|NDT.net - February 2001, Vol. 6 No. 2|
Recent Advances in Ultrasonic Inspection of Railway -Axles and - WheelsH. Wüstenberg, A. Erhard, N. Bertus, T. Hauser (BAM - Berlin)
H. Hintze, M. Schüßler (DB AG Kirchmöser )
Corresponding Author Contact:
Email: Hermann.Wuestenberg@bam.de, Web: http://www.bam.de/g3_viii.html
The present report will try to demonstrate this with the help of two examples for automatic ultrasonic inspection devices which had been planned and applied in Germany during the last two years, the one being focused to the old problem of ultrasonic axle inspection which useally takes11/2 to 2 hours if carried out with a manual scanning on non dismantled sets of axles. In Germany the inspection of axles without an inner hole will in the near future be carried out with phased array probes. The experimental solution and first experiences will be presented (Fig.1).
|Fig 1: Ultrasonic Inspection of Railway Axles with Phased Arrays|
Fig 2: Ultrasonic Inspection of the Wheel Disk with a Multiprobe Arrangement
Fig 3: Inspection Concept V-Transfer with Shear Waves
Some difficulties are given by the fact that the waves must travel through disk areas with different thicknesses. This requires a careful position adjustment of the probes. An additional difficulty is given by the restricted access for the nondestructive inspection of the whole wheel .
Fig 5: Laboratory Experiments
Fig 6: Highly contoured Wheel Construction
Fig 7a: Multi Probe Inspection System
Fig 7b: Fitness for practical Application
Fig 4: In Situ NDT Access at the Wheel
It must be considered that many of the wheels are equiped with brake discs, so that an access for an alternative NDT inspection e.g. by magnetic particle testing or by an eddy current scanning is not possible. For the in situ inspection the wheel is slightly lifted by an appropriate machinery built into the rails; the lifted wheel set can then be turned. A suitable mechanism is coupling a probe set simular to the one demonstrated in fig.5 to the wheel rim. During the rotation all the data are recorded by a multi channel ultrasonic equipment which offers the possibility to record data for up to 64 channels. The A-scan data are digitized either as HF- signal or as rectified signals with a propriate filtering.The later version offers the opportunity to store the A-scan data in the so called pixelized format that means in constant distance samples. The shape of the A-scan is digitally stored. Based on those data an echotomographic presentation of the received data can be presented on a screen and evaluated by the operator. Fig.6 shows such an echotomographic presentation. Fig.7a and b are presenting photographs of an experimental set and the realized inspection tool which is in operation since beginning of July 2000 in Munich.
Fig 8: Shear Wave Phased Array Probe for the Axel Inspection
Fig 9: Shear Wave Directivity Pattern in the Plan of Incidence
Fig 10: TD-Scans and A-Scans of different Incidence Angles
Fig 11: Indication of a Saw Cut at a Railyway Axel
Fig 12: Detection of Saw Cuts in the Journal Area of the Axel
Fig 13: Detection of Notches by a Phased Array Probe
The operator has to overcome those restrictions by a careful observation and scanning of the wheel surface, which may take up to two hours for one axle. A simple automatisation allowing an imagelike representation of the circumferential surface of the axle like shown in the upper part of fig.10 enhances the discrimination between geometric and defect indications. This is further enhanced by a suitable choice of optimal angles for the different areas, which especially is made possible and easy to realize by the application of the phased array technique. (see Fig. 8). This technique allows to cover a large portion of the endangered areas from a limited number of probe positions with a much better discrimination between interfering indications and defect echos.
The basic experimental investigations for this approach have been carried out at BAM during February and March 2000, the first inspection equipment will be established at the beginning of 2001. Fig.11 shows a set of typical interfering indications generated by waves traveling into the brake disk. It is nevertheless possible to recognize cracklike defects even under those difficult conditions as shown in fig.12. Since a fast inspection requires an easy interpretation of the results, we have tried to enhance this part of the job by the presentation of so called time-displacement scans received during one circumferential scanning on a wheel axle as shown in fig.13. For the different angles of incidence the time-displacement patterns (produced by the sequential arrangement of A-scans as they are received during one circumferential scanning movement). This presentation shows the possibility to detect all kinds of defects as they are represented by the set of sawcuts choosen for this problem. A further improvement is expected by the application a reference image from a flawless axle and an automatisized comparison.
The technical data of this inspection are described by an axle as presented in Fig.9 and Fig.11 and containing 9 different sawcuts in typical positions. The required sensitivity is a 1mm sawcut which corresponds to a 6 dB lower amplitude than the one received by a 2mm sawcut. At all 9 positions for a 1mm sawcut the signal to noise ratio is better than 8dB.
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