VISVESVARAYA TECHNOLOGICAL UNIVERSITY VISVESVARAYA TECHNOLOGICAL UNIVERSITY
“Jnana Sangama”, Belgaum-590014, Karnataka
A Seminar Report on
“ULTRASONIC TECHNIQUES FOR HIDDEN CORROSION DETECTION”
Submitted in partial fulfillment of the requirement for the award of the degree of
MASTER OF TECHNOLOGY MASTER OF TECHNOLOGYIn
MACHINE DESIGN MACHINE DESIGNBy
THEJAS NUnder the guidance of
Dr.M.Venkatarama ReddyDr.M.Venkatarama Reddy Professor and Head, Professor and Head,
Dept. of Mechanical EngineeringDept. of Mechanical EngineeringBIT, BangaloreBIT, Bangalore
Department of Mechanical EngineeringDepartment of Mechanical EngineeringBANGALORE INSTITUTE OF TECHNOLOGYBANGALORE INSTITUTE OF TECHNOLOGY
K R Road, V V Puram, Bangalore(2009-2010)
Ultrasonic techniques for hidden corrosion detection
BANGALORE INSTITUTE OF TECHNOLOGY
(K R Road, V V Puram, Bangalore)
Department of Mechanical Engineering (M-Tech)
CERTIFICATE
This is to certify that the seminar topic entitled “ULTRASONIC TECHNIQUES FOR
HIDDEN CORROSION DETECTION” is a bonafied work carried out by THEJAS N
bearing university seal number 1BI09MMD17 of 1st semester M.Tech in Machine Design at
Bangalore Institute of Technology, Bangalore in partial fulfillment of the Master of Technology
in Mechanical engineering (Machine Design) of Visveswaraiah Technological University,
Belgaum, during the year Sep2009-Jan2010. It is certified that all correction and deposited in
the department library. The seminar report has been approved as it satisfies the academic
requirement in respect of seminar work prescribed for the Master of Technology Degree.
PLACE :DATE : SIGNATURE OF STUDENT
SIGNATURE SIGNATURE SIGNATUREOF GUIDE OF H.O.D OF PRINCIPAL
Ultrasonic techniques for hidden corrosion detection
ACKNOWLEGEMENTS
A great deal of time and effort has gone into successful execution of my seminar and
preparation of this seminar report. I would like to acknowledge the contribution from various
sources and people who have been instrumental in the success of this endeavor.
I wish to place on record my heart-felt thanks to Dr.M.Venkata Rama Reddy,
Professor and Head, Department of Mechanical Engineering, BIT for his ever inspiring support
and encouragement.
I sincerely express my gratitude to All Staff, Machine Design, and BIT for their kind
co-operation in guiding me in the seminar.
Thanks & Regards,
(THEJAS N)
Ultrasonic techniques for hidden corrosion detection
CONTENTS
1. INTRODUCTION
2. HIDDEN CORROSION
3. TYPES OF CORROSION
3.1 Galvanic Corrosion
3.2 Pitting Corrosion
3.3 Erosion-Corrosion
3.4 Crevice Corrosion
3.5 Intergranular Corrosion
4. POTENTIAL DAMAGE AREAS OF CORROSION
5. ULTRASONIC TECHNIQUES
5.1 BASIC PRINCIPLE OF ULTRASONIC INSPECTION
5.2 MODES OF SOUND WAVE PROPAGATION
5.3 GUIDED ULTRASONIC WAVES FOR CORROSION DETECTION
CASE STUDY
6. CORROSION DETECTION IN AIRCRAFT STRUCTURES USING GUIDED LAMB WAVES
6.1 WHY GUIDED LAMB WAVES
6.2 OBJECTIVES
6.3 CONVENTIONAL ULTRASONIC INSPECTION
6.4 GUIDED WAVE INSPECTION
6.5 EXPERIMENTAL SETUP
7. APPLICATIONS OF ULTRASONIC TECHNIQUES
8. REFERENCES
Ultrasonic techniques for hidden corrosion detection
1. INTRODUCTIONCorrosion is one of the serious problem affecting air force and other aviation industries.
It affects the aircraft on its wings, surface, between joints and fasteners. The presences of
corrosion underneath the paints of surface and between joints are not easy to be detected. The
unnoticed presence of corrosion may cause the aircraft to crash leading to human and money
loses. To detect the corrosion present on the metal surface, various methods and tests are used.
