NASA CASE NO. LAR 15046-1

PRINT FIG. _!_1

NOTICE

The invention disclosed in this document resulted from research in aeronautical and

space activities performed under programs of the National Aeronautics and Space

Administration. The invention is owned by NASA and is, therefore, available for

licensing in accordance with the NASA Patent Licensing Regulation (14 Code of

Federal Regulations 1245.2).

To encourage commercial utilization of NASA-owned inventions, it is NASA policy

to grant licenses to commercial concerns.

nonexclusive licensing to promote competition

Although NASA encourages

and achieve the widest possible

utilization, NASA will provide the necessary incentive to the licensee to achieve

early practical application of the invention.

Address inquiries and all applications for license for this invention to NASA Patent

Counsel, Langley Research Center, Code 143, Hampton, Virginia 23681-0001.

Serial No.: 08/134,444

10/12/93 LaRC

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(NASA-Case-LAR-I5046-1)

FLUX-FOCUSING EDDY CURRENT PROBE

ANO METHO0 FOR FLAW DETECTION

Patent Application (NASA. Langley

Research Center) 24 p

G3/38

N94-29366

Uncl as

0003648
FLUX-FOCUSING EDDY CURRENT PROBE AND METHOD 

FOR FLAW DETECTION 

AWARDS ABSTRACT 

NASA Case No. LAR 15046-1 

A flux -focusing electromagnetic sensor which uses a 

ferromagnetic flux-focusing lens simplifies inspections and 

increases detectability of fatigue cracks and material loss in 

high conductivity material. The unique feature of the device is 

the ferrous shield isolating a high-turn pick-up coil from an 

excitation coil. The use of the magnetic shield is shown to 

produce a null voltage output across the receiving coil in the 

presence of an unflawed sample. A redistribution of the current 

flow in the sample caused by the presence of flaws, however, 

eliminates the shielding condition and a large output voltage is 

produced, yielding a clear unambiguous flaw signal. 

The maximum sensor output is obtained when positioned 

symmetrically above the crack. Hence, by obtaining the position 

of the maximum sensor output, it is possible to track the fault 

and locate the area surrounding its tip using an algorithm 

similar to the grid-search chi-square minimization scheme. The 

precise position of the crack tip can then be found using a 

simple numerical process. The accuracy of tip location is 

enhanced by two unique features of the sensor; a very high 

signal-to-noise ratio of the probe's output which results in an 

extremely smooth signal peak across the fault, and a rapidly 

decaying sensor output outside a small area surrounding the crack 

tip which enables the region for searching to be clearly defined. 

Under low frequency operation, material thinning due to corrosion 

damage causes an incomplete shielding of the pick-up coil. The 

low frequency output voltage of the probe is therefore a direct 

indicator of the thickness of the test sample. A swept frequency 

measurement performed on manufactured thickness calibration 

standards was able to clearly delineate all samples tested, 

showing a resolution of less that 0.5% of the sample thickness. 

In addition, simulated inter-layer corrosion in a lap-joint 

sample has been detected using a single frequency mode of 

operation. The results clearly identified all material loss from 

the back surface of the panel. 

Inventors: John W. Simpson C. Gerald Clendenin 

Address : 916 Yorktown Road 121 Sharps Road 

Tabb. VA 23693 WiIliamsbura. VA 23188 

Employer LESC NASA LaRC 
Names : James P. Fulton Buzz A. Wincheski 

Address : 142 Marcella Road 305 Cobble Stone 

3185 

Employer: AS&M AS&M 

Ronald G. Todhunter Min Namkung 

200 Lakewood Circle 112 Villa Way 

Grafton, VA 23692 Yorktown, VA 23693 

AS &M NASA LaRC 

Initial Evaluator: Joseph S. Heyman 

Serial No.: 08/134,444 

Filed: October 12, 1993 
LAR 15046-1 -1- PATENT APPLICATION

FLUX-FOCUSING EDDY CURRENT PROBE AND METHOD

FOR FLAW DETECTION

5

10

Oriqin of the Invention

The invention described herein was jointly made by employees of the

United States Government and contract employees during the performance

of work under NASA contracts and is subject to the provisions of Section

305 of the National Aeronautics and Space Act of 1958, as amended, Public

Law 85-568 (72 Stat. 435; 42 U.S.C. 2457) and 35 USC 202 in which the

contractor elected not to retain title.

