E-Journal of Advanced Maintenance Vol.4 No.2 (2012) 96-104
Japan Society of Maintenology

Nondestructive Investigation of Wall Thinning in Doubled Layer Tube by
Magnetic Adaptive Testing
Gábor VÉRTESY1,†,*, Ivan TOMÁŠ2, Tetsuya UCHIMOTO1 and Toshiyuki TAKAGI1
1
2

Institute of Fluid Science, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
Institute of Physics, Na Slovance 2, Praha 182 2, Czech Republic(Received; For the use of JSM)
Abstract. A recently developed nondestructive method, called Magnetic Adaptive Testing, which is based on
systematic measurement and evaluation of minor magnetic hysteresis loops was applied for detection of local
wall thinning in carbon steel, ferromagnetic T-shape tubes. Presently no effective inspection method is available
for detection of wall thinning of a T-tube with reinforcing plate. It was shown that an artificially made slot canbe reliable detected with good signal/noise ratio from the other side of the specimen even through the support
plate.

KEYWORDS: Magnetic NDT, Wall thinning, Magnetic Adaptive Testing1.

Introduction

For pipes used in industry, e.g. in chemical and power plants, wall thinning is one of the mostserious defects [1, 2]. Detection and evaluation of the thickness reduction of pipes are very important
issues for prediction of lifetime of the pipes in order to avoid severe accidents. Recently many
nondestructive testing techniques, such as X-ray [3], microwave [4] ultrasonic [5-7], acoustic
emission [8-10], eddy current [11], thermal infrared measurements [12] have been used for themeasurement of pipe wall thinning. Currently the magnetic flux leakage (MFL) method is the mostcommonly used pipeline inspection technique [13-16].
Local wall thinning on the inner surface of a pipe (e.g. in a nuclear power plant) may occur due
to the stream of coolant flowing inside the pipe, causing a serious problem of maintenance of thepiping systems. The inspection should be done from the outer side of the pipe. In Japan there is a
special concern on the local wall thinning at locations under an enforcement shield that covers outsideof the pipe, where a branch pipe is connected to the main one. Because the enforcement shield and thepipe wall form two layers of metal, it is difficult to inspect inside of the pipe under the enforcement
shield by ultrasonic thickness gauge. Pulsed eddy current testing technology was developed in recent
years to detect wall thinning [17-19]. Because of its rich frequency components and applicability oflarge electric current, the pulsed eddy current method may show promising capability in detection andevaluation the defect in deep regions of the material. The feasibility of magnetic flux leakage method
for estimation of wall thinning on pipes under reinforcing plates in nuclear power plants was also
discussed. Assessment of size of a slit fabricated on underlayer of the layered specimen was shown to
be possible from the flux leakage profile. Conditions of wall thinning under reinforcing plates can bemonitored by MFL methods [20-21].
Another, recently developed nondestructive magnetic method (Magnetic Adaptive Testing,MAT) can also be successfully applied for the inspection of wall thinning in layered ferromagnetic
materials [22,23]. MAT introduces a large number of magnetic descriptors to diverse variations in
non-magnetic properties of ferromagnetic materials, from which those, optimally adapted to the just*Corresponding author, E-mail: vertesy.gabor@ttk.mta.hu
On leave from Research Institute for Technical Physics and Materials Science, H-1121 Budapest, Konkoly
Thege ut 29-33, Hungary†

ISSN – 1883 – 9894/09 © 2009 – JSM and the authors. All rights reserved.

96

G. Vértesy et al./ Nondestructive investigation of wall thinninginvestigated property and material, can be picked up. It was shown in [24] that Magnetic Adaptive
Testing was an effective and promising tool for nondestructive detection of local thinning of a platefrom the other side of the specimen. The method gave good results also in a layered ferromagnetic
plates. It was proved by these model experiments, that a 9x2 mm2 slot, made in a 3 mm thickferromagnetic material could be well detected with good signal/noise ratio through one (or two)
air-gap(s) and through 3-6 mm additional ferromagnetic material. The slot is seen not only in casewhen the measuring yoke is positioned exactly above it, but from about ±10 mm distance, too, with anacceptable signal/noise ratio.
The purpose of the present work is to apply MAT for detection of wall thinning in a big size, T
shape, layered tube (used in nuclear power plants) made of ferromagnetic material.
2.

