ASM Metals HandBook Vol. 17 - Nondestructive Evaluation and Quality Control

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The equipment required consists of an impedance-plane, flying-dot, multipurpose eddy current instrument. In some cases,

the instrument may be too bulky for access to the inner wing surface. If this situation exists, one operator can manipulate

the probe using an extended lead connected to the remotely located instrument monitored by a second operator. The probe

operator should periodically scan the defect in the reference standard during the inspection to validate the instrument

operator's interpretation. The two operators must be in constant communication with each other to ensure defect detection

and to prevent inspection in areas where sealant thickness is beyond the range of the equipment calibration. Thick layers

of sealant do require trimming to acceptable dimensions, as shown in Fig. 51. The probe used is a reflectance-type model

that is operated at 20 kHz. It measures 25.4 mm (1.0 in.) high by 7.6 mm (0.30 in.) in diameter.

Fig. 51

Sealant trimming requirements for eddy current inspection. (a) Sealant buildup that hinders detection of

fatigue cracks in spar caps because probe capability is exceeded. (b) Sealant trimmed to dime

nsion thin enough

to allow probe to be positioned properly around fastener. Source: Ref 13

Example 13: Cracks Under Nonmagnetic Bushings Used in Clevis/Lug Attachments.

There are places in light piston-powered aircraft where bushings are installed by staking or interference fit, and it is

difficult and costly to remove them for crack inspection. Bushings are usually used at clevis/lug attachments for assembly

union with wing, horizontal stabilizer, and vertical stabilizer box sections with spar caps. A typical joint is illustrated in

Fig. 52(a). The bushings in fitting 1 are identified as 2, and the bushings in clevis 5 are identified as 4. The bolt, which is

removed, is identified as 3.

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.