Patent Publication Number: US-10308344-B2

Title: Disbond resistant composite stiffener runout

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional patent application of U.S. patent application Ser. No. 14/641,723, now U.S. Pat. No. 9,676,470 entitled “Disbond Resistant Composite Stiffener Runout,” which in turn is a divisional patent application of U.S. patent application Ser. No. 13/455,217, filed on Apr. 25, 2012, now U.S. Pat. No. 8,974,886 entitled “Disbond Resistant Composite Stiffener Runout,” the entire disclosures of which are expressly incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     Stringers are structural components that are often used to stiffen panels and other structures. With composite panels, composite stringers may be co-bonded, or secondarily bonded, onto the panel to prevent the panel from buckling or otherwise failing when subjected to compressive loads. The combined panel and stringers may then be secured to a sub structure. 
     While stiffeners provide adequate reinforcement of the corresponding composite panel with respect to compressive loads, the stiffener runouts, or areas at which the stringers terminate, may experience disbonding between the stringers and the panel under certain conditions. The sudden discontinuity of the load path, combined with a low composite interlaminar toughness, may result in a structural component that may delaminate or disbond at operational loads. 
     Traditionally, the disbond potential that exists at the stiffener runout locations of a composite structure has been accommodated via various techniques. For example, the architecture of a composite panel may be designed to move the stiffener terminations to an edge of the panel. However, doing so may negatively affect the cost and weight of the corresponding panel. According to an alternative traditional solution, the disbond may be allowed to occur at operational loads and fasteners used to arrest the disbond. With this solution, extensive certification testing is required and any amount of disbond, even though non-critical, may be disconcerting to a customer. 
     Another conventional method for controlling disbond at stiffener runout locations includes the use of flat metallic radius fillers secured to the stiffeners at the termination ends. However, these conventional radius fillers are not efficient at arresting the disbond under the flange, and provide no arrest capabilities under a web of a stringer. Relatively large, mechanically attached metallic fittings may also be used to transfer loads from stringers into the attached composite structure. A disadvantage of these types of fittings is that the fittings increase the weight of the corresponding structure and may be expensive to install, while not completely suppressing the disbond at operational loads. 
     It is with respect to these considerations and others that the disclosure made herein is presented. 
     SUMMARY 
     It should be appreciated that this Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to be used to limit the scope of the claimed subject matter. 
     Apparatus and methods described herein provide for disbond resistant stiffener runouts. According to one aspect, a stiffener runout may include a web and a base flange. The web may be configured in a vertical orientation having a front surface, a rear surface, a bottom edge, a web termination point that is positioned at an outermost location of the web, and web height that decreases from a full height at a location prior to the web termination point to a reduced height at the web termination point. The base flange may include two flanges extending horizontally from the front and rear surfaces of the web, respectively. The base flange may include a flange termination point that is positioned at an outermost location of the base flange, which is forward of the web termination point. 
     According to another aspect, a method of controlling bondline separation between a composite stiffener and a composite structure may be provided. The method may include providing a composite stiffener having a web and a base flange. The web may terminate at a web termination point, while the base flange terminates at a flange termination point that is forward of the web termination point. The base flange may include rounded corners. The web may be trimmed from a full height prior to the web termination point to a reduced height at the web termination point. A recessed notch may be provided within the base flange at the flange termination point, with the web termination point being positioned at the center of the recessed notch. The base flange may be bonded to the composite structure. 
     According to yet another aspect, a stiffener runout assembly may include a stiffener runout, a composite structure bonded to the stiffener runout, a pair of perimeter clamp radius fillers, and a number of fasteners. The stiffener runout may include a web trimmed from a full height to a reduced height at the web termination point at an outermost location of the web. The stiffener runout may also include the base flange that extends horizontally in opposite directions from the web and includes a flange termination point at an outermost location of the base flange. The flange termination point may be positioned forward of the web termination point, with the web termination point positioned within a recessed notch of the base flange. 