Background of the Invention

15 The present invention relates generally to a device and method for

detecting material flaws using the eddy current principle, and more

particularly to 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, for detecting different types of flaws or

20 discontinuities at various depths within an electrically conductive material,

and an automated method of monitoring crack growth rate and trajectory

during fatigue testing. The automated method is not limitative and the

invention can also be used for manual testing and fault isolation. The eddy

current probe of this invention is simple in construction and operation while

25 providing unambiguous signals of fault indication simplifying support

equipment requirements, supporting self nulling capability in no fault

conditions during surface fault detection negating the need for calibration

standards, maintaining highly resistent lift-off characteristics, and providing

highest signal levels at fault tips.

30 The use of eddy currents to inspect electrically conductive material for
i

LAR 15046-1 -2- PATENT APPLICATION

flaws is known in the art. Such methods are particularly useful in the nondestructive evaluation of conducting material for surface or internal faults and

material thinning. An eddy current probe generally consists of a coil

electrically connected to a current generator producing an alternating current

5 within the coil. This generates a time-varying primary magnetic field which

in turn induces current flow (eddy currents) in an electrically conductive

material positioned in the vicinity of the coil• As described by Lenz's Law, the

eddy currents, during their return path, create an opposing secondary

magnetic field superimposed on the coil's primary magnetic field decreasing

10 the coil's flux and modifying the coil's voltage effecting the impedance of the

coil.

The eddy current's power and direction are dependent upon the

specific impedance of the conductive material. In unflawed conditions, eddy

current flow is generally parallel to the magnetizing coil's windings. Flaws,

15 such as cracks, pits, and material thinning, in the conductive material create

regions of higher resistances at the flaw location which affect eddy current

flow. The direction change in eddy current flow reduces the opposing

secondary magnetic field and consequently the voltage in the coil. An

impedance detecting circuit, which may take the form of an inductive bridge,

20 monitors coil voltage which, if different from an established no-fault

condition, indicates a conductive material flaw. Sharp changes in impedance

over a localized area would indicate the existence of cracks or other relatively

small area flaws, whereas gradual changes in impedance over a broad region

of the conductive material might indicate large-area flaws such as a grain

25 change in the metal, an area of material creep, or material thinning. Though

traditional eddy current probes use the same coil for magnetizing the

conductive material and for detecting impedance variations caused by

changes to eddy current flow, use of separate magnetizing and pick-up coils

is known in the prior art. The same principle, however, of monitoring for

30 variations in coil impedance as indicative of a conductive material flaw is
LAR 15046-1 -3- PATENT APPLICATION

applied. This conventional eddy current flaw detection technique often

involves complex impedance planes necessitating special test electronics to

achieve null balancing and known standards to calibrate probe responses to

each type of flaw.

5 Multiple coils within a single probe can be used as separate

magnetizing and sensing means, or they may be used in a more traditional

fashion and independently operate the coils as bi-function magnetizingsensing coils. In the case of a multiple coil probe, the coils can be juxtaposed

in a matrix to provide a large detection area, or concentrically arranged to

10 simultaneously detect flaws at various depths. Each coil of multiple coil

probes is energized and monitored for impedance variations independently.

Multiple coil probes use a shielding material high in magnetic permeability to

provide a low reluctance path and divert potentially interfering magnetic fields

to separate the coils from one another. The shielding is designed to isolate

15 all interfering signals so that each coil's impedance can be independently

balanced. This design provides an ability to perform material testing at

several frequencies simultaneously. The conventional eddy current flaw

detection techniques are employed which require special test equipment to

analyze probe signals.

20 As the conventional eddy current probe separates from the test

material, the eddy currents induced within the material rapidly decrease

resulting in a similarly decreasing opposing magnetic field which directly

affects the resistance and inductance of the probe's coil. Abrupt changes

occur to the impedance balance being monitored for fault indications making

25 traditional eddy current probes unusable during these lift-off conditions.

