NASA CASE NO.

PRINT FIG.

NQTJCE

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. Although NASA encourages nonexclusive

licensing to promote competition 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 212, Hampton, Virginia 23681-0001.

Serial No.: 08/343,740

11/22/94 LaRC

(NASA-Case-LAR-15231-I-SB)

FLUX-FOCUSING E00Y CURRENT PROBE

AND ROTATING PROBE METHOD F0R FLAW

OETECTION Patent Application

(IYASA. Langley Research Center)

34 p

G3138

N95-20764

unclas

0039289

LAR 1 5231-1 -1 - PATENT APPLICATION

FLUX-FOCUSING EDDY CURRENT PROBE AND ROTATING

PROBE METHOD FOR FLAW DETECTION

5

Cross-Reference

This is a continuation-in-part of application Serial No. 08/134,444, filed

October 12, 1993.

10

15

Ori,qin 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.

Back,qround of the Invention

20 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 discontinuities at various depths

25 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. In addition, the present invention relates to

a method of detecting flaws about circular fasteners or other circular
LAR 15231-1 -2- PATENT APPLICATION

inhomogeneities in electrically conductive material. The eddy current probe of

this invention is simple in construction and operation while providing

unambiguous signals of fault indication simplifying support equipment

requirements, supporting self nulling capability in no fault conditions during

5 surface fault detection negating the need for calibration standards, maintaining

highly resistant lift-off characteristics, and providing highest signal levels at fault

tips.

The use of eddy currents to inspect electrically conductive material for

flaws is known in the art. Such methods are particularly useful in the non10 destructive 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 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

15 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 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

20 impedance of the conductive material. In unflawed conditions, eddy current flow

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

25 consequently the voltage in the coil. An impedance detecting circuit, which may

take the form of an inductive bridge, 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
LAR 15231-1 -3- PATENT APPLICATION

impedanceover a broad region of the conductive material might indicate largearea flaws such as a grain 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

5 caused by changes to eddy current flow, use of separate magnetizing and pickup coils is known in the prior art. The same principle, however, of monitoring for

variations in coil impedance as indicative of a conductive material flaw is

applied. This conventional eddy current flaw detection technique often involves

complex impedance planes necessitating special test electronics to achieve null

10 balancing and known standards to calibrate probe responses to each type of

flaw.

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 magnetizing-sensing coils. In the

15 case of a multiple coil probe, the coils can be juxtaposed in a matrix to provide

a large detection area, or concentrically arranged to 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

20 divert potentially interfering magnetic fields to separate the coils from one

another. The shielding is designed to isolate 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

25 special test equipment to analyze probe signals.

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
LAR 15231-1 -4- PATENT APPLICATION

balance being monitored for fault indications making 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

5 to predict material fatigue failure limits. Current approaches to testing fatigue

cracking include optical methods and other length measuring techniques such

as crack mouth opening displacement gauge and four point 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

10 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.

15 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

20 tracking the crack tip which supports crack trajectory as well as crack growth

rate.

A variety of non-destructive evaluation techniques are currently used to

inspect rivet joints. However, eddy current testing is currently the most widely

used technique to find flaws which are not readily visible. Several different types

25 of eddy current probes have been developed for the specific purpose of rivet

inspection. The Sliding Probe is one such probe. A method using pencil probes

and templates to trace around the fastener head is also used.

The sensitivity of the Sliding Probe is often lower than required by

industry standards. In addition, a preferred orientation of this probe may lead
LAR 15231-1 -5- PATENT APPLICATION

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.

5 Obiects

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

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

detection characteristics provide unambiguous fault detection signals enhancing

10 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, a single probe design

and instrument configuration performs surface and subsurface eddy current fault

15 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 growth rates and

trajectory during the fatigue process as well as automatically controlling

20 conducting material loading conditions based on measured fatigue data and

experimental designs.

It is a further object of the invention to establish a device and method for

detecting flaws about circular fasteners and other circular inhomogeneities in

electrically conductive material.

25

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
LAR 15231-1 -6- PATENT APPLICATION

need for specialtest equipment. The flux-focusing eddy current probe performs

non-destructive evaluation of electrically conductive material. The probe uses

an excitationcoil to induce eddy currents in conducting material perpendicularly

oriented to the coil's longitudinal axis. A separate pick-up coil, surrounded by

5 the excitationcoil, is used to detect generatedfields. Betweenthe excitation coil

and pick-up coil is a flux focusing lens which magnetically separates the two

coils and produces high flux density at the 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

10 system performs fatigue evaluation of conducting material and is dependent

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

computerinterpretsthe signalfrom the probe to controlthe probe's position over

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

under test. A methodfor performingthe automatedtesting requires a load to be

15 applied to the conducting material to produce a fault. The flux-focusing eddy

current probe scans the conducting material and signals the 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.

