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Recent Developments of Eddy Current Probes Benoît MARCHANDa, Chiara ZORNIa , Jean-Marc DECITREa and, Olivier CASULAa CEA, LIST, F-91191 Gif-sur-Yvette, France Abstract. Eddy Currents (EC) technique is a powerful means of detection of defects in conductive components. The spectrum of industrial needs is large and EC probes have to be optimized to deal with different issues. Thanks to the CIVA- simulation platform, the CEA LIST develops high resolution arrays and high sensitive flexible probes to bring adapted solutions. On a first part, an original scheme, composed with two one-millimeter coils etched on a kapton film is presented. Experimental results coming from a flexible 32 elements of this sensor are shown and demonstrate the high performances of this technology in terms of sensitivity and spatial resolution. The second part of this paper concerns probes based on magnetic sensors. Two large arrays using respectively 22 Giant-Magneto Resistance sensors and 96 Anisotropic Magneto-Resistance sensors have been developed for the detection of very small surface flaws. Their high integration leads to a spatial resolution as small as 100µm. Experimental tests are performed and demonstrate their efficiency. Eventually, flexible magnetic sensor probes recently developed in the labs are presented and their performances in the detection of deep defects are valued. Keywords. Eddy Currents, multi-elements, flexible, micro-coils, magnetic sensors. Introduction Eddy Currents (EC) technique is a powerful mean of defects detection in conductive components. The spectrum of industrial needs is large and EC probes have to be optimized to deal with different issue. The first one is the detection of very small surface flaws. This objective required high sensitivity and high spatial resolution for fast and accurate inspection. The second one is the detection of deep defects that need high sensitivity at low frequency. Planar or complex parts are concerned by both issues and that is the reason why flexible probes have to be developed. Thanks to the CIVA-simulation platform, the CEA LIST develops high resolution arrays and high sensitive flexible probes to bring solutions. In this paper we present recent developments of such probes achieved in the laboratory. 1. Flexible Array Probes Based on Micro-Coils Classical winding coils probes present limits coming from their technology itself: They are hardly able to be integrated in arrays with small pitch, meaning high spatial resolution. Too rigid, they can hardly be used in flexible probes for the inspection of complex parts. To overcome these limits, the CEA-LIST has developed a new kind of sensor that allows flexibility, high resolution and sensitivity at the same time. This sensor consists in two micro-coils, one as emitter and the other one as receiver, etched on both sides of a kapton film. Simulations, performed with CIVA software has led to a special and new pattern, presenting an overlapping of the coils. As the efficiency of this original sensor has been demonstrated by several experimental testing, large arrays have been developed. Thanks to the small size of the micro-coils and using an innovative staggered pattern, the measurement pitch has been reduced to only 350µm. Furthermore, the kapton film is flexible and can easily be put on a silicone roll, whose shape fits with the part that has to be inspected. Another consequence of this flexibility is the reduction of the lift-off noise, since these sensors remain in contact with the inspected surface . Figure 1 is a photo of the 32 elements (meaning 64 micro-coils) array probe, presenting a matrix setting (4 rows and 8 lines) that allows the scanning of a 11mm width strip. Amplifiers have been put close to the sensor to improve the signal to noise ratio (SNR). Figure 1. Flexible micro-coils array probe with 32 elements. This probe is driven by the Eddy-Current device Multi-X, developed by M2M Company. It allows the driving of 32 independent and multi-frequencies emitting channels and 64 independent digital demodulators as receivers. Figure 2 is an experimental testing of the probe. The objective is the detection of a 4mm long surface notch, located in Inconel elbow mock-up. The 32 overlapped Lissajous curves can be seen in the impedance plane, and on the right size the defect itself appears. On the top part, the CSCAN corresponds to data coming from a time encoder trigger. Since the pattern of the sensor is 4 staggered rows, the signature of the defect is a dot line. Thanks to mechanical encoders, a real-time shift is carried out to give a continuous CSCAN of the defect, as shown on the bottom part of the figure. Figure 2. Experimental testing: detection of a 4mm long surface notch 2. Probes Based on Magnetic Sensors Magnetic sensors present a very interesting trade-off between size and sensitivity. Since their sensitive area is only few micrometers, large high spatial resolution arrays can be achieved . Furthermore, their large frequency bandwidth allows to using them for the detection of small surface breaking flaws, when high frequency is required, or else, for the detection of deeply buried defects, at low frequency . 