Abstract:
A method of analyzing ultrasonic inspection data from turbine wheel or bucket dovetail fingers for a crack about a ledge thereof with the turbine wheel or bucket having a number of adjacent holes therethrough. The method may include inserting an ultrasonic probe into a first hole, rotating an ultrasonic beam of the ultrasonic probe to scan the adjacent holes, scanning each adjacent hole, and determining the presence of the crack in the ledge by the failure to receive a signal from one or more of the adjacent holes.

Description:
TECHNICAL FIELD 
   The present application relates generally to turbines and more particularly relates to a method of inspecting turbine wheel finger dovetail ledges for cracks via the use of an ultrasonic probe. 
   BACKGROUND OF THE INVENTION 
   The rims of turbine wheels are often provided with axially spaced, annularly extending fingers that define dovetails. These dovetails receive generally complementary shaped finger dovetails on buckets that are to be secured to the wheel. A number of pinholes may be aligned axially through the bucket fingers and the wheel fingers along the margin of the wheel. The pins may be axially inserted through these pinholes to secure the buckets to the wheel. 
   Over time and extended use, radial loading on the pins and the presence of a corrosive environment in the turbine may cause stress corrosion cracks to develop about the pinholes. In addition, stress corrosion cracking can occur on the wheel finger ledges where the wheel fingers and bucket fingers fit together. These cracks appear to initiate mid-way between columns of pinholes on the wheel fingers. At these locations, the fingers of adjacent buckets butt together and form a crevice extending between neighboring wheel fingers. The cracks may grow circumferentially along the wheel finger ledges toward the nearest pinholes while also growing axially through the finger. These cracks may lead to the failure of a finger and potential damage to the turbine as a whole. 
   As a result of this possible damage, periodic inspections of the wheel and the bucket finger dovetails are indicated. These inspections generally involve driving the existing pins out of the pinholes to allow the buckets to be removed from the wheel. A florescent magnetic particle inspection of the finger surfaces or other types of inspections then may be performed. For example, the magnetic particles collect around any surface breaking cracks in the presence of an applied magnetic field. The inspector may then illuminate the area with a black light such that the magnetic particles fluoresce. Any cracks present in the finger then may be visually identified. This inspection method, however, requires extensive disassembly of the wheel and the buckets such that the method is labor intensive, time consuming, and hence, costly. 
   More recent improvements have led to inspecting the turbine wheel and bucket finger dovetails via a phased array ultrasonic probe inserted within a pinhole. Unlike the magnetic particle inspection, the buckets need not be removed for the ultrasonic inspection and only a fraction of the pins must be removed for probe insertion. The phased array probe is designed to produce an ultrasonic beam directed radially outward from the pinhole that is electronically rotated to inspect the surrounding finger material. A similar inspection may be performed by an ultrasonic probe in which the radially-directed beam produced by a single transducer element is rotated mechanically. This inspection method may detect cracks occurring at the adjacent pinholes and along the finger ledges. However, at the ledges the orientation of the cracks may prevent the ultrasonic beam from reflecting back to the probe. Specifically, cracks that occur on the finger ledges are generally located between adjacent pinholes and have an axial-circumferential orientation. The ultrasonic beam has an angle of incidence on these cracks that results in the beam being reflected away from the probe. Consequently, these cracks may not be identified by the same analysis method used to identify cracks occurring at the adjacent pinholes. 
   There is a desire, therefore, for an improved method of analyzing the ultrasonic inspection data from turbine wheel and bucket finger dovetails, particularly at the ledge region. 
   SUMMARY OF THE INVENTION 
   The present application thus describes a method of analyzing ultrasonic inspection data from turbine wheel or bucket dovetail fingers for a crack about a ledge thereof with the turbine wheel or bucket having a number of adjacent holes therethrough. The method may include inserting an ultrasonic probe into a first hole, rotating an ultrasonic beam of the ultrasonic probe to scan the adjacent holes, receiving reflected signals from each adjacent hole, and determining the presence of the crack in the ledge by the failure to receive a signal from one or more of the adjacent holes. 
   The present application further describes a method of analyzing ultrasonic inspection data from turbine wheel or bucket dovetail fingers for a crack about a ledge thereof with the turbine wheel or bucket having a number of adjacent holes therethrough. The method may include inserting an ultrasonic probe into a first hole, rotating an ultrasonic beam of the ultrasonic probe to scan the adjacent holes, receiving reflected signals from each adjacent hole, and determining the absence of the crack in the ledge by receiving a signal from each of the adjacent holes. 