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

20 probe that satisfies the need to detect flaws about any circular fasteners or

inhomogeneitiesin a material. This system performsnon-destructive evaluation

of conducting materialand is dependent upon the use and characteristics of the

flux-focusing eddy current probe. The signal due to current flow about the

fastener is detected as the probe is rotated around the fastener, keeping a

25 constant distance between the center of the probe and the center of the

fastener. Changes in the current distribution, such as due to fatigue cracks at

a rivet joint, are detectedas an increase in the output voltage above that due to

the flow about the rivet head.
LAR 15231-1

Brief Description of the Drawin,qs

-7- PATENT APPLICATION

These and other features, aspects, and advantages of the present

invention will become better understood with regard to the following description,

5 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 accordance

with the principles of this invention;

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

10 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 energized

in free space;

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

15 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 excitation

coil which generates eddy currents in conductive material as focused by the

ferromagnetic lens;

20 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 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

25 conductive material where the flaw tip lies within the probe's circumference;

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
LAR 15231-1 -8- PATENT APPLICATION

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

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;

5 FIG. 6A 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 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

10 a 1 mm unflawed aluminum alloy plate;

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 FIG. 10 shows the induced current in an unflawed sample;

FIG. 11 shows the induced current flow about a rivet in an otherwise

uniform sample;

FIG. 12 shows the induced current flow about a fatigue crack in an

otherwise uniform sample;
10

LAR 15231-1 -9- PATENT APPLICATION

FIG. 13A shows a top view of the eddy current distributions about an

unflawed rivet and a rivet above a buried fatigue crack;

FIG. 13B shows a side view of the eddy current distributions about an

unflawed rivet and a rivet above a buried fatigue crack;

FIG. 14 shows the pickup coil voltage for a probe rotating around a

fastener;

FIG. 15A shows probe misalignment with respect to a circular fastener;

FIG. 15B shows fastener misfit;

FIG. 15C shows voltage variations caused by probe misalignment and

fastener misfit at the frequency of the probe rotation;

FIG. 16 illustrates the effect of spatial filtering to distinguish signals due

to geometrical effects from signals due to fatigue damage; and

FIG. 17 illustrates the probe attached to a rotator.

15 Detailed Description

20

25

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 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 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 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.
LAR 1 5231-1 -10- PATENT APPLICATION

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 the

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

5 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 location information of

identified faults, though it must be of sufficient size to support test frequencies

10 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 when the probe 10 is in free space or

a flaw in the conductive material 16 crosses the boundary established by the

15 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 avoided when the thickness of the lens is several times the

skin depth of the magnetic flux within the lens. The size of the pick-up coil 13

20 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 detectable

flaw is approximately equal in size to one-half the inner diameter of the fluxfocusing lens 12.

25 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 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,
LAR 1 5231-1 -1 1- PATENT APPLICATION

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 at the surface of the material, whereas

inspection for interior or back surface flaws requires low frequency such that the

5 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 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

10 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.

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

the magnetic flux 31 around the lens 12 results, FIG. 3A. The leakage flux 32

15 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 separation of the pickup coil 13 from the excitation coil 11 can be achieved, FIG. 3B. The flux 33 is

concentrated within the conductive material 16 and generates eddy currents 42,

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

voltmeter.

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

25 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 pickup coil resulting in an alternating current being established in the pick-up 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.
LAR 15231-1 -12- PATENT APPLICATION

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 by in air leakage flux 32

resulting in a higher voltage signal level, FIG. 5C, thus providing a maximum

5 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 variations. As material

thins, eddy currents in the conducting material are directed to within the area

10 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 of the material's actual

thickness.

15 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 around

the outside edge of the probe to prevent eddy currents from reflecting off a

20 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.

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

25 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 plurality

of pick-up coils 73A and 73B within a single excitation coil 11 and flux focusing
LAR 15231-1 -13- PATENT APPLICATION

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 and no fault indication

5 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 eddy

10 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 computer 82

15 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

20 concentrations as the sample undergoes fatigue. Upon detection of crack

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 throughout the fatigue process.

