Abstract:
An eddy current probe adapted for detecting cracks in material directly beneath a raised-head fastener is disclosed. The probe comprises an eddy current coil and a support for carrying the coil in an orientation suitable for introducing eddy currents into material directly beneath a raised-head fastener.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Non-provisional Application of U.S. Provisional Application No. 60/617,311 filed on 8 Oct. 2004, titled “Rivet Rotating Probe,” and is a U.S. National-stage application of International Application PCT/US2005/036070, filed on 7 Oct. 2005, titled “Rivet Rotating Probe”. 
    
    
     TECHNICAL FIELD 
     The present invention relates in general to the field of non-destructive evaluation. In particular, the present invention relates to eddy current probes. 
     DESCRIPTION OF THE PRIOR ART 
     The outer skin of many aircraft comprise overlapping metal sheets joined together by inserting fasteners through overlapping portions of the metal sheets. A common problem related to using overlapping metal sheets is the undesirable existence and/or formation of cracks within the metal sheets and/or the fasteners which hold the sheets together. The cracks may exist in the sheets and/or fasteners as initially produced or the cracks may form after production. The cracks which form in the metal sheets and/or fasteners after initial production are commonly categorized as fatigue cracks caused by highly repetitious cyclic deformation of the metal sheets and/or fasteners. Since any crack in the metal sheets and/or fasteners may lead to catastrophic failure of the aircraft skin and subsequently a crash of the aircraft, detection of any crack is of the utmost importance. 
     One method well known and often used for detecting cracks in the metal sheets of an aircraft outer skin is to move a sliding eddy current probe along the surface of the metal sheet. A sliding eddy current probe typically comprises an electrical coil oriented parallel to a sliding surface of the sliding probe where the sliding surface is the surface placed in contact with the metal sheet. As the sliding eddy current probe passes over a crack, the presence of a crack is typically indicated by the presence of a crack signature viewable on a display screen of a connected probe controller. Cracks of different sizes, geometries, and locations often present crack signatures of different shapes, intensities, and/or amplitudes on the display screen. Further, a particular crack may present a variety of crack signatures depending upon a number of probe operating variables including the frequency at which the probe controller excites the electrical coil, the amplitude of the electrical signal transmitted to the probe from the probe controller, the geometry of the movements made with the probe along the surface of the metal sheet, and the speed with which the probe is manipulated. The sliding eddy current probe may be well suited for detecting some cracks located both, directly below the probe and in the metal sheet actually in contact with the probe; however, there are many scenarios where sliding eddy current probes do not offer adequate crack detection. 
     In helicopter construction, it is common practice to overlap metal sheets and join them together by inserting the raised-head fasteners through overlapping portions of the metal sheets. The raised-head fasteners are typically rivets comprising aerodynamic heads with smooth finishes. It is also common for the raised-head fasteners to be located substantially close together when used for securing the metal sheets of helicopter outer skins. Also, while the sliding eddy current probe may suitably detect cracks oriented in a manner aligned lengthwise with the direction of the lap joint, the sliding eddy current probe performs poorly in detecting cracks oriented transverse to the direction of the lap joint due to eddy current edge effects. Finally, the sliding eddy current probe is not well suited for detecting cracks where the sizes, geometries, and locations of a plurality of cracks may unfortunately be such that the probe response to the plurality of cracks results in no recognizable crack signature being displayed due to crack signature cancellation effects. 
     Although sliding eddy current probes are able to detect some cracks, many shortcomings remain. 
     SUMMARY OF THE INVENTION 
     There is a need for crack detection apparatus capable of detecting cracks beneath the head of a raised-head fastener. 
     Therefore, it is an object of the present invention to provide an eddy current probe having features suitable for detecting cracks beneath the head of a raised-head fastener. 
     This object is achieved by providing an eddy current probe comprising an eddy current coil and a support means for orienting the coil such that the eddy currents created by the coil are directed into the area of material beneath the head of a raised-head fastener. 
     The present invention provides significant advantages, including providing an eddy current probe that can: (1) detect cracks in metal sheets having closely located raised-head fasteners or raised-head fasteners in a closely located staggered pattern, (2) detect cracks located in metal sheets directly below the head of a raised-head fastener, (3) detect cracks in the head of a raised-head fastener, and (4) detect cracks which would otherwise go undetected by typical sliding probes due to crack signature cancellation and/or edge effects. 
