Design and assembly of an apparatus system based on the villari effect for detecting stress concentration zone on ferromagnetic materials
PETROLEUM TECHNOLOGIES
PETROVIETNAM JOURNAL
Volume 10/2020, p. 60 - 66
ISSN 2615-9902
DESIGN AND ASSEMBLY OF AN APPARATUS SYSTEM BASED ON
THE VILLARI EFFECT FOR DETECTING STRESS CONCENTRATION
ZONE ON FERROMAGNETIC MATERIALS
Doan Thanh Dat, Le Thi Hong Giang, Nguyen Dinh Dung, Hoang Anh Tuan, Nguyen Thi Le Hien
Vietnam Petroleum Institute
Email: hienntl@vpi.pvn.vn
Summary
This paper presents the study results on the fabrication of a structural integrity assessment apparatus by determining stress
concentration zones in pressure pipeline and equipment. The apparatus uses a triaxial magnetic field sensor to measure magnetic field
components in three axes Ox, Oy, and Oz, in the working range of the magnetic field from -300 µT to 300 µT. The investigation of the self-
magnetic leakage field by this apparatus in the API 5L steel specimens under tensile stress shows a high variation of the magnetic field
at a steel elongation lower than 1 mm (corresponding to the elastic deformation state of the material). In the case of an artificial defect,
the apparatus can detect a change in the magnetic field caused by stress concentration.
Key words: Magnetic field apparatus, stress concentration zone, integrity assessment, defect detection, self-magnetic flux leakage.
1. Introduction
Non-destructive testing (NDT) is a technology widely
The advantage of NDT methods is that they
may inspect the pipe and equipment online during
operation. However, these methods need direct contact
with the metal surface, whereas it is difficult to access
some locations such as underground or/and submerge
pipelines; pipes and equipment under insulation or under
support, etc. To satisfy practical requirements, the study,
development and testing of a non-contact NDT apparatus
are necessary. Among the non-contact testing methods,
the magneto-mechanical methods are used with a wide
range of applications.
used in the inspection of the integrity of equipment and
pipes in operation. It is used by the industry to evaluate
the properties of a material, component, structure, or
system for characteristic differences or welding defects
and discontinuities without causing damage to the
original part. Early detection of defects in metal structure
is very important, allowing timely replacement and repair.
Therefore, factories can operate safely, save repair costs,
and avoid possible disasters [1]. Non-destructive testing
consists of different methods, usually divided into two
main groups according to their ability to detect defects:
Joule magnetostriction is a property of magnetic
materials that causes them to change their shape or
dimensions during the process of magnetisation [2]. The
structure of ferrous material is divided into small magnetic
domains, whicharerandomlyorientatedwhenthematerial
is not exposed to a magnetic field. When a magnetic field is
applied, the magnetic domains shift and rotate causing a
change in the material’s dimensions as shown in Figure 1.
NDT methods can detect the defects and discontinuity
in and/or on the metal surface of inspected component
and structure: Radiographic testing (RT), Ultrasonic
testing (UT).
Other NDT techniques can detect only the defects on
the metal surface or near the surface: Liquid penetrant
testing (PT), Magnetic particle testing (MT), Eddy current
testing (ET).
The inverse magnetostrictive effect, magnetoelastic
effect or Villari effect characterises the change of the
magnetic susceptibility of a material when subjected to
mechanical stress [3]. Pressure pipelines and equipment
are typically subjected to internal or external corrosion,
which can weaken their structural integrity. The pressure
Date of receipt: 18/10/2020. Date of review and editing: 19 - 20/10/2020.
Date of approval: 21/10/2020.
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and bending stresses applied to the corroded sections
will result in stress concentration. Measurement of the
stress concentration zone (SCZ), as well as detection of
the micro-defect growth, has been of high importance
for predicting the location of mechanical failures and
evaluating the remaining useful lifetime (RUL) of pipelines
[4].
