19th World Conference on Non-Destructive Testing 2016

1 License: http://creativecommons.org/licenses/by-nd/3.0/

Interactive Education in Eddy Currents

Gerhard MOOK 1, Yury SIMONIN 1 1 Otto-von-Guericke-Universität Magdeburg, Magdeburg, Germany

Contact e-mail: mook@ovgu.de

Abstract. The paper presents an interactive eddy current (EC) tool for learning and teaching. It aims to help beginners to understand this non-destructive inspection method. The tool consists of • easy to handle interactive computer simulation for pick-up probes, • all-digital EC instrument with USB interface, • extended set of reference pieces and

• instructions for training. All typical tasks like sorting, crack inspection, hidden defect and corrosion inspection and even dynamic inspection using low and high pass filtering and signal indication in XY- and Yt-mode may be learned . For teachers and trainers the tool offers best opportunities to demonstrate an EC instrument using data projectors by switching between the lecture material and the instrument “on the flight”. For eddy current imaging an easy to handle sensor array is provided. The reference piece is an engraved aluminium sheet to be read by the array from both sides.

1. Introduction

Eddy current (EC) method uses electromagnetic energy to inspect the near-surface region of conductive material for different kinds of anomalies very efficiently. No couplant is needed and the inspection speed is very high. Unfortunately, the signal generation in EC technique is hard to understand. In contrary to x-rays or ultrasonics no obvious shadow models or echo phenomena can explain it. To overcome this drawback and to make EC technique more popular in education and innovation some tools were developed making the signal generation more transparent [1-9].

The first tool is an all-digital EC instrument with USB interface. Transparent probes give an insight into their inner construction. They are easy to handle and produce evident signals of typical defects on reference test objects provided together with the instrument. The instrument allows learning inspection for surface and hidden defects, material sorting and wall or layering thickness assessment. For experienced inspectors it opens the chance to optimize settings and different types of filtering in x-y- and y-t-mode. For teachers the tool offers new opportunities to demonstrate an EC instrument using common data projectors and to switch between the lecture material and the instrument operation “on the flight”.

The second tool is a semi-quantitative simulation code showing the correspondence between probe action and signal behaviour. The student guides a virtual probe over a virtual work piece and may adjust lift-off effects, conductivity, frequency to become familiar with the signals of surface defects and hidden defects.

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2. The Instrument

2.1 Hard- and Software

For practising eddy current inspection stand-alone instruments are too expensive to give every student a single instrument. Moreover, a student first time facing a real pick-up probe and a real eddy current instrument is completely concentrated on the probe handling and will not be able to generate a regular signal on the screen.

For this reason a kit was developed containing all necessary components for teaching and learning the basics of eddy current inspection. This kit is called EddyCation

(derived from eddy current and education). Its main component is a small USB box to be connected to a PC, notebook or Macbook running Windows. The software is copied from the USB stick. It does not need nor installation, nor annoying passwords nor permissions. EddyCation converts the notebook into an easy to use eddy current instrument with a huge xy-plane display. The student stays in his well known environment and may focus on signal recording and interpreting. Figure 1 a) shows the case with the USB box, the probes and the references, Figure 1 b) displays the user interface on a notebook screen.

Fig. 1. a) Instrument, probes and references of an EddyCation kit, b) user interface on a notebook

How does EddyCation work? The core of the EddyCation system is the computer

generating and processing the signals. The USB box includes DA and AD converters with frequency ranges up to 20 kHz (for low frequency kits) and up to 5 MHz (for high frequency kits).

2.2 Absolute Probes

Material sorting is a good introduction to get into the correlation between the normalized impedance plane and the xy-plane indication of an eddy current instrument. For that, EddyCation comes with an absolute probe and seven round blanks from materials of different conductivity and permeability. Figure 2a depicts lift-off signals up to the air point. The hodograph of conductivity may be imagined by joining the end points of the lift-off trajectories. The large influence of the magnetic permeability also becomes obvious.

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The special sorting problem of coin rejection is investigated by the students most carefully. It can be shown that the Euro-coins are well selected combinations of materials of determined conductivity and permeability. With low effort these coins may be sorted.

Fig. 2. a) Sorting of 7 materials, b) crack detection using the absolute probe on the aluminium reference

EddyCation covers the wide topic of crack inspection by a single reference piece.