These tests conducted should be such that it does not destroy or disassemble the plane to parts
or damage its surface. Hence for the further use of the plane, Non-destructive tests (NDT) are
carried out.
Non-destructive testing as the name suggests is testing procedure without any damage to
the part being tested. The various non-destructive testing methods used are:
1) Visual inspection2) X-ray inspection3) Die (liquid) penetration inspection4) Magnetic particle inspection5) Eddy current inspection6) Ultrasonic inspection
Ultrasonic inspection is conventionally used for corrosion detection in aircraft wings. But the conventional inspection method carries with it certain defects like: (i) It scans perpendicular to the surface and hence rate of scanning (from point to point) is less and hence highly time consuming.(ii) Conventional method is not capable of detecting disbonds between layers and cracks at fastener holes.These defects are over come by a newly developed inspection method using guided ultrasonic waves.
Guided waves demonstrate an attractive solution where conventional ultrasonic
inspection techniques are less sensitive to defects such as corrosion/disbonds in thin
multilayered wing skin structures and hidden exfoliation under wing skin fasteners. Moreover,
with their multimode character, selection of guided wave modes can be optimized for detection
of particular types of defects. Mode optimization can be done by selecting modes with
maximum group velocities (minimum dispersion), or analysis of their wave mode structures
(particle displacements, stresses and power distributions). Guided Lamb modes have been used
for long range/large area corrosion detection and the evaluation of adhesively bonded
structures.
Ultrasonic guided waves are promising but require procedure development to ensure
high sensitivity and reliable transducer coupling and to provide a mechanism to transport the
Ultrasonic techniques for hidden corrosion detection
probe(s) over the area to be scanned. This paper describes some practical inspection setups and
procedures based on guided wave modes for corrosion damage detection in single and
multilayered wing skin structures and exfoliation detection immediately adjacent to fasteners in
aircraft wing skin. It describes the results of their application to detection of corrosion in
simulated and real components of aircraft wing skin. Using a tone burst system, the wave
modes are selected, excited and tested in pulse echo and pitch catch setups. Launch angles were
obtained from the calculated dispersion curves. Theoretical group velocities were compared to
tested group velocities to confirm the excited modes at frequency thickness product and launch
angle. The simulated corrosion in single and multilayered wing skin structures and exfoliation
located under several rivets was successfully detected. Some guided Lamb modes proved to be
more sensitive to corrosion type defects and produced better results
2. HIDDEN CORROSIONHidden corrosion is a type of electro-chemical material degradation that is not readily or
directly detectable visually or by any other surface measurement technique. It can often be
detected and quantified in terms of reduction of wall thickness or structural discontinuities such
as pits, flaws and voids. When attempting to detect material degradation due to electro-
chemical processes, the corrosion products (e.g., iron oxides, aluminum oxides, etc.) must be
identified so that an appropriate energy source can be selected for detection.
3. TYPES OF CORROSION
Corrosion in aircraft may appear in various forms depending on the alloy, product form,
corrodent, general conditions and residual stress. This complicates the metrics of corrosion and
therefore also complicates the quantification of detection reliability. Some corrosion types are
listed below
3.1 Galvanic Corrosion
Galvanic corrosion is a very common form of corrosion that results from contact
between dissimilar metals. A difference in the electrode potential of the two metals and the
difference in the surface area of the dissimilar metals drive the process. Galvanic corrosion is
responsible for much of the corrosion in aircraft.
3.2 Pitting Corrosion
Ultrasonic techniques for hidden corrosion detection
Pitting is another form of corrosion that results when the anodic site in the
electrochemical reaction corresponds to a local micro structural discontinuity, such as an
inclusion, grain boundary, or even a scratch, on an otherwise large cathodic surface area.
3.3 Erosion-Corrosion
Erosion-corrosion, as its name suggests, results from the actions of corrosion and
erosion in the presence of a moving corrosive fluid, causing accelerated loss of the metal.
