U.S. patent application number 09/855460 was filed with the patent office on 2001-09-20 for method and apparatus for short term inspection or long term structural health monitoring.
Invention is credited to Kim, Sang-Young, Kwun, Hegeon, Light, Glenn M., Spinks, Robert L. JR..
Application Number | 20010022514 09/855460 |
Document ID | / |
Family ID | 25321316 |
Filed Date | 2001-09-20 |
United States Patent
Application |
20010022514 |
Kind Code |
A1 |
Light, Glenn M. ; et
al. |
September 20, 2001 |
Method and apparatus for short term inspection or long term
structural health monitoring
Abstract
A method and apparatus is shown for implementing
magnetostrictive sensor techniques for the nondestructive short
term inspection or long term monitoring of a structure. A plurality
of magnetostrictive sensors are arranged in parallel on the
structure and includes (a) a thin ferromagnetic strip that has
residual magnetization, (b) that is coupled to the structure with a
couplant, and (c) a coil located adjacent the thin ferromagnetic
strip. By a transmitting coil, guided waves are generated in a
transmitting strip and coupled to the structure and propagate along
the length of the structure. For detection, the reflected guided
waves in the structure are coupled to a receiving strip and are
detected by a receiving magnetostrictive coil. Reflected guided
waves may represent defects in the structure.
Inventors: |
Light, Glenn M.; (San
Antonio, TX) ; Kwun, Hegeon; (San Antonio, TX)
; Kim, Sang-Young; (San Antonio, TX) ; Spinks,
Robert L. JR.; (San Antonio, TX) |
Correspondence
Address: |
TED D. LEE
GUNN, LEE & KEELING
Suite 1500
700 N. St. Mary's Street
San Antonio
TX
78205
US
|
Family ID: |
25321316 |
Appl. No.: |
09/855460 |
Filed: |
May 15, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09855460 |
May 15, 2001 |
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09815219 |
Mar 22, 2001 |
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09815219 |
Mar 22, 2001 |
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09519530 |
Feb 25, 2000 |
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60124763 |
Mar 17, 1999 |
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Current U.S.
Class: |
324/240 |
Current CPC
Class: |
G01N 29/2412 20130101;
G01N 2291/2632 20130101; G01N 2291/0421 20130101; G01N 2291/2634
20130101; G01N 17/006 20130101; G01N 29/11 20130101; G01N 2291/0422
20130101; G01N 2291/044 20130101; G01N 2291/0428 20130101 |
Class at
Publication: |
324/240 |
International
Class: |
G01R 033/10 |
Claims
1. A method of nondestructive, short term inspection or long term
monitoring of a structure to determine if said structure (a) has a
defect such as a crack, corrosion or erosion or (b) has a transient
stress signal due to vibrations, cracking or mechanical impacts,
said method comprising the following steps: preparing a plurality
of thin strips of ferromagnetic material of appropriate width and
length; inducing residual magnetization along said length of said
thin strips by applying an external magnetic field and thereafter
removing said external magnetic field; coupling said thin strips in
parallel to said structure; installing a magnetostrictive probe on
each of said thin strips; generating a pulse signal in a
transmitter control circuit and delivering said pulse signal to a
first of said magnetostrictive probes to create guided waves in a
first of said thin strips, which guided waves are coupled to said
structure for propagation therein; magnetostrictively detecting any
reflected waves by a second of said magnetostrictive probes in
combination with a second of said thin strips, said reflected waves
being coupled from said structure to said second of said thin
strips; and determining if said reflected waves are due to said
defect or said transient stress signal.
2. The method of nondestructive, short term inspection or long term
monitoring of said structure as recited in claim 1 wherein said
coupling step includes bonding said thin strips to said
structure.
3. The method of nondestructive, short term inspection or long term
monitoring of said structure as recited in claim 2 wherein said
guided waves are shear waves.
4. The method of nondestructive, short term inspection or long term
monitoring of said structure as recited in claim 2 wherein said
structure is a pipe and said guided waves are torsional waves.
5. The method of nondestructive, short term inspection or long term
monitoring of said structure as recited in claim 1 wherein said
coupling step includes using a thick, viscous material as a
couplant.
6. The method of nondestructive, short term inspection or long term
monitoring of said structure as recited in claim 4 wherein said
determining step includes storing a reference reflected wave and,
after an appropriate period of time, repeating said generating step
and said magnetostrictively detecting step, comparing a second
reflected wave with said reference reflected wave to determine if
defects have occurred during said appropriate period of time.
7. The method of nondestructive, short term inspection or long term
monitoring of said structure as recited in claim 6 wherein said
determining step includes subtracting said reference reflected wave
from said second reflected wave.
8. The method of nondestructive, short term inspection or long term
monitoring of said structure as recited in claim 1 wherein said
determining step includes storing a reference reflected wave,
thereafter repeating said generating step and said
magnetostrictively detecting step and comparing subsequent
reflected waves with said reference reflected wave to determine if
said transient stress signal has occurred.
9. The method of nondestructive, short term inspection or long term
monitoring of said structure as recited in claim 8 wherein said
reference reflected wave is continually updated.
10. The method of nondestructive, short term inspection or long
term monitoring of said structure as recited in claim 1 wherein
said thin strip may be selected from the group of ferromagnetic
materials having appropriate magnetostrictive coefficients
consisting of nickel, grain-oriented silicon steel or
TERFENDOL-D.RTM..
11. The method of nondestructive, short term inspection or long
term monitoring of said structure as recited in claim 5 wherein
said couplant is honey.
12. An apparatus for nondestructive, short term inspection or long
term monitoring of a structure to determine if said structure (a)
has a defect, such as a crack, corrosion or erosion, or (b) has a
transient stress signal due to vibrations, cracking or mechanical
impact, said apparatus comprising: a plurality of thin
ferromagnetic strips that have residual magnetization therein, said
thin ferromagnetic strips being coupled in parallel to said
structure; a transmitter coil being located adjacent to a first of
said thin ferromagnetic strips; a receiver coil being located
adjacent to a second of said thin ferromagnetic strips; a
transmitter control circuit connected to said transmitter coil for
generating a pulse signal and delivering said pulse signal to said
transmitter coil, said transmitter coil creating magnetostrictively
a guided wave that is coupled from said first thin ferromagnetic
strip to said structure to propagate along said structure; said
receiver coil magnetostrictively detecting said guided wave and any
reflected signals, including any caused by defect or transient
stress signals in said structure; said transmitter coil and said
receiver coil being wound adjacent said first and second thin
ferromagnetic strips, respectively, said guided waves moving
perpendicular to said first and second thin ferromagnetic
strips.
13. An apparatus for nondestructive, short term inspection or long
term monitoring of a structure as recited in claim 12 wherein said
residual magnetization is in a lengthwise direction of said thin
ferromagnetic strips and said guided wave is a shear wave.
14. An apparatus for nondestructive, short term inspection or long
term monitoring of a structure as recited in claim 13 further
including a computer for storing a first of said reflected signals
and, after appropriate periods of time, comparing new reflected
signals with said stored reflected signal to determine if changes
have occurred.
15. An apparatus for nondestructive, short term inspection or long
term monitoring of a structure as recited in claim 14 wherein a
couplant for said coupling of said plurality of said thin
ferromagnetic strips is a bonding material, such as epoxy.
16. An apparatus for nondestructive, short term inspection or long
term monitoring of a structure as recited in claim 12 wherein a
couplant for said coupling of said plurality of said thin
ferromagnetic strips is a thick, viscous material, such as
honey.
17. An apparatus for nondestructive, short term inspection or long
term monitoring of a structure as recited in claim 12 wherein said
plurality of thin ferromagnetic strips retain said residual
magnetization for a long period of time, such as nickel,
grain-oriented silicon steel, or TERFENDOL.RTM..
18. An apparatus for nondestructive, short term inspection or long
term monitoring of a structure as recited in claim 15 wherein at
least four of said thin ferromagnetic strips are used, two for
transmitting and two for receiving, so that direction of travel of
said guided wave can be determined, therefore only said reflected
signals in a given direction being stored in said computer.
19. An apparatus for nondestructive, short term inspection or long
term monitoring of a structure recited in claim 15 wherein said
transmitter coil and said receiver coil are coils on a flexible
printed circuit board which generates or receives said guided wave
in said ferromagnetic strips.