concentration [8 - 9]. This technique is a promising tool
for inspecting early damage due to stress concentration
in ferromagnetic components by testing and analysing
the magnetic leakage field (MLF) above the surfaces of
the components in the geomagnetic field [10 - 13]. The
technique seems rather similar to the magnetic flux
leakage testing technique since both need to measure
the magnetic field surrounding the ferromagnetic
components, but their discrepancy is remarkable. The
magnetic flux leakage testing must impose a high
intensity magnetic field and is mostly used to inspect
geometrical defects such as holes; whereas the MMM
technique only utilises the natural weak geomagnetic
field (the Earth's magnetic field) and is more sensitive to
stress. Usually, there are possibly both geometrical defects
and local stress concentration zones in ferromagnetic
components. In fact, the MLF signals incorporate the
effects of geometrical defects and stress concentration
zones, in which the former perturbs the MLF path and
the latter induces the local magnetic anisotropy [10]. A
significant advantage of this MMM method is that it can
detect the SCZ, and thus high areas of stress, giving it the
ability to predict regions where anomalies are developing
before they become failures. The ability to detect SCZ
means that using this method, it is possible to analyse all
areas that are exposed to high stress, including anomalies,
corrosion and bending stresses, making the method more
comprehensive than the existing technologies. However,
the magnetic field variation caused by SCZ is small, of
the order of 10 μT, which is a variation in the background
(Earth) field of 40 - 60 μT, and as such the measurement of
thisappearsdaunting[4].Withtheadventofsmallportable
magnetometers, the measurement and interpretation of
these signals have become a practical proposition.
The metal magnetic memory (MMM) technique, which
was originally developed in Russia in 1997 [5], is based on
the Villari effect [6 - 7], in which the application of stress
on a ferromagnetic material causes the rearrangement of
magnetic domains. When this occurs in the presence of
an external magnetic field such as the Earth’s magnetic
field, a relatively large magnetisation change will be
caused (Figure 2). By measuring the residual magnetic
leakage field (RMLF) distribution on the material surface,
the MMM method has been implemented as a periodic
screening inspection tool, evaluating the degree of stress
ε
H = 0
H
Figure 1. The magnetostriction effect - an applied magnetic field causes the alignment
of magnetic dipoles and thus the change in length of a given sample.
Magnetic field
Sensor
Magnetic field
Sensor
N
S
Magnetic material
S
N
Magnetic material
(a)
Figure 2. Magnetic leakage field from the magnetic material without (a) and with (b) a defect.
(b)
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Villari eꢀect
Stress
concentration
zone (SCZ)
Detect by magnetic
ꢀeld measuring
apparatus
Defects on the
structure under
stress
Anormal or
SMLF anomalies
Figure 3. Diagram of the principle of applying the Villari effect in the defect detection.
Sensor
ADC 32Bit
MCU
Software
IN
ADC
OUT
Figure 4. Block diagram of apparatus.
In Vietnam, studies and development of the
magnetoelastic Villari effect are still very limited [15].
It is, therefore, necessary to design and fabricate a
device or apparatus which can detect the “natural”
SMLF that escapes from anomalies and SCZ without the
requirement of an applied external magnetic field. This
paper introduces the magnetic field measuring apparatus
fabricated by the Vietnam Petroleum Institute (VPI) and
some initial results of detecting the stress concentration
on ferromagnetic speciments under tensile stress.
A communication block controls a bi-direction
connection between device and computer for monitoring,
controlling and data collecting.
2.2. Investigation of magnetic sensor characteristics
The sensor is a linear magnetic field transducer
whose output is a voltage proportional to a magnetic
field applied perpendicularly to the package top surface.
To verify the characteristic of the sensor, the relationship
between the output voltage of the sensor and the applied
external magnetic field is investigated.
2. Design and assembly of magnetic field measuring
apparatus
The applied magnetic field is uniform, generated by
Helmholtz coils, in which the intensity of the magnetic
field is controlled by a constant current source; the
sensor's output signal is picked up via a high-precision
voltmeter, which connects to the computer. The
relationship between the sensor output signal and the
external magnetic field is described in Figure 5.
2.1. Apparatus design
The main board of the apparatus is a control
intergrated unit which includes sensor block, amplifier
into noise filter, analog-to-digital converter (ADC) and
communication block.
The sensor is a three-dimensional (3D) type of
Honeywell with high sensitivity, a magnetic field sensor
according to the Giant magnetic field - GMR. This is a
hybrid sensor consisting of one dual-axis sensor and one
single-axis sensor with a measuring range of 2 Gs.
The obtained results show that the variation of the
output voltage of the magnetic sensor following the
applied external magnetic field is linear in a magnetic
range from -3Gs to 3Gs (corresponding to -300 µT - 300 µT).
