Placement of prepreg tows in high angle transition regions

Prepreg tows are placed on a substrate having a bend with a curvature extending over a transition region in the substrate. The tows are steered and laid on the substrate in at least a first section and a second section within the transition region, wherein each of the first and second sections has an angular orientation that is less than the curvature of the bend in order to reduce gathering of the tows.

BACKGROUND INFORMATION

The present disclosure generally relates to the fabrication of composite laminate structures, and deals more particularly with automated placement of prepreg tows in high angle transition regions of a structure.

Numeric computer controlled advanced fiber placement (AFP) machines may be used to layup large-scale, complex-shaped composite laminate structures. For example, in the aircraft industry, AFP machines may be used to layup composite airframe components such as spars and stringers. These AFP machines typically have one or more material placement heads that may be manipulated to apply and compact multiple prepreg tows on a mandrel or similar tool. Each tow comprises a formed tow prepreg or narrow strip cut from unidirectional tape which includes a bundle of fibers pre-impregnated with resin. In order to form nonlinear features or details of a structure, the material placement head is sometimes programmed to follow constant radius paths.

Automated layup of composite structures as discussed above can be challenging where the structure is highly contoured or has sharp geometrical features. For example, limitations on machine programming and/or material placement head movement may prevent layup of material around relatively sharp corners or highly angled bends, hereinafter referred to as “high angle transition regions” or “transition regions”. Material characteristics such as, without limitation, material width, tack and fiber stiffness may also limit material placement in high angle transition regions. Another problem that may be encountered when laying tows in high angle transition regions is wrinkling, buckling and/or distortion of the tows. Steering the tows around sharp, constant radius paths in these transition regions causes the inside radius of the tows to be placed in compression. This inside radius compression may force the fibers of the tow to gather, resulting in wrinkles, buckles and/or fiber distortion that may have an undesired effect on the mechanical performance of the structure.

Tow gathering in high angle transition regions may be reduced to some degree by using narrower tows, however the use of narrower tows reduces the rate at which material can be laid, thus reducing production efficiency, and may not be practical in some applications. Employing narrow tows may require the use of compensating reinforcements such as additional plies because narrow tows may cause undesired knockdown in some mechanical properties of a structure. These compensating reinforcements add undesired weight to the structure and may increase manufacturing costs.

In order to overcome the problem of material gathering when wider tows are used, and/or limitations on the movement of material placement heads, composite laminate structures having high angle transition regions are currently produced using a multi-step process in which a joint containing the high angle transition region is separately fabricated and then joined to straight sections of the structure. This solution to the problem is time-consuming, labor intensive and requires multiple, complex and expensive tools. Moreover, use of a separately fabricated joint may require the use of additional reinforcements in order to achieve structural performance requirements.

Accordingly, there is a need for a method of placing composite material in high angle transition regions which permits formation of complex geometrical features or details of a structure and which reduces or eliminates material wrinkling, bucking and/or fiber distortion, while permitting fabrication of the structure as a single component. There is also a need for a method of automated placement of prepreg tows within high angle regions and sharp corners or highly angled bends that is not limited by AFP machine programming capabilities and/or AFP material application head movements. Further, there is a need for a method of fabricating composite laminate structures having high angle transition regions that obviates the need for separately fabricated joints and multiple tools.

SUMMARY

The disclosed embodiments provide a method of placing prepreg tows in high angle transition regions on a substrate during automated layup of composite laminate structures. The method substantially reduces or eliminates wrinkling, buckling and/or fiber distortion of the tows in the high angle transition regions. The reduction or elimination of tow wrinkling may reduce or eliminate localized stresses in a laminate, which may lead to improved performance of the structure. Complex and/or difficult-to-form geometric features such as sharp bends of a structure may be laid up, which may otherwise not be possible because of limitations on AFP machine programming and/or material placement head movements, and/or material properties. The disclosed method may allow the use of wider tows in order to increase the material application rate, and thus may increase production efficiency. A composite laminate structure having high angle transition regions may be fabricated as a single component, rather than multiple components which require multiple assembly tools.

