Abstract:
A tool for inspecting the integrity of fasteners in their environment of use and methods of performing such inspections are disclosed and claimed. The tool includes a probe that matches the internal socket by which the fastener is coupled to the workpiece. The probe contains ultrasonic transducers on flat portions corresponding to flat portions of the socket. The transducers induce angled ultrasonic beams into the fastener to detect flaws therein. The beams are angled so they can be directed to the areas of interest at the head to shank region of the fastener. The presence of a defect such as a crack is determined based on the reply/echoes of the imparted ultrasonic beams.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application claims the benefit of U.S. Provisional Patent Application No. 61/648,681 filed on May 18, 2012, which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to inspecting the integrity of fasteners, and, more particularly, the present invention relates to a device fir and method of inspecting fasteners in their environment of use. 2. Description of the Related Art 
         [0004]    A typical requirement for industrial facilities, regardless of the type of industry, is that the equipment must be inspected on a routine basis to ensure that the structural integrity of the equipment is within acceptable guidelines. While the frequency of such inspections may vary, the need to prevent failure or break-down of the equipment is a common requirement across industry. For exemplary purposes, the environment of a nuclear utility will be discussed herein. 
         [0005]    Nuclear utilities have a need to verify the integrity of their aging components within nuclear reactors and other plant systems. In Pressurized Water Reactors (PWRs), one exemplary set of components required to be inspected for plant life extension and compliance with regulatory requirements are the baffle bolts that are part of the reactor internal assembly. Due to the bolt pre-loads, age, and radiation fluence through the bolts, these fasteners are susceptible to loosening and cracking. Moreover, due to radiation embrittlement of the bolt materials (irradiation-assisted stress corrosion cracking, or IASCC), once a crack begins, it can grow quickly due to the reduced toughness of the embrittled material. Other influences may subject equipment in other types of facilities to similar degradation. 
         [0006]    Industrial facilities typically have reduced access to fasteners once the equipment and related components are placed in service. In the environment of a nuclear power plant, retaining bars and/or washers are welded to the top of fasteners to prevent them from falling out if they become loose or if the head portion becomes detached from the shaft. These safety precautions make fastener inspection much more difficult and complex. Without a qualified and reliable inspection of the fasteners in their use configuration, the safety components must be removed to physically access the fastener. Thus, the facility operators will replace these fasteners regardless of whether they actually need to be replaced. Replacement, of course, is a much more costly and time consuming undertaking. 
         [0007]    Thus, there is a need for a reliable and capable inspection system and method for fasteners with limited access. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention relates to a tool for inspecting the integrity of fasteners in their environment of use. Baffle bolts of a nuclear reactor head are specifically contemplated as fasteners to be inspected. The tool includes a probe that matches the internal socket by which the fastener is coupled to the workpiece. The probe contains ultrasonic (UT) transducers on flat portions corresponding to fiat portions of the socket. The transducers induce angled ultrasonic beams into the fastener to detect flaws therein. The beams are angled so they can be directed to the areas of interest at the head to shank region of the fastener. The presence of a defect such as a crack is determined based on the reply/echoes of the imparted ultrasonic beams. 
         [0009]    When coupled with a multichannel UT instrument and associated software, the beams are activated in specified sequences to inject and receive the ultrasonic energy on various transducer elements to provide coverage of the entire circumference of the bolt. Various ultrasonic techniques are contemplated for use, including dual “side-by-side,” dual “opposite” transducer pairs, and single element “pulse-echo.” The “side-by-side” technique fires on one element and receives on the adjacent element. This sequence is incremented by one element around the probe until each of the elements has been fired. The “opposite” technique fires on one element and receives on the opposite element. This sequence is incremented once for each set of element pairs. The “pulse-echo” technique fires and receives on the same element, one at a time in sequence until all elements have been fired. All of these sequences repeat until data acquisition stops. The data files are stoned for off-line analysis. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0010]    The present invention is described with reference to the accompanying drawings, in which like reference characters reference like elements, and wherein: 
           [0011]      FIG. 1  shows a typical baffle bolt; 
           [0012]      FIG. 2  shows a side view of a fastener inspection tool of the present invention; 
           [0013]      FIG. 3  shows a top view of the fastener inspection tool of  FIG. 2 ; 
           [0014]      FIG. 4  shows the fastener inspection tool of  FIG. 2  in its use position within the socket of the baffle bolt of  FIG. 1 ; 
           [0015]      FIG. 5  shows an example dual side-by-side mode of operation of the inspection tool of  FIG. 2 ; 
           [0016]      FIG. 6  shows an example flaw that is centered on the intersection of two bolt socket flats; 
           [0017]      FIG. 7  shows an example flaw that is centered on only one bolt socket flat; 
           [0018]      FIGS. 8-11  each show typical baffle bolt UT responses from flaws of varying tilt angles and flaw depths measured with the dual side-by-side technique of the present invention; 
           [0019]      FIG. 12  shows an example pulse-echo mode of operation of the inspection tool of  FIG. 2 ; 
           [0020]      FIG. 13  shows an example flaw that is centered on the intersection of two bolt socket flats; 
           [0021]      FIG. 14  shows an example flaw that is centered on only one bolt socket flat; 
           [0022]      FIGS. 15-17  each show typical baffle bolt UT responses from flaws of varying tilt angles and flaw depths measured with the pulse-echo technique of the present invention; 
           [0023]      FIG. 18  shows an example dual-opposite mode of operation of the inspection tool of  FIG. 2 ; 
           [0024]      FIG. 19  shows an example flaw that is centered on the intersection of two bolt socket flats; 
