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The technique of eddy currents is based on the principle of electromagnetism that an exciting coil with an alternating current can generate a magnetic field around the rail surface when placed perpendicular to the rail component; then the eddy currents are induced within the skin depth of the rail surface layer and the eddy currents also generate a secondary magnetic field opposite to the first magnetic field, as shown in Figure 2.15a [109]. The sensing coil can measure the impedance change of the system and any defects present at the surface or sub-surface of the rail can lead to a disturbance in the secondary magnetic field, causing the impedance deviate from Z1 to Z2, as shown in Figure 2.15b [18].

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As the eddy current technique, based on the skin effect, is sensitive to any changes at the surface or near-surface of the rail, it has been used in rail inspection system, as a supplementary technique to the conventional ultrasonic transducer for surface crack detection and sizing. The technique has better ability, compared with ultrasonics, of detecting defects such as RCF cracking, squats, shelling and surface corrugation [13, 18, 110, 111]. A summary of the ability of eddy currents in detecting various surface defects of rails is shown in Table 2.1 but the details of accuracy were not given in [13] .

A hybrid inspection system has been developed and used by Deutsche Bahn DB (German Rail) AG that combines ultrasonic techniques with eddy current sensors for surface and sub-surface defects inspection [13, 18, 112]. The system consists of 10 ultrasonic probes at angles from -70° to 70° and 4 eddy current sensors located at the gauge side 25 mm of the railhead where RCF cracks usually occur (as shown in Figure 2.16 [13]). The inspection speeds can be up to 100 km/h with the eddy current signal remaining unaffected by speed [14]. The eddy current sensors have also been implemented on a grinding train focusing on the detection of head checking and providing position and depth information to on-line control of the rail grinding machine [113].

In order to give accurate depth of the damage in rails, the inspection system was calibrated by grinding a rail with gauge corner defects at a relative low material removal rate; eddy current measurements and cross-sectional measurements were performed to determine the signal and associated amount of material removed. As a result a calibration curve was established which allows the crack depth determined on the basis of the detected eddy current signal [114]. The system was not able to identify cracks with crack spacing smaller than 2 mm as the calibration curve was based on single cracks. A statistical method of practical field measurements was used to compensate the signal allowing the assessment of multiple cracks. However, the research [115] mentioned that the crack propagating angle (vertical angle) is necessary for determination of the crack depth but the details of how vertical angle can be measured was not given, or whether an assumed value was used. The system, known as HC Grinding Scanner, has been continually developed under Speno International SA.

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Figure 2.15 (a) Illustration of eddy current generation [109]; (b) variation of the sensing coil’s impedance due to the presence of a crack (vertical and horizontal axis represent the imaginary and real part of the impedance, respectively) [18].

Table 2.1 Sensitivity of eddy current sensors to defects summarised by German Rail (details of accuracy was not provided; belgrospi’s refers to indentations and cracks on crests of short wave corrugation) [13].

Category detectability Statement

Head checking Very good Quantity, location, depth Wheel-burns Very good Location, extent Grinding marks Very good Quantity, location, period

Rail joints Very good Location, kind Indentures Very good Quantity, location, period

Squats Good Quantity, location

Short/long pitch corrugation Good Location, period Welds Good Location, kind, lack of fusion

Belgrospi’s Good Quantity, location

Unlike conventional ultrasonic techniques, EC does not require any contact with the rail surface, which makes it suitable for high-speed inspection. However, as the

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exciting coil is normally perpendicular to the inspected surface, the magnetic flux that can penetrate into the surface decreases rapidly as the senor lift off increases. The eddy current signal subsequently diminishes with sensor lift-off; lift-off of eddy current sensors is typically no more than 2 mm for rail defect inspection [14]. It is a drawback for analysing EC signals that calibration blocks are needed to estimate the detected crack characteristics.

Instead of the empirical methods being used at present, the mathematical models for eddy current systems have allowed accurate predictions of the sensor responses to crack characterisation. Burke [116] reviewed the crack sizing modelling method (the swept-frequency method), which uses the absolute values of the change in the coil impedance due to the crack as a function of the coil position and frequency; the method measures over a range of frequency and uses an approximate solution to Maxwell’s equations for the determination of the crack depth. The method was tested for a range of single rectangular and semi-elliptical EDM slots (vertical angle of 90°) in Al alloy plates and results showed that the depth was determined within about 15 %; it was also tested for a range of fatigue cracks in compact tension specimens but the depth of smaller cracks was underestimated due to the crack-face contact effects (i.e. two crack surfaces connected and eddy currents flow through the connection rather than the crack tip, resulting in underestimate of the crack depth); no results about any crack vertical angle measurement or the performance predicting angled cracks were reported. Bentoumi, et al. [111] compared three signal processing algorithms for on-line evaluation of minor rail defects (shellings and welded joints) with a design of a double-coils and double-frequencies sensor in a subway train that give 8 complementary eddy current signals; the wavelet approach showed best results but wrongly classified a shelling defect as a welded joint.

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Figure 2.16 A set of 4 eddy current sensors (arrowed) installed on the inspection train combined with the ultrasonic technique as the high speed inspection system used in German Rail [13].