According to one disclosed embodiment, a method is provided of placing a prepreg tow on a substrate having a bend with a curvature extending over a transition region in the substrate. The tow is laid in at least a first section and a second section within the transition region. Each of the first and second sections has an angular orientation that is less than the curvature of the bend. The first section and the second section of the tow at least partially span the transition region. The bend has a bend angle, and each of the first and second sections of the tow may be curved and have an angle of curvature that is less than the bend angle. The shape of the tow approximates an ideal shape of the bend in the transition region. In one variation, each of the first and second sections of the tow is a substantially straight section. The method may further comprise programming a numeric controller, and using the controller to control an advanced fiber placement machine. Laying the tow is performed by the advanced fiber placement machine, and may include steering the tow in a first direction from a beginning point along the first section to an ending point along the first section, changing the direction of steering of the tow at the end of the ending point of the first section, and steering the tow in a second direction from the ending point of the first section to an ending point of the second section. The method may be employed to form a composite laminate structure, such as an aircraft spar.

According to another disclosed embodiment, a method is provided of placing prepreg tows on a substrate having a bend angle extending over a transition region. Each of the tows is placed on the substrate in a plurality of sections, wherein at least certain of the sections of each of the tows has an angular orientation that is less than the bend angle. At least one of the sections is a substantially straight section, and placing the tows may include steering the tow in a direction along the section from a beginning point of the section to an ending point of the section, and changing the direction of steering at each of the ending points. In one variation, the sections include at least two curved sections and a substantially straight section connecting the two curved sections. In another variation, each of the sections is curved while in a further variation, each of the sections is substantially straight.

According to a further disclosed embodiment, a method is provided of placing a prepreg tow on a substrate having a bend angle θ extending over a transition region. The method comprises dividing the bend angle θ into n individual sections, wherein each of the sections has an angle of curvature of approximately θ/n, and placing the tow on the substrate includes steering the tow along each of the sections.

According to still another disclosed embodiment, a method is provided of producing a one-piece composite structure having at least one transition region containing a bend angle. The method comprises forming a composite laminate layup by laying up prepreg tows on a substrate, including dividing the bend angle into multiple sections and steering the prepreg tows along each of the sections. Steering the prepreg tows along each of the sections includes steering the tows along angles that are each less than the bend angle. The steering may be performed using a numerically controlled, advanced fiber placement machine. The method may also include programming a numeric controller to automatically control the numerically controlled, advanced fiber placement machine, including programming the numeric controller to steer the prepreg tows within each of the sections of the bend angle, and curing the layup. The method may be employed to produce a composite laminate aircraft airframe member.

DETAILED DESCRIPTION

The disclosed embodiments involve a method of fabricating composite laminate structures, such as composite laminate aircraft spars, that have high angle transition regions using automated placement of prepreg tows. As will be discussed below, the disclosed method helps reduce or eliminate gathering and wrinkling of the tows and/or stress concentrations in the structure.

FIGS. 1, 2, 3 and 5illustrate a typical one-piece composite laminate structure20fabricated in accordance with the disclosed method. The composite laminate structure20is elongate and includes a pair of flanges22integrally formed with a web24. The flanges22have a height “H” and transition into the web24along integral radiused corners26. The composite laminate structure20includes two generally straight, elongate portions30connected by a curved transition region28, sometimes also referred to below as a “high angle transition region”28or “transition region”28. As used herein, “high angle transition region”, and “transition region” refer to a region of the composite laminate structure20having one or more curves, contours or changes in angles or other geometry feature or details along which it may be difficult to steer and place one or more of the tows32, or in which the tow32may be subject to gathering, wrinkling, buckling and/or fiber distortion. The illustrated composite structure20may be, for example and without limitation, a spar or a stringer forming part of an airframe90(FIG. 13), but is merely illustrative of a wide range of one-piece composite laminate structures having one or more transitions regions28that may be fabricated using the disclosed method.

