U.S. patent application number 11/292146 was filed with the patent office on 2007-05-24 for magnetic field characterization of stresses and properties in materials.
Invention is credited to Neil J. Goldfine, David C. Grundy, Robert J. Lyons, Darrell E. Schlicker, Ian C. Shay, Vladimir Tsukernik, Andrew P. Washabaugh, Vladimir A. Zilberstein.
Application Number | 20070114993 11/292146 |
Document ID | / |
Family ID | 31999560 |
Filed Date | 2007-05-24 |
United States Patent
Application |
20070114993 |
Kind Code |
A1 |
Goldfine; Neil J. ; et
al. |
May 24, 2007 |
Magnetic field characterization of stresses and properties in
materials
Abstract
Described are methods for monitoring of stresses and other
material properties. These methods use measurements of effective
electrical properties, such as magnetic permeability and electrical
conductivity, to infer the state of the test material, such as the
stress, temperature, or overload condition. The sensors, which can
be single element sensors or sensor arrays, can be used to
periodically inspect selected locations, mounted to the test
material, or scanned over the test material to generate
two-dimensional images of the material properties. Magnetic field
or eddy current based inductive and giant magnetoresistive sensors
may be used on magnetizable and/or conducting materials, while
capacitive sensors can be used for dielectric materials. Methods
are also described for the use of state-sensitive layers to
determine the state of materials of interest. These methods allow
the weight of articles, such as aircraft, to be determined.
Inventors: |
Goldfine; Neil J.; (Newton,
MA) ; Shay; Ian C.; (Cambridge, MA) ;
Schlicker; Darrell E.; (Watertown, MA) ; Washabaugh;
Andrew P.; (Chula Vista, CA) ; Grundy; David C.;
(Reading, MA) ; Lyons; Robert J.; (Boston, MA)
; Zilberstein; Vladimir A.; (Chestnut Hill, MA) ;
Tsukernik; Vladimir; (West Roxbury, MA) |
Correspondence
Address: |
HAMILTON, BROOK, SMITH & REYNOLDS, P.C.
530 VIRGINIA ROAD
P.O. BOX 9133
CONCORD
MA
01742-9133
US
|
Family ID: |
31999560 |
Appl. No.: |
11/292146 |
Filed: |
November 30, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10441976 |
May 20, 2003 |
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11292146 |
Nov 30, 2005 |
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60382447 |
May 21, 2002 |
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60384006 |
May 28, 2002 |
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60388103 |
Jun 11, 2002 |
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Current U.S.
Class: |
324/239 |
Current CPC
Class: |
G01N 27/9013
20130101 |
Class at
Publication: |
324/239 |
International
Class: |
G01R 33/12 20060101
G01R033/12 |
Goverment Interests
GOVERNMENT SUPPORT
[0002] The invention was supported, in whole or in part, by a grant
F09650-01-M-0956 from the U.S. Air Force, and by NASA under
contract NAS1-02022. The Government has certain rights in the
invention.
Claims
1. A method for determining weight of an article, said method
comprising: disposing a sensor array comprising at least one linear
drive conductor with a plurality of sense elements proximate to a
surface of a magnetizable test material, the magnetizable material
transferring a load from the article; passing a time-varying
electric current through the drive conductor to create a magnetic
field; measuring the response from each sense element to determine
magnetic permeability; and, using a correlation between this
magnetic permeability and article weight to determine the article
weight.
2. The method as claimed in claim 1 wherein the article is an
aircraft.
3. The method as claimed in claim 1 wherein the properties are
monitored at selected locations on the article.
4. The method as claimed in claim 1 wherein the sensor is scanned
over the surface of the article.
5. The method as claimed in claim 1 wherein the sensor is mounted
to the surface of the article.
6. The method as claimed in claim 1 wherein the sensor is oriented
to measure the response parallel to the maximum principal
stresses.
7. The method as claimed in claim 2 wherein the correlation to
weight is determined by performing measurements as the aircraft is
loaded.
8. The method as claimed in claim 1 wherein the magnetizable test
material is a landing gear component.
9. A weight monitor for determining the weight of an article, the
weight monitor comprising: a sensor array, the sensor array
comprising at least one drive conductor with a plurality of sense
elements, a time-varying current being passed through the drive
conductor to create a magnetic field; a magnetizable test material,
the plurality of sense elements being proximate to a surface of the
magnetizable test material, and the magnetizable test material
transferring a load from the article; and correlation electronics
using a measured response from each sense element to determine a
magnetic permeability and correlate the magnetic permeability to
determine the article weight.
10. The weight monitor of claim 9 wherein the article is an
aircraft.
11. The weight monitor of claim 10 wherein the correlation to
weight is determined by performing measurements as the aircraft is
loaded.
12. The weight monitor of claim 9 wherein the magnetizable test
material is a loading gear component.
13. The weight monitor of claim 9 wherein the sensor is oriented to
measure the response parallel to maximum principal stresses.
14. A weight monitor for determining the weight of an article, the
weight monitor comprising: a magnetizable receiving means for
receiving a load from an article, the load changing a magnetic
permeability of the magnetizable receiving means; a sensing means
for sensing the magnetic permeability of the magnetizable receiving
means; and a correlation means for correlating the magnetic
permeability to the weight of the article.
Description
RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 10/441,976, filed May 20, 2003, which claims the benefit of
U.S. Provisional Application No. 60/382,447, filed May 21, 2002,
U.S. Provisional Application No. 60/384,006, filed May 28, 2002,
and U.S. Provisional Application No. 60/388,103, filed Jun. 11,
2002. The entire teachings of the above applications are
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0003] The technical field of this application is that of
nondestructive materials characterization, particularly
quantitative, model-based characterization of surface,
near-surface, and bulk material condition for flat and curved parts
or components using magnetic field based or eddy-current sensors.
Characterization of bulk material condition includes (1)
measurement of changes in material state, i.e., degradation/damage
caused by fatigue damage, creep damage, thermal exposure, or
plastic deformation; (2) assessment of residual stresses and
applied loads; and (3) assessment of processing-related conditions,
for example from aggressive grinding, shot peening, roll
burnishing, thermal-spray coating, welding or heat treatment. It
also includes measurements characterizing material, such as alloy
type, and material states, such as porosity and temperature.
Characterization of surface and near-surface conditions includes
measurements of surface roughness, displacement or changes in
relative position, coating thickness, temperature and coating
condition. Each of these includes detection of electromagnetic
property changes associated with either microstructural and/or
compositional changes, or electronic structure (e.g., Fermi
surface) or magnetic structure (e.g., domain orientation) changes,
or with single or multiple cracks, cracks or stress variations in
magnitude, orientation or distribution.
[0004] A specific application of these techniques is the inspection
of high-strength steel components with the goal of measuring
applied and residual stresses and detecting early stage fatigue
damage or hydrogen embrittlement. Highly stressed aircraft
components, such as landing gear components, require the use of
steels such as 4340M and 300M heat treated to very high strength
levels. The integrity of these components is critical to the safe
operation of aircraft and for maintaining readiness of military
aircraft. However, unintentional loading of these components, such
as a hard landing, can impart residual stresses that compromise the
integrity of the component. Similarly, the mechanical properties of
these ultra-high strength steels can be seriously degraded as a
result of the ingress of hydrogen. Hydrogen ingress can occur
during pickling or plating operations and also during cleaning with
citric acid based maintenance solutions. The resulting hydrogen
embrittlement is unpredictable and can cause catastrophic failure
of the component. Hydrogen embrittlement has been established as
the direct cause of numerous landing gear failures. This similarly
applies to related degradation mechanisms such as temper
embrittlement, creep and other degradation processes that reduce a
materials functional behavior.
[0005] The detrimental effects of hydrogen on material properties
and component integrity have been observed in a wide range of
metals, as described for example in Interrante and in Hydrogen in
Metals. Management of high-strength steel components embrittled by
hydrogen is made more difficult by the fact that failures are
typically delayed, occurring some time after ingress of atomic
hydrogen. The delay between exposure to hydrogen and failure of a
high strength steel component depends on a number of factors. Among
these are the levels of hydrogen concentration, tensile stress,
temperature, stress gradients, and certain impurities in the steel,
as well as the type, concentration, and size of certain crystal
lattice defects and inclusions. Moreover, susceptibility to
hydrogen embrittlement can vary significantly between different
heats of steels and between different pours from a given heat, as
described by Lawrence. Hydrogen concentration on the order of a few
parts per million is sufficient to cause hydrogen embrittlement and
delayed fracture. Once atomic hydrogen enters the steel, excess
hydrogen atoms diffuse to inclusions, preexisting defects, and
zones of high dislocation density. Some hydrogen atoms, as a result
of stress-assisted diffusion, can cluster and form "platelets"
leading to initiation of microcracks. When such platelets form in
front of a crack tip, they facilitate crack extension. Critical
regions where hydrogen cracks are more likely to initiate are
notches or other stress raisers where local hydrogen concentration
is higher due to enhanced diffusion into the triaxially stressed
region in front of a stress raiser. Cracks at these critical
locations often initiate close to but beneath the surface, making
them more difficult to detect.
[0006] A recent review of existing magnetic/electromagnetic,
diffraction, ultrasonic and other methods for assessment of
residual stresses in steel components by Bray highlighted strengths
and weaknesses of the available methods. This review also indicated
that practical and cost-effective methods for assessment of
residual stresses as well as for monitoring of applied stresses
over wide areas in steel components are not yet available.
Typically, discrete strain gages are mounted directly onto the
material under test (MUT). However this requires intimate fixed
contact between the strain gage and the MUT and individual
connections to each of the strain gages, both of which limit the
potential usefulness for monitoring stress over large areas.
Furthermore, strain gages are limited in durability and do not
always provide sufficient warning of gage failure or malfunction.
Possible correlations between magnetic properties and stresses in
ferromagnetic materials have been studied for over 100 years, as
reviewed by Bozorth. Magnetostriction effect data suggests that,
depending on the magnitude and sign of the magnetostriction
coefficient, correlation between stress and magnetic permeability
within certain ranges of the magnetic field should be present.
However, attempts to use conventional inductive, i.e., eddy-current
sensors for assessment of residual stresses as well as for a number
of other applications have shown serious limitations, particularly
for complex geometry components.
[0007] Conventional eddy-current sensing involves the excitation of
a conducting winding, the primary, with an electric current source
of prescribed frequency. This produces a time-varying magnetic
field at the same frequency, which in turn is detected with a
sensing winding, the secondary. The spatial distribution of the
magnetic field and the field measured by the secondary is
influenced by the proximity and physical properties (electrical
conductivity and magnetic permeability) of nearby materials. When
the sensor is intentionally placed in close proximity to a test
material, the physical properties of the material can be deduced
from measurements of the impedance between the primary and
secondary windings. Traditionally, scanning of eddy-current sensors
across the material surface is then used to detect flaws, such as
cracks. Conventional eddy-current sensors widely used in
nondestructive testing applications are effective at examining near
surface properties of materials, but have a limited capability to
examine material property variations deep within a material. In
contrast, ultrasonic techniques that are also widely used are
effective at measuring property variations deep within a material,
but have limited sensitivity near the surface and behind some
geometric features such as air gaps.
