Abstract:
Techniques for detecting defects in the proximity of a hole of a laminate structure include inserting a generally cylindrical body portion into a hole such that a first coil of wire will reside in a plane substantially parallel to a first electrically conductive layer of the laminate material. A magnetic field produced by the first coil of wire will produce eddy-currents in the conductive layer in the plane of the first conductive layer, but damaged laminate materials will fail to produce similar eddy-currents. As the differences in eddy-currents between damages and undamaged laminate layers can be measured, damage to such laminate materials can be determined.

Description:
FIELD OF THE INVENTION 
   This invention relates to methods and systems for nondestructive testing of materials. 
   BACKGROUND OF THE INVENTION 
   Originally, naturally available and relatively light materials, such as wood, were the most common materials used for constructing aircraft. However, with the development of new alloys the aircraft construction industry shifted from one of carpentry to one of metal shaping. 
   Relatively recently, a new generation of materials known as “composites” or “composite materials” were developed. Certain composite materials often provide an excellent strength-to-weight ratio as compared to metals, and their acceptance into the various aircraft industries is near universal. 
   Generally, there are two major genres of composite materials: honeycomb structures and laminates. Honeycomb structures are exceeding light materials that provide unequalled structural support (for their weight) when placed in wings and other strategic locations in a given aircraft. Laminate materials, while usually not as light as honeycomb structures, are often lighter than any commercially viable metal equivalent, and typically far stronger than any honeycomb structure. 
   As with all materials, laminates are subject to the normal “wear and tear” of everyday use. For example, over the course of everyday usage, cracks and other defects can develop around laminate-mechanical interfaces, such as bolt-holes. Such damage can not always be seen. While various diagnostic tools, such as ultrasonic imagers, are available to assess such hidden damage, these existing tools can be very expensive and require a substantial amount of training to properly use. Accordingly, new methods and systems for detecting damage in laminate structures are desirable. 
   SUMMARY OF THE INVENTION 
   In one aspect, an apparatus for detecting defects in the proximity of a hole of a laminate structure includes a generally cylindrical body portion having a diameter substantially close to the diameter of the hole and capable of being inserted into the hole, and a first coil of wire wrapped about the cylindrical body portion such that, when the body portion is inserted into the hole, the first coil of wire resides in a plane substantially parallel to a first electrically conductive layer of the laminate material such that a magnetic field produced by the first coil of wire will produce eddy-currents in the first conductive layer in the plane of the first conductive layer. 
   In another aspect, an apparatus for detecting defects in the proximity of a hole of a laminate structure includes a generally cylindrical body portion having a diameter substantially close in the diameter of the hole and capable of being inserted into the hole, and one or more first coils embedded within the body portion with each of the first coils having a portion residing at or near the surface of the body portion at different angular ranges, where each angular range is less than 360 
   In still another aspect, a method of diagnosing a crack in the proximity of a hole of a laminate structure includes positioning a first coil in the hole with the first coil residing in a plane substantially parallel to the plane of the electrically conductive layer, exciting the first coil in a manner as to produce appreciable eddy-currents flowing in the plane of the electrically conductive layer, and measuring the impedance of the first coil, wherein the impedance of the first coil changes as a function of eddy-current activity occurring close to the first coil. 
   In yet another aspect, an apparatus for detecting defects in the proximity of a hole of a laminate structure includes a first body and an inducing means for inducing eddy-currents coupled to the first body, and a measuring means for measuring an impedance of the inducing means. 
   There has thus been outlined, rather broadly, certain embodiments of the invention in order that the detailed description thereof herein may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional embodiments of the invention that will be described or referred to below and which will form the subject matter of the claims appended hereto. 
   In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of embodiments in addition to those described and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting. 
   As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  depicts a laminate material with a bolt-hole. 
       FIG. 2  depicts possible eddy-current paths in the laminate material of  FIG. 1 . 
       FIG. 3  depicts eddy-current paths in an undamaged laminate material. 
       FIG. 4  depicts eddy-current paths in a damaged laminate material. 
       FIG. 5A  depicts a first bolt-hole probe useful for detecting bolt-hole damage. 
