Abstract:
A self-aligning probe assembly has one or more ultrasonic transducers adapted to transmit an ultrasonic shear wave into the inside surface of the hollow structure such as an aircraft fuse pin and to receive the shear wave emerging from the inside surface of the hollow structure. The nature of the received shear wave indicates the presence of flaws or damage in the inspected part. The probe assembly can be used to inspect fuse pins on aircraft without having to remove the aircraft engines.

Description:
TECHNICAL FIELD  
       [0001]    This disclosure relates to inspection of parts for defects or damage. More particularly, this disclosure relates to ultrasonic inspection of hollow structures such as aircraft fuse pins. 
       BACKGROUND  
       [0002]    During hard landings, the hollow fuse pins that hold the engines onto some aircraft such as the KC-135 can be damaged and need to be inspected. The typical damage is an outer diameter (OD) crack or offset (step), or a gradual bending. Currently, the pin must be removed in order to inspect for these types of damage, which requires significant labor and time. A prime example of where this is a significant problem is the KC-135 aircraft. These aircraft have four fuse pins on every strut: over-wing, at the diagonal brace, and mid-spar, all of which need to be inspected. While an optical in-bore method has been conceived to measure bending of the pins and offsets down to less than 0.005″, no method currently exists to find the OD cracks with the pins still mounted. 
       SUMMARY  
       [0003]    A device and method in accordance with the invention allows for on-aircraft ultrasonic inspection of engine hollow fuse pins for OD circumferential cracks and offsets caused by hard landings. This is a rapid, low cost method to inspect fuse pins without removing engines which will save significant maintenance and inspection costs. The device and method of the invention also can be used to inspect other kinds of hollow structures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0004]      FIGS. 1   a  and  1   b  illustrate the principles used by the invention to ultrasonically inspect for cracks and other damage in aircraft fuse pins. 
           [0005]      FIGS. 2   a  and  2   b  illustrate lateral adjustment of the ultrasonic transmitter and ultrasonic receiver in the probe assembly of  FIG. 1 . 
           [0006]      FIGS. 3 and 3   a  illustrate an embodiment of the invention involving a hand turned ultrasonic probe. 
           [0007]      FIG. 4  illustrates an embodiment of the invention involving another hand turned ultrasonic probe. 
           [0008]      FIG. 5  illustrates an embodiment of the invention involving motorized turning of an ultrasonic probe. 
       
    
    