Description
[0001] This is a continuation-in-part patent application depending
from U.S. patent application Ser. No. 09/815,219, filed Mar. 22,
2001, which is a continuation-in-part patent application depending
from U.S. patent application Ser. No. 09/519,530, filed Feb. 25,
2000, which depends on provisional Patent Application Ser. No.
60/124,763, filed on Mar. 17, 1999.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to methods and
devices for short term inspection of structures, or long term
monitoring of the health of a structure. The present invention
relates more specifically to a magnetostrictive sensor based system
for short term inspection or long term monitoring of the health of
a structure.
[0004] 2. Description of the Related Art
[0005] Magnetostrictive effect refers to the phenomena of a
physical dimension change in ferromagnetic materials that occurs
through variations in magnetization. In magnetostrictive
applications, the generation and detection of mechanical waves is
typically achieved by introducing a pulse current into a
transmitting coil adjacent to a ferromagnetic material. The change
in magnetization within the material located near the transmitting
coil causes the material to change its length locally in a
direction parallel to the applied field. This abrupt local
dimension change, which is the magnetostrictive effect, generates a
mechanical wave (called guided wave) that travels through the
ferromagnetic material with a certain fixed speed (which is usually
less than the speed of sound). When the mechanical wave is
reflected back from the end of the ferromagnetic material, or from
a defect in the ferromagnetic material, and reaches a detection
coil, the mechanical wave generates a changing magnetic flux in the
detection coil as a result of the inversed magnetostrictive effect.
This changing magnetic flux induces an electric voltage within the
detection coil that is proportional to the magnitude of the
mechanical wave. The transmitting coil and the detection coil can
be identical.
[0006] Advantages of using the magnetostrictive effect in
nondestructive evaluation (NDE) applications include (a) the
sensitivity of the magnetostrictive sensors, (b) durability of the
magnetostrictive sensors, (c) no need to couple the sensor to the
material being investigated, (d) long range of the mechanical waves
in the material under investigation, (e) ease of implementation,
and (f) low cost of implementation.
[0007] The use of magnetostrictive sensors (MsS) in the
nondestructive evaluation (NDE) of materials has proven to be very
effective in characterizing defects, inclusions, and corrosion
within various types of ferromagnetic and non-ferromagnetic
structures. A MsS launches a short duration (or a pulse) of guided
waves in the structure under investigation and detects guided wave
signals reflected from anomalies such as defects in the structure.
Since guided waves can propagate long distances (typically 100 feet
or more), the MsS technique can inspect a global area of a
structure very quickly. In comparison, other conventional NDE
techniques such as ultrasonics and eddy current inspect only the
local area immediately adjacent to the probes used. Therefore, the
use of magnetostrictive sensors offers a very cost effective means
for inspecting large areas of steel structures such as strands,
cables, pipes, and tubes quickly with minimum support requirements
such as surface preparation, scaffolding, and insulation removal.
The ability to use magnetostrictive sensors with little preparation
of the object under inspection derives from the fact that direct
physical contact between the sensors and the material is not
required.
[0008] Efforts have been made in the past to utilize
magnetostrictive sensor technologies in association with the
inspection of both ferromagnetic and non-ferromagnetic materials.
Included in these efforts are systems described in U.S. Pat. Nos.
5,456,113; 5,457,994; and 5,501,037, which are each commonly owned
by the assignee of the present invention. The disclosures of U.S.
Pat. Nos. 5,456,113; 5,457,994; and 5,501,037, provide background
on the magnetostrictive effect and its use in NDE and are therefore
incorporated herein by reference. These efforts in the past have
focused primarily on the inspection of pipe, tubing and steel
strands/cables wherein the geometry of the structure is such that
the cross-sectional diameter is small in comparison to the length
of the structure. While these systems and their application to
longitudinal structures find significant applications, there are
yet other structures that could benefit from the use of
magnetostrictive based NDE.
[0009] Other efforts have been made in the past to utilize sensors
that measure magnetic flux and/or acoustic waves in structural
materials. These efforts have included those described in the
following patents:
[0010] U.S. Pat. No. 3,555,887 issued to Wood on Jan. 19, 1971
entitled Apparatus for Electroacoustically Inspecting Tubular
Members for Anomalies Using the Magnetostrictive Effect and for
Measuring Wall Thickness. This patent describes a system designed
to direct a mechanical wave through the thickness dimension of a
long tubular member. The sensitivity of the device is limited to
the directing of a wavefront normal to the surface of the material
under inspection and immediately back to a sensor when reflected
from an opposite wall or an anomaly.
[0011] U.S. Pat. No. 4,881,031 issued to Pfisterer, et al. on Nov.
14, 1989 entitled Eddy Current Method and Apparatus for Determining
Structure Defects in a Metal Object Without Removing Surface Films
or Coatings. This patent describes a method for establishing
localized eddy currents within ferromagnetic materials and
recognizes the presence and effect of a coating in order to
identify and quantify corrosion beneath the coating. As with other
eddy current methods, the ability to inspect a material is limited
to the area immediately adjacent to the sensor.
[0012] U.S. Pat. No. 5,544,207 issued to Ara, et al. on Aug. 6,
1996 entitled Apparatus for Measuring the Thickness of the Overlay
Clad in a Pressure Vessel of a Nuclear Reactor. This patent
describes a system directed solely to the measurement of magnetic
field variations that result from the distribution of the magnetic
field through overlays of varying thickness. The system utilizes a
magnetic yoke that is placed in close contact with the surface of
the overlay clad of the pressure vessel.
[0013] U.S. Pat. No. 5,687,204 issued to Ara, et al. on Nov. 11,
1997 entitled Method of and Apparatus for Checking the Degradation
of a Pressure Vessel of a Nuclear Reactor. This patent describes a
system similar to the earlier issued Ara, et al. patent and
utilizes a magnetic yoke having an excitation coil and a magnetic
flux measuring coil that are placed in close contact with the inner
wall of the pressure vessel. The hysteresis magnetization
characteristics formed by the magnetic yoke and the pressure vessel
wall are measured. Degradation of the material comprising the
pressure vessel is inferred from a determination of the hardness of
the material which is determined from the coercive forces obtained
by analyzing the hysteresis characteristics of the
magnetization.
[0014] In general, a magnetostrictive sensor consists of a
conductive coil and a means for providing a DC bias magnetic field
in the structure under inspection. The means for providing a bias
magnetic field can include the use of either permanent magnets or
electromagnets. In a transmitting magnetostrictive sensor, an AC
electric current pulse is applied to the coil. The resulting AC
magnetic field (a changing magnetic field) produces guided waves in
an adjacent ferromagnetic material through the magnetostrictive
effect. For pipes, cables, tubes, and the like, the waves are
typically launched along the length of the longitudinal structure.
In the receiving magnetostrictive sensor, a responsive electric
voltage signal is produced in the conductive coil when the guided
waves (transmitted or reflected from anomalies within the material)
pass the sensor location, through the inverse magnetostrictive
effect.
[0015] With MsS techniques, defects are typically detected by using
the pulse-echo method well known in the field of ultrasonics. Since
the sensor relies on the magnetostrictive behavior found in
ferrogmagnetic materials, this technology is primarily applicable
to the inspection of ferromagnetic components such as carbon steel
piping or steel strands. It is also applicable, however, to the
inspection of nonferrous components if a thin layer of
ferromagnetic material, such as nickel, is plated or coupled onto
the component in the area adjacent to the magnetostrictive
sensors.
[0016] The magnetostrictive sensor technique has the advantage of
being able to inspect a large area of material from a single sensor
location. Such sensors have, for example, been used to accurately
inspect a length of pipe or cable of significantly more than 100
feet. Further, magnetostrictive sensor techniques are comprehensive
in their inspection in that the methods can detect both internal
and external defects, thereby providing a 100% volumetric
inspection. The techniques are also quite sensitive, being capable
of detecting a defect with a cross-section less than 1% of the
total metallic cross-section of cylindrical structures such as
pipes, tubes, or rods. Finally, as indicated above,
magnetostrictive sensor techniques do not require direct physical
contact between the component surface and the sensor itself. This
eliminates the need for surface preparation or the use of a
couplant.