Beyond this working range, the sensor is in a saturated
state. This selected sensor is suitable for detecting the
self-magnetic leakage field.
The amplifier into the noise filter block is a component
that filters noise from the environment. In the research to
fabricate this device, three-level amplification was used.
The testing apparatus was assembled and configured
to determine the stress concentration zone of the defected
steel specimens.
The analog-to-digital converter (ADC) is a system
that converts the analog physical quantity continuously
received from the sensor to a digital value to represent
the magnitude of that quantity. Magnetic sensors can
receive magnetic signals through output voltage signals.
ADS1262 IC ADC with very high resolution, programmable
amplifier and good noise resistance was selected.
3. Experimental condition
3.1. Specimen preparation
The testing specimens are made of API 5L steel,
which is the material widely used to manufacture the
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6
5
5
4
3
2
4
3
2
1
0
1
0
-3
-2
-1
0
1
2
3
-5 -4 -3 -2 -1
0
1
2
3
4
5
Applied magnetic field (Gs)
Applied magnetic field (Gs)
(a)
(b)
Figure 5. Relationship between the output voltage signal of the sensor and the applied external magnetic field in two axes x (a) and y (b).
(a)
(b)
Figure 6. Testing specimens and measuring points (a) and the artificial defect (b).
transportation pipelines of oil and gas. The shape of the
flat specimen is shown in Figure 6. Its surface roughness
Ra is 1.6 mm; two parallel lines marked with 6 mm of
space vertically are drawn on the surface. There are 11
points with 10 mm intervals on every horizontal line
chosen for magnetic field measurement. The length of
each measured line is 100 mm.
the specimen through the elongation of the specimen.
The magnetic field values of all points on each measured
line were collected at a predetermined elongation. After
the measurement, the specimen was loaded again to a
higher elongation, and the above procedure was repeated
until the specimen broke.
3.3. Magnetoelastic measurement
To create a stress concentration zone, an artificial
defect is made in the middle of the specimen and then
tensile stress is applied to the specimen. To avoid residual
stress in the specimen affecting the results of the test, the
artificial defects are created by electrochemical corrosion.
The shape, size and depth of the defects are controlled
by the active surface of the metal specimen (the surface
contacts directly with electrolyte) and the electric charge
flowing through the specimen.
The magnetic field values were measured by our
self-fabricated apparatus. The magnetic sensor of the
apparatus is fixed at a distance of 2 cm from the specimen
surface and could move along the direction of measured
lines vertically, and the measurement data are collected
at 11 points (from point -5 to point 5 beside the defect
(point 0)).
4. Results
3.2. Tensile stress
The change of the self-magnetic leakage field along
the length of the metal specimen with artificial defects
under tensile stress has been investigated by the self-
fabricated device. The sensor was moved parallel to
the specimen surface at a distance of 2 cm away. The
magnetic field signals were collected in three axes of Ox,
Oy and Oz of the sensor (Ox is the vertical direction, Oy is a
To evaluate the ability of the apparatus to detect
stress concentrations by the magnetoelastic effect, the
specimens were stretched by a mechanical DLR testing
machine. During the tensile test, each specimen is
positioned vertically between the upper and lower grip
holders of the testing machine. Tensile stress is loaded on
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horizontal direction and Oz is the
perpendicular direction to the
specimen surface). The values of
the magnetic field at points -5,
-4, -3, -2, -1 and 1, 2, 3, 4, 5 were
compared to artificial defects
(point 0) at different levels of
specimen elongation.
Axis Ox
350
300
250
200
150
100
Point -5
Point -4
Point -3
Point -2
Point -1
Point 0
defect
Point 1
Point 2
Point 3
Point 4
Point 5
Figure 7 shows the variation
of the magnetic field with the
elongation of the specimen,
corresponding
to
tensile
strength. The obtained results
showed that the magnetic
field varied very strongly at an
elongation range lower than 1
mm. When the specimen was
stretched with an elongation
higherthan1mm,corresponding
to a relative elongation above
0.5%, the magnetic field tended
to be less variable. Therefore,
we can confirm that the metal
specimen is at an elastic
deformation state when relative
elongation is lower than 0.5%.