In addition to detecting existing flaws in conductive material,

determination of fatigue crack growth criteria in structural materials is

important to predict material fatigue failure limits. Current approaches to

testing fatigue cracking include optical methods and other length measuring

30 techniques such as crack mouth opening displacement gauge and four point
5

10

15

LAR 15046-1 -4- PATENT APPLICATION

potential drop method. Each of these procedures requires long periods of

continuous monitoring by well trained operators to record fatigue crack tip

locations and to adjust experimental controls in compliance with experimental

designs. Though the crack mouth opening displacement gauge and four point

potential drop method could potentially be automated to reduce operator time

requirements, only overall crack length data can be provided and the fatiguing

process must usually be stopped to make the crack length measurements.

Eddy current devices have also been used to monitor fatigue crack

growth during fatigue. This approach to monitoring fatigue crack growth has

been automated, though, again, only overall crack length data can be

provided. The traditional eddy current probe which implements impedance

measurement techniques to identify the presence of test material faults is not

conducive to tracking the crack tip which supports crack trajectory as well as

crack growth rate.

Ob!ects

Therefore, it is an object of the present invention to overcome or

mitigate these problems and to provide a flux-focusing eddy current probe

20 whose fault detection characteristics provide unambiguous fault detection

signals enhancing the confidence level of fault detection and reducing the

need for special test electronics. Through the use of the present invention,

a flux-focusing characteristic of probe signals during surface fault detection

eliminates the need for known standards of specific types of flaws. Further,

25 a single probe design and instrument configuration performs surface and

subsurface eddy current fault detection.

It is a further object of the invention to establish a method which

supports automated monitoring of faults in conducting material. The use of

this method will perform continuous tracking and archiving of fatigue crack

30 growth rates and trajectory during the fatigue process as well as
LAR 15046-1 -5- PATENT APPLICATION

automatically controlling conducting material loading conditions based on

measured fatigue data and experimental designs.

5

Summary of the Invention

The present invention is directed to a flux-focusing eddy current probe

that satisfies the need for unambiguous signals of fault detection, reducing

the need for special test equipment. The flux-focusing eddy current probe

performs non-destructive evaluation of electrically conductive material. The

10 probe uses an excitation coil to induce eddy currents in conducting material

perpendicularly oriented to the coil's longitudinal axis. A separate pick-up

coil, surrounded by the excitation coil, is used to detect generated fields.

Between the excitation coil and pick-up coil is a flux focusing lens which

magnetically separates the two coils and produces high flux density at the

15 outer edge of the pick-up coil.

The present invention is further directed to an automated eddy current

system that satisfies the need to perform continuous fatigue testing. This

system performs fatigue evaluation of conducting material and is dependent

upon the use and characteristics of the flux-focusing eddy current probe. A

20 computer interprets the signal from the probe to control the probe's position

over the conducting material and to control the loading of the conducting

material under test. A method for performing the automated testing requires

a load to be applied to the conducting material to produce a fault. The fluxfocusing eddy current probe scans the conducting material and signals the

25 computer once a fault is detected. The computer then adjusts the loading on

the material and position of the probe to continue the test to completion.

Brief Description of the Drawings

30 These and other features, aspects, and advantages of the present
LAR 15046-1 -6- PATENT APPLICATION

invention will become better understood with regard to the following

description, appended claims, and accompanying drawings where:

FIG. 1 shows a cross sectional view across line 1-1 of FIG. 2 for an

elevation view of a flux-focusing eddy current probe constructed in

5 accordance with the principles of this invention;

FIG. 2 shows a plan view of the flux-focusing eddy current probe

illustrated in FIG. 1 with the receiving coil shown in phantom;

FIG. 3A shows a cross sectional view across line 3-3 of FIGS. 1 and

2 of the magnetic flux lines when the flux-focusing eddy current probe is

10 energized in free space;

FIG. 3B shows a cross sectional view across line 3-3 of FIGS. 1 and 2

of the magnetic flux lines when the flux-focusing eddy current probe is

energized and in contact with an unflawed electrically conductive material;

FIG. 4A shows an elevation view for the magnetic field of the

15 excitation coil which generates eddy currents in conductive material as

focused by the ferromagnetic lens;

FIG. 4B shows a plan view of the eddy current flow in unflawed

electrically conductive material;

