Abstract:
A method of inspecting a preselected area of an electrically conductive component to determine whether flaws are present. The method includes the steps of permanently mounting an eddy current element on the component over the preselected area and energizing the element to generate alternating magnetic fields proximate the component. An electrical signal generated by a secondary magnetic field formed proximate the component is detected using the element and the detected electrical signal is compared to a reference signal to determine whether the detected signal is different than the reference signal. Differences indicate the presence of a flaw in the component. Inspection apparatus for performing this method is also disclosed.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to eddy current inspection, and more particularly to components having permanently affixed eddy current elements. 
     Eddy current inspection is commonly used to detect flaws in electrically conductive components such as aluminum aircraft fuel tanks. Electromagnetic induction is used in this type of inspection to induce eddy currents in the component being inspected. Generally, a probe having one or more coils is used to generate alternating magnetic fields which induce the eddy currents in the component. When flaws are present in the component, the flow of eddy currents is altered. The altered eddy currents produce changes in a secondary magnetic field which are detected by the probe. The probe generates an electrical signal in response to the altered secondary magnetic field. The amplitude and phase of the electrical signal is generally proportionate to the size of the flaw. 
     As previously mentioned, a probe having one or more coils was used in the past to perform the inspections. The probe was positioned adjacent to the surface being inspected. Using a probe to inspect interior surfaces of components such as bulkheads forming fuel tanks inside aircraft wings required disassembly of the structure to position the probe adjacent the surface. Depending upon the complexity of the structure, disassembly, inspection and reassembly can take several hours, days, weeks or longer. During this time, the structure is unavailable. Further, the cost of labor required to perform these tasks can be high. Accordingly, a need exists for a method and apparatus for performing eddy current inspection of interior surfaces of complex structures without disassembling the structures. 
     SUMMARY OF THE INVENTION 
     Among the several features of the present invention may be noted the provision of a method of inspecting a preselected area of an electrically conductive component to determine whether flaws are present therein. The method comprises the steps of permanently mounting an eddy current element on the component over the preselected area and energizing the element to generate alternating magnetic fields proximate the component thereby inducing eddy currents in the component. An electrical signal generated by a secondary magnetic field formed proximate the component by the eddy currents is detected by the element, and the detected electrical signal is compared to a reference signal to determine whether the detected signal is different than the reference signal. A difference indicates a flaw is present in the component. 
     In another aspect, a method of the present invention for installing inspection apparatus on a component comprises permanently mounting an eddy current element on the component and attaching a conduit to the component. A lead is attached to the eddy current element and threaded through the conduit for selectively connecting the eddy current element to remote eddy current inspection equipment. 
     In still another aspect, the present invention includes inspection apparatus for detecting flaws in a preselected area of an electrically conductive component. The apparatus includes a substrate sized and shaped for covering the preselected area of the component. The substrate includes an adhesive for attaching the substrate to the component over the preselected area. Further, the apparatus includes a primary eddy current element mounted on the substrate sized and shaped for covering at least a portion of the preselected area to detect flaws in the component. 
     Yet another aspect of the present invention includes an electrically conductive component having an area selected for inspection in combination with apparatus for detecting flaws in the selected area of the component. The apparatus comprises a substrate mounted on the component over the area selected for inspection and a primary eddy current element mounted on the substrate over at least a portion of the selected area for detecting flaws in the area. 
     Other features of the present invention will be in part apparent and in part pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary perspective of a component having inspection apparatus of the present invention; 
     FIG. 2 is a front elevation of inspection apparatus of the present invention; 
     FIG. 3 is a schematic showing the inspection apparatus and a response from eddy current equipment for a component having no flaws; 
     FIG. 4 is a schematic showing the apparatus and response for a component having a small flaw; and 
     FIG. 5 is a schematic showing the apparatus and response for a component having a larger flaw. 
     Corresponding reference characters indicate corresponding parts throughout the several views of the drawings. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings and in particular to FIG. 1, a electrically conductive component such as a portion of a bulkhead used to form an aircraft fuel tank is generally designated by the reference number  10 . The component  10  is conventional in all respects and will not be described in further detail. 
     As further illustrated in FIG. 1, inspection apparatus (generally designated by  12 ) is mounted on the component  10  for detecting flaws (e.g., a crack  14 ) in a preselected area  16  of the component. Although only a small portion of the component  10  is covered by the apparatus  12  in FIG. 1, those skilled in the art will appreciate that the apparatus may be positioned over each critical portion of the component or those portions which are particularly susceptible to failure. Further, the entire component  10  (or those portions which are inspectible by eddy current inspection) may be covered by the apparatus  12  without departing from the scope of the present invention. 
     As illustrated in FIG. 2, the apparatus  12  includes a substrate  20  sized and shaped for covering the preselected area  16  of the component  10 . Although the substrate may have other sizes and shapes without departing from the scope of the present invention, the substrate  20  of the preferred embodiment is rectangular, having a width of about 12.5 mm and a length of about 112.5 mm. Further, although the substrate may be made of other materials without departing from the scope of the present invention, the substrate  20  of the preferred embodiment is a sheet of Kapton tape having an adhesive backing for attaching the substrate to the component  10  over the preselected area  16 . Kapton is a U.S. federally registered trademark of E. I. du Pont de Nemours and Company of Wilmington, Del. Alternatively, a separate adhesive tape (not shown) may be used to attach the substrate  20  to the component  10 . 