25 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 continued until the new crack tip is found. Upon
LAR 15231-1 -14- PATENT APPLICATION

locatingthe 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 branchingare avoided. As the crack grows to predetermined lengths,

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

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

completelyautomatedexperimentalcontrol of fatigue studies which saves time,

allows continuous unattended testing, and archives fatigue growth rates and

trajectories.

10 An alternate embodiment of the present invention supports detecting

flaws about any circular fastener such as rivets, or about circular

inhomogeneities. When the probe is scanned over a rivet joint the discontinuity

inthe materialat the rivet will cause some of the current induced in the sample

to flow underthe pickup coil 13. This will result in a voltage being measured at

15 the A.C. voltmeter14 as explainedabove. It has beenfound that this rivet signal

can mask the signal from a small flaw originating from the shank of the rivet.

Figs. 10-12 display the induced current distribution in unflawed material, at a

rivet joint, and about a fatigue crack, respectively. The location of the pickup

coil 13 is indicated by a black circle in each figure. The lighter rings are the

20 current streamlines90, where the induced currentwill flow along the streamlines

90, with the current density inversely proportional to the spacing between

adjacent streamlines.

It is customary in eddy currenttesting to use lowerfrequencies (i.e., <100

kHz) to inspect for subsurface flaws in highly conducting materials such as

25 aluminum. This is becausethe depth of penetrationof the eddy currents into the

sampledecays exponentiallywithfrequency. However, use of the flux-focusing

probe of the present invention permits the use of a higher frequency drive (i.e.,

~750 kHz) to inspect for fatigue damage buried under the rivet head. The use

of a high frequency field is preferable to low frequency because the pickup coil
LAR 15231-1 -1 5- PATENT APPLICATION

output of the probe is proportional to the frequency, and because spatial

resolution is improved at high frequencies.

The distribution of the induced current pictured in Fig. 10 is characteristic

of the flux-focusing eddy current probe of the present invention. A high density

5 eddy current ring is located just beyond the outer diameter of the pickup coil 13.

In Fig. 11, conformal mapping techniques were used to determine the flow of the

current when a circular obstacle such as a rivet 92 is introduced into the flow

displayed in Fig. 10. A portion of the induced current now flows directly under

the pickup coil 13, as shown by the large concentration of streamlines 90 which

10 travel into the interior of the pickup coil location. In Fig. 12 the current

distribution about a fatigue crack 94 is shown. The current flow once again

travels beneath the pickup coil location, although a lower current density is seen

to extend beneath the pickup coil as compared to the flow pictured in Fig. 11.

The resulting output voltage would therefore be smaller for the fatigue crack (-4

15 mm length) of Fig. 12 than for the rivet (-3 mm) of Fig. 11.

Figs. 13a and 13b illustrate the use of the probe to inspect under a rivet

head. The eddy current distributions 96 are shown in Figs. 13a and 13b for an

unflawed rivet 98 and a rivet 100 above a buried fatigue crack 102. As shown,

the presence of a crack pushes the eddy current distribution 96 further under the

20 probe so that a local increase in the probe output will be detected at the angular

location corresponding to the crack position.

Since the rivet fit may vary greatly from one rivet to the next, changes in

the rivet signal are expected and have been seen experimentally. In some

cases, the change in the signal from one rivet to the next is larger than the

25 contribution due to a fatigue crack at the rivet joint. For this reason, small flaws

can be hidden under the larger output voltage due to the current flow around the

rivet head.

It has been determined that by maintaining a constant distance between

the probe center and rivet center the signal due to the current flow about the
LAR 1 5231-1 -1 6- PATENT APPLICATION

rivet can be held constant. Any further changes in the current distribution, such

as 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

5 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.

Fig. 14 displays the results of scanning a 12 mm inside diameter probe

about a 8 mm diameter rivet head. The crack detection limit for isolated flaws

has been shown to be approximately 4mm. The data of Fig. 14 shows that the

10 rotating probe method is able to clearly identify an EDM notch extending less

than 0.3 mm beyond the rivet head. A mechanical translation table was used to

accurately position the probe a fixed radial distance from the center of the rivet

and an A.C. voltmeter recorded the induced voltage across the pickup coil as the

circumference of the rivet was traversed. Any suitable means may be used to

15 rotate the probe at a constant distance from the center of the rivet or circular

inhomogeneity. For example, FIG. 17 illustrates placement of the probe 10 on

a rotator 110 comprising a pivot leg 112 positioned at the center of a rivet 113.