   The present application further describes a method of inspecting analyzing ultrasonic inspection data from in situ turbine wheel or bucket dovetail fingers for a crack about a ledge thereof with the turbine wheel or bucket having a number of adjacent holes therethrough. The method may include the steps of removing a pin from a first hole, inserting an ultrasonic probe into the first hole, scanning each adjacent hole with a rotating ultrasonic beam, and determining the presence of the crack in the ledge by the failure to receive a signal from one or more of the adjacent holes. 
   These and other features of the present application will become apparent to one of ordinary skill in the art upon review of the following detailed description when taken in conjunction with the several drawings and the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an exploded view of a turbine wheel and a bucket as may be used herein. 
       FIG. 2  is a cross-sectional view of a wheel finger of the turbine wheel of  FIG. 1 . 
       FIG. 3  perspective view of a turbine wheel and a bucket with a number of ultrasonic probes. 
       FIG. 4  is a schematic view of the beam from the ultrasonic probe as it is incident on the adjacent pinholes at different times during its rotation. 
       FIG. 5  is a schematic view of a turbine wheel pinhole test with the probe located in a middle pinhole and showing the ultrasonic beam and direction of beam rotation. 
       FIG. 6  is a data set showing the pattern of reflected pinhole signals of  FIG. 5 . 
       FIG. 7  is a schematic view of a turbine wheel pinhole test with a probe in the middle pinhole and crack at an outer ledge. 
       FIG. 8  is a data set showing the pattern of reflected pinhole signals of the scan of  FIG. 7 . 
       FIG. 9  is a schematic view of a turbine wheel pinhole test with a probe in the middle pinhole and a crack at an inner ledge. 
       FIG. 10  is a data set showing the pattern of reflected pinhole signals of the scan of  FIG. 9 . 
       FIG. 11  is a schematic view of a turbine wheel pinhole test with the probe inserted into an inner pinhole. 
       FIG. 12  is a data set showing the pattern of reflected pinhole signals of the scan of  FIG. 11 . 
       FIG. 13  is a schematic view of a turbine wheel pinhole test with the probe inserted into an outer pinhole. 
       FIG. 14  is a data set showing the pattern of reflected pinhole signals of the scan of  FIG. 13 . 
   

   DETAILED DESCRIPTION 
   Referring now to the drawings, in which like numerals refer to like elements throughout the several views,  FIG. 1  shows an exploded view of a rotor wheel  10  for mounting a number of buckets  12  thereon. The rotor wheel  10  includes a circumferentially extending dovetail area  13 . The dovetail area  13  includes a number of circumferentially extending, radially outward projecting fingers  14 . These fingers  14  define grooves  16  therebetween. The grooves  16  receive a complementary shaped dovetail  17  with a number of fingers  18  extending from a base  20 . 
   The fingers  14 ,  16  of the wheel  10  and the bucket  12  have a number of axially extending pinholes, a number or wheel pinholes  22  and a number of bucket pinholes  24 . Generally, columns of three (3) radially aligned holes  22 ,  24  are used, although any number may be used. A number of pins  26  are used to secure the buckets  12  to the wheels  10  via the pinholes  22 ,  24 . 
     FIG. 2  shows a side cross-sectional view of a single finger  14  of the wheel  10 . The wheel pinholes  22  also are shown. In this case, an inner pinhole  28 , a middle pinhole  30 , and an outer pinhole  32 . The finger  14  also has an inner ledge  34  and an outer ledge  36 . The ledges  34 ,  36  denote a transition in the thickness of the finger  14 . As is shown, a number of cracks  38  may form at these ledges  34 ,  36 . 
   In the known methods, one or more of the pins  26  are removed from the pinholes  22 . As is shown in  FIGS. 3 and 4 , a probe  40  then may be inserted into one of the wheel pinholes  22  to inspect the fingers  14  and/or fingers  18  for cracks therein. The probe  40  may be a discrete ultrasonic probe having one or more piezoelectric elements that are rotated mechanically to produce a rotating beam  41  or a phased array ultrasonic probe that electronically creates a rotating beam. The probe  40  provides a full 360-degree circumferential scan of the adjacent pinholes  22 . The ultrasonic beam  41  from probe  40  continually rotates past the surface of the adjacent pinholes  22  as the probe  40  travels axially through each finger  14 . 
