Nondestructive Evaluation and Quality Control (1998) Part 5 pps

Fig. 52 Eddy current inspection of cracks located under installed bushings. (a) Schematic of typical assembly employing interference-fit bushings in a clevis/lug attachment asse mbly. (b) Reference standard incorporating an electrical discharge machined corner notch. (c) Probe coil positioned in bolthole and encircled by bushing. (d) CRT display of a crack located under a ferromagnetic bushing. Source: Ref 13 A reference standard was made from material of the proper thickness, and the electrical discharge machined corner notch was made at the edge of the appropriate-size hole. The bushing was then installed in the reference standard, as shown in Fig. 52(b). The proper-size bolthole probe was selected and inserted into the bushed hole, and the operating frequency was selected to allow the eddy current to penetrate through the bushing in order to detect the notch (Fig. 52c and d). After calibration, the bolthole probe was inserted into the appropriate bushed hole in the lug or crevis on the aircraft. The probe was inserted at increments of about 1.59 mm (0.0625 in.) and rotated 360° through each hole to be inspected. The bushing, made of a copper alloy, had a thickness of about 1.5 mm (0.060 in.) and a conductivity between 25 and 30% IACS, which is easily penetrated at a frequency of 1 to 2 kHz. Example 14: Detection of Fatigue Cracks in Aircraft Splice Joints. Surface and subsurface fatigue cracks usually occur at areas of high stress concentration, such as splice joints between aircraft components or subassemblies. High-frequency (100 to 300 kHz) eddy current inspection was performed to detect surface cracks with shielded small-diameter probes. A reference standard was made from typical materials, and a small electrical discharge machining notch was placed at the corner of the external surface adjacent to a typical fastener. The high-frequency probe was scanned around the periphery of the fastener using a circle template for a guide, as illustrated in Fig. 53(a). Fig. 53 High-frequency eddy current inspection of surface and subsurface cracks in aircraft splice joints. (a) Calibration procedure involves introducing an electrical discharge machining notch in the reference standard to scan the fastener periphery using a circle template to guide the probe. (b) CRT trace on an oscilloscope of typical cracks in both skin and spar cap sections shown in (c). Source: Ref 13 When subsurface cracks are to be detected, low-frequency eddy current techniques are employed. Basically stated, the thicker the structure to be penetrated, the lower the eddy current operating frequency that is required. However, the detectable flaw size usually becomes larger as the frequency is lowered. Example 15: Hidden Subsurface Corrosion in Windowbelt Panels. There are various areas of the aircraft where subsurface (hidden) corrosion may occur. If such corrosion is detected, usually during heavy maintenance teardown, a nondestructive testing method can be developed to inspect these areas in the remainder of the fleet. Following is an example of subsurface corrosion detected by low-frequency (<10 kHz) eddy current check of the windowbelt panels. Such inspection is applicable at each window on both sides of the aircraft. Moisture intrudes past the window seal into the inboard side of the windowbelt panel and causes corrosion thinning of the inner surface (Fig. 54). The eddy current inspection is performed using a phase-sensitive instrument operating at 1 to 2.5 kHz and either a 6.4 or 9.5 mm (0.25 or 0.375 in.) surface probe. Fig. 54 Location of subsurface corrosion in aircraft windowbelt panels. Source: Ref 13 The edge of the inner surface of the windowbelt (where corrosion occurs) tapers from 4.06 to 2.0 mm (0.160 to 0.080 in.) over a distance of 19 mm (0.750 in.). A reference standard simulating various degrees of corrosion thinning (or, in reality, remaining material thickness) is used to calibrate the eddy current instrument (Fig. 55a). The instrument phase is rotated slightly so that probe lift-off response is in the horizontal direction of the CRT. As the probe is scanned across the steps in the standard, the eddy current response is in a vertical direction on the CRT. The amplitude of the response increases as the material thickness decreases (Fig. 55b). As each step in the standard is scanned, the eddy current response may be offset, as shown in Fig. 55(c). Fig. 55 Eddy current calibration procedure to detect subsurface