3.4 Crevice Corrosion
Crevice corrosion is a form of localized corrosion that occurs near an area of a metal
surface adjacent to another metal that is sheltered from full exposure to the environment. The
reaction between the oxygen in the crevice and the rest of the metal causes a gradient in the
oxygen concentration, and thus a difference in electrode potentials and a flow of current.
3.5 Intergranular Corrosion
Intergranular corrosion occurs at or adjacent to the grain boundaries of a metal or alloy.
The actual mechanism of the corrosion varies with metal system. This attack at the grain
boundaries can cause entire metal grains to become dislodged. Leakage of corrosive fluids, loss
of effective cross sectional area, and mechanical failure can result.
4. POTENTIAL DAMAGE AREAS OF CORROSION The severity of corrosion attacks varies with aircraft type, design techniques, operating environments, operators and maintenance programs. Common areas of corrosion problems are listed below
1. Floor and structure in the vicinity of lavatory systems and galleries,
2. Structures surrounding doors, particularly landing gear doors,
3. wing skin adjacent to counter fastener heads,
4. Wing to body joint fittings,
5. Fuselage lower structure (bilge area),
6. Areas having environmentally unstable materials,
7. Structures susceptible to protective treatment damage during installation and repair, abrasion, fretting and erosion.
5. ULTRASONIC TECHNIQUES
Ultrasonic techniques for hidden corrosion detection
5.1 BASIC PRINCIPLE OF ULTRASONIC INSPECTION
Fig1. Ultrasonic testing equipment
Ultrasonic Testing (UT) uses high frequency sound energy to conduct examinations and
make measurements. Ultrasonic inspection can be used for flaw detection/evaluation,
dimensional measurements, material characterization, and more.
A typical UT inspection system consists of several functional units, such as the
pulser/receiver, transducer, and display devices. A pulser/receiver is an electronic device that
can produce high voltage electrical pulses. Driven by the pulser, the transducer generates high
frequency ultrasonic energy. The sound energy is introduced and propagates through the
materials in the form of waves. When there is a discontinuity (such as a crack) in the wave path,
part of the energy will be reflected back from the flaw surface. The reflected wave signal is
transformed into an electrical signal by the transducer and is displayed on a screen.
5.2 MODES OF SOUND WAVE PROPAGATIONIn air, sound travels by the compression and rarefaction of air molecules in the direction
of travel. However, in solids, molecules can support vibrations in other directions, hence, a
number of different types of sound waves are possible. Waves can be characterized in space by
oscillatory patterns that are capable of maintaining their shape and propagating in a stable
manner. The propagation of waves is often described in terms of what are called “wave
modes.” The different types of modes of sound wave propagation in solids are listed below
5.2.1 Longitudinal waves and shear waves
Ultrasonic techniques for hidden corrosion detection
Fig2. Longitudinal and shear wavesIn longitudinal waves, the oscillations occur in the longitudinal direction or the direction
of wave propagation. Since compressional and dilational forces are active in these waves, they
are also called pressure or compressional waves. They are also sometimes called density waves
because their particle density fluctuates as they move. Compression waves can be generated in
liquids, as well as solids because the energy travels through the atomic structure by a series of
compressions and expansion (rarefaction) movements.
In the transverse or shear wave, the particles oscillate at a right angle or transverse to the
direction of propagation. Shear waves require an acoustically solid material for effective
propagation, and therefore, are not effectively propagated in materials such as liquids or gasses.
Shear waves are relatively weak when compared to longitudinal waves. In fact, shear waves are
usually generated in materials using some of the energy from longitudinal waves.
5.2.2 Surface waves
Fig3. Surface waves
Surface (or Rayleigh) waves travel the surface of a relatively thick solid material
penetrating to a depth of one wavelength. Surface waves combine both a longitudinal and
transverse motion to create an elliptic orbit motion as shown in the image and animation below.
The major axis of the ellipse is perpendicular to the surface of the solid. As the depth of an
Ultrasonic techniques for hidden corrosion detection
individual atom from the surface increases the width of its elliptical motion decreases. Surface
waves are generated when a longitudinal wave intersects a surface near the second critical angle
and they travel at a velocity between .87 and .95 of a shear wave. Rayleigh waves are useful
because they are very sensitive to surface defects (and other surface features) and they follow
the surface around curves. Because of this, Rayleigh waves can be used to inspect areas that
other waves might have difficulty reaching.