     The composite structure may be bonded to the base flange of the stiffener runout at a scarf joint. The pair of perimeter clamp radius fillers may be positioned on a top surface of the base flange on opposing sides of the web. Each perimeter clamp radius filler may include a bottom surface that is configured to contact the top surface of the base flange around the perimeter of the top surface to create a contact area between the perimeter clamp radius filler and the perimeter of the top surface. The contact area may define a relief cavity between the perimeter clamp radius filler and the top surface of the base flange. The fasteners may secure the perimeter clamp radius fillers to the top surface of the base flange, extending through a top surface of the perimeter clamp radius fillers, through the relief cavities, through the base flange, and through the composite structure. 
     The features, functions, and advantages that have been discussed can be achieved independently in various embodiments of the present disclosure or may be combined in yet other embodiments, further details of which can be seen with reference to the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front view of a stiffener runout assembly illustrating aspects of a stiffener runout for controlling disbond according to various embodiments presented herein; 
         FIG. 2  is a top view of a stiffener runout illustrating aspects of the stiffener runout for controlling disbond according to various embodiments presented herein; 
         FIG. 3  is a side view of a stiffener runout assembly illustrating aspects of a stiffener runout for controlling disbond according to various embodiments presented herein; 
         FIG. 4  is a side view of a stiffener runout assembly illustrating aspects of a stiffener runout and scarf joint for controlling disbond according to various embodiments presented herein; 
         FIG. 5  is a side view of a stiffener runout assembly illustrating aspects of a stiffener runout and alternative scarf joint for controlling disbond according to various embodiments presented herein; and 
         FIG. 6  is a flow diagram showing a method of controlling bondline separation between a composite stiffener and a composite structure according to various embodiments presented herein. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is directed to controlling bondline separation between a composite stiffener and a composite structure. As discussed above, delamination or disbond may occur between stringers and co-bonded composite structures at stringer runout locations when subjected to operational loads. As composite materials are increasingly used in the construction of vehicles such as aircraft, controlling delamination and disbonding at the stringer runout locations is a primary concern. Solutions to the stringer disbonding issue should contemplate weight factors, as well as cost and complexity of manufacturing, particularly within the aircraft industry. 
     Utilizing the concepts and technologies described herein, stringer runouts may incorporate various stringer runout aspects, alone or in combination, to prevent disbonding between composite stringers and co-bonded composite structures while minimizing weight and manufacturing complexity and cost. The various embodiments described below will be described within the context of a composite aircraft stringer co-bonded with composite aircraft skin. It should be appreciated that the various embodiments are not limited to these specific components, or to use within aircraft. Rather, the concepts described below may equally apply to any implementation in which a stringer or similar stiffening component is bonded to a composite structure. 
     In the following detailed description, references are made to the accompanying drawings that form a part hereof, and which are shown by way of illustration, specific embodiments, or examples. Referring now to the drawings, in which like numerals represent like elements through the several figures, a disbond resistant stiffener runout and assembly, as well as methods form controlling disbond between a composite stiffener and a composite structure, will be described. 
       FIG. 1  shows a front view of a stiffener runout assembly  100 . According to this illustrative embodiment, the stiffener runout assembly  100  includes a stiffener runout  102  co-bonded onto a composite structure  104 . The composite structure  104  may be any composite panel, sheet, or other component for which stiffening would be desirable, such as an aircraft skin. The stiffener runout  102  includes a web  106 , a base flange  108  that includes a first flange portion  108 A and a second flange portion  108 B, and a cap  110 . A noodle  114  is bonded within the region created between the web  106  and the first and second flange portions  108 A and  108 B. 
     As shown in  FIG. 1 , the web may extend vertically in relation to the composite structure  104 . The base flange  108  may be positioned at a bottom edge of the web  106 , with the first flange portion  108 A extending substantially horizontally from a front surface  120  of the web  106  and the second flange portion  108 B extending substantially horizontally from a rear surface  122  of the web  106 . The base flange  108  is bonded to the composite structure  104 . According to various embodiments, the stiffener runout assembly  100  may include perimeter clamp radius fillers  112  positioned on the first and second flange portions  108 A and  108 B to assist with preventing disbond at the forward edges of the stiffener runout  102 . The various features of the perimeter clamp radius fillers  112  will be described in greater detail below with respect to  FIGS. 2 and 3 . It should be appreciated that the stiffener runout  102  may include additional features and/or varying proportions or configurations as compared to the stiffener runout  102  shown in the figures without departing from the scope of this disclosure. The various figures may not be drawn to scale and are shown for illustrated purposes only. 
