stringer containing these stresses. In the case of a contoured stringer having a hat-shaped cross section ("hat stringer"), the stresses tend to form in the contoured regions of the stringer, and cause buckling or wrinkling of the cap. Buckling or wrinkling of the cap is undesirable because it may affect the structural performance of the stringer, resulting in the need for stringer rework or alteration of the stringer design that may increase labor and/or material costs.

Accordingly, it would be desirable to provide a method and apparatus for making contoured hat stringers that reduces or eliminates buckling and/or wrinkling of the cap, particularly in contoured sections of the stringer.

Document <CIT>, according to its abstract, discloses a method for forming composite materials including providing a composite charge wider than a first surface of a mandrel, and positioning the composite charge across the first surface of the mandrel. The portion of the composite charge overhanging the first surface of the mandrel is supported and urged against the mandrel while supporting the unbent portion of the composite charge substantially parallel to the first surface of the mandrel.

Document <CIT>, according to its abstract, discloses how a flexible punch and die are used to form a flat composite laminate charge into a stiffener having a desired cross sectional shape. A desired contour is formed in the stiffener by bending the punch and die. Ply wrinkling is avoided during contouring by maintaining those portions of the stiffener subject to wrinkling in tension as the contouring is being performed. Bridging of the plies during the contouring process is avoided by first contouring those sections of the stiffener that are subject to wrinkling, and then contouring the remaining sections of the stiffener.

Document <CIT>, according to its abstract, discloses a manufacturing system including a first mandrel, a second mandrel, and laminate securing mechanisms. The first mandrel has a first mandrel surface and a first mandrel surface edge. The second mandrel has a second mandrel surface and a second mandrel surface edge, and is positionable in a closed position in which the first mandrel surface edge and the second mandrel surface edge are in contact to form a continuous mandrel surface collectively defined by the first mandrel surface and the second mandrel surface. The second mandrel translates to an open position defining a gap between the first mandrel surface edge and the second mandrel surface edge for receiving a forming die. The laminate securing mechanisms secure the composite laminate on at least one of the first mandrel and the second mandrel during trimming and/or forming of the composite laminate.

Document <CIT>, according to its abstract, discloses a method of manufacturing a composite component comprising a main body and an integral flange, the method comprising applying fibre-reinforcement material on a tool having a main body portion and a flange-forming portion to provide a pre-form comprising a body region and a longitudinally adjacent flange region. The pre-form extends generally longitudinally between two longitudinal ends (<NUM>); and a trailing ply of the pre-form extends generally longitudinally between the longitudinal end closest to the flange region and an inner ply end located in the flange region or partway into the body region.

The disclosure relates in general to manufacturing composite parts, and more specifically to methods and apparatus for producing contoured composite hat stringers having reduced wrinkling.

More particularly, apparatus is provided for making a contoured composite hat stringer comprising the features disclosed at claim <NUM>. The dependent claims outline advantageous forms of embodiment of the apparatus.

According to another aspect, a method is provided of forming a contoured composite hat stringer having reduced wrinkling, comprising the steps described at claim <NUM>. The dependent claims outline advantageous ways of carrying out the method.

According to still another aspect, which goes beyond the scope of the appended claims, a method is provided of forming a contoured composite hat stringer heading a hat section including sides and a cap. The method comprises placing a flat composite charge on a pair of dies defining a die cavity, and forcing the flat composite charge into the die cavity to form a hat section of the stringer having sides and a cap. The method further comprises contouring the die cavity, and reducing wrinkling of the cap during forming of the hat section by transferring stress in the cap away from the cap.

One of the advantages of the disclosed method and apparatus is that contoured composite hat stringers can be produced in which wrinkling in contoured areas of the cap is reduced or eliminated. Another advantage is that the method can be implemented with only minor modifications of existing stringer forming equipment. Still another advantage is that rework and attendant labor and material costs caused by stringer wrinkling may be reduced or eliminated. A further advantage is that reduced cap wrinkling may lead to improved structural performance of hat stringers.

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

The novel features believed characteristic of the illustrative examples are set forth in the appended claims. The illustrative examples, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative examples of the present disclosure when read in conjunction with the accompanying drawings, wherein:.

Referring first to <FIG>, a hat stringer <NUM> having one or more out-of-plane curvatures comprises a hat section <NUM> and a pair of outwardly turned flanges <NUM>. The hat section <NUM> includes a substantially flat cap <NUM> coupled with the flanges <NUM> by a pair of inclined sides <NUM>, sometimes referred to as webs. The hat stringer <NUM> may possess other cross sectional shapes. For example, <FIG> illustrates a hat stringer <NUM> having a hat section <NUM> with a rounded cap <NUM> that smoothly transitions into the sides <NUM>.

