·Home ·Table of Contents ·Materials Characterization and testing

Elaboration Software and Material Characterization by Eddy-Current Method M. Zergoug, S Mebrek, T . A . Hadjaoui, N. Boucherrou , A .Hamouda, A . Haddad F. Sellidj , A. Benchaala, G. kamel laboratoire d'electronique et d'electrotechnique Centre de soudage et de contrôle, Route de Dely Ibrahim, B.P:64, Chéraga, Tipaza Tel : (213)(02) 36 18 54 à 55, Télex : 71 179, Fax: (213)(02) 36 18 50 Contact

ABSTRACT

The application of eddy current in non destructive testing was very developed during the last years. From the characterization of defect, currents studies deal with the material metallurgy evaluation. The sensitivity to defects and other parameters of control can be improved by the optimal choice of the probe.
A software has been elaborated ,allowing to automate this control and to reconstitute images having two and three dimensions of the controlled samples.
The realized experimentation's have shown large possibilities of analysis by the Eddy current method, the evolution of the various metallurgic characteristics. They allow , besides the verification of the coating and the determination of its thickness .The study and realization of probes allowing an optimal coating control of ferromagnetic material,Material control, material and coating evaluation from the software Control and realization of coating, defects by the restitution of the structure image. The testing data and conditions are directly provided to the software .The results appear under the from of a color or relief graphical chart representing the variation with three dimensions for each one of chosen parameters .
This visualization is realized in comparison with the display parameter in respect to that of reference previously taken for each one of the sweeping points. Besides the color chart is affected by a fixed sensibility that corresponds to a color set. This one is function of the minimal required variation .
This results is very important because it allows from the impedance value of a sample of the same structure and processing to determine defects identification and cementation characterization.
This results is important because it allows by this software to characterize the metallurgic states of materials and we can to determine the defects dimensions

INTRODUCTION

The electromagnetic properties of steels depend on their composition, their microstructures and applied stresses. It is therefore natural to try to use magnetic and electrical parameters of steels to evaluate their microstructure.
A lot of progress has been made in the theoretical and practical aspects of eddy current testing. One of this part is to calculate the electromagnetic field produced by eddy current control, the second approach is to determine the impedance changes produced when the eddy current field is perturbed by a flaw
A software has been elaborated allowing the automation of this control and the reconstitution of 2D and 3D images of the controlled samples.
The realized experiments have shown the large possibilities of analysis by the eddy currents method. The present problem involves a wide variety of defects types and orientations wich would make it very easy and to facility the result interpretation.[1-4]

TESTING BY EDDY CURRENTS

Induction laws and experiments show that the impedance of a coil crossed by an AC current put near a conductive piece is modified by the creation of eddy currents. The presence of an anomaly in this material structure modifies the impedance of the generating coil. The impedance variation measure is at the root of non destructive testing by eddy currents. Any variation inside a piece (variation of conductivity or permeability) modifies the intensity and the course of the eddy currents and consequently the coil impedance.

CONSTRUCTION OF EDDY CURRENT PROBES[6,7,8]

The sensibility to defects and other testing parameters of pieces can be modified by the geometry of the piece to be controlled and the conception of the probe. It is sufficient to set the direction of circulation of eddy currents, regulate the magnetic field intensity and choose the coil of the appropriate size.
A number of methods may be used to display the changes in complex impedance measured in eddy current testing.
The impedance diagram is a representation in a complex plan of the coil impedance . The real and imaginary parts of this impedance are function of the different parameter variation governing the construction of a sensor. The curves representing parameters influencing in the plan of impedance can be normalized in a standard curve called impedance diagram normalized.

Software allows the processing of results of the eddy current data acquisition. This computer processing facilitates the interpretation of results about the probe and indicate the optimum frequency used for a specific material

SURFACE TESTING BY X, Y and Z SWEEPINGS

This probe is put in a mechanical system that allows, using three stepping motors, the movement following the three axes : X, Y and Z. A software allows the acquisition of the measure parameters of the impedance bridge, by means of an IEEE 488 interface, and the mechanical system control by means of the RS 232 interface. The testing data and conditions are directly provided to the software. The results are displayed in a graphical color map, or in relieves representing a 3D variation for each of the chosen parameters. This visualization is realized in comparison with the display parameter in respect to that of reference previously taken for each sweeping point. Moreover, the color map is affected by a fixed sensibility that corresponds to a color palette. This one is function of the minimal required variation. In order to optimize the testing, it is necessary to consider certain parameters :
The excitation current was fixed for the realized probe at 1mA. The computed field resulting for this current value is lower than 100 Am^-1, in order to be located in the linear zone of the hysterisis diagram (Rayleigh). Whatever the type of the chosen probe or the excitation frequency, the same zone is controlled

CONTROL CONDITION

The testing probe realization by eddy currents was satisfactory at the sight of control and testing results of coating samples .The fundamental parameters to keep in mind for a probe construction are
• To diminish the reluctance of the measure circuit
• to allow an exchange between the probe and the material to be tested optimal energy
• A magnetic probe shell be realized in a material of a high magnetic permeability and a low electric conductivity
• Sensibility increases with this geometry

The resulting field at the contact point with the material to be tested can be assimiled to a material point .The influence of the lateral field is considerably minimized.