FIG. 4C shows a plan view of the eddy current flow in flawed

20 electrically conductive material where the flaw divides the probe's

circumference;

FIG. 4D shows a plan view of the eddy current flow in flawed

electrically conductive material where the flaw tip lies within the probe's

circumference;

25 FIG. 5A shows a flux-focusing eddy current probe's signal level

response for a 50 KHz, 10.0 V p-p excitation coil signal when the probe is in

contact with an unflawed aluminum alloy plate;

FIG. 5B shows a flux-focusing eddy current probe's signal level

response for a 50 KHz, 10.0 V p-p excitation coil signal when the probe is in

30 free space;
LAR 15046-1 -7- PATENT APPLICATION

FIG. 5C shows a flux-focusing eddy current probe's signal level

response for a 50 KHz, 10.0 V p-p excitation coil signal when the probe is in

contact with a flawed aluminum alloy plate;

FIG. 6A shows a flux-focusing eddy current probe's signal level

5 response for a 2 KHz, 10.0 V p-p excitation coil signal when the probe is in

contact with a 2 mm unflawed aluminum alloy plate;

FIG. 6B shows a flux-focusing eddy current probe's signal level

response for a 2 KHz, 10.0 V p-p excitation coil signal when the probe is in

contact with a 1 mm unflawed aluminum alloy plate;

10 FIG. 6C shows a flux-focusing eddy current probe's signal level

response for a 2 KHz, 10.0 V p-p excitation coil signal when the probe is in

free space;

FIG. 6D shows a flux-focusing eddy current probe's signal level

response to thickness variations in aluminum as a function of frequency;

15 FIG. 7 shows another version of the flux-focusing eddy current probe

embodying features of the present invention which provides additional

resistance to edge effects and other conductive material discontinuities.

FIG. 8A shows another version of the flux-focusing eddy current probe

embodying features of the present invention which provides additional

20 resistance to lift-off response indicating conductive material fault;

FIG. 8B shows another version of the flux-focusing eddy current probe

embodying features of the present invention which provides additional

resistance to lift-off response indicating conductive material fault; and

FIG. 9 shows the eddy current probe system for locating crack tips.

25

Detailed Description

3O

With reference now to FIG. 1, wherein like numbers designate like

components throughout all the several figures, the flux-focusing eddy current

probe of the invention is particularly well adapted for non-destructive
LAR 15046-1 -8- PATENT APPLICATION

evaluation, and for fatigue testing of electrically conductive material. The

flux-focusing eddy current probe generally designated at 10 includes an

excitation coil, a flux focusing lens, and a pick-up coil. The probe 10 applies

the eddy current principle to evaluate electrically conductive material 16 for

5 faults. An alternating current supplied by a current source 15 electrically

connected to the excitation coil 11 produces eddy currents within conductive

material 16 placed in proximity with the probe 10. Magnetic fields created

in the pick-up coil 13 are registered by an A.C. voltmeter 14 electrically

connected to the pick-up coil 13. The flux focusing lens 12 magnetically

10 separates the excitation coil 11 from the pick-up coil 13 and produces high

flux density at the edge of the pick-up coil 13.

With reference now to figure 2, the excitation coil 11, the flux focusing

lens 12, and the pick-up coil 13 are circular and concentrically arranged with

the excitation coil 11 at the exterior of the probe 10, the pick-up coil 13 at

15 the innermost position of the probe 10, and the flux focusing lens 12

between the coils 11 and 13. The overall size of the probe 10 is primarily

determined by the diameter of the flux focusing lens 12 which is a function

of fault depth and fault isolation accuracy. The diameter of the lens 12 is

minimized to reduce the overall size of the probe 10 and to provide accurate

20 location information of identified faults, though it must be of sufficient size

to support test frequencies of the applied current from the current source 15

and to maximize search area covered by the probe. Likewise, the thickness

of the flux focusing lens is minimized to ensure energy created by the

magnetic field of the excitation coil 11 produces energy in the pick-up coil 13

25 when the probe 10 is in free space or a flaw in the conductive material 16

crosses the boundary established by the lens 12, though the lens 12 must

provide isolation from direct magnetic energy of the excitation coil 11 from

producing an alternating current in the pick-up coil 13 when the probe 10 is

in contact with unflawed conductive material 16. Direct energy transfer is

30 avoided when the len's thickness is several times the skin depth of the
LAR 15046-1 -9- PATENT APPLICATION

magnetic flux within the lens. The size of the pick-up coil 13 is simply

maximized to achieve the largest possible surface area for greatest signal

sensitivity within the constraints dictated by the excitation coil 11 and the

flux focusing lens 12. It must be noted, however, that the smallest

5 detectable flaw is approximately equal in size to one-half the inner diameter

of the flux-focusing lens 12.