[0017] Application to Plate Type and Containment Structures
[0018] In recent years, there have been many reported occurrences
of steel containment liners degrading at commercial nuclear power
plants. Due to the aging of such facilities and the increased
requirements for inspection, incidents of degradation are likely to
increase. The structural degradation of these liners, especially
corrosion damage, is an important concern since the liners are
designed to provide a leak-tight pressure boundary for the nuclear
containment. Many other industrial uses of plate type ferromagnetic
materials could benefit from more frequent inspections to determine
the state of deterioration, the location of faults, and the
likelihood of failure. In most instances in the past, inspections
of large plate type objects (such as large aboveground storage
tanks) have required either very expensive off-line inspections or
statistical samplings of randomly selected local areas that are for
the most part less than reliable. It has heretofore been difficult
to carry out a thorough inspection of a plate type structure, or a
structure comprised of a plurality of plate type sheets of
material, without high cost and long down time for the object under
inspection. It would be desirable to use the magnetostrictive
sensor technique for detecting and locating various anomaly
characteristics within plate type materials. Such techniques could
be used for detecting and locating wall thickness reductions in
liners, such as those described above, that might be caused by
corrosion over time. If such a system were applicable, it would be
possible to inspect otherwise inaccessible regions of containment
liners and the like that are either imbedded in concrete or
adjacent to flooring or equipment that cannot be moved.
[0019] It would therefore be desirable to implement
magnetostrictive sensor techniques in conjunction with plate type
structures in a manner similar to, and with the accuracy of, such
systems utilized in conjunction with cylindrical structures. It
would be desirable if an inspection of plate type and cylindrical
structures could be carried out in an efficient manner that did not
require full access to the surface of the plate or the inner or
outer surface of cylindrical structures such as pipes and tubes.
Such a magnetostrictive sensor system would be able to investigate
large volumes of a plate type or cylindrical structure, including
pipes and tubes, and would provide a cost effective global
inspection of the structure.
SUMMARY OF THE PRESENT INVENTION
[0020] It is therefore an object of the present invention to
provide a sensor device for implementing magnetostrictive based NDE
in association with pipes and tubes in order to evaluate the
condition of the structures and to determine the presence of
anomalies indicative of fractures, deteriorations, and the
like.
[0021] It is a further object of the present invention to provide a
magnetostrictive sensor appropriate for use in conjunction with the
inspection of pipes and tubes that is capable of transmitting and
receiving guided waves within the pipes and tubes and generating
signals representative of the characteristics of such waves
appropriate for the analysis and detection of anomalies
therein.
[0022] It is a further object of the present invention to provide
magnetostrictive sensor devices appropriate for use in conjunction
with the inspection of pipes and tubes that inspect the entire
structure for anomalies, corrosion, fractures, and the like in a
cost effective manner.
[0023] It is a further object of the present invention to provide a
method for the inspection of pipes and tubes that includes the use
of a magnetostrictive sensor specifically adapted for directing
guided waves along the length of the pipe or tube and detecting
such waves as may be reflected from anomalies along the pipe or
tube.
[0024] It is yet another object of the present invention to provide
a method and apparatus for nondestructive evaluation of pipes and
tubes utilizing magnetostrictive sensors that generate and detect
shear horizontal waves along the length of the item being
inspected.
[0025] It is yet another object of the present invention to provide
a magnetostrictive sensor that is suitable for low frequency
operation (200 kHz or less), has good sensitivity and long
inspection range, and is relatively tolerate to liftoff.
[0026] It is still another object of the present invention to
provide a method and apparatus for nondestructive evaluation of
pipes using magnetostrictive sensors that propagate guided waves in
a circumferential direction around the pipe.
[0027] Another object of the present invention is to provide a
method and apparatus for nondestructive evaluation of pipes and
tubes using magnetostrictive sensors with torsional waves that has
better defect detectability particularly in liquid filled pipes or
tubes.
[0028] Still another object of the present invention is to provide
a method and apparatus for the nondestructive evaluation of pipes
and tubes that requires no permanent DC bias magnets or
electromagnets and, thus is easier to apply.
[0029] Another object of the present invention is to provide a
method and apparatus for the nondestructive evaluation of pipes and
tubes that has a reduced setup time and therefore a lower
inspection cost.
[0030] In fulfillment of these and other objectives, the present
invention provides a method and apparatus for implementing
magnetostrictive sensor techniques for the nondestructive
evaluation of plate type structures such as walls, vessels,
enclosures, and the like. The system includes magnetostrictive
sensors specifically designed for application in conjunction with
welded plate type structures that generate guided waves in the
plates which travel through the plate in a direction parallel to
the surface of the plate. Similarly structured sensors are
positioned to detect the guided waves (both incident and reflected)
and generate signals representative of the characteristics of the
guided waves detected. The system anticipates the use of either
discrete magnetostrictive transmitters and receivers or the use of
a single magnetostrictive sensor that operates to both transmit and
detect the guided waves. The sensor structure is longitudinal in
nature and generates a guided wave having a wavefront parallel to
the longitudinal direction of the sensor. Appropriate electronics
associated with the process of generating the guided waves and
controlling the propagation direction of the generated wave through
the magnetostrictive transmitter as well as detecting, filtering,
and amplifying the guided waves at the magnetostrictive receiver,
are implemented as is well known in the art. Signal analysis
techniques, also known in the art, are utilized to identify
anomalies within the plate type structure. The method utilizes
pattern recognition techniques as well as comparisons between
signal signatures gathered over time from the installation of the
structure under investigation to a later point after deterioration
and degradation may have occurred.
[0031] The magnetostrictive sensors can also be used to detect
defects in cylindrical structures such as to detect defects in
electric resistance welding, such as in pipes that are welded along
a seam thereof. For example, a magnetostrictive transmitter can be
placed on one side of the pipe being investigated and a
magnetostrictive receiver on the other side of the pipe. By
propagating a guided wave in circumferential direction around the
pipe, any defects in the pipe can immediately be detected, such as
in the area of the weld.
[0032] For generation and detection of the symmetrical (S) or the
anti-symmetrical (A) Lamb wave mode in a plate type structure, the
DC magnet or field required for MsS operation is applied parallel
to the direction of wave propagation. For generation and detection
of the shear horizontal (SH) wave mode, the DC magnetic field
required for MsS operation is applied perpendicular to the
direction of wave propagation. Due to the enclosed nature of
cylindrical structures such as pipes and tubes, the shear
horizontal wave can be induced to act as a torsional wave along the
length of the pipe or tube. The generation of a shear horizontal or
torsional wave along the length of the pipe or tube allows defect
detectability that will not be hampered by the presence of liquid
in the pipe or tube.
[0033] Current flow along the longitudinal axis of a pipe or tube
will cause magnetization of a ferromagnetic pipe or tube in the
circumferential direction. This magnetization can be used for the
transmission and detection of torsional waves that flow along the
pipe and tube and any reflections thereof. The reflections may be
from anomalies or defects in the pipe or tube.
[0034] Also, a thin ferromagnetic strip that is magnetized in the
circumferential direction may be wrapped around and held tightly
against the pipe or tube. Thereafter, a torsional wave may be
generated or detected where the ferromagnetic strips are located
along the pipe or tube. The circumferential magnetization around
the pipe or tube is in the ferromagnetic strip. It is very
important to hold the ferromagnetic strip in tight surface contact
with the pipe or tube so that the full effect of the torsional wave
can be felt and detected in either the transmitter or receiver
coils adjacent thereto.
[0035] In another embodiment of the present invention, a thin strip
of a ferromagnetic material that can retain residual magnetization,
such as nickel, is prepared an appropriate width and length. The
width of the strip depends upon the operating frequency of the
magnetostrictive device. The length of the strip depends upon the
structure to be monitored. For example, for a pipe, the length is
slightly shorter than the circumference of the pipe. For a
plate-type structure, the length is typically 10 inches or less.
Residual magnetization is induced along the length of the strip by
applying an external magnetic field to the strip along its length
and then removing the external magnetic field. Afterwards, the
strip is coupled to the structure to be monitored with an
appropriate material, such as epoxy. For a pipe, the strip is
bonded around the circumference of the pipe. For a plate-type
structure, the strip is bonded normal to the direction of wave
propagation to be used for inspection. For short term inspection, a
viscous couplant, such as honey, may be used to couple the strip on
a temporary basis to the structure being inspected.
[0036] After coupling the strip to the structure to be inspected, a
coil is either wrapped around or placed adjacent to the magnetized
strip. A minimum of two strips and coils are used, one for
transmitting and one for receiving the magnetostrictive
signals.