corrosion in the aircraft windowbelt panels illustrated in Fig. 51 . (a) Reference standard used to simulate varying degrees of corrosion thinning from 0.5 to 2.0 mm (0.020 to 0.08 in.) in 0.5 mm (0.020 in.) increments. (b) Plot of CRT display at 2.25- kHz test frequency. (c) CRT offset display permits resolution of amplitudes at the various material thicknesses. Source: Ref 13 After calibrating the instrument, the inspector scans along the inner edge of the window and monitors the CRT for thinning responses, which are indicative of internal corrosion. When thinning responses are noted, the inspection marks the extent of the corrosion and determines the relative remaining thickness. Results are marked on a plastic overlay or sketch and submitted to the engineering department for disposition. The extent of severe corrosion and whether or not thinning has occurred are determined by removing the internal panels, window, and insulation to expose the corroded areas. The corrosion products are removed, and the thickness is measured using an ultrasonic thickness gage or depth dial indicator. Effect of Test Frequency on Detectable Flaw Size. Very small surface cracks, extending outward from fastener holes, are detectable using high-frequency, small-diameter eddy current probes. However, the detection of subsurface cracks requires a reduction in operating frequency that also necessitates an increase in the coil (probe) diameter resulting in a larger detectable crack. Because the depth of eddy current penetration is a function of operating frequency, material conductivity, and material magnetic permeability, increased penetration can only be accomplished by lowering the operating frequency. Therefore, the thicker the part to be penetrated, the lower the frequency to be used. Most of the subsurface crack detection is accomplished with advanced-technology phase-sensitive CRT instruments and reflection (driver/receiver) type eddy current probes. To demonstrate the capability of this technology to detect subsurface cracks in aluminum structures adjacent to fastener holes, Fig. 56 shows a plot of operating frequency versus detectable crack size. Fig. 56 Plot of operating frequency versus detectable crack length in aluminum structures using reflectance- type (transmit-receive) eddy current probes. Source: Ref 13 Figure 56 illustrates that the detectable flaw size increases as the frequency is reduced. The simulated subsurface flaws range in length from 4.8 to 12.7 mm (0.1875 to 0.50 in.), and the operating frequency band is from 100 Hz to 10 kHz. In addition, Fig. 57 shows a plot of detectable crack size versus thickness of the aluminum layer penetrated before the eddy currents intercepted the crack in the underlying layer. Fig. 57 Plot of detectable crack length versus thickness of overlying aluminum layer for reflectance- type eddy current probes. Source: Ref 13 The simulated subsurface cracks range in length from 4.8 to 12.7 mm (0.1875 to 0.50 in.). The thickness of the aluminum penetrated before the crack was reached ranged from 1.3 to 7.62 mm (0.050 to 0.300 in.). Although Fig. 57 shows only one overlying layer, the actual specimens contain from one to three layers on top of the layer containing the crack. From Fig. 57, it can be seen that the detectable crack size increases as the overlying layer increases in thickness. Reference cited in this section 13. D. Hagemaier, B. Bates, and A. Steinberg, "On- Aircraft Eddy Current Inspection," Paper 7680, McDonnell Douglas Corporation, March 1986 Note cited in this section 6 Example 8was prepared by J. Pellicer, Staveley Instruments. Eddy Current Inspection Revised by the ASM Committee on Eddy Current Inspection * References 1. M.L. Burrows, "A Theory of Eddy Current Flaw Detection," University Microfilms, Inc., 1964 2. C.V. Dodd, W.E. Deeds, and W.G. Spoeri, Optimizing Defect Detection in Eddy Current Testing, Mater. Eval., March 1971, p 59-63 3. C.V. Dodd and W.E. Deeds, Analytical Solutions to Eddy-Current Probe-Coil Problems, J. Appl. Phys., Vol 39 (No. 6), May 1968, p 2829-2838 Remote-Field Eddy Current Inspection J.L. Fisher, Southwest Research Institute Introduction REMOTE-FIELD EDDY CURRENT (RFEC) INSPECTION is a nondestructive examination technique suitable for the examination of conducting tubular goods using a probe from the inner surface. Because of the RFEC effect, the technique provides what is, in effect, a through-wall examination using only the interior probe. Although the technique is applicable to