Patent Publication Number: US-2018045608-A1

Title: Method of facilitating visual detection of a crack in a component of a gas turbine engine

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a divisional application of U.S. patent application Ser. No. 14/191,689 filed on Feb. 27, 2014, incorporated herewith by reference. 
    
    
     TECHNICAL FIELD 
     The application relates generally to components of gas turbine engines and more specifically to detection of cracks in those components. 
     BACKGROUND OF THE ART 
     Parts of a gas turbine engine can become scratched or cracked by normal wear, stress loading, or by incident such as foreign object impact at high velocity. Some of the parts of the gas turbine engine are made of a substrate and of at least one coating over the substrate. Cracks in the coating can expose the substrate, which in turn can leave the component vulnerable to stress fractures, such as stress corrosion cracks whereby in the presence of stress and corrosion the component would crack and fracture at a stress level below the tensile strength of the substrate. 
     SUMMARY 
     In one aspect is provided a component of a gas turbine engine, the component comprising: a substrate; a corrosion resistant top layer; and an intermediate corrodible layer disposed between the corrosion resistant top layer and the substrate, when corroding, the intermediate layer having a color contrasting with a color of the top layer. 
     In another aspect is provided method of detecting a crack in a component of a gas turbine engine having a corrosion resistant top layer and an intermediate corrodible layer, the method comprising, in sequence: observing that at least one area of the component has a color contrasting with that of the top layer; determining that the color of the at least one area is a result of corrosion of the intermediate corrodible layer; and determining that the top layer has a crack therethrough as a result of determining corrosion of the intermediate corrodible layer. 
     In yet another aspect, there is provided a method of facilitating crack detection in a component of a gas turbine engine, the method comprising: obtaining a substrate of the component; depositing an intermediate corrodible layer onto the substrate; and depositing a corrosion resistant top layer onto the intermediate corrodible layer, wherein when a crack in the top layer exposes the intermediate corrodible layer, the corroded intermediate layer corroding in a color contrasting with a color of the top layer. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the accompanying figures in which: 
         FIG. 1  is a schematic cross-sectional view of a gas turbine engine; 
         FIG. 2  is a schematic cross-section view of a component of the gas turbine engine of  FIG. 1  having a substrate, a corrosion resistant top layer and an intermediate corrodible layer; 
         FIG. 3  is a schematic cross-section view of the component of  FIG. 2  showing a crack penetrated through the top layer; 
         FIG. 4  is a schematic cross-section view of the component of  FIG. 3  showing the corroded intermediate layer expanded through the crack in the top layer and partially covering a surface of the component; and 
         FIG. 5  is a flow chart of a method of detecting a crack such as the crack in the component of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a gas turbine engine  10  of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication a fan  12  through which ambient air is propelled, a compressor section  14  for pressurizing the air, a combustor  16  in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section  18  for extracting energy from the combustion gases. 
     Referring to  FIG. 2 , a component  20  of the gas turbine engine  10  includes a corrosion resistant top layer  22  and a substrate  26 . An intermediate corrodible layer  24  is disposed between the substrate  26  and the top layer  22  and allows detecting cracks in the top layer  22  before the substrate  26  becomes exposed. 
