This study has investigated the applicability of the ACFM (alternating current field measurement) technique for the detection and evaluation of the severity of surface-breaking RCF(rolling contact fatigue) defects as part of an integrated rail inspection platform under development through the INTERAIL project. Small-scale high-speed laboratory tests with a turning lathe which was used to spin a specially designed rotary sample up to 3000 r/m showed that the probe could successfully detect and rank the artificially induced surface-breaking defects at speeds up to 121.5 km/h. The signal-to-noise ratio remained largely unaffected by the increases in speed under constant lift-off. The turning lathe tests also proved that the defects remained detectable at significant lift-offs exceeding 5 mm although ranking the severity of the defects became more complicated as lift-off increased. Tests using different lift-off values indicated that the intensity of the signal obtained for a given notch was reduced by the square of the lift-off as expected. Actual rail inspection conditions at high speed were simulated using a special spinning rig at the University of Birmingham. Manual tests with a 5 kHz ACFM micro pencil probe showed that all the artificial defects induced on the test rails were detectable. However, ranking became quite complicated when clustered defects were concerned: the ACFM signal amplitude being affected by both the order of the defects as well as the spacing between them. This is an important finding since RCF cracks are predominantly found in the form of clusters. The efficiency of ACFM arrays in resolving individual defects at speeds is currently under investigation. The ACFM probe used for the turning lathe tests was installed on a special trolley. Unfortunately, this experimental configuration could not prevent lift-off variations that occurred during the spinning rail rig tests at higher speeds due to safety precautions that limited the efficiency of the trolley. Analysis of the obtained results showed that the ACFM sensor successfully detected all artificially induced defects at low speeds and a qualitative indication of their severity was obtained. However, as the speed increased, lift-off variations became significant reaching and exceeding 7mm during tests. At 48 km/h only some of the defects could be successfully detected and reliable ranking of the detected defects was not possible. Tests were also performed on a rail sample removed from the network which contained real life mild RCF cracks. The ACFM technique was capable of detecting all RCF cracks successfully.
Detection and evaluation of rail surface defects using alternating current field measurement techniques
2012
12 Seiten, 19 Bilder, 19 Quellen
Article (Journal)
English
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