     Other objects and advantages will become apparent from the detailed description that follows. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, including its features and advantages, reference is now made to the detailed description of the invention taken in conjunction with the accompanying drawings in which like numerals identify like parts, and in which: 
         FIG. 1  is a top view of a probe according to the present invention; 
         FIG. 2  is a sectional side view of the probe of  FIG. 1 ; 
         FIG. 3  is an enlarged sectional side view the probe of  FIG. 1 ; 
         FIG. 4  is a top view of the preferred embodiment of a probe according to the present invention; 
         FIG. 5  is a bottom view of the probe of  FIG. 4 ; and 
         FIG. 6  is an enlarged sectional side view of the probe of  FIG. 4 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIGS. 1-3  in the drawings, an embodiment of the probe of the present invention is illustrated.  FIG. 1  is a top view.  FIG. 2  is a side view.  FIG. 3  is an enlarged side view. Probe  11  comprises an eddy current coil system  13  and a support means  15  for supporting coil system  13 .  FIGS. 1 and 2  display sectional views of probe  11  as taken at cutting line AA of  FIG. 1  and viewed in the direction indicated by the arrows attached to cutting line AA. For clarity and readability, only support means  15  is cross-hatched in  FIG. 2 . Probe  11  is situated atop a joint  17  joining an upper sheet  19  and a lower sheet  21 . Sheets  19 , 21  are constructed of aluminum, but may alternatively be constructed of any other metal composition. Upper sheet  19  comprises an upper sheet top surface  23  and an upper sheet bottom surface  25 . Lower sheet  21  comprises a lower sheet top surface  27  and a lower sheet bottom surface  29 . 
     Eddy current coil system  13  comprises a disc-shaped coil  31  having a coil centerline  33  most clearly shown in  FIG. 3 . Upper sheet  19  and lower sheet  21  are joined together with raised-head fasteners  35  such that upper sheet bottom surface  25  is in substantial contact with lower sheet top surface  27 . Raised-head fasteners  35  are constructed of aluminum, but may alternatively be constructed of any other metal composition. Raised-head fasteners  35  each have a fastener centerline  37 . Fastener centerlines  37  are substantially orthogonal to upper sheet top surface  23 . It will be appreciated that in some cases sheets  19 , 21  may be bent to form a contoured outer skin. Where sheets  19 , 21  are bent (not shown), fastener centerline  37  may not be substantially orthogonal to the entire upper sheet top surface  23  but rather substantially orthogonal to a smaller area of upper sheet top surface  23 . 
     Support means  15  is a tubular member  39  having a substantially annular cross-section and having an outer wall  41 , an inner wall  43 , a top face  45 , and a bottom face  47 . The diameter of inner wall  43  is selected to substantially match the diameter of the head of fasteners  35  while providing proper tolerances for allowing rotation of probe  11  about fastener centerline  37  and/or along the perimeter of the head of fastener  35  while bottom face  47  of member  39  remains in contact with upper sheet top surface  23 . Member  39  accepts fastener  35 ′ within the inner void space of member  39  while bottom face  47  of member  39  remains in contact with upper sheet top surface  23 . 
     Coil system  13  further comprises an electrical conductor  49  for connecting coil  31  to a modular electrical connector  51 . Conductor  49  is substantially disposed between inner wall  43  and outer wall  41 . Connector  51  is a “microdot” connector commonly used for connecting to a standard eddy current inspection unit (not shown). Connector  33  is located on top face  45 . While typical eddy current probes are designed such that the centerline of a typical disc-shaped coil is oriented substantially orthogonal to the top surface of a sheet being inspected, coil  31  is oriented such that coil centerline  33  rests at an angle  53  of about 23 degrees from fastener centerline  37 ′ as most clearly shown in  FIG. 3 . It will be appreciated that in other embodiments of the present invention, angle  53  may be a value different from 23 degrees. Coil  31  is substantially disposed between inner wall  43  and outer wall  41 . Further, coil  31  is located such that while bottom face  47  of member  39  remains in contact with upper sheet top surface  23 , eddy currents caused by coil  31  penetrate not only upper sheet  19  but also through the edge of the head of the fastener. 