           [0025]      FIG. 20  shows an example flaw that is centered on only one bolt socket flat; and 
           [0026]      FIGS. 21-24  each show typical baffle bolt UT responses from flaws with no tilt angle and approximately 25% flaw depths measured with the dual-opposite technique of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    The present invention provides a tool for and method of inspecting the integrity of fasteners in their environment of use. While the present invention can be used with any type of fastener, baffle bolts of a nuclear reactor head will be discussed herein for exemplary purposes. Baffle bolts number in the hundreds in the reactor vessel and hold baffle plates together. The baffle plates allow a cylindrical vessel interior to accommodate the fuel, which is in square bundles. They also provide a boundary between incoming cold reactor coolant and the heated reactor coolant flowing on the outside and inside of the cylinder. 
         [0028]      FIG. 1  shows a typical baffle bolt  10 . The bolt  10  includes a head  11  and a shaft  14 , which includes a threaded portion  15 . The head  11  contains a socket  12  therein for engagement with a tool to engage or disengage the bolt  10 . As shown in the illustrative example of  FIG. 1 , baffle bolts  10  typically have hexagonal sockets  12  having six flat sides or “flats”  13  by which the insertion/extraction tool provides torque to the bolt  10 . 
         [0029]    Typical ultrasonic inspection systems apply UT energy through the bottom of the sockets  12 . The socket bottoms typically have a concave shape. However, the socket bottom typically is not controlled during the manufacturing process so the exact curvature is not known. Moreover, the manufacturing process by its nature, coupled with variations caused by machinery becoming worn and dulled during manufacture of the bolts  10 , can cause a wide variation in the geometries of the socket bottoms This is problematic for UT inspection, as the surface on which the transducer is placed must be well matched to the transducer contour in order to perform ultrasonic examinations. Thus, typical UT inspection systems are not able to provide accurate, reliable results for such widely varying socket bottom geometries. 
         [0030]    The instant invention overcomes these measurement problems by applying UT energy o the bolt  10  through the socket flats  13 .  FIG. 2  shows a side view of a tool  20  of the present invention, and  FIG. 3  shows a top view thereof. The tool  20  includes a probe head  21  coupled to a probe body  22 . The probe head  21  is hexagonally shaped to match the socket  12  of the baffle bolts  10 . Thus, the probe head  21  can access and inspect the baffle bolts  10  even when the top perimeter area of the bolt head  11  is inaccessible due to a retaining washer or the like. 
         [0031]    The probe head  21  contains six transducers A-F, one transducer positioned on each of the sides corresponding to the socket flats  13 . Wires  23  independently connect each transducer A-F to its corresponding controller  24 A- 24 F. Thus, each transducer A-F can be independently controlled to impart or receive UT energy. The power supply, processing equipment, and other components are not shown. 
         [0032]    The transducers A-F are configured to impart UT energy at a downward angle to the baffle bolt shaft  14 . The imparted UT energy preferably is directed at an angle α of approximately 35°-55° relative to the longitudinal axis  16  of the bolt  10 , with approximately 40°-45° being more preferred. The transducers A-F preferably are mounted in a housing that provides protection from damage and wear. The transducers A-F preferably are positioned so that the beam exit point is close to the tip of the probe head  21  to provide proper positioning of the sound beams when the probe  20  is seated in the bolt socket  12 . Preferably, the transducers A-F are within approximately 0.12 in. from the tip of the probe head  21 . 
         [0033]      FIG. 4  shows the probe  20  in its use position within the socket  12  of a baffle bolt  10 . The socket  12  includes a number of flats  13  that are substantially parallel to the fastener longitudinal axis  16 . As illustrated in the example embodiment of  FIG. 4 , the probe head  21  is configured to substantially fill the socket  12 . In the event that the socket  12  is deeper and the probe head  21  does not extend to the bottom of the socket  12 , it could be said that the probe head  21  substantially fills an elevation of the socket  12 . 
         [0034]    The probe head  21  includes a number flats corresponding to the number of socket flats  13  with a transducer positioned on each head flat such that when the probe head  21  is positioned within the fastener socket  12 , a transducer is positioned adjacent each of the socket flats  13 . Preferably, each transducer A-F spans 40%-60% of its respective socket flat  13 . The transducers A-F are arranged in an array and are substantially evenly spaced adjacent the internal surface of the socket  12 . Thus, the probe  20  can impart or receive UT waves  28  through any of the socket flats  13 . The imparted sound beam  28  is reflected off of the outer diameter of the baffle bolt head  11  to the shaft  14  to detect any cracks  30  therein. This reflection is at the same angle as the incident angle imparted by the transducer. The beam  28  reflects off the crack and is sensed by the receiving transducer. A preferred nominal frequency for the beam  28  is approximately 4-6 MHz, and more ‘preferably 5 MHz, which represents a compromise between high frequency for better resolution and lower frequency for less sensitivity to coupling interferences and better tolerance for flaw mis-orientation. Preferably, the transducer is a six channel transmitter/receiver. 
         [0035]    When coupled with a multichannel UT instrument and associated software, the beams are activated in specified sequences to inject and receive the ultrasonic energy on various transducer elements to provide coverage of the entire circumference of the bolt. The ultrasonic techniques used with the probe  20  include dual “side-by-side” and dual “opposite” transducer pairs and single element “pulse-echo.” The side-by-side technique fires on one element and receives on the adjacent element. This sequence is incremented by one element around the probe until each of the six elements has been fired. The opposite technique fires on one element and receives on the opposite element. This sequence is incremented once for each set of element pairs. The pulse-echo technique fires and receives on the same element, one at a time in sequence until all six elements have been fired. All of these sequences repeat until data acquisition stops. The data files are stored for off-line analysis. Table 1 below provides a summary of the ultrasonic techniques: 
         [0000]    
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Acquisition Channel Naming Structure 
               