[0037] For long term monitoring, the transmitters and receivers are
encased or covered in a manner to protect them from the
environment. Wires from the magnetostrictive transmitters and
receivers are easily accessible whereby the transmitters and
sensors can be electrically monitored by appropriate
magnetostrictive instrumentation.
[0038] For short term inspection, the signal obtained indicates if
there is a defect in the structure being inspected. For long term
monitoring, a baseline signal is obtained and stored in the
computer. Thereafter, additional signals are obtained periodically
from the magnetostrictive transmitters and receivers with changes
in the signal indicating changes in the structure being
inspected.
[0039] The guided wave normally used in the method just described
for piping applications is a torsional wave and in plate-type
structures is a shear horizontal wave.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] FIG. 1 is a schematic block diagram showing the components
of the system of the present invention.
[0041] FIG. 2 is a perspective view of a magnetostrictive sensor of
the present invention.
[0042] FIG. 3 is a cross-sectional view of the implementation of
the sensors of the present invention in conjunction with a plate
type structure.
[0043] FIG. 4 is a plot of a signal received through the system of
the present invention utilizing a 60 kHz symmetric (S.sub.0) wave
mode signal in a 4 foot wide, 20 foot long, 0.25 inch thick steel
plate.
[0044] FIG. 5 is a plot of a signal received through the system of
the present invention in conjunction with the structure associated
with FIG. 4 for a 40 kHz anti-symmetric (A.sub.0) wave mode
signal.
[0045] FIG. 6 is a plot of three signals received through the
system of the present invention utilizing a 40 kHz symmetric
(S.sub.0) wave mode signal in a 4 foot wide, 20 foot long, 0.25
inch thick steel plate.
[0046] FIG. 7 is a plot of three signals received through the
system of the present invention utilizing a 20 kHz anti-symmetric
(A.sub.0) wave mode signal in a 4 foot wide, 20 foot long, 0.25
inch thick steel plate.
[0047] FIGS. 8(a) and (b) are plots of a shear horizontal (SH) wave
received through the system of the present invention utilizing an
80 kHz wave in a 4 foot wide, 20 foot long 0.25 inch thick steel
plate, before and after a 0.05 inch hole is cut therein.
[0048] FIG. 9 is a pictorial end view of a welded pipe being
inspected using a magnetostrictive transmitting probe and a
magnetostrictive receiving probe on opposite sides of the pipe for
transmission and receipt of Lamb or SH waves.
[0049] FIGS. 10(a) and (b) are plots of signals received through
the system of the present invention when used to test a pipe as
shown in FIG. 9, utilizing a 150 kHz SH wave mode in a 4.5 inch
outside diameter steel pipe having a 0.337 inch thick wall before
and after cutting a notch therein.
[0050] FIG. 11 is a pictorial view of a pipe being inspected using
a magnetostrictive transmitting probe and a magnetostrictive
receiving probe for transmission and receipt of torsional waves
with a high DC electric current for circumferential
magnetization.
[0051] FIG. 12 are plots of torsional wave signals received through
the system of the present invention depicted in FIG. 11 when used
to test a pipe filled with water, utilizing a 32 kHz torsional wave
mode in a 4.5 inch outside diameter steel pipe having a 0.337 inch
thick wall and 168 foot length.
[0052] FIG. 13 is an illustration of different types of
magnetostrictive waves in a plate to illustrate dimensional changes
in the plate.
[0053] FIG. 14 is a cross-sectional view of a transmitter or
receiver attached to a pipe for transmission or receipt of
torsional waves.
[0054] FIG. 15 is another embodiment of a cross-sectional view of a
transmitter or receiver attached to a pipe for transmission or
receipt of torsional waves.
[0055] FIG. 16 is a plot of a signal received using the embodiment
as shown in FIG. 14 on a 9.3 foot long pipe having 4 inch outside
diameter and a 0.224 inch thick wall, with the transmitters and
receivers being located on each end of the pipe.
[0056] FIG. 17 is yet another embodiment of a cross-sectional view
of a transmitter and receiver attached to a tube for transmission
or receipt of torsional waves from inside the tube.
[0057] FIG. 18 is another embodiment of a cross-sectional view of a
transmitter or receiver attached to a pipe for transmission or
receipt of torsional waves.
[0058] FIG. 19 is a pictorial diagram showing a use of the present
invention for long term monitoring of a pipe.
[0059] FIG. 20 is a pictorial view of the present invention as used
on a test pipe.
[0060] FIG. 21 is plots of signals received using the test pipe as
shown in FIG. 20, using a differential algorithm.
[0061] FIG. 22 is plots of signals received using the test pipe as
shown in FIG. 20, using another type of differential algorithm.
[0062] FIG. 23 is a cross-sectional view of the embodiment shown in
FIG. 18 as applied to a plate.
[0063] FIG. 24 is a top view of FIG. 23.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0064] As indicated above, the present invention utilizes the basic
methodological approach of earlier developed magnetostrictive
sensor techniques associated with the inspection of cylindrical
structures such as pipe, tubes, and the like. The basic system of
such techniques is combined with a novel magnetostrictive sensor
for application to plate type structures. Reference is made first
to FIG. 1 for a general description of the complete system utilized
to carry on the inspection of a plate type structure. Inspection
system 10 includes a magnetostrictive sensor transmitter control 12
and an associated transmitter coil/core 14. Transmitter coil/core
14 is positioned adjacent to the surface of plate type structure
34. Also positioned near the surface of plate type structure 34 is
receiver coil/core 20. Receiver coil/core 20 is positioned to
detect reflected waves within plate type structure 34 and to
thereby generate a signal representative of the wave
characteristics that are reflected from a defect present in the
structure. Receiver coil/core 20 is connected to preamp/filter 18
which in turn is connected to computer system 16.
[0065] Magnetostrictive sensor transmitter control 12 is comprised
of function generator 22, power amplifier 24, and synchronization
circuitry 26. These elements together generate an appropriate
signal for driving transmitter coil/core 14 and thereby generate
guided waves within plate type structure 34.
[0066] Computer system 16 is comprised of memory 28, digital
processor 30, and analog to digital converter 32. These components
together receive, digitize, and analyze the signal received from
receiver coil/core 20. The signal contains wave characteristics
indicative of the characteristics of the reflected guided waves
present in plate type structure 34.
[0067] Both transmitter coil/core 14 and receiver coil/core 20 have
associated with them bias magnets 36 and 38, respectively. Bias
magnets 36 and 38 are positioned adjacent the coils/cores 14 and 20
near plate type structure 34 in order to establish a bias magnetic
field to facilitate both the generation of guided waves within
structure 34 and the appropriate detection of reflected guided
waves.
[0068] Reference is now made to FIG. 2 for a detailed description
of the novel magnetostrictive sensor structure utilized in the
present invention. Magnetostrictive sensor 11 as shown in FIG. 2
could be utilized as either transmitter coil/core 14 or receiver
coil/core 20 described above in FIG. 1. Magnetostrictive sensor 11
is comprised of a plurality of U-shaped cross-sectional cores
stacked in a lengthwise direction to form a sensor with a
longitudinal axis that is long in comparison to its cross-section.
Core elements 15a through 15n in the preferred embodiment may be
made from a stack of U-shaped ferrites, transformer steel sheets,
mild steel, or permanent magnets. The core elements 18a through 15n
could have other shapes; however, U-shaped or E-shaped core
elements have been found to be more efficient. If an E-shaped core
is used, a transmitter may be located on one part of the E with a
receiver on the other part of the E.
[0069] Surrounding the stack of U-shaped cores 15a through 15n is
wire coil 17. The number of turns for coil 17 is dependent upon the
driving current and the magnetic permeability of core 15 and may be
varied as is well known in the art.
[0070] FIG. 3 shows in cross-sectional view the application of a
pair of sensors structured as shown in FIG. 2 and implemented in
conjunction with the methods of the present invention. In FIG. 3, a
cross-section of plate type structure 34 is shown with transmitter
coil/core 14 and receiver coil/core 20 positioned on the plate. The
view in FIG. 3 of both transmitter coil/core 14 and receiver
coil/core 20 is cross-sectional in nature in order to show the
establishment of a magnetic flux within plate type structure 34.
Associated with each of the coils/cores 14 and 20 are bias magnets
36 and 38. In FIG. 3, bias magnets 36 and 38 are shown placed over
coils/cores 14 and 20. It is understood that in the actual
implementation of the present invention, bias magnets 36 and 38 may
be one or two magnets. What is necessary is that a magnetic field
be generated in plate type structure 34 under the transmitter
coil/core 14 and the receiver coil/core 20. It is only critical
that the DC bias magnetic fields established by bias magnets 36 and
38 are established within the volume of plate type structure 34
under transmitter coil/core 14 and under receiver coil/core 20 as
appropriate.