Referring toFIG. 4, the composite laminate structure20may be laid up on a substrate such as a layup mandrel31, using a numerically controlled advanced fiber placement (AFP) machine25operated by a controller33having one or control programs29containing program instructions (not shown). The AFP machine25may include a material placement head27that steers, places and compacts a bandwidth of prepreg tows32onto the layup mandrel31, or onto a substrate defined by underlying layers or plies formed by the tows32. Steering the prepreg tows32as they are placed is controlled by the controller33using control programs29that are suitable for the application.

FIGS. 5 and 6illustrate a desired, ideal geometry of one typical tow32that has been steered around a bend34within the transition region28and placed on one of the flanges22. The bend35is connected to and is continuous with the straight portions30of the tow32. The straight portions30form a bend angle θ relative to each other. In the illustrated application, the bend35has a substantially constant radius of curvature R, however in other applications, the radius of curvature R of the bend35may or may not be constant. In other words, the bend35may or may not have a constant curvature. Where the bend35has a constant radius of curvature R, such as in the example illustrated inFIG. 5, the transition region28may be considered as comprising the arc length of the bend35, or the length of the tow32over which the bend angle θ extends.

Referring particularly toFIG. 6, due to the curvature of the bend35, the inside radius36of the tow32is in compression37, while the outside radius34of the tow32is in tension. In accordance with the disclosed method discussed below, the tow32is steered and placed in a manner that reduces the inside radius compression39to the point that possible gathering, wrinkling, buckling and/or distortion of the tow32within the transition region28is reduced or eliminated.

Referring toFIG. 7, in one embodiment, the disclosed method broadly comprises laying the tow32in a plurality of sections, for example, sections42,44,46, around a bend35in the transition region28. Each of the sections42,44,46has an angular orientation that is less than the bend angle θ. The sections42,44,46may at least partially span, or may fully span the transition region28. In the embodiment shown inFIG. 7, sections42,44are curved and have angular orientations or angles of curvatures ϕ that are less than the curvature of the bend35, while section46is a straight section that has an angular orientation that is less than the bend angle θ. In effect, the bend angle θ is broken-up into multiple sections, for example sections42,44,46(FIG. 7) which together, form an approximation of the desired bend35. More particularly, the bend angle θ is broken-up into n individual sections that may be either straight or curved, where n is a number that is two or more. The n number of sections may or may not be connected by or include one or more straight sections46, and together, may span the entire transition region28.

In another embodiment, the method comprises laying the tow32in at least a first section and a second section within the transition region28, where each of the first and second sections has an angular orientation that is less than the curvature of the bend35. In one variation discussed below, each of the first and second sections is a curved section58(FIG. 9) and has an angle of curvature ϕ that is less than the bend angle θ. In another embodiment discussed below, each of the first and second sections is a substantially straight section50(FIG. 10) having an angular orientation that is less than the bend angle θ. In still other embodiments, the tow32may be laid within the transition region in any combination of straight sections50and curved sections58, each having an angular orientation that is less than the bend angle θ. As will be discussed below in more detail, the use of one or more straight sections46may be unnecessary where the bend angle θ is broken into many relatively short, gently curved bend sections. In one embodiment, the bend angle θ may be divided into n individual sections42,44, wherein each of the sections42,44has an angle of curvature ϕ of approximately θ/n. Although placed in n individual sections, each of the tows32is continuous throughout the bend35.

For example,FIG. 7illustrates a bend35in a continuous tow32placed within a transition region28between two substantially straight portions30of the tow32. The bend35in the tow32is formed by steering the prepreg tow32in a direction along two curved sections42,44and a substantially straight section46extending between the two curved sections42,44. Each of the two curved sections42,44may or may not have a constant radius of curvature R1and each has an angle of curvature ϕ that is less in magnitude than the bend angle θ. The radius of curvature R1and the angle of curvature ϕ of the two curved sections42,46may be substantially identical, or may be different from each other.

