Faculty of Natural Science and Technology

Internship report MAT3

BLARDONE Allan
Material science department POLYTECH GRENOBLE

Main Volume
AND

Annex

May 29, 2017 to August 21, 2017

ACKNOWLEDGMENT
First of all, I wish to thank all the peoples who permitted me to live in this great experience.
I'd like to begin with Dr. CHENEVIER, director of research at CNRS (Grenoble) and Senior
University Research Administrator at Okayama University. Without his help, this internshipwouldn’t have been possible. He really cares about making internships in Japan possible forforeign students. Conversely, he also allows Japanese students from Okayama University toexperience studying abroad.
Indeed, he submitted me to Dr. KIWA, an Associate professor at Okayama University, who
was my interlocutor while I was in France and the organiser of my internship. He reallyfacilitated me all administrative formalities to officialize my internship, and introduced me to myinternship tutor, Professor TSUKADA, who was my tutor during this internship.
I especially want to render thanks to Dr. SAKAI, an assistant professor at Okayama University.
He was my main interlocutor during this internship and he guided me throughout my work. He
was sincerely concerned about my comfort, and in spite of his tight schedule he spent a lot oftime talking with me about my work as well as my experience in Japan.
I finally want to thank all laboratory professors and students, who were cordial and welcoming
towards me and the other international student Tom, from Taiwan. They even organised two
welcome parties for us, in traditional Japanese restaurants. A big thank you to students
Kentaro, Tsubasa, Yuki and Tetsuro with whom we spent a great trip in Himeji, and who helped
us to buy bus and train tickets, which is not an easy task for a foreigner in Japan.

ありがとう ございます!
(aligatô gozaimasu !)BLARDONE ALLAN

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TABLE OF CONTENTSACKNOWLEDGMENT ............................................................................................................... 2
INTRODUCTION ........................................................................................................................ 4
I.

CONTEXT OF MY INTERNSHIP ....................................................................................... 5
1.1.

Few Words about Okayama University................................................................................... 5

1.2.

Principle of Non-Destructive Evaluations ................................................................................ 6II.

BACKGROUND AND THEORY OF THE RESEARCH ..................................................... 7
2.1.

Theory of our magnetic testing ............................................................................................... 7

2.2.

Envisaged improvement: reduce the size of the probe. .......................................................... 9

III.

EXPERIMENTAL TECHNIQUES.................................................................................. 10

3.1.

Equipment used .................................................................................................................... 10

3.2.

Safety Rules ......................................................................................................................... 11

3.3.

Measurement protocol of the control .................................................................................... 11IV.

RESULTS AND INTERPRETATIONS .......................................................................... 12

4.1.

New probes created.............................................................................................................. 12

4.2.

Results of New Probes ......................................................................................................... 13

4.3.

Interpretation ........................................................................................................................ 14V.

CONCLUSION .................................................................................................................. 15

VI.

ANNEXES ..................................................................................................................... 16

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INTRODUCTION
Recently, a new method of hot press for steel plates has been developed, allowing us to create“non-hardened” and “hardened” regions at the desired position on a steel plate. Due to thedifference in hardness of these durable and non-durable regions, this plate has the ability todramatically reduce the fracture in the accident of collisions: this makes this type of materialvery interesting in the automotive sector, for the bodywork (example in figure 1). According toa study conducted by some companies such as GESTAMP, the use of these steel plates withdifferent levels of hardness could reduce the weight of the body work by 20%. While keeping
users safe is a case of accident, this technology allows to reduce the amount of steel used forcar manufacturing, as well as fuel consumption (indeed, the weight of a car determines its
consumption).
In order to put this new technology into practice, industrialists must have the means to ensurethat hard and soft regions have been properly prepared at the desired position. However, theconventional hardness measurement method requires long inspection time and is difficult toapply in the case of an industrial production line. A reliable and fast non-destructive testing
method is required for identifying hardened and non-hardened areas in a steel plate for the
implementation of this method in industry.
In a laboratory of the Okayama University, in which my internship took place, the group ofProfessor TSUKADA discovered that it is possible to carry out this evaluation by a magnetic
method, using a process derived from the Eddy Current Testing. They developed ameasurement method and a prototype probe which gives good results. The main purpose of
my work during this internship was to reduce the size of this prototype probe while keeping thesame quality of signals.
I will first introduce the context of my internship and the background of the research, then I will
explain the experimental procedure of this non-destructive evaluation, and finish with theresults we could obtain and interpret them.