       FIG. 5B  depicts the circuitry of the first bolt-hole probe. 
       FIG. 6A  depicts a second bolt-hole probe useful for detecting bolt-hole damage. 
       FIG. 6B  depicts the circuitry of the second bolt-hole probe. 
       FIG. 7A  depicts an exemplary circuit interface for the first bolt-hole probe. 
       FIG. 7B  depicts an exemplary circuit interface for the second bolt-hole probe. 
       FIG. 8  depicts an exemplary third bolt-hole probe. 
       FIG. 9  depicts an exemplary fourth bolt-hole probe. 
       FIG. 10  is a flowchart outlining an exemplary operation for detecting bolt-hole damage. 
   

   DETAILED DESCRIPTION 
   The disclosed methods and systems provide new diagnostic tools for relatively new materials.  FIG. 1  depicts one such material known as a laminate. The laminate structure  110  of  FIG. 1  includes four laminate layers  112 - 118 , and further includes a bolt-hole  120  in the center. While a bolt-hole is generally defined by its purpose as well as its structure, for the purpose of this disclosure, a “bolt-hole” can be any through-hole in a laminate structure whether or not the hole is ultimately used in conjunction with bolts. The laminate structure  110  of  FIG. 1  can be composed of any number of known or later developed material, such as carbon-fiber, titanium, titanium alloy etc, some of which can conduct electricity. 
   While many laminate layer materials are conductive, it should be appreciated that the great majority of bonding materials used to join laminate layers are typically ester-based resin and other poor conductors. Accordingly, it should be appreciated that the vast majority of laminate structures will only conduct electricity well in the individual thin planes of its laminate layers. Conductivity between layers, i.e., across the thickness of a laminate structure, will be poor or nonexistent. 
     FIG. 2  depicts various eddy-currents that might be induced into the laminate structure  110  of  FIG. 1 . As shown in  FIG. 2 , a first eddy-current  220  is capable of flowing in the plane of the top laminate layer. Assuming that the top-layer is conductive, an incumbent magnetic field of the proper orientation can easily generate such eddy-currents. 
   While a second eddy-current  210  is also depicted as flowing in a direction perpendicular to the plane of the top laminate layer, the path of the second eddy-current  210  is limited to the thickness of a layer, which may be but a few 100ths of an inch. As a result, even a strong and correctly oriented magnetic field is likely to induce but a minute amount of current in the path of eddy-current  210 . 
     FIG. 3  better depicts the eddy-current activity that can be generated using an appropriately configured magnetic field, i.e., a magnetic field having lines of flux extending in a direction perpendicular to the plane of structure  110 . As shown in  FIG. 3 , eddy-currents  310  travel in closed paths, which will be generally circular when current is unimpeded. In contrast,  FIG. 4  depicts the resultant eddy-current activity when a conductive plane is damaged of flawed. As is depicted in  FIG. 4 , crack  410  can impede any local, smaller eddy-currents and/or cause the paths of larger eddy currents to change dramatically. 
     FIG. 5A  depicts a first testing-device  510  capable of diagnosing flaws/damage in the vicinity of a bolt-hole of a laminate structure. Generally, the testing-device  510  operates by inducing appreciable eddy-currents in the plane of a laminate structure and detecting whether the expected amounts of eddy-currents are present. As shown in  FIG. 5A , testing-device  510  has a cylindrical main body portion  510  and a test-coil  522  wrapped around the body portion  520  and slightly inset such that the test-coil  522  is flush or slightly inset of the body portion  520 . The testing-device  510  further includes a head portion  530 . The diameter of the body portion D 2  can be slightly less than the diameter of a bolt-hole for which the testing-device  510  is designed to make insertion possible but still keep the coil  522  as close to the inner-wall of the bolt-hole as possible. As shown in  FIG. 5B , the coil  522  is primarily an inductor capable of generating a magnetic field. 
   In operation, coil  522  can be excited with any of a range of frequencies such that, when place in the plane of a conductive laminate layer, the coil  522  will induce eddy-currents about the plane of the laminate material. As discussed above with respect to  FIGS. 1-4 , the eddy-currents induced in an undamaged laminate layer will vary from those of a damaged layer. That is, as the inductance of a coil will generally vary when place in different environment, and the eddy-current paths of damaged and undamaged laminate layers will produce different environments from one another, it should be appreciated that the impedance across the coil  522  will be different when placed in the plane of an undamaged laminate layer as compared to when placed in the plane of a damaged laminate layer. 