     The component  20  can be used in various parts of a gas turbine engine. In one embodiment, the substrate  26  is an aluminum alloy, the intermediate layer  24  a ferrous alloy and the top layer  22  a metal plating of cobalt. The component  20  may be coated, plated, or the like. The substrate  26 , the intermediate layer  24 , and the top layer  22  may be various materials depending of the intended use of the component  20 . Examples of materials for the top layer  22  include non-exhaustively a metal alloy, and a non-metal. Yet, for any choice of the top layer  22 , intermediate layer  24  and substrate  26 , the top layer  22  is chosen to be corrosion resistant and the intermediate layer  24  to be corrodible. The substrate  26 , however, may or may not be corrosion resistant. In addition, the intermediate layer  24  is selected to have a corroded color contrasting with that of the top layer  22  for allowing visual detection of cracks of the top layer  22 , as will be described below. By “contrasting”, it should be understood any color difference which would be detectable without confusion by a normal healthy human eye. Examples of color contrasting include a dark color over a light background, a vivid color over a pastel background. Colors that could not be easily dateable such as green over red for daltonians are preferably avoided. In one embodiment where the top layer  22  is cobalt and the substrate  26  is aluminum, the top layer  22  has a generally light grey color and the substrate  26  has a generally silver to dull grey color, and a crack in the top layer  22  may be difficult to detect during visual inspection. The presence of the intermediate layer  24  having a corroded color contrasting with that of the top layer  22  facilitates visual detection of cracks in the top layer  22 . Cracks are undesirable because they may lead to fracture of the component  20 . For example, the substrate  26  and the top layer  22  may not be galvanic compatible, and if the top layer  22  is scratched and the substrate  26  exposed, a fluid could involuntary connect the substrate  26  to the top layer  22  and cause galvanic corrosion. It is therefore undesirable to leave the component  20  with a portion of the substrate  26  exposed. Exposing involuntarily a portion of the substrate  26  could also cause stress corrosion cracking thereby inducing potentially cracking and fracture of the component  20  at a lower stress level than it otherwise would. 
     In the embodiment described herein, the top layer  22  and the intermediate layer  24  may be formed by a specific electrolytic deposition to deposit metal to a grain size in the nanometer range, for example 10 to 15 nanometers. However, the top layer  22  and the intermediate layer  24  may be formed using plating techniques other than the above described electrolytic deposition process, and the cobalt grain size of the top layer  22  and/or the ferrous grain size of the intermediate layer  24  used in the electrolytic process may have a grain size other than in the nanometer size range. The top layer  22  and/or intermediate layer  24  may also not be plated. 
     The top layer  22  has a thickness  23 , the intermediate layer  24  has a thickness  25  and the substrate  26  has a thickness  27 .  FIG. 2  is schematic and the top layer  22 , intermediate layer  24  and substrate  26  may have dimensions relative to each other different than the ones shown in  FIG. 2 . For example, the intermediate layer  24  may be thicker than the top layer  22 . 
       FIG. 3  schematically shows the component  20  having a crack  30  in the top layer  22 . The crack  30  could originate from various events, including normal wear, stress loading, or foreign object impact at high velocity. The crack  30  extends through the thickness  23  of the top layer  22  and exposes the intermediate layer  24 .  FIG. 3  is schematic, and the crack  30  may have other shapes and dimensions as the ones shown in  FIG. 3 . For example, the crack  30  could be linear shaped or non linear shaped. The crack  30  may also extend through a portion of the intermediate layer  24  before exposing the substrate  26 . 
     The intermediate layer  24  is a corrodible material selected to have a corrosion color contrasting with that of the top layer  22  in order to enhance visual detection of the crack  30 . In the example described herein, the intermediate layer  24  is a ferrous alloy and corrodes in a generally red color. One example of a ferrous alloy is an alloy of 75 to 80% iron by weight. When corroding, the ferrous alloy turns into a red color, while the top layer  22  remains grey. As a consequence, a user inspecting the component  20  would visually detect the red areas. Knowing that the red areas are a consequence of corrosion of the intermediate layer  24  (as opposed to areas of different color not related to the crack detection described herein), the user can deduce that these areas or stains are locations where a crack is present and that the crack  30  had penetrated through the thickness of the top layer  22  which may cause the component  20  to be subject to retirement from the engine  10 . In one embodiment, the thickness  25  of the intermediate layer  24  is selected so that the crack  30  would not propagate through the thickness  25  of the intermediate layer  24  of the component  20  and would not reach the substrate  26  for at least two scheduled inspection intervals. This could correspond to 1,500 to 3,000 hours of engine operation. The component  20  could then be able to have a margin of safety to operate to the second scheduled inspection knowing the crack  30  would not be able to reach and expose the substrate  26  to corrosion and stress corrosion if the crack  30  misses detection at the first scheduled inspection. The thickness  25  of the intermediate layer  24  could be determined by crack propagation methodology. If the user detects no stains related to corrosion of the intermediate layer  24 , the user deduces that no crack  30  is present or that the crack  30  is at an early stage and yet not penetrated through the thickness  23  of the top layer  22  that would otherwise makes the crack visually visible. 