SUMMARY OF THE EMBODIMENT OF THE INVENTION
[0008] Aspects of the embodiment of the invention described herein
involve novel sensors and sensor arrays for the measurement of the
near surface properties of conducting and/or magnetic materials.
These sensors and arrays use novel geometries for the primary
winding and sensing elements that promote accurate modeling of the
response and provide enhanced observability of property changes of
the test material.
[0009] Methods are described herein for the monitoring of material
properties as they are changed during processing. This can involve
disposing an eddy current sensor or sensor array in proximity to
the test material and converting the response of each sensor or
sense element into an effective material property. In one
embodiment, the sense elements are sensing coils the respond to
absolute changes in the magnetic field response. Preferably, these
coils are rectangular. In another embodiment, the sense elements
incorporate GMR sensors. In a preferred embodiment, the leads to
the sense elements also have a proximate set of leads that permit
cancellation of the stray magnetic flux to the leads and permits
the use of small sense elements. In one embodiment of the
invention, the process being monitored is the heat treatment of a
material. In one embodiment, the effective property is the
electrical conductivity. In another, it is the lift-off. In one
embodiment, the measurements are performed at multiple excitation
frequencies. In another embodiment, the sensor is not in contact
with the surface of the test material, which helps to minimize any
effects the monitoring system may have on the environment around
the material being processed.
[0010] Another aspect of the invention includes methods for
monitoring stresses in materials. Preferably, this is performed
with eddy current sensors disposed in proximity to the surface of
the test material or embedded within layers of the test material.
In one embodiment, measurements are performed with multiple
orientations of the sensor relative the stress distribution. In
another embodiment, the measurement orientations are perpendicular.
Preferably, the orientations correspond to the maximum and minimum
principal stresses, or to maximum and minimum property values that
are to be determined. In yet another embodiment, the sensor or
sensor array is mounted on a flexible substrate. The sensor can
have a foam backing, which permits sensor conformability to the
test material surface, or a rigid backing that approximates the
shape of the material surface and permits contact or non-contact
measurements. In one embodiment, the sensor has a plurality of
sense elements which may be aligned with one another. In another
embodiment, the sensor is scanned over the surface, preferably in
multiple orientations, so that the entire stress distribution and
orientation of stresses over the material can be resolved. In a
preferred embodiment, the sensor arrays are scanned in two mutually
perpendicular orientations, preferably with the orientations
corresponding to the directions of maximum and minimum principal
stresses.
[0011] Alternatively, the sensors or sensor arrays can be mounted
in one or more locations to monitor the stresses. When at least two
sensors are mounted in different locations, the sensors can have
different orientations. Preferably, the orientations are mutually
perpendicular, which also permits the monitoring of maximum and
minimum principal stresses. In one embodiment, stress measurements
are performed at multiple excitation frequencies. In another
embodiment, the electrical property of interest is the magnetic
permeability.
[0012] Other aspects of the invention include methods for the
inspection of magnetic materials. In one embodiment, an eddy
current sensor is placed proximate to a test material to measure
the magnetic permeability. In one embodiment, the sensor response
is measured at multiple locations to determine the magnetic
permeability distribution of the material surface. In another
embodiment, the permeability is measured with different sensor
orientations, in another it is measured with sensors placed at
different locations. In one embodiment, the sensor has a plurality
of sense elements that may be aligned with one another. In another
embodiment, the sensor is scanned over the surface, preferably in
multiple orientations, so that the material property distribution
and orientation over the material can be resolved. In another
embodiment, the sensor array is mounted to the surface. Preferably,
at least one more sensor array is mounted with a different
orientation.
[0013] Yet another method is described for the monitoring of the
weight of an article, by measuring the magnetic permeability of a
portion of the test material that transfers the mechanical load
from the article and correlating this permeability with the article
weight. Preferably, the article is an aircraft. In one embodiment,
measurements are performed at several select locations on the
material. In one embodiment, the sensors are mounted to the
material In another, the sensors are scanned over the surface of
the material. Preferably, the sensors are scanned so that the
permeability is measured in a direction that is parallel to the
direction of maximum principal stress.
[0014] Also described are methods for monitoring of the state of a
material or a test article. In one embodiment, a sensor is placed
in proximity to a test material to measure an electrical property
of the state of the article. In one embodiment, the electrical
property is the magnetic permeability and in another the electrical
property is the electrical conductivity. In one embodiment, the
state is stress and in another it is temperature. In another
embodiment the state is an overload condition. Preferably, this
overload condition results from excessive temperature expose as a
thermal overload or from over stressing as a mechanical overload.
Alternatively, the state can reflect the accumulation of fatigue
damage of the presence of a crack within the test article. In
another embodiment, the state of the article may be inferred from
state-sensitive material layers. These layers may be placed on the
surface of the article or embedded within layers of the article. In
one embodiment, the state-sensitive material can be split into
strips or some other geometry that facilitates a rapid inspection
or indication of state changes. Preferably, the strips have
different orientations with depth into the test article so that the
depth of the state change can also be readily determined. In a
further embodiment, these methods are used to remotely monitor the
state or properties of a hidden material so that other material
layers are present between the sensor and the material layer of
interest.
[0015] In one embodiment for monitoring the properties of the
materials, the sensors can be eddy current sensors. In one
embodiment, the sensors have separate layers for the drive winding
and sense elements. In another embodiment, the conductors for the
drive and sense elements are placed in different layers on the test
article itself. In another embodiment, the sensor is an array of
eddy current sensors. These sensors can be mounted to the surface
of the material or scanned over the surface of the article. In
another embodiment, the sensor is a dielectric sensor. In another,
it is a GMR sensor. These sensors can also be protected from
environmental damage by durable layers. Suitable durable materials
depend upon the type of sensor and the environment. In one
embodiment, the durable medium is a ceramic. In another, it is a
stainless steel.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The foregoing and other objects, features and advantages of
the embodiment of the invention will be apparent from the following
more particular description of preferred embodiments of the
invention, as illustrated in the accompanying drawings in which
like reference characters refer to the same parts throughout the
different views. The drawings are not necessarily to scale,
emphasis instead being placed upon illustrating the principles of
the embodiment of the invention.
[0017] FIG. 1 is a drawing of a spatially periodic field
eddy-current sensor.
[0018] FIG. 2 is an expanded view of the drive and sense elements
for an eddy-current array having offset rows of sensing
elements.
[0019] FIG. 3 is an expanded view of the drive and sense elements
for an eddy-current array having a single row of sensing
elements.
[0020] FIG. 4 is an expanded view of an eddy-current array where
the locations of the sensing elements along the array are
staggered.
[0021] FIG. 5 is an expanded view of an eddy current array with a
single rectangular loop drive winding and a linear row of sense
elements on the outside of the extended portion of the loop.
[0022] FIG. 6 shows a representative measurement grid relating the
magnitude and phase of the sensor terminal impedance to the
lift-off and magnetic permeability.
[0023] FIG. 7 shows a representative measurement grid relating the
magnitude and phase of the sensor terminal impedance to the
lift-off and electrical conductivity.
[0024] FIG. 8 shows a plot of MWM measured permeability scans along
the axis of a 4340 steel tensile specimen containing semicircular
notches, at two levels of applied stress. The distance along the
scan is in inches.
[0025] FIG. 9 shows a plot of MWM measured permeability for a five
load-unload sequence.
[0026] FIG. 10 shows a plot of MWM measured permeability for the
loads achieved on the increasing and decreasing portions of a
load-unload sequence.
[0027] FIG. 11 shows the MWM measured magnetic permeability versus
bending stress in a high-strength steel specimen at stresses from
-700 to 700 MPa. The specimen was shot peened.
[0028] FIG. 12 shows MWM measured transverse permeability changes
at incrementally increasing and decreasing tensile load (maximum
load=53.4 kN (12,000 lbs); increment=8.9 kN (2,000 lbs)).
[0029] FIG. 13 shows a plot of MWM measured transverse permeability
changes for a cyclically changing tensile load.
[0030] FIG. 14 shows a plot of normalized permeability against the
number of fatigue cycles for a shot peened 4340 steel specimen.
[0031] FIG. 15 shows an image of the MWM measured permeability of
the fatigue damage zone at the end of the fatigue test.
[0032] FIG. 16 shows the effective permeability variation with
applied stress for coated and uncoated samples.
[0033] FIG. 17 shows MWM-Array measured permeability changes during
the complete loading/hydrogen exposure/unloading test cycle.
[0034] FIG. 18 shows an illustration of a lap joint with a
stress-sensitive material and a sensor array.
[0035] FIG. 19 shows an illustration of a non-contact measurement
of a stress-sensitive material.
[0036] FIG. 20 shows a layout for a single turn Cartesian geometry
GMR magnetomer.
[0037] FIG. 21 shows a layout for a multi-element GMR array.
[0038] FIG. 22 shows a schematic for remotely monitoring the
temperature of a plate.
[0039] FIG. 23 shows the top plate conductivity as a function of
temperature with and without compensation for changes in the
conductivity of the bottom plate, which is between the top plate
and the sensor.
[0040] FIG. 24 shows a plot of the top plate relative permeability
as a function of the top plate strain at varying levels of applied
stress.
[0041] FIG. 25 shows the layout of a GMR sensor and 6.4 mm thick
sample plate for simulating material loss.
[0042] FIG. 26 shows an image of material loss in a 6.4 mm
(0.25-in.) thick aluminum plate, generated with the GMR probe at
100 Hz. The four regions represent 3%, 5%, 10%, and 20% loss.
Distances and thickness estimates are in millimeters.
[0043] FIG. 27 shows the normalized thickness estimate for two
lengthwise scans through the centers of the 10% and 3% regions, and
the 20% and 5% regions.
[0044] FIG. 28 shows the MWM measured conductivity changes for Al
2024 at temperatures up to 270.degree. C.
[0045] FIG. 29 shows the temperature and conductivity history for
an Al 2024 coupon heat treatment.
[0046] FIG. 30 shows the MWM measured conductivity transient of Al
7075 during retrogression.
[0047] FIG. 31 shows a normalized permeability image obtained from
an MWM-Array scanned over a double-notched 4340 low-alloy steel
tensile specimen after failure in a tension test, with an
excitation frequency of 1 MHz and the extended portions of the
primary winding oriented parallel to the loading axis.
[0048] FIG. 32 shows a normalized permeability image obtained from
an MWM-Array scanned over a double-notched 4340 low-alloy steel
tensile specimen after failure in a tension test, with an
excitation frequency of 158 kHz and the extended portions of the
primary winding oriented parallel to the loading axis.
[0049] FIG. 33 shows a normalized permeability image obtained from
an MWM-Array scanned over a double-notched 4340 low-alloy steel
tensile specimen after failure in a tension test, with an
excitation frequency of 1 MHz and the extended portions of the
primary winding oriented perpendicular to the loading axis.
