U.S. patent application number 09/682902 was filed with the patent office on 2003-05-01 for embedded eddy current inspection apparatus, system, and method.
Invention is credited to Batzinger, Thomas James, Herd, Kenneth Gordon, Nath, Shridhar Champaknath.
Application Number | 20030080736 09/682902 |
Document ID | / |
Family ID | 24741681 |
Filed Date | 2003-05-01 |
United States Patent
Application |
20030080736 |
Kind Code |
A1 |
Batzinger, Thomas James ; et
al. |
May 1, 2003 |
EMBEDDED EDDY CURRENT INSPECTION APPARATUS, SYSTEM, AND METHOD
Abstract
An embedded eddy current inspection apparatus includes a
substrate having an opening, and a test eddy current coil ("test
coil") affixed to the substrate near the opening. An internally
inspected multilayer component structure includes an upper layer, a
lower layer, and an eddy current probe embedded between the upper
and lower layers. The eddy current probe includes the test coil
facing a subject layer selected from the upper and lower layers. A
method of inspecting a multilayer component structure includes
simultaneously energizing the test coil and a reference eddy
current coil ("reference coil") embedded between the upper and
lower layers and facing the subject layer. The reference coil is
located in a reference region of the multilayer structure. A test
signal from the test coil is compared with a reference signal from
the reference coil, to determine whether a flaw is present in the
subject layer near the test coil.
Inventors: |
Batzinger, Thomas James;
(Burnt Hills, NY) ; Nath, Shridhar Champaknath;
(Niskayuna, NY) ; Herd, Kenneth Gordon;
(Niskayuna, NY) |
Correspondence
Address: |
GENERAL ELECTRIC COMPANY
GLOBAL RESEARCH CENTER
PATENT DOCKET RM. 4A59
PO BOX 8, BLDG. K-1 ROSS
NISKAYUNA
NY
12309
US
|
Family ID: |
24741681 |
Appl. No.: |
09/682902 |
Filed: |
October 31, 2001 |
Current U.S.
Class: |
324/238 ;
324/234 |
Current CPC
Class: |
G01N 27/9046
20130101 |
Class at
Publication: |
324/238 ;
324/234 |
International
Class: |
G01N 027/82 |
Claims
1. An embedded eddy current inspection apparatus comprising: a
substrate having an opening; and a test eddy current coil affixed
to said substrate near said opening.
2. The embedded eddy current inspection apparatus of claim 1,
wherein said substrate includes a reference area, and said embedded
eddy current inspection apparatus further comprises a reference
eddy current coil affixed to said substrate near said opening and
in said reference area.
3. The embedded eddy current inspection apparatus of claim 2,
further comprising an opposing test eddy current coil affixed to
said substrate near said opening, said opposing test eddy current
coil being positioned across from said test eddy current coil.
4. The embedded eddy current inspection apparatus of claim 3,
further comprising: a second test eddy current coil affixed to said
substrate and positioned further from a center of said opening than
is said test eddy current coil; and a second opposing test eddy
current coil affixed to said substrate and positioned further from
the center of said opening than is said opposing test eddy current
coil.
5. The embedded eddy current inspection apparatus of claim 2,
further comprising: a plurality of pairs of electrical contacts
formed on said substrate; and a plurality of pairs of leads formed
on said substrate, each pair of leads connecting a respective one
of said reference eddy current coil and said test eddy current coil
to a respective pair of said electrical contacts.
6. The embedded eddy current inspection apparatus of claim 5,
wherein said test eddy current coil and said reference eddy current
coil comprise single eddy current array probes (SECAPs).
7. The embedded eddy current inspection apparatus of claim 5,
wherein said test eddy current coil and said reference eddy current
coil comprise eddy current array probes (ECAPs).
8. The embedded eddy current inspection apparatus of claim 5,
wherein said test eddy current coil comprises an ECAP and said
reference eddy current coil comprises a SECAP.
9. The embedded eddy current inspection apparatus of claim 5,
wherein said substrate comprises a flexible organic polymer.
10. A method of inspecting a multilayer component structure
including an upper and a lower layer, said method comprising:
energizing a test eddy current coil embedded between the upper and
lower layers, the test eddy current coil facing a subject layer
selected from the upper and lower layers; energizing a reference
eddy current coil simultaneously with the test eddy current coil,
the reference eddy current coil coil being embedded between the
upper and the lower layers in a reference region of the multilayer
structure and facing the subject layer; and comparing a test signal
from the test eddy current coil with a reference signal from the
reference eddy current coil to determine whether a flaw is present
in the subject layer near the test eddy current coil.
11. The inspection method of claim 10 wherein the test and
reference eddy current coils are positioned near a fastener
extending through the upper and the lower layers.