             
          
           
               
                 Transmit 
                 Receive 
                 Channel Name 
                 Technique 
               
               
                   
               
               
                 A 
                 B 
                 AB 
                 Dual Side-by-Side 
               
               
                 B 
                 C 
                 BC 
                 Dual Side-by-Side 
               
               
                 C 
                 D 
                 CD 
                 Dual Side-by-Side 
               
               
                 D 
                 E 
                 DE 
                 Dual Side-by-Side 
               
               
                 E 
                 F 
                 EF 
                 Dual Side-by-Side 
               
               
                 F 
                 A 
                 FA 
                 Dual Side-by-Side 
               
               
                 A 
                 D 
                 AD 
                 Dual Opposite 
               
               
                 B 
                 E 
                 BE 
                 Dual Opposite 
               
               
                 C 
                 F 
                 CF 
                 Dual Opposite 
               
               
                 A 
                 A 
                 A 
                 Pulse Echo 
               
               
                 B 
                 B 
                 B 
                 Pulse Echo 
               
               
                 C 
                 C 
                 C 
                 Pulse Echo 
               
               
                 D 
                 D 
                 D 
                 Pulse Echo 
               
               
                 E 
                 E 
                 E 
                 Pulse Echo 
               
               
                 F 
                 F 
                 F 
                 Pulse Echo 
               
               
                   
               
             
          
         
       
     