[0050] FIG. 34 shows an illustration of segments of state-sensitive
material embedded between layers of a test article.
[0051] FIG. 35 is a representative single wavelength interdigitated
electode dielectrometer with spatially periodic driven and sensing
electrodes of wavelength .lamda. that can measure dielectric
properties of the adjacent material.
DETAILED DESCRIPTION PREFERRED EMBODIMENTS
[0052] A description of preferred embodiments of the invention
follows.
[0053] The use of conformable and nonconformable eddy-current
sensors and sensor arrays is described herein for the
nondestructive characterization of materials, particularly as it
applies to the characterization of applied and residual stresses.
This includes surface mounted and scanning, contact and non contact
configuration. This sensing approach can be used to monitor the
material characteristics at a given location with single or
multiple sensing element sensors, sensor arrays and/or networks of
surface mounted sensors using hand-held probes, mounted into
automated scanners or as part of an embedded network. The sensors
can be mounted into a structure in proximity to a material under
test for monitoring the property changes while the material is
being stressed and fatigued. Alternatively, such embedded sensors
can be queried with instrumentation on a scheduled or unscheduled
basis with either no electronics on board or minimal electronics on
board, and by plugging in at an easy access location. The sensors
can also be used to detect process related changes in the material
properties, such as grinding burns in steels either as a part of
in-process monitoring or at any time after processing, i.e., during
quality control inspections or in service.
[0054] Aspects of this embodiment of the invention address
measurement of damage, conditions (e.g., from manufacture, rework
or repair), stresses microstructure changes and other material
properties through scanning in contact on non-contact surface
mounting, non contact mounting and even monitoring one layer or on
internal material property through another layer or external
material closer to the sensor. Applications include but are not
limited to test materials, coupons, components or systems, bridges,
aircraft (e.g., landing gear), towers, construction equipment,
nuclear reactor nozzles (e.g., I.D. or O.D., or J-welds),
submarines (e.g., temper embrittlement near welds) automobile
components, tires (e.g., internal steel components), medical
devices or implants, and weapon casings.
[0055] A conformable eddy-current sensor suitable for these
measurements, the Meandering Winding Magnetometer (MWM.RTM.), is
described in U.S. Pat. Nos. 5,015,951, 5,453,689, and 5,793,206.
The MWM is a "planar," conformable eddy-current sensor that was
designed to support quantitative and autonomous data interpretation
methods. These methods, called grid measurement methods, permit
crack detection on curved surfaces without the use of crack
standards, and provide quantitative images of absolute electrical
properties (conductivity and permeability) and coating thickness
without requiring field reference standards (i.e., calibration is
performed in "air," away from conducting surfaces). MWM sensors and
MWM-Arrays can be used for a number of applications, including
fatigue monitoring and inspection of structural components for
detection of flaws, degradation and microstructural variations as
well as for characterization of coatings and process-induced
surface layers. Characteristics of these sensors and sensor arrays
include directional multi-frequency magnetic permeability or
electrical conductivity measurements over a wide range of
frequencies, e.g., from 250 Hz to 40 MHz with the same MWM sensor
or MWM-Array, high-resolution imaging of measured permeability or
conductivity, rapid permeability or conductivity measurements with
or without a contact with the surface, and a measurement capability
on complex surfaces with a hand-held probe or with an automated
scanner. This allows the assessment of applied and residual
stresses as well as permeability variations in a component
introduced from processes such as grinding operations.
[0056] FIG. 1 illustrates the basic geometry of an the MWM sensor
16, a detailed description of which is given in U.S. Pat. Nos.
5,453,689, 5,793,206, and 6,188,218 and U.S. patent application
Ser. Nos. 09/666,879 and 09/666,524, both filed on Sep. 20, 2000,
the entire teachings of which are incorporated herein by reference.
The sensor includes a primary winding 10 having extended portions
for creating the magnetic field and secondary windings 12 within
the primary winding for sensing the response. The primary winding
is fabricated in a spatially periodic pattern with the dimension of
the spatial periodicity termed the spatial wavelength .lamda.. A
current is applied to the primary winding to create a magnetic
field and the response of the MUT to the magnetic field is
determined through the voltage measured at the terminals of the
secondary windings. This geometry creates a magnetic field
distribution similar to that of a single meandering winding. A
single element sensor has all of the sensing elements connected
together. The magnetic vector potential produced by the current in
the primary can be accurately modeled as a Fourier series summation
of spatial sinusoids, with the dominant mode having the spatial
wavelength .lamda.. For an MWM-Array, the responses from individual
or combinations of the secondary windings can be used to provide a
plurality of sense signals for a single primary winding construct
as described in U.S. Pat. Nos. 5,793,206 and Re. 36,986.
[0057] In another embodiment, eddy-current sensor arrays comprised
of one or more drive windings, (possibly a single rectangle) and
multiple sensing elements are used to inspect the test material.
Example sensor arrays are shown in FIG. 2 through FIG. 4 some
embodiments of which are described in detail in U.S. patent
application Ser. Nos. 10/102,620, filed Mar. 19, 2002, and
10/010,062, filed Mar. 13, 2001, the entire teachings of which are
incorporated herein by reference. These arrays include a primary
winding 70 having extended portions for creating the magnetic field
and a plurality of secondary elements 76 within the primary winding
for sensing the response to the MUT. The secondary elements are
pulled back from the connecting portions of the primary winding to
minimize end effect coupling of the magnetic field. Dummy elements
74 can be placed between the meanders of the primary to maintain
the symmetry of the magnetic field, as described in U.S. Pat. No.
6,188,218. When the sensor is scanned across a part or when a crack
propagates across the sensor, perpendicular to the extended
portions of the primary winding, secondary elements 72 in a primary
winding loop adjacent to the first array of sense elements 76
provide a complementary measurement of the part properties. These
arrays of secondary elements 72 can be aligned with the first array
of elements 76 so that images of the material properties will be
duplicated by the second array (improving signal-to-noise through
combining the responses or providing sensitivity on opposite sides
of a feature such as a fastener as described in-U.S. patent
application Ser. Nos. 10/102,620 and 10/010,062. Alternatively, to
provide complete coverage when the sensor is scanned across a part
the sensing elements, can be offset along the length of the primary
loop or when a crack propagates across the sensor, perpendicular to
the extended portions of the primary winding, as illustrated in
FIG. 2.
[0058] The dimensions for the sensor array geometry and the
placement of the sensing elements can be adjusted to improve
sensitivity for a specific inspection. For example, the effective
spatial wavelength or four times the distance 80 between the
central conductors 71 and the sensing elements 72 can be altered to
adjust the sensitivity of a measurement for a particular
inspection. For the sensor array of FIG. 2, the distance 80 between
the secondary elements 72 and the central conductors 71 is smaller
than the distance 81 between the sensing elements 72 and the return
conductor 91. An optimum response can be determined with models,
empirically, or with some combination of the two. An example of a
modified sensor design is shown FIG. 3. In this sensor array, all
of the sensing elements 76 are on one side of the central drive
windings 71. The size of the sensing elements and the gap distance
80 to the central drive windings 71 are the same as in the sensor
array of FIG. 2. However, the distance 81 to the return of the
drive winding has been increased, as has the drive winding width to
accommodate the additional elements in the single row of elements.
Increasing the distance to the return reduces the size of the
response when the return crosses a feature of interest such as a
crack. Another example of a modified design is shown in FIG. 4.
Here, most of the sensing elements 76 are located in a single row
to provide the basic image of the material properties. A small
number of sensing elements 72 are offset from this row to create a
higher image resolution in a specific location. Other sensing
elements are distant from the main grouping of sensing elements at
the center of the drive windings to measure relatively distant
material properties, such as the base material properties for
plates at a lap joint or a weld.
[0059] In one embodiment, the number of conductors used in the
primary winding can be reduced further so that a single rectangular
drive is used. As shown in FIG. 5, a single loop having extended
portions is used for the primary winding. A row of sensing elements
75 is placed on the outside of one of the extended portions. This
is similar to designs described in U.S. Pat. No. 5,453,689 where
the effective wavelength of the dominant spatial field mode is
related to the spacing between the drive winding and sensing
elements. This spacing can be varied to change the depth of
sensitivity to properties and defects. In one embodiment this
distance is optimized using models to maximize sensitivity to a
feature of interest such as a buried crack or stress at a specific
depth. Advantages of the design in FIG. 42 include a narrow drive
and sense structure that allows measurements close to material
edges and non-crossing conductor pathways so that a single layer
design can be used with all of the conductors in the sensing region
in the same plane. The width of the conductor 91 farthest from the
sensing elements can be made wider in order to reduce an ohmic
heating from large currents being driven through the drive winding.
In another embodiment additional rows of sense elements can be
placed on the opposite side of the drive 71 at the same or
different distances from the drive. In another embodiment sensing
elements can be placed in different layers to provide multiple
lift-offs at the same or different positions.
[0060] The MWM sensor and sensor array structure can be produced
using micro-fabrication techniques typically employed in integrated
circuit and flexible circuit manufacture. This results in highly
reliable and highly repeatable (i.e., essentially identical)
sensors, which has inherent advantages over the coils used in
conventional eddy-current sensors. As indicated by Auld and
Moulder, for conventional eddy-current sensors "nominally identical
probes have been found to give signals that differ by as much as
35%, even though the probe inductances were identical to better
than 2%"[Auld, 1999]. This lack of reproducibility with
conventional coils introduces severe requirements for calibration
of the sensors (e.g., matched sensor/calibration block sets). In
contrast, duplicate MWM sensor tips have nearly identical magnetic
field distributions around the windings as standard
micro-fabrication (etching) techniques have both high spatial
reproducibility and resolution. As the sensor was also designed to
produce a spatially periodic magnetic field in the MUT, the sensor
response can be accurately modeled which dramatically reduces
calibration requirements. For example, calibration in air can be
used to measure an absolute electrical conductivity without
calibration standards, which makes the sensor geometry well-suited
to surface mounted or embedded applications where calibration
requirements will be necessarily relaxed.
[0061] For applications at temperatures up to 120.degree. C.
(250.degree. F.), the windings are typically mounted on a thin and
flexible substrate, producing a conformable sensor. A higher
temperature version has shown a good performance up to about
270.degree. C. (520.degree. F.). In another embodiment these
sensors might be fabricated on ceramic substrates or with platinum
leads and Boron Nitrite coatings or other means to extend their
operating temperature range. The sensors, which are produced by
microfabrication techniques, are essentially identical resulting in
highly reliable and highly repeatable performance with inherent
advantages over the coils used in conventional eddy-current sensors
providing both high spatial reproducibility and resolution. For
conformable sensors, the insulating layers can be a flexible
material such as Kapton.TM., a polyimide available from E. I.
DuPont de Nemours Company, while for high temperature applications
the insulating layers can be a ceramic such as alumina.