As shown inFIG. 7, the sections42,44,46of the continuous tow32respectively have lengths L1, L2, L3that may vary, depending on the application and the geometry of the bend35. When placing the continuous tow32within the high angle transition region28, the tow32is steered in a curved path from a beginning point45at the end of one of the straight portions30, along one of the curved sections42to an ending point47, and then in a substantially straight line from the ending point47along the straight section46to the beginning point49of the second curved section44. The tow32is then steered in a curved path from the beginning point49to the ending point55of the curved section44. In an embodiment where the tow32is placed by an AFP machine25(FIG. 4), the beginning and ending points, e.g.45,47,49,55represent a change in direction of the material placement head27.

In the example shown inFIG. 7, the bend35in the tow32within the transition region28is formed by breaking the transition region28into three sections42,44,46, however, it may be possible to form the bend35in as few as two sections of the tow32, for example, into a single curved section42and single straight section46. By using curved sections42that have an angle of curvature ϕ less than the bend angle θ, the amount of compression37(FIG. 6) on the inside radius of the tow32within the transition is reduced, thereby reducing possible gathering of the tow and related wrinkling and/or fiber distortion. Similarly, the use of one or more curved sections42in combination with one or more straight sections46within the transition region28likewise reduces possible wrinkling and/or fiber distortion because each of the straight sections46provides an opportunity for the tow32to “relax” by reducing compressive forces that may build up in the tow32due to having been steered along the curved sections42,44.

FIG. 8illustrates a bend35within a high angle transition region28, wherein the tow32is alternately steered between a plurality of curved sections58and a plurality of straight sections50between the curved sections58. The points at which tow steering is altered between a straight path (i.e. straight sections50) and curved paths (i.e. curved sections58) are indicated at62. The length L1of the straight sections50and the length L2of the curved sections58will depend upon the particular application, including the bend angle θ. Some or all of the lengths L1may be equal or unequal to each other. Similarly, some or all of the lengths L2may be equal or unequal to each other. As in previous examples, each of the curved sections58has an angle of curvature ϕ that is less than the bend angle θ.

Referring now toFIG. 9, it may be possible to place the tow32around the bend35within the high angle transition region28by steering the tow32along a plurality of successive curved sections58each of which has an angle ϕ that is less than the bend angle θ. The angles ϕ of curvature of the curved sections58, as well as the lengths L2of the curved sections58, may be the same or different from each other.

FIG. 10illustrates a further example of a tow32that has been steered around a bend35within the transition region28in a plurality of sections50in order to reduce or eliminate tow wrinkling and/or distortion. In this embodiment, each of the sections50is a straight section50and has a length L1. The lengths L1of the straight sections50may be the same or may be different from each other in magnitude. Each of the straight sections50has an angular orientation ϕ relative to a reference axis65that is less than the bend angle θ. The number of sections50will vary with the application, but generally, use of a greater number sections50results in a closer approximation of a desired curvature of the tow32to form the bend35.

Attention is now directed toFIG. 11which broadly illustrates the overall steps of a method of fabricating a one-piece composite structure20having a bend35in a high angle transition region28thereof, using automated placement of prepreg tows32. The method may begin at step64with programming a numeric controller33to form the bend35in a one-piece composite structure20by placing each tow32in differing sections42,44,46within the transition region28, wherein each of the sections42,44,46has an angle of curvature ϕ that is less in magnitude than angle θ of the bend. Although placed in differing sections42,44,46, each of the tows32is continuous throughout the transition region28. Programming the numeric controller includes programming the numeric controller33to steer the continuous prepreg tows within each of the sections of the bend angle θ. At step66, a numerically controlled, advanced fiber placement machine25operated by the controller33is used to layup the composite structure20. Step66includes using the controller33to form the bend35by placing each tow32in n differing sections within the transition region28. At68, the one-piece composite laminate layup20is cured, and at70, the cured, one-piece composite structure20may be trimmed and finished, as required.