   Accordingly, by comparing the impedance across the coil  522  with some form of reference(s), the testing device  510  can determine whether the coil is in free space, placed next to an undamaged laminate layer of a known material or placed next to a damaged laminate layer. Further, by employing the appropriate resolution and using a variety of references (or meter), it can be possible to determine whether any damage to a laminate layer is slight, appreciable or severe. Still further, by measuring the depth of the coil  522  as placed within a bolt-hole, the particular layer, as well as the damaged, can be determined. 
     FIG. 6A  depicts a second testing-device  610  capable of detecting flaws/damage in the vicinity of a bolt-hole of a laminate structure. As with the device of  FIG. 5A , the second testing-device  610  has a main body portion  510  and a test-coil  522  wrapped around the body portion  520 . However, unlike the device of  FIG. 5A , the second testing-device  610  further includes a second coil  622  that can act as a reference to the first coil  522 . That is, by employing a reference-coil that has the same resistive and reactive properties as a test-coil, and exciting both coils using the same frequency signals, the impedance of the reference-coil should track the impedance of the test-coil barring certain external factors. Such external factors include, for example, differences in temperature, external magnetic fields and proximity to conductive materials can change the relative impedance of two or more coils that might otherwise be identical. 
   In view of such external factors, it should be appreciated that the material used for the body  520  should be a good conductor of heat to keep temperatures relatively even. While metals are good conductors of heat and can provide shielding against wayward electromagnetic interference, tests have shown that better performance can be had by using a body made from materials that are highly conductive of heat, but poor conductors of electricity. One plastic that has proved useful is Delrin (any of a number of acetal polyoxymethylene (POM) resins) due to its conduction properties, its high strength and stiffness, dimensional stability and low friction/high wear. 
     FIG. 6B  depicts a useful circuit configuration for use in certain sensors known as a “bridge” or a “voltage divider”. The exemplary bridge  650  is constructed of coils  522  and  622  having respective inductances L 1  and L 2 . When L 1  and L 2  are known, Vout can be a determinable fraction of Vin. As the impedance of coil  522  is expected to deterministically change when placed in the vicinity of an undamaged conductive laminate structure, the ratio of Vout/Vin also will change deterministically. In contrast, when coil  522  is placed in the vicinity of a damaged conductive laminate structure, the impedance of coil  522  will not change as it would when placed near an undamaged laminate structure. That is, the ratio of Vout/Vin (damaged structure) will not likely equal Vout/Vin (undamaged structure). Given that the temperature and other external factors to coils  522  and  622  are expected to remain nearly identical, the bridge  650  of  FIG. 6B  presents itself as an extremely useful instrument for measuring induced eddy-current activity, and thus useful for determining damage for conductive laminate layers. 
     FIG. 7A  depicts a schematic of the test-coil  522  of  FIG. 5A  together with an appropriately designed sensor circuitry  710 . The exemplary sensor circuitry  710  can be designed to excite coil  522  with a voltage having a sinusoidal frequency f 1  and measure the impedance of the coil  522  under such conditions. In the present embodiment, the driving circuitry (not shown) has a characteristic impedance X 1 . As the impedance of an inductor will vary with frequency, it should be appreciated that the impedance of the coil  522  can be made to match the impedance of the driving circuitry if frequency f 1 , is chosen to be X 1 /2πL 1 , where L 1  is the inductance of coil  522 . By matching the impedances, the performance of the testing device as a whole can be expected to improve. 