[0062] For measuring the response of the individual sensing
elements in an array, multiplexing between the elements can be
performed. However, this can significantly reduce the data
acquisition rate so a more preferably approach is to use an
impedance measurement architecture that effectively allows the
acquisition of data from all of the sense elements in parallel.
Furthermore, ability to measure the MUT properties at multiple
frequencies extends the capability of the inspection to better
characterize the material and/or geometric properties under
investigation. This type of instrument is described in detail in
U.S. patent application Ser. No. 10/155,887, filed May 23, 2002,
the entire teachings of which are incorporated herein by reference.
The use of multiple sensing elements with one meandering drive and
parallel architecture measurement instrumentation then permits high
image resolution in real-time and sensitivity with relatively deep
penetration of fields into MUT.
[0063] An efficient method for converting the response of the MWM
sensor into material or geometric properties is to use grid
measurement methods. These methods map the magnitude and phase of
the sensor impedance into the properties to be determined and
provide for a real-time measurement capability. The measurement
grids are two-dimensional databases that can be visualized as
"grids" that relate two measured parameters to two unknowns, such
as the magnetic permeability (or electrical conductivity) and
lift-off (where lift-off is defined as the proximity of the MUT to
the plane of the MWM windings). For the characterization of
coatings or surface layer properties, three- (or more)-dimensional
versions of the measurement grids called lattices and hypercubes,
respectively, can be used. Alternatively, the surface layer
parameters can be determined from numerical algorithms that
minimize the least-squares error between the measurements and the
predicted responses from the sensor, or by intelligent
interpolation search methods within the grids, lattices or
hypercubes.
[0064] An advantage of the measurement grid method is that it
allows for real-time measurements of the absolute electrical
properties of the material and geometric parameters of interest.
The database of the sensor responses can be generated prior to the
data acquisition on the part itself, so that only table lookup and
interpolation operations, which are relatively fast, needs to be
performed. Furthermore, grids can be generated for the individual
elements in an array so that each individual element can be
lift-off compensated to provide absolute property measurements,
such as the electrical conductivity. This again reduces the need
for extensive calibration standards. In contrast, conventional
eddy-current methods that use empirical correlation tables that
relate the amplitude and phase of a lift-off compensated signal to
parameters or properties of interest, such as crack size or
hardness, require extensive calibrations using standards and
instrument preparation.
[0065] For ferromagnetic materials, such as most steels, a
measurement grid provides conversion of raw data to magnetic
permeability and lift-off. A representative measurement grid for
ferromagnetic materials (e.g., carbon and alloy steels) is
illustrated in FIG. 6. A representative measurement grid for a
low-conductivity nonmagnetic alloy (e.g., titanium alloys, some
superalloys, and austenitic stainless steels) is illustrated in
FIG. 7. For coated materials, such as cadmium and cadmium alloys on
steels, the properties of the coatings can be incorporated into the
model response for the sensor so that the measurement grid
accurately reflects, for example, the permeability variations of
substrate material with stress and the lift-off. Lattices and
hypercubes can be used to include variations in coating properties
(thickness, conductivity, permeability), over the imaging region of
interest. In one embodiment, the variation in the coating is
corrected at each point in the image to improve the measurement of
permeability in the substrate for the purpose of imaging
stresses.
[0066] Robust directional magnetic permeability measurements by MWM
sensors and MWM-Arrays with grid methods allow estimation of
stresses by taking advantage of the magnetostriction effect. For
steels, at magnetic fields typical of those used for MWM, the
magnetostriction coefficient generally is positive, so that the
magnetic permeability increases with stress. Thus, once a
correlation between stress and MWM measured magnetic permeability
is established, stresses can be estimated as long as baseline
information is available. In another embodiment, bias fields or DC
offsets in the drive current (possibly using a multiple turn wound
or etched drive winding) can be used to move up the B-H curve away
from the zero field location to improve performance.
[0067] An example of a permeability measurement scan with a single
element sensor over a 4340 steel dogbone specimen with semicircular
notches installed in a 90 kN (20,000-lb) Instron frame is shown in
FIG. 8. In this case, the highest stress is expected at the
1.75-in. position with the highest estimated nominal stress in the
narrow section between the semicircular notches at 16 and 32 ksi,
respectively. Magnetic permeability measurements were performed
prior to each loading sequence, i.e., at no load and at various
levels of tensile load in an incremental load-unload sequence. The
results shown in FIG. 8 were obtained at a frequency of 1 MHz.
Multiple frequency MWM measurements can provide information on
stress distribution with depth. FIG. 9 shows permeability changes
in five load-unload sequences to a maximum estimated nominal stress
of 8, 24, 32, 48, and 49 ksi. The pattern of the magnetic
permeability changes actually reflects the loading pattern. The
permeability-load curves shown in FIG. 10 illustrates a hysteresis
between permeability measured at loads achieved on the increasing
and decreasing portions of a loading sequence. This hysteresis is
caused by a "delay" in rotation of magnetic domains on
unloading.
[0068] MWM permeability measurements on 300M high-strength steel
specimens under fully reversed bending loading provide further
indication of the capability of MWM sensors to perform stress
measurements. The tests were performed on flat shot-peened
specimens installed in a bending fixture. The stress range used in
the test was between -700 MPa in compression and 700 MPa in
tension. The stresses were determined from strains measured with a
BLH strain gage using BLH instrumentation. The strain gages were
attached to the "back" side. MWM magnetic permeability measurements
were performed with the longer segments of the MWM drive winding
perpendicular to the bending stress direction. In this orientation,
the MWM measures permeability in the specimen longitudinal
direction. FIG. 11 shows how the permeability measured at
frequencies of 40 kHz, 100 kHz, and 1 MHz changes with applied
bending stress. The data illustrate the sensitivity and quality of
the permeability measurements for stress measurements in high
strength steels over a wide range of stresses. The results clearly
show the sensitivity of the MWM measurements to stress changes and
reasonably small hysteresis, particularly in the compressive stress
range.
[0069] The capability to perform directional permeability
measurements allows characterization of both uniaxial and biaxial
stresses, as described for example in U.S. patent application Ser.
No. 10/351,978, filed Jan. 24, 2003, the entire teachings of which
are incorporated herein by reference. In the latter case, the MWM
permeability measurements at various sensor orientations reveal the
directions of the principal stresses. Furthermore, permeability
data from multifrequency MWM measurements can be used for
reconstruction of stress distribution with depth. For typical
excitation frequencies in the several kHz to several MHz range, the
depth of penetration of the magnetic field is limited to a fairly
thin layer near the surface, e.g., the first 0.5 mm (0.02 in.).
However, lowering the excitation frequency, for example down to
several Hz, and using alternative sensing elements such as
magnetoresistive or giant magnetoresistive sensors, as described
for example in U.S. patent application Ser. No. 10/045,650, filed
Nov. 8, 2001, the entire teachings of which are incorporated herein
by reference, permits measurements to a significantly greater
depth. Also, MWM-Arrays allow imaging of stress distributions over
wide areas.
[0070] The single layer designs of the drive and sensing elements
supports low cost fabrication without introducing excessive
requirements to align multiple layers. This significantly reduces
manufacturing costs and increases the number of suppliers that can
fabricate the sensors. However, to obtain reasonable signal to
noise levels for such single turn coils (simple rectangles) at low
frequencies, it is necessary to apply more current than is typical
for conventional eddy current sensors, e.g., 1 A. One practical
limitation on the sensing element size is fabrication costs (e.g.,
75 .mu.m line widths and larger are low cost with many suppliers,
while smaller line widths is more costly and limits available
suppliers). Another limitation is the relative contribution to the
signal of the flux coupled by the active sensing area to the flux
coupled by the relatively long leads. Thus, these leads are kept
close together and the novel "flux cancellation" design is used to
literally cancel the contribution from these long leads (thus
instead of two conductors entering each sensing element, there are
actually four conductors--two to sense the flux linked by the
sensing elements and the leads themselves, and the other two to
cancel the contribution from the leads, leaving just the response
of the sensing elements).
[0071] FIG. 12 shows the results of another set of tests
illustrating the magnetic permeability changes due to the Poisson's
effect or the transverse contraction under tensile axial load. A
7-channel MWM-Array was mounted on the specimen with the longer
segments of the MWM-Array drive oriented along the specimen axis,
i.e., parallel to tensile load orientation during tests, so that
the magnetic permeability in the transverse direction is measured.
In this test, the tensile load was first incrementally increased by
8.9 kN (2,000 lbs) to the maximun tensile load of 53.4 kN (12,000
lbs) and then incrementally decreased to 0. The estimated maximum
axial stress in the center of the area was about 700 MPa (100 ksi).
After each load increment, a constant load was maintained for a
period of time. The loading pattern and MWM-Array measured
transverse permeability in all seven channels is shown in FIG. 12.
The observed change in MWM-Array measured transverse permeability
appears to mimic changes in transverse strain. The lowest
permeability changes occur near the center. The results emphasize
the importance of permeability measurements and suggest that
bidirectional permeability measurements are critical to stress
measurements even under uniaxial loading. Similar results are
obtained with the cyclic loading pattern of FIG. 13, which had a
mean load of 8,000 lbs and load amplitude progressively increasing
from 1,000 lbs (load range of 2,000 lbs) to 4,000 lbs (load range
of 8,000 lbs).
[0072] The ability to detect and image stress distributions has
implications for the detection and imaging of early stage fatigue
damage as well. Fatigue tests of 4340 steel specimens revealed the
capability to detect precrack damage early in the fatigue life.
These specimens were designed with a cylindrical cavity in the gage
section, where an MWM-Array could be mounted, and reinforcement
ribs on the back side. This provides a nonuniform stress
distribution with the maximum stress in the central portion of the
cavity, as verified by a finite element analysis, beneath the
footprint of the MWM-Array. The shape and stress distribution
within the cylindrical cavity can be varied to simulate the
geometry of high strength steel components of interest. In one
embodiment MWM or MWM-Array sensors are oriented with their longer
winding segments aligned parallel or perpendicular to the direction
of likely fatigue crack orientation. The sensor aligned
perpendicular to this direction is most sensitive to fatigue damage
and crack monitoring, while the sensor with longer drive segments
parallel to this direction is most sensitive to stress (i.e.,
magnetic permeability is measured dominantly in the direction
perpendicular to the longer drive segments, while conductivity, or
induced current flow, is sensed dominantly parallel to the
direction or the longer winding segments). In another embodiment,
multiple series connected or multiplexed eddy current sensors, such
as MWM-Arrays, are mounted at selected critical and non critical
locations to support both fatigue and stress monitoring either
continuously or periodically or on a scheduled or unscheduled basis
depending on convenience or loading/fatigue/overload events.
[0073] In another embodiment, MWM-Arrays we used in a surface
mounted or even non contact (where lift-off is measured using grid
methods) to monitor stress and proximity (or vibrations). As with
strain gages or extensometers this information can be used to
control load frames, monitor changes in material properties or
structures, or monitor in service behavior and damage. Integration
of information with that from strain gages or extensometers can be
used to support decisions regarding fitness for service, material
life or to assess material performance in fatigue tests.