       FIGS. 2 and 3  show top and side views, respectively, of a stiffener runout  102  according to one embodiment. As described above, the stiffener runout  102  may include a web  106  and a base flange  108 . There are multiple features of the stiffener runout  102  that independently and collectively mitigate or eliminate any disbond between the stiffener runout  102  and the composite structure  104 . One such feature includes the rounded corners  202  of the base flange  108 . Conventional stiffeners include base flanges that have squared off corners. The square corners are easier to manufacture. Historically, delamination tends to occur at the corners of the base flange first, particularly when a partial shear loading exists. However, by trimming or otherwise creating the rounded corners  202  as shown in  FIG. 2 , the loads through the corners of the base flange  108  are more easily distributed, reducing the potential for delamination at those locations. 
     Another feature disclosed herein to control disbonding includes the recessed notch  204  in base flange  108 . According to various embodiments, the base flange  108  terminates at a flange termination point  206 . The flange termination point  206  may be the most forward location of the stiffener runout  102 . Similarly, the web  106  terminates at a web termination point  208 . Conventional stiffeners include webs and base flanges that terminate at identical locations. The forward edge of a stringer runout is typically stiffer under the web  106  and noodle  114  as compared to the base flange  108 . This stiffer central location of the typical stringer forward edge allows for disbonds to occur at this location before occurring at other locations of the base flange  108 . 
     However, as shown clearly in  FIG. 2 , the stiffener runout  102  of the various embodiments disclosed herein includes a flange termination point  206  that is forward of the web termination point  208 . The web termination point  208  is located within a recessed notch  204 , which may be on a semicircular or arcuate edge of the base flange  108  aft of the flange termination point  206 . In doing so, any curvature between the termination end of the stiffener runout  102  and the composite structure  104  may be matched, which mitigates the disbonding under the web  106  and noodle  114 . 
     Similarly, as seen in  FIG. 3 , the web  106  may be trimmed from a full height  304  to a reduced height  306  at the web termination point  208 . According to one embodiment, the reduced height  306  may be substantially flush with a top surface of the base flange  108 . By tapering or reducing the height of the web  106 , the loads within the stringer  102  are reduced at the flange termination point  206 , which reduces the potential for disbond. Additionally, reducing the height of the web  106  reduces the stiffness of the stiffener runout  102  along the central axis of the stiffener, which similar to the benefits described above with respect to the recessed notch  204 , mitigates the disbonding under the web  106  and noodle  114 . According to one embodiment, a leading edge  302  of the web  106  is tapered according to a curved configuration as shown in  FIG. 3 . 
     An additional feature used to control disbonding includes the use of the perimeter clamp radius fillers  112 , as shown in  FIGS. 2 and 3 . As discussed above, traditional radius fillers are flat metallic components that may be secured to a base flange in order to provide pressure to the flange and corresponding bond between the flange and the underlying composite structure in an effort to mitigate disbonding. However, traditional radius fillers do not efficiently handle panel deformation. For example, when a composite structure  104  or panel deforms under load such that the panel under a portion of the traditional radius filler flexes downward, the traditional radius filler will tend to lift off of the surface, either over the portion of the panel that is deflecting downward, or as that portion of the radius filler deflects downward with the panel, an opposing edge of the traditional radius filler may lift off of the surface. 
     The perimeter clamp radius fillers  112  described herein alleviate these deficiencies of traditional radius fillers by concentrating the clamping force around the perimeter of the base flange  108  where the disbonding is most likely to occur. Moreover, a relief cavity  212  within a central portion of the perimeter clamp radius filler  112  accommodates any deformation of the composite structure  104  and the corresponding base flange  108 . The perimeter clamp radius fillers  112  may be metallic, or any other suitable material. 