         [0036]    Stress corrosion cracking typically consists of a sequence of facets and eventually branching in the bolt material. This structure creates scattering of the ultrasonic beam, which effect aids in flaw detection of tilted flaws using the dual side-by-side and pulse echo techniques described. In contrast, a smooth flaw is optimum for the dual-opposite technique. It should be noted, however, that a rough flaw would be expected to provide a lower signal to noise ratio response. 
         [0037]    In a preferred method of operation, the software configures the UT instrument and acquires the data through the use of pre-established system setup files. These settings can be defined, for example, through experimentation on bolt sets containing flaws in order to optimize performance. After the data has been acquired, it may be stored on a hard drive attached to the data acquisition computer for archival purposes and for off-line analysis of the data. During the data acquisition process, the entire waveform may he digitally recorded for each channel with 12-bit resolution. Analysis of the data can also be performed with the software. Analysis of the data is performed by reviewing the displays for each of the data channels. 
         [0038]    The probe  20  can be delivered to each bolt  10  to be examined using a variety of means. One preferred means is a remote controlled submarine. The delivery vehicle should have sufficient maneuverability to be able to deliver the probe  20  into the bolt socket  12 . This may require that the probe  20  be rotated to align with the socket  12  and positioned relatively normal to the longitudinal axis of the bolt  10  to enable insertion. The delivery vehicle may preferably include a rail system and a transducer holder assembly that allows the transducer to traverse from side to side and to rotate the probe  20  about its axis to align with socket  12  of the bolt  10 . Once the probe  20  is seated in the socket  12 , the data acquisition sequence is started and the probe  20  is withdrawn from the socket  10 . The data acquisition cycle may be timed such that the probe  20  has time to be withdrawn from the socket before recording stops. Tracking of the delivery vehicle may be accomplished using a map of the core barrel and auxiliary cameras to ensure the correct quadrant, plate, column, and former elevation are examined. 
         [0039]    The basic principle for the examination of bolts  10  is the propagation of a sound field from the internal hex flat  13  through the side wall of the head to the outer diameter surface, and then back toward the center of the bolt  10  below the socket  12  in the head  11  to shank  14  region of the bolt  10 . If there is a separation (crack)  30  in the material, which reflects part of the sound energy, the result is that signals will be detected by the techniques described herein and the bolt  10  will be identified as a cracked bolt  10 . If a bolt  10  is determined to be cracked, it is considered non-functional and no further characterization is necessary. 
         [0040]    A high signal-to-noise ratio (SNR) is optimal for flaw detection with minimal false calls. Because the grain structure is fine and the sound path to the flaws of interest is short, flaw response is expected to be good. It is expected that most bolts should provide on average a 3% to 5% noise level response. 
         [0041]    Either shear wave or longitudinal wave angle beams modes can be used for the inventive application. However, the shear wave mode is preferred. 
         [0042]    The dual side-by-side technique uses each transducer A-F as a transmitter and the adjacent transducer as a receiver as shown in Table 1 to create six channels of data. This technique can be used to detect angled flaws in the head to shank region of the bolt  10 . During the data acquisition cycle, the transmitter and receiver pairs are electronically incremented around the probe  10  to activate all transducer pairs sequentially. This technique is illustrated in  FIG. 5 , which shows a transmitting transducer T adjacent a receiving transducer R. The left side of  FIG. 5  presents a cross-sectional view showing the beam  28  reflecting off of the outer diameter surface of the head  11  and traveling to an area of interest. If a flaw  30  is in the beam path and tilted at an angle with sufficient circumferential extent, it will reflect some of the energy back to the receiver on the adjacent flat  13  and be detected. The face of tilted flaws is expected to be somewhat spherical in shape because the crack will follow the circumference of the bolt  10  as it progresses upward and toward the center of the bolt  10 . This shape helps direct the reflected energy toward the receiver R and aids in detection of the flaw  28 . However, to be detected with this technique, the flaw circumferential position relative to the flats  13  must extend at least partially across two flats (approximately 60°) as shown in the left side of  FIG. 5 . 
         [0043]      FIGS. 6 and 7  present top views from the bolt head  11  and show the effect of flaw  28  position relative the flats  13  for detection for the dual side-by-side technique.  FIG. 6  illustrates an elliptically shaped flaw  28  that is centered on the intersection of two flats  13 . The flaw  28  is approximately 25% deep. This is the optimum flaw position for this technique.  FIG. 7  illustrates the same flaw as  FIG. 6 , but it is on only one of the flats  13 . This is not an optimum flaw position for this technique because the reflected energy may not reach the receiver. Detection of these flaws  28  is addressed by the pulse-echo technique described below. 