Combination with new fiber optic strain gages is also useful.
[0074] FIG. 14 shows the permeability changes during a test using a
7-channel MWM-Array. There is virtually no change in the measured
permeability up to 7,000 cycles. The change in the slope of MWM
measured permeability in the four centrally located channels at
about 7,000 cycles is most likely associated with residual stress
relaxation and precrack fatigue damage. This fatigue damage stage
extends, perhaps, up to 17,000 cycles followed by initiation and
extension of multiple microcracks. Two of the channels show a
significant permeability increase at 32,000 cycles indicating
coalescence of closely spaced cracks and faster crack growth. SEM
analysis on this specimen revealed a few small cracks, with the
longest crack approximately 200 .mu.m (0.008 in.) long. This crack
was also confirmed by fluorescent liquid penetrant inspection
(FPI). The FPI indication appeared as a tiny "speck" judged to be
on the order of 0.25-mm (0.01-in.) long.
[0075] The fatigue critical area of this specimen was also scanned
with an imaging MWM-Array, with the drive oriented perpendicular to
the axis of the coupon cavity. This orientation is perpendicular to
anticipated predominant orientation of fatigue cracks, and is the
same as in fatigue test monitoring of FIG. 14. FIG. 15 shows a
permeability image and aligned intermittent regions of increased
permeability having a combined length of about 20 mm (0.75-in.)
Three of these regions appear to contain short indications
characterized by the highest measured permeability. The other
relatively high permeability regions are likely to indicate stress
relaxation due to the cyclic loading and fatigue damage prior to
formation of detectable cracks. These regions of enhanced
permeability are also consistent with the higher stress region of
the component from the finite element analysis.
[0076] For cadmium-plated high-strength steel components, it is
important to account for the effect of the cadmium layer on the MWM
measurements. This is illustrated in FIG. 16, where a coating model
was applied to the multiple frequency data obtained from MWM
measurements on the 300M high strength steel specimens.
Qualitatively, this data (from 39.8 kHz to 1 MHz) showed a decrease
in the effective permeability and lift-off compared to measurements
on the uncoated specimen. This is consistent with the presence of a
nonmagnetic conducting surface layer on magnetizable substrate. The
model assumed a Cd layer (electrical conductivity of 22%IACS, 12.76
MS/m) on top of a magnetizable substrate (electrical conductivity
of 3.4%IACS, 2 MS/m), so that the unknowns in this model were the
lift-off, Cd layer thickness, and permeability of the substrate
(steel). The stress distribution, and hence the magnetic
permeability, is not necessarily uniform with depth into the
substrate and definitely not uniform for a shot peened steel. As
the first step, the thickness of the Cd layer on an unstressed
sample was estimated using a least-squares minimization routine on
the multiple frequency data. In another embodiment a fast table
lookup within a lattice is used. Assuming a substrate permeability
of 57.1, the Cd thickness was estimate to be 1.5 .mu.m. Using this
thickness, substrate permeability/lift-off grids were then
generated so that the effective permeability of the substrate could
be determined. FIG. 16 shows permeability vs. stress curves for
non-plated steel, for Cd-plated steel using a model that does not
account for the Cd layer, and for Cd-plated steel using a model
that does account for the Cd layer. As shown in FIG. 16, using
grids that have a thin Cd layer can provide estimates of the
permeability that are similar between the coated and uncoated
samples. Without this compensation for the presence of the Cd
coating, the permeability estimates are significantly reduced for
the coated sample.
[0077] The numerical value for the Cd layer thickness of 1.5 .mu.m
is small compared to the nominal thickness of 10-20 .mu.m because
of the assumed conductivity for the layer. For these relatively
thin layers and intermediate excitation frequencies, the
measurements are essentially sensitive to the product of the layer
thickness and electrical conductivity. For alloy layers (e.g.,
cadmium-titanium alloys) or for microstructural variations due, for
example, from porosity introduced during the coating process, the
electrical conductivity can be lower, in the range of 1.2-7.0 MS/m
(2-12 %IACS) and the corresponding thickness larger. The thicker Cd
layer values can be accommodated, without appreciably affecting the
permeability estimates, if a lower conductivity is used for the Cd
layer.
[0078] The properties of the coating material layer and even the
base material itself can be obtained from multiple parameter
estimation approaches. The use of multiple frequencies allows more
than two parameters to be estimated. As an example, three, four and
five parameter estimation routines have been developed for
determining the properties of coatings, such as MCrAlY coatings
used on turbine blades and vanes. As described in more detail in
the DOE Phase II proposal "Intelligent Probes for Enhanced
Non-Destructive Determination of Degradation in Hot-Gas-Path
Components," the entire teachings of which are incorporated herein
by reference, a four parameter estimation routine is used for
determining the coating electrical conductivity and thickness, the
sensor lift-off, and the substrate electrical conductivity for
nonmagnetizable materials. A five parameter algorithm that allows
determination of an additional parameter, e.g., magnetic
permeability when one of the layers is magnetizable is also
described. Clearly, this multiple parameter estimation approach can
be applied to different combinations of electrical and geometric
properties for the various layers. This also applies to shot peened
materials or materials with a degraded near-surface layer. For
example, it was successfully demonstrated for turbine blades with
two types of coatings. Since the processing time typically
increases as the number of properties to be estimated increases,
this approach for determining the layer and substrate properties
can be performed prior to continuous stress measurements so that
simpler models or measurement grids can be used while the
measurements are being performed. For example, this can involve
characterizing the coating thickness and conductivity using the
multiple parameter algorithm, using this information to generate
substrate permeability-lift-off grids, and then performing the
continuous stress measurements. Alternatively, rapid table look up
in lattices or hypercubes can be performed.
[0079] Measurements can also be performed while the hydrogen
embrittlement occurs. As an example, cathodic charging was used to
introduce hydrogen into the steel using a setup similar to some
previously described [Grendahl, 2002]. Here, a flat 4340 steel
specimen is placed in the bending fixture with a 0.4-liter vessel
with a 25 mm hole in the bottom mounted and sealed on the top of
the 4340 steel specimen. The specimen is loaded to stresses as high
as 150 ksi. A strain gage mounted to the compressive side of the
specimen provides an independent measure of the strain so the
bending stresses during loading and cathodic charging may be
monitored. After loading to the desired stress level, the
permeability is measured until an initial transient period passes.
Then approximately 0.3 liters of a 3.5% NaCl solution is poured
into the vessel. An electrochemical cell is formed with the 4340
specimen as the working electrode. A graphite rod serves as the
counter electrode and a standard calomel electrode (SCE) provides a
reference point for the electrochemical potential. The working
electrode, counter electrode, and SCE were connected to a
Schlumberger SI 1286 potentiostat and a selected potential between
-1.15 V and -1.25 V was applied to effect cathodic charging.
Magnetic permeability measurements with an MWM-Array can be
performed in either scanning mode or with a permanently mounted
MWM-Array, on the exposed (tension) side of the test specimen. This
type of in-process monitoring can be performed for other processes
that include material property changes.
[0080] FIG. 17 shows MWM-Array measured permeability changes during
the complete test cycle for one specimen. The normalized
permeability measurements were made with the 7-channel MWM-Array
attached on the tension side of the specimen. In this test, the
specimen was first monitored with the mounted MWM-Array before a
bending load was applied to the specimen. Then, the specimen was
loaded in bending to 700 MPa (100 ksi) stress level as measured
with a strain gage. The MWM-Array measured permeability increased
due to the stress from point a to point b. Between points b and c,
the specimen was monitored by the MWM-Array at the constant stress
level. Then, at point c, the vessel was filled with 3.5% NaCl
solution, the initial open circuit potential was measured,
adjustments made, and -1.15 V potential (relative to the standard
calomel reference electrode) was applied at point d. After an
initial period of a fairly stable magnetic permeability between
sets 120 and 160, i.e., for about three hours, a distinct and
steady increase in the MWM-Array measured permeability was observed
for the next nine hours, i.e., up to the time when polarization was
discontinued (up to point g). Note that the potential was changed
to -1.25 V at point f Between points g and h, the stress was
maintained, while there was no applied potential. Finally at point
h, the specimen unloading started.
[0081] Example sensor arrays are the MWM-Arrays shown in FIG. 2
through FIG. 5, although other array formats can also be used, such
as those described in U.S. patent application Ser. Nos. 09/666,879
and 09/666,524. These applications also describe using a magnetic
material in combination with the sensor as a load cell and
adjusting the sensitivity of the response through the selection of
the type and dimensions of the material. Alternatively, if the
changes in the stress distribution occur relatively slowly,
periodic measurement of the stress distribution can be performed.
This can be accomplished with occasional measurements with an
MWM-Array that has been mounted to a surface or by scanning
eddy-current sensing arrays over the surface to provide a complete
mapping of the material properties over the entire surface. In
addition, measurements in multiple orientations, preferably two
orthogonal orientations, can be performed to determine anisotropic
material property variations associated with changes in stresses in
the fasteners and can be determined with directional eddy-current
sensor arrays. The MWM-Array is one such example as the sensing
elements respond preferentially to the magnetic permeability
oriented perpendicular to the extended segments comprising the
primary winding. In another embodiment the larger drive segments
are oriented at an angle, e.g., 45 degrees, relative to the scan
direction. If cracks are likely to form in the direction
perpendicular to the scan direction, then this is necessary to
increase sensitivity to crack size or precrack damage.
[0082] In another embodiment an array is configured in an x or v so
that two drive orientations are provided in as two or one layer
respectively. An example of this configuration is described in U.S.
patent application Ser. No. 10/419,702, filed Apr. 18, 2003, the
entire teachings of which are incorporated herein by reference. In
one embodiment the legs of the v or x are at different angles
relative to the scan direction (or the x or v is rotated relative
to the scan direction) so that cracks respond differently and
material variations that are isotropic respond the same. This
improves discrimination capability. This can also be used for
buried object imaging.
[0083] Conventional eddy-current designs are not ideal for
permanent mounting. Conventional eddy-current techniques require
varying the proximity of the sensor (or lift-off) to the test
material or reference part by rocking the sensor back and forth or
scanning across a surface to configure the equipment settings and
display. For example, for crack detection the lift-off variations
are generally displayed as a horizontal line, running from right to
left, so that cracks or other material property variations appear
on the vertical axis. Affixing or mounting the sensors against a
test surface precludes this calibration routine. The probe-to-probe
variability of conventional eddy-current sensors prevents
calibrating with one sensor and then reconnecting the
instrumentation to a second (e.g., mounted) sensor for the test
material measurements. These shortcomings are overcome with
conformable eddy-current sensors that provide absolute property
measurements and are reproduced reliably using micro-fabrication
techniques. Calibrations can also be performed with duplicate
spatially periodic field sensors using the response in air or on
reference parts, which may simply be different areas of the same
component, prior to making the connection with the surface mounted
sensor. The capability to characterize fatigue damage in structural
materials, along with the continuous monitoring of crack initiation
and growth, has been demonstrated, as described in U.S. patent
application Ser. Nos. 09/666,879, 09/666,524, and 10/102,620. This
inspection capability is suitable for on-line fatigue tests for
coupons and complex components, as well as for monitoring of
difficult-to-access locations on both military and commercial
aircraft. In another embodiment, mounted sensors are removed to
perform other examinations (e.g., acetate replicas to defect small
cracks), and must be reattached using a reference calibration to
the previously recorded value. By recalibrating the sensor with the
same values as before removal, then the continuation of monitoring
will begin with the same values, e.g., permeability, as before the
sensor was removed and reinstalled.