Magnetic Adaptive Testing

Magnetic Adaptive Testing is based on systematic measurement and evaluation of minor
magnetic hysteresis loops. For this purpose a specially designed Permeameter [25] with amagnetizing/sensing yoke is applied for measurement of families of minor loops of the magnetic
circuit differential permeability. The block-scheme of the device and the sketch of the yoke can beseen in Fig. 1.
Function
generatorPoweramplifierResistor

Driving coilSampleA/D in/Out Card

ComputerCurrent
indication
amplifierSignalamplifierSignal coil
Yoke

Fig. 1: Block-scheme of the Permeameter and sketch of the yoke.

The magnetizing coil wound on the yoke gets a triangular waveform current with step-wiseincreasing amplitudes and with a fixed slope magnitude in all the triangles. The voltage signal in the
pick-up coil is proportional to the differential permeability of the magnetic circuit. The Permeameterworks under full control of a PC computer, which sends the steering information to the function
generator, and collects the measured data. An input-output data acquisition card accomplishes themeasurement. The computer registers data-files for each measured family of the minor permeability
loops. They contain detailed information about all the pre-selected parameters of the voltage signal
induced in the pick-up coil. Before each measurement the sample is demagnetized by triangular
waveform decreasing current. The raw data, which is registered for each sample characterized by adegradation parameter, k, is a family of permeability loops, which is then processed by a special
data-evaluation program. The program filters the experimental noise and interpolates the data into aregular square grid of elements, ij(Fi,Aj,k), of a -matrix with a pre-selected magnetizing fieldstep F=A. Here Aj is field amplitude of the j-th minor loop and Fi is current field value within thatminor loop. The consecutive series of (k)-matrices, each taken for one sample with a value of the
independent variable, k, describes the magnetic reflection of the material degradation, . ( can be
any independent parameter, which characterize material modification, e.g. plastic deformation, fatigue,
irradiation, thermal ageing, etc.) The matrices are processed by a matrix-evaluation program, which
normalizes them by the matrix of a reference sample (=0) chosen as the reference matrix, andarranges all the mutually corresponding elements ij(k) of all the evaluated (k)-matrices into a table.

97

G. Vértesy et al./ Nondestructive investigation of wall thinning

Each ij()-column of the table numerically represents dependence of one ij-matrix element on the
independent variable of the material degradation, . The dependence is referred to as theij()-degradation function. The most sensitive of all the degradation functions is found and it is thenused for highly sensitive characterization of the investigated material.
Once the degradation functions are computed, the next task is to find the optimum degradation
function(s) for the most sensitive and enough robust description of the investigated material
degradation. A 3D-plot of sensitivity of the degradation functions can substantially help to choose theoptimum one(s). The matrix-evaluation program calculates also sensitivity of each degradation
function and draws their “sensitivity map” in the plane of the field coordinates (Fi,Aj)(i,j). A 3D-plotof sensitivity of the degradation functions can substantially help to choose the optimum one(s). Thismap shows relative sensitivity of each ij()-degradation function with respect to the independent
variable, , of the investigated material. Sensitivity of each degradation function is computed as theslope of its linear regression and it is expressed by a color and/or shade in the sensitivity map figure.
The sensitivity map gives useful information about the relative change of the investigated magnetic
descriptor with respect to the independent variable (the top of the “hills” are those area(s) from where
the most sensitive MAT-degradation functions can be taken). On the other side it also indicates, howreliably the parameter can be determined if the measurement is repeated (large plateaus are favorablefrom this point of view, where parameters depend only very slightly from the actual choice of theexact field coordinates values Fi and Ai).
By integration and/or differentiation of permeability along the field, Fi, B-matrices and/or
’-matrices can be obtained. The B- and ’-matrices contain in principle the same information as the-matrices, however, the corresponding B()- and ’()-degradation functions are different andsometimes more advantageous.
The paragraphs above describe the method of MAT as it is used for determination of the mostsensitive characterization (i.e. of the most sensitive degradation function) of variation of the material
properties of different samples, typically referred to as the “material degradation”. In the case of theinvestigation of local thinning of a tube of ferromagnetic material, the sample should be scanned by
the magnetizing/sensing yoke in a sequence of pre-determined positions, xk, of the yoke on the surfaceof the tube. Measurement at each position, xk, is then characterized by one (xk)-matrix, and similarly
as in the above paragraph ij(x)-degradation functions are constructed. Then, the most sensitive of all
the ij(x)-degradation functions is found and used for highly sensitive determination of the yoke
position xD, under which the thinning of the tube (i.e. the defect) is situated. Most suitably, theposition of the yoke x0, under which there is no defect, and which is far from the specimen boarders, isused for normalization.
The measuring conditions form an open magnetic circuit, where the exact value of magnetizing
fields is difficult to express. Because of this, magnitudes of the magnetizing field, Fi, and of the minor
loop amplitude, Ai, will be characterized by corresponding values of the applied magnetizing currents,
given in Amps, in all the following text.
3.