     Depending on the choices of the top layer  22 , intermediate layer  24  and substrate  26 , the intermediate layer  24  can also act as a stiffener and supports structural loads for the component  20 . In the example described herein, the ferrous alloy of the intermediate layer  24  is a stiffener and a structural load supporting element for the component  20  made of a cobalt top layer  22  and an aluminum alloy of the substrate  26 . The intermediate layer  24  may not only serves to aid crack detection but also support the structural loads at the top layer  22  and the substrate  26  and provides stiffening to the component  20 . A ferrous intermediate layer  24  tensile modulus and tensile strength may be respectively three times and over two times that of a high strength aluminum alloy of the substrate  26 . It is contemplated that the intermediate layer  24  also acts to stiffen and strengthen the component  20 . 
     Turning to  FIG. 4 , the corroded intermediate layer  24  may expand in the crack  30  through the top layer  22  which could further enhance visual detection of the crack  30 .  FIG. 4  is schematic, and the corroded intermediate layer  24  may expand more or less than shown in  FIG. 4 . For example, the corroded intermediate layer  24  may expand only partially in the crack  30  at onset of corrosion in the intermediate layer  24 . In the schematic of  FIG. 4 , the corroded intermediate layer  24  is shown to completely fill crack  30  in the top layer  22  and onto a surface  21  of the component  20 , but it contemplated that the corroded intermediate layer  24  could be contained within the crack  30  at the onset of corrosion in the intermediate layer  24 . 
     Turning to  FIG. 5 , a method  40  of detecting the crack  30  that had penetrated the thickness  23  of the top layer  22  in the component  20  starts at step  42  with observing that at least one area of the component  20  has a color different from a rest of the top layer  22 . In the example described herein, one may observe red color stains over a gray color background. From step  42 , the method goes to step  44 , where it is determined that the color is a result of corrosion of the intermediate layer  24 . This can be achieved by knowing that the color is a typical color of corrosion of the intermediate layer or by sampling the corrosion residue and determining that it is indeed a product of the corrosion. From step  44 , the method goes to step  46  where it is determined that the crack  30  had penetrated the top layer  22  as a result of determining corrosion of the intermediate corrodible layer  24 . 
     A manufacturer of the component  20  of the gas turbine engine  10  may perform a method of facilitating crack detection by obtaining the substrate  26  of the component  20 ; depositing the intermediate corrodible layer  24  onto the substrate  26 ; and depositing the corrosion resistant top layer  22  onto the intermediate corrodible layer  26 . When the crack  30  in the top layer  22  exposes the intermediate corrodible layer  24 , the corroded intermediate layer  26  corrodes in the color contrasting with a color of the top layer  22 . 
     The above described layered component and method allow detecting cracks that penetrated the top layer before the substrate becomes exposed by using an intermediate layer of corrodible material that has a corrosion color contrasting with that of the top layer. The intermediate layer corrodes at an accelerated rate from galvanic corrosion with the top layer thereby provides earlier crack warnings than it otherwise would. The substrate being susceptible to stress corrosion cracking, i.e. at the presence of both stress and corrosion, it may fail at a stress corrosion crack stress level below its ultimate tensile strength. Earlier crack detection is thus enabled which is desired since crack tip stress is lower for smaller cracks and cracks grows over time from repeated cyclic stressing. The above detection method does not require detection tools, and relies only on the visual detection of the stains which can be done during the routine checks. The better the contrast between the top layer and the corroded intermediate layer, the easier the visual detection. In addition, the intermediate layer may not be parasitic, but rather enhances the stiffness, strength, and fatigue endurance life of the component. 
     The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. Still other modifications which fall within the scope of the present invention 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 appended claims.