[0084] Another aspect of this embodiment of the invention is the
inspection of nickel alloy engine materials, such as Alloy 738 or
Alloy 718, where shot peening and/or heat treatment may produce
near surface relative permeability greater than 1.0. This is also
described in U.S patent application Ser. No. 10/419,702. Higher
permeability regions are typically created near the surface by the
shot peening and/or heat treatment process. At sufficiently high
frequencies, the magnetic field is confined near the surface of the
MUT and reflects only the permeability (and stress) of the surface
region. At lower frequencies, the magnetic field can penetrate
through this region and the average or effective permeability is
reduced. At sufficiently low frequencies, the magnetic field
penetrates far enough into the base material that the permeability
approaches 1.0. High resolution images of permeability can then be
used to map residual stress variations to qualify shot peening or
other manufacturing processes or to assess material aging/material
degradation, as described in more detail in U.S. patent application
Ser. No. 10/351,978. Then, regions with unacceptable residual
stresses might be reworked (e.g., blending and reshot peening) to
extend life.
[0085] Another aspect of this embodiment of the invention relates
to the application of a stress-sensitive material to a test
material and monitoring the properties of this stress-sensitive
material to infer the stress distribution or mechanical load on the
test article. The stress sensitive material could be a magnetic
material in which the magnetic permeability changes significantly
with stress, as illustrated in FIG. 11. An alternative
stress-sensitive material is one whose electrical conductivity
changes significantly with stress. Also, different layers with
different sensitivity might be used, for example some layers with
permeability being sensitive, and other layers with conductivity
being sensitive. These materials could be magnetic or nonmagnetic;
In general, according to the literature on strain gages, metals
typically have a gage factor reflecting change in resistance per
unit strain of between 2 and 4. Representative values are listed in
Table 1. Preferable materials for nonmagnetic stress-sensitive
materials are platinum and platinum alloys because of the
relatively large gage factors. It should be noted that conductivity
variation with strain tends to become nonlinear for large strains
and the listed gage factors are most applicable to situations of
low strains. The choice of the stress-sensitive material can
therefore depend on the strains anticipated for the inspection.
TABLE-US-00001 TABLE 1 Gage factors for stress-sensitive conducting
materials. Material Composition Gage Factor Platinum 100% Pt 6.1
Platinum-Iridium 95% Pt, 5% Ir 5.1 Platinum-Tungsten 92% Pt, 8% W
4.0 Isoelastic 55.5% Fe, 36% Ni, 8% Cr, 0.5% Mo 3.6 Karma 74% Ni,
20% Cr, 3% Al, 3% Fe 2.4 Constantan 55% Cu, 45% Ni 2.0 Nichrome 80%
Ni, 20% Cu 2.0 Monel 67% Ni, 33% Cu 1.9 Manganin 84% Cu, 12% Mn, 4%
Ni 0.47 Nickel 100% Ni -12.1
[0086] Monitoring the properties of a stress-sensitive material
attached to a test material is most useful in situations where
direct nondestructive measurements of the stresses in the test
material are relatively difficult, such as in aluminum with
eddy-current sensors. In contrast, monitoring the permeability
changes of a layer of magnetic material or electrical conductivity
changes of a layer of stress-sensitive non-magnetic material
integrally attached to the test article can offer substantially
greater sensitivity. The properties of the attached layer material
can be monitored using a permanently mounted sensor or with a
scanning sensor array to create images of the stress distribution.
An illustration of this approach is given in FIG. 18, where the
stress-sensitive material 130 is affixed to the back of the lap
joint 100, which has layers 104 and 106 joined by fasteners 108.
The sensing elements 122 are shown in a linear array, but they
could also be distributed among and around the fasteners as well.
In this case, the drive winding is not shown. The measurements can
also be performed in a non-contact fashion, as shown in FIG. 19,
where an air gap 134 is maintained between the sensor or sensor
array 136 and the test material 132. In both FIG. 18 and FIG. 19,
the magnetic fields generated by the eddy-current sensor are
projected through the test material so that the remote fields
interact with the attached stress-sensitive layer and the sensor
and attached layer material effectively operate as a load cell. In
one embodiment, stress sensitive materials (or materials more
sensitive to other affects such as temperature) might be located on
the near and far side of a layer or at multiple depths within a
multiple layered construct. Also, as described latex strips,
composites or other directionally sensitive media might be used to
measure stress in different directions or to simply aid in
differentiating between affects at different depths.
[0087] The sensitivity of this measurement approach is affected by
the electrical and geometric properties of the stress-sensitive
layer attached to the test material. The material should be
selected so that the permeability or conductivity change for an
anticipated stress level is detectable with the sensor and
instrumentation. Furthermore, the material should be relatively
thin to better reflect the stress distribution of the test
material. However, it should also be thick enough to provide a
measurable signal with the sensor or sensor array. Selection of the
thickness of the layer must therefore balance these competing
effects. The magnetic or non-magnetic stress-sensitive material can
also be applied to the surface of the test material near the
sensor.
[0088] The properties of the stress-sensitive material and even the
base material itself that the coating is applied to can be obtained
from multiple parameter estimation approaches. The use of multiple
frequencies allows more than two parameters to be estimated. As an
example, three, four and five parameter estimation routines have
been developed for determining the properties of coatings, such as
MCrAlY coatings used on turbine blades and vanes, as described
above.
[0089] This type of state sensitive material layer could also be
applied other inspection or monitoring applications. Instead of
stress, the sensitive layer may have a greater temperature
sensitivity so that the temperature of the material can be
monitored. It may also permit the detection and characterization of
thermal or mechanical overload events that can compromise the
future use of the article.
[0090] As another alternative embodiment, in addition to inductive
coils, other types of sensing elements, such as Hall effect
sensors, magnetoresistive sensors, SQUIDS, and giant
magnetoresistive (GMR) sensors, can also be used for the
measurements. The use of GMR sensors for characterization of
materials is described in more detail in U.S. patent application
Ser. No. 10/045,650. While conventional eddy-current sensors are
effective at examining near surface properties of materials, but
have a limited capability to examine material property variations
deep within a material. GMR sensors respond to magnetic fields
directly, rather than through an induced response on sensing coils,
which permits operation at low frequencies, even DC, and deep
penetration of the magnetic fields into the test material. The GMR
sensors can be used in place of sensing coils, conventional
eddy-current drive coils, or sensor arrays. Thus, the GMR-based
sensors can be considered an extension of conventional eddy-current
technology that provides a greater depth of sensitivity to hidden
features and are not deleteriously affected by the presence of
hidden air gaps or delaminations. In an alternative embodiment
electric field sensors, IDEDs described in U.S. Pat. Nos. 4,814,690
and 6,380,747 and in U.S. patent application Ser. Nos. 10/040,797,
filed Jan. 7, 2002, and 10/225,406, filed Aug. 20, 2002, the entire
teachings of which are hereby incorporated by reference might be
used to monitor stress or temperature, moisture content or
contamination or overload of fatigue in adhesives, epoxies, glass,
oil, plastics and in single or multiple layered media. Here the
conductivity and dielectric constant or complex permittivity and
layer thicknesses are measured using the same methods as for
magnetic field sensing. In one such electric field method multiple
layers of material are added on top of a sensor each sensitive to
different chemicals or biological materials. In another embodiment
coatings on pharmaceuticals are characterized for manufactured
quality, such as density, moisture content or density of suspended
particles in a time release coating.
[0091] There are a variety of inspection applications that could
take advantage of the capabilities of a GMR-based sensing
technology. One is the imaging of damage, such as cracks,
inclusions, and corrosion, deep below the surface of conducting
materials typically found in aging aircraft. These measurements can
also be performed on magnetic materials where the skin depth is
relatively small when conventional eddy-current technology and
excitation frequencies are used. Since the GMR sensor responds to
magnetic field variation and the magnetic field created by a drive
coil or primary winding can readily penetrate air gaps, these
sensors can be used for non-contact and remote measurements where
air-gaps are present between conducting and/or magnetic material
layers. For example, measurements of the electrical properties,
such as the electrical conductivity or magnetic permeability, of
inaccessible materials can be used with correlation curves to
assess the temperature or stress of the hidden materials. As an
example, the use of direct temperature measurements for
characterizing the thermal response of inaccessible surfaces is
considered an ill-posed inverse heat conduction problem. In
contrast, low-frequency magnetic fields can penetrate the test
materials to inspect the remote surface with simultaneous
measurements at higher frequencies used to compensate for
near-surface property variations. This new capability may be
suitable for health monitoring for engines. An example is the
monitoring of internal temperatures in aircraft engines, for
example, in the compressor or turbine stages. This technology may
also be suitable for the non-contact remote monitoring of stresses
in ferrous materials, such as landing gear after a hard landing or
for monitoring the stresses on steel bolt in an aluminum structure
through the aluminum at relatively low frequencies or even at DC or
in other steel structures.
[0092] GMR-based magnetometers can be incorporated into a variety
of primary winding geometries, such as rectangular, circular, or
elliptical coils. An example rectangular or Cartesian-geometry
GMR-based magnetometer is illustrated in FIG. 20. A GMR array is
shown in FIG. 21. The parallel primary windings are laid out in a
fashion similar to the MWM-Array designs, which has shown
advantages in the generation of C-scan images.
[0093] The winding layout of FIG. 20 allows the relative polarity
of the two constituent current loops to be changed to generate a
structure with twice the effective wavelength. This permits
inspection at two different depths, at a single frequency. For the
results presented here, the currents were directed as shown, with
the center leg of the winding carrying twice the current of the two
edge (or return) windings. The two half-wavelength current loops
have identical areas and oppositely directed currents of equal
magnitude, so that their effective dipole moments cancel out,
resulting in no net dipole moment in the far field. Here the
dominant far-field moment is a quadrupole. This elimination of the
net dipole moment of the sensor improves measurement reliability
because it reduces the sensitivity of the magnetometer to objects
outside its nominal range of sensitivity.
[0094] The GMR sensing element requires biasing with a constant
magnetic field. To provide this bias, and to address the high
nonlinearity of the sensor's transfer characteristic, it is placed
in a feedback configuration with a secondary biasing coil. In this
way, the magnetic field at the GMR sensor remains nearly constant
during operation, eliminating the effect of the nonlinear transfer
characteristic, while maintaining sensitivity at low frequencies.