12. The inspection method of claim 11 further comprising:
energizing an opposing test eddy current coil and the reference
eddy current coil simultaneously, the opposing test eddy current
coil being embedded between the upper and lower layers, facing the
subject layer, and positioned across from the test eddy current
coil; energizing a second test eddy current coil and the reference
eddy current coil simultaneously, the second test eddy current coil
being embedded between the upper and lower layers, facing the
subject layer, and positioned further from a center of the opening
than is the test eddy current coil; energizing a second opposing
test eddy current coil and the reference eddy current coil
simultaneously, the second opposing test eddy current coil being
embedded between the upper and lower layers, facing the subject
layer, and positioned further from the center of the opening than
is the opposing test eddy current coil; comparing a test signal
from the opposing test eddy current coil with a reference signal
from said simultaneous energizing of the reference eddy current
coil to determine whether a flaw is present in the subject layer
near the opposing test eddy current coil; comparing a test signal
from the second test eddy current coil with a reference signal from
said simultaneous energizing of the reference eddy current coil to
determine whether the flaw in the subject layer near the test eddy
current coil extends to the second test eddy current coil; and
comparing a test signal from the second opposing test eddy current
coil with a reference signal from said simultaneous energizing of
the reference eddy current coil to determine whether the flaw in
the subject layer near the opposing test eddy current coil extends
to the second opposing test eddy current coil.
13. A method of assembling an internally inspected multilayer
component structure, said method comprising: embedding an eddy
current probe between an upper and a lower layer of the multilayer
component structure, the eddy current probe comprising a test eddy
current coil facing a subject layer selected from the upper and
lower layers.
14. The assembly method of claim 13, wherein the eddy current probe
further comprises a reference eddy current coil positioned in a
reference region of the multilayer component structure and facing
the subject layer.
15. The assembly method of claim 14, wherein the eddy current probe
further comprises a substrate having an opening and a reference
area, the test and reference eddy current coils being affixed to
the substrate and positioned near the opening, and the reference
eddy current coil being positioned in the reference area, said
method further comprising: extending a fastener through the upper
and lower layers and through the opening of the substrate.
16. The assembly method of claim 14, wherein the eddy current probe
further comprises an opposing test eddy current coil affixed to the
substrate, facing the subject layer, and positioned near the
opening and across from the test eddy current coil.
17. The assembly method of claim 16, wherein the eddy current probe
further comprises a second test eddy current coil and a second
opposing test eddy current coil affixed to the substrate, facing
the subject layer, and positioned further from a center of the
opening than are the test and the opposing test eddy current coils,
respectively.
18. An internally inspected multilayer component structure
comprising: an upper layer; a lower layer; and an eddy current
probe embedded between said upper and lower layers, said eddy
current probe comprising a test eddy current coil facing a subject
layer selected from said upper and lower layers.
19. The internally inspected multilayer component structure of
claim 20, wherein said eddy current probe further comprises: a
reference eddy current coil facing the subject layer and positioned
in a reference region of said multilayer component structure.
20. The internally inspected multilayer component structure of
claim 19, wherein said eddy current probe further comprises: a
substrate including a reference area, wherein said test and
reference eddy current coils are affixed to said substrate and said
reference eddy current coil is positioned in the reference
area.
21. The internally inspected multilayer component structure of
claim 20, wherein said eddy current probe further comprises: a
plurality of pairs of electrical contacts formed on said substrate;
and a plurality of pairs of leads formed on said substrate, each
pair of leads connecting a respective one of said reference eddy
current coil and said test eddy current coil to a respective pair
of said electrical contacts.
22. The internally inspected multilayer component structure of
claim 21, wherein said substrate comprises a flexible organic
polymer.
23. The internally inspected multilayer component structure of
claim 20, wherein said test eddy current coil and said reference
eddy current coil comprise single eddy current array probes
(SECAPs).
24. The internally inspected multilayer component structure of
claim 20, wherein said test eddy current coil and said reference
eddy current coil comprise eddy current array probes (ECAPs).
25. The internally inspected multilayer component structure of
claim 20, wherein said test eddy current coil comprises an ECAP and
said reference eddy current coil comprises a SECAP.
26. The internally inspected multilayer component structure of
claim 20, wherein said substrate has an opening, said test and
reference eddy current coils being positioned near said opening,
said internally inspected multilayer component structure further
comprising a fastener extending through said upper and lower layers
and through said opening.
27. The internally inspected multilayer component structure of
claim 26, wherein said eddy current probe further comprises an
opposing test eddy current coil affixed to said substrate near said
opening, said opposing test eddy current coil being positioned
across from said test eddy current coil.
28. The internally inspected multilayer component structure of
claim 27, wherein aid eddy current probe further comprises: a
second test eddy current coil affixed to said substrate and
positioned further from a center of said opening than is said test
eddy current coil; and a second opposing test eddy current coil
affixed to said substrate and positioned further from a center of
said opening than is said opposing test eddy current coil.
29. The internally inspected multilayer component structure of
claim 27, further comprising: a supplemental eddy current probe
embedded between said upper and lower layers, said supplemental
eddy current probe comprising: a supplemental substrate including a
supplemental opening and a supplemental reference area, wherein
said fastener extends through said supplemental opening; a
supplemental test eddy current coil affixed to said supplemental
substrate, facing a remaining layer of said upper and lower layers,
and positioned near said supplemental opening; and a supplemental
reference eddy current coil affixed to said supplemental substrate,
facing the remaining layer, and positioned near said supplemental
opening and in said supplemental reference area.
30. The internally inspected multilayer component structure of
claim 29, wherein said supplemental eddy current probe further
comprises: a supplemental opposing test eddy current coil affixed
to said supplemental substrate near said supplemental opening and
facing the remaining layer, said supplemental opposing test eddy
current coil being positioned across from said supplemental test
eddy current coil.
31. An internally inspected multilayer component structure
comprising: an upper layer; at least one intermediate layer; a
lower layer; and an eddy current probe embedded between a pair of
adjacent layers, selected from said lower, intermediate, and upper
layers, said eddy current probe comprising a test eddy current coil
facing a subject layer selected from the pair of adjacent
layers.