FIG. 12illustrates a method of placing a prepreg tow on a substrate having a bend with a curvature extending over a transition region in the substrate. At74, the tow is laid in a first section within the transition region, wherein the first section has an angular orientation that is less than the curvature of the bend in the tow. At76, the tow is laid in a second section within the transition region, wherein the second section has an angular orientation that is less than the curvature of the bend in the tow. At78, a numeric controller is programmed, and at80, the numeric controller is used to control an advanced fiber placement machine which is employed to lay the tow in the first and second sections.FIG. 13illustrates a method of carrying out steps74and76in which the tow is laid in sections. As shown in step82, the tow is steered in a first direction from a beginning point along the first section to an ending point along the first section. In some embodiments, the first section may be curved. At step84, the direction of steering of the tow is changed at the ending point of the first section. At step86, the tow is steered in a second direction from the ending point of the first section to an ending point of the second section. In some embodiments, the second section may be straight.

Attention is now directed toFIG. 14which illustrates a method of placing prepreg tows on a substrate having a bend angle extending over a transition region. As shown at88, the method comprises placing each of the tows on the substrate in a plurality of sections, wherein at least certain of the sections of each of the tows has an angle of curvature that is less than the bend angle.

FIG. 15illustrates a method of placing a prepreg tow on a substrate having a bend angle θ extending over a transition region. At90, the bend angle θ is divided into n individual sections, wherein each of the sections has an angle of curvature of approximately θ/n. At step92, the tow is placed on the substrate and is steered along each of the sections.

Attention is now directed toFIG. 16which illustrates the steps of a method of producing a one-piece composite structure having at least one transition region containing a bend angle. The method comprises, at step94, forming a composite laminate layup by laying prepreg tows on a substrate, including dividing the bend angle into multiple sections and steering the prepreg tows along each of the sections. At step96, a numeric controller may be programmed to automatically control a numerically controlled advanced fiber placement machine to steer the prepreg tows within each of the sections of the bend angle.

Embodiments 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 one-piece curved or contoured composite structural members, such as spars, stringers and similar stiffeners, may be used. These structural members may have one or more high angle transition regions. Thus, referring now toFIGS. 17 and 18, embodiments of the disclosure may be used in the context of an aircraft manufacturing and service method98as shown inFIG. 17and an aircraft100as shown inFIG. 18. Aircraft applications of the disclosed embodiments may include, for example, without limitation, various components of an airframe116(FIG. 18) such as spars87and stringers89having high angle transition regions28. During pre-production, exemplary method98may include specification and design102of the aircraft100and material procurement104. During production, component and subassembly manufacturing106and system integration108of the aircraft100takes place. Thereafter, the aircraft100may go through certification and delivery110in order to be placed in service112. While in service by a customer, the aircraft100is scheduled for routine maintenance and service114, which may also include modification, reconfiguration, refurbishment, and so on. One-piece, curved or contoured composite laminate structures may be used as replacement components during the routine maintenance and service114of the aircraft100.

As shown inFIG. 18, the aircraft100produced by exemplary method98may include an airframe116with a plurality of systems118and an interior120. As noted previously, the airframe116may include spars87, stringers89or a variety of other contoured, one-piece structural members fabricated according to the disclosed method described above. One or more of the spars87and/or the stringers89may have one or more high angle transition regions28. Examples of high-level systems118include one or more of a propulsion system122, an electrical system124, a hydraulic system126and an environmental system128. 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.

Systems and methods embodied herein may be employed during any one or more of the stages of the production and service method98. For example, components or subassemblies corresponding to production process106may be fabricated or manufactured in a manner similar to components or subassemblies produced while the aircraft100is in service. Also, one or more apparatus embodiments, method embodiments, or a combination thereof may be utilized during the production stages106and108, for example, by substantially expediting assembly of or reducing the cost of an aircraft100. Similarly, one or more of apparatus embodiments, method embodiments, or a combination thereof may be utilized to fabricate one-piece, contoured composite structures having high angle transition regions28used in the maintenance and service88of the aircraft100.