The hat stringer <NUM> may comprises a composite laminate formed of multiple plies of a fiber reinforced polymer such as a thermoset or thermoplastic. As will be discussed below, the hat stringer <NUM> may have one or more out-of-plane contours or curvatures along its length. In the example shown in <FIG>, the hat stringer <NUM> possesses a single constant curvature in the X-Z plane within the coordinate system shown at <NUM>. <FIG> illustrates another example of a hat stringer <NUM> having contours in both the X-Z and X-Y planes. Referring particularly to <FIG>, as will be discussed later, during forming of the hat stringer <NUM> to the desired cross-sectional shape, regions <NUM> of the cap <NUM> along inside radius of the hat section <NUM> may be placed in compression <NUM> as a result of the forming and contouring process. In one example, contouring of the hat stringer <NUM> occurs as the hat stringer is being formed to the desired cross section shape. Alternatively, in another example, the hat stringer <NUM> is contoured along its length in a separate operation after it has been formed to the desired cross sectional shape. In each of these examples, due to the contouring, the cap <NUM> is placed in compression <NUM> along the regions <NUM> of its length that are contoured. Compression <NUM> of the cap <NUM> in this manner during stringer forming may cause the cap <NUM> to buckle or wrinkle.

Attention is now directed to <FIG>, which illustrate an apparatus <NUM> for forming a flat composite charge <NUM> into a contoured hat stringer <NUM> having reduced wrinkling of the cap <NUM>. The apparatus <NUM> includes a tool set <NUM> comprising a punch <NUM>, and a pair of dies <NUM> that are spaced apart to form a die cavity <NUM>. Referring to <FIG>, the punch <NUM> may be segmented along its length to allow it to flex, and is mounted on the bottom of a flexible top plate <NUM>. The cross sectional shape of the punch <NUM> substantially matches the IML (inner mold line) of the hat section <NUM> (<FIG>). In other examples of the tool set <NUM>, an inflatable bladder (not shown) may be used in lieu of the punch <NUM> to form the flat composite charge <NUM> down into the die cavity <NUM>. The tool set <NUM> may be installed in a press (not shown) having press platens (not shown) that move the top and bottom plates <NUM>, <NUM> relative to each other, causing displacement of the punch <NUM> into the die cavity <NUM> at a desired rate and with a desired amount of force F.

The dies <NUM> are mounted for lateral movement <NUM> on a flexible bottom plate <NUM>. In one example, the dies <NUM> are segmented along their lengths, allowing them to flex out-of-plane. In another example, the dies <NUM> may each comprise a series of individual die blocks (not shown), likewise permitting the dies <NUM> to flex out-of-plane. A pair of side rails <NUM> are secured to the bottom plate <NUM>, outboard of the dies <NUM>. Inflatable side bladders <NUM> are respectively located between the dies <NUM> and the side rails <NUM>. The side bladders <NUM> may be inflated with a fluid such as air, and function to control the outward lateral movement of the dies <NUM> during the forming process. In other examples, as shown in <FIG>, a mechanical actuator <NUM> or similar mechanism coupled with each of the dies <NUM> by a drive rod <NUM> may be employed to control lateral movement of the dies <NUM>.

Referring to <FIG>, an inflatable pinch bladder <NUM> is installed within the die cavity <NUM> which may rest on the bottom plate <NUM> until it is inflated with air or another suitable pressurized fluid. The cap pinch bladder <NUM> may be formed of any suitable material such as an elastomer having rigidity that is sufficient to apply pressure to and constrain the cap <NUM> of the hat section <NUM> during the forming process. In order to achieve the required rigidity, in some examples, the cap pinch bladder <NUM> may include local reinforcements (not shown). In the illustrated example, the cap pinch bladder <NUM> is sized to completely fill the die cavity <NUM>, extends along the entire length of the die cavity <NUM>. However in other examples, the pinch bladder <NUM> may be located in only a section of the die cavity <NUM> where compression of the cap <NUM> may be sufficient to cause buckling or wrinkling of the cap <NUM>.

A stringer forming operation begins with the tool set <NUM> arranged as shown in <FIG>, with the punch <NUM> in a raised position. Next, as shown in <FIG>, the cap pinch bladder <NUM> is installed within the die cavity <NUM> in a deflated or partially deflated condition. Then, a flat composite charge <NUM> is placed on the dies <NUM>, spanning the die cavity <NUM>. Next, a pair of flange pinch bladders <NUM> are installed between the outer edges <NUM> of the composite charge <NUM> and the top plate <NUM>. The flange pinch bladders <NUM> may be pressurized with air or other suitable fluid, and as will be described below, function to pinch the outer edges <NUM> of the composite charge <NUM> against the dies <NUM> as the punch <NUM> "punches" the composite charge <NUM> down into the die cavity <NUM>.