32. The internally inspected multilayer component structure of
claim 31, wherein said eddy current probe further comprises a
reference eddy current coil facing the subject layer.
33. The internally inspected multilayer component structure of
claim 32, wherein said eddy current probe further comprises a
substrate having an opening and a reference area, wherein said test
and reference eddy current coils are affixed to said substrate and
positioned near said opening, said reference eddy current coil
being positioned in said reference area, and said internally
inspected multilayer structure further comprises a fastener
extending through said upper, intermediate, and lower layers and
through said opening.
34. The internally inspected multilayer component structure of
claim 31, wherein said eddy current probe further comprises an
opposing test current coil affixed to said substrate and positioned
near said opening and facing the subject layer, said opposing test
eddy current coil being across from said test eddy current
coil.
35. An embedded eddy current inspection system for inspecting a
multilayer component structure comprising an upper and a lower
layer, said system comprising: an eddy current probe embedded
between the upper and lower layers, said eddy current probe
comprising at least one test eddy current coil; a signal generator
configured to energize said test eddy current coil; and a
comparison module for comparing a test signal received from said
test coil and a reference signal and outputting a compared
signal.
36. The embedded eddy current inspection system of claim 35,
wherein said eddy current probe further comprises a reference eddy
current coil, wherein said signal generator is further configured
to energize said reference eddy current coil simultaneously with
said test eddy current coil, and wherein said reference eddy
current coil is configured to supply the reference signal.
37. The embedded inspection system of claim 36, further comprising
a computer configured to control said signal generator.
Description
BACKGROUND OF INVENTION
[0001] The present invention relates generally to eddy current
inspection and, more particularly, to eddy current inspection of
aircraft structures near fasteners.
[0002] Aircraft structures are generally constructed using multiple
layers of material utilizing lap joints and fasteners, such as
rivets. Sealants are often disposed between the layers to prevent
corrosive materials from collecting in the joint. In response to
stress exerted on the aircraft structure during operation, cracks
occasionally develop in the layers comprising the aircraft
structure in the vicinity of the fasteners.
[0003] In order to ensure the structural integrity of the aircraft,
aircraft structures are repeatedly inspected during the life of the
aircraft, to determine that cracks have not formed near the rivet
hole. However, these inspections can be difficult to perform, due
to the reduced access to critical inspection areas, such as
portions of the aircraft structure surrounding the rivets. The
inspections are further complicated by the fact that cracks
internal to lap joints can be very difficult to detect.
[0004] Currently, aircraft structures are inspected for cracks
internal to lap joints using x-ray, eddy current, and ultrasonic
inspection techniques applied to the external surfaces of the lap
joints. However, application of these inspection techniques is
often impeded by the internal structure of the aircraft and
frequently requires considerable aircraft disassembly. In addition,
ultrasonic signals can be difficult to interpret because of the
complicated geometry near the rivet and the number of interfaces in
the lap joint. Eddy current inspection may undesirably require
removal of the rivet and disassembly of the structure and is
further complicated by the presence of sealants in the joints.
X-ray inspection creates radiation exposure problems and requires
evacuation of operators during testing, preventing concurrent
maintenance and inspections.
[0005] By way of further background, eddy current inspection is
based on the principle of electromagnetic induction in which a
drive coil is employed to induce eddy currents within the material
under inspection, and secondary magnetic fields resulting from the
eddy currents are detected by a sense coil, generating signals
which are subsequently processed. Eddy current inspection detects
flaws as follows. The presence of a discontinuity or a crack in the
surface of the component under inspection changes the flow of the
eddy currents within the test specimen. The altered eddy current,
in turn, produces a modified secondary magnetic field, which is
detected by the sense coil, thereby generating a signal which
indicates the presence of the flaw upon subsequent processing.
[0006] Previous eddy current inspection applications to aircraft
structure involved positioning a single eddy current coil probe
adjacent to the surface of the aircraft structure. Although this
technique is adequate for external and easily accessible surfaces,
it is not desirable for interior surfaces. Due to gaps between
aircraft structure layers filled with air or sealants, low
frequency eddy current inspection must be used to inspect interior
surfaces from an external position. However, low frequency eddy
current inspection provides limited resolution. Thus, in order to
perform high resolution eddy current inspection of interior
surfaces of a lap joint using this technique, the rivets would have
to be removed and the structure disassembled. Accordingly,
inspection times would be considerable to disassemble, inspect, and
reassemble the aircraft structure. Further, the cost of labor
required to perform these tasks would be high.
[0007] Moreover, the conventional eddy current inspection technique
is performed during maintenance periods when the aircraft is taken
out of use, disassembled, inspected, and reassembled. In
particular, this conventional technique does not inspect for crack
formation during flight operations.
[0008] Accordingly, it would be desirable to provide a method and
apparatus for performing eddy current inspection on interior
surfaces of multilayer structures that does not require disassembly
of the multilayer structure. In addition, it would be desirable to
provide a method and an apparatus for performing eddy current
inspection on a lap joint near a fastener, that does not require
disassembly of the fastener or the lap joint. It would further be
desirable for the method and apparatus to permit periodic eddy
current inspection of the lap joints and other multilayer
structures, that provides information about the presence and size
of cracks, such as cracks near fasteners (e.g., rivets) in the
multilayer structure. In addition it would be desirable to provide
a method and apparatus for performing eddy current inspection
during flight operations.