With reference now to FIGS. 3, 4 and 5, an alternating current applied

to the excitation coil 11 creates a magnetic field 41 which in turn creates

eddy currents within conductive material 16, FIG. 4A. The depth of

10 penetration of the magnetic field 41 into the test material 16 is dependent

upon the conductivity of the material 16 and the frequency of the applied

current source. Consequently, the frequency of the drive signal is predetermined by the type of inspection being performed. Inspection for surface

breaking flaws requires high frequency so as to concentrate the eddy currents

15 at the surface of the material, whereas inspection for interior or back surface

flaws requires low frequency such that the skin depth of the induced currents

penetrate to the desired depth.

The magnetic field 41 is also established in the flux focusing lens 12.

The lens 12 is formed of conducting material high in magnetic permeability

20 which provides a low reluctance path to divert the magnetic field away from

the pick-up coil 13. The point of maximum penetration of the concentrated

magnetic field 41 within the lens 12 is at one half the height of the excitation

coil 11, FIG. 4A. In the preferred embodiment, the top of the pick-up coil 13

falls below this maximum penetration point.

25 In the absence of a conducting material test sample 16, some leakage

of the magnetic flux 31 around the lens 1 2 results, FIG. 3A. The leakage flux

32 produces a current in the pick-up coil 13 which provides a signal, FIG. 5B,

to the A.C. voltmeter. When the probe is placed above a non-flawed

electrically conductive surface, however, a complete electromagnetic

30 separation of the pick-up coil 13 from the excitation coil 11 can be achieved,
LAR 15046-1 -10- PATENT APPLICATION

FIG. 3B. The flux 33 is concentrated within the conductive material 16 and

generates eddy currents 42, FIG. 4B. The induced eddy currents 42 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 12 is canceled, resulting in a null

5 signal, FIG. 5A, to the A.C. voltmeter.

In the presence of a conductive material fault 43 which divides the area

covered by the probe 10, a change in eddy current flow results, FIG. 4C. The

field produced by the eddy currents 44 and 45 pass through the area of the

pick-up coil resulting in an alternating current being established in the pick-up

10 coil 13. The presence of a current in the pick-up coil 13 provides an

unambiguous voltage signal as indicative of the presence of a conductive

material fault. Where the tip of a fault 46 falls within the area covered by the

pick-up coil 13, FIG. 4D, the field produced by eddy current flow 47 is

additive to the field of the excitation coil 11 and is greater than that produced

15 by in air leakage flux 32 resulting in a higher voltage signal level, FIG. 5C,

thus providing a maximum signal level.

With reference now to FIG. 6, operation of the flux-focusing eddy

current probe at reduced frequencies increases the penetration of the

magnetic field and supports evaluation of conductive material thickness

20 variations. As material thins, eddy currents in the conducting material are

directed to within the area covered by the pick-up coil producing a voltage

increase in the coil. Consequently, decreased material thickness produces

increased flux-focusing eddy current probe output levels. The output level is

directly related to the thickness of a specific material allowing measurement

25 of the material's actual thickness.

With reference now to FIG. 7, an alternate probe configuration supports

detecting faults close to an edge of the conductive material under test or near

other conductive material discontinuities. An exterior shield 70 made of

conductive material high in magnetic permeability focuses magnetic flux

30 around the outside edge of the probe to prevent eddy currents from reflecting
LAR 15046-1 -11- PATENT APPLICATION

off a nearby conductive material edge and into the area of the pick-up coil.

This allows the flux-focusing eddy current probe to be used near conductive

material edges, however, the exterior shield 70 reduces the probe's overall

sensitivity performance.