   For the middle pinhole  30 , there may be eight (8) adjacent pinholes  22  that surround it. In a crack-free finger  14 , reflected signals  42  are received by the probe  40  from each of the eight (8) adjacent pinholes  22 . The presence of a crack  38  on the inner ledge  34  or the outer ledge  36  of a finger  14 , however, may block the ultrasonic beam from reaching one or more of the adjacent pinholes  22  when the crack is sufficiently deep. The result may be an absence of one or more of the reflected pinhole signals  42 . Alternatively, the reference signal  42  may be reduced in amplitude and less than full strength rather than being completely blocked if the crack  38  is shallow. As such, the absence of a pinhole signal  42  or a weak pinhole signal  42  may indicate that a crack  38  is present in one of the ledges  34 ,  36 . 
     FIG. 5  shows a section of rotor wheel  10  with a number of the pinholes  22 . The probe  40  is positioned within the middle pinhole  30  and a scan is taken by the probe  40  of each of the surrounding eight (8) pinholes  22 .  FIG. 6  shows a data set of the reflected signals  42  received by the probe  40 . Specifically, signals  42  concerning each of the surrounding eight (8) pinholes  22  are received in terms of distance and angular position. In a crack-free finger  14  as is shown, data signals  42  will be received from each of the eight (8) surrounding pinholes  22 . 
     FIG. 7  shows one of the fingers  14  of the rotor wheel  10  with a crack  38  on the outer ledge  36 . The probe  40  again is positioned within the middle pinhole  30  and the eight (8) surrounding pinholes  22  are scanned.  FIG. 8  shows the data set of the signals  42  received by the probe  40 . Given the presence of the crack  38 , a signal  42  is not received from pinhole number  2  by the probe  40 . Rather, the ultrasonic beam  41  is deflected by the crack  38 . Likewise, a crack  38  on the inner ledge  34  is shown in  FIG. 9  and the accompanying data set is shown in  FIG. 10 . As a result of the crack  38 , the ultrasonic beam  41  is again deflected such that no data is received from pinhole number  4 . The absence of a particular pinhole signal is used to determine on which ledge a crack is located. The absence of either pinhole signal number  2  or  8  in  FIG. 6  indicates a crack is located on the outer ledge. Whereas the absence of pinhole signal number  4  or  6  indicates a crack is located on the inner ledge. The circumferential location of the crack along a particular ledge can also be determined from the particular pinhole signal that is absent. The absence of pinhole signal  2  indicates not only that the crack is at the outer ledge but also that it is between pinhole numbers  1  and  2 , rather than between pinhole numbers  1  and  8 . Likewise, the absence of pinhole number  6  indicates a crack on the inner ledge between pinhole numbers  5  and  6 , rather than between pinhole numbers  4  and  5 . In general, the key signals on which to concentrate are the ones from the pinholes located diagonally from the hole in which the probe is inserted and across the ledge that is being evaluated. 
   This analysis also can be used in evaluating ultrasonic data obtained with the probe  40  in an inner pinhole  28  or an outer pinhole  22 . In the example of  FIG. 11 , with the probe  40  in inner pinhole  22  reflected signals from only seven (7) surrounding pinholes  22  would be obtained. The presence of a crack  38 , however, would still be found through the absence of a signal  42 . A data set from the scan of the seven (7) pinholes  22  is shown in  FIG. 12 . In this case, the absence of pinhole signal numbers  2  and/or  7  would indicate a crack(s) located on the outer ledge while the absence of pinhole signal numbers  3  and/or  6  would indicate a crack(s) on the inner ledge. In the example of  FIG. 13 , with the probe  40  in outer pinhole  22  reflected signals again from only seven (7) surrounding pinholes  22  would be obtained. The presence of a crack  38 , however, would still be found through the absence of a signal  42 . A data set from the scan of the seven (7) pinholes  22  is shown in  FIG. 14 . In this case, the absence of pinhole signal numbers  3  and/or  5  would indicate a crack(s) located on the inner ledge while the absence of pinhole signal numbers  2  and/or  6  would indicate a crack(s) on the inner ledge. 
   Although the techniques used herein have been described in the context of the fingers  14  of a wheel  10 , the techniques are equally applicable to the fingers  18  of the bucket  12  as well. The cracks  38  in the ledges  34 ,  36  of the fingers  14 ,  18  thus may be detected by the absence of a signal  42  from the probe  40 . Likewise, the techniques herein may be combined with known ultrasonic testing methods for the remaining areas of the fingers  14 ,  18  as are described above. 
   It should be apparent that the foregoing relates only to certain embodiments of the present application and that numerous changes and modifications may be made herein by one of ordinary skill in the art without departing from the general spirit and scope of the invention as defined by the following claims and the equivalents thereof.