SUMMARY OF INVENTION
[0009] Briefly, in accordance with one embodiment of the present
invention, an embedded eddy current inspection apparatus includes a
substrate having an opening, and a test eddy current coil affixed
to the substrate near the opening.
[0010] In accordance with another embodiment, an internally
inspected multilayer component structure includes an upper layer
and a lower layer. The multilayer component structure further
includes an eddy current probe embedded between the upper and lower
layers. The eddy current probe includes a test eddy current coil
facing a subject layer selected from the upper and lower
layers.
[0011] In accordance with another embodiment, a method of
inspecting a multilayer component structure including an upper and
a lower layer is provided. The inspection method includes
energizing a test eddy current coil embedded between the upper and
lower layers. The test eddy current coil faces a subject layer
selected from the upper and lower layers. The inspection method
further includes energizing a reference eddy current coil
simultaneously with the test coil. The reference eddy current coil
is embedded between the upper and the lower layers in a reference
region of the multilayer structure and faces the subject layer. The
inspection method further includes comparing a test signal from the
test eddy current coil with a reference signal from the reference
eddy current coil, to determine whether a flaw is present in the
subject layer near the test eddy current coil.
[0012] In accordance with another embodiment of the invention, an
embedded eddy current inspection system is provided for inspecting
a multilayer component structure including an upper and a lower
layer. The inspection system includes an eddy current probe
embedded between the upper and lower layers. The eddy current probe
includes at least one test eddy current coil. The system further
includes a signal generator configured to energize the test eddy
current coil. The system also includes a comparison module for
comparing a test signal received from the test coil and a reference
signal and outputting a compared signal.
BRIEF DESCRIPTION OF DRAWINGS
[0013] These and other features, aspects, and advantages of the
present invention will become better understood when the following
detailed description is read with reference to the accompanying
drawings in which like characters represent like parts throughout
the drawings, wherein:
[0014] FIG. 1 is a top view of an embedded inspection apparatus
according to one embodiment of the invention;
[0015] FIG. 2 depicts illustrative reference and testing areas;
[0016] FIG. 3 is an enlarged view of the embedded inspection
apparatus configured to monitor crack length;
[0017] FIG. 4 is a cross-sectional view of a multilayer component
structure according to another embodiment of the invention, which
includes the embedded inspection apparatus of FIG. 1;
[0018] FIG. 5 is a top view of the multilayer component structure
shown in FIG. 4, where the upper layer(s) of the component
structure have been removed to expose the embedded inspection
apparatus;
[0019] FIG. 6 is a cross-sectional view of a multilayer component
structure similar to that shown in FIG. 4, which includes two
embedded inspection apparatuses;
[0020] FIG. 7 is a cross-sectional view of a multilayer component
structure according to yet another embodiment of the invention;
[0021] FIG. 8 is a cross-sectional view of a multilayer component
structure similar to that shown in FIG. 7, which includes more than
one embedded inspection apparatuses;
[0022] FIG. 9 schematically depicts an embedded eddy current
inspection system according to another embodiment of the invention;
and
[0023] FIG. 10 depicts illustrative reference and testing
areas.
DETAILED DESCRIPTION
[0024] An embedded inspection apparatus 10 embodiment of the
present invention is shown in FIG. 1. For reference purposes only,
cracks 50 of an adjacent component layer (not shown) are indicated
in FIG. 1, even though these cracks are not part of the embedded
inspection apparatus 10. The embedded inspection apparatus is
configured for embedding in a multilayer component structure 100,
as illustrated for example in FIG. 4.
[0025] The embedded inspection apparatus 10 includes a substrate 20
with an opening 30, as illustrated in FIG. 1. The opening is
configured to receive a fastener 70, such as a rivet. Other
exemplary fasteners include bolts. An exemplary fastener is shown
in cross-sectional view in FIG. 4. The embedded inspection
apparatus further includes a test eddy current coil 40 (which is
also referred to as a "test coil" herein) affixed to the substrate
near the opening. The phrase "near the opening" means that the test
coil is close enough to the opening that it can detect cracks 50
formed in an adjacent component layer 110 propagating outward from
the fastener 70 extending through the opening and through the
component layer. The precise distance between the test coil and the
opening is determined by the engineering design criteria. For
example, the shorter the allowable length of cracks extending from
the fastener, the closer the test coil will be to the opening.
However, an exemplary distance between the opening and the test
coil is greater than about 0.5 mm.
[0026] According to one embodiment of the embedded inspection
apparatus 10, the substrate 20 further includes a reference area
22. Exemplary reference areas are depicted in FIGS. 2 and 10. As
suggested by the cracks 50 shown in FIG. 1, cracks in riveted
component structures, such as the multilayer component structure
illustrated in FIG. 4, preferentially form in certain directions
based on the stresses on the component structures at the fastener
(rivet) 70. Accordingly, testing and reference areas 24, 22 are
designated on the substrate 20 around the opening 30 to correspond
to the areas in which cracking can and cannot occur in the riveted
component structure, respectively. As known to those skilled in the
art, crack formation at a rivet hole is not necessarily
reflectively symmetric about an axis A, shown for example in FIGS.