5.2.3 Plate waves (or lamb waves)
Fig4. Lamb wave
Plate waves are similar to surface waves except they can only be generated in materials
a few wavelengths thick. Lamb waves are the most commonly used plate waves in NDT.
Lamb waves are complex vibrational waves that propagate parallel to the test surface
throughout the thickness of the material. Propagation of Lamb waves depends on the density
and the elastic material properties of a component. They are also influenced a great deal by the
test frequency and material thickness. Lamb waves are generated at an incident angle in which
the parallel component of the velocity of the wave in the source is equal to the velocity of the
wave in the test material. Lamb waves will travel several meters in steel and so are useful to
scan plate, wire, and tubes. With Lamb waves, a number of modes of particle vibration are
possible, but the two most common are symmetrical and asymmetrical. The complex motion of
the particles is similar to the elliptical orbits for surface waves. Symmetrical Lamb waves
move in a symmetrical fashion about the median plane of the plate. This is sometimes called
the extensional mode because the wave is “stretching and compressing” the plate in the wave
motion direction. Wave motion in the symmetrical mode is most efficiently produced when the
exciting force is parallel to the plate. The asymmetrical Lamb wave mode is often called the
“flexural mode” because a large portion of the motion moves in a normal direction to the plate,
Ultrasonic techniques for hidden corrosion detection
and a little motion occurs in the direction parallel to the plate. In this mode, the body of the
plate bends as the two surfaces move in the same direction.
5.3 GUIDED ULTRASONIC WAVES FOR CORROSION DETECTION
Guided ultrasonic wave NDE offers the potential for a cost effective methodology for
inspection of hidden corrosion in large and sometimes difficult to access areas, such as
insulated piping. The field of guided waves has reached some degree of maturity, but
unfortunately the number of practical applications compared to the number of research papers is
rather small. Guided waves can be used in three regimes, depending on inspection distance:
• Short range (<< 1 m)
• Medium range (up to about 5 m)
• Long range (up to around 100 m)
The short range methods include high frequency surface wave scanning with rayleigh
waves, leaky lamb waves, and acoustic microscopy in which a leaky surface wave is generated
by the lens. The medium range methods typically use frequencies in the 250 kHz to 1 MHz
range and are applicable to plate, tube and pipe testing. The long range method generally uses
long range frequencies below 100 kHz, and the primary advantage is that it allows a large area
to be tested from a single transducer location without tedious scanning. Long range testing,
which has the greatest potential utility in field-testing, is usually carried out in the pulse echo
mode
5.3.1 Basic principle of ultrasonic guided waves
Fig5. Basic principle of ultrasonic guided waves
The major difference between bulk wave propagation and guided wave propagation is
the fact that a boundary is required for guided wave propagation. As a result of a boundary
along a thin plate or interface, we can imagine a variety of different waves reflecting and mode
converting inside a structure and superimposing with areas of constructive and destructive
Ultrasonic techniques for hidden corrosion detection
interference that finally leads to the nicely behaved guided wave packets that can travel in the
structure.
Fig5 shows the angle beam transducer for the generation of guided waves by pulsing a
piezoelectric element on the wedge placed on a test surface. As a result of refraction at the
interface between the wedge and the test specimen, a variety of different waves can propagate
in the structure and by way of mode conversion and reflection from the surfaces of the structure
can lead to interference patterns as a resulting wave vector propagates along the structure.
Snell’s law can be used to calculate the resulting phase velocity, sometimes referred to as a
‘‘Cremer hypothesis.’’ In doing calculations of finding out what interference packages might
come about in the material, one can produce a so-called ‘‘dispersion curve’’ that shows the
wave propagation possibilities of phase velocity and frequency that could possibly propagate in
the structure.
