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This article asserts that the applied ultrasonic testing method does not effectively recognize all possible defects of the wheel. For that reason a task optimized the technique which showed first good results and plans for the necessary modifications are underway for a long term investigation at the service station in Hamburg.
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The eddy current method and the ultrasonic method were used for testing 34 wheel pairs at intervals of 240.000, 360.000 and 480.000 km operation time. At the end of the investigation the wheels were tested using destructive methods.
|After 60 Tkm, all surfaces already showed more or less significant surface damage. Typically, two damage zones develop. (Fig 1). The zone of greater damage lies between 30-60 mm and zone of less damage lies between 80-90 mm from the outer edge of the wheel. At the early stage of damage development, the damage is visible only with the help of glasses or chemical surface treatment. The small cracks in zone 1 are mostly transversally oriented. In zone 2 no preferences of orientation are visible. After 360 Tkm the damaged zones are doubled in width and are clear visible to the naked eye. After 480 Tkm, some of the wheels were already damaged enough to meet rejection criteria. The rest did not develop higher damage levels. The number and size of the cracking increased clearly.|| |
Fig1. Position of the damaged zones on the wheel surface
|Multi channel eddy current test equipment was used and 3 sensors were placed at 10 mm, 47 mm and 85 mm distance from the wheel edge (Fig.2). After 240 and 360 Tkm three sensors showed a higher noise level, which is a sign of damage. After 360 and 480 Tkm high noise together with single indications were measured.|| |
Fig2. Position of the ECT probes
Fig 3. Principle of EMAT generated ultrasound
|At all wheel pairs the echo amplitude was recorded during the operation time intervals. Measures indicated that until the 360 Tkm interval the echo amplitude declined and from that point no change was measured. Fig 4 shows the relation between UT and ECT.|| |
Fig 4. Relation between UT and ECT measurement
|The method was integrated into test intervals for trains and performed every 3 to 5 days. 410 kHz EMUS probes with no coupling medium generated waves which propagated 7 mm depth. The probes were mounted at the track so that an automated test could be possible by driving the train at a speed of 1,5 to 8 km/h over the probes (Fig. 5).||
Fig 5. Installation of the UT probes
The experience of two units showed that ultrasonic testing with this configuration could not be used. After a relatively short operation time the wheel material characteristics changed by hardening in such a way that sound was already absorbed and no evaluation could be performed for future inspection. For that reason a task together with the FhG IzfP optimized the technique. The optimization recognized:
1. Railway testing in the Ultrasonic Testing Encyclopaedia
2. H.-J. Salzburger, Saarbrücken; H. Hintze, Kirchmöser Tiefenbestimmung von Laufflächenfehlern an Eisenbahnrädern unter Nutzung linear polarisierter Transversalwellen - Verfahrens-Optimierung und -Erprobung. DGZfP Jahrestagung 1996.
The paper was presented at the German-French Workshop in Aachen in April '97. Published in Berichtsband 57 ISBN 3-931381-13-7: Vorträge 2. Deutsch-Französisches Seminar Automatisierung und Modellierung für die Zerstörungsfreie Prüfung. Page 105 - 112. DGZfP Deutsche Gesellschaft für Zerstörungsfeie Prüfung e.V. Berlin 1997.
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