     A primary eddy current element, generally designated by  22 , comprising several eddy current coils  24  is mounted on the substrate  20  (FIG.  2 ). Although other numbers and patterns of primary coils  24  may be used without departing from the scope of the present invention, the primary element  22  of the preferred embodiment has an array of coils formed by nine rows of coils containing three overlapping coils each. Although other coil sizes and shapes may be used without departing from the scope of the present invention, each of the coils of the preferred embodiment is rectangular, having a width of about 0.75 mm and a length of about 42.5 mm. Thus, the element  22  of the preferred embodiment is sized and shaped for covering at least a portion of the preselected area  16  to detect flaws in the component. Further, the coils  24  in each row of the preferred embodiment are overlapped by a distance of about 7.5 mm. Although the coils may be made of other materials and by other processes without departing from the scope of the present invention, the coils of the preferred embodiment are copper and are etched in the substrate by a conventional photolithographic process. 
     As further illustrated in FIG. 2, a reference eddy current element  26  comprising eddy current coils  28  is mounted on the substrate  20  below the lowermost row of primary eddy current coils  24 . As will be appreciated by those skilled in the art, since both the primary eddy current element  22  and the reference eddy current element  26  are spaced from the component  10  by the substrate  20 , these elements are spaced from the component by a substantially equal and constant distance (i.e., the thickness of the substrate). Although other numbers and patterns of reference coils may be used without departing from the scope of the present invention, the reference element  26  of the preferred embodiment has two separated coils  28  positioned over a reference area  30  (FIG. 1) of the component  10  located outside the area selected for inspection  16 . Preferably, the reference element  26  is positioned so it obtains a reference signal corresponding to a portion of the component  10  without flaws. Alternatively, it is envisioned that the primary coils  24  may be scanned for a coil producing a nominal signal and that coil can be used as a reference coil. Thus, under some circumstances the reference element may be located inside the selected area  16  rather than outside of it. 
     Instrumentation leads  32  are connected to each primary coil  24  and each reference coil  28  as shown in FIG.  2 . These leads  32  are bundled and fed through a protective tube or conduit  34  leading to an electrical connector  36  (FIG. 1) positioned for access by technicians to selectively connect the primary element  22  and reference element  26  to conventional eddy current equipment (generally designated by  40  in FIG.  3 ). Although the tube  34  may have other configurations without departing from the scope of the present invention in one preferred embodiment the tube is a cylindrical tube having an outer diameter of about 5 mm. Further, although other means of attaching the the  34  to the component may be used without departing from the scope of the present invention, in one embodiment the tube is attached to the component with a suitable conventional adhesive. Holes and/or grooves or other openings may be formed in low stress regions of the component  10  to accommodate the tube  34 . Further, the ends of the tube  34  may be sealed with a suitable conventional sealant to prevent contaminates from entering the tube and component  10 . Still further, it is envisioned that openings may be formed in the side of the tube  34  to provide access for the leads  32 . 
     As will be appreciated by those skilled in the art, the apparatus  12  described above may be used to inspect a preselected area  16  of an electrically conductive component  10  to determine whether flaws (e.g., a crack  14 ) are present. First an eddy current element  22  is permanently mounted on the component  10  over the preselected area  16 . When the preselected area is tested, conventional eddy current equipment  40  is connected to the element  22  using the connector  36 . The equipment  40  energizes the element  22  to generate alternating magnetic fields proximate the component  10  thereby inducing eddy currents in the component. As will be understood by those skilled in the art, the element  22  detects an electrical signal generated by a secondary magnetic field formed proximate the component  10  by the eddy currents. The detected electrical signal is compared to a reference signal to determine whether the detected signal is different than the reference signal. Such a difference indicates the presence of a flaw  14  in the component  10 . 
     As illustrated in FIG. 3, if no flaws are present the electrical signals received by the primary coils (e.g., coils  42 ,  44 ) are equal to the reference signals received by the reference coils  28 . Thus, when the impedance of coil  42  is compared to the impedance of the reference coils  28  on a corresponding display  46  of the eddy current equipment  40 , the difference is zero. Likewise, when the impedance of coil  44  is compared to the impedance of the reference coils  28  on a corresponding display  48  of the eddy current equipment  40 , the difference is zero. However, when a flaw such as a crack  14  grows to a length as shown in FIG. 4, the display  46  shows a difference in impedance between coil  42  and coils  28 . Since the length of the flaw does not extend under the coil  44 , the corresponding display  48  displays a null reading. As the crack grows longer as shown in FIG. 5, displays  46  and  48  both show a difference in impedance between the respective coils. Thus, the location and the length of any flaws may be detected using the apparatus  12  and method described above. 
     When introducing elements of the present invention or the preferred embodiment(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
     As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.