5.3.2 Air coupled ultrasonic guided wave for hidden corrosion detection in multilayer aircraft structures
Fig6.Air coupled ultrasonic guide waves
Non-contact air-coupled transducers can be used to apply guided waves to the
inspection of thinning in aluminum plates. In this application, a pair of micro machined gas
(air)-coupled capacitive transducers is used for the generation and detection of guided plate
modes. Features in the dispersive behavior of selected guided wave modes were used for the
detection of plate thinning. Mode cutoff measurements provided a qualitative detection of plate
thinning, while frequency shift measurements were able to provide a quantitative measure of
plate thinning. The experimental setup with air coupled transducers is shown in Figure6.Non-
contacting electromagnetic acoustic transducers (EMATs) can also be used to generate and
detect shear horizontal (SH)-guide waves for inspection and mapping of corrosion in pipe walls
and plates. The SH waves have a pure shear-motion parallel to the surfaces and perpendicular to
Ultrasonic techniques for hidden corrosion detection
the plane of incidence. SH-guided waves have a unique feature in contrast to guided waves with
in plane polarization; the lowest order mode has no dispersion and the dispersion of the higher
order modes is much weaker than modes with polarization in the plane of incidence. As a result,
SH-guided waves could be economically and reliably used to detect and map corrosion in plates
and pipes. Couplant free excitation and the resultant simplified waveforms add to the versatility
and usefulness of the technique.
5.3.3 Advantages of guided ultrasonic waves Inspection over long distances from a single probe position.
By mode and frequency tuning, to establish wave resonances and excellent overall defect detection potential
Ability to detect structures under water, coatings, insulation, multilayer structures or concretes with an excellent sensitivity.
Potential with multimode and frequency lamb type, surface or horizontal shear waves to detect, locate, classify and size defects.
Cost effectiveness because of simplicity and speed
CASE STUDY
6. CORROSION DETECTION IN AIRCRAFT STRUCTURES USING GUIDED LAMB WAVES6.1 WHY GUIDED LAMB WAVES
Lamb wave are used because they offer an improved inspection potential due to their:
variable mode structure and distributions
multimode character
sensitivity to different type of flaws
propagation for long distances
Guiding character which enables them to follow curvature and reach hidden and/or
buried parts.
6.2 OBJECTIVES
Non-destructive testing methods for simple, rapid and reliable corrosion detection in
complex metallic assemblies is an on-going challenge; this is due to the size and
geometric complexity of these assemblies.
Ultrasonic techniques for hidden corrosion detection
Nondestructive ultrasonic testing technique based on velocity change, attenuation and
backscattering is been successfully applied by using ultrasonic bulk waves. However,
plate waves whose velocity changes with frequency and thickness product can equally
be used to detect defects and corrosion in multilayered metallic structures.
This work demonstrates the benefits of Lamb waves for detecting corrosion in
aluminum multilayered structures.
The main objective therefore, was to develop NDT method, through a theoretical and
experimental work, to detect corrosion in multilayered aluminum structures.
Experimentally elaborated Guided wave testing method to determine quickly and
reliably where these multilayered structurally significant parts are in need of repair.
To demonstrate detectability and sensitivity of guided wave techniques, experiments
were performed on 1.0 -2.0 mm thick aluminum plates and facilitate interpretation
results are presented through imaging.
6.3 CONVENTIONAL ULTRASONIC INSPECTION
Fig7.Conventional ultrasonic inspection
With Conventional ultrasonic method like C-scan the area under interrogation at any
instant is limited to region covered by the transducer. Therefore, it is a localized point
by point inspection technique.
This method is an efficient conventional inspection technique but it is very time
consuming for large structural areas.
C-scans also have difficulty to inspect non uniform and buried structures, since with a
C-scan; a transducer needs access to each point of the inspected area.
Ultrasonic techniques for hidden corrosion detection
6.4 GUIDED WAVE INSPECTION
Fig8. Lamb wave inspection technique
Lamb waves also known as plate waves are based on plate wave natural resonant
modes. Lamb waves are two dimensional stress waves are guided by the geometry of
the plate-like structures whose surfaces are free of stresses. They can propagate in plate-
like structures that are only a few wavelengths thick (d<=3l) where l represents the
incident wavelength.
Particle displacements and stresses in the Lamb waves occur throughout the thickness of
the plate. Their propagation properties depend on the density, the elastic properties and
geometrical structure of the inspected object and are also influenced by the thickness of
the material and the wave cyclic frequency.