The pivot leg 112 is connected to an arm 114 which supports the probe 10 as

it circles the rivet and passes over a crack 115.

20 The presence of rivet misfit or probe misalignment, as illustrated in Figs.

15a and 15b, respectively, will cause voltage variations at the frequency of the

probe rotation, as illustrated in Fig. 15c. In Fig. 15a, the rivet 104 is not

centered with respect to the path of the probe center 108. In Fig. 15b, the rivet

is tilted with respect to the path of the probe center 108. Spatial filtering can be

25 used to separate signals due to the effects of rivet tilt and misfit from those of

fatigue damage at the rivet interface. This decreases the minimum detectable

flaw size. Spatial filtering is illustrated in Fig. 16 which shows voltage increases

caused by the presence of a fatigue crack 112 will occur at a frequency given

by:
5

LAR

where:

15231-1 -17-

fF (fr*( 2nRr, JP do,))12

ff = flaw frequency

fr = rotation frequency

Pd_a= probe diameter

R,ev = radius of probe revolution.

PATENT APPLICATION

High pass filtering of the data with a cutoff frequency determined by the probe

diameter, rotation frequency and radius of probe revolution will therefore

distinguish signals due to geometrical effects from those due to fatigue damage.

10 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.

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

the United States is:
°_i s

i_ _

.3

I

il

I--'1_ Z
0

........ , ....... I....I

..,,..,..,,.......... I-_1

I I ltt"_" I i

.-.._._.

I

'..

t.; .; t : , ;... _.'__-'-'_'..... _:.

31

"_ 3_
12

13 41

11

I I FIG. 4A

10

12

FIG. 4B

12 FIG.

43

45

4C

46

' 47

t FIG.

_ 12

,i

4D
lo.o#S/di,,

0.I0 V/div

FIG. 5A

t i

\ /

\/

V

A /\

t\

\t /

/\

/ \

\/

\/ \/

v

#-,,%

/\

\

FIG. 5B

m

/ \

Ik

A

/

l/

t

\

\/

W W

FIG. 5C

/ i

10.0 ilIS/div

0.10 V/div
J

/

,._/

\

\

\

/

\

FIG. 6A

, \

0.20mS/div

0.10 V/div

/

/

//

,..J

/\

\

/

/

/

I

\

\

\

,11

0.20mS/div

O. I0 V/div

FIG. 6B

/

/

\ .J

f..-.., /

\

,,,,.

FIG. 6C

/

""X

,, /

0.20mS/div

0.I0 V/div
FIG.

7

13
71_

v

11

81

14

FIG. 8A

72

J._

1 1 : ,I,,",//

73A _--73B

FIG. 8B
85

\

/

16B

X AXIS

MOTOR DRIVER

Y AXIS

MOTOR DRIVER

SYSTEM CONTROLLER

LOADED FLAME CONTROLLER

84

16

FIG. 9
cto

13

f l&. /o

13

ctz

90

F/&. it
-- .

q

90

m_. iJ,
<1f' I00

X °

\

+/.I

\

i

f m

'_/ I -/ ,,,,<7

//--

\ ./

i

F/6.. 13
E

"I:

¢.)

.J::

OO

l--d

O

l.u

°/

¢)

I,<

O

O

O

¢.)

;>

f

f

L

| i ] i i

,--- (_I

O

O

f

I i

db

b

c.)

C_J

r_

O

O

(IJ

(suu AUU) a6e].10A _nd3,no

_3

O_

o

" O

u5
epnl!IdwV

d'

-Fl

--_D

_L

L_

O

tCO'_

n

0

epm!Idwv
°. ,

.o

°. .

i

I

"-----_ !_,

aJ5

Fi&, 17
LAR 15231-1 PATENT APPLICATION

FLUX-FOCUSING EDDY CURRENT PROBE AND ROTATING

METHOD FOR FLAW DETECTION

5

Abstract of the Disclosure

A flux-focusing electromagnetic sensor which uses a ferromagnetic fluxfocusing lens simplifies inspections and increases detectability of fatigue cracks

about circular fasteners and other circular inhomogeneities in high conductivity

material. The unique feature of the device is the ferrous shield isolating a high10 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.

15 By rotating the probe in a path around a circular fastener such as a rivet

while maintaining a constant distance between the probe and the center of a

rivet, the signal due to current flow about the rivet can be held constant. Any

further changes in the current distribution, such as due to a fatigue crack at the

rivet joint, can be detected as an increase in the output voltage above that due

20 to the flow about the rivet head.