[0071] Transmitter coil/core 14 is comprised of core material 40
and coil windings 42. Together these components, as driven by the
magnetostrictive sensor transmitter control (not shown), operate to
generate changes in the magnetic field established by bias magnet
36 within plate type structure 34. This time-varying or AC magnetic
field within plate type structure 34 generates a guided wave that
propagates in a direction parallel to the surface of plate type
structure 34. This guided wave is depicted as wave 50 in FIG. 3 and
propagates in a direction away from transmitter coil/core 14. If,
as shown in FIG. 3, transmitter coil/core 14 is placed on the
surface of plate type structure 34, with the longitudinal axis of
coil/core 14 directed into the drawing page in the view shown, wave
50 would propagate in two directions away from the longitudinal
axis of coil/core 14 and through plate type structure 34. This
would serve to investigate the volume of plate type structure 34
bounded by the length (long axis) of the magnetostrictive sensor
utilized. In this manner, an inspection "sweep" of a volume of
plate type structure 34 can be carried out generally equal in width
to the length of the magnetostrictive sensor.
[0072] The arrangement of the magnetostrictive sensor utilized as
the detection coil in the present invention is essentially the same
as the arrangement for the transmitter coil. In FIG. 3, receiver
coil/core 20 is comprised of core material 44, shown in
cross-section, as well as coil windings 46. Bias magnet 38 is
likewise positioned over receiver coil/core 20. This arrangement
establishes a bias magnetic field within plate type structure 34
that fluctuates according to the presence of reflected guided waves
within the material adjacent the sensor. In FIG. 3, reflected
guided waves are depicted as 52 proximate to receiver coil/core 20
and are detected thereby. In this manner, guided waves passing
through plate type structure 34 under receiver coil/core 20 are
detected and "translated" into voltage fluctuations in coil 46 in a
manner that generates an appropriate signal for analysis by the
balance of the electronics of the system of the present invention
(not shown).
[0073] As indicated above, the methods and apparatus of the present
invention can be utilized in conjunction with discrete
magnetostrictive transmitters and receivers or in conjunction with
a single magnetostrictive sensor operable as both a transmitter and
a receiver. In the latter case, the structures described in FIG. 3
would be limited to a single magnetostrictive sensor of the
configuration shown for either transmitter coil/core 14 or receiver
coil/core 20.
[0074] In another alternative approach, one with greater practical
application, two transmitter sensors and two receiver sensors may
be used when the sensors are controlled by appropriate phasing. In
this manner, the direction of the interrogating beam may be
controlled. As an example, when the transmitter generates the wave
in a first position (+) direction, the return signals may be
detected by a receiver controlled to detect waves traveling in the
negative (-) direction. As mentioned above, this control is
achieved by phasing the two sensors appropriately, a process well
known in the field of NDE techniques. In this manner, an inspection
of the plate may be carried out first to one side of the
transmitting sensor and then by simply switching the sensor
instrumentation an inspection may be carried out to the opposite
side of the transmitting sensor. Various other inspection
techniques known and used with magnetostrictive sensors may
likewise apply with the methods and structures of the present
invention.
[0075] Reference is now made to FIGS. 4 and 5 for a detailed
description of sample data acquired from a 0.25 inch thick, 20 foot
long, and 4 foot wide steel plate investigated by the devices and
methods of the present invention.
[0076] The signal represented in FIG. 4 shows the first symmetric
wave mode (S.sub.0) in the plate while the signal depicted in FIG.
5 shows the first anti-symmetric wave mode (A.sub.0). FIG. 4 is a
time varying amplitude plot of a 60 kHz magnetostrictive sensor
signal taken from the above described steel plate geometry. The
wave is directed through appropriate orientation of the sensor and
propagates in the long direction within the steel plate. The signal
components identified in FIG. 4 include the initial pulse 60, end
reflected signal 62, and trailing signals 64. Likewise in FIG. 5,
initial pulse 70 is indicated, as are end reflected signals 72.
[0077] Anomalies within the path of the guided wave generated
within the material would, as is known in the art, generate signal
components having amplitudes sufficient for identification within
either of the two signals shown in FIGS. 4 and 5. In this manner,
characteristics of anomalies detected within the plate type
structure can be identified and located in the direction of wave
propagation away from the magnetostrictive sensor. As is known in
the art, the relative location of an anomaly may be identified by
the position of the signal characteristic indicative of the anomaly
in time relationship with the initial pulse (indicative of the
position of the sensor) and the end reflected signals 62 and
72.
[0078] Examples of such signals are shown in FIGS. 6 and 7. FIG. 6
shows pulse-echo magnetostrictive sensor data for a 40 kHz S.sub.0
wave mode signal obtained in a 4 foot wide, 20 foot long, 0.25 inch
thick steel plate. Three signals are shown for data collected with
a 4 inch long, 8 inch long, and 12 inch long notch cut in the plate
at a point approximately two-thirds of the length of the plate away
from the sensor.
[0079] FIG. 7 shows pulse-echo magnetostrictive sensor data for a
20 kHz A.sub.0 wave mode signal obtained in a 4 foot wide, 20 foot
long, 0.25 inch thick steel plate. Three signals are also shown for
data collected with a 4 inch long, 8 inch long, and 12 inch long
notch cut in the plate at a point approximately two-thirds of the
length of the plate away from the sensor.
[0080] In each case, the notch is not only detectable but may be
characterized as to size and position. Various signal analysis
techniques may be applied to these signals to discern and
characterize other types of anomalies found in such plate-type
structures. Discrete fractures and the like are typically
identified by isolated reflected waves, while broad deteriorations
or corrosions in the plate might be identified by grouped waves
received over a period of time. In addition, it is anticipated that
signature signals of a particular plate type structure might be
acquired prior to implementation of the structure into service. In
this manner subsequent signatures may be acquired periodically and
compared with the initial base line reference signature to
determine the presence of developing anomalies within the
plate.
[0081] To prove the invention works, symmetric (S.sub.0) and
anti-symmetric (A.sub.0) longitudinal wave mode signals were
generated and detected using a 12 inch long magnetostrictive probe
such as shown in FIG. 2. To generate and detect these wave modes,
the bias magnets 36 and 38 are applied in the direction parallel to
the direction of wave propagation (perpendicular to the lengthwise
length of the magnetostrictive probe). The same probe as shown in
FIG. 2 can be used to generate and detect shear horizontal waves in
a plate by applying DC bias magnetic fields in a direction
perpendicular to the wave of propagation (or parallel to the
lengthwise direction of the magnetostrictive probe).
[0082] Using a 4 inch long magnetostrictive probe, a signal was
induced in a 0.25 inch thick, 4 foot wide, 20 feet long, steel
plate. FIG. 8(a) shows the signal as generated and reflected over
time. The initial pulse 100 is generated by the magnetostrictive
transmitter controller 12 until it reaches the far end of the sheet
and a signal from the far end 102 is received by the receiver
coil/core 20. A signal from the near end 104 is received due to the
imperfect directionality control of the system.
[0083] After drilling a 0.25 inch hole about two-thirds of the way
down the sheet, another initial pulse 100 is sent down the sheet.
Again, a signal is received from the near end 104 due to imperfect
directionality control. Also, a signal 102 from the far end is
received. However, now a signal 106 is received that indicates the
0.25 inch hole in the sheet. Therefore, FIGS. 8(a) and (b) in
combination clearly illustrate that shear horizontal waves can be
used in the magnetostrictive inspection techniques and probes of
the current invention. Also, the magnetostrictive testing of the
large plate structures is suitable for low frequency operation (200
kHz or less), has good sensitivity and long range inspection, and
is relatively tolerate to liftoff. This is not the case if the
inspection technique had used other common nondestructive
evaluation techniques, such as electromagnetic acoustic
transducers.
