·Table of Contents
·Terrestrial and Marine Transportation
Pioneering Inspection of Railroad Rails with Eddy Currents
H. - M. Thomas, BAM Berlin (Federal Bureau of Material Testing)
M. Junger, ROHMANN GmbH Frankenthal
H. Hintze, R. Krull, Deutsche Bahn AG, Kirchmöser
S. Rühe, Prüftechnik Linke & Rühe, Magdeburg
Using a rail-inspection train, the Deutsche Bahn's system of railroads is continuously checked for traffic-induced defects. Until now, only ultrasonic inspection was used. In addition, eddy-current inspection could successfully and efficiently be used to detect and evaluate certain surface cracks - so-called head checks. There have already been reports about laboratory tests which proved the general suitability of this test method [1,2]. The next step - described here - consisted of practical trials under real-life conditions. A test system which is able to fulfil the strict requirements (test speeds of up to 100 km/h, complex probe guidance, suppression of electrical and mechanical interference, recording of data, filtering and evaluation of data) was developed. As part of the rail-inspection train and as a manual system, this inspection system was successfully tested on the Deutsche Bahn's system of railroads under real-life conditions.
2. Problem Definition
Head checks are scaly cracks on the surface which mainly occur in curves along the guiding surface of the outer rails (see figure 1). Their distance is generally 2...7 mm. They grow at different angles into the inside of the material. As previously reported, laboratory tests showed that it is possible to use eddy currents to make a statement about the depth of a crack. It was now to be proven that such a statement could also be made under real-life conditions. The tests mainly focussed on:
- Laboratory tests regarding the possible speed of the inspection
- Design of a suitable system to guide the probes
- Supplying suitable software and hardware tools to store and evaluate the measured data
- Installation and trials of the system in the rail-inspection train of the DB AG
- Supplying a manual inspection system 
Fig 1: Top: Sketch of Head Checks in a Rail Section
Right: Cross Section of a Head Check
The test system should enable the user to monitor the development of a defect, optimally plan the repair of defects and check on their success. Thus, a modern quality-management of the system of rails becomes possible and valuable pieces of information regarding the future design of track systems are gained.
3. Laboratory Tests regarding High-Speed Inspection
Figure 2 depicts the schematic diagram of the inspection system that was used. A commercially available 2-channel eddy-current instrument was used. The probes were custom-designed for this application. They have a very small length of coverage, but offer an unusually deep penetration of the material. In addition, the lift-off effect of these probes is very small. To reliably evaluate the measured signals, at least five measured values must be saved within the length of coverage of the probe. Based on a length of coverage of 3 mm and a test speed of 28 m/s (100 km/h) the scan rate is approximately 50 KS/s. If 6 channels are used (2 EC-probes featuring 2 channels each; 1 channel for the local trigger and 1 channel for additional information) at 12 bit each, 1 MB of data are generated per second respectively 3.6 GB per hour. An A/D-board with 8 differential channels and 1.25 MS/sec and a Pentium III computer (550 MHz, 256 MB onboard, 18 GB SCSI hard drive) were used. A CD-RW-burner and a HP-DAT-streamer were to be used for data storage and the subsequent evaluation of the data. This configuration ensured that the data were recorded by the computer.
Fig 2: Schematic Diagram of the Test System
To find specific indications again, a GPS (global positioning system) was used in addition to the path data supplied by the train. The coordinates are saved as part of a fixed interrelationship with the measured data. This is a very useful quality-management measure which makes it possible to easily allocate the positions later on.
To simulate the high speeds of the rail-inspection train, a rotary test standard as depicted in figure 3 was manufactured. When rotated at 1,800 rpm, the standard which has a diameter of 230 mm reaches a peripheral speed of approx. 22 m/sec (78 km/h); this is about the same as the nominal test speed of the rail-inspection train. A turning lathe was used to rotate the part. The maximum speed of 3,000 rpm made it possible to simulate a test speed of 36 m/sec (130 km/h).
Fig 3: Rotary Test Standard featuring EDM-Notches
Fig 4: Depth Inspection Pattern top: 0.07 km/h; bottom: 78 km/h test speed
The rounded circumference of this test standard represents a model of the gauge side of the rails. Two crack patterns were electro-eroded into the circumference. Such crack pattern were used earlier for laboratory tests; they were, however, used in plates which were scanned with linear units. Illustration 4 and 5 depict the consistency of the signal course of the various samples. Except for an amplitudinal factor the amplitudinal proportions and the signal course are almost identical. During the rotary test, the height of the signal remained independent of the speed up to a speed of 78 km/h. At a speed of 100 km/h the signal was reduced by approximately 3 dB. This was caused by the eddy-currents system's low-pass filter. If tests will regularly be carried out at such a speed later on, the dimensions of the low pass should be modified.
Fig 5: Resolution Test Pattern top: 0.07 km/h; bottom: 78 km/h test speed
4. Probe Guidance with the Rail-Inspection Train
It is very important to guide the probe accurately relative to the test surface. So the signals are not influenced and the sensitivity does not fluctuate because of the lift-off effect. The test situation is especially complex, since it must be ensured that the probe is positioned at an angle relative to the guiding surface. In addition, there are the high speed and the difficult surface conditions that are due to the varying degrees of wear and tear. Two probe carriers were tried. One was a modified sliding block which is usually used to guide the ultrasonic probes. It was equipped with two probes (guiding surface and center position) (please refer to figure 6). The second carrier was a device which guided the probes across the guiding surface at a distance of 0.5 mm; here the probes were located between two rollers.
Fig 6: Probe Carrier with two Eddy-Current Probes on the Rail-Inspection Train
Fig 8: Using the Eddy-Current Test System with an Inspection Trolley to Verify the Indications of Head Checks
Fig 7: Indications of Head Checks on a Ground Rail. The excerpt of the measured data shown here represents 1,596 mm.
5.Results of the Rail-Inspection
- Junger, M. (Frankenthal); Thomas H. - M. (Berlin); Krull, R. (Kirchmöser):
Das Wirbelstrom-Verfahren auf die Schiene gebracht: Prüfung betriebsbedingter Schädigungen an Eisenbahnschienen mit Wirbelstrom-Verfahren
(Eddy-Current Technology on Rails: Inspection of Traffic-Induced Defects on Railroad Rails with Eddy-Current Technology)
Annual DGZfP Convention Celle/Germany (May 10 to 12, 1999)
- Junger, M.; Thomas, H. - M. and Krull, R.:
Wirbelstromprüfung betriebsbedingter Schädigungen an Eisenbahnschienen
(Eddy-Current Inspection of Traffic-Induced Defects on Railroad Rails)
"Stahl und Eisen" 119 (1999), issue 12, pages 107 - 110
Verlag Stahleisen GmbH, Düsseldorf/Germany
- Rühe, S.; von Hornhardt, A. and Thomas, H. - M.:
Wirbelstrom-Handprüfsystem für Eisenbahnschienen
(Eddy-Current Based Manual Inspection System for Railroad Rails)
DGZfP/DACH Convention Innsbruck/Austria (May 29 to 31, 2000)