Abstract:
A giant magnetoresistive flux focusing eddy current device effectively detects deep flaws in thick multilayer conductive materials. The probe uses an excitation coil to induce eddy currents in conducting material perpendicularly oriented to the coil&#39;s longitudinal axis. A giant magnetoresistive (GMR) sensor, surrounded by the excitation coil, is used to detect generated fields. Between the excitation coil and GMR sensor is a highly permeable flux focusing lens which magnetically separates the GMR sensor and excitation coil and produces high flux density at the outer edge of the GMR sensor. The use of feedback inside the flux focusing lens enables complete cancellation of the leakage fields at the GMR sensor location and biasing of the GMR sensor to a location of high magnetic field sensitivity. In an alternate embodiment, a permanent magnet is positioned adjacent to the GMR sensor to accomplish the biasing. Experimental results have demonstrated identification of flaws up to 1 cm deep in aluminum alloy structures. To detect deep flaws about circular fasteners or inhomogeneities in thick multilayer conductive materials, the device is mounted in a hand-held rotating probe assembly that is connected to a computer for system control, data acquisition, processing and storage.

Description:
CLAIM OF BENEFIT OF PROVISIONAL APPLICATION 
   Pursuant to 35 U.S.C. §119, the benefit of priority from provisional application 60/253,352, with a filing date of Nov. 28, 2000, is claimed for this non-provisional application. 

   ORIGIN OF THE INVENTION 
   The invention described herein was jointly made by an employee of the United States Government and by a contractor during the performance of work under a NASA contract and is subject to provisions of Section 305 of the National Aeronautics and Space Act of 1958, as amended, Public Law 85-568 (72 Stat. 435; 42 USC 2457). 