[0084] Pipes can be considered as plates that are simply bent in a
circle. Pipes are literally made from sheet metal that is bent into
a circle and welded on one side thereof utilizing electric
resistance welding. Magnetostrictive inspection techniques may be
used to inspect such pipes as shown and explained in connection
with FIG. 9, including the electric resistance welding. A pipe 200
is shown with a weld line 202. A transmitter coil/core 14 is
located on one side of the pipe 200 and a receiver coil/core 20 is
located 180.degree. on the opposite side of the large diameter pipe
200. While not shown, magnetic bias is provided adjacent to the
transmitter coil/core 14 and the receiver coil/core 20. Using the
inspection system 10 as shown in FIG. 1, an initial pulse 206 is
started around the pipe as shown in FIG. 10(a). Each time the pulse
passes the receiver coil/core 20, a signal 208 is received. The
signal 208 dies out over a period of time and after repeated
revolutions around the pipe 200.
[0085] If the transmitter coil/core 14 is 180.degree. around the
pipe 200 from the receiver coil/core 20, the two opposite going
waves add constructively producing a single large amplitude signal.
Once generated, the initial pulse 206 keeps revolving around the
circumference of the pipe 200 until all of its energy is
dissipated. Therefore, the generated wave produces signals at
regular intervals which are equal to the transient time of the
shear horizontal wave to travel around the full circumference of
the pipe 200. If there are any defects at the weld line 202, they
will clearly be indicated as defect signals. If the weld line is
approximately 90.degree. from transmitter coil/core 14, then the
defect would be approximately midway between the signals 208 as
received by the receiver coil/core 20.
[0086] To prove the measuring of the defects, the applicant, after
measuring the signal as shown in FIG. 10(a), cut a notch in the
pipe 200. The test was then repeated with an initial pulse 206
inducing a shear horizontal wave around the circumference of the
pipe 200. Again, signals 208 indicate each time the shear
horizontal wave reaches the receiver coil/core 20. However, in
addition, there are notch signals 210 that are created by a
reflected signal from the notch that has been induced in the pipe
200. The notch signal 210 increases in amplitude with time because
each time the initial wave revolves around the pipe 200, it passes
the notch defect thereby producing a notch defect signal 210 which
is then added to the previous notch defect signal 210. The
increasing of the notch signal 210 occurs for a period of time and
then it will decrease until its energy is dissipated, the same as
signal 208.
[0087] It is possible to get a comparative indication as to the
size of the defect by the ratio between the first initial wave
signal amplitude 208 and the first defect signal amplitude 210. In
the example illustrated in FIG. 10(b), the notch is approximately
8% of the cross-sectional area. This compares well to the ratio of
signal 208 to 210 being approximately 10%. This is intended to be a
rough generalization as to the size of the notch. Obviously, other
factors would be considered, such as whether the notch is
perpendicular or parallel to the direction of travel of the shear
horizontal wave.
[0088] By use of the method as just described, the present
invention can be used to inspect pipes for longitudinal defects and
corrosion defects. In the present method, the magnetostrictive
probes are moved along the length of pipe to determine any defects
in the pipe. In manufacturing facilities, the magnetostrictive
transmitters or receivers may be stationary with the pipes moving
therebetween and simultaneously being inspected for any
defects.
[0089] While one of the advantages of the present invention is the
ability to carry out broad inspections of large volumes of a plate
type structure from a single positioning of the sensor, it is
anticipated that the complete investigation of a containment vessel
or the like would require multiple placements of the sensor in a
variety of positions and orientations. For example, a containment
vessel might require the placement of the sensor in a sequential
plurality of positions along a predetermined scan line (which could
be either horizontal or vertical to the floor) that best achieves
the inspection of the entire structure. In this manner, a
progressive inspection of an entire containment vessel is carried
out without the requirement that all surfaces of the vessel be
accessed.
[0090] FIG. 11 is a pictorial view of a pipe 300 being inspected
using a magnetostrictive transmitter 314 and a magnetostrictive
receiver 320 on the pipe 300 for transmission and receipt of
torsional waves. A current source 322 is applied to the pipe 300 at
contact points 324 and 326 that connect around the entire pipe 300.
The current source 322 can be either a DC source or a low frequency
AC (approximately 10 Hz).
[0091] At a given frequency, more than one longitudinal (L) wave
mode can exist in a pipe or tube. The defect detectability of the
MsS technology has been found to be hampered by the presence of
extraneous wave modes that were produced by the MsS itself and/or
by mode conversion of the transmitted wave at geometric features in
pipelines, such as welds, elbows and tees. In addition, when the
pipe 300 under inspection is filled with a liquid, the liquid
interacts with the L-wave mode and causes many extraneous signals
to be produced, which can significantly degrade defect
detectability.
[0092] In order to overcome these deficiencies in detecting defects
in pipes or tubes containing a liquid, a torsional wave is used for
the inspection. The torsional wave is a shear wave that propagates
along the length of the pipe 300 or tube. Because the torsional
wave is a shear wave in a pipe or tube, its interaction with a
liquid is negligible (unless the liquid is viscous). Therefore, the
defect detectability of torsional waves will not be hampered by the
presence of liquid in the pipe 300. In addition, the torsional wave
exists as a single mode up to a considerable frequency and,
consequently, has minimal problems in defect detectability due to
the presence of extraneous wave modes.
[0093] The torsional wave therefore is expected to have
significantly better defect detectability than the longitudinal
wave modes.
[0094] To explain why a torsional wave would not be hampered by the
presence of liquid in pipe 300, an explanation of the dimensional
changes in the material due to magnetization and the waves
generated therefrom is provided in conjunction with FIG. 13.
Referring to FIG. 13, the larger arrows 350 shown in FIGS. 13a, b
and c represent the direction of propagation of the wave front.
Referring to FIG. 13a, the dotted lines 352 give an exaggerated
representation of the dimensional changes in the ferromagnetic
plate 354 when a shear wave is projecting in direction 350. Arrows
356 represent the oscillations occurring by the dimensional changes
illustrated by waves 352. For the purposes of illustration, the
dimensional changes due to magnetization caused by waves 352 and
illustrated by arrows 356 have been exaggerated.
[0095] Referring to FIGS. 13b and 13c, Lamb waves are projecting
along the ferrogmagnetic plate 354. In FIG. 13b, the dimensional
changes due to a symmetrical Lamb wave propagating in direction 350
is illustrated in an exaggerated form. The smaller arrows shown in
FIG. 13b represent the dimensional changes of the plate 354. FIG.
13c shows an asymmetrical Lamb wave that would propagate along
plate 354, again with the small arrows representing dimensional
changes of the plate 354. As can be seen in FIG. 13, the
dimensional changes in the Lamb waves shown in FIGS. 13b and 13c
will react against any liquid contained in a pipe or container.
However, the use of a shear wave or a torsional wave as shown in
FIG. 13a, because the dimensional change is in the same plane of
the plate 354, there would be no reaction or interference by the
liquid contained in any pipe or container. Therefore, the shear or
torsional wave is the ideal waveform to use if the plate or pipe is
being checked that may contain a fluid.
[0096] As illustrated in FIG. 11, coil windings 342 and 346,
transmitter 314 and receiver 320 that are used in the existing MsS
L-wave inspection are installed around pipe 300. A high ampere
electric current is applied to pipe 300 by current source 322
applied at contact points 324 and 326 along the length of pipe 300.
The electric current flowing along pipe 300 sets up a DC bias
magnetization in the circumferential direction of the pipe 300
necessary for MsS generation and detection of torsional waves in
the wall of pipe 300. The generated torsional waves propagate along
the length of pipe 300, and signals reflected from defects in pipe
300 are detected in the same manner used for L-wave pipe
inspection. The results of experimentation on this aspect of the
invention are contained in FIG. 12.
[0097] FIGS. 12a-c are plots of signals received through the system
of the present invention utilizing torsional waves when used to
test pipe 300 filled with water shown in FIG. 11. The data were
obtained using a 32 kHz torsional wave mode in a 4.5 inch outside
diameter steel pipe having a 0.337 inch thick wall and 168 foot
length. The sample contained several simulated defects. The DC
current applied was approximately 150 amps, and the frequency of
the MsS was 32 kHz. Signals from small simulated defects (whose
cross sections were about one percent of the total pipe 300 wall
cross section) were not recognizable in these data. It is however
expected that the application of a higher DC current would permit
detection of the small defects. The data showed no effects of
water.
[0098] Referring to the waveform shown in FIGS. 12b and 12c,
numerals 1 through 12 represent the defects that occur in the pipe.