6.5 EXPERIMENTAL SETUP
Ultrasonic techniques for hidden corrosion detection
Fig9. Tone burst pulser/receiver system
The basic equipment in the instrumentation set up was a tone-burst pulser/receiver
system that can be used to excite a high power narrow-band width guided wave mode.
The schematic diagram of a generalized tone-burst system set up is given in the above
Figure.
The output of system is a tone-burst waveform which is fed to the sender (transmitting
transducer). This burst is initially formed of a continuous sine wave from the function
generator which is gated and subsequently amplified. The received signal (received
from the receiving transducer) is transferred to the broadband receiver through the
attenuator and the digital oscilloscope.
The flexibility of guided wave approach is based on mode selection, criteria which are
dictated by launch angle and excitation frequency.
The most widely employed method of guided wave excitation is the wedge or prismatic
coupling block method, which is based on conversion (Cook, Valkenburg and Minton
1954).
Ultrasonic techniques for hidden corrosion detection
Dispersion curves describe the natural resonance of a specific structure. Commonly
presented as a plot of phase group velocity versus the frequency thickness of the
structure.
They depend upon material properties and the particular geometrical model selected.
We use the dispersion curves to determine the incidence angle of the transducer and to
select any desired mode.
Another type of dispersion curve is presented as a plot of group velocity versus
frequency*thickness product of the structure.
These graphs are essential for signal interpretation and identification
Detectability of corrosion was investigated in two aluminum specimens with two types of
simulated corrosion. The first specimen was an aluminum plate with dimension 460x405x1 mm
with controlled thinning in designated areas. This first type of corrosion is named open surface
corrosion because corrosion is visible to the naked eye. To demonstrate the sensibility of the
excited wave modes, corrosions were induced in three places with different level of thinning
(10%, 15% and 25%). Measurements were made using the pitch-catch setup which consists of
two variable angle broadband transducers with central frequencies at 3.5 MHz, one of the
transducers acts as transmitter used to generate the guided wave mode and the other one used to
receive the generated mode and its interaction with the corroded structure. The transducers are
driven by a tone-burst pulser/receiver system. The first set of tests demonstrates detectability of
the open corrosion on the aluminum plate using the pitch- catch setup with piezo-composite
transducers.
The inspection of bonded structure with pitch-catch setup is based on the following principles.
A guided Lamb wave mode once generated will travel from sender to receiver,
producing relatively high amplitude RF signal when a disbond exists between the two
bonded layers; otherwise it will leak in the tear strap if the bond is good, preventing the
generated wave mode from being received by the receiver.
Relative amplitude changes which occur in the transmitted wave mode through bonded
structures are an indication for the existence of disbond, corrosion or even missed tear
strap.
Ultrasonic techniques for hidden corrosion detection
Thus,
The propagation of Lamb guided waves in plates and multilayered structures has been
presented.
The generation of pure modes and appropriate mode selection has been discussed.
Also demonstrated the tools to properly control, predict and launch guided waves in
aluminum.
Under different conditions, three sets of experimental tests utilizing So, S1 and A1
modes with different frequency-thickness product were performed.
Showed how the ultrasonic guided waves method can be used as a highly sensitive, fast
and cost effective inspection technique for disbond and corrosion detection.
Demonstrated that the ultrasonic guided wave method can be used as appropriate and
promising inspection technique.
Lamb wave inspection can detect disbond in lap splice joints and tear straps in a single
scan and the procedure is suitable for presentation of the results as an image.