   BACKGROUND OF THE INVENTION 
   Nondestructive evaluation (NDE) technologies have recently been challenged to inspect thick, layered, conducting materials for fatigue and corrosion damage. Structures that fall into this class, such as airframe wings, pose significant difficulties for conventional inspection techniques, and especially challenging is the detection of deeply buried flaws at airframe fasteners. Reflections of ultrasound at layer boundaries cause serious problems for the application of ultrasonic inspection methods. Conventional eddy current inspection techniques are also compromised due to the exponential decay of electromagnetic energy with depth into a conductor. 
   Eddy current techniques, using detection that is sensitive to the time rate of change of the magnetic field level, are currently the most widely used method for the detection of hidden damage in thin conductors. The sensitivity of the method, however, is severely limited as material thickness increases. Eddy currents decay exponentially with both depth into the material and the square root of the applied frequency. Sufficient field penetration requires the reduction of the excitation frequency. This reduction in frequency, however, limits the sensitivity of the inductive pickup sensor whose output is proportional to the frequency. U.S. Pat. No. 5,648,721, which is hereby incorporated by reference, teaches a flux focusing eddy current probe having separate excitation and pick-up coils, magnetically isolated from one another by a highly permeable flux focusing lens. 
   The use of Giant Magnetoresistive (GMR) sensors for electromagnetic nondestructive evaluation has grown considerably in the last few years. Technological advances in the research and development of giant magnetoresistive materials has led to commercially available GMR sensors with many qualities well suited for electromagnetic NDE. Low cost GMR magnetometers are now available which are highly sensitive to the magnitude of the external magnetic field, have a small package size, consume little power, and operate at room temperature. Incorporation of these sensors into electromagnetic NDE probes has widened the application range of the field. In particular, the low frequency sensitivity of the devices provides a practical means to perform electromagnetic inspections on thick-layered conducting structures. 
   There are several other eddy current type methods which use detection sensitive to the magnetic field level as opposed to the time rate of change of the field. Magneto-Optic and Hall sensors are two common techniques. These devices, however, typically require substantial instrumentation and have sensitivities much lower than that of the GMR sensor. Super Conducting Quantum Interface Devices (SQUIDS) have sensitivities better than the GMR devices but the requirement of cryogenic temperatures imposes difficulties in application and typically greatly increases the sensor size. Flux Gate Magnetometers have sensitivities similar to GMR sensors and although they do not require a cryogenic temperature, they do require substantial instrumentation and typically relatively large probe size. 
   Ultrasonic techniques have been shown to be useful for detecting flaws in thick materials, but have limited success in multilayered structures due to reflection of the wave at the interfaces of the layers. Thermographic techniques have difficulties with metallic and thick materials, and typically require greater amounts of instrumentation for application and data analysis. X-ray methods pose environmental concerns and are not typically portable due to the large instrumentation requirements. 
   A variety of non-destructive evaluation techniques are currently used to inspect rivet joints, with eddy current testing being the most widely used technique. Several different types of eddy current probes have been developed for the specific purpose of rivet inspection. U.S. Pat. No. 5,648,721, herein incorporated by reference, teaches a probe having a flux focusing lens positioned between an excitation coil and a pick-up coil which are isolated from one another by a flux focusing lens, and a means for rotating the probe about the circular inhomogeneity. The device is very accurate for the detection of near surface fatigue cracks but the performance for deeper inspections is limited due to several features of the design, including flux leakage around the flux focusing lens and decreased detection sensitivity with lowered frequency. Other techniques include a Sliding Probe and a method using pencil probes and templates to trace around the fastener head. The sensititivy of the Sliding Probe is often low and a preferred orientation of the probe may lead to false calls or undetected flaws for rivets which are not aligned in a row. Template methods using pencil probes are very time consuming, and lift off or probe wobble can produce false signals. 
   OBJECTS 
   Therefore, it is an object of the present invention to effectively detect flaws in thick conductive materials. 
   It is a further object of the present invention to effectively detect flaws in multilayer conductive materials. 
   It is another object of the present invention to provide high sensitivity to low frequency magnetic fields. 
   It is another object of the present invention to enable the detection of deeply buried flaws. 
   It is yet another object of the present invention to provide an eddy current probe that is compact in size, operates at room temperature and utilizes simple electronics to provide ease of use and low cost. 