         [0044]    Deeper flaws are likely o span multiple flats  13  and be detected with multiple side-by-side channels. In this regard, the number of channels that detect a flaw may be an indicator of the flaw depth; however, flaw size is not relevant to disposition the condition of the bolt  10 . The method used to minimize false calls with this technique involves a second acquisition of the data but with the probe rotated at least 60°. If the indication is relevant, it will move to the channel(s) corresponding to the probe rotation. For example, if the indication was detected on channels CD and DE during the first acquisition and then after rotating the probe 60° the indication appears on channels DE and EF, the indication will be considered relevant and the bolt considered cracked. If the indication does not change channels corresponding to the probe rotation position, it can be considered non-relevant. 
         [0045]      FIGS. 8-11  each show typical baffle bolt UT responses from flaws of varying tilt angles and flaw depths measured with the dual side-by-side technique. Responses obtained from non-flawed areas of the bolt are also shown. The images include the displays from each of the six channels used for this technique. As evidenced by the responses, flaws ranging from 15° to 30° tilt are easily detectable with good signal to noise ratio. 
         [0046]    The pulse-echo technique uses each of the six transducers A-F separately as transmitters/receivers in a pulse-echo mode. Table 1 lists each of the six channels. This technique can be used to supplement the dual side-by-side technique to detect angled flaws at the head to shank region of the bolt  10  that are restricted primarily to only one of the flats  13 . This condition exists for shallow flaws where the circumferential extent is short. During the data acquisition cycle, the transducers A-F are electronically incremented around the probe  20  to activate all transducers sequentially. This technique is illustrated in  FIG. 12 . 
         [0047]    The left side of  FIG. 12  presents a cross-sectional view showing the beans  28  reflecting off of the outer diameter surface of the head  10  and traveling to an area of interest. If a flaw  30  is in the beam path and tilted at an angle with sufficient circumferential extent, it will reflect some of the energy back to the transducer A-F and be detected. Because detection is based on the beam  28  traveling along the same path for the transmitted and reflected beams, this technique is the most sensitive to small flaws. 
         [0048]      FIGS. 13 and 14  present top views from the bolt head  11  and shows the effect of flaw  30  position relative the flats  13  for detection with the pulse-echo technique.  FIG. 13  illustrates an elliptically shaped flaw  30  that is approximately 25% deep and centered between two flats  13 . This is the least optimum position for this technique because the sound beam  28  only interacts with a small fraction of the flaw surface area. Very shallow flaws with this orientation may escape detection with this technique because the reflected energy will likely not reach the receiver R.  FIG. 14  illustrates the same flaw as  FIG. 13 , but it is on only one of the flats  13 . Shallow flaws with this alignment will likely be detected because the reflecting surface of the flaw  30  will direct most of the energy back to the transducer. 
         [0049]    Deeper flaws are likely to span multiple flats  13  and be detected with multiple pulse-echo channels. In this regard, the number of channels that detect a flaw may be all indicator of the flaw depth; however, flaw size is not relevant to disposition the bolt  10 . The method used to minimize false calls with this technique involves a second acquisition of the data but with the probe rotated at least 60°. If the indication is relevant, it will move to the channels corresponding to the probe rotation. For example, if the indication was detected on channels C and D during the first acquisition and then after rotating the probe 60° the indication appears on channels D and E, the indication will be considered relevant and the bolt considered cracked. If the indication does not change channels corresponding to the probe rotation position, it can be considered non-relevant. 
         [0050]      FIGS. 15-17  each show typical UT responses from flaws with various tilt angles and 25% flaw depths with the pulse-echo technique. The images include the displays from each of the six channels used for this technique. As evidenced by the responses, flaws are easily detectable with good signal to noise ratio. 
         [0051]    The dual-opposite technique uses three of the transducers A-F as transmitters T and the three (opposed) transducers as receivers R configured in pairs as shown in Table 1 to create three channels of data. This technique can be used to detect flat flaws (little or no tilt angle) in the head to shank region of the bolt  10 . During the data acquisition cycle, the transmitter T and receiver R pairs are electronically incremented around the probe to activate all transducer pairs sequentially. This technique is illustrated in  FIG. 18 . 