FIGURE 1: EXAMPLE OF UTILISATION OF A METAL PLATE WITH DIFFERENT LEVELS OF HARDNESS,
FOR THE REAR FRAME OF A CAR

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I. CONTEXT OF MY INTERNSHIPAt the end of my third year of study in material and science engineering at Polytech Grenoble(3rd year of a 5-year Master's degree), I performed a twelve-week internship in a laboratory of
research at Okayama University from May 29, 2017 to August 21, 2017. I was working on thesubject “Non-destructive evaluation using low-frequency magnetic field”, as a research
assistant.
I had heard about this opportunity thanks to the International relations of Polytech Grenoble: I
immediately got interested because I wanted to discover the working way in a laboratory andfind out about NDE. I was also interested in Japanese culture and way of life.

1.1. Few Words about Okayama University
Okayama University is a national and public university in Japan founded in 1870 and locatedin the north of Okayama City. This university contains about 14,000 national and international
students and about approximately 2,600 staff are engaged in educational and research
activities, and university management.
Okayama University contains 4 campuses (Tsushima, Shikata, Misasa and Kurashiki) which
are composed of 11 faculties:

Faculty of LettersFaculty of Education

Faculty of Law

Faculty of Economics



Faculty of ScienceMedical SchoolDental SchoolFaculty of
Pharmaceutical
SciencesFaculty of EngineeringFaculty ofEnvironmental Scienceand Technology

Faculty of AgricultureMatching Program Course

BLARDONE ALLAN

FIGURE 2: LOCATION OF OKAYAMA UNIVERSITY

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I carried out my internship in the Graduate school of Natural Science and Technology which islocated in the Tsushima campus.

FIGURE 3: MAP OF TSUSHIMA CAMPUS

1.2. Principle of Non-Destructive Evaluations
The objective of a non-destructive evaluation is to be able to control a sample without having
to destroy it. NDEs are therefore a set of methods for characterising the integrity of structuralparts and the condition of their materials without degrading them. These examinations can be
carried out either during production, during use or in the context of maintenance: the piece
may then be accepted or rejected according to the standards of the specifications.
Whatever the control technique used, the following five steps are identified:
1. The implementation of a physical process on the part → excitation (Electromagnetic wave,
vibration, penetrating liquid, magnetic field ...)
2. The alteration of this process due to the anomaly contained in the part → perturbation
3. The disclosure of this perturbation by a suitable detector (Eye, sensor, imaging system ...)
4. The conversion of this variation into a form adapted to the processing of information
5. Interpretation of the information obtained.
Non-destructive testing therefore include a flow transmitter and a receiver or detector.
According to the type of defaults tracked, the type of material of the piece, the required time
and the cost of the control, a type of NDE is chosen among dozens of different methods. In
our case, we do not use a NDE to detect a flaw, but to reveal irregularities of properties in a
metal: the change of hardness due to the change of microstructure.
We can also reveal these irregularities using the traditional hardness testing: it consists ofdriving a penetrator, whose shapes and dimensions vary according to the type of test, in themetal to be tested. The load is constant and the surface or depth of the impression left in thematerial is measured: the impression will be all the more important as the material will be soft.
The laboratory made a test of Vickers hardness on the steel plate that I used during my tests,
in order to have a reference for the next experiments:
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FIGURE 4: STEEL PLATE USED FOR EXPERIMENTS

Position on the sample (mm)FIGURE 5: VICKERS HARDNESS OF THE STEEL PLATE IN FUNCTION OF THE POSITION (THE 5 CURVESREPRESENT THE 5 LINES OF MEASUREMENT)II.

BACKGROUND AND THEORY OF THE RESEARCH
2.1. Theory of our magnetic testing
As I mentioned in the introduction of this report, this conventional hardness control method is
too long for a usage in industry and difficult to automatise. The laboratory is thus developing amagnetic method for this purpose, which should be easily automatised in industries. The
principle of this magnetic method is based on the one of Eddy Current Testing:
1. An electric current is applied to sendingcoils
2. It emits an induced magnetic field thanks
to Lenz's law
3. This primary magnetic field crosses thesteel plate and creates induced currents(called Eddy currents)
4. These Eddy currents
secondary magnetic field

generatea5. The secondary magnetic field is partiallytransformed in electric current by thereceiving coil.
FIGURE 6: GENERAL EDDY CURRENT PRINCIPLE

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In a steel plate, if the hardness increases, the permeability decreases and the resistivity
increases. It implies that the intensity of induced magnetic field will be higher in non-hardened
areas than in hardened areas.

FIGURE 7: IDEAL SHAPE OF THE INTENSITY OF INDUCED MAGNETIC FIELD IN FUNCTION OF THE
POSITION ON THE SAMPLE

To evaluate properly the steel plate used for automobile components, which is approximately
2 mm thick, we have to consider the skin depth effect: this electromagnetic phenomenon
makes that alternating currents flows mostly near the outer surface of an electrical conductor,
such as our steel plate. This effect becomes more and more apparent as the frequency
increases, following this equation:
𝜹 the skin depth
𝝈 the electrical conductivity of the steel plate𝟏
𝝁 the magnetic permeability of the steel plate
with:
√𝝈𝝁𝝅𝒇
𝒇 the frequency of the magnetic field.

𝜹=

We can estimate the skin depth of the magnetic field inside the steel plate in function of thefrequency, knowing the electrical conductivity and the magnetic permeability of hard and soft
regions:

Skin depth in function of frequency
12,0
Hard regions

Soft regions

Skin depth (mm)

10,0
8,0
6,0

4,0
2,0
0,0
1

10

100

1000

10000

Frequency (Hz)FIGURE 8: THEORETICAL SKIN DEPTH OF THE MAGNETIC FIELD THROUGH THE STEEL PLATE IN
FUNCTION OF FREQUENCY

We will consequently work with a frequency of 100 Hz to be sure to analyse the whole thicknessof the steel plate. The law of Lenz-Faraday allows us to express the electromotive force (emf)which appears at the terminals of the coil during the experiment, since it becomes a generator:

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𝒆=−

𝒅𝜱
𝒅𝒕

with :

𝒆 the electromotive force at the terminals of the coil
𝜱 the magnetic field flux vector through the coil.

As we are working with relatively low-frequency magnetic field (100 Hz), the output voltage(emf) of the receiving coil will have a low intensity, which will drastically reduce the sensitivityof our probe. Professor TSUKADA therefore had the idea to substitute the receiving coil by asmall magnetic sensor, which sensitivity does not depend on the frequency of the magnetic
field. In this study, we used magneto resistive (MR) sensor which has high sensitivity as low
as 30 mT, because they are small, low-cost, highly sensitive and reliable.

2.2. Envisaged improvement: reduce the size of the probe.
Using the former measurement probe using a magnetic sensor (shown in figure 10), which
was created before my arrival in the laboratory, we can already distinguish properly hard andsoft regions on the steel plate:

FIGURE 9: MAPPING OF THE INTENSITY OF INDUCED MAGNETIC FIELD USING THE PROBE N°1

By applying the measurement protocol explained later in this report with the first probe, we
obtain this mapping (figure 9), showing the intensity of induced magnetic field in μTesla infunction of the position, showed in millimetres. The red part shows a high intensity resulting
from the non-hardened area of the plate. This result is totally in agreement with the Vickers
test made on the sample (figure 5), which means that this probe is reliable.
However, this probe is relatively big for use in industry.
Considering the measurement of automobile
components, there are many kinds of shapes forcomponents and the size of the probe should be
reduced.
For example, an automobile industry uses this kind ofcomponents:

FIGURE 10: PROBE N°1

BLARDONE ALLAN

FIGURE 11: EXAMPLE OF SHAPE OF COMPONENTS
TO ANALYSE

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FIGURE 12: ACTUAL COMPONENTS FOR AUTOMOBILES, MADE BY BENDING A STEELPLATE WITH DIFFERENT LEVELS OF HARDNESSTo properly evaluate this component, we need to have a spatial resolution below 15 mm. In
order to respond as best as possible to industry demands, the main purpose of my work during
this internship is to reduce the size of the probe.
A simulation model has been made to investigate the effect of coil size on the magnetic field
distribution generated from this sample, displayed in ANNEXE 1 (pages 16 to 17).

III.

EXPERIMENTAL TECHNIQUES
3.1.

Equipment used
To carry out the measurement, we create the alternative
current with a low frequency generator: we chose 100
Hz and 1V peak-to-peak. We also use DC generators tooperate the magnetic sensor. An oscilloscope can beused to check the output of the magnetic sensor(detected magnetic field). The current flows theinduction coil of the probe which is fixed on a XYZ stage.
A laser displacement sensor is fixed on the probe: when
activated, it will be used to determine the operation
amount of a motor fixed on the Z axis to maintain aconstant and predefined distance between the probe
and the sample. Before starting the analysis, we reset
the magnet sensor by sending pulsed current to align its
magnetic domains.

Laser

The output voltage which will be emitted by thismagnetic sensor will have a small intensity and has tobe optimised: we will use a lock-in amplifier. It has many
advantages: it detects very small AC signals by
extracting a signal of known frequency, amplifies it until
109, delete most part of noises and it is able to measure
the variation of a phase of the alternative signal.

FIGURE 13: XYZ STAGE AND LASERBLARDONE ALLAN

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3.2. Safety Rules

We use relatively high currents (about 0.3 A) to produce intense magnetic fields. It isnecessary to check the connections when circuits are switched on and not todisconnect them while circuits are switched on.
The metal samples to be tested are rough and sharp, therefore the use of protective
gloves when handling them is recommended.

3.3. Measurement protocol of the control
To evaluate the positions of hardened and non-hardened regions of a steel plate, we start by
fixing the probe on the motorised platform. Then, we chose the number of measurement points
and lines, as well as the distance interval between each point. We set these settings on theinterface of the motorised platform and on the computer, using the software LabVIEW. Then,
we set up all generators according to chosen experimental conditions, and adjust the liftoff ofthe probe if we want to use the laser for Z-axis correction. After resetting the magnet sensor
and check the calibration of the lock-in amplifier, we can finally start the analysis.
Using Excel, we obtain the graph of the output signal of magnetic sensors, in mV. Then weconvert this signal into μT using the documentation of the magnetic sensor, to get the intensityof the induced magnetic field in function of the position on the sample (an example in figure
14).

Intensity of induced magnetic field in function of the position
15,4

Magnetic field (μTesla)

15,2
15
14,8
14,6
14,4
14,2
14
13,8
13,6
0

50

100

150

200

250

300

Position (mm)FIGURE 14: GRAPH OF THE INTENSITY OF INDUCED MAGNETIC FIELD IN FUNCTION OF THE POSITION,
USING THE PROBE N°1

If we analysed several lines on the sample, we can use the software Origin to have the mappingof the induced magnetic field as previously shown in figure 9.

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IV. RESULTS AND INTERPRETATIONS
4.1. New probes createdDuring the internship, we prepared 2 new probes, which are smaller than the former probe n°1:

FIGURE 15: PROBE N°2

FIGURE 16: PROBE N°3

These two probes are smaller than the probe n°1 in diameter and in length, especially for theprobe n°3. These probes use a rubber ferrite with an induction coil: the magnet sensor issandwiched between two layers of a rubber plate for the probe n°2, and only one ferrite plate
for the probe n°3.

The interest of ferrite is its highpermeability in weak fields (up to tens of
thousands): the magnetic field isconcentrated inside the ferrite, to reduce
its potential.

