TECHNICAL FIELD  
       [0001]    The technical field relates generally to gas turbine engines, and more particularly to the repair of composite bypass ducts for aero turbofans. 
       BACKGROUND  
       [0002]    Some models of gas turbine engines have bypass ducts provided with parts that are made of one or more composite materials. Composite materials, however, often tend to be somewhat less tolerant to mishandling compared to other materials. For instance, composite parts can be susceptible to fraying if they are hit by a tool, such as a chisel or a drift, during a maintenance operation. An entire bypass duct can then be inadvertently ruined if one of its composite parts is damaged by such mishandling. 
         [0003]    Bypass ducts may include studs for holding wiring or for similar fixing purposes. The studs often bear no significant loads when a gas turbine engine is operated, but they can nevertheless be damaged, particularly in the case of composite bypass ducts, such as during a maintenance operation. For instance, a stud can be inadvertently bent, have its threads damaged, or become loose or be broken off. A damaged stud may require that the entire composite bypass be replaced by a new one. Room for improvements thus exists. 
       SUMMARY 
       [0004]    In one aspect, the present concept provides a method of replacing a stud bonded to a composite bypass duct of a gas turbine engine, the stud comprising a stem attached to a base, the method comprising the steps of: removing substantially all of the stem of the stud from the base; reducing the base in thickness to a desired thickness; and then bonding a replacement stud over a remaining portion of the base. 
         [0005]    In another aspect, the present concept provides a method of replacing a stud bonded to an outer surface of a bypass duct of a turbofan gas turbine engine, the surface being made of a composite material, the stud having a base and had a stem that has been previously removed by separating the stem from the base, the method comprising the steps of: reducing the base in thickness; and then bonding a replacement stud over a remaining portion of the base. 
         [0006]    Further details on these and other aspects will be apparent from the detailed description and figures included below. 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0007]      FIG. 1  is an isometric and partially exploded view showing an example of a composite bypass duct with studs, the studs being shown before being bonded to the composite bypass duct; 
           [0008]      FIG. 2  is a side view showing an example of a stud; 
           [0009]      FIG. 3  is a view similar to  FIG. 2 , showing an example of a damaged bonded stud; 
           [0010]      FIG. 4  is a semi-schematic view showing the damaged bonded stud of  FIG. 3  being replaced; 
           [0011]      FIG. 5  is a side view showing an example of a replacement stud being bonded over what is left of the base plate of a previously bonded stud; 
           [0012]      FIG. 6  is a semi-schematic longitudinal cross section of the composite bypass duct showing a bonded stud provided over a curved surface portion; and 
           [0013]      FIG. 7  is a view similar to  FIG. 6 , showing a bonded stud provided over a flat surface portion. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]      FIG. 1  is an isometric and partially exploded view showing an example of a bypass duct  10  of a gas turbine engine. The bypass duct  10 , hereafter referred to as the “composite bypass duct”, comprises at least some parts that are made of one or more composite materials. In the illustrated example, the composite bypass duct  10  comprises a wall  12  made of such material. The wall  12  defines an outer surface  14  to which two studs  16  are attached once the composite bypass duct  10  is fully assembled. It should by noted that the composite bypass duct  10  can have parts that are not made of a composite material. 
         [0015]      FIG. 2  is a side view showing an example of a stud  16 . Other stud models are also possible. The illustrated stud  16  comprises a stem  18 , in this case a stem entirely threaded along its length. The stem  18  has a bottom end  18   a  that is embedded into a base, which base can be for instance a base plate  20 , as in the illustrated example. The bottom end  18   a  of the stem  18  can be for instance molded in the base plate  20 . Other attachment methods are also possible. The base plate  20  of the illustrated example has a circular shape, a flat bottom surface  22  and a chamfered upper rim  24 . Other shapes and surface designs are possible as well. Studs  16  can be provided with a stem made of a relatively rigid material and a base plate made of a different material. For instance, the stem  18  can be made of metal, such as stainless steel, and the base plate  20  can be made of a polymer or a composite material, such as fiberglass. Other materials are also possible. It is also possible to use the same material for both the stem  18  and the base plate  20 . A person skilled in the art will know how to design studs and therefore, the design of studs need not be discussed in further details. 