The MsS transmitter 314 and receiver 320 along with coil windings
342 and 346 are located at 54 feet down the pipe 300 from one end
represented by end F1. The other end of the pipe is represented by
end F2. There are three welds in the pipe represented by W1, W2 and
W3, respectively, at 42 feet, 84 feet, and 126 feet. When the
torsional wave is propagated down the pipe towards end F2, there
will be some small amount of reflection of the signal from end F1
because of imperfect direction control as can be seen in FIG. 12b.
Likewise, when the waveform is propagated towards end F1, there is
some reflection of the signal from end F2 as shown in FIG. 12c.
Therefore, in FIG. 12b, the torsional wave signal is first directed
towards end F2. In FIG. 12c, the signal is directed towards end F1.
Also, as can be seen in the signals, some of the simulated defects
are so small they can hardly be distinguished. Other simulated
defects that are larger in cross-sectional area can be seen in the
reflected signals shown in FIGS. 12b and 12c.
[0099] Referring now to FIG. 14, an alternative way of creating the
circumferential magnetic field in a pipe 400 is illustrated.
Wrapped around the pipe 400 is a ferromagnetic strip 402 that
contains residual magnetization. The ferromagnetic strip 402 would
typically be about an inch wide and wrapped almost around pipe 400,
with the exception of a small gap 404 at one end thereof. The
ferromagnetic strip 402 may be made from any material that has good
magnetization characteristics, such as nickel, grain-oriented
silicon steel, or a magnetostrictive material, such as
TERFENDOL-D.RTM.. The objective is to have a flexible strip of
material that has good magnetization characteristics (ability to
retain residual magnetization and high magnetostrictive
coefficient) for wrapping around pipe 400. The residual
magnetization in the ferromagnetic strip 402 is induced prior to
wrapping around the pipe 400 by applying an external magnetic field
to the ferromagnetic strip 402 and then removing the external field
(not shown). After wrapping the ferromagnetic strip 402 around pipe
400, a magnetostrictive coil 406 is placed around the magnetized
ferromagnetic strip 402. The coil 406 may be of the common ribbon
type with a coil adapter 408 connecting the two ends of the ribbon
type coil 406.
[0100] To press the magnetized ferromagnetic strip 402 against pipe
400, some type of external pressure is necessary. The embodiment
shown in FIG. 14 is a flexible strap 410 wrapping around both
ferromagnetic strip 402 and coil 406. The flexible strap 410 is
pulled tight by means of buckle 412, which in turn presses the
ferromagnetic strip 402 against the pipe 400. The guided waves are
then generated in the ferromagnetic strip 402 and coupled into the
pipe 400. For detection, the guided waves in the pipe 400 are
coupled to the ferromagnetic strip 402, which guided waves are
subsequently detected by the MsS coil 406 placed over the
ferromagnetic strip 402.
[0101] For torsional wave generation and detection, the residual
magnetization is induced along the lengthwise direction of the
ferromagnetic strip 402. For longitudinal wave generation and
detection, the residual magnetization is induced along the width of
the ferrogmagnetic strip 402. The pressing on the ferromagnetic
strip 402 provides a mechanical coupling of the guided waves
between the pipe 400 and the ferromagnetic strip 402. The
illustration as shown in FIG. 14 can be either a transmitter or a
receiver of guided waves (either longitudinal or torsional wave
modes) that are propagated along the pipe 400.
[0102] Referring now to FIG. 15, another alternative is shown as to
how to create a guided wave in pipe 500. Just as in FIG. 14, in
FIG. 15, a magnetized ferromagnetic strip 502 is wrapped around the
pipe 500. Again, a gap 504 will exist between two ends of the
ferromagnetic strip 502. Also, the same as is the case in FIG. 14,
a coil 506 is wrapped around the ferromagnetic strip 502, which
coil 506 is of the ribbon type and connected by a coil adaptor 508.
However, the means of applying pressure against the ferromagnetic
strip 502 to press it against the pipe 500 is different in FIG. 15
from FIG. 14. In FIG. 15, a metal case or container 510 encircles
the ferromagnetic strip 502 and coil 506. The metal case or
container 510 is held together by clamp 512. Inside of the metal
case or container 510 is located a pneumatic or hydraulic tube 514
that may be inflated. By inflating the tube 514, it presses the
coil 506 and ferromagnetic strip 502 against the pipe 500. Again,
the embodiment as just explained in conjunction with FIG. 15 may be
used as either a transmitter or receiver of guided waves being
propagated along pipe 500.
[0103] The width of the magnetized ferromagnetic strips 402 or 502
is adjusted depending on the frequency and the mode of the guided
waves. For high frequencies, the magnetized ferromagnetic strips
402 or 502 should be narrower; for lower frequencies, the
magnetized ferromagnetic strips 402 or 502 should be wider.
[0104] The feasibility of the approach explained in FIGS. 14 or 15
has been proven in the laboratory as illustrated in conjunction
with FIG. 16. Using a 4-inch outside diameter pipe with a 0.224
inch wall thickness pipe which was 9.3 feet long, a crude test was
performed. The magnetized ferromagnetic strip 402 or 502 was made
of 0.01 inch thick nickel foil. The magnetized ferromagnetic strips
402 or 502 were placed circumferentially around each end of the
pipe sample. The magnetized ferromagnetic strips 402 or 502 were
mechanically coupled to the outside surface of the pipe and in this
case strapped using the method as shown in FIG. 14. FIG. 16 shows
the data acquired at 64 kHz by transmitting the torsional wave from
one end of the pipe and detecting the signals at the other end of
the pipe. The data clearly indicates FIG. 14 as being an acceptable
method for generating and detecting guided waves in pipes.
[0105] Referring to FIG. 17, a probe for generating and detecting
guided waves in a tube 600 from inside the tube 600, which uses the
same principle as the present invention, is illustrated. A
pneumatic tire 602 has ferromagnetic strips 604 and 606 bonded
therearound. In FIG. 17, ferromagnetic strips 604 and 606 represent
a transmitter and a receiver, respectively, of the torsional waves.
The pneumatic tire 602 has a pressure valve 608 for
inflating/deflating.
[0106] Inside of the pneumatic tire 602 are two bobbin type cores
610 and 612 about which a transmitting coil 614 and receiving coil
616 are wound, respectively. To hold everything together in their
respective locations, the cores 610 and 612 are mounted on rod
618.
[0107] By inflating the pneumatic tire 602 through pressure valve
608, the magnetized ferromagnetic strips 604 and 606 are pressed
against the inside of tube 600. Thereafter, the guided wave
generated by transmitting coil 614 in the ferromagnetic strip 604
is coupled to the tube 600 and propagates along the tube 600.
Reflected signals from defects in tube 600 are received back
through the ferromagnetic strip 606 and detected by receiving coil
616. The type of signal that will be generated will be a guided
wave that propagates along tube 600. It is envisioned that the
configuration as shown in FIG. 17 will be inserted in the end of a
tube 600 to propagate a signal down the entire length of the tube
to detect flaws or defects that may exist in the tube 600. The
cores 610 and 612 are ferrite or ferromagnetic steel to aid in the
transmission and receiving of magnetostrictive signals to and from
the tube 600.
[0108] Another embodiment of the present invention that has been
found useful for either short term inspection or long term
monitoring of pipelines is illustrated in the embodiment shown in
FIG. 18. A pipe 700 has a thin ferromagnetic strip 702 attached to
its outer surface by a suitable couplant 704. Wrapped around the
outside of the thin ferromagnetic strip 702 is a coil 706 that has
external connections 708 and 710.
[0109] As previously described in connection with FIG. 14, the thin
ferromagnetic strip 702 is about one-half inch to one inch wide and
has a gap 712 between the respective ends thereof. The thin
ferromagnetic strip 702 may be made from any material that has good
magnetization characteristics, such as nickel, grain-oriented
silicon steel or a magnetostrictive material, such as
TERFENDOL-D.RTM.. The thin ferromagnetic strip 702 should have the
flexibility that it can be wrapped around the pipe 700. Also, it is
important that the thin ferromagnetic strip 702 retain residual
magnetization and have a high magnetostrictive coefficient.
[0110] The couplant 704 may vary depending upon whether the use is
for a short term inspection or a long term monitoring. If the use
is for short term inspection, the couplant 704 would be of a thick,
highly viscous material, such as honey, that would stick the thin
ferromagnetic strip 702 to the pipe 700. Also, the coil 706 would
have a coil adapter (similar to those described in connection with
FIGS. 14 and 15) so that the coil 706 can be quickly removed.
