1. Introduction

The replacement of classical NDT inspection tools for rails, wheels and axles by modernized concepts of method and equipment is a challenging task due to the fact that most of the NDT problems at railway material have found during the last half century traditional solutions, which today are often regarded as undoubted standards. It is therefore somewhat difficult to introduce the possibilities of modern techniques in an atmosphere largely characterized by a certain conservativism. But the remarquable increase of damages due to modified operating conditions, e.g. highspeed trains and high density trafic e.g. in suburban areas gave to certain damaging phenomena on rails, wheels and axles a stronger relevance than considered e.g. fifteen years before.

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


The second concept is a new inspection approach with ultrasound for the disk area of railway wheels via ultrasonic probes coupled at the wheel rim.

2. Ultrasonic inspection of the disk at railway wheels

Fig 2: Ultrasonic Inspection of the Wheel Disk with a Multiprobe Arrangement Fig 3: Inspection Concept V-Transfer with Shear Waves

The ultrasonic inspection of the wheel disk is carried out with a multiprobe arrangement. In order to avoid to strong interferences by wave mode conversions one is forced to strictly rely on shearwaves with a polarisation more or less parallel to the disk plane (fig.2).
The probe positions at the wheel rim must be choosen very carefully in order to achieve a sufficient detectability of cracks within the different zones of the wheel disk area by V-transfer probe positions for more or less circumferentially oriented cracklike defects. and by probe positions with a direct reflection from more or less radially oriented cracklike defects like shown in fig.3. The exitation of shearwaves at the rim is possible by EMATS and by probes coupled with a liquid. Probes coupled with a liquid are restricting the exitation of shearwaves to inclined incidence angles between approxmatively 33 ° and 75° . But for the disk inspection angles between 25 and 55 degree had to be choosen. Of course it is not possible to exite shearwaves with high intensities for angles smaller than 35° of incidence in steel, but since in the arrangement of Fig.3 a V-transfer is used, even smaller shearwave energies are sufficient to guarantee the detection of the corresponding circumferential cracks. During the experiments the cracks had been represented by spark eroded sawcuts with 4mm depth and 20mm length. The exitation of the required shearwaves with a polarisation parallel to the disc plane with the help of EMATS is basicly possible but requires a well defined not damaged rim surface. Since this is not the case for the in situ inspection we have decided to choose the liquid coupling situation with the consequence of a V-transfer or quasi tandem inspection technique as shown in fig.3.

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

This is shown in fig.4 where the restrictions are indicated. The accessability areas are marked by the arrows. Fig.5 shows a set of probes adapted to the detection of the different defects which had to be detected.

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.

3. Ultrasonic inspection of railway axles with phased array probes

An example of a new ultrasonic inspection technique for wheel axles is demonstrated in Fig. 8. An ultrasonic phased array probe coupled in the space between the wheel and one of the brake disks emits ultrasonic shearwaves under different angles. This enables the scanning of a large portion of the surface area endangered by fatigue crack generation.

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

Fig.9 shows the directivity patterns which can be produced by the probe. That probe, especially tuned for this purpose, is able to exite angles between 33° up to 72°. The amplitude of grating lobes for the different incidence angles is minimized by a suitable choice of the element sizes and a slight focusing superimposed on the repartition of the delay times. Fig. 10 shows the specific difficulty which an operator encounters during the traditional manual scanning of the axles. The observation of an A-scan on the screen of a standard ultrasonic pulse-echo-equipment has to distinguish between real defect indications and indications due to the axle geometry and due to echos produced from the brake discs or wheels by wave portions travelling through the interface between the axle and the wheel or brake disc.

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.

4. Conclusion

An increasing demand for automatic nondestructive testing inspections at railway materials have promoted the development of different ultrasonic inspection techniques. The described inspection techniques for an in situ test of wheels and an off -line test of axles demonstrates the advantages of the use of modern multiprobe arrangements and phased array probes for railway materials. The application of automatic inspection techniques will reduce the inspection time needed for the wheel disk or an axle to less than 10 minutes. The documentation and evaluation of the results can be based on the video presentation of time-displacement scans or ultrasonic echotomograms which can also be stored for archiving purposes. The detectability depends on the possibilities of the inspection techniques and the needs of the specific inservice inspection on railway materials.

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