Influence of the sample thickness on the measured MAT signal

In order to show background of the technique, preliminary measurements were performed on asystem of carbon steel plates, demonstrating influence of the sample thickness on the measured MAT
signal. It was studied, as presented in [24], what modifications of the measured signal can be expected
if the compared samples have different thickness.
(a) Generally it can be expected that thick or large samples, if magnetized by a single yoke
attached from one side, are magnetized less uniformly than thin or small samples. As a consequenceof this, the induced signal can be expected more “blurred“ (“lower and wider”) on thick and large
samples than on thin and small ones, where it can be expected sharp and narrow. Signal on samples oflarge dimensions can be understood as integrated from all the large body of the sample, where themagnetic conditions are not very uniform.
(b) However, also generally, it can be probably expected, that signal of thick and large samples
will be larger than signal measured by the same magnetizing head on thin and small samples. The
reason for this statement is that the cross-section of the thick and large samples is larger, i.e. their98

G. Vértesy et al./ Nondestructive investigation of wall thinningmagnetic resistance is smaller. Evidently, the tendencies (a) and (b) do not work exactly in the same
direction, actually they can work to some extent against each other and some work on optimization of
the yoke dimensions with respect to different samples will be needed.
For checking validity of the above considerations and to see the influence of sample thickness on
the measured signal, two flat samples with different thicknesses (3 and 6 mm) and the same lateraldimensions (100 mm x 100 mm) were measured by MAT. The samples were prepared fromcommercial low carbon steel (CSN 12050) material. Each sample is one piece of a “continuous” steel.
The samples were measured by attaching a magnetizing yoke on the surface of the sample. The yoke
was a C-shaped laminated Fe-Si transformer core. Fig. 2 shows comparison of the two samples, where
the primarily measured MAT signal is shown for the two differently thick plates. As clearly seen, thesignal of thicker samples is wider and larger than signal of the thinner samples. Both (a) and (b) rules
hold. These preliminary experiments verified that different signal – and different MAT degradation
functions – are expected if the thickness of the measured plate sample is locally modified. These
considerations mean the theoretical background of wall thinning measurements.
0,015

sample 6mm
sample 3mm

Pick-up coil signal (V)

0,010

0,005

0,000

-0,005-0,010-0,015
-0,8

-0,6

-0,4

-0,2

0,0

0,2

0,4

0,6

0,8

Magnetizing current (A)Fig. 2. Sample thickness effect on MAT signal, measured on two flat plates with different, 3 and 6 mm
thicknesses [24].

4.

Experimental

Experiments were performed on a layered T-shape carbon steel tube (outer layer is thereinforcing plate) which simulates the T-tubes in nuclear power plants. It contained an artificial slot.
The main tube is STB410 500A sch60 (JIS) whose diameter is 508 mm. The branch tube installed tothe main tube perpendicularly is STB410 400A sch60 (JIS) and its diameter is 406 mm. Circular
cover plate made of SB410 (JIS) with the diameter of 400 mm were forged and welded to the mainand branch tubes. The wall of the main tube is 20 mm thick, and the cover plate is 10 mm thick. The
photograph of the tube is given in Fig. 3. The magnetizing yoke, used for magnetic measurement, canalso be seen in the picture. (The arrow indicates the direction of the scanning of the yoke. See also Fig.
4.) An artifial slot was fabricated in the inner side of the main tube (not seen on the picture, but it islocated moreorless under the magnetizing yoke). Its area is 50x20 mm2, and it is 10 mm deep.
The measurement was performed on the tube of Fig. 3, as shown in Fig. 4. The magnetizing yoke
was placed outside, as it is seen in Fig. 3. The yoke was a C-shaped laminated Fe-Si transformer core.
The total outside length of the yoke is 105 mm (perpendicular to x direction), the legs of the yoke are
40x30 mm2, the gap between legs is 45 mm. A magnetizing coil (N=105) and a pick-up coil (N=35)
are wound on the legs of the core. The yoke was moved step by step along the tube’s surface,perpendicular to the axis of the large tube, as indicated in Figs. 3 and 4 by the arrow, andmeasurements were performed at each position. The sketch of the tube and the measurement
arrangement is shown in Fig. 4. The distance from the yoke to the branch tube was 23 mm, and the
distance from the yoke to the plate edge was 44 mm. The presence of the slot was determined byscanning the yoke along the „Measurement“ line. A reference measurement was also performed alongthe „Reference measurement“ line.