However, for the purposes of this illustration, assume that long
term monitoring is desired. For long term monitoring, the couplant
704 would be made from a couplant that becomes a rigid material,
such as epoxy, to physically bond the thin ferromagnetic strip 702
to the pipe 700. For long term monitoring, it is important that the
couplant 704 maintain a good bond with the pipe 700 over an
extended period of time.
[0111] The thin ferromagnetic strip 702, prior to placing on the
pipe 700, has residual magnetization induced therein. Because a
torsional wave is ideal for long term monitoring of a pipe,
especially a pipe that may be filled with fluid, the residual
magnetization in the thin ferromagnetic strip 702 is induced in the
lengthwise direction of the thin ferromagnetic strip 702.
Thereafter, the thin ferromagnetic strip 702 is ready for bonding
to the pipe 700 with the couplant 704. After bonding, the coil 706
is wrapped around the thin ferromagnetic strip 702, with the
external connections 708 and 710 being available for
monitoring.
[0112] An ideal situation for the use of the magnetostrictive
sensor monitoring technology is involving gas pipelines. It has
been found that gas pipelines have a tendency to accumulate fluids
inside the gas pipeline along any low point in the gas pipeline,
which fluid accumulation will tend to cause corrosion. Referring to
FIG. 19, a gas pipeline 714 is buried under ground 716 so that a
low point 718 exists in the gas pipeline 714. At the bottom of the
low point 718 is a corrosion defect 720. If there is some way to
monitor the low point 718 in the gas pipeline 714, the corrosion
defect 720 can be determined before catastrophic results, such as
explosion of the pipeline.
[0113] Some distance from the low point 718 (typically up to 50
feet), magnetostrictive probes 722 (similar to those described in
FIG. 18) are mounted around the gas pipeline 714. At least two
magnetostrictive probes have to be used, but to determine
directionality, a minimum of four magnetostrictive probes are
necessary to make use of phased array interference principals so
that direction of the signals can be determined. In the present
illustration as shown in FIG. 19, four magnetostrictive probes 722
are illustrated.
[0114] Because the magnetostrictive probes 722 are buried under
ground 716, and may be left buried for long periods of time with
just periodic monitoring, some type of shielding cover 724 is
necessary to protect the magnetostrictive probes 722. Electrical
wires 726 connect to a junction box 728 located at the surface 730
of the ground 716.
[0115] In actual use, periodically magnetostrictive sensor
monitoring electronics 732, similar to that described in
conjunction with FIG. 1, is connected to the junction box 728 at
the surface 730. The magnetostrictive sensor monitoring electronics
732 generates a signal that is fed through the electric wires 726
to the magnetostrictive probes 722 that causes a guided wave 734 to
propagate along the gas pipeline 714. If there is a corrosion
defect 720 in the gas pipeline 714, a defect signal 736 will be
reflected back to the magnetostrictive probes 722 for detection by
the magnetostrictive sensor monitoring electronics 732 via the
electric wires 726 and connection box 728.
[0116] This system as just described in conjunction with FIG. 19 is
envisioned for use along low points of gas pipelines that need to
be monitored on an infrequent basis, such as every six months. By
use of a permanent reference signal and comparing future signals
against the reference signal, very small changes due to corrosion
can be detected. Using this technique, corrosion defects as small
as 0.2 percent of the cross-sectional area of the gas pipeline 714
can be detected.
[0117] This invention has been proven in the laboratory as will be
explained in conjunction with FIG. 20 and the waveforms shown in
FIGS. 21 and 22. A pipe 738 is shown that is 29.4 feet long and 4.5
inches in outside diameter having a 0.337 inch thick wall, each end
being represented by E1 and E2. At 4 feet from E1 are located the
magnetostrictive probes 722. The magnetostrictive probes 722
generate a guided wave 734 that propagates along the pipe 738. A
corrosion defect 740 causes a reflected defect signal 742. The
reflected defect signal 742 is 19 feet from end E1.
[0118] Referring to FIG. 21, waveform 1 shows the result of
subtracting the interference to form a waveform collected prior to
any corrosion being applied. The reference signal subtracted from a
second waveform obtained at a time different from when the
reference signal is obtained is called the difference signal. An
initial pulse 744 is applied to the pipe 738 and the directionality
of the generated wave is controlled using electronics designed on
phased array principals. The reflected end signals (E1 and E2) are
canceled out because they are in both the reference and the
waveform collected before any corrosion is applied. Then after
applying a defect at corrosion defect point 740 that is
approximately 0.26 percent of the cross-sectional area of the pipe
wall, the difference signal is obtained for a 0.26 percent defect,
the data shown in waveform 2 is obtained. This shows that the 0.26
percent defect signal 736 is just becoming detectable. Once the
corrosion defect 740 is increased to 0.48 percent of the
cross-sectional area, shown in waveform 3, the defect signal 736 is
clearly detectable. By increasing the size of the corrosion defect
to 0.74 percent of the cross-sectional area, the difference signal
becomes even larger as shown in waveform 4. By the time the
corrosion defect 740 reaches 0.98 percent of the cross-sectional
area as shown in waveform 5, the defect signal 736 is clearly
visible.
[0119] Also as the temperature of pipe 738 varies, the signal will
travel at different speeds in the pipe. Therefore, when obtaining
the difference signal, sometimes there is not a perfect match in
the two signals due to the difference of speed of the signal moving
along the pipe 738 caused by temperature changes. Referring to FIG.
21, the end signals E2 begin to increase in waveforms 2-5 due to
the temperature change. However, for buried pipelines as
illustrated in FIG. 19, the temperature underground is relatively
constant and is close to the average mean temperature for the
area.
[0120] To give an even clearer indication as to a defect, certain
processing can be applied to the waveforms as shown in FIG. 21. For
example, the signal can be squared and then averaged over a short
window of time to give waveforms 1-5 as shown in FIG. 22. In this
manner, the difference signal for the defect signal 736 is even
clearer.
[0121] The difference signal 736 becomes detectable at slightly
over 0.2 percent loss of the cross-sectional area of the pipe being
monitored.
[0122] By use of the techniques as just described in conjunction
with FIGS. 18-22, detection of defects can occur in pipes other
than ferromagnetic pipes. For example, the pipe could be plastic
with the torsional wave being transmitted to the plastic pipe
through the coupling. In other words, the torsional wave set up in
the thin ferromagnetic strip 702 is transferred to any type of pipe
700 as long as the pipe is rigid with a high modulus of
elasticity.
[0123] The same principle can be used for plate-type structures as
is shown in conjunction with FIG. 23. A plate 746 has a thin
ferromagnetic strip 748 coupled thereto by a couplant 750. Again,
the thin ferromagnetic strip 748 may be of nickel or other
materials described in conjunction with FIG. 18. On top of the thin
ferromagnetic strip 748 is located a plate magnetostrictive probe
752. The plate magnetostrictive probe 752 could be either the type
illustrated in FIG. 2 or a coil laid on a printed circuit board as
illustrated in FIG. 24.
[0124] The couplant 750 is made from any thick material that will
couple the thin ferromagnetic strip 748 to the plate 746. If a
permanent monitoring feature is desired, the couplant 750 would be
made from a coupling material that becomes rigid, such as epoxy.
However, if it is desirable to periodically inspect the plate 746,
and thereafter remove the magnetostrictive probe, the couplant 750
may be made from a thick viscous material, such as honey. However,
other types of thick viscous material that will allow the
magnetostrictive probe to be removed can be used.
[0125] The guided wave to be used in conjunction with FIGS. 18-22
for piping applications is a torsional wave. The guided wave to be
used for plate-type structures would be a shear horizontal wave.
The method and apparatus as described in conjunction with FIGS.
18-23 may be used not only to detect corrosion, but can also be
used to detect transient stress signals due to vibration, cracking
or mechanical impacts (for example, crash event of a passenger car
for air bag operation). By use of the system as just described, it
is inexpensive to implement by the end user.
[0126] Although a description of a preferred embodiment of the
apparatus and method of the present invention has been described,
it is anticipated that variations in the manner in which the basic
sensor structure of the present invention may be utilized are
possible. No specific dimensions for the sensor structure described
have been identified as such would be dependent upon the specific
plate type structures to be investigated. It is anticipated that
sensors of a variety of lengths could be utilized depending upon
the requirements of the environment of investigation. It is
anticipated that other applications of the basic sensor structure
described herein will be discerned by those skilled in the art of
nondestructive evaluation of materials.
* * * * *