Ferrite stem

FIGURE 17: MAGNETIC FIELD LINES CLOSE TO AFERRITE STEMBLARDONE ALLAN

The main drawback of ferrite is its poor
mechanical properties: it is really hardand brittle. To overcome this lack offlexibility, we use a rubber containing
ferrite.

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Because of this, we can easily change the bottom shape of ferrite rubber which is close to thesample. This leads to reduce the probe size markedly.

4.2. Results of New Probes
To highlight the concentration of the magnetic field, we mapped the distribution of the magnetic
field generated from each probe. To do this, we applied the same experimental protocol as forthe measurement of a steel plate, but the one magnetic sensor was placed below the probe.
We can then make the mapping of the distribution of the magnetic field for each probe, in μT:

We can see that the area ofstrong magnetic field insmaller for the probe 2 thanfor the probe 1, and evensmaller for the probe 3. The
unit
of
positioniscentimetres, and the unit of
the magnetic field is 𝜇T.

~2.3
cm

FIGURE 18: MAGNETIC FIELD
DISTRIBUTION OF THE PROBE N°1~1.4 cm

~1.0 cm

FIGURE 19: MAGNETIC FIELD DISTRIBUTION OF THE PROBE
N°2

FIGURE 20: MAGNETIC FIELD DISTRIBUTION OF THE PROBE
N°3

The next step was to evaluate the hardness of the steel plate with the new probes. We thenapplied the measurement protocol, with an input current of 100 Hz and 0.1A, without liftoff: theZ-axis correction was removed, and for each measurement point the probe touched the sample.

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The measurement was done with 3 lines of 34 points, with an interval of 1 cm between eachpoint and each line, and the intensity is given in μT. We can compare all probes:

FIGURE 21: MAPPING OF THE INTENSITY OF INDUCED MAGNETIC FIELD USING THE PROBE N°1

FIGURE 22: MAPPING OF THE INTENSITY OF INDUCED MAGNETIC FIELD USING THE PROBE N°2

FIGURE 23: MAPPING OF THE INTENSITY OF INDUCED MAGNETIC FIELD USING THE PROBE N°3

4.3. Interpretation
As shown in Fig. 18, 19, 20, we can see that the reduction of the coil diameter and the use of
ferrite was efficient, to reduce the area of applied magnetic field.
The mapping of probe n°1 and probe n°3 are quite similar and both good, while the mappingof the probe n°2 is a bit more unstable.
We can say that we achieved our objective: we obtained a result as well as with the former
probe, with a new probe (n°3) which is approximately three times smaller in diameter and inlength.

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V. CONCLUSION
During this internship, I’ve had the opportunity to work in a research laboratory on a concrete
problem, which is truly stimulating: the outline was improving a probe for non-destructive
evaluation which will allow a significant optimisation of weight of vehicle body shells. The main
objective of my work was attained: we have reduced the size of the probe by about three times,
and we are able to acquire an accurate mapping of the steel plate hardness.
Naturally, there is still a lot of remaining work before the implantation of the method in industry.
For instance, a probe that touches the metal precisely should be considered, because themagnetic field intensity is very sensitive to the spacing with the sample, the miniaturisation of
the probe should be continued, possibly in considering new techniques, etc.
During my internship, my level of autonomy increased gradually: at the beginning I had to learn
experimental protocols while experimenting with Professor SAKAI and student ITO, with theaim of being able to experiment all by myself thereafter. This requires good methodology andmemory, since everything is written in Japanese on interfaces. I’ve had to deal with a lot of
data, I’ve consequently improved my organisation of work and my ability to synthesise results.
I was also initiated to LabVIEW and Origin, I improved my skill on Excel and had to read somescientific publications written by my professors in English: this requires a good focusing.
Writing this report was also challenging and rewarding: I accomplished a lot of different
experience that I didn’t mention in this report, but which took me a lot of time. For example, I
studied the impact of the input current frequency, the impact of Z-axis correction or the impact
of demagnetisation of the steel plate, on the resulting signal. Among all of these works, it has
been interesting to focus the report on only one common thread: the reduction of the size ofthe probe.
In conclusion, this 3-month internship in Japan has really been a wonderful and enrichingexperience. I enjoyed experiencing the work in a scientific research laboratory. Working in aforeign country permits to learn different ways of working and thinking, and I benefited from myfree time to travel in a lot of places: the cultural disorientation is really fulfilling and make usreconsider what we thought evident or taken for granted.

