What is SICOR?
SICOR is an acronym for “Speed Induced Current On Rails”. It describes a new measuring procedure for the examination of fitted railway rails for sub-surface (hidden) defects on the rail head, especially at depths of 1-10 mm below the surface. Thus the previously designated “problem zone”, too deep for the usual eddy current procedures and not deep enough for the usual ultrasonic methods, can be tested. The principles of the SICOR technique are explained below.
Because of time dependency of the rail magnetization, B(t), at point ‘P’, the magnetization can appear to the stationary observer to be a local magnetic impulse excitation. Altogether, the chain of connections described above, brings forth measuring effect which demonstrably show that, with suitable testing systems, very deep defects, up to ca. 10 mm below the surface of the rails, can be found. The detector sensitivity is therefore dependent on the size of the "material defect separation" and its depth. Initial trials of the SICOR procedure proved very successful, further investigations are currently taking place. Surface breaking defects can, of course, still be found using existing eddy current procedures. Using the SICOR probe system, one possibility would be to use a dual frequency technique to find surface defects though the magnetisation can offer an improved correlation to the damage depth. For the sake of the completeness, it must be mentioned that, due to its outstanding results, a worldwide patent of the SICOR procedure, which can naturally be used to increase the safety/reliability of railway wheels, for example on the train’s rolling contact surfaces during maintenance and overhaul work, has been secured by Rohmann.In order to understand how SICOR works and on which physical principles the measuring effect is based, we first need to look at the reciprocal effect between magnetic fields and electrically conductive materials, particularly the penetration characteristics in metallic materials. In simplistic form the SICOR principle can be broken down into three different effects.
1. Firstly is the generally known phenomenon that magnetic fields which penetrate electrically conductive materials produce electric currents in those materials. This is described as magnetic induction. In order to achieve this, the magnetic flow – or more precisely, the density of the magnetic flow – changes in the material plausibly represented by the notion that the “density of the magnetic field lines” may themselves change in the material. In the case in consideration there are two possibilities; either the strength of the magnetic field changes or the electrical conductivity of the material changes relative to the magnetic field. In the case of SICOR, it is the second of these possibilities: magnetic field and material, in this case the rail, change in relation to one another and thus a magnet with a known speed is moved over the rail. This phenomenon works with eddy current brakes as well and has the advantage that it is contactless and free from wear. Well known applications are, for example, the braking mechanisms of “Free Fall Towers” in amusement parks or the steplessly-adjustable magnetic brakes on exercise bicycles or medical ergo meter bicycles. One can simply be convinced of the effectiveness of this principle by stroking a magnetic clamp over an electrically conductive, but not magnetic (e.g. aluminium, copper, brass) surface; there is a noticeable resistance against the direction of movement of the magnetic clamp which is stronger the faster the magnet moves over the metallic surface. Inside the conductive material, currents, which themselves produce a magnetic field, are excited by the movement of the magnet. The braking action is caused by the fact that the magnetic fields of the excited currents and the moving magnet are in opposition. This phenomenon also functions in magnetic materials only as long as they are electrically conductive. The attraction force of the magnet on magnetisable materials however usually masks the effect of the braking action. It is the braking action against the direction of motion of the magnet which interests us here and is the first basic principle that the SICOR procedure demonstrates. Moving the magnet over the metallic material creates currents in the material itself and thereby a magnetic field. This magnetic field is in opposition to the magnetic field of the moving magnets and causes a braking force on it. The magnetic field in the metal and also therefore of the induced currents is shifted/distorted in such a way by the movement of the magnet that it is dragged “behind the magnet” – we are talking here of the magnetic tow effect.
2. Secondly it is known that slots in metals obstruct the formation of induced currents. The closer the slots are to one another in conductive materials, the fewer eddy currents are able to flow. The cores of electrical transformers are therefore made up of thin sheets. This prevents the transformer core from heating itself excessively and leads to a higher efficiency of the transformer by avoiding these losses. The eddy currents do not usually get completely prevented in the transformer core. This effect can also be exemplified on a metal surface with many slots: in the area of the slots the braking action on the magnets is clearly lower than in the area without slots and that means that in the area of the slots fewer eddy currents can be formed. With the SICOR procedure, material separations, e.g. cracks, still affect the formation of currents even at deeper levels; these are however not the currents which the SICOR procedure itself detects in the material.