To perform a forming operation, the top plate <NUM> moves down, causing the punch <NUM> to initially contact the composite charge <NUM>. Immediately prior to the punch <NUM> contacting the composite charge <NUM>, the cap pinch bladder <NUM> is inflated, causing it to come into contact with and apply pressure against the bottom of the composite charge <NUM>. Continued downward movement of the top plate <NUM> (<FIG>) causes the punch <NUM> to begin forming the composite charge <NUM> into the die cavity <NUM>. As the composite charge <NUM> is being formed into the die cavity <NUM>, the flange pinch bladders <NUM> apply pressure to the outer edges <NUM> of the composite charge <NUM>, holding them flat against the dies <NUM>, while allowing them to slip over the dies <NUM> during the remainder of the forming process.

As the punch <NUM> forms the composite charge <NUM> into the die cavity <NUM>, the pressurized cap pinch bladder <NUM> bears against and applies pressure on the cap <NUM>, thereby restraining the cap <NUM> against buckling or wrinkling. Continued movement of punch <NUM> into the die cavity <NUM> results in the cap pinch bladder <NUM> moving onto and applying pressure to the sides <NUM> of the hat section <NUM> (see <FIG>), until eventually the entire area of the hat section <NUM> is nearly enveloped and constrained by the cap pinch bladder <NUM> (<FIG>). Depending on the application, it may be necessary to coordinate the level of pressurization of the cap pinch bladder <NUM> with the desired rate of movement of both the punch <NUM> and the dies <NUM> to achieve optimum results.

In one example, as previously mentioned, prior to forming the composite charge <NUM> to the desired cross-section, the tool set <NUM> is contoured along its length using, for example, a later discussed contour changing mechanism <NUM> (<FIG>). Consequently, in this example of the forming sequence, the composite charge <NUM> is formed into the die cavity <NUM> that has been previously contoured along its length, consequently, compression of contoured regions <NUM> of the cap <NUM> occurs as the cross-sectional shape of the hat section <NUM> is being formed. In another example of the forming sequence, the composite charge <NUM> is formed into the die cavity before the tool set (including the die cavity <NUM>) is contoured along its length. In this latter example, compression of the contoured regions <NUM> of the cap <NUM> occurs after the cross-sectional shape of the hat section <NUM> has been formed, as the hat section <NUM> is being formed to its final contour along its length. Regardless of which of these forming sequences is used, the cap pinch bladder constrains the cap <NUM>, causing stresses in the cap <NUM> caused by compression to be swept away to other areas of the hat section <NUM> that may not subject to buckling or wrinkling.

<FIG> graphically illustrates how the use of the cap pinch bladder <NUM> reduces or eliminates wrinkling of the cap <NUM> when forming the hat stringer <NUM> to a desired contour. As previously discussed, when forming the hat section <NUM> within contoured regions of the hat stringer <NUM>, the cap <NUM> is placed in compression, resulting in the creation of stresses <NUM> within the cap <NUM> that can cause buckling or wrinkling of the cap <NUM>. To prevent such buckling or wrinkling, the force <NUM> applied to the cap <NUM> by the cap pinch bladder <NUM> constraints cap <NUM>, forcing the cap stresses <NUM> to shift <NUM> and spread to other locations in the hat section <NUM> that are not be subject to buckling or wrinkling. For example, the cap stresses <NUM> may shift <NUM> onto the sides <NUM> of the hat section <NUM>, or may shift longitudinally along the cap <NUM> to cap regions where the cap <NUM> is not under compression. In other words, the constraint imposed on the cap <NUM> by the cap pinch bladder <NUM> effectively relieves these stresses by sweeping them to other areas of the hat stringer <NUM>.

Referring to <FIG>, the force <NUM> applied to the cap <NUM> by the cap pinch bladder <NUM> in contoured regions <NUM> of the hat section <NUM> causes material in the composite charge <NUM> that is in contact with the punch <NUM> to strain <NUM> into at least some of the slits <NUM> in the punch <NUM>. Strain of the material within the cap <NUM> into the slits <NUM> assists in relieving some of the stress <NUM> in the cap <NUM>, further reducing the possibility of buckling or wrinkling of the cap <NUM>.