   It is a further object of the present invention to detect deep flaws about circular fasteners and other circular inhomogeneities in electrically conductive material. 
   Additional objects and advantages are apparent from the drawings and specification that follow. 
   SUMMARY OF THE INVENTION 
   According to the present invention, the foregoing and other objects and advantages are attained by providing a giant magnetoresistive eddy current device that effectively detects deep flaws in thick multilayer conductive materials. The probe uses an excitation coil to induce eddy currents in conducting material perpendicularly oriented to the coil&#39;s longitudinal axis. A giant magnetoresistive (GMR) sensor, surrounded by an excitation coil, is used to detect generated fields. Between the excitation coil and the GMR sensor is a highly permeable flux focusing lens which magnetically separates the GMR sensor and excitation coil and produces high flux density at the outer edge of the GMR sensor. The use of feedback inside the flux focusing lens enables complete cancellation of the leakage fields at the GMR sensor location and biasing of the GMR sensor to a location of high magnetic field sensitivity. In an alternate embodiment, a permanent magnet is positioned adjacent to the GMR sensor to accomplish the biasing. Experimental results have demonstrated identification of flaws up to 1 cm deep in aluminum alloy structures. 
   The present invention is further directed to a magnetoresistive flux focusing eddy current device that effectively detects deep flaws about circular fasteners or inhomogeneities in thick multilayer conductive materials. A giant magnetoresistive flux focusing eddy current device, as described above, is mounted in a hand-held rotating probe assembly that is connected to a computer for system control, data acquisition, processing and storage. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
       FIG. 1A  shows a cross sectional view of a magnetoresistive flux focusing eddy current probe constructed in accordance with the principles of this invention; 
       FIG. 1B  shows an alternate embodiment of the probe having a permanent magnet for biasing; 
       FIG. 2  shows a schematic of a commercially available GMR sensor; 
       FIG. 3  shows calibration data for the GMR sensor depicted in  FIG. 2 ; 
       FIG. 4  shows an alternate embodiment of the magnetoresistive flux focusing eddy current probe for detecting faults close to an edge; 
       FIG. 5A  shows an alternate embodiment of the magnetoresistive flux focusing eddy current probe that provides additional resistance to lift-off; 
       FIG. 5B  shows a further alternate embodiment of the magnetoresistive flux focusing eddy current probe that provides additional resistance to lift-off; 
       FIG. 6A  shows excitation coil and GMR sensor output waveforms for the magnetoresistive flux focusing eddy current probe, without feedback to the GMR sensor, for a 185 Hz drive with 15 volts to the GMR bridge and 40 dB preamplification when the probe is in contact with a conductive sample; 
       FIG. 6B  shows the change in GMR sensor output as the feedback source is adjusted; 
       FIG. 7A  shows a two-layer aluminum alloy test sample having a 4.76 mm thick unflawed top layer and a 1 mm thick lower layer having fatigue cracks grown from either side of a drilled center hole; 
       FIG. 7B  shows the geometry and surface plot of the experimentally measured probe output amplitude for the  FIG. 7A  sample; 
       FIG. 8A  shows a 13-layer aluminum test sample having a flaw in the 10th layer, beneath 9 mm of unflawed aluminum; 
       FIG. 8B  shows the geometry and surface plot of the experimentally measured probe output amplitude for the  FIG. 8A  sample; 
       FIG. 9A  shows the sensor output amplitude for a 1 cm deep flaw for the sample of  FIGS. 8A and 8B ;  FIG. 9B  shows the subsequent phase rotated output amplitude of  FIG. 9A ; 
       FIG. 9C  shows the flattened image of  FIG. 9B ; 
       FIG. 9D  shows the low pass filtered data of  FIG. 9C ; 
       FIG. 10  shows a schematic diagram of the rotating probe embodiment; 
       FIG. 11  shows a multilayer experimental sample used for the rotating magnetoresistive flux focusing eddy current probe embodiment; 
       FIG. 12A  shows amplified output for a 2.5 mm third layer flaw; 
       FIG. 12B  shows filtered output corresponding to  FIG. 12A ; 
       FIG. 13A  shows amplified output for an unflawed rivet joint in a three layer sample; 
       FIG. 13B  shows filtered output corresponding to  FIG. 13A ; 
       FIG. 14  shows rotating probe outputs normalized to inspection results for an unflawed fastener; 
       FIG. 15  shows circuitry for the rotating probe;  FIG. 16  shows signal detection circuitry; 
       FIG. 17  shows the sliding filter technique for the rotating probe; 
       FIG. 18A  shows amplified output for a 3.8 mm, 4th layer flaw using the detection electronics of  FIG. 16 ; 
       FIG. 18B  shows output for a 3.8 mm, 4th layer flaw using spatial Fourier filtering; 
       FIG. 19A  shows the results of applying the sliding filter to a 1.5 mm, 4 th  layer flaw; 
       FIG. 19B  shows the results of applying the sliding filter to an unflawed rivet joint; and 
       FIG. 20  shows comparison of normalized outputs for 4th layer flaw detection at an airframe fastener. 
   