99G. Vértesy et al./ Nondestructive investigation of wall thinning

Fig. 3. Photograph of the T-shape tube with the magnetizing yoke placed on the top tube. The arrowshows the direction of movement of the magnetizing yoke (x direction).

Fig. 4. The sketch of the tube and the measurement. Dashed lines show how the center of the yoke moves
during the measurement (along x direction).

5.

Results and discussion

As described above, the measurement was performed by positioning of the yoke along the x line.
The signals of the pick up-coil for three different positions of the yoke are shown in Fig. 5. x = -60mm means the position of the yoke, farest from the slot, while at x = 0 mm the center of yoke wasabove the center of the slot. The magnetic contact between the tube’s surface and magnetizing yoke
was not good, because the leg of the yoke was flat, which touched the surface of the tube only along aline. Nevertheless, the measured permeability loops were acceptable, the sample was magnetized
relatively well, as it is reflected on the measured permeability plots, shown in Fig. 5. For future100

G. Vértesy et al./ Nondestructive investigation of wall thinningapplications, curved surface of the yoke’s leg (according to the radius of the tube) should be prepared,
which probably will result better quality magnetic contact and perhaps even better results.
The modification of the shape of permeability loops, due to the existence of the slot under themagnetizing yoke is seen very well, and it is in good correlation with the phenomenon, observed on
samples having different thickness (see Fig. 2). Theory works well.
MAT degradation functions were evaluated as functions of the yoke position. The center of theslot is at x = 0 mm. Each MAT degradation function is normalized by the corresponding one, which
belongs to the x = -60 mm position (farest from the slot). The best degradation functions wereobtained if the reciprocal values of ’(x)-degradation functions were considered. “Best” means thatlargest sensitivity and best reproducibility can be achieved in the case of these degradation functions.
The optimally chosen degradation functions are shown in Fig. 6, while the correspondig sensitivity
map is given in Fig. 7. This map does not give directly the values of 1/ij’(x)-degradation function, butshows the relative sensitivity of each 1/ij’(x)-degradation function with respect to the position of theyoke, x. Sensitivity of each degradation function is computed as the slope of its linear regression and
it is expressed by a color and/or shade in the sensitivity map figure. Red areas corresponds to the top
sensitivity values. It gives (Fi,Aj) pairs, and in the knowledge of these values most sensitive 1/ij’(x)values can be determined. In our case optimal MAT degradation function can be taken from two areasof the sensitivity map, which are identical with each other from this point of view. These areas appear
with red color in Fig. 7, and represents those degradation functions, which exhibit the largest
sensitivity of the modification of MAT parameters while the yoke is moving along the measurement
line. The crosses of lines in the sensitivity map of the 1/’-degradation functions show the centers of
the areas, from where the points of Fig. 6 are taken (Fi=90mA, Ai=240mA) and (Fi=240mA,
Ai=300mA). It is important to mention that the areas of these top sensitivity degradation functions arerather large, and the “hill” is smooth enough, which ensures good reproducibility and reliability of thechosen MAT descriptors. (The area between F=0 and F=75 mA is not suitable for choosing reliable
MAT parameters, in spite of the large sensitivity, because in this area the reproducibility of parameters
is poor due to the large scatter of points.)
x = -60 mm
x = -20 mm
x = 0 mm

0,04

Signal of pick-up coil (V)

0,03
0,02
0,01
0,00
-0,01
-0,02
-0,03
-0,04
-4

-3

-2

-1

0

1

2

3

4

Magnetizing current (A)Fig. 5. Examples of families of the -shaped loops vs. magnetizing current measured at three different
positions of the magnetizing yoke. The positive and negative parts of the signal correspond to theincreasing and decreasing parts of the triangular waveform of the current (field), respectively.

It is demonstrated very well in the figure, that significant difference was found in MAT
parameters between the reference position of the yoke and its position over the slot. The measurement
was repeated several times with identical results, demonstrating reliability and reproducibility of thistechnique. About 40% difference is measured, which is caused by the local wall thinning. The largest
values of the measured curve is over the center of the slot.