2 and 10. Rather, based on the position of the fastener relative to
other fasteners and on any curvature of the aircraft structure near
the fastener, crack formation may be asymmetric, as shown for
example in FIG. 10. Accordingly, the test and reference areas 24,
22 are selected based on the engineering criteria for the desired
application.
[0027] Where the substrate 20 includes a reference area 22, the
embedded inspection apparatus 10 further includes a reference eddy
current coil 60 (also referred to as a "reference coil" herein)
affixed to the substrate and positioned in the reference area 22
near the opening 30. In addition, the test coil 40 is positioned in
the testing area 24. An exemplary reference coil is positioned at
about the same radial distance from the opening as the test coil.
Advantageously, this exemplary configuration is approximately
symmetric with respect to the fastener 70 extending through the
opening. In this manner, the effect of the fastener on eddy
currents detected by the test and reference coils are similar.
[0028] According to another embodiment, the embedded inspection
apparatus 10 further includes an opposing test eddy current coil 42
(or "opposing test coil"), as shown in FIGS. 1 and 2. The opposing
test coil is affixed to the substrate 20 near the opening 30 and is
positioned across from the test coil 40. In this manner, the test
coil and opposing test coils are configured to detect cracks on
opposite sides of the opening. The phrase "across from the test
coil" encompasses positions within the opposite test area 24. An
exemplary opposing test coil is positioned approximately 1 80
degrees from the test coil, as illustrated in FIGS. 1 and 2. In
addition, the exemplary opposing test coil is positioned at about
the same radial distance from the opening as the test coil.
However, as noted above, crack formation at a rivet hole is not
necessarily symmetric about the axis A. Accordingly, the phrase
"across from the test coil" further encompasses positions within an
asymmetric opposite test area, as shown for example in FIG. 10. As
noted above, the test areas are selected based on engineering
criteria for the desired application.
[0029] In order to monitor the crack length, the embedded eddy
current inspection apparatus 10 according to another embodiment
further includes a second test eddy current coil 140 ("second test
coil") affixed to the substrate 20 and positioned further from the
center of the opening than is the test coil, as illustrated in FIG.
3. By the phrase "center of the opening," the center of area (or
centroid) of the opening is meant. An exemplary second test coil is
positioned radially outward from the test coil (i.e., essentially
no rotation relative to the test coil), as shown in FIG. 3.
However, where engineering criteria determine that the crack 50
will propagate in a non-radial fashion between the test and the
second test coils, the second test coil is rotationally offset from
the test coil by an amount determined by the engineering
criteria.
[0030] In addition to the second test eddy current coil 140, the
embedded eddy current inspection apparatus 10 further includes a
second opposing test eddy current coil 142 ("second opposing test
coil") affixed to the substrate 20 and positioned further from the
center of the opening 30 than is the opposing test coil 42, as
shown for example in FIG. 3. Advantageously, the embedded eddy
current inspection apparatus of this embodiment not only is
configured to detect cracks 50 propagating outward from a fastener
(not shown), but is further configured to monitor the length of the
cracks. Depending on the engineering design criteria (namely, the
allowable crack length), this principle can be extended to include
third, fourth, fifth etc. sets of test coils (not shown) positioned
still further from the center of the opening, in the manner shown
in FIG. 3. For example, if the engineering design criteria specify
an allowable crack length of less than about 2.5 mm, five pairs of
test coils (not shown) spaced at about 0.5 mm intervals could be
employed to monitor the crack length. However, those skilled in the
art will readily recognize that both the number of pairs of test
coils and spacing therebetween are purely illustrative and do not
limit the embedded inspection apparatus of the present
invention.
[0031] The substrate 20 is desirably formed of a flexible material,
such as a flexible organic polymer. An exemplary flexible organic
polymer is polyimide, one example of which is KAPTON.RTM..
KAPTON.RTM. is a federally registered trademark of E.I. du Pont de
Nemours and Company of Wilmington, Del. An exemplary substrate has
a thickness of about 25 .mu.m to about 100 .mu.m, for example a 25
.mu.m thick KAPTON.RTM. substrate. Advantageously, a flexible
substrate is easy to process and is robust.
[0032] Exemplary reference and test coils 60, 40, 42, 140, 142
include single eddy current array probes (SECAPs). SECAPs are
single, conducting coils formed on the substrate 20 by known
photolithographic methods. A variety of conductive materials, such
as copper, silver, and gold are used to form SECAPS. One benefit of
SECAPs is that they are compatible with existing eddy current
instrumentation, such as an eddy current instrument 220, which is
shown in FIG. 9.
[0033] Other exemplary reference and test coils 60, 40, 42, 140,
142 are eddy current array probes (ECAPs). ECAPs are arrays of
conducting coils (not shown) disposed on the substrate 20. The
coils are formed of conductive materials, examples of which include
platinum and copper. ECAPs are fabricated using photolithography
techniques that are capable of achieving precision and uniformity
at small dimensions. An overview of an exemplary fabrication
process is provided in commonly assigned U.S. Pat. No. 5,389,876,
entitled "Flexible Eddy Current Surface Measurement Array for
Detecting Near Surface Flaws in a Conductive Part," by Kristina H.
V. Hedengren, et al . An exemplary ECAP includes 24 differential
pick up coils which extend approximately 25 mm, with each coil
being about 1.8 mm in length and about 0.9 mm in width. Thus, the
use of ECAPs accommodates inspecting an area covered by the active
area of the array . Exemplary ECAPs further include one and two
dimensional arrays of coils.