The magnitude of the current in the secondary winding is taken as
the output signal, and since the relationship between this current
and the magnetic field for an air-core winding is perfectly linear,
so is the transfer characteristic of the entire hybrid sensor
structure.
[0095] One example application using a GMR sensor is for monitoring
properties through intermediate layers of metal. In this case, the
absolute electrical properties are measured through thick metal
plates and then related to other physical properties of interest.
FIG. 22 shows one such layered geometry, with a low frequency (100
Hz) measurement used to remotely monitor the temperature dependent
conductivity variation of an aluminum plate through a 0.25-in.
thick aluminum plate. The thickness of the upper plate (remote from
the sensor), the conductivity and thickness of the bottom plate
(near the sensor), as well as its lift-off (proximity) from the
sensor windings, are incorporated in the model used to generate the
appropriate measurement grids. The two unknown properties are the
conductivity of the upper plate and the thickness of the thermally
insulating nonconducting spacer between the two plates, which also
varied significantly with the temperature of the upper plate. The
ability to measure the two unknown parameters independently is
demonstrated by taking measurements at room temperature with
spacers of varying thickness and demonstrating that the data follow
a constant-conductivity line in the grid. To verify and record the
actual plate temperatures, thermocouples were attached to both
metal plates. The top plate was initially chilled and then
gradually heated with a hot air gun. The data of FIG. 23 shows that
both the conductivity and spacer thickness are affected by the
plate temperature.
[0096] In this experiment, the temperature of the bottom plate also
increased, despite the thermal insulation. Ignoring this effect
yields the plot in FIG. 23 with cross symbols. To compensate for
the temperature variation of the bottom plate, data were also taken
at 10 kHz simultaneous with the 100 Hz measurement. At this higher
frequency the bottom plate appears infinitely thick since it is
more than several skin depths thick and a simple
conductivity/lift-off grid can be used to independently determine
the bottom plate's conductivity. Once this value is obtained, it
can be used in the estimation of the upper plate conductivity via a
three-dimensional measurement grid, called a grid lattice. Using
this method, the data shown with squares in FIG. 23 are obtained.
As expected, it follows a linear relationship. Example applications
of such remote temperature and stress measurement include: 1)
measuring temperature of carbon fibers in an epoxy matrix, 2)
measuring stress or temperature on fibers in a composite where the
fibers are coated with a magnetic or other material that can be
sensed using magnetic or electric fields, 3) measuring temperature
on the outside of a wing or other cavity from the inside using
sensors mounted on the inside with or without an added more
sensitive on the outside of the metal (or other material) layers,
4) measuring stress on a bearing race at the outside diameter (or
rolling surface) from the inside diameter or using an embedded
sensor, 5) measuring stress or temperature as a function of depth,
6) measuring stress or temperature variations with time and/or
space near an internal geometric feature such as a cooling hole or
at the inlet of an aircraft engine.
[0097] Another example of demonstrated measurements with a GMR
sensor is the monitoring of stress. In this case, measurements were
performed on a hidden steel layer in a thick structure. A 1.4 mm
thick steel plate was suspended over a 6.7 mm Al 6061 plate using a
3 mm thick spacer located in the center. A 5 kg weight was used to
keep the center part of the plate from moving. The measurement grid
used in this case was a permeability/spacer thickness grid. The
spacer thickness was one of the unknowns since it varied as the
steel plate was deformed under the applied force. Zero stress is
registered when the plate is placed on a flat surface. The measured
relative permeability as a function of the applied stress at the
bottom of the plate are shown in FIG. 24. This illustrates the
capability to measure stress (or strain) on a buried steel layer
through relatively thick intermediate aluminum and insulating
layers.
[0098] GMR-based sensors can also be used to measure material loss
of relatively thick parts. A representative sample is shown in FIG.
25. In this case, four flat-bottom circular areas were milled out
of a 6.4 mm (0.25-in.) aluminum alloy plate. The depths of the four
depressions, simulating material loss, are 3%, 5%, 10%, and 20%,
respectively. Using the GMR probe and an excitation frequency of
100 Hz, the thickness scan image in FIG. 26 was generated. The
orientation of the sensor probe is such that the primary windings
lie along the vertical direction, with the secondary positioned in
the left half-wavelength. The "double hump" signature produced by
this sensor is apparent. The plot in FIG. 27 shows the normalized
thickness estimate for two of the 220 individual lengthwise scans
that comprise the area scan, through the centers of the 10% and 3%
regions, and the 20% and 5% regions. The source of the
"double-hump" response to what is a square pulse in thickness
change, stems from the fact that the largest change to the signal
is observed when the perturbation (e.g., thickness change) falls
under a primary winding. The main lobe is generated by the response
to the center winding, which carries twice the current and is
closer to the secondary than the two outer winding legs. The second
smaller lobe is the response to the outer (or return) primary
winding on the opposite side of the secondary. It has the same
polarity, because both the current direction and the relative
position with respect to the secondary are reversed with respect to
the center winding. The response to the third leg of the primary
winding is weaker still and appears to the left of the main lobe.
Its polarity is reversed because while the current flows in the
opposite direction, it is on the same side of the secondary as the
center winding.
[0099] The edge effect, i.e. when part of the sensor footprint is
outside the sample area, is very prominent both in the plot and in
the scan image. It is much more pronounced in the horizontal
direction, because of the geometry of the probe, which is built to
appear uniform (and in principle infinite) in the direction
parallel to the longer drive winding segments. The edge effect is
more prominent on the left side, since the secondary sensing
element is closer to the left edge of the probe. The polarity of
the edge effect is opposite on the two sides as described above.
The vertical edge effect is not present in the scan image because
the image was normalized by the thickness in the area between the
140- and 150-mm positions.
[0100] The MWM sensors and sensor arrays can also be used to
monitor heat treatment of metals and for monitoring high
temperature tests on metals and alloys, metal processing and the
condition of elevated temperature components. FIG. 28 shows the
results of standard MWM configuration lift-off and conductivity
measurements of an aluminum 2024 coupon during various heat
treatments at temperatures up to 270.degree. C. The plots shows
conductivity versus temperature for an aluminum 2024 coupon as the
original T3 condition was overaged and then reheated. Following a
solution anneal to produce the T42 condition, the coupon was again
heated to monitor conductivity changes as the T62, T72 and overaged
conditions were achieved. These measurements used a single-channel
MWM sensor constructed of copper conductors on a Kapton.TM.
substrate. The sensors can also be fabricated onto ceramic
substrates, such as alumina, for even higher temperature operation.
Although not generally flexible, these ceramic substrate arrays can
be molded into the shape of the test article. FIG. 29 shows the
corresponding temperature and conductivity history, which
illustrates the capability of the sensor to monitor the processing
of the metal and the condition during the treatment. Similarly,
FIG. 30 illustrates the alloy condition changes during
retrogression. The conductivity drops dramatically after the coupon
is placed in the furnace. Then, after dwelling at or exposing the
coupon to an elevated temperature such as 400.degree. F. the coupon
is quenched. This results in a higher electrical conductivity than
was present initially, with the effect depending on the exposure
duration.
[0101] MWM-Arrays also provide a capability to perform
bi-directional magnetic permeability measurements in a scanning
mode. FIG. 31 through FIG. 33 provide images of the magnetic
permeability for a broken tensile specimen of 4340 low alloy steel.
The MWM-Array was scanned across and along the gage section of a
specimen broken in a tensile test and the permeability was measured
at two frequencies, 158 kHz for FIG. 32 and 1 MHz for FIG. 31 and
FIG. 33. In FIG. 31 and FIG. 32 the extended portions of the
primary winding were oriented parallel to the loading axis. In FIG.
33 the extended portions of the primary winding were oriented
perpendicular to the loading axis. This illustrates the potential
to map residual stress variations produced in parts fabricated from
carbon and low alloy steels, for example by a hard landing in the
latter case. Notice that the permeability images at low and high
frequencies reveal stress changes with distance from the surface. A
high residual stress region near the fracture is indicated in the
images of FIG. 33. To create these images, a permeability/lift-off
measurement grid was used, assuming a known conductivity and an
infinite half-space (i.e., the steel layer is assumed to be
infinitely thick). Since the lift-off or distance between the
sensing windings and the test material is being measured through
the measurement grids, the residual stress measurement can be
performed in a non-contact mode, which ensures that the sensor and
probe assembly do not influence the stress distribution on the
component.
[0102] These eddy current sensors and sensor arrays permit the
measurement and monitoring of stresses (applied and residual) in
steel components. The sensors can be used to inspect selected
locations on a part by placing the sensor over the area of
interest, scanning over the area, or permanently mounting or
affixing the sensor to the surface. By measuring with multiple
sensor orientations, the permeability and stress distribution can
be inferred. Preferably, the orientations are perpendicular to one
another so that the biaxial stress distribution is obtained. The
anisotropy is most easily obtained when the orientation of the
sensor or sensor array has the direction of greatest sensitivity
aligned with the directions of the maximum and minimum principal
stresses in the materials. When the sensors are flexible and can
conform to the complex geometry surfaces, the sensors can be
supported by a bottom foam support that makes the sensor
essentially flat until placed onto the surface. Alternatively, the
sensors can be molded into a fixture that conforms to or has a
shape similar to the geometry of the test material.
[0103] These same techniques can be used to detect and characterize
overload effects on components. As examples, excessive mechanical
or thermal loading on a component can compromise the structural
integrity of a component so that it fails during subsequent use. In
magnetizable materials such as most steels, patterns of the
magnetic permeability distribution over the surface of critical
components can reflect these overload conditions. The permeability
can be measured using single element eddy current sensors placed at
selected locations and in different orientations, eddy current
sensor arrays permanently mounted at selected locations in
different orientations, and by imaging sensors, such as MWM-Arrays,
scanned in different orientations.
[0104] The capability to monitor applied and residual stress on a
component also permits other properties to be inferred, such as the
weight of an article. For example, the weight of an aircraft or
changes in the weight in the aircraft can be inferred from magnetic
permeability measurement performed on steel landing gear
components. Eddy current sensor arrays can be scanned or mounted at
selected locations that reflect the load transfer from the weight
of the aircraft about the landing gear. The arrays should be
mounted or scanned in the orientation parallel to the direction of
maximum principal stresses. Other orientations could also be
included to provide a more complete observation of the stress
distribution.
[0105] Combinations of permanently mounted and scanning sensors and
sensor arrays can be used to image and monitor stress distributions
on simple and complex surfaces. The surfaces can be flat or
non-flat geometry and may be on layered materials, such as lap
joints. Measurements can also be performed in contact or
non-contact modes, with parts of the sensor mounted on an opposing
surface.
[0106] These can be in the form of layers, gradient materials with
properties varying with depth or near critical features such as
holes, fibers or coated fibers, weld material or entire components
such as a fitting, bolt, or joint. State-sensitive materials can
also be used in the design and use of critical components. These
materials can be ones whose magnetic permeability, dielectric
permittivity, or electrical conductivity varies with stress,
temperature, thermal or mechanical overload, fatigue damage, crack
presence, or some combination of these effects. The materials would
be selected for use with a particular component based on the
sensitivity of the materials and the properties of the component.