7. APPLICATIONS OF ULTRASONIC TECHNIQUESSome of the applications of ultrasonic techniques for detecting hidden corrosion are as
follows
1. ULTRASONIC TECHNIQUE FOR THE DETECTION OF LINER CORROSION IN
A HB-53 HELICOPTER FUEL TANKS
Fig10. HB-53 fuel tank
2. ULTRASONIC TECHNIQUES FOR THE DETECTION OF DEFECTS IN RAILS
AND WELDED RAIL JOINTS
Ultrasonic techniques for hidden corrosion detection
Fig11a. Shelling Fig11b.Flaking
Fig11. Surface defects
3. ULTRASONIC TECHNIQUES FOR RAPID ASSESSMENT OF CORROSION
DETECTION IN PIPING
Fig12.Electromagnetic acoustic transducer assembly on a pipe run4. ULTRASONIC TECHNIQUES FOR CORROSION DETECTION IN A STORAGE
TANKS
Fig13. Storage tank
Ultrasonic techniques for hidden corrosion detection
8. REFERENCES “Corrosion Detection Technologies”, BDM Federal Inc., March 1998
L.E. Soley and J.L. Rose, “Ultrasonic Guided Waves for the Detection of Defects and
Corrosion in Multi-Layer Structures,” Review of Progress in Quantitative
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D. Tuzzeo and F. Lanza di Scalea, “Noncontact Air-Coupled Guided Wave Ultrasonic
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Evaluation, Vol. 13, No. 2, 2001, pp. 61-77
Y. Bar-Cohen, A.K. Mal and M. Lasser, “NDE of Hidden Flaws in Aging Aircraft
Structures Using Obliquely Backscattered Ultrasonic Signals (OBUS),” The SPIE
Conference on Nondestructive Evaluation of Aging Aircraft, Airports, and Aerospace
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J.C.I. Chang, “Aging Aircraft Science and Technology Issues and Challenges and USAF
Aging Aircraft Program”, Structural Integrity in Aging Aircraft, ASME: AD-Vol. 47,
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R.P. Dalton, P. Cawley and M.J.S. Lowe, “Propagation of Acoustic Emission Signals in
Metallic Fuselage Structure,” IEEE Proceedings: Science, Measurement and
Technology, Vol. 148, 2001a, pp. 169-177.
R.P. Dalton, P. Cawley and M.J.S. Lowe, “The Potential of Guided Waves for
Monitoring Large Areas of Metallic Aircraft Fuselage Structure,” Journal of NDE, Vol.
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M.J. Quarry and J.L. Rose; “Multimode Guided Wave Inspection of Piping Using Comb
Transducers,” Materials Evaluation, Vol. 57, No. 10, October 1999, pp.1089-1090.
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Corrosion in Multi-Layer Structures,” Review of Progress in Quantitative
Nondestructive Evaluation, Vol. 19B, 2000, pp. 1801-1808.
Ultrasonic techniques for hidden corrosion detection
D. Tuzzeo and F. Lanza di Scalea, “Noncontact Air-Coupled Guided Wave Ultrasonic
for Detection of Thinning Defects in Aluminum Plates,” Research in Nondestructive
Evaluation, Vol. 13, No. 2, 2001, pp. 61-77.
H.J. Salzburger, “Long Range Detection of Corrosion by Guided Shear Horizontal
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D.J. Barnard and D.K. Hsu, “Detection and Quantification of Intergranular Corrosion
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P.S. Rutherford, “NDI Method to Locate Intergranular Corrosion Around Fastener
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pp. 78-84.
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Structures Using Obliquely Backscattered Ultrasonic Signals (OBUS),” The SPIE
Conference on Nondestructive Evaluation of Aging Aircraft, Airports, and Aerospace
Hardware III, Vol. 3586, 1999, pp. 347-353.
R. Rempt, “Scanning with Magnetoresistive Sensors for Subsurface Corrosion,” Review
of Quantitative Nondestructive Evaluation, Vol. 21B, 2002, pp. 1771-1778.
Ultrasonic techniques for hidden corrosion detection
S. Giguere, B.A. Lepine and J.M.S. Dubois, “Pulsed Eddy Current Technology:
Characterizing Material Loss with Gap and Lift-off Variations,” Research in
Nondestructive Evaluation, Vol. 13, No.3, 2001, pp. 119-129.
M.S. Safizadeh, Z. Liu, C. Mandache, D.S. Forsyth and A. Fahr, “Automated Pulsed
Eddy Current Method for Detection and Classification of Hidden Corrosion,” Proc. Vth
International Workshop, Advances in Signal Processing for Non Destructive Evaluation
of Materials, Quebec City (Canada), 2-4 Aug. 2005, X. Maldague ed., E. du Cao, 2006,
pp. 75-84.
Y.S. Sun, T. Ouang and S. Upda, “Remote Field Eddy Current Testing: One of the
Potential Solutions for Detecting Deeply Embedded Discontinuities in Thick and
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