[0034] According to one specific embodiment, the test eddy current
coil is an ECAP, and the reference eddy current coil is a SECAP. By
configuring the ECAP such that an array of coils (not shown)
extends outward from the opening 30, the ECAP can be used to detect
crack formation near a fastener, to monitor their propagation from
the fastener, and to estimate the crack length.
[0035] According to a more specific embodiment, the embedded eddy
current inspection apparatus 10 further includes a plurality of
pairs of electrical contacts 26 formed on the substrate 20.
According to this aspect, a plurality of pairs of leads 28 is
formed on the substrate for connecting the reference and the test
eddy current coils 60, 40, 42 to the electrical contacts. A pair of
contacts and a pair of leads are provided for each reference and
test coil.
[0036] An internally inspected multilayer component structure 100
embodiment of the invention is illustrated in FIG. 4 in
cross-sectional view. A top view of the multilayer component
structure is shown in FIG. 5, where the upper layer 110 of the
component structure is removed. As the multilayer component
structure incorporates many aspects of the embedded inspection
apparatus 10, a detailed description of these features will not be
repeated.
[0037] The internally inspected multilayer component structure 100
includes an upper layer 110 and a lower layer 120, as illustrated
in FIG. 4. For multilayer component structures that do not include
intermediate layers, either the upper or the lower layer is
conductive. Herein, the phrase "conductive layer " means that eddy
currents can be generated in the conductive layer. Where the
multilayer component structure includes intermediate layers 150,
for example as shown in FIG. 6, neither the upper nor the lower
layer need be conductive.
[0038] The internally inspected multilayer component structure 100
further includes an eddy current probe (also indicated by reference
number 10) embedded between the upper and lower layers 110, 120, as
illustrated in FIG. 4. The eddy current probe includes the test
eddy current coil 40, which faces a subject layer selected from the
upper and lower layers. For the configuration illustrated in FIG.
4, the subject layer is the upper layer 110. The subject layer 110
is conductive.
[0039] Although the multilayer component structure 100 is
illustrated in FIGS. 4 and 5 as including a fastener 70, the
multilayer component structure encompasses multilayer component
structures held together by other means. Advantageously, the eddy
current probe 10 can be embedded between upper and lower layers
110, 120, where the subject layer is difficult to access without
disassembling the multilayer component structure. For example, the
test coil 40 can be positioned to face any stress point on either
of the layers.
[0040] According to one embodiment, the eddy current probe 10
further includes the reference eddy current coil 60 positioned in a
reference region of the multilayer component structure 100, as
illustrated in FIG. 5. The reference coil faces the subject layer
110. The reference region is a region in which cracks are known not
to form. As explained above in the description of the first
embodiment, cracks 50 in multilayer component structures
preferentially form in certain directions based on the stresses on
the component structure at fasteners 70 or other stress points. The
reference region is selected based on known engineering principles
to avoid such crack-prone areas.
[0041] Exemplary test and reference coils 40, 60 include SECAPs and
ECAPs, as discussed above.
[0042] According to another embodiment, the eddy current probe 10
further includes the substrate 20, as for example shown in FIG. 5.
As described above, the substrate includes the reference area 22,
and the test and reference eddy current coils 40, 60 are affixed to
the substrate. The reference coil is positioned in the reference
area, which is illustrated in FIG. 2 and described in detail above.
Exemplary substrates are formed from a flexible material, such as a
flexible organic polymer.
[0043] As shown in FIG. 4, a sealant 130 can be disposed between
the upper and lower layers 110, 120 and between the substrate 20
and the lower layer, for the component structure shown in FIG. 4.
The sealant reduces the collection of corrosive materials in the
component structure.
[0044] As noted above the multilayer component structure 100 can be
held together by means other than the fastener 70. However,
according to the embodiment illustrated in FIG. 4, the multilayer
component structure includes the fastener. For this embodiment, the
substrate 20 has the opening 30 to accommodate the fastener, as
illustrated in FIGS. 4 and 5. The test and reference eddy current
coils 40, 60 are positioned near the opening, and the fastener
extends through the upper and lower layers and through the opening,
as illustrated in FIG. 4.
[0045] In order to monitor cracks on opposite sides of the
fastener, the eddy current probe 10 for another embodiment of the
multilayer component structure 100 includes an opposing test eddy
current coil 42 positioned across from the test coil 40, as
illustrated in FIG. 5 and as discussed above.
[0046] In order to monitor crack length, the eddy current probe 10
for yet another embodiment of the multilayer component structure
100 further includes a second test eddy current coil 140 and a
second opposing test eddy current coil 142 affixed to the substrate
20 and positioned further from the center of the opening 30 than
are the test coil 40 and the opposing test coil 42, respectively,
as exemplarily shown in FIG. 3. As discussed above, this
arrangement of test coils is advantageous in that it is configured
both to detect cracks 50 and to monitor their propagation in the
subject layer 110 away from the fastener 70. Further, this
principle can be extended to include third, fourth, fifth, etc.
sets of test coils (not shown) positioned still further out from
the center of the opening, in the manner shown in FIG. 3, depending
on the design criteria (i.e., the allowable crack length).