2.1. Large Magnetic Sensors Arrays Two large magnetic sensors arrays have been developed by the CEA LIST. The first is based on Giant Magneto-Resistance (GMR) sensors. It’s a linear array, composed with 22 GMR, which sensitive area is a rectangle of 70x8µm2. The 100µm only pitch gives to this probe a very high spatial resolution, which makes the inspection fast and efficient. The emitter is a current foil, put above the array. Figure 3 presents experimental results that show the efficiency of the probe in the detection of small surface breaking flaws. An inconel mock-up in which three defects of different length (200x100x200 µm3, 100x100x200 µm3 and 100x100x100 µm3) has been used. The second magnetic sensor array has been developed in the framework of the IMAGINE project, supported by the French National Research Agency. It consists of a linear array composed with 96 Anisotropic Magneto-Resistance sensors. The challenge of this probe was the integration of so many sensors, with a pitch as small as 100µm to give to the probe its high spatial resolution. Therefore, almost a 10mm width strip is inspected which reduces the scanning time but not the efficiency of the probe considering the small space between two consecutive sensors. The emitter is a large current foil. Besides, to improve the SNR, a special PCB containing amplifiers has been added next to the sensors. Figure 4 is a photo of the probe and a zoom of its sensitive area. As the objective is the detection of small surface defects, the magnetic sensor array has been put as close as possible of the inspected surface. Voie : test Interpolation : Non Composante : Amplitude Extraction : Oui Voie : test Interpolation : Non Voie : test Interpolation : Non Equilibrage : aucun Axe Orthonormé : Oui Composante : Amplitude Extraction : Oui Composante : Amplitude Extraction : Oui Normalisation : 1.000V 0.000deg Echelle de Couleur Fixée : Non Equilibrage : aucun Axe Orthonormé : Oui Equilibrage : aucun Axe Orthonormé : Oui test Normalisation : 1.000V 0.000deg Echelle de Couleur Fixée : Non Normalisation : 1.000V 0.000deg Echelle de Couleur Fixée : Non 2.4306 test test 4 1.1358 0.56645 3.4 3.8 0.55 1.1 2.6 3.2 3.6 2 1 0.5 3 2.4 3.4 0.9 0.45 2.8 0.8 0.4 3.2 1.5 2.2 2.6 0.7 0.35 mm 3 mm mm 0.6 2 0.3 2.4 2.8 1 0.5 0.25 2.2 2.6 1.8 0.4 0.2 2 2.4 0.5 0.3 0.15 1.6 1.8 2.2 0.2 0.1 1.6 0.1 1.4 0.05 2 0 25 25.2 25.4 25.6 25.8 26 26.2 26.4 26.6 26.8 27 0.0074744 mm 45.2 45.4 45.6 45.8 46 46.2 46.4 46.6 46.8 47 65.4 65.6 65.8 66 66.2 66.4 66.6 66.8 0.0092172 0.0089227 mm mm 200x100x200 100x100x200 100x100x100 Défaut n°1 Défaut n°2 Défaut n°3 µm3 200x100x200 µm3 µm3100x100x200 µm3 µm3 100x100x100 µm3 Figure 3. Detection of three small flaws in an inconel mock-up. Figure 4. 96 AMRs sensor probe. Both magnetic sensor arrays can be driven by the Multi-X Eddy Current device. 2.2. Flexible Magnetic Sensor Probes A common issue in ECT is the detection of defect buried in planar or in complex geometry parts. For this particular need, the CEA LIST has developed three flexible probes based on magnetic sensors : Giant-Magnetic Resistance (GMR), Giant- Magnetic Impedance (GMI) and µfluxgate. The GMR one is shown on Figure 5. Their design is the following: two coils can be used as emitter, the first one is 15mm from the sensor and the other one is 20mm distant. This configuration allows to improving the inspection depending on the deepness of the defect to look for. Coils and magnetic sensor are embedded in silicone that gives flexibility. Parts with a curvature radius up to 30mm can be inspected. The sensitivity axis of the magnetic sensor is perpendicular to the surface. CSCANs presented in Figure 6 are experimental data obtained using the probe in which the sensor is a GMR, 15 mm distant from the winding coil emitter. The inspected is a multilayer Inconel 600 mock-up, containing a 10x0.1x0.78 mm3 flaw. This mock-up allows to studying the detection efficiency of the probe, regarding the deepness of the defect. For each configuration the frequency is first valuated by using the skin depth relationship given by: f 1 2 and then experimentally optimized. magnetic sensor : GMI, μFluxgate or GMR flexible emitter : Winding coils Figure 5. Flexible GMR probe for detection of deeply buried defects. Figure 6. Detection of a 10 mm long defect vs ligament – experimental data. Signal-to-noise ratios have been computed and reveal an almost linearly decrease with respect to the ligament of the defect, considering the ligament slot [0 – 4] mm, as shown on Figure 7 Experimental results Figure 7. SNR vs ligament - GMR sensor probe - Experimental results. 3. Conclusions The innovative pattern, based on staggered rows and presenting an overlapping of the micro-coils, has been designed using CIVA software. It shows great interests in NDT probe development. Several flexible array probes based on this sensor technology have already been achieved. As show in this paper, their high spatial resolution and lift-off noise reduction, coming from their flexibility, allow them to detect small surface breaking flaws with good SNR. As the silicone roll can be changed and adapted to the inspected area, they are able to inspect a large spectrum of complex parts. The advantages of magnetic sensors in terms of size and frequency bandwidth sensitivity have been used to develop high performances probes for special needs: on one hand, large arrays for the fast and accurate surface inspection, on the other hand flexible probe for the inspection of deep buried defects in complex parts. Experimental results have been shown to demonstrate the good performances of the advanced NDT probes. References  Gilles-Pascaud C, Vacher F, Decitre JM, Cattiaux G, EC Array Probe Development For Complex Geometries, 5th ICNDE, San Diego, July 2006.  T.Dogaru and S. T. Smith, Giant Magnetoresistance-Based Eddy-Current Sensor, IEEE Transactions on Magnetics, Vol. 37, No. 4, pp.2790-2793, 2001.  Marchand B, Vacher F, Decitre JM, Gilles-Pascaud C, Fermon C, High Resolution Eddy Current Probes For Non Destructive Testing, QNDE 2007, Golden, July 2007.  Decitre JM, Casula O, Non Destructive Testing with GMR Magnetic Sensor Arrays, ECNDT, Berlin, 27-29 September 2006.
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