3. To understand the SICOR procedure another effect must be considered: magnetisable materials can only be magnetized up to a certain level, depending on the composition of the material and its pre-treatment. Above this level, the so called saturation level, the material cannot be magnetised any more strongly and it behaves as though it were not magnetic; it behaves quasi-like as a non-magnetised material. How strongly the “test rail” is magnetised locally depends on one hand on the external magnetisation and, on the other hand, the internal damage within the material. At this point the third basic principle of the SICOR procedure comes into play, namely that induced currents can penetrate more deeply into non-magnetic materials than into magnetic materials. Therefore a suitable probe system is used which examines the “test rail” in a zone in which the “displaced” currents produced by the strong magnet move. This sensor system can, on the one hand, “see”, because of the saturation effect of the external magnetisation, very much deeper inside the steel of the rail than would be possible without magnetisation. On the other hand, it also "sees" the effects produced inside the material by deep defects, (e.g. cracks). As already explained above, fewer currents are produced in areas of material separation, therefore fewer magnetic fields and thus less of a screening effect in the material arises. The saturation effect caused by the external magnetic field and the smaller screening effect caused by a smaller induced current in the damaged area aid one another.
4. The length ‘L’ of the moving magnet and/or magnetisation yoke has an additional influence on the formation and depth range of the induced currents in the SICOR procedure. One can visualize the magnetising device as a horizontal bar magnet, which is moved lengthwise over the test material. Now if a certain place, ‘P’, in the test material moves over the magnet with a constant speed, ‘v’, the test material sees firstly one magnetic pole then the other, it therefore changes the magnetic polarization of the test material, for example from "north" to "south", dependent on the length of the magnet and how quickly it is moved over the material. This means, however, that the local magnetisation of the test material can be designated as the “equivalent magnetising frequency”, ‘f’: long magnets and low speeds result in low frequencies, short magnets and high speeds mean high frequencies.
Fig. 1 – Moving a magnet with length ‘L’ and speed ‘v’ over a rail:
The “equivalent magnetising frequency”, ‘f’ can be derived from Fig. 1. At the point ‘P’ the value of the strength of the magnetic field ‘B (t)’ is at a maximum when ‘t=1’ a minimum when ‘t=3’ and the time between them is ‘Dt’. On the assumption that the magnetization within this range has a sine-wave-like appearance, -the time ‘Dt’ can be regarded as a half-period duration; the entire time duration is therefore 2D’t’. The time ‘Dt’ is a result of the speed ‘v’ and the length of the magnet ‘L’. As the frequency ‘f’ is equal to 1/’t’ the following relationships are created:
If (2) and (3) are used in (1) then (4) is the “equivalent magnetising frequency” ‘f’.
Because the depth of penetration ‚dE’ in conductive materials is dependent on the frequency of the magnetic excitation ‘f’, ’the penetration of long magnets is greater than that of shorter magnets at the same speed. The depth of penetration of the induced currents, produced by the magnet, can be targeted according to the length of the (electro-) magnet ‘L’ and the speed of movement ‘v’. For steel materials, using two magnets of different length and the resulting “equivalent magnetising frequency”, the following approximate values for the depth of penetration ‘dE’ can be calculated (Fig 2):
Fig 2. Table for typical depth of penetration ‚dE’ using the SICOR principle
Because of time dependency of the rail magnetization, B(t), at point ‘P’, the magnetization can appear to the stationary observer to be a local magnetic impulse excitation.
Altogether, the chain of connections described above, brings forth measuring effect which demonstrably show that, with suitable testing systems, very deep defects, up to ca. 10 mm below the surface of the rails, can be found. The detector sensitivity is therefore dependent on the size of the "material defect separation" and its depth. Initial trials of the SICOR procedure proved very successful, further investigations are currently taking place.
Surface breaking defects can, of course, still be found using existing eddy current procedures. Using the SICOR probe system, one possibility would be to use a dual frequency technique to find surface defects though the magnetisation can offer an improved correlation to the damage depth.
For the sake of the completeness, it must be mentioned that, due to its outstanding results, a worldwide patent of the SICOR procedure, which can naturally be used to increase the safety/reliability of railway wheels, for example on the train’s rolling contact surfaces during maintenance and overhaul work, has been secured by Rohmann.