Abstract:
A method of refurbishing a gas turbine engine rotor is disclosed and includes an initial step of inspecting a rotor for defects such as cracks. A strain is then generated around any detected cracks in the rotor to create enhanced plasticized zones about the detected defects that delay defect propagation. Strain is generated by rotating the rotor at speeds greater than operational speeds to induce the desired strain and delay the propagation of defects.

Description:
BACKGROUND 
     A gas turbine engine typically includes a fan section, a compressor section, a combustor section and a turbine section. Air entering the compressor section is compressed and delivered into the combustion section where it is mixed with fuel and ignited to generate a high-speed exhaust gas flow. The high-speed exhaust gas flow expands through the turbine section to drive the compressor and the fan section. The compressor section typically includes low and high pressure compressors, and the turbine section includes low and high pressure turbines. 
     Gas turbine engines are removed from service and each part including compressor and turbine rotors are inspected for defects including cracks. Crack propagation rates for specific rotor materials, temperature and loading characteristics limit the life of a rotor. Once parts reach a determined limit, they are scrapped. Compressor and turbine rotors are costly parts and therefore any additional use or extension of life provides substantial value. 
     SUMMARY 
     A method of extending operational life of a gas turbine engine rotor according to an exemplary embodiment of this disclosure, among other possible things includes inspecting a rotor for defects, and generating an enhanced compressive residual stress around any defects in the rotor to create plasticized zones about the defects that delay defect propagation. 
     In a further embodiment of the foregoing method, includes rotating the rotor at a speed above a normal operational speed for generating strain around the defects. 
     In a further embodiment of any of the foregoing methods, the rotating speed of the rotor greater than about 30% above a normal operating speed. 
     In a further embodiment of any of the foregoing methods, the rotating speed of the rotor is between about 5% and 45% above a normal operational speed. 
     In a further embodiment of any of the foregoing methods, the rotating speed of the rotor is between about 25% and 35% above a normal operational speed. 
     In a further embodiment of any of the foregoing methods, includes rotating the rotor for a time representative of a flight cycle. 
     In a further embodiment of any of the foregoing methods, includes maintaining the temperature within the range replicating operational temperatures of the rotor during rotation of the rotor. 
     In a further embodiment of any of the foregoing methods, includes heating the rotor to a temperature within a range between 70° F. and 1250° F. 
     In a further embodiment of any of the foregoing methods, includes placing blades and/or representations of blades onto the rotor during rotation of the rotor. 
     In a further embodiment of any of the foregoing methods, the rotor comprises one of a compressor rotor and a turbine rotor. 
     In a further embodiment of any of the foregoing methods, includes evaluating the defect in view of a predefined criteria and conducting rotor refurbishment when the defects within the rotor meet the criteria. 
     In a further embodiment of any of the foregoing methods, includes inspecting the rotor after creation of the plasticized zones to confirm that the defects fall within desired operational criteria. 
     A method of retarding defect growth in a rotor according to an exemplary embodiment of this disclosure, among other possible things includes spinning the rotor at an over-speed between of at least 30% above a normal operational speed, and cycling the rotor between a normal operational speed and the over-speed for a duration of time representative of a flight cycle. 
     In a further embodiment of the foregoing method, includes generating a plasticized area about a defect with the strain generated by spinning the rotor at the over-speed. 
     In a further embodiment of any of the foregoing methods, the rotor comprises one of a turbine rotor and a compressor rotor. 
     Although the different examples have the specific components shown in the illustrations, embodiments of this invention are not limited to those particular combinations. It is possible to use some of the components or features from one of the examples in combination with features or components from another one of the examples. 
     These and other features disclosed herein can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a portion of a disclosed method of extending operational life of rotating parts of a gas turbine engine. 
         FIG. 2  is a schematic view of an inspection step of the example method. 
         FIG. 3  is a schematic representation of a life extension method. 
         FIG. 4  is a schematic representation of the treatment areas provided by the example method. 
         FIG. 5  is a graph illustrating a cyclical loading of a rotor for according to the example method. 
