BS EN 16729-2-2020 Railway applications一Infrastructure – Non-destructive testing on rails in track Part 2: Eddy current testing of rails in track.
5 Basic principles
With the help of eddy current testing, surface cracks can be detected in electrically conducting materials (ferromagnetic or non-ferromagnetic materials).
Eddy current testing is based on the induction of electric current in electrically conducting material. The measured and analysed size refers to the distribution of the induced currents (eddy currents).
The distribution of the eddy currents in the material is governed by physical laws; the density of these eddy currents drastically decreases with the distance to the surface.
To determine the depth of a head check crack, the pocket length of the crack and the penetrating angle are used. The penetrating angle can vary depending on steel grade, traffic, tonnage and support conditions of the rail. The penetrating angle a (see Figure 3) commonly used for testing head checks in rail is 25 0
Parameters influencing the testing results are:
— conductivity of the material;
— permeability of the material;
— dimension and geometry of the defect;
— geometry between the eddy current probe and the material surface;
— design and size of the probe;
— setting of the excitation frequency.
More detailed information on the eddy current method can he found in EN ISO 15549.
5.2 Purpose of testing
The purpose of eddy current testing on rails is to detect, quantif’ or classit’ rail defects in the material surface mainly due to rolling contact fatigue which can affect the structural integrity of the rails in track and which are defined in EN 1323 1-5. This is not limited to, but generally covers the following defects:
— head checks;
6 Requirements for the eddy current testing system on rails
Inspection of the rail surface can be carried out manually or by using mechanized devices.
When testing with an eddy current probe, it is placed above the rail head. The impedance of the probe is influenced by the interaction of the electromagnetic field generated by the probe and the rail head. This influence recorded by the complex voltage of the probe can be calculated to the severity of cracking on rail surface.
The minimum requirements for a test system are:
— suitable for the test task;
— capable of dealing with various material characteristics (e.g. welds, steel grades);
— probes optimized to the position, direction, depth and distance between cracks;
— probes optimized to test non-planar rail surfaces;
— capable of testing rough and rusty surfaces;
— capable of recording the correct location of the defect;
— capable of testing forwards and backwards in track independent of the direction of the traffic (if mounted on vehicle, independent of the orientation of the vehicle).BS EN 16729-2 pdf download.