   DETAILED DESCRIPTION 
   With reference now to  FIG. 1A , wherein like numbers designate like components throughout all the several figures, the GMR sensor-based flux focusing eddy current probe of the present invention is particularly well adapted for non-destructive evaluation of electrically conductive thick materials having multiple layers and deep cracks. Although particularly useful for such detection, it may also be used effectively for closer-surface flaw detection. The probe generally designated at  100  includes an excitation (drive) coil  110 , a flux focusing lens  120 , a giant magnetoresistive (GMR) sensor  130 , and a feedback coil  140 . The probe  100  applies the eddy current principle to evaluate electrically conductive material  150  for faults. An alternating current supplied by a current source  160  electrically connected to the excitation coil  110  produces eddy currents within conductive material  150  placed in proximity with the probe  100 . Voltage  190  is supplied to the GMR sensor  130 . Magnetic fields created in the GMR sensor  130  generate an electrical potential amplified by a differential preamplifier  185  and registered by an A.C. voltmeter  180  electrically connected to the GMR sensor  130 . The flux focusing lens  120  magnetically separates the excitation coil  110  from the GMR sensor  130  and produces high flux density at the edge of the GMR sensor  130 . The use of active feedback inside the flux focusing lens  120  enables a complete cancellation of the sinusoidal stray fields at the GMR sensor  130  location. At low frequencies the magnetic flux leakage around the bottom of the flux focusing lens  120  results in a large background signal level. This background signal is removed by applying a feedback signal via feedback source  170  at the same frequency as the current source  160  but 180 degrees out of phase with the GMR sensor  130  output to the feedback coil  140 . The feedback coil  140  also shifts the operating point of the GMR sensor  130  out of the low field region where the sensitivity of the probe  100  is low. For this, a D.C. voltage  175  is applied to the feedback coil  140  to provide a sufficient static background field level to bias the GMR sensor  130  in the linear region. 
   In an alternate embodiment, a permanent magnet  195  is positioned on the substrate directly behind the GMR sensor  130 , as illustrated in FIG.  1 B. The permanent magnet  195  is used to provide a sufficient D.C. bias to the sensor to keep the device in the linear response region. The feedback coil  140  is then used solely for A.C. feedback, rather than both sensor biasing and A.C. feedback. The incorporation of the permanent magnet  195  greatly reduces field generation requirements of the feedback coil  140 , enabling construction of a smaller device with lower power requirements. A D.C. bias from approximately 0.5 Oersted to approximately 0.75 Oersted was used in the experimental examples discussed herein. 
   Electromagnetic inspection of thick conducting materials requires a low frequency excitation due to the skin depth relationship, which is given in CGS units by 
     B   z   =B   0  exp(− z /δ)  (1) 
                 δ   =     c   /       2   ⁢   πμωσ                 (   2   )               
where B z  is the magnetic field at depth z into the material under test, B o  is the magnetic field at the surface, c is the speed of light, μ is the permeability, ω is the angular frequency, σ is the conductivity, and δ is the skin depth. The skin depth of a highly conducting material will be quite small unless the frequency of operation is also lowered. In aluminum alloys typically used in airframe construction with μ=1 and σ=1.7×10 17  sec −1 , for example, a frequency of 130 Hz is required for the skin depth to reach 1 centimeter into the material under test. At such frequencies, pickup coil type sensors lose sensitivity due to Faraday&#39;s law of electromagnetic induction that states 
             ɛ   =         -     A   c       ⁢       ⅆ   B       ⅆ   t         ∝     ω   ⁢           ⁢   sin   ⁢           ⁢   ω   ⁢           ⁢   t               (   3   )               
The electromotive force, ε, induced around a circuit is proportional to area enclosed, A, times the time rate of change of the magnetic field through the circuit. As the excitation frequency is lowered, the induced voltage across the pickup coil is reduced, reducing the effectiveness of the inspection.
 