         FIG. 6  is a graph illustrating the extension of life provided by the example method. 
         FIG. 7  is schematic view of an example method of utilizing a treated rotor in a short use gas turbine engine. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , an example gas turbine engine includes at least one rotor  12 . The rotor  12  is periodically removed from the gas turbine engine and inspected according a specific maintenance schedule as is schematically indicated at  14 . Once removed, the rotor  12  is inspected for defects such as cracks and micro-cracks. The disclosed example describes the rotor  12  as a turbine rotor  12 ; however, the disclosed method could be applied to a compressor rotor, or any other rotating component of the gas turbine engine  10 . 
     Referring to  FIG. 2  with continued reference to  FIG. 1 , the inspection process generally indicated at  16  includes the identification of defects such as cracks  18  within the rotor  12 . Once the cracks  18  are detected they are measured and a specific crack propagation schedule is utilized to determine the remaining life cycle limit for the inspected rotor  12 . The life cycle limit for a rotor  12  that includes cracks  18  is dependent on the rotor materials that the temperature in which it operates along with loading characteristics experienced during operation. Rotor  12  will then be utilized until it reaches that determined life cycle limit. At that life cycle limit the rotor  12  will be removed from the gas turbine engine and scrapped. As appreciated, the scrapping of a rotor is undesirable as rotors are costly, highly engineered components. Accordingly, a disclosed method includes treatment of rotors  12  including cracks  18  detected during scheduled maintenance that extends the cycle life for a specific rotor beyond a cycle life for non-treated rotors. 
     In this example, the sample rotor  12  is a turbine rotor removed from the turbine section of the gas turbine engine  10 . Cracks  18  are detected within a portion of the rotor  12 . The cracks illustrated in  FIG. 2  are schematically shown and are not indicative of any specific location or length. The identification of the cracks  18  during the inspection process  16  is utilized as a prompt to indicate that treatment should be utilized to extend the useful life of the example rotor  12 . 
     Referring to  FIG. 3  with continued reference to  FIG. 2 , the example treatment process includes mounting the rotor  12  to a spin machine  22  that is disposed within a controlled environment  20 . The rotor  12  is spun as indicated at  36  to introduce beneficiary compressive stresses at a tip of each of the cracks  18 . 
     In this example, the rotor  12  includes blades shown schematically at  15  to induce a further load on the rotor to aid in generation of the beneficial compressive stresses about the crack  18 . Moreover, the blades  15  could be replaced with blanks replicating the blades configuration and mass to provide for the desired additional load without the use of actual blades  15 . It is also within the contemplation of this disclosure that the rotor  12  could be treated without the blades or representations of the blades  15 . 
     The example method can be performed at room temperature. Moreover in one example disclosed embodiment, the rotor  12  is brought to an elevated temperature for treatment within the controlled environment  20 . In one example, the temperature is between 550° F. and 1250° F. As appreciated, heating of the rotor  12  may not be required, but is a step that may be utilized to simulate operational conditions. Furthermore, the temperature at which the rotor  12  is soaked within the controlled environment  20  is dependent on operation parameters and can vary beyond those presented in this disclosure. 
     Referring to  FIG. 4  with continued reference to  FIG. 3 , the rotor  12  is spun according to a specific cycle schedule to generate beneficiary compressive stress regions around the tips of each of the cracks  18 .  FIG. 4  illustrates plasticize regions around the cracks  18  that prevent or slow the propagation of those cracks  18  to extend the cycle life of the rotor  12 . 
     Referring to  FIG. 5  with continued reference to  FIG. 3 , the machine  22  spins the rotor  12  according to a cycle schedule generally indicated by graph  30 . In this example, the load comprises rotations  36  of the rotor  12  on the machine  22 . The rotations are provided at a first normal level  26  for a first span of cycles. The rotor  36  is then sped up to an over speed condition indicated at  28 . The over speed condition comes at specific cycle intervals  32 . 