Various mechanisms may be used to configure the tool set <NUM> for forming one or more out-of-plane contours in the hat stringer <NUM>. <FIG> illustrates one example of a contour changing mechanism <NUM> used to configure the tool set <NUM> to form a hat stringer <NUM> that is contoured along its length in the X-Z plane (<FIG>). The contour changing mechanism <NUM> may comprise, for example and without limitation, a press <NUM>. The press <NUM> includes a plurality of individual, spaced apart actuators <NUM> respectively mounted on opposite press plates <NUM> that are adapted for movement toward and away from each other, as indicated by the arrows <NUM>. The tool set <NUM> is positioned between the press plates <NUM>. The press plates <NUM> may be coupled with any suitable power operated mechanisms such as cylinder actuators (not shown) which displace the press plates <NUM> to open/close the tool set <NUM> during a composite charge forming operation. Each of the actuators <NUM> includes a drive rod <NUM> coupled with one of the top and bottom flexible plates <NUM>, <NUM>. The drive rods <NUM> displace the flexible top and bottom plates <NUM>, <NUM> which in turn bend (contour) the dies <NUM>, thereby contouring the hat stringer <NUM>. As previously discussed, the dies <NUM> may be contoured before the composite charge <NUM> is "punched" into the die cavity <NUM>. Alternatively, however, the composite charge <NUM> may be punched into the die cavity <NUM> after the dies <NUM> have been contoured by the contour changing mechanism <NUM>.

Attention is now directed to <FIG>, broadly illustrates the components of an apparatus <NUM> for making contoured hat stringers <NUM> with reduced cap wrinkling. A controller <NUM> is coupled with and is operable to control operation of the press <NUM>, the contour changing mechanism <NUM>, as well as a pressurizing system <NUM>. The controller <NUM> may comprise a PC (personal computer) or a programmable controller operating under the control of one or more software programs <NUM>. A pressurizing system <NUM> may comprise any suitable pump and fluid reservoir (not shown) that are operable to independently pressurize/depressurize (inflate/deflate) the side bladders <NUM>, flange pinch bladders <NUM>, and the cap pinch bladder <NUM>. The controller <NUM> controls and coordinates operation of the contour changing mechanism <NUM>, the press <NUM> and a pressurizing system <NUM>, such that the forming and contouring processes are carried out at a controlled rate. The controller <NUM> may also coordinate and synchronize inflation of the cap pinch bladder <NUM> with the movement of the punch <NUM> and the dies <NUM>.

<FIG> broadly illustrates the steps of a method of making composite hat stringers <NUM> with reduced wrinkling. Beginning at <NUM>, a hat section <NUM> of the stringer having sides <NUM> and a cap <NUM> is formed by forcing a composite charge <NUM> into a die cavity <NUM>. At <NUM>, stresses in the cap <NUM> produced during the forming are reduced by constraining cap <NUM> the as a composite charge <NUM> is being forced into the die cavity <NUM>.

<FIG> illustrates the steps of another example of a method of making composite hat stringers <NUM> possessing a hat section <NUM> having a cap <NUM> and sides <NUM>. At <NUM>, a flat composite charge <NUM> is placed on a pair of dies <NUM> defining a die cavity <NUM> therebetween. At <NUM>, the flat composite charge <NUM> is forced into the die cavity <NUM> to form a hat section <NUM> of the hat stringer <NUM> having a cap <NUM> and sides <NUM>. At <NUM>, the hat section <NUM> is contoured along its length. At <NUM>, stresses in the cap <NUM> are directed away from the cap <NUM> by applying pressure against the cap <NUM> as the hat section <NUM> is being contoured.

Examples of the disclosure may find use in a variety of potential applications, particularly in the transportation industry, including for example, aerospace, marine, automotive applications and other application where hat stringers may be used. Thus, referring now to <FIG>, examples of the disclosure may be used in the context of an aircraft manufacturing and service method <NUM> as shown in <FIG> and an aircraft <NUM> as shown in <FIG>. Aircraft applications of the disclosed examples may include a variety of contoured hat-type stringers used the airframe <NUM> of the aircraft <NUM>. During pre-production, exemplary method <NUM> may include specification and design <NUM> of the aircraft <NUM> and material procurement <NUM>. During production, component and subassembly manufacturing <NUM> and system integration <NUM> of the aircraft <NUM> takes place. Thereafter, the aircraft <NUM> may go through certification and delivery <NUM> in order to be placed in service <NUM>. While in service by a customer, the aircraft <NUM> is scheduled for routine maintenance and service <NUM>, which may also include modification, reconfiguration, refurbishment, and so on.

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

Claim 1:
Apparatus (<NUM>) for making a contoured composite hat stringer (<NUM>), comprising:
a pair of dies (<NUM>) configured to be contoured along a length and defining a die cavity (<NUM>) into which a flat composite charge (<NUM>) may be formed into a contoured hat section (<NUM>) having a cap (<NUM>);
a punch (<NUM>) configured to form the composite charge (<NUM>) into the die cavity (<NUM>); and
a bladder (<NUM>) located within the die cavity (<NUM>) between the dies (<NUM>) and configured to be arranged beneath the composite charge (<NUM>), the bladder (<NUM>) being sized to completely fill the die cavity (<NUM>) when the bladder (<NUM>) is inflated and being configured to constrain the cap (<NUM>) as the contoured hat section (<NUM>) is being formed.