any conducting tubular material, it has been primarily applied to ferromagnetics because conventional eddy current testing techniques are not suitable for detecting opposite-wall defects in such material unless the material can be magnetically saturated. In this case, corrosion/erosion wall thinning and pitting as well as cracking are the flaws of interest. One advantage of RFEC inspection for either ferromagnetic or nonferromagnetic material inspection is that the probe can be made more flexible than saturation eddy current or magnetic probes, thus facilitating the examination of tubes with bends or diameter changes. Another advantage of RFEC inspection is that it is approximately equal (within a factor of 2) in sensitivity to axially and circumferentially oriented flaws in ferromagnetic material. The major disadvantage of RFEC inspection is that, when applied to nonferromagnetic material, it is not generally as sensitive or accurate as traditional eddy current testing techniques. Remote-Field Eddy Current Inspection J.L. Fisher, Southwest Research Institute Theory of the Remote-Field Eddy Current Effect In a tubular geometry, an axis-encircling exciter coil generates eddy currents in the circumferential direction (see the article "Eddy Current Inspection" in this Volume). The electromagnetic skin effect causes the density of eddy currents to decrease with distance into the wall of the conducting tube. However, at typical nondestructive examination frequencies (in which the skin depth is approximately equal to the wall thickness), substantial current density exists at the outer wall. The tubular geometry allows the induced eddy currents to rapidly cancel the magnetic field from the exciter coil inside the tube, but does not shield as efficiently the magnetic field from the eddy currents that are generated on the outer surface of 4. R. Halmshaw, Nondestructive Testing, Edward Arnold, 1987 5. R.L. Brown, The Eddy Current Slide Rule, in Proceedings of the 27th National Conference, American Society for Nondestructive Testing, Oct 1967 6. H.L. Libby, Introduction to Electromagnetic Nondestructive Test Methods, John Wiley & Sons, 1971 7. E.M. Franklin, Eddy-Current Inspection Frequency Selection, Mater. Eval., Vol 40, Sept 1982, p 1008 8. L.C. Wilcox, Jr., Prerequisites for Qualitative Eddy Current Testing, in Proceedings of the 26th National Conference, American Society for Nondestructive Testing, Nov 1966 9. F. Foerster, Principles of Eddy Current Testing, Met. Prog., Jan 1959, p 101 10. E.M. Franklin, Eddy-Current Examination of Breeder Reactor Fuel Elements, in Electromagnetic Testing, Vol 4, Nondestructive Testing Handbook, American Society for Nondestructive Testing, 1986, p 444 11. H.W. Ghent, "A Novel Eddy Current Surface Probe," AECL- 7518, Atomic Energy of Canada Limited, Oct 1981 12. "Nondestructive Testing: A Survey," NASA SP- 5113, National Aeronautics and Space Administration, 1973 13. D. Hagemaier, B. Bates, and A. Steinberg, "On- Aircraft Eddy Current Inspection," Paper 7680, McDonnell Douglas Corporation, March 1986 the tube. Therefore, two sources of magnetic flux are created in the tube interior; the primary source is from the coil itself, and the secondary source is from eddy currents generated in the pipe wall (Fig. 1). At locations in the interior near the exciter coil, the first source is dominant, but at larger distances, the wall current source dominates. A sensor placed in this second, or remote field, region is thus picking up flux from currents through the pipe wall. The magnitude and phase of the sensed voltage depend on the wall thickness, the magnetic permeability and electrical conductivity of tube material, and the possible presence of discontinuities in the pipe wall. Typical magnetic field lines are shown in Fig. 2. Fig. 1 Schematic showing location of remote-field zone in relation to exciter coil and direct coupling zone Fig. 2 Instantaneous field lines shown with a log spacing that allows field lines to be seen in all regions. This spacing also emphasizes the difference between the near-field region and the remote- field region in the pipe. The near- field region consists of the more closely spaced lines near the exciter coil in the pipe interior, and the remote-field region is the less dense region further away from the exciter. Probe Operation The RFEC probe consists of an exciter coil and one or more sensing elements. In most reported implementations, the exciter coil encircles the pipe axis. The sensing elements can be