         [0016]      FIG. 3  is a side view showing an example of a damaged stud  16  bonded to a surface made of a composite material in a bypass duct, for instance the outer surface  14  of the wall  12  ( FIG. 1 ). In this example, the stem  18  has damaged threads  26  in the zone identified with reference numeral  28 . It should be noted that bonded studs can also have other kinds of damages, including for instance a stem being loose or being broken off from their base plate. 
         [0017]    In the case of the damaged bonded stud  16  of  FIG. 3 , the proposed stud replacement method is initially carried out by removing the stem  18  from the base plate  20 . This can be done, for instance, by using a tool such as locking pliers or a similar tool, and by forcing the stem  18  out of the base plate  20 . Once this is done, the base plate  20  of the damaged bonded stud  16  will look somewhat similar to what is shown in  FIG. 4 , the base plate  20  having a hole  20   a  where the end  18   a  of the stem  18  was attached. 
         [0018]    The proposed replacement method further involves not to remove entirely the base plate  20  of the damaged bonded stud  16 . Instead, the base plate  20  is machined or is otherwise worked to reduce its thickness compared to its original value. One possible way to achieve this goal is to grind it using a power tool or a hand-held tool so as to remove material from the upper surface of the base plate  20  and leave only a fraction of its original thickness. The base plate  20  is grinded off to a thickness that is within a predetermined range of values. For instance, the base plate  20  can be ground to leave only about 0.015 to 0.020 inch (0.38 to 0.51 mm) of the original base plate  20 . This removes the chamfer of the upper rim  24  of the base plate  20 . The base plate  20 , however, is not entirely removed to prevent the surface  14  to be damaged by the machining. 
         [0019]    Next, as shown in  FIG. 5 , a replacement stud  16 ′ is bonded directly over the remaining portion of the base plate  20  of the damaged bonded stud  16 . The replacement stud  16 ′ can be bonded using an adhesive. The replacement bonded stud  16 ′ can be positioned prior to its attachment using a positioning jig  30 . An example of a positioning jig is shown in  FIG. 5 . The positioning jig  30  can be used to place the replacement stud  16 ′ so that its central axis (i.e. the axis at the center of the stem  18 ) is in registry with the geometric center of the base plate  20  of the damaged bonded stud  16 . The positioning jig  30  has a base  32  that is temporarily attached to the composite surface  14 , using for instance removable adhesive pads  34 , and the replacement stud  16 ′ is held by a corresponding holder  36  slidingly connected to the base  32 . The holder  36  is movable when a positive force F is applied on it. This force will bring the bottom surface of the base plate  20 ′ of the replacement stud  16 ′ in contact with the upper surface of the machined base plate  20  of the damaged bonded stud. The force can be maintained for a period of time sufficient for curing the adhesive. 
         [0020]    The present method can be used to replace bonded studs in a wide variety of locations on the bypass duct. For instance, a replacement bonded stud  16 ′ can replace a damaged bonded stud that was connected to a curved surface portion  38 , as shown semi-schematically in  FIG. 6 , or to a flat surface portion  40 , as shown semi-schematically in  FIG. 7 . Wiring or other components can later be reattached to the stems of the replacement bonded studs  16 ′ once the adhesive is cured. 
         [0021]    As can be appreciated, the above-mentioned method mitigate the likelihood of damaging a composite bypass duct upon removing a damaged bonded stud, in particular its base plate, in preparation for the installation of a replacement stud. 
         [0022]    Overall, the above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to what is described while still remaining within the same concept. For example, studs can have another shape than that shown in the figures. Studs can have a stem whose end is adhesively attached to the base plate instead of being molded therein. The illustrated positioning jig is only one example and other kinds of jigs can be used as well. The replacement studs can be bonded using an adhesive or another kind of connection initially provided on the remaining portion of the base plate of the damaged bonded stud instead of under the base plate of the replacement stud. A replacement stud can be bonded, in some instances, at a location on the remaining portion of the base plate of the previous bonded stud where the central axis of the replacement stud will be slightly offset with reference to the geometric center of the base plate of the previous bonded stud. Still, replacement studs do not necessarily need to be identical in shapes and/or sizes to the previous stud they each replace. Still other modifications will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the scope of the appended claims.
A method of replacing a stud bonded to an outer surface of a bypass duct of a turbofan gas turbine engine is disclosed. The surface is made of a composite material. The stud has a base and had a stem that has been previously removed by separating the stem from the base. The method includes: reducing the base in thickness; and then bonding a replacement stud over a remaining portion of the base.