     According to one embodiment, each perimeter clamp radius filler  112  includes a bottom surface that projects downward from a portion of the perimeter of the radius filler to create a contact area  210  between the perimeter clamp radius filler  112  and a corresponding portion of the top surface around a perimeter of the base flange  108 . The contact area  210  defines the boundaries of the relief cavity  212 . Pressure is applied to the base flange  108  by the perimeter clamp radius filler  112  within the contact area  210  to prevent disbonding at the edges of the stiffener runout  102 . Because pressure applied to the perimeter clamp radius filler  112  is distributed and concentrated around the perimeter of the base flange  108  while allowing for deformation within the relief cavity  212 , the perimeter clamp radius fillers  112  provide a significant advantage over the less effective conventional radius fillers. 
     According to various embodiments, the contact area  210  may be substantially C-shaped with an opening  216  that provides access to the relief cavity  212  from one edge of the base flange  108 . The opening  216  may be positioned along the edge of the base flange  108  opposite the web  106  as shown in  FIGS. 2 and 3  since that portion of the edge opposite the web  106  is not highly susceptible to disbonding. The opening  216  may alternatively be positioned at the most rearward edge or any other edge of the base flange  108 , and may be configured according to any size and shape. According to one embodiment, the relief cavity  212  may be filled with a compliant filler, such as a sealant or other material appropriate to the particular application, that allows for deformation without applying pressure to the base flange  108 . The opening  216  may provide a passage for excess compliant filler to escape from the relief cavity  212  during installation. 
     Fasteners  214  may be threaded or otherwise placed through corresponding apertures in the perimeter clamp radius filler  112 , base flange  108 , and composite structure  104 . Torque applied to the fasteners  214  transfers force from the perimeter clamp radius filler  112  to the perimeter of the base flange  108  via the contact area  210  to prevent disbonding. Although two fasteners  214  are shown for each perimeter clamp radius filler  112 , any number of fasteners  214  may be used without departing from the scope of this disclosure. 
       FIGS. 4 and 5  illustrate two alternative embodiments in which a type of scarf joint  402  is created between the stiffener runout  102  and the composite structure  104 . For the purposes of this disclosure, the term “scarf joint” will be used to refer to a coupling between two or more components in which two of the components are manufactured or modified to have an angled surface that is engaged with a complementary angled surface of the other component.  FIG. 4  shows a first embodiment in which the composite structure  104  has a uniform thickness, but is bonded to a fiberglass wedge  404 . The fiberglass wedge  404  is hatched in the  FIG. 4  to highlight the wedge for clarity purposes only. The fiberglass wedge  404  provides an angled wedge surface  406  that is co-cured onto an angled flange surface  408  of the base flange  108 . The angled flange surface  408  may be created by reducing the plies of the composite material according to the desired angle and joint length. The fiberglass wedge  404  is highly compliant and is configured to shear load into the composite structure  104  over a longer length, thereby reducing the local moments that may cause disbonding. Although the fiberglass wedge  404  is described herein as being fiberglass, it should be appreciated that any suitable material that is more compliant (having less stiffness) than the material of the stiffener runout  102  and composite structure  104  may be used to create the wedge. 
       FIG. 5  shows the second embodiment in which the scarf joint  402  is created by reducing the plies of the composite material according to the desired angle of the angled flange surface  408  and length of the scarf joint  402 . Similarly, a corresponding angled structure surface  502  may be created by increasing plies of the composite material according to the desired angle of the angled structure surface  502  and length of the scarf joint  402 . The result is a non-uniform thickness of the base flange  108  that complements a non-uniform thickness of the composite structure  104  within the scarf joint  402 . According to an example embodiment, the run to rise ratio associated with the angled structure surface  502  and angled flange surface  408  of the scarf joint  402  is approximately 20:1. According to various embodiments, the ratio may be between 20:1-30:1. Similar to the effect of a scarf joint  402  created with the fiberglass wedge  404  discussed above with respect to  FIG. 4 , the effect of the scarf joint  402  created by altering the plies of the base flange  108  and composite structure  104  as shown in  FIG. 5  is to shear the loads from the stiffener into the composite structure  104  over a relatively long distance. Doing so minimizes the detrimental local moments by reducing the offset in the load line that may initiate disbonding. 