coils with axes parallel to the pipe axis, although sensing coils with axes normal to the pipe axis can also be used for the examination of localized defects. In its simplest configuration, a single axis-encircling sensing coil is used. Interest in this technique is increasing, probably because of a discovery by Schmidt (Ref 1). He found that the technique could be made much more sensitive to localized flaws by the use of multiple sector coils spaced around the inner circumference with axes parallel to the tube axis. This modern RFEC configuration is shown in Fig. 3. Fig. 3 RFEC configuration with exciter coil and multiple sector receiver coils The use of separate exciter and sensor elements means that the RFEC probe operates naturally in a driver-pickup mode instead of the impedance-measuring mode of traditional eddy current testing probes. Three conditions must be met to make the probe work: • The exciter and sensor must be spaced relatively far apart (approximately two or more tube diameters) along the tube axis • An extremely weak signal at the sensor must be amplified with minimum noise generation or coupling to other signals. Exciter and sensing coils may co nsist of several hundred turns of wire in order to maximize signal strength • The correct frequency must be used. The inspection frequency is generally such that the standard depth of penetration (skin depth) is the same order of magnitude as the wall thick ness (typically 1 to 3 wall thicknesses) When these conditions are met, changes in the phase of the sensor signal with respect to the exciter are directly proportional to the sum of the wall thicknesses at the exciter and sensor. Localized changes in wall thickness cause phase and amplitude changes that can be used to detect such defects as cracks, corrosion thinning, and pitting. Instrumentation Instrumentation includes a recording device, a signal generator, an amplifier (because the exciter signal is of much greater power than that typically used in eddy current testing), and a detector. The detector can be used to determine exciter/sensor phase lag or can generate an impedance-plane type of output such as that obtained with conventional driver-pickup eddy current testing instruments. Instrumentation developed specifically for use with RFEC probes is commercially available. Conventional eddy current instruments capable of operating in the driver-pickup mode and at low frequencies can also be used. In this latter case, an external amplifier is usually provided at the output of the eddy current instrument to increase the drive voltage. The amplifier can be an audio amplifier designed to drive loudspeakers if the exciter impedance is not too high. Most audio amplifiers are designed to drive a 4- to 8- load. Limitations Operating Frequency. The speed of inspection is limited by the low operating frequency. For example, the inspection of standard 50 mm (2 in.) carbon steel pipe with a wall thickness of 3.6 mm (0. 14 in.) requires frequencies as low as 40 Hz. If the phase of this signal is measured (and a phase measurement can be made once per cycle), then only 40 measurements per second are obtained. If a measurement is desired every 2.5 mm (0.1 in.) of probe travel, the maximum probe speed is 102 mm/s (4 in./s), or 6 m/min (20 ft/min). Although this speed may be satisfactory for many applications, the speed must decrease directly in proportion to the spatial resolution required and inversely (approximation is based on simple skin effect model and is generally valid when the skin depth is greater than the wall thickness) with the square of the wall thickness. This limitation is illustrated in Fig. 4 for a range of wall thicknesses. Fig. 4 Relationship between maximum probe speed and tube wall thickness for nominal assumptions of resolution and tube characteristics Effect of Material Permeability. Another limitation is that the magnitude and phase of the sensor signal are affected by changes in the permeability of the material being examined. This is probably the limiting factor in determining the absolute response to wall thickness and the sensitivity to localized damage in ferromagnetic material. This disadvantage can be overcome by applying a large magnetic field to saturate the material, but a bulkier probe that is not easily made flexible would be required. [...]... 