Slots of different depth have been eroded into an anodized aluminium strip. This strip may be inspected from both sides to simulate open and hidden cracks. The absolute probe is designed for easy handling with big foot avoiding tilting. The software offers the opportunity to record the signal in different colours. Figure 2b) and c) show that open and hidden cracks produce very different signal orientations. These signals easily can be distinguished using the method of phase discrimination.

Fig. 3. Surface crack inspection using the high frequency absolute probe

The latest version of EddyCation works in a frequency range up to 5 MHz and

comes with an appropriate high frequency probe. Figure 3 displays surface crack signals obtained from a 4 layer Rohmann crack reference [10] from ferritic and austenitic steel, aluminium and titanium. Even on the low conducting titanium reference the surface crack signals are easily distinguishable. The crack signal provides a significant angle to the lift-off signal enough for separating both signals.

A further important topic is the detection of wall reductions caused by hidden corrosion for instance. Within EddyCation a special aluminium strip simulates this defect by milled grooves. The students learn to interpret eddy current signals according to their phase shift. With increasing underlying the phase shift increases. Figure 4 brings up that this circumstance permits to estimate the remaining wall. Valuable information about eddy current behaviour may be gathered with different inspection frequencies.

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Fig. 4. Signals of hidden wall reductions at different frequencies. The specimen is tested from the smooth

side. The signal turns clockwise with increasing frequency but the amplitude reduces.

2.3 Differential Probes

Differential probes are able to detect local differences in the material properties. If correctly guided they bring up very small defects like cracks or pores even below the surface. But the operation of these probes is not intuitive. The students should take care of the orientation of this probe type. Like with the absolute probe, the contact area is big. The probe easy slides over the anodized aluminium sheet.

Fig. 5. Differential probe signals from surface and hidden cracks in aluminium

The differential probe provides the signal pattern in Figure 5. The signal magnitude of the open slots increases significantly with increasing depth but only slightly turns clockwise. The magnitude of the hidden slot signals is much weaker (by 16 dB) but the phase behaviour is a suitable measure for the underlying of the slot. The students learn to guide the probe with correct orientation.

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2.4 Dynamic Inspection

Filtering is a big problem for beginners because there is no understanding for the effect of filters on the shape of the signals. In commercial eddy current training courses beginners start to learn filtering with a hand-held rotating probe. It is hard to manage the probes handling and at the same time to adjust the low and the high pass filters. For this goal a rotating specimen has been developed.

Fig. 6. Rotating aluminium disc below glass fiber cover

Figure 6 shows this specimen. An aluminium disc with a slot rotates with an adjustable speed below a glass fibre plate. Any eddy current probe may be placed on this plate not touching the rotation disc. The eddy current signal can be displayed in the common xy-plane (small image on the left side) but also in the yt-mode in time domain (small image on the right side). For yt-mode it is necessary to synchronize the indication with the revolutions of the disc. The eddy current signal is processed by the software that catches the slot signal and automatically synchronized the indication. When the probe is lifted the image freezes for reporting. No other connection to the notebook is necessary.

The student may now concentrate on filter adjustment. Often the question arises how to adjust the low pass filter? Is it better to open it widely or better to close it as much as possible? For that question a switching power supply with a light band was installed in the housing. The power supply sends disturbances to the probe clearly visible in the signal. Figure 7 a) shows the complete signal.

Fig. 7. Signals of one disc revolution at different stages of signal processing

The slot signal is the biggest followed by the four controller signals of every disc revolution. The small riffle is produced by the switching power supply. Now the student

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clearly can understand to what extend to open or close the low pass filter. He closes this filter until the riffle has vanished and the slot signal remains as big as it was before. The motor controller signals are reduced but did not vanish. If the low pass is closed further the controller signals reduce but also the slot signal decreases. This situation has to be avoided. Instead, the high pass filter is closed as far as the slot signal does not get its symmetric shape. Now it is easy to define a threshold distinguishing the slot signal from the motor controller signals. After this lesson the student can manage the more difficult task of hole inspection by a hand rotor probe and a suitable kit of EddyCation. Figure 8 d) shows the Rohmann reference and the signals in different indication modes.

Fig. 8. Hand rotor signals of a test defect in aluminium

Figure 8 e) brings up the principle. A small differential probe rotates along the

hole’s wall. The student moves the probe slowly into the hole thus covering its whole surface. Figure 8 a) displays the xy-plane signal after band pass filtering. The threshold is crossed. Figure 8 b) shows one single turn of the probe with correctly adjusted filters. Figure 8 c) is a non-standard indication with coincidence of defect and indication orientation.