   Giant magnetoresistive sensors respond to the magnitude of the external field instead of the time rate of change of the field and therefore do not lose sensitivity at low frequencies. In the absence of an applied field, the resistivity of the GMR sensor is high due to scattering between oppositely polarized electrons in the antiferromagnetically coupled multi-layers of the device. An external field aligns the magnetic moments of the ferromagnetic layers, eliminating this scattering mechanism and thereby reducing the resistivity of the material. A schematic diagram of a commercially available GMR sensor  130  is displayed in FIG.  2 . The design uses four GMR elements  210  connected in a Wheatstone bridge configuration. Two elements with shielding  220  are shielded from the external field. In the absence of an external field, all four elements will have the same resistance and no voltage will be detected across the center of the bridge. When an external field is present, the resistivity of the two unshielded GMR elements will drop and a positive voltage will be detected across the bridge. The four GMR elements  210  along with the flux concentrators  230 , which increase the sensitivity of the sensor, are packaged in an eight pin integrated circuit chip. Alternatively, the GMR sensor  130  can be incorporated into the probe  100  in its unpackaged die form. This form may be preferred if smaller sizes are desired. The die form of the GMR sensor  130  can be used with both embodiments of the active feedback discussed earlier. 
     FIG. 3  shows the calibration data for GMR sensor  130  of FIG.  2 . This data was acquired with the GMR sensor  130  placed in the center of a Helmholtz pair driven by a precision current source. Fifteen volts were placed across the GMR bridge, and a 20 dB differential preamplifier was used to amplify the output voltage of the GMR sensor  130 . The output increases with the magnitude of the magnetic field and the sensitivity, the slope of the output voltage versus applied field, drops dramatically in the low field region between approximately ±0.5 Oe. GMR sensors are available in various ranges and sensitivites. Their selection is based on the desired size and voltage response per field change for particular flaw measurements. 
   With further reference now to  FIGS. 1A and 1B , the excitation coil  110 , the flux focusing lens  120 , and the GMR sensor  130  are concentrically arranged with the excitation coil  110  at the exterior of the probe  100 , the GMR sensor  130  at the innermost position of the probe  100 , and the flux focusing lens  120  between the excitation coil  110  and the GMR sensor  130 . The overall size of the probe  100  is primarily determined by the diameter of the flux focusing lens  120 , which is a function of fault depth and fault isolation accuracy, and the size of the GMR sensor  130 . The diameter of the lens  120  is minimized to reduce the overall size of the probe  100  and to provide accurate location information of identified faults, though it must be of sufficient size to support test frequencies of the applied current from the current source  160  and to maximize search area covered by the probe  100 . Likewise, the thickness of the lens  120  is minimized to ensure energy created by the magnetic field of the excitation coil  110  produces energy in the GMR sensor  130  when the probe  100  is in free space or a flaw in the conductive material  150  crosses the boundary established by the lens  120 , though the lens  120  must provide isolation from direct magnetic energy of the excitation coil  110  from producing an alternating current in the GMR sensor  130  when the probe  100  is in contact with unflawed conductive material  150 . Direct energy transfer is avoided when the lens  120  thickness is several times the skin depth of the magnetic flux within the lens  120 . Generally, a lens  120  wall thickness of at least three skin depths and a sensor  130  height less than the half height of the lens  120  is desired. 
   An alternating current  160  applied to the excitation coil  110  creates a magnetic field which in turn creates eddy currents within conductive material  150 . The depth of penetration of the magnetic field into the test material  150  is dependent upon the conductivity of the material  150  and the frequency of the applied current source  160 . Consequently, the frequency of the drive signal is predetermined by the type of inspection being performed, with inspection for deeper flaws requiring lower frequencies. 
   The magnetic field is also established in the flux focusing lens  120 . The lens  120  is formed of conducting material high in magnetic permeability which provides a low reluctance path to divert the magnetic field away from the GMR sensor  130 . The point of maximum penetration of the concentrated magnetic field within the lens  120  is at one half the height of the excitation coil  110 . In the preferred embodiment, the top of the GMR sensor  130  falls below this maximum penetration point. 
   In the absence of a conducting material test sample  150 , some leakage of the magnetic flux around the lens  120  results. The leakage flux produces a potential in the GMR sensor  130  which provides a signal to the A.C. voltmeter  180 . When the probe  100  is placed above a non-flawed electrically conductive surface, however, a complete electromagnetic separation of the GMR sensor  130  from the excitation coil  110  can be achieved. The flux is concentrated within the conductive sample  150  and generates eddy currents. The induced eddy currents work to stop any change in the magnetic state of the system so that the leakage field within the interior of the flux focusing lens  120  is canceled, resulting in a null signal to the A.C. voltmeter  180 . 
   In the presence of a conductive material fault that divides the area covered by the probe  100 , a change in eddy current flow results. The field produced by the eddy currents pass through the area of the GMR sensor  130  resulting in a potential being established across the GMR sensor  130 . This provides an unambiguous voltage signal as indicative of the presence of a conductive material fault. 