     Turning now to  FIG. 6 , an illustrative routine  600  for controlling bondline separation between a stiffener runout  102  and a composite structure  104  will now be described in detail. It should be appreciated that more or fewer operations may be performed than shown in the  FIG. 6  and described herein. Moreover, these operations may also be performed in a different order than those described herein. The routine  600  begins at operation  602 , where the stiffener runout  102  having a web  106  and a base flange  108  of composite materials is created. It should be appreciated that other features of a stiffener that are not germane to this disclosure may additionally be created during this or any other operation of the routine  600 . For example, creation of the stiffener may commonly include bonding the cap  110  to the top edge of the web  106 , as well as bonding the noodle  114  in the appropriate position between the bottom edge of the web  106  and the first and second flange portions  108 A and  108 B. 
     From operation  602 , the routine  600  continues to operation  604 , where the rounded corners  202  of the base flange  108  are created. This operation may be performed by trimming conventional square corners after curing of the base flange  108 , or may be a part of the composite molding process. At operation  606 , the recessed notch  204  is created in the base flange  108 . The recessed notch  204  may be created via trimming material from the base flange  108  or may be created during the material composite molding process. The size and depth of the recessed notch  204  may depend on the particular implementation and the desired distance between the flange termination point  206  and the web termination point  208 . The routine  600  continues from operation  606  to operation  608 , where the height of the web  106  is trimmed from the full height  304  to the reduced height  306  at the web termination point  208 . The leading edge  302  of the web  106  may be trimmed according to a curved configuration or linearly tapered to the reduced height  306 . 
     From operation  608 , the routine  600  continues to operation  610 , where the scarf joint  402  is created between the stiffener runout  102  and the composite structure  104 . As discussed above with respect to  FIG. 4 , a first method for creating the scarf joint  402  involves utilizing a fiberglass wedge  404  bonded to a top surface of the composite structure  104 , providing an angled wedge surface  406  that is co-cured into an angled flange surface  408  of the base flange  108 . The angled flange surface  408  of the base flange  108  may be created by dropping plies to create the desired 20:1-30:1 or other ratio at the scarf joint  402 . Also as discussed above with respect to  FIG. 5 , a second method for creating the scarf joint  402  includes dropping plies in the base flange  108  to create the angled flange surface  408  and increasing plies in the composite structure  104  to create the angled structure surface  502 . The angled flange surface  408  and the angled structure surface  502  are then co-cured to create the scarf joint  402 . 
     The routine  600  continues from operation  610  to operation  612 , where the perimeter clamp radius fillers  112  are secured to the base flange  108  with fasteners  214 . Torque is applied to the fasteners  214  to concentrate a clamping force within the contact area  210  against the perimeter of the base flange  108  to mitigate disbonding in these areas. At operation  614 , the relief cavity  212  is filled with a compliant sealant or other material and the routine  600  ends. 
     During use, loads from the stiffener runout  102  may be sheared into the composite structure  104  at the scarf joint  402  over a relatively long distance to reduce the offset in the load line that could initiate a disbond between the stiffener runout  102  and the composite structure  104 . These loads may include a first loading condition in which static loads are experienced, such as during a steady straight and level aircraft flight operation. The loads may shift to a second loading condition in which dynamic loads are experienced, such as during a climb, turn, or velocity change. During this load shift, the scarf joint  402  and other features of the stiffener runout  102  described above serve to distribute the loads to the composite structure  104  in a manner that prevents or delays the disbonding that may be present with conventional stringer runouts. 
     It should be clear from discussion above that the concepts described herein may be used independently or in combination to mitigate disbonding at the stiffener runout  102  under operational loads. The subject matter described above is provided by way of illustration only and should not be construed as limiting. Various modifications and changes may be made to the subject matter described herein without following the example embodiments and applications illustrated and described, and without departing from the true spirit and scope of the present disclosure, which is set forth in the following claims.