5 With reference now to FIG. 8, though the preferred embodiment is

resistant to lift-off problems, alternate probe configurations are possible which

provide further protection from lift-off conditions incorrectly indicating the

presence of a conductive material fault. In the alternate embodiments, signals

from a plurality of probes 10 combined within a casing 81, FIG. 8A, or a

10 plurality of pick-up coils 73A and 73B within a single excitation coil 11 and

flux focusing lens 12, FIG. 8B, are compared to one another. Equal, non-zero

signal levels signify probe lift-off from the test material. Pick-up coils 13 or

73A and 73B are electrically connected to a differential amplifier 72. Lift-off

conditions result in equal signal level outputs such that the difference is zero

15 and no fault indication is provided by the A.C. voltmeter 14.

With reference now to FIG. 9, the eddy current probe system for

fatigue evaluation of conductive material incorporates the flux-focusing eddy

current probe 10 as a means of tracking fatigue crack growth in a conductive

material 16 installed in a load frame 81. The unambiguous flaw signal of the

20 eddy current probe supports automated computer searching and identifying

fatigue cracks. The probe 10 is used to scan conductive material 16 installed

in the load frame 81. A computer 82 controls the probe's location over the

conductive material by means of x-axis 83 and y-axis 84 motor drives which

power their respective stepping motors 85 and 86. Additionally, the

25 computer 82 continuously monitors the output of the probe 10 as it is

scanned about the stress riser 16B in the sample and directs the load frame

controller 87 as it cyclically fatigues the conductive material sample.

The area of the probe scan is initiated around a notch 16A in the

conductive material sample. This location gives rise to high stress

30 concentrations as the sample undergoes fatigue. Upon detection of crack
LAR 15046-1 -12- PATENT APPLICATION

growth, a large output signal from the probe 10 is received by the computer

82. The computer adjusts the position of the probe such that it is centered

about the region of crack initiation. As the crack continues to grow, the

computer updates the probe's position to trace the fatigue crack tip

5 throughout the fatigue process. The position of the crack tip is determined

by the peak output level of the probe as the crack path is traversed. The

peak search routine using an algorithm similar to the grid-search chi-square

minimization scheme increments the probe's location in the direction of the

crack growth as determined by previous crack tip locations. The scan is

10 continued until the new crack tip is found. Upon locating the crack tip, the

probe is moved a short distance back along the path of the crack and the

search is repeated. By continuously reevaluating the path of the crack near

the crack tip, errors in crack tip location due to signal noise and crack

branching are avoided. As the crack grows to predetermined lengths,

15 experimental parameters such as the load level applied to the conductive

material sample are adjusted by the computer. This method allows for

completely automated experimental control of fatigue studies which saves

time, allows continuous unattended testing, and archives fatigue growth rates

and trajectories.

20 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.

25 What is claimed as new and desired to be secured by Letters Patent of

the United States is:
=°

LAR 15046-1 PATENT APPLICATION

FLUX-FOCUSING EDDY CURRENT PROBE AND METHOD

FOR FLAW DETECTION

5

Abstract of the Disclosure

A flux-focusing electromagnetic sensor which uses a ferromagnetic

flux-focusing lens simplifies inspections and increases detectability of fatigue

cracks and material loss in high conductivity material. The unique feature of

the device is the ferrous shield isolating a high-turn pick-up coil from an

10 excitation coil. The use of the magnetic shield is shown to produce a null

voltage output across the receiving coil in the presence of an unflawed

sample. A redistribution of the current flow in the sample caused by the

presence of flaws, however, eliminates the shielding condition and a large

output voltage is produced, yielding a clear unambiguous flaw signal.

15 The maximum sensor output is obtained when positioned symmetrically

above the crack. Hence, by obtaining the position of the maximum sensor

output, it is possible to track the fault and locate the area surrounding its tip.

The accuracy of tip location is enhanced by two unique features of the

sensor; a very high signal-to-noise ratio of the probe's output which results

20 in an extremely smooth signal peak across the fault, and a rapidly decaying

sensor output outside a small area surrounding the crack tip which enables

the region for searching to be clearly defined. Under low frequency operation,

material thinning due to corrosion damage causes an incomplete shielding of

the pick-up coil. The low frequency output voltage of the probe is therefore

25 a direct indicator of the thickness of the test sample.
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