   With reference now to  FIG. 4 , an alternate probe configuration supports detecting faults close to an edge of the conductive material  150  under test or near other conductive material discontinuities. An exterior shield  410  made of conductive material high in magnetic permeability focuses magnetic flux around the outside edge of the probe to prevent eddy currents from reflecting off a nearby conductive material edge and into the area of the GMR sensor  130 . This allows the probe  100  to be used near conductive material edges, but reduces the probe&#39;s overall sensitivity performance. 
   With reference now to  FIGS. 5A and 5B , alternate probe configurations are possible which provide further protection from lift-off conditions incorrectly indicating the presence of a conductive material fault. Signals from a plurality of probes  100  combined within a casing  510 , illustrated in  FIG. 5A , or a plurality of GMR sensors  130 A and  130 B within a single excitation coil  110  and flux focusing lens  120 , illustrated in  FIG. 5B , are compared to one another. Equal, non-zero signal levels signify probe lift-off from the test material. GMR sensors  130  or  130 A and  130 B are electrically connected to differential amplifiers  185  and  520 . Lift-off conditions result in equal signal level outputs such that the difference is zero and no fault indication is provided by the A.C. voltmeter  180 . 
     FIGS. 6A and 6B  display the excitation coil  110  and GMR sensor  130  output waveforms at various stages of feedback to the GMR sensor  130 . The data were acquired at 185 Hz drive from current source  160  with 15 volts from D.C. voltage source  190  to the GMR bridge and 40 dB preamplification  185 . The waveforms in  FIG. 6A  were taken without any feedback. The output signal is rectified due to the insensitivity of the GMR sensor  130  to the direction of the applied field. A significant drop in the output voltage is observed when the probe is placed on the conducting sample  150 , although a relatively large output voltage is still observed. 
     FIG. 6B  displays the change in the output as the feedback is adjusted. With the probe  100  on a sample  150 , the application of a D.C. bias from D.C. voltage source  190  to the feedback coil  140  shifts the operating point of the sensor away from the zero crossing area and up the calibration curve to a location of higher sensitivity. This results in an increased output amplitude of the GMR sensor  130 . Also, the signal is now at the same frequency as the current source  160 , with the rectification of the signal eliminated. The final step is then to apply a sinusoidal signal from the feedback source  170  to the feedback coil  140  at the excitation coil  110  frequency but 180° out of phase with the GMR sensor  130  output. The resulting output is a D.C. shifted amplitude with only a very small A.C. component. Adjusting the feedback in this manner cancels the leakage magnetic fields in the center of the probe and biases the GMR sensor  130  in order to obtain maximum sensitivity to small changes in the magnetic field caused by deeply buried defects. 
   Once the feedback source  170  is adjusted as described above and illustrated in  FIG. 6B , the sample  150  under test can be scanned. Lock-in amplifier  185  referenced to the excitation coil  110  is used to record both the amplitude and phase of the probe output as a function of position on the sample  150  surface. 
     FIG. 7B  shows experimental results from the test of a two-layer aluminum alloy lay-up  700  shown in FIG.  7 A. Referencing  FIG. 7A , the top layer  710  was formed of an unflawed 4.76 mm thick aluminum alloy plate. The lower layer  720  was formed from a 1 mm thick aluminum alloy plate with fatigue cracks  740  grown from either side of a drilled center hole  730 . The sample was scanned from the unflawed side with the probe operating at 375 Hz. 
     FIG. 7B  displays the geometry and a surface plot of the experimentally measured probe  100  output amplitude for the sample. The edge of the bottom plate  720 , center hole  730 , and fatigue cracks  740  emanating from either side of the hole  730  are all clearly visible in the surface plot. In addition, a peak in the output amplitude at the location of the fatigue crack tips is evident in the data. 
   A second experimental sample  800 , shown in  FIG. 8A , was fabricated to examine the detection capabilities of the probe  100  for flaws at varying depths in thick layered conductors such as airframe wings. A set of thirteen, 1 mm thick, aluminum plates  810  with a net cross section of 15×15 cm 2  was obtained. An EDM notch  820  of 1.4×0.0127 cm 2  was placed in the center of one of the plates. The location of this flawed layer within the stack of unflawed plates could then be varied, and data acquired over the flawed area for each case. Results using the probe  100  without the feedback coil  140  showed that the flaw could be clearly detected in the 6 th  and 7 th  layer, but that the signal to noise ratio dropped dramatically for deeper layers. The plotted data  860  were acquired at an operating frequency of 185 Hz with the flaw  820  placed in the 10 th  layer  830  of the sample  800 , beneath 9 mm of unflawed aluminum. The flaw location  820  is clearly imaged in the experimental data. A double peak indication is detected since the notch length of 1.4 cm was less than the outside diameter of the GMR based probe (1.9 cm). A peak in the output is observed when the probe is centered over either tip of the flaw  820 . When the probe is centered over the midpoint of the notch, however, the majority of the induced current will flow unperturbed around the flaw tips. 
   The results shown in  FIG. 8B  for the 10 th  layer flaw were achieved by using additional data analysis and image processing that were not required for shallower flaws such as those depicted in FIG.  7 A. In  FIG. 7A  the amplitude of the probe output voltage was used to identify the flaw location. In  FIG. 8A , however, the amplitude data was combined with the phase information in order to obtain a phase rotated amplitude. The probe output voltage was calculated as
 