[0047] The internally inspected multilayer component structure 100
can further be configured to inspect crack formation on both the
upper and the lower layers 110, 120, where both layers are
conductive. According to this embodiment, the internally inspected
multilayer component structure further includes a supplemental eddy
current probe (also indicated by reference numeral 10) embedded
between the upper and lower layers, as illustrated in FIG. 6. The
supplemental eddy current probe is positioned and configured to
inspect the remaining layer. For example, if the subject layer
coincides with the upper layer 110 (as for the configuration of
FIG. 4), the remaining layer is the lower layer 120. Although this
numbering scheme is employed for convenience, this embodiment is
symmetric in that the multilayer component structure is configured
to inspect crack formation in both the upper and lower layers.
Accordingly, either layer could be termed the subject or remaining
layer.
[0048] The supplemental eddy current probe 10 includes a
supplemental substrate 20 including a supplemental opening 30 and a
supplemental reference area 22, illustrated in FIGS. 1 and 2. As
illustrated in FIG. 6, the fastener 70 extends through the
supplemental opening. The supplemental eddy current probe further
includes supplemental test and reference eddy current coils 40, 60
affixed to the supplemental substrate and facing the remaining
layer 120. The supplemental test and reference coils are configured
as discussed above with respect to the test and reference
coils.
[0049] In order to monitor cracks on opposite sides of the fastener
70, the supplemental eddy current probe 10 can further include a
supplemental opposing test eddy current coil 42 positioned across
from the supplemental test coil 40. As with the embedded inspection
apparatus 10 discussed above, the supplemental eddy current probe
can include second, third, fourth, etc. pairs (not shown) of test
eddy current coils to monitor the length of cracks 50 in the
remaining layer 120.
[0050] According to yet another embodiment, pairs of electrical
contacts 26 are formed on the substrate 20 and on the supplemental
substrate 20. The test and reference coils 40, 42, 140, 142, 60
(and supplemental test and reference coils) are connected to the
respective pairs of electrical contacts by pairs of leads 28, as
illustrated in FIG. 5 (for the substrate).
[0051] An internally inspected multilayer component structure 80,
as shown for example in FIGS. 7 and 8 is similar to the multilayer
component structure 100 in many aspects. Accordingly, only features
of the multilayer component structure not previously will be
discussed in detail. As exemplarily illustrated in FIG. 7, the
multilayer component structure 80 includes an upper layer 110, at
least one intermediate layer 150, and a lower layer 120. The
multilayer component structure further includes the eddy current
probe 10 embedded between a pair of adjacent layers selected from
the lower, intermediate and upper layers. For example, for the
component structure shown in FIG. 7, the pair of adjacent layers
comprises the upper layer and the intermediate layer. The eddy
current probe includes the test coil 40 facing a subject layer
selected from the pair of adjacent layers. For the component
structure shown in FIG. 7, the subject layer is the upper layer.
For convenience the subject layer will be designated by the same
reference number 110. However, in general the subject layer can
comprise any conductive layer of the component structure.
[0052] The eddy current probe 10 is discussed above and previous
descriptions will not be repeated in detail. According to one
embodiment of the multilayer component structure 80, the eddy
current probe 10 further includes the reference eddy current coil
60 facing the subject layer (110 for the structure of FIG. 7).
[0053] According to another embodiment of the multilayer component
structure 80, the eddy current probe 10 further includes the
substrate 20 having the opening 30 and the reference area 22. The
multilayer component structure further includes the fastener 70
extending through the upper, intermediate, and lower layers 110,
150, 120 and through the opening, as shown in FIG. 7.
[0054] To monitor cracks on opposite sides of the fastener 70, the
eddy current probe 10 for another embodiment of the multilayer
component structure 80 further includes the opposing test current
coil 42 positioned across from the test eddy current coil 40, as
exemplarily shown in FIG. 7.
[0055] The internally inspected multilayer component structure 80
of the third embodiment encompasses the case of a plurality of
intermediate layers 150 and a plurality of eddy current probes 10,
as exemplarily shown in FIG. 8. In addition, second, third, fourth,
etc. pairs of test coils can be included on each of the eddy
current probes 10 to monitor crack propagation away from the
fastener in the subject layers(s).
[0056] Because the internally inspected multilayer component
structures 100, 80 incorporate embedded eddy current probes 10,
these multilayer component structures are suitable for in flight
monitoring of crack formation in aircraft structures. Namely, by
incorporating the multilayer component structures into an aircraft
structure, eddy current inspection can be performed at any time
during the life of the aircraft structure, including during flight
operations. This provides a distinct advantage relative to
conventional eddy current inspection techniques, which require
disassembly and re-assembly of the aircraft structure.
[0057] An inspection method embodiment of the invention for
inspecting a multilayer component structure 100, including an upper
and a lower layer 110, 120, is discussed with respect to FIGS. 4
and 5. The inspection method includes energizing the test eddy
current coil 40 embedded between the upper and lower layers. The
test coil faces a subject layer selected from the upper and lower
layers. As discussed above, for the component structure of FIGS. 4
and 5, the subject layer is the upper layer 110. The inspection
method further includes energizing the reference coil 60
simultaneously with the test coil 40 to obtain the reference
signal. The reference coil is discussed above. A test signal from
the test coil is compared with a reference signal from the
reference coil to determine whether a flaw is present in the
subject layer near the test eddy current coil. More precisely, a
difference signal is obtained by subtracting the reference and the
test signals. Reference and test coils that are in electrical
balance (i.e., a difference signal is obtained that is
approximately zero to within the precision of the measurements),
indicate that there is no flaw near the test coil. When the
reference and test coils are out of balance (i.e., a nonzero
difference signal is obtained), a flaw near the test coil is
indicated.