For example, Table 1 listed several materials suitable for an eddy
current sensor but these same materials would not be suitable for a
dielectric sensor.
[0107] These state-sensitive materials could be placed on the
surface or embedded between material layers of the test component.
FIG. 34 shows a sensor 160 above the top layer 150, middle layer
152, and bottom layer 154 of a three layer component.
State-sensitive materials are embedded between the top and middle
layers 156 and between the middle and bottom layers 158. For
monitoring stress or fatigue, the state-sensitive layers can
contain laminates of multiple strips or some other pattern that has
different orientations at the different depths in the material.
When the properties of the layers (156 and 158) are measured, the
pattern for the effective property (e.g., permeability) will change
depending upon the state (e.g., stress) on the component. This can
also be applied to the monitoring of stress around fasteners, as
described in U.S. patent application Ser. No. 10/351,978.
[0108] In a similar fashion, eddy current sensors can be mounted
between layers to measure fatigue damage and stresses or for
detection of cracks. This can also include placing the electrical
conductors for the drive and sense windings on different layers of
the component as well. The sensors themselves can also be supported
with durable substrates, such as stainless steel or ceramic to
prevent damage to the sensor conductors. The sensors may even be
embedded within these durable supports. In one embodiment one or
two sets of drive winding with or without a magnetic substrate and
with or without a protective coating to limit fretting damage are
embedded between layers with or without an embedded array of
sensing elements. An array of sense elements is then scanned across
the outer surface to measure the field. This increases sensitivity
to cracks or stress changes between the layers because the applied
field does not have to diffuse through the metal and then back
again when compared to a non embedded drive, scanning method. In
another embodiment with one embedded sensor or sensitive material,
protective layers are used to limit fretting damage at the faxing
surface.
[0109] A variety of sensors can be used to measure the response of
the state sensitive materials. For example, for insulating or
weakly conducting materials such as fiberglass composites,
capacitive or dielectric sensors can be used. The sensors are the
electromagnetic dual to the inductive sensors, with electric fields
taking the place of magnetic fields for inspecting the materials. A
representative single sided sensor geometry is shown in FIG. 35.
The application of a sinusoidally varying potential of complex
magnitude v and angular frequency .omega.=2.pi.f results in the
flow of a terminal current with complex amplitude I, whose
magnitude and phase is dependent on the complex permittivity of the
material. The capacitive sensor 100 in one preferred embodiment has
interdigitated electrodes as presented in U.S. Pat. Nos. 4,814,690
and 6,380,747 and in U.S. patent application Ser. Nos. 10/040,797,
filed Jan. 7, 2002, and 10/225,406, filed Aug. 20, 2002, the entire
teachings of which are hereby incorporated by reference. This
sensor 102 utilizes a pair of interdigitated electrodes 104 and 106
to produce a spatially periodic electric field. The electrodes are
adjacent to the material of interest with an insulating substrate
and a ground plane on the other side of the substrate. One of the
two electrodes, 104, is driven with a sinusoidally varying voltage,
V.sub.D, while the other, 106, is connected to a high-impedance
buffer used to measure the magnitude and phase of the floating
potential, V.sub.s. The periodicity of the electrode structure is
denoted by the spatial wavelength .lamda.=2.pi./k, where k is the
wavenumber.
[0110] In another embodiment sensor diagnostics and/or
recalibration is performed for individual surface mounted or
embedded sensors or networks of sensors by performing an act or
exposing the sensor to a condition change that changes one or more
unknowns while not changing at least one other unknown (e.g.,
lift-off). In one such method the temperature is varied using a
heat source or measurements are made at different ambient
temperatures. This changes conductivity and or permeability with
lift-off constant. In another such method, stress (or external
load) is altered to again vary conductivity or permeability of the
MUT or added sensitive material with lift-off and layer thicknesses
constant. In another embodiment another layer is added keeping all
else constant. In another embodiment the scanning of a drive or
sense array relative to an embedded drive or sense array is
accomplished at different lift-offs, temperatures or applied
stresses. In still another method a bias field is used to change
the permeability, including directional variations if practical.
Both for performance check and measurement enhancements. The sensor
performance is verified by comparing to expected behavior.
Recalibration is performed relative to expected results by
monitoring temperature or stress with thermostats/thermocouples or
strain gages or some other means or model prediction relative to
load/source of change.
[0111] While the embodiments of the invention have been
particularly shown and described with reference to preferred
embodiments thereof, it will be understood to those skilled in the
art that various changes in form and details may be made therein
without departing from the spirit and scope of the embodiment of
the invention as defined by the appended claims.
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paper titled "In-Situ Crack Detection and Depth Discrimination for
Coated Turbine Blade Contact Faces," presented at ASNT Spring
Conference, Orlando, Fla., Mar. 10-14, 2003. [0130] 12. Technical
paper titled "MWM Eddy Current Sensor Array Characterization of
Aging Structures Including Hidden Damage Imaging," presented at the
NACE Conference, San Diego; Calif., Mar. 17-19, 2003. [0131] 13.
Technical paper titled "Material Condition Monitoring Using
Embedded and Scanning Sensors for Prognostics," presentation at the
57.sup.th MFPT Conference, Virginia Beach, Va.; April 2003. [0132]
14. Technical paper titled "Nondestructive Evaluation for CBM and
PHM of Legacy and New Platforms," 57.sup.th MFPT Conference,
Virginia Beach, Va.; April 2003. [0133] 15. Technical paper titled
"Validation of Multi-Frequency Eddy Current MWM Sensors and
MWM-Arrays for Coating Production Quality and Refurbishment
Assessment," submitted for the proceedings of the ASME/IGTI Turbo
conference, June 2003, Atlanta, Ga. [0134] 16. Technical paper
titled "Corrosion Detection and Prioritization Using Scanning and
Permanently Mounted MWM Eddy-Current Arrays", Tri-Service Corrosion
Conference, January 2002. [0135] 17. Technical paper titled
"Surface Mounted and Scanning Periodic Field Eddy-Current Sensors
for Structural Health Monitoring," presented at IEEE Aerospace
Conference: Prognostics & Health Management for Aging Aircraft;
March 2002. [0136] 18. Technical paper titled "MWM-Array
Characterization and Imaging of Combustion Turbine Components,"
presented at EPRI International Conference on Advances in Life
Assessment and Optimization of Fossil Power Plants, Orlando, Fla.;
March 2002. [0137] 19. Technical paper titled "Corrosion Detection
and Prioritization Using Scanning and Permanently Mountable MWM
Eddy-Current Arrays," presented at U.S.
[0138] Army Corrosion Summit, March 2002. [0139] 20. Technical
paper titled "MWM-Array Characterization and Imaging of combustion
Turbine Components," presented at EPRI International Conference on
Advances in Life Assessment and Optimization of Fossil Fuel Power
Plants, March 2002. [0140] 21. Technical paper titled "Application
of MWM Sensors and MWM-Arrays for Inspection of Aircraft
Components," presented at NAVAIR NDT Working Group, April 2002.
[0141] 22. Technical paper titled "Application of MWM.RTM.
Eddy-Current Technology during Production of Coated Gas Turbine
Components," presented at 11.sup.th International Symposium on
Nondestructive Characterization of Materials, Berlin, Germany; June
2002. [0142] 23. Technical paper titled "High Temperature
Eddy-Current Sensors for Heat Treatment Monitoring," presented at
AeroMat, June 2002. [0143] 24. Technical paper titled "Eddy-Current
Array Volumetric Imaging of Microstructure and Flaws for Thick
Components," presented at AeroMat, June 2002. [0144] 25. Technical
paper titled "Absolute Electrical Property Imaging using High
Resolution Inductive, Magnetoresistive and Capacitive Sensor Arrays
for Materials Characterization," presented at 11.sup.th
International Symposium on Nondestructive Characterization of
Materials, Berlin, Germany; June, 2002. [0145] 26. Technical paper
titled "Friction Stir Weld Inspection for Lack of Penetration
Defects via Conductivity Mapping using MWM Eddy Current Sensor
Arrays," presented at AeroMat, June 2002. [0146] 27. Technical
paper titled "Eddy Current Sensor Technology for Real-Time State
Monitoring, Condition Based Maintenance, and Life Management,"
presented at ONR Prognostics Workshop, July 2002. [0147] 28.
Technical paper titled "MWM-Array Eddy Current Sensors for
Detection of Cracks in Regions with Fretting Damage," published in
ASNT Materials Evaluation, Volume 60, No. 7, pp 870-877; July 2002.
[0148] 29. Technical paper titled "Residual and Applied Stress
Estimation from Directional Magnetic Permeability Measurements with
MWM Sensors," published in ASME Journal of Pressure Vessel
Technology, Volume 124, pp 375-381; August 2002. [0149] 30.
Technical paper titled "Rapid High-Resolution Corrosion Imaging and
Detection with MWM Eddy Current Arrays," presented at 5.sup.th
International Aircraft Corrosion Conference, August 2002. [0150]
31. Technical paper titled "High-Resolution, Deep Penetration and
Rapid GMR/Eddy Current Array Imaging of Weld Condition and Quality"
presented at ASNT Structural Materials Technology--NDE/NDT for
Highways and Bridges, September 2002. [0151] 32. Technical paper
titled "High Resolution MWM-Array Imaging of Cracks in Fretting
Regions of Engine Disk Slots," presented at the 6.sup.th Joint
FAA/DoD/NASA Aging Aircraft Conference, September, 2002. [0152] 33.
Technical paper titled "Multi-Site Damage Imaging of 3.sup.rd Layer
Cracks in Lap Joints using MWM-Arrays," presented at the 6.sup.th
Joint FAA/DoD/NASA Aging Aircraft Conference, September, 2002.
[0153] 34. Technical paper titled "Conformable Eddy-Current Sensors
and Arrays for Fleet-wide Gas Turbine Component Quality
Assessment," published in ASME Journal of Engineering for Gas
Turbines and Power, Volume 124, No. 4, pp 904-909; October 2002.
[0154] 35. Technical paper titled "MWM-Array Eddy Current Testing
for Corrosion and Fatigue Damage," presented at Air Transport
Association, October 2002. [0155] 36. Technical paper titled
"Characterization and Imaging of Coated Turbine Components with MWM
Sensors and MWM-Arrays," presented at ASM Heat Treat/Surface
Engineering, October 2002. [0156] 37. Technical paper titled "Thin,
Conformable Eddy Current Sensor Arrays for Difficult-to-Access
Location Inspections," presented at Defense Working Group, November
2002. [0157] 38. Technical paper titled "Health Monitoring for
Landing Gear and Other Critical Components," presented at ASIP
2002, December, 2002. [0158] 39. Technical paper titled
"MWM-Eddy-Current Arrays for Crack Initiation and Growth
Monitoring," submitted to International Journal of Fatigue, from
the International Conference on Fatigue Damage of Structural
Materials IV, Hyannis, Mass., 2002.
* * * * *