 V   out   =A  Cos(φ+β)  (4)
 
were V out  is the probe output voltage, A is the root mean square output amplitude,  100   is the phase of the output waveform with reference to the drive signal, and β is a constant phase shift applied uniformly to all data points. The phase shift is rotated so as to minimize output voltage changes due to lift-off effects and maximize the signal due to the deeply buried flaws. The method is very similar to the standard eddy current practice of rotating the impedance plane diagram so as to provide a horizontal signal for lift-off variations and then monitoring the vertical component of the change in impedance.
 
   After the amplitude and phase data were combined, a simple image processing routine was used to enhance the image quality for greater flaw detectability. The image was first flattened, in order to remove any linear drift in the data caused by the presence of a small angle between the scanning plane and the sample surface or electronics drift during the scan. A low pass two-dimensional Fourier filter was then applied across the sample set in order to remove any high frequency noise caused by the scanning system.  FIGS. 9A through 9D  detail the data analysis and image processing steps as applied to the data presented in FIG.  8 B.  FIG. 9A  shows the GMR sensor output amplitude,  FIG. 9B  shows the phase rotated amplitude,  FIG. 9C  shows the flattened image and  FIG. 9D  shows the low pass Fourier filtered amplitude. 
   Another embodiment of the present invention, shown schematically in  FIG. 10 , supports detecting deep flaws about any circular fastener such as rivets, or about circular inhomogeneities. Sensor  130  output is amplified  185  and read by peak to peak detector  1010 . The sensor  130  output is monitored as the probe is scanned about the circumference of the fastener. Scanning is performed through stepper motor control with a typical scan frequency of 3 Hz. Spatial Fourier filtering applied to the probe output is then used to highlight voltage variations caused by the presence of fatigue cracks as well as to provide feedback on the rotation center for probe alignment. The total system gain is approximately 52 dB. 
   Maintaining a constant distance between the probe center and rivet center enables the signal due to the current flow about the rivet to be held constant. Any further changes in the current distribution, such as that due to a fatigue crack at the rivet joint, can then be detected as an increase in the output voltage above that due to the flow about the rivet head. In addition, the high current density at the location along the rivet circumference where the radius vector is along the line joining the rivet and probe centers enhances the flaw detection capabilities of the probe over that of an isolated flaw. 
   The probe is positioned a fixed radial distance from the center of the rivet and peak to peak detector  1010  measures the amplified voltage as the circumference of the rivet is traversed. Any suitable means, shown generally as rotation assembly  1040 , may be used to rotate the probe at a constant distance from the center of the rivet or circular inhomogeneity. For example, as described in U.S. Pat. No. 5,648,721, the probe  100  may be placed on a rotator comprising a pivot leg positioned at the center of a fastener, with the pivot leg connected to an arm that supports the probe  100  as it circles the fastener and passes over a crack. 
   The flaw detection capabilities of the rotating embodiment were examined experimentally using a riveted multilayer aluminum sample. As pictured in  FIG. 11 , the sample  150  consisted of a set of 1 mm thick Aluminum  2024  plates  1110 . The top plate was drilled and countersunk for 4 mm 100° rivets  1120 . EDM notches  1130  of lengths 1.5, 2.5, and 3.8 mm were machined at the rivet holes in the flawed layer. The rivet pattern was drilled into three additional plates that could be added to the sample layup, increasing the depth of the flawed layer. Nonmagnetic fasteners were attached to the bottom of the threaded rivets in order to secure the sample layers. 
     FIGS. 12A and 12B  show the panel displays for inspection of a third layer 2.5 mm EDM notch.  FIG. 12A  displays a polar plot of the amplified probe output voltage during a scan about the fastener.  FIG. 12B  shows the spatial Fourier filtered signal in millivolts versus the angular position in radians. The flaw is clearly visible as a high amplitude oscillation at −1.5 radians.  FIGS. 13A and 13B  display the results for a scan of an unflawed rivet joint in the three-layer sample. Data are acquired as the probe rotates about the fastener. A rotation frequency of 2.5 Hz was used such that a complete scan was acquired in 0.4 seconds. The amplified output voltage is nearly constant at 1 volt in both  FIGS. 12A and 13A . A slight increase in the raw data is observed at the 9:00 position in  FIG. 12A , corresponding to the location of the third layer flaw. A background signal caused by the varying magnetic field around the stepper motor is also observed. The twelve pole pairs of the permanent magnet rotor within the motor produce a cyclic field variation with 12 cycles per revolution. The specifications of the peak to peak detector were selected to eliminate most of the effects of this variation. The drive signal  160 , although low frequency, is much higher than the 30 Hz signal due to the stepper motor magnets (12 cycles/revolution×2.5 revolutions/second). The peak to peak circuit then measures V p-p  of the drive signal. Only a small distortion due to the stepper motor field remains. This distortion is manifested as a twelve-sided polygon in the background of the polar plots. 
     FIGS. 12B and 13B  display the filtered results in units of millivolts versus radians. For these results, a band pass spatial Fourier filter around the predicted flaw frequency is applied to the data. The spatial Fourier filter is described in Wincheski, B., Fulton, J., Todhunter, R., Simpson, J., 1996,  Review of Progress in Quantitative NDE , Plenum Press, 2133, herein incorporated by reference. The filter removes the large D.C. background and minimizes the stepper motor effects described above as well as other anomalies such as rivet tilt or misfit. The resulting display clearly identifies the 2.5 mm third layer flaw and produces a nearly flat background for the unflawed sample. The peak amplitude of the flawed signal is 38.8 mV, as compared to 2.4 mV for the unflawed sample. This peak output voltage is recorded and compared to the value of a predetermined threshold for impartial flaw identification. 
   The flaw detection capabilities for various flaw depths are summarized in Table 1. The operating frequency was set at 1.5, 1.3 and 0.9 kHz for third, fourth, and fifth layer inspections, respectively. 
   A plot of the filtered output probe voltage versus flaw length for the three and four layer samples is displayed is FIG.  14 . The data is normalized to the peak amplitude for the scan of an unflawed rivet. The 1.5 mm and 2.5 mm flaws in the fifth layer were not detectable above the background. 
   
     
       
             
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
               Flaw Layer 
               Unflawed Fastener 
               1.5 mm Flaw 
               2.5 mm Flaw 
               3.8 mm Flaw 
             
             
                 
             
           
           
             
               Third 
               2.4 mV 
               23.3 mV 
               38.8 mV 
               69.2 mV 
             
             
               Fourth 
               2.2 mV 
                6.9 mV 
               13.3 mV 
               22.8 mV 
             
             
               Fifth 
               2.1 mV 
               — 
               — 
                7.9 mV 
             
             
                 
             
           
        
       
     
   
   In another embodiment of the rotating probe, a permanent magnet is positioned on the substrate directly behind the GMR sensor, as described earlier. 
     FIG. 15  illustrates system electronics for the embodiment of the rotating probe having a permanent magnet for D.C. biasing. Two synchronized direct digital synthesis chips give full computer  1030  control of the amplitude, frequency, and phase angle of the source signals (feedback source  170  and current source  160 ) to the excitation  110  and feedback  140  coils. This level of programming control allows for the proper nulling field to the GMR sensor  130 , and is an alternative to the manual method described earlier. In particular, the relative phase angle between the two sources is directly related to the inspection depth by the skin depth equation, given in CGS units by:
   B   z   =B   0  exp(− z/δ )exp( izδ−iωt )  (5)               δ   =     c   /       2   ⁢   πμωσ                 (   6   )               
where B z  is the magnetic field at depth z into the material under test, B 0  is the magnetic field at the surface, c is the speed of light, μ is the permeability, ω is the angular frequency, σ is the conductivity, and δ is the skin depth. Equation 5 shows that the phase of the magnetic field at depth z lags the source signal by angle δ. Optimization of the device for detection of defects at this depth requires careful control of the phase of the feedback source  170  so as to maximize the change in amplitude from the defect. The rotating probe assembly  1020  comprises the GMR sensor  130 , excitation coil  110  and feedback coil  140 , as well as the stepper motor, Hall sensor and push button. The stepper motor and driver, push button and Hall sensor are located on the probe head  1050 . The push button and key press detector are used for data display. The Hall sensor and Hall sensor detector are used to detect the probe&#39;s  100  position. A D.C. shift of the feedback coil signal would be added when the feedback coil  140  is used for both A.C. feedback and sensor biasing.
 