2.5 Thresholds and Reporting

Different shapes of thresholds may be chosen. For material sorting up to 7 elliptical thresholds may be used. They define the area of coincidence and can be used for demonstration of coin rejection.

Single thresholds are provided for absolute probe, symmetric double thresholds for differential probes. For pure amplitude assessment a circle threshold is useful.

Many stand-alone instruments may be connected to a printer for reporting. EddyCation lets you generate a MS-Word® report automatically. All settings and the xy-plane image are included. The student can concentrate on the essentials e.g. the interpretation of the eddy current signals. Directly in MS-Word® he can add comments and conclusions. The report can be printed like any other document. If the PC is connected to a network, the reports can be gathered, compared, transmitted or archived. Anyhow, the

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report remains on the hard disc of the student’s notebook and he can repeat what he has learned. 3. Homework

The effect of electrical, magnetic and geometric parameters on the eddy current signal is described in the normalized impedance plane (Foerster plot) independently from the probe. But it is not easy to understand the complicated hodographs and trajectories of the point movement mirroring the real and imaginary part of the impedance related to the reactance of the “empty” probe. A real eddy current instrument cannot demonstrate these trajectories exactly. Neither the normalization nor the influence of the test frequency or the conductivity may be displayed directly.

Fig. 9. a) Simulated inspection, left: pick-up probe over non-ferromagnetic material; right: lift-off signal in

the normalized impedance plane. Signals of b) reduced wall an c) surface slot.

To bridge this gap a semi-quantitative tool was developed by the students V. v. Hintzenstern, T. Haase, A. Goldammer und S. Andres of the Otto-von-Guericke-University Magdeburg (Germany). It is able to visualize the most important effects avoiding time consuming and expensive numerical modelling algorithms. It forgoes exact field theory on behalf of speed and handling. The tool wants to be a textbook for playing.

Figure 9 a) displays the set-up. On the left-hand side a virtual probe is moved over a virtual reference piece by three position wheels. The reference piece contains slots (to simulate cracks) and local wall reductions from the back side. On the right-hand side a virtual eddy current instrument visualizes all signals in the normalized impedance plane. The user may zoom in to recognise details of the point movement. Additionally the frequency and the material under inspection may be selected. The impedance variation caused by wall reduction and surface cracks is shown in Figure 9 b) and c).

4. Conclusion EddyCation (www.eddycation.de) makes eddy current teaching and learning more easy. The teacher clearly may demonstrate facts and methods via notebook and data projector. The student keeps motivation and concentration over long terms due to the interactivity and diversity of the work. Eddy current inspection becomes playing easy to learn.

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References [1] Mook, G.: Die Wirbelstromprüfung “spielend” erlernen! ZfP-Zeitung 103 (2007) 2,

pp. 35-38 [2] Mook, G.: Eddy current inspection - learning by playing, Insight, 49 (2007) 12, pp.

733-736 [3] Mook, G.; Simonin, J.: Eddy current tools for education and innovation, 17th World

Conference on Non-destructive Testing, paper 379, Shanghai, 2008 [4] Mook, G.: Tools for Education in Eddy Currents, 10th International Conference on

Application of Contemporary Non-Destructive Testing in Engineering, Ljubljana, Slovenia, 2009

[5] Mook, G.; Simonin, J.: Eddy current tools for education and innovation, 10th European Conference on Non-Destructive Testing, Moscow, 2010

[6] Mook, G.; Simonin, J.: Neue Geräteentwicklungen zur ET-Ausbildung. ZfP in Forschung, Entwicklung und Anwendung, DGZfP-Jahrestagung, Bremen, 2011

[7] Mook, G.; Simonin, J.: Wirbelstromprüfung – lehren und lernen, DACH-Tagung der Deutschen, Österreichischen und Schweizerischen Gesellschaft für Zerstörungsfreie Prüfung, Graz, 2012

[8] Mook, G, Simonin, J.: Dynamische Wirbelstromprüfung mit EddyCation, DGZfP Jahrestagung, Potsdam, 2014

[9] Mook, G.; Nowack, H.; Rühe, S.; Simonin, J.: Innovative Testkörper für die Wirbelstromausbildung, DACH-Jahrestagung Zerstörungsfreie Materialprüfung, Salzburg, 2015

[10] http://www.rohmann.de/blog/portfolio/testkoerper/ am 10.02.2016