10-14 1 05 3 × 1020 10-12 1.6 × 10-13 106 Infrared X-ray and -ray radiation 3 × 1021 Cosmic ray radiation 10-13 1.6 × 10-12 107 3 × 1022 10-14 1.6 × 10-11 108 Table 2 Microwave frequency bands Band designator Frequency range, GHz UHF 0.30-1 p 0.23-1 L 1-2 S 2-4 C 4-8 X 8-12 .5 Ku 12 .5- 18 K 18-26 .5 Ka 26 .5- 40 Q 33 -50 U 40-60 V 50 - 75 E 60-90 W 75- 110 F 90-140 D 110-170 G 140-220 Y 170-260 J 220-3 25 Source:... Holography" and "Acoustical Holography" in this Volume Fig 9 Microwave holography with locally produced nonradiated reference wave References cited in this section 4 R.P Dooley, X-Band Holography, Proc IEEE, Vol 53 (No 11), Nov 19 65, p 1733-17 35 5 W.E Kock, A Photographic Method for Displaying Sound Wave and Microwave Space Patterns, Bell Syst Tech J., Vol 30, July 1 951 , p 56 4 -58 7 6 E.N Leith and J Upatnieks,... directions and interfere with each other The result is the formation of a total field whose maximum and minimum points remain in a fixed or standing position Both component waves are still traveling, and only the resultant wave pattern is fixed A simple way to form standing waves is to transmit a coherent wave normal to a surface The incident and reflected waves interfere and cause a standing wave The standing... pressure tube and calandria tube In addition, at low test frequency, there is poor signal discrimination because of variations in lift-off, electrical resistivity, wall thickness, and gap In practice, 90° phase separation between lift-off and gap signals gives optimum signal-to-noise and signal discrimination For this application, the optimum test frequency was found to be between 3 and 4 kHz Typical... 2 J.L Fisher, S.T Cain, and R.E Beissner, Remote Field Eddy Current Model, in Proceedings of the 16th Symposium on Nondestructive Evaluation (San Antonio, TX), Nondestructive Testing Information Analysis Center, 1987 3 W Lord, Y.S Sun, and S.S Udpa, Physics of the Remote Field Eddy Current Effect, in Reviews of Progress in Quantitative NDE, Plenum Press, 1987 4 D.L Atherton and S Sullivan, The Remote-Field... Table 1 Major subintervals of the microwave frequency band are designated by various letters; these are listed in Table 2 The microwave frequency region is between 300 MHz and 3 25 GHz This frequency range corresponds to wavelengths in free space between 1000 cm and 1 mm (40 and 0.04 in.) Table 1 Divisions of radiation, frequencies, wavelengths, and photon energies of the electromagnetic spectrum Division... Radio waves (FM and TV) 3 × 108 1 1.6 × 10- 25 10-6 Microwaves 3 × 109 10-1 1.6 × 10-24 10 -5 3 × 1010 10-2 1.6 × 10-23 10-4 3 × 1011 10-3 1.6 × 10-22 10-3 3 × 1012 10-4 1.6 × 10-21 10-2 3 × 1013 10 -5 1.6 × 10-20 10-1 Visible light 3 × 1014 10-6 1.6 × 10-19 1 Ultraviolet light 3 × 10 15 10-7 1.6 × 10-18 10 3 × 1016 10-8 1.6 × 10-17 102 3 × 1017 10-9 1.6 × 10-16 103 3 × 1018 10-10 1.6 × 10- 15 104 3 × 1019... has been concerned with interpreting and modeling the remote-field effect without flaws in order to explain the basic phenomenon and to demonstrate flaw detection results The no-flaw case has been successfully modeled by several researchers, including Fisher et al (Ref 2), Lord (Ref 3), Atherton and Sullivan (Ref 4), and Palanissimy (Ref 5) , using both analytical and finite-element techniques This work... to outside-diameter and inside-diameter slots has been modeled with two-dimensional finite-element programs (Ref 3) References cited in this section 2 J.L Fisher, S.T Cain, and R.E Beissner, Remote Field Eddy Current Model, in Proceedings of the 16th Symposium on Nondestructive Evaluation (San Antonio, TX), Nondestructive Testing Information Analysis Center, 1987 3 W Lord, Y.S Sun, and S.S Udpa, Physics... both the pressure tube and the calandria tube (Fig 10a) The gap component of the signal is obtained by subtracting the pressure tube signal from the total signal Because the probe is near or in contact with the pressure tube and nominally 13 mm ( in.) away from the calandria tube, most of the signal comes from the pressure tube In addition, the calandria tube is much thinner and of higher electrical . 1 .59 mm (0.06 25 in.) and rotated 360° through each hole to be inspected. The bushing, made of a copper alloy, had a thickness of about 1 .5 mm (0.060 in.) and a conductivity between 25 and 30% IACS,. as shown in Fig. 55 (c). Fig. 55 Eddy current calibration procedure to detect subsurface corrosion in the aircraft windowbelt panels illustrated in Fig. 51 . (a) Reference standard used to. panel and causes corrosion thinning of the inner surface (Fig. 54 ). The eddy current inspection is performed using a phase-sensitive instrument operating at 1 to 2 .5 kHz and either a 6.4 or 9.5