   The signal detector circuitry of  FIG. 15  is expanded in the detector embodiment illustrated in FIG.  16 . As discussed for an earlier embodiment, a major source of signal interference for the GMR-based device is stepper motor noise. An oscillation in the magnetic field is observed as the twelve pole pairs of the permanent magnet rotor rotate within the stepper motor. The high Q notch filter  1610  in  FIG. 16  is therefore centered at 12×(rotation frequency). A probe rotation frequency of 3 Hz was used, such that the notch filter  1610  was centered at 36 Hz. The amplitude modulation detector  1630 , requiring biasing  1620 , is then used to capture the change in amplitude of the signal due to the probe-to-sample interaction. A low pass filter  1640  is next applied to highlight the effects of fatigue cracks as well as probe alignment position. Finally, this signal is amplified  1650  and sent as output  1660  to an A-D converter to be read by the computer and processed. The  FIG. 16  detection circuitry replaces differential amplifier  185  and peak to peak detector  1010 . 
   A further embodiment, shown in  FIG. 17  incorporates a simple “sliding” filtering scheme for improved response of the system. In all cases, the angular position of a flaw correlates with a peak in the output voltage, while two minima are observed on either side lobe of this peak. The position of these side lobe minima relates directly to the spatial frequency of the flaw which is a direct function of the rotation radius and probe footprint. As illustrated schematically in  FIG. 17 , the processed data amplitude is windowed into two segments  1710 A and  1710 B with width equal to one half the period of the flaw frequency. The spacing between the two windows is set such that a full period separates the center position of the two arrays. The data in one of the arrays is then reversed and the two are multiplied point by point. The average value of the array product is then calculated and added to the probe response amplitude at the point centered directly between the two windows. The processing is applied equally to each point as the two windows slide in unison through the data. 
   The incorporation of the additional detection electronics in  FIG. 16  provides the control computer  1030  with a deep flaw signal response very similar to that of the near surface response of the probe described in U.S. Pat. No. 5,617,024. The standard filtering techniques developed for that pickup coil-based instrument are thus directly applicable to the present invention. Spatial Fourier filtering can be applied to the amplitude of the sensor output. The use of lower drive frequencies requires only a slight adjustment in the band pass parameters in order to account for the more diffusive lower frequency eddy current distribution. 
   Experimental results were obtained with a riveted aluminum alloy sample consisting of a stack of 1.0 mm thick aluminum  2024  plates fastened together with threaded rivets to enable inspection of flaws at variable depths. 
     FIGS. 18A and 18B  display the raw and processed (spatially Fourier filtered) response, output  1660 , of the system for a 3.8 mm long 4 th  layer flaw. An operating frequency of 1.3 kHz, rotation radius of 5.5 mm, and spatial bandpass filter between 2.5 and 7.5 cycles per revolution were used in acquiring the data. The polar plot,  FIG. 18A , displays the raw amplified probe output amplitude while  FIG. 18B  shows the processed data as a function of angular position in radians. The characteristic flaw signal in the processed data is clearly evident at an angular position of approximately −1.5 radians. 
     FIGS. 19A and 19B  show the results of applying the sliding filter to a 1.5 mm 4 th  layer flaw as well as an unflawed rivet joint. The filter is seen to clearly enhance the system output for the flawed fastener, FIG.  19 A. The peak output value increased from 8.4 mV to 10.9 mV. The filtered results show a slightly narrower response to the flaw, with peak position remaining at the angular location of the 4 th  layer EDM notch. Data away from the flaw center show very little effect from the sliding filter as positive and negative fluctuations across the windowed arrays average out. This is also clearly displayed in the data for the unflawed fastener, FIG.  19 B. The sliding filter has very little effect as it passes through the data. 
   The performance of the probe for deep flaw detection is summarized in FIG.  20 . This graph displays the system output voltages for 1.5, 2.5, and 3.8 mm long EDM notches in the 4 th  layer of the airframe lap-splice joint sample described earlier. The data are normalized to the output recorded for an unflawed fastener. 
   Although our invention has been illustrated and described with reference to the preferred embodiment thereof, we wish to have it understood that it is in no way limited to the details of such embodiment, but is capable of numerous modifications for many mechanisms, and is capable of numerous modifications within the scope of the appended claims.