     FIG. 7B  depicts a schematic of the test-coil of  FIG. 6A  together with a second appropriately designed sensor circuitry  720  designed to excite coils  522  and  622  with a voltage having a sinusoidal frequency f 2  while measuring the voltage of the bridge node between the two coils  522  and  622 . As with the circuit in  FIG. 7A , the sensor circuit&#39;s driving circuitry has a characteristic impedance X 2 , and it can be beneficial to match the impedance of the coils  522  and  622  with the impedance of the driving circuitry. Assuming the bridge node between the two coils  522  and  622  is high-impedance, the optimal frequency f 2  can be expected to be X 2 /2π(L 1 +L 2 ) where L 1  and L 2  are the respective inductances of coils  522  and  622 . Should the feedback node be a lower impedance, the optimal frequency f 2  may vary but it can still be a function of L 1  and L 2 . With respect to  FIGS. 7A and 7B , laboratory results show that good operating frequencies for f 1  and f 2  are found in the 100 KHz range±500%, and good values for the coils L 1  and L 2  tend to be in the 100 uH range±200% 
     FIG. 8  depicts a third embodiment of a testing device for testing bolt-holes in laminate structure. As with the device of  FIGS. 6A and 6B , the testing device  810  shown in  FIG. 8 , has a first coil  522  and a reference-coil  622  (not shown in  FIG. 8 ), but also has a metal shield  822  covering the second coil. The metal shield  822  causes the reference-coil  622  to better match the impedance of the first coil  522  when the first coil  522  is places against a metallic laminate layer. That is, by matching the metal of the shield  822  to that of an laminate layer, a reference-coil can be expected to provide an even better impedance match to a test-coil. 
     FIG. 9  depicts a cross-section of a fourth embodiment of a testing device for testing bolt-holes in laminate structure. As shown in  FIG. 9 , the testing device  910  has a coil  922  that does not circumvent the body  920  but only spans an angular range A. Additionally, although not shown in  FIG. 9 , the coil  922  may include one or more coils disposed above and/or below the coil  922  that span the angular range A. Accordingly, the coil  922  (with surface portion  924 ) can detect laminate damage only across range A, which effectively provides the testing device  910  with angular resolution about its cylindrical axis. By placing multiple coils with differing angular ranges, a testing device may simultaneously detect laminate flaws/damage to a particular range about its body. 
     FIG. 10  is a flowchart outlining an exemplary operation according to the present disclosure for diagnosing a bolt-hole. The process starts at step  1002  where a number of coils in a test-body are excited. As discussed above, the excitation frequency of the coils should can be between 50 KHz and 5 MHz and/or at a frequency to cause the reactive impedance of the coils to be close to that of the characteristic impedance of the circuitry driving the coils. Next, in step  1004 , the test-body is inserted into a bolt-hole. Then, in step  1006 , the test-body is manipulated such that a first coil of the test-body is positioned in a plane roughly equivalent to the plane of a laminate layer of interest. Control continues to step  1008 . 
   In step  1008 , the impedance of the first coil is measured. As discussed above, the impedance of the first coil can be measured against an expected value, a reference value or against the impedance produced by a reference-coil. Next, in step  1010 , a determination is made as to whether the laminate layer is flawed/damaged based on the impedance measurement of step  1008 . Then, in step  1012 , the results of the determination of step  1010  are displayed, and control continues to step  1050  where the process stops. 
   In various embodiments where the above-described systems and/or methods can in part be implemented using a programmable device, such as a computer-based system or programmable logic, it should be appreciated that the above-described systems and methods can be implemented using any of various known or later developed programming languages, such as “C”, “C++”, “FORTRAN”, Pascal”, “VHDL” and the like. 
   Accordingly, various storage media, such as magnetic computer disks, optical disks, electronic memories and the like, can be prepared that can contain information that can direct a device, such as a computer, to implement the above-described systems and/or methods. Once an appropriate device has access to the information and programs contained on the storage media, the storage media can provide the information and programs to the device, thus enabling the device to perform the above-described systems and/or methods. 
   For example, if a computer disk containing appropriate materials, such as a source file, an object file, an executable file or the like, were provided to a computer, the computer could receive the information, appropriately configure itself and perform the functions of the various systems and methods outlined in the diagrams and flowcharts above to implement the various functions. That is, the computer could receive various portions of information from the disk relating to different elements of the above-described systems and/or methods, implement the individual systems and/or methods and coordinate the functions of the individual systems and/or methods related to laminate testing. 
   The many features and advantages of the invention are apparent from the detailed specification, and thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.