     DETAILED DESCRIPTION  
       [0009]    Fuse pin circumferential crack inspection involves an ultrasonic shear wave probe assembly shown generally at reference numeral  10  in  FIGS. 1   a  and  1   b . Cracks and other damage in a metal structure  12  may often be found using shear wave ultrasonic-based inspection in accordance with this invention. With this technique, an ultrasonic stress wave is produced by a transducer  14  containing a piezoelectric material that receives an electric pulse from an ultrasonic test system not shown in  FIGS. 1   a  and  1   b . The transducer  14  can be mounted so as to insert the stress wave  15  into adjacent structure  12 . If it is mounted at an angle, a particular type of stress wave called a “shear wave” or “S-wave” is produced that has a shearing type of action as it travels through a material of the structure  12 . The shear wave will reflect and/or transmit at impedance boundaries according to the laws of wave physics. As shown in  FIG. 1   a , when the structure  12  is flawless, the magnitude and phase of the shear wave can be sensed using an appropriately positioned second ultrasonic transducer  16  that picks up the signal and converts it back into an electric signal, for measurement and analysis in the ultrasonic test system. When the structure  12  contains a crack or other defect  17 , as in  FIG. 1   b , the shear wave  15  is partially or completely reflected or scattered by the flaw and thus either is not received by the sensor  16  or is received at reduced amplitude, an event recognized by the ultrasonic test system as area of damage or other flaw. 
         [0010]    The displacement between the transducers  14  and  16  may be horizontally adjusted, as shown in  FIGS. 2   a  and  2   b . The displacement between the transducers  14  and  16  should be such that the transducer  16  receives a sufficient amplitude signal from transducer  14  when a good part is being inpected. The thickness of the part under inspection and the transmission and reflection angles will determine the appropriate distance between transducers  14  and  16 . 
         [0011]    As discussed above, the presence of a flaw will reduce the amplitude of the signal from transducer  14  received by transducer  16 . An adjustable fitting  24  can hold the probes  14  and  16  at the selected distance. The horizontal adjustment mechanism for the probe assembly of  FIGS. 2   a  and  2   b  comprises a pin  18  on probe  14  and a pin on probe  16  that ride in a slot  22  formed in the fitting  24  of the probe assembly  10 . 
         [0012]    The invention is not limited to use with any particular ultrasonic transducer. For example, the transducers  14  and  16  may be ultrasonic transducers made by Krautkramer. The transducers  14  and  16  may be part of an ultrasonic test system such as a USN 60 ultrasonic flaw dedector also made by Krautkramer. 
         [0013]    In one particular embodiment of the invention shown in  FIG. 3 , a self-aligning inspection probe assembly  26  that contains a sending ultrasonic transducer  14  and a receiving ultrasonic transducer  16 . The transducers  14  and  16  are mounted on shoes  28  and  30  pressed against a portion of the inner diameter of a cylindrical fuse pin  32  using a spring loaded standoff  34  which presses against a diametrically opposite portion of the inner diameter of the fuse pin  32  to align the probe assembly  26  inside the fuse pin  32 . The probe assembly  26  is designed to fit inside the hollow fuse pin  32 , and will self-align and self-center inside the fuse pin  32 , for proper orientation of the transducers  14  and  16 , as shown most clearly in  FIG. 3   a.    
         [0014]    The ultrasonic signal from the “sending” transducer  14  enters the pin wall  36  at an angle, bounces off the outer wall  38 , and returns to the “receiving” transducer  16 . A circumferential crack will provide a barrier to the shear wave traveling between transducers  14  and  16 , and will reduce the signal amplitude at the receiving transducer  16  and the corresponding electrical signal output by the transducer  16 . 
         [0015]    The probe end holding the transducers may be rotated  360  degrees around the inside of the pin  32 , and carefully indexed axially to cover all potentially damaged areas of the fuse pin  32 . This can be done by hand using the handle or crank  40  at the end of a threaded rod or screw  42 , extending through a rubbber end cap  44 , which allows for the rotation and advancement of the probe assembly completely around and through the fuse pin  32 . The inspector visually monitors the ultrasonic test equipment display (or sets an audible or visual alarm) for a pre-determined drop in the amplitude. The amount of ultrasonic amplitude drop is pre-determined using a calibration standard with known crack sizes. 
         [0016]    The transducer mount  47  supporting the transducers  14  and  16  in the assembly  26  provides the capability of adjusting the location of the transducers  14  and  16  axially and radially in the inside of the fuse in  32 . The transducer  14  in  FIG. 3  can be adjusted axially in the assembly  26  by means of a pin  48  fixed to the transducer  14  that slides in a slot  50  formed in the transducer mount  47 . The transducer  14  in  FIG. 3  can be also be adjusted radially in the assembly  26  by means of a pin  51  fixed to the transducer  14  that slides in a slot  53  in the mount  47 . The transducer  16  in  FIG. 3  can be adjusted axially by means of a pin  52  fixed to the transducer  16  that slides in the slot  50  and radially by means of a pin  54  fixed to the transducer  16  that slides in the slot  55 . The pins  48 ,  51 ,  52 , and  54  may be threaded members that each have a nut that is used to tighten its respective rod to the mount  47  to fix the location of the transducers  14  and  16  in the assembly  26  after adjustment. 
         [0017]    A liquid or gel couplant may be used to get the ultrasonic stress wave more effectively from the probe assembly  26  into and out of the structure being tested. There are several options. A gel specially designed for ultrasonic testing can be squirted into the center of the pin, and spread around so that the shoes always have couplant between them and the pin wall. This gel would need to be cleaned out after the test. Or, a grease can be used that can be left in the pin. The rubber end caps  44  and  46  with threaded sleeves in  FIG. 3  can be used to hold a bath of water or other liquid couplant that will wet the transducers each time they rotate through it. Or, when end caps are not desired, water can be dribbled out through a hole at the end of the shoe using low pressure water fed through a plastic tube. In this case, the inspector would need to have a means to control the water draining out of the pin, with rags or some sort of a makeshift dam at the sides of the pin. 
         [0018]    A second embodiment of the invention is shown in  FIG. 4 , which is a hand-held (non-screw) concept of ultrasonic bore inspection tool. The tool can be moved axially or circumferentially by hand. End caps can also be used to contain a couplant bath in the fuse pin  32  to keep the couplant from dripping out during inspection. The same inspection can be done with a hand-held system without the end caps or a hand crank. In this embodiment, the probe assembly  26  is held at one end of an unthreaded rod  56 . The other end of the rod  56  has a handle  58  held by the operator when the probe assembly  26  is inserted into the fuse pin  32  and slid axially from one end of the fuse pin  32  to the other end of the fuse pin  32 . In addition to being slid axially toward one end or the other of the fuse pin  32 , the probe assembly  26  is also rotated circumferentially around the inside of the fuse pin  32  at each axial location to search for defects on the entire inside of the fuse pin  32 . 
         [0019]    A third embodiment of the invention shown in  FIG. 5  which involves mounting the probe assembly  26  on a rod  60  that is rotated with a low speed drill-type motor  62 . This approach is the least operator-dependent, and the quickest, but requires battery or A/C power. 
         [0020]    It should be noted that an alternative configuration requiring only a single transducer used in a pulse-echo mode is possible. This configuration can be used with any of the three embodiments. Instead of setting up a test to look for loss of transmitted signal amplitude, the operator sets up the test to look for any return signal within a certain time frame. No signal will be returned in a “good” area, but a crack will produce a reflection that will be picked up by the single transducer. Dual transducer, pitch-catch configurations shown in  FIGS. 1-5  are preferred for this type of inspection, however, since they are generally more reliable than single transducer implementations. The reason for this is that if a single transducer is set up incorrectly, a lack of signal will not necessarily indicate a good part. 
         [0021]    Advantages of rotating shear wave probe in accordance with this invention include the fact that the probe assembly  26  can be attached to an off-the-shelf existing ultrasonic test system, such as the Krautkramer USN 60 ultrasonic test system mentioned above. A probe in accordance with this invention is a low cost, rapid solution to the problem of finding both internal and external cracks in hollow cylindrical structures such as aircraft fuse pins. As described above, the probe can be rotated by hand or attached to a crank or drill motor. The probe can be used to inspect different size hollow structures. For example, fuse pins with a range of inside diameters from about 0.6″ to 1.1″ can be inspected. The probe can also be configured to inspect other hollow structures, such as piping and conduits. 
         [0022]    The Title, Technical Field, Background, Summary, Brief Description of the Drawings, Detailed Description, and Abstract are meant to illustrate the preferred embodiments of the invention and are not in any way intended to limit the scope of the invention. The scope of the invention is solely defined and limited by the claims set forth below.