| Wheels of rail vehicles have to withstand extremely high loads: large variations in temperature in addition to huge mechanical stresses. A modern test bed using ultrasound makes it possible to test the wheels rapidly and simply as well as reliably. |
 
In order to ensure the highest possible quality, the International Railway Union (UIC: Union Internationale des Chemins de Fer) places strict requirements on the manufacturers of railway wheels and their steel tyres. Non-destructive test methods such as the sonogram are required to test each part during production. In accordance with UIC rules the ultrasonic waves are targeted in two directions at right angles to the main deformation axes. Inhomogeneities also have to be taken into account, such as can occur at the inner edge of the tyres. Engineers working at the Fraunhofer Technology Development Group (TEG) in Stuttgart and the Fraunhofer Institute for Non-Destructive Testing (IZFP) in Saarbrücken have jointly developed a rapid test system in which the wheels and tyres can be tested for flaws in the material. The system can easily be incorporated in the production process. The procedure is simple. While a lifting device takes the wheel off the production line, sensors measure its diameter and thickness, so that it can be positionedt on the test rig in the optimum way. Then it is immersed in a bath of water, which is used as a coupling medium because it is superior to air as a transmitter of the high-frequency waves. Several ultrasonic heads are adjusted independently as required, and matched to different sizes of wheel and different requirements, when they are moved into position. If the steel is fault-free, then the signals pass through the wheel undistorted. But if there are flaws in the material, such as cavities, cracks, or inclusions, they reflect the sound waves and alter the waveform. The engineers can see on the monitor the position of the flaw and its size and nature. After testing, the wheel is returned to the production line or rejected; the entire test takes no more than 60 seconds. »The outstanding feature of our method is the combination of perpendicular and oblique incident waves«, explains Friedhelm Walte of the IZFP. » In this way we can even show up longitudinal cracks in the material, which are otherwise hard to detect.« The fully automatic procedure greatly reduces the cost of testing. It now makes up only a negligible fraction of the total production costs. » Ultrasonic fault detection is not restricted to railway wheels«, as Joachim Montnacher of the TEG points out. »The technique could well be applied to other products where particularly high quality is of paramount importance.«
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