[0058] According to another embodiment, the test and reference
coils 40, 60 are positioned near the fastener 70 extending through
the upper and lower layers 110, 120.
[0059] In order to monitor crack length, the inspection method
according to another embodiment further includes energizing the
opposing test coil 42, the second test coil 140, and the second
opposing test coil 142, which are discussed above. Each of the
opposing, second, and second opposing test coils is energized
simultaneously with the reference coil 60. However, the test coils
are energized sequentially, with the reference coil being energized
each time one of the test coils is energized. A test signal from
the opposing test coil is compared with a reference signal from the
simultaneously energized reference coil to determine whether a flaw
50 is present in the subject layer (110 in FIG. 4) near the
opposing test eddy current coil. In this manner, a difference
signal is obtained. A nonzero difference signal indicates the
presence of a flaw near the opposing test eddy current coil. A test
signal from the second test coil is also compared with a reference
signal obtained from the simultaneous energizing of the reference
coil to determine whether the flaw in the subject layer near the
test coil extends to the second test coil. The comparison produces
a difference signal, and a nonzero difference signal indicates the
presence of a flaw near the second test coil. In addition, a test
signal from the second opposing test coil is compared with a
reference signal obtained from the simultaneous energizing of the
reference coil to determine whether the flaw near the opposing test
coil extends to the second opposing test coil. The comparison
produces a difference signal, which if nonzero, indicates the
presence of a flaw near the second opposing test coil.
[0060] An assembly method embodiment of the invention for
assembling an internally inspected multilayer component structure
100 is described with respect to FIGS. 4 and 5. The assembly method
includes embedding the eddy current probe 10 between the upper and
the lower layer 110, 120 of the multilayer component structure,
such that the test coil 40 faces the subject layer selected from
the upper and lower layers. As discussed above, for the component
structure of FIG. 4, the subject layer is the upper layer 110.
[0061] According to one embodiment of the assembly method, the eddy
current probe 10 further includes the reference eddy current coil
60, as discussed above.
[0062] To accommodate the fastener 70, according to another
embodiment the test and reference coils 40, 60 are affixed to the
substrate 20 with the opening 30 and the reference area 22, as
discussed above and as exemplarily shown in FIG. 5. The assembly
method of this embodiment further includes extending the fastener
70 through the upper and lower layers and through the opening of
the substrate.
[0063] As discussed above, a sealant 130 can be disposed between
the upper and lower layers 110, 120 to reduce the collection of
corrosive materials in the component structure 100.
[0064] Advantageously, by embedding the eddy current probe 10 in
the multilayer component structure 100, inaccessible conductive
layers of the component structure can be inspected without
requiring disassembly of the component structure.
[0065] To monitor cracks on opposite sides of the fastener 70,
according to another embodiment of the assembly method the eddy
current probe 10 further includes the opposing test eddy current
coil 42, as discussed above.
[0066] To monitor crack length, the eddy current probe 10 according
to yet another embodiment of the assembly method further includes
the second test eddy current coil 140 and the second opposing test
eddy current coil 142. Advantageously, this arrangement is
configured to detect cracks, monitor their propagation from the
fastener, and estimate the crack length.
[0067] An embedded eddy current inspection system 200 embodiment of
the invention includes the eddy current probe 10 embedded between
the upper and the lower layer 110, 120, as exemplarily illustrated
in FIG. 9. The eddy current probe includes the test eddy current
coil 40. Exemplary test coils include SECAPs and ECAPs. The
embedded eddy current inspection system further includes a signal
generator 210, as schematically illustrated in FIG. 9, for
energizing the test coil. An exemplary signal generator supplies an
AC signal. SECAP and ECAP coils are energized by signals having
amplitudes of about 5V to about 10 V and frequencies in the range
of about 500 KHz to about 6 MHz.
[0068] The embedded inspection system 200 further includes a
comparison module 220, for comparing a signal received from the
test coil 40 with a reference signal and outputting a compared
signal. One exemplary comparison module is an eddy current
instrument (also indicated by reference number 220).
[0069] According to embodiment of the embedded eddy current
inspection system 200, the eddy current probe 10 further includes
the reference eddy current coil 60. Exemplary reference coils
include SECAPs and ECAPs. The reference coil is energized by the
signal generator simultaneously with the test coil 40 and supplies
the reference signal(s) to the comparison module.
[0070] According to a more specific embodiment, the embedded
inspection system 200 further includes a computer 230, as
illustrated in FIG. 9. The computer is configured to control the
signal generator 210.
[0071] Advantageously, the embedded inspection system 200 can be
used to inspect otherwise inaccessible conductive layers of a
multilayer component structure 100 without requiring disassembly of
the component structure. Consequently, the embedded inspection
system permits repeated eddy current inspection of lap joints and
other component structures including rivets or other fasteners or
stress points, without requiring time, money, and labor intensive
disassembly of the overall structure, such as an aircraft.
[0072] While only certain features of the invention have been
illustrated and described herein, many modifications and changes
will occur to those skilled in the art. It is, therefore, to be
understood that the appended claims are intended to cover all such
modifications and changes as fall within the true spirit of the
invention.
* * * * *