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G. Vértesy et al./ Nondestructive investigation of wall thinning

F90A240F240A300
RefF90A240
RefF240A300

1,5
Measurement

1/' degradation functions

1,4
1,3
1,2
1,1

Reference measurement
1,0
0,9
0,8
-60

-40

-20

0

20

40

60

Position of the center of the yoke, x (mm)Fig. 6. The optimally chosen MAT degradation functions, taken from the two indicated areas ofsensitivity map.

The fact, that a maximum is observed over the slot shows by itself that this is a real effect, notonly an error of measurement. However, the complicated shape of the measured tube can influence
the magnetic flux distribution inside the wall. In principle – as the worst case – it can happen thatedge of the cover plate and/or existence of the perpendicular tube (which is rather close to the area of
measurement) can perturb the measured permeability as a function of the yoke position. To be surethat really the existence of the inner slot was detected, the same measurement was also done on theother, symmetrical side of the perpendicular tube, where no artificial slot on the inner wall existed.
This “reference measurement” line is shown in Fig. 4. For the reference evaluation the same (Fi,Aj)
points, as in the case of “real” evaluation were used, as indicated in Fig. 6. In this case no differenceof any MAT degradation function vs. the yoke position was detected, the measured point scatter overthe normalized value. This was a direct and evident proof that the measurement really detected theslot.
-0,02500-0,02208-0,01917-0,01625-0,01333

Minor loop amplitude, Ai (mA)

250

-0,01042-0,007500-0,004583-0,001667

200

0,001250
0,004167
0,007083
0,01000

150

100

50

1/' sensitivity map
0
0

50

100

150

200

250

Magnetizing current, Fi (mA)

Fig. 7. Map of relative sensitivity of the 1/’(x)-degradation functions. The crossing point of the lines
indicate field coordinates of the most sensitive ij’()-degradation function, from where MAT degradation
function of Fig. 6 were taken.

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G. Vértesy et al./ Nondestructive investigation of wall thinning

The determination of wall thinning by MAT measurements are affected by modifying the tube
thickness and the size of magnetizing yoke. Many measurement series were performed previously onflat samples to find the optimal relation between the thickness of the sample and the size of the yoke.
The above described experiment was performed in a given tube, with given tube thickness. The sizeof magnetizing yoke was chosen according to this tube thickness This size is believed to be close to
the optimal one.
By applying this type of magnetic measurements, the results are also affected both by the surface
conditions of the tube and also by the clearance between the top plate and the main tube. Results are
influenced by existence of the uncontrolled little air gap between the applied yokes and the samples.
Quality of magnetic contact between the sample surface and faces of the yoke has a strong influenceon the measured signal and especially with unpolished surfaces it is difficult to be reproduced.
However, if the conditions are similar in all the tested cases (there are no big differences of surface
roughness from measurement to measurement) the measurements can be performed with good
reproducibility and reliability. This is the case in the presently shown experimens. For future
measurements, where the surface conditions are not rigorously kept constant, the scatter of points canbe reduced significantly by applying a nonmagnetic spacer between the sample surface and yoke. It
has been demonstrated recently [26], that spacers placed between the sample surface and faces of theyokes decreased fluctuation of quality of the magnetic contact from sample to sample. The spacers
smoothed down scatter of the measured signal values, the measurement became more reliable, but
sensitivity of the -degradation functions was decreased at the same time. However, presence of thespacers also modified shapes of the signals and their derivatives. As a result ’-degradation functions
computed from the first derivative of the signal with an optimally chosen spacer acquired extreme
sensitivity, while keeping still very satisfactory scatter of the measured points.
The influence of the clearance between the tubes was not investigated in the present case,because the tube system was given. However, many measurements were also performed to study thisphnomenon, and it was found that up to a certain size of clearance between ferromagnetic plates MAT
can be successfully applied.
6.

Conclusions

The above outlined result illustrates that Magnetic Adaptive Testing is a suitable technique forinspection of wall thinning in thick, layered carbon steel tubes. The existence of an artifical slot could
be detected with good signal/noise ratio and with good reliability even under an enforcement shieldthat covers outside of the pipe. Further experiments are needed to optimize the size and shape of themagnetizing yoke, and to determine the smallest detectable size of the thinned area of the wall.

Acknowledgements
This study was performed as a part of “Project on enhancement of aging management andmaintenance of NPPs” supported by nuclear and industrial safety agency, METI, JAPAN. The work
was also supported by Hungarian Scientific Research Fund (project A08 CK 80173), by projectNo.101/09/1323 of the Grant Agency of the Czech Republic and by the Hungarian-Czech BilateralIntergovernmental S&T Cooperation project.

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