0
0.5
1
1.5
2
2.5
3
Elongation (mm)
Axis Oz
Point -5
Point -4
Point -3
Point -2
Point -1
450
300
150
Point 0
defect
Point 1
Point 2
Point 3
Point 4
Point 5
0
The variation of the magnetic
field obtained along with
the metal sample at different
measured points was shown
in Figure 8. Since the direction
of the tensile force (along the
specimen length) is in the same
direction of the Ox axis, the self-
magnetic leakage field in the Ox
direction is the greatest variation.
When the relative elongation of
the specimen is lower than 0.5%,
corresponding to the elastic
deformation state of the metal
specimen, it is visible to detect
the defect position through the
change of the self-magnetic
leakage field escaping from
the defect. However, when the
elongation is more than 0.5%,
the metal may transform to a
plastic deformation state, the
-150
0
0.5
1
1.5
2
2.5
3
Elongation (mm)
Axis Oy
-90
-100
-110
-120
-130
-140
-150
Point -5
Point -4
Point -3
Point -2
Point -1
Point 0
defect
Point 1
Point 2
Point 3
Point 4
Point 5
0
0.5
1
1.5
2
2.5
3
Elongation (mm)
Figure 7. Variation of magnetic field with elongation of the specimen.
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magnetic moment reaches a saturated state
so the magnetic field is less variable and the
difference of the magnetic field is not visibly
observed at the defect position.
Axis Ox
350
300
250
200
150
100
Elongation 0%
Elongation 0.2%
Elongation 0.4%
Elongation 0.5%
Elongation 0.7%
Elongation 0.9%
Elongation 1%
The pressure pipelines and equipment
always operate in the elastic deformation
state of the material. Therefore, the fabricated
apparatus can determine the abnormal
magnetic leakage field at or near the defects
of the ferromagnetic components, allowing
the practical application.
-5 -4 -3 -2 -1
0
1
2
3
4
5
Measured point
In addition to the Earth's magnetic
field, the measured self-magnetic leakage
field escaping from stress concentration of
the ferromagnetic specimen, also includes
the magnetic fields of other sources in the
environment such as electric resources, other
magnetic materials, if any). So, to eliminate
undesired magnetic fields and observe
only the self-magnetic leakage field from
the defect, the magnetisation gradient was
determined along the length of the testing
specimen as described in Figure 9. The results
also show that in the elastic deformation
zone, the highest gradient magnetic field can
be observed at the defect location (point 0),
corresponding to the stress concentration
zone. This confirms the ability of the apparatus
to detect the defect through abnormal
magnetisation positions.
Axis Oy
-90
-100
-110
-120
-130
-140
-150
Elongation 0%
Elongation 0.2%
Elongation 0.4%
Elongation 0.5%
Elongation 0.7%
Elongation 0.9%
Elongation 1%
-5 -4 -3 -2 -1
0
1
2
3
4
5
Measured point
Axis Oz
500
400
300
200
100
0
Elongation 0%
Elongation 0.2%
Elongation 0.4%
Elongation 0.5%
Elongation 0.7%
Elongation 0.9%
Elongation 1%
5. Conclusion
-100
-200
Based on the magnetoelastic Villari effect,
a magnetisation measurement apparatus has
been successfully designed, assembled and
tested, with a working range from -300 µT to
300 µT. The investigation of the self-magnetic
leakage field has been carried out by this
apparatus in the API 5L steel with an artificial
defect under tensile stress.
-5 -4 -3 -2 -1
0
1
2
3
4
5
Measured point
Figure 8. Variation of magnetic field along the specimen length under tensile stress.
15
12
9
Elongation 0%
Elongation 0.2%
Elongation 0.4%
Elongation 0.5%
Elongation 0.7%
Elongation 0.9%
Elongation 1%
6
The results show that a high variation
of the magnetic field can be observed at an
elongation lower than 1 mm (corresponding
to the elastic deformation state of the
material). In case of a defect, the apparatus
can detect a change in the magnetic field
due to the stress concentration. Therefore,
3
0
-3
-5
-3
-1
1
3
5
Measured point
Figure 9. Variation of magnetisation gradient along the specimen length under tensile stress.
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this apparatus can be developed and used to inspect the
integrity of equipment and pipes without direct contact.
[9] Shi Changliang, Dong Shiyun, Xu Binshi, He
Peng, “Stress concentration degree affects spontaneous
magnetic signals of ferromagnetic steel under dynamic
tension load”, NDT & E International, Vol. 43, No. 1, pp. 8 -
12, 2010. DOI: 10.1016/j.ndteint.2009.08.002.
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