     It will be appreciated that while coil  31  is described as an absolute single turn type coil, coil  31  could be replaced in alternative embodiments of the present invention by (1) multiple winding coils such as differential coil units, (2) multiple coil configurations such as reflection coil units, or (3) any other suitable eddy current coil configuration suitable for introducing eddy currents beneath the head of the fastener. It will also be understood that additional and/or different conductors  49  and connectors  51  may be incorporated to allow operation of the above mentioned different coil configurations. 
     In use, an operator would connect probe  11  to a standard eddy current inspection unit with proper electrical conductors between connector  51  and the inspection unit. Next, to search for a crack under a chosen raised-head fastener, the operator would place probe  11  over the chosen raised-head fastener  35  and lower member  39  down in a manner such that the head of the chosen raised-head fastener  35  is substantially within the interior void of probe  11  as defined by inner wall  43  and such that bottom face  47  of member  39  substantially abuts upper sheet top surface  23 . After calibrating the inspection unit, the user will rotate probe  11  about centerline  37  of the a chosen raised-head fastener  35  while ensuring that bottom face  47  of member  39  remains substantially abutted to upper sheet top surface  23 . If the probe detects a crack, a crack signature will appear on a display screen of the inspection unit. While the above described use of probe  11  is described as a manually operated procedure, it will be appreciated that rotation of probe  11  and detection of cracks may be automated through the use of motors, computers, and/or other automation devices. For example, probe  11  may be robotically controlled to inspect for cracks at one or more fasteners  35  while crack identification is performed by a computer adapted for interpreting the signals received by the inspection unit. Of course any such automation may log or otherwise record crack detection results for later retrieval. 
     As explained above, probe  11  is well suited for crack detection in single wall and multi-layered aerospace structural members secured using raised-head fasteners. Probe  11  is adapted for detecting cracks under the fastener  35  head where the cracks have a thickness of up to about 0.080 inches and where crack thickness is defined as the distance from the edge of a hole in a metal sheet and extending radially outward from the centerline of the hole. Probe  11  is adapted for being driven by an oscillatory signal having a frequency of about 1 kHz to about 100 kHz. Of course, like other eddy current probes, probe  11  is driven at higher frequencies to more accurately detect cracks near upper sheet top surface  23 . Similarly, probe  11  is driven at lower frequencies to more accurately detect cracks near lower sheet bottom surface  29 . 
     Referring now to  FIGS. 4-6  in the drawings, the preferred embodiment of a probe according to the present invention is illustrated.  FIG. 4  is a top view.  FIG. 3  is a bottom view.  FIG. 3  is an enlarged side view of a cross-section taken at cutting line BB of  FIG. 4  and viewed in the direction of the arrows connected to cutting line BB. Probe  111  comprises an eddy current coil system  113  and a support means  115  for supporting coil system  113 . Eddy current coil system  113  is substantially similar in form, function, location, and construction to eddy current coil system  13 . Support means  115  is substantially a solid cylinder having eddy current coil system  113  embedded within the cylinder structure. It will be appreciated that in other embodiments of the present invention, the support means may be shaped differently. Support means  115  comprises an outer wall  117 , a top face  119 , a bottom face  121 , and a substantially concave receptacle  123  for receiving the head of a raised-head fastener  35  (see  FIGS. 1-3 ). Coil system  113  comprises an eddy current coil  125  substantially similar to coil  31 , a conductor  127  substantially similar to conductor  49 , and a connector  129  substantially similar to connector  51  located on top face  119 . Receptacle  123  is especially well suited for receiving the smooth heads of raised-head fasteners  35  and in particular, rivets having uniform and smooth heads. Receptacle  123  may be sized and shaped to accommodate reception of a myriad of rivet heads. Support means  115  comprises a support means centerline  131 . Coil  125  is oriented such that a coil centerline  133  rests at an angle  135  of about 23 degrees from support means centerline  131 . 
     Use of probe  111  is substantially similar to use of probe  11 . During use of probe  111 , an operator should ensure that bottom face  121  is substantially abutted to upper sheet top surface  23 . Opportunity for inadvertent removal of bottom face  121  from upper sheet top surface  23  during use is reduced for use of probe  111  as compared to probe  11  since probe  111  provides more surface area contact between support means  115  and the head of a raised-head fastener  35  than the amount of surface area contact provided between support means  15  and the head of a raised-head fastener  35 . Consequently, probe  111  may wobble less and provide more accurate crack detection than probe  11  if probe  11  is wobbled during use. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description.