         [0052]    The left side of  FIG. 18  presents a cross-sectional view showing the beam  28  reflecting off of the outer diameter surface of the head  10  and traveling to an area of interest. If a flaw  30  is in the beam path and relatively flat with sufficient radial extent, it will reflect the energy toward the bottom of the socket  12 , reflect off the bottom of the socket  12  to the opposite shoulder, reflect to the bolt head outer diameter surface, and then to the receiving transducer R and be detected. In the absence of a flaw, some energy will reflect off of the shoulder and be directed toward the opposite side due to the width of the bolt shoulder. If the bottom of the socket  12  is conical, much of this energy will be scattered and little response will be detected on the opposite side of the bolt  10 . However, if the bottom of the socket  12  is relatively flat, a greater portion of the energy will be reflected to the opposite side and will be detected. This amplitude response is expected to be relatively uniform in non-flawed bolts  10  for all three channels because the bolt head  11  and socket  12  geometry are symmetric. The presence of a flat flaw at the head to shank intersection will cause a change in the symmetry of the bolt head shoulder and cause a change in the amount of reflected energy that is transmitted to the receiver R. The effect will be an indication response that is greater than the amplitude observed on the non-affected channel(s). Depending on the actual flaw shape, detection may also be evident on the dual side-by-side or pulse-echo channels as well. 
         [0053]    To determine if the response is due to a flaw or “flat” bottom socket geometry, the bolt socket  12  will need to be reviewed with video. If the bottom socket geometry is conical then it can be concluded that the response is due to a flaw. If the bottom so&amp;et geometry is observed to be relatively flat and the amplitude of the indication is greater than that of an unflawed bolt with flat bottom geometry, then it can be concluded that the bolt is cracked. The depth of the flaw affects the amplitude of the flaw up until the flaw size exceeds the beam width because the increase in flaw size increases the reflecting surface resulting in more energy reaching the receiver. Depending on flaw roughness, the dual side-by-side or the pulse echo technique may also see the flaw at a lower amplitude response. If a flaw response is also present with the dual side-by-side or the pulse-echo techniques, then video confirmation of the socket geometry is not necessary. 
         [0054]      FIGS. 19 and 20  present top views from the bolt head  11  and shows the effect of flaw  28  position relative the flats  13  for detection with the dual-opposite technique.  FIG. 19  illustrates an elliptically shaped flaw  28  that is approximately 25% deep and centered between two flats  13 . This is the least optimum position for this technique because only a small portion of the flaw  28  is contributing to the reflection of the beam  28  across the bolt  10 . Shallow flaws with this orientation may escape detection with this technique because the reflected energy may not be of sufficient strength to distinguish it from a non-flawed area of the bolt.  FIG. 20  illustrates the same flaw as  FIG. 19 , but it is on only one of the flats  13 . Shallow flaws with this alignment will likely be detected because the beam is centered on the flaw and provides the maximum reflecting surface. 
         [0055]    Deeper flaws are likely to span multiple flats  13  and be detected with multiple dual-opposite channels. In this regard, the number of channels that detect the flaw may be an indicator of the flaw depth; however, flaw size is not relevant to disposition the condition of the bolt. The method used to minimize false calls with this technique involves a second acquisition of the data but with the probe rotated at least 60°. If the indication is relevant and present on only one or two channels, it will move to the channels corresponding to the probe rotation. For example, if the indication was detected on channel BE during the first acquisition and then after rotating the probe 60° the indication appears on channel CF, the indication will be considered relevant and the bolt considered cracked. If the indication does not change channels corresponding to the probe rotation position it will be considered non-relevant. 
         [0056]      FIGS. 21-24  each show typical UT responses from flaws with no tilt angles (flat) and 25% flaw depths with the dual-opposite technique. Responses obtained from two non-flawed bolts are also shown for comparison. The images include the displays from each of the three channels used for this technique. As evidenced by the responses, flaws are easily detectable with good signal to noise ratio. 
         [0057]    While the preferred embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not of limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus the present invention should not be limited by the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. Furthermore, while certain advantages of the invention have been described herein, it is to be understood that not necessarily all such advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein.