BAM has developed a test method for testing the surface of installed railway rails in co-operation with the Deutsche Bahn AG and other industrial partners.
The primary purpose of this new test method is to detect and determine the depth of crack-like damage in the gauge corner of the rail. One cause for this is the so-called rolling contact fatigue of the rail material due to high speeds and large axle loads.
The new test method is based on the eddy current principle.
The eddy current test method is based on the principles of magnetic induction. An alternating magnetic field is produced by the test sensor which induces electrical eddy currents in a test object. Direct contact between the sensor and the test object is not necessary. When inhomogenities or damage are found, the flux of the eddy current field is effected. These changes are measured with the test sensor.
Principle of the eddy current testing
The eddy current procedure exhibits some remarkable characteristics, making it particularly suited for use as a test method for rail surfaces:
At present the eddy current test method has been installed on two rail test vehicles. The data collected on the condition of the rail rolling surfaces are mainly evaluated automatically. Test reports are generated, describing the condition of the tested rails. Recently a remote maintenance system has been made available for rail test and grinding vehicles. Via an UMTS connection BAM’s development laboratory can be directly connected to the test trains. With it fault-tracing and updates can be performed comfortably.
Rail testing facilities must not interfer with railway facilities - in particular those of control and safety engineering. Eddy current sensors emit magnetic fields of a certain frequency. Since the eddy current test sensors are positioned at the gauge corner of the rail, a potential influence of the wheel sensors and axle counters arranged in close proximity has to be considered.
Extensive tests were undertaken, which led to an understanding of how to avoid influencing the control and safety engineering components:
Of special importance is the detection of cracks, particularly in mobile switching gear, e.g. tongues and toes of points, which can lead to failure of the unit. Therefore a mobile eddy current test system was installed onto a light manual trolley. Spring-actuated locks ensure that this technology can be removed from the track within a few seconds. Thus an examination on active railway tracks is possible without interrupting normal service. A global positioning system (GPS) was integrated into the manual trolley as well as the eddy current test system of the rail test vehicle. By saving the data supplied from the GPS a precise location is ensured.
Division 8.4 | Reference defects for NDT procedures