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VI. ANNEXESANNEXE 1: SIMULATION MODEL TO INVESTIGATE THE EFFECT OF COIL SIZE (PART 1)

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ANNEXE 1: SIMULATION MODEL TO INVESTIGATE THE EFFECT OF COIL SIZE (PART 2)

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DOS DU RAPPORT
Étudiant : BLARDONE Allan

Année d’étude dans la spécialité : 1A

Entreprise :Adresse complète :Téléphone :Okayama University, Tsushima Campus
3-1-1 Tsushima-naka, Kita-ku, Okayama 700-8530+81-86-252-1111

Responsable administratif :Téléphone :Courriel:

Hirofumi Makino, general administration division

Tuteur de stage :Téléphone :Courriel :Professor TSUKADA, Keiji+86-251-8129

Enseignant-référent :Téléphone :Courriel:

Pascal JAY
04 56 52 01 84+81-86-251-7004
aax7007@adm.okayama-u.ac.jp

tsukada@cc.okayama-u.ac.jp

pascal.jay@univ-grenoble-alpes.fr

Title: Non-destructive evaluation of steel plates containing different hardness using low-frequency magnetic fields
Résumé :Récemment, une nouvelle méthode de presse à chaud est née et permet de créer des régions "durcies" et"non durcies" à la position souhaitée dans une plaque d'acier. La différence de dureté entre ces régions permetde réduire considérablement l'impact d'un choc, ce qui prédispose ces plaques d’acier à une utilisation dansles structures automobiles. Avant de les implanter dans l'industrie, une méthode de contrôle non destructif
fiable et rapide est requise afin de contrôler la position des régions durcies et non durcies à l’intérieur desplaques d’acier. L'équipe du professeur TSUKADA, à l’université d’Okayama, a alors mis au point une méthoded'évaluation magnétique permettant cette évaluation : ils ont mis au point une sonde permettant de détecter
fidèlement l’emplacement de ces régions. Ce rapport relate de mon stage en laboratoire au sein de cetteéquipe de recherche, travaillant sur l'amélioration de cette méthode de contrôle. Mon objectif est de créer
une sonde plus petite que la sonde prototype afin d'augmenter sa résolution spatiale, pour permettre lecontrôle de pièces industrielles comprenant des régions non durcies plus petites. Je présente dans un premier
temps le contexte de mon stage et de la recherche, ainsi que les intérêts de réduire la taille de la sonde.
J'explique ensuite la procédure expérimentale d'un contrôle de dureté d'une plaque d'acier. Puis, je présente
les deux nouvelles probes créées et je finis par montrer et interpréter leurs résultats.
Summary :Since recently, a new method of hot pressing makes it possible to create "hardened" and "non-hardened"regions at the desired position in a steel plate. The difference in hardness between these regions considerably
reduces the impact in the event of a shock, which predisposes these steel plates to use in automobiles
structures. Before implanting them in industries, a reliable and rapid non-destructive testing method isrequired to control the position of the hardened and non-hardened regions within the steel plate. Professor
TSUKADA's team, from Okayama University, has developed a magnetic evaluation method which permits thisevaluation: they developed a probe allowing an accurate localisation of these regions. This report recounts
my internship in the laboratory within this research team, working on the improvement of this method ofcontrol. My goal is to create a probe smaller than the prototype probe to increase its spatial resolution,
allowing the control of industrial components containing smaller hardened and soft regions. Firstly, I present
the context of my internship and of the research, as well as the interests of reducing the size of the probe.
Then, I explain the experimental procedure of the hardness control of a steel plate. Finally, I present the two
new probes created and I end up by showing and interpreting their results.

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