     The over speed condition  28  represents rotations of the rotor  12  above a speed that is normally encountered during operation. In one example disclosed embodiment, the over speed condition  28  represents a speed between about 5% and 45% above the normal operational speed indicated at  26 . As appreciated, operational speed of a gas turbine engine can vary during operation. The over speed condition  28  is as compared to a cruise condition of the engine. During operation the rotors of the gas turbine engine will vary in speeds and therefore the cruise condition is utilized as the normal operational speeds to provide a baseline to determine the proper over speed condition  28 . However, it should be understood that other baseline speeds could be utilized and are within the contemplation of this disclosure. 
     In a second disclosed embodiment, the over speed condition  28  comprises a speed between about 25% and 35% greater than the normal rotation speed encountered by the rotor  12 . In another disclosed embodiment the speed is at least 30% above normal operational speed of the rotor  12 . 
     The over speed condition  28  is encountered at specific cycle intervals indicated at  32 . The cycle intervals  32  are related to operational characteristics and encountered over the life of the rotor. The cycle intervals indicated at  32  are related to a length of time in which the rotor is rotated. 
     A typical period for operation can vary depending on the type of engine and specific use. In some instance, such as in military uses, the gas turbine engine may normally operate for about 2 hours. Moreover, commercial engines may operate for a longer time such as approximately 5 hours, or even up to 9 hours in some instances. Accordingly, the time at which the rotor is rotated at the over speed condition is selected depending on the specific engine and known use. In one example the rotor is rotated at an interval of between 2 and 5 hours. In another example, the rotor is rotated at an interval of between 7 and 10 hours. It is further within the contemplation of this disclosure that other intervals may be utilized that represent a normal operational period for a particular gas turbine engine. 
     The method includes mounting of the rotor  12  to the machine  22  within an environment  20  that heats the rotor  12  to a temperature between 550-1250° F. The rotor  12  is then spun at a rotation that simulates operational characteristics and cycles. At specified intervals indicated at  32  on graph  30 , the rotor  12  is rotated at a speed greater than the normal operational speed between 25-35% greater than normal operational speed as is indicated by  28 . This is repeated over the treatment cycle range to improve and generate the plasticized regions  24  ( FIG. 4 ) around the cracks  18  that slow crack propagation. 
     Referring to  FIG. 6 , a graph  38  schematically illustrates improvement and cycle length increases indicated at  50  that are provided by the example plasticity enhancement around the detected crack  18 . The graph  38  includes a line  40  that is indicative of a normal non-treated rotor where cracks have been detected at  18 . 
     Cracks propagate and reach a limit indicated at  46  after a first number of cycles. The dashed line indicated at  42  represents a treated rotor where the cracks  18  have been over stressed to generate the enhanced plasticized regions  24 . The overall operational life of the rotor  12  is thereby increased as indicated at  50  to a new cycle life duration indicated at  48 . The new duration  48  is determined by testing of rotors  12  for the specific defect plasticity enhancement schedule. As appreciated, the defect plasticity enhancement schedule including the temperature and load placed on a specific rotor are determined for each rotor material configuration, geometry and specific part number. A new operational life cycle limit is determined that accounts for the enhance plasticity condition with an increase in usable life of the rotor  12  beyond that that would be acceptable for a non-treated rotor. Each of the normal and the extended life cycles limit the cycle of the rotor  12  such that a crack length will not extend beyond an acceptable length indicated by the line  44 . 
     Referring to  FIG. 7 , in this disclosed method, treated rotors  54  will have an extended cycle life as is indicated in the graph  38 . However, this extended life is still shorter than original parts and are of the refurbished and treated rotors  54 . Accordingly, the rotors  54  are installed into gas turbine engines as indicated at  56  for a limited life cycle operation. Such limited operational life cycle gas turbine engines  56  can be utilized for alternate functions and uses such that the rotors  12  can be utilized beyond the original cycle life once a crack is detected within the rotor  12 . 
     Accordingly, the example method provides assistance for treating and over stressing a rotor  12  with defects and/or cracks  18  to elongate the operational cycle life and allow for further use of expensive components. 
     Although an example embodiment has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this disclosure. For that reason, the following claims should be studied to determine the scope and content of this disclosure.