EXPERIMENTATION

The achieved software allows the probe characterization and the reproduction of image in 2D (color or levels of gray) and 3D of the controlled samples. This has the practical advantages that is easy to establish the characteristic for your control by simply raising the probe from the surface .The method for this experiment are
• Control of grooves on pieces in steel, aluminum and copper.
• Control of a piece in steel, with small defect less 100 micron.
• Control of pieces having sudden of different thickness cementation.

RESULTS AND INTERPRETATION

A more complete presentation can be made by superposing a number of parallel traverses in the x direction with a displacement in the y direction between each traverse and same information is displayed as a grey or color pixel .in the c scan (Fig 10).

Fig 10: Scan system

The size and the dimension defect are identically by this software .the dimension and the orientation of grooves are determinate in 2 dimensions or the 03 dimensions. for small defect (Fig 1-6).

Fig 1: Image 2D color of steel sample	Fig 2: Image 2D color of aluminium sample
Fig 3: Image 2D level Grey of steel sample	Fig 4: Image 2D level grey of aliminium sample
Fig 5: Image 3D of steel sample	Fig 6: Image 3D of aliminium sample

Fig 7: Image 2D color of sample impression

Fig 8: Image 3D of sample impression

Fig 9: Image 2D level grey of sample impression

The software allows by judicious choice of the pixel to determine the defect(Fig 7-9)

• The symmetry of the images clearly related to the defect shape
• We can estimate the defect grandeur by analyzing the impedance
• We can determine the nature of defects
• For linear defects, we can immediately find the orientation by looking the image

The realized experiments have shown the large possibilities of analysis by the eddy currents method. They allow, besides the verification of the coating and the determination of its thickness, to give an overview on the metallurgic characteristics of the controlled samples coating by a deepened processing. (Fig 11-12)
Moreover, the color map is affected by a fixed sensibility that corresponds to a color palette. This one is function of the minimal required variation. In order to optimize the testing, it is necessary to consider certain parameters :[3]

Fig 11: Image 2d of cementation

Fig 12: Image 3d of cementation

CONCLUSION

Software allows the processing of results of the eddy current acquisition This computer processing facilitates the interpretation of results
We now consider the relative merits of these display methods and their potential utility for defects identification (size ,orientation ,dimension) in real time
The results found for the coating controlled parts allow to consider during the testing the exploitation of the information provided by the measured variation impedance for the processed products by the coating thickness determination and low frequency homogeneity and to evaluate the high frequency coating nature.
This software permit also the probe characterisation by the impedance diagram, frequentiel diagram,...

REFERENCES

1. M. ZERGOUG, F. SELLIDJ, F. H. MANSOURI, N. CHEIKH, L. GACI, N. HANED, Automatisation d'une chaÎne de contrôle. 2ème Conférence maghrébine sur l'automatique, L'électrotechnique et l'électronique industrielle, Décembre 1996, Tlemcen, Algérie.
2. M. ZERGOUG, F. SELLIDJ, A. HAMMOUDA, S. MEBREK, élaboration d 'un modèle physique par courant de Foucault et son application dans la caractérisation des défauts longs dans les produits cylindriques creux non ferromagnétiques. 3ème congrés de mécanique, Avril 1997, Tetouan, Maroc.
3. M. ZERGOUG, F. SELLIDJ, A. HAMMOUDA, Caractérisation non destructive par courant de foucault de couches de rechargement et de revêtement dans les matériaux conducteurs. First Arab mechanics congress CAM 97, June 1997, Damascus, Syria.
4. M. ZERGOUG, F. SELLIDJ, A. HAMMOUDA, S. MEBREK, Réalisation des sondes et caractérisation non destructive par courant de Foucault de ouches de rechargement et de revêtement dans les matériaux conducteurs. Conférence maghrébine sur le contrôle non destructif, Juin 1997, Alger, Algérie.
5. Non destructive testing handbook, Vol. 4 Electromagnetic testing, American society for non destructive testing, Second edition, USA, 1986.
6. DOD (C.V.), CHENG (C.C.), SIMPSON (W.A.), DEEDA (D.A.), SMITH (J.H.), The analysis of reflection type coils for eddy current testing, DAK Ridge Normal Laboratory, US atomic energy commission.
7. WATCHE (G.), LASNIER (J.), MANSIR (M.), BURAIS (N.), NICOLAS (A.), Définition d'un logiciel adaptéau calcul de l'impédance d'un capteur à courants de Foucault, CEDRAT recherche, 1985.
8. FISET (M.), CHOUINARD (G.), Caractérisation des revêtements nitrurés et cémentés sur l'acier par courants de Foucault, University of Laval, 1992.

© AIPnD , created by NDT.net

|Home|    |Top|