Head Check Measurement – a Fully-operational System on a Rail Grinder

R. Meierhofer1, R. Pohl2

1Speno International, Geneva, Switzerland, 2Federal Institute for Materials Research and Testing, Berlin, Germany

Abstract Rolling contact fatigue of the rail surface can lead to multiple hairline cracks known as head checking or gauge corner cracking. The condition is a potential threat to rail integrity. It constitutes a growing problem for networks [1,2]. Rail grinding offers a remedy, but its deployment is complex. Key to successful on-site treatment of head checking is knowledge of the presence of the defect. Speno International has developed a working system (called HC-Grinding Scanner) that provides – in real-time during rail grinding – information on head check presence with details of position and dimensions. The emphasis of this paper is on the application of the equipment and the specifically developed software. Introduction Head check detection by eddy-current (EC) techniques is a known manual application. Speno International has achieved important progress by installing continuous head check measurement equipment on a grinding machine. The company has adapted standard equipment and developed entirely new data analysis software in line with the specific nature of the grinding activity. The purpose of the system is to steer the grinding process while balancing opposing constraints. The HC cracks must be removed (or at least substantially reduced), while safeguarding the capital of rail metal. Knowledge of the depth and position of rail surface defects permits a controlled elimination of the fatigued material with minimum metal removal. The system supplies documentation giving a complete overview of rail conditions before and after grinding, and permitting constant monitoring of the rectification process. The system described has been employed successfully in real-life conditions on German Railways (DB) tracks since March 2005 and has proven reliable. These long term trials are providing practical information that will enable finalization of the application and its formal acceptance by DB. Convention of expressions used Figure 1 clarifies expressions used in this paper.

Figure 1 : Convention of expressions used

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Figure 2 : Measuring trolley and electrical cabinet with Eddy Current devices

System configuration There are four detection probes over each rail. They are mounted on a guidance device carried by a measuring trolley. This arrangement ensures perfect curve inscription and keeps the probes clear of contact with the rail. The guidance device assures that the probes are maintained within a distance of 2.5mm from the rail surface. The probes possess highly developed electrical properties and are robustly engineered. Excellent signal quality and minimum interference are assured by the close position of the four-channel EC devices to the probes, and special electrical connection to the cabin computer. The EC devices are placed under the frame of the train in a heated and dust-sealed cabinet which has proven absolutely reliable even in hard weather conditions (see figure 2). The HC-Grinding Scanner touch screen, specially designed computer and printing devices are grouped in the cabin close to the main train and grinding controls (see figure 3).

Figure 3 : View in the operation cabin

The four probes are mounted on a modular mechanism which permits individual setting of their measurement ranges so as to cover the affected zone on the rail transverse profile. Each probe has a range of measurement of 6mm which allows optimal coverage. The software compensates distance variations in order to be independent of profile changes due to grinding. The probes have been developed to measure up to 10mm depth of crack [3]. A special calibration device has been constructed on which the measuring trolley and the guidance devices can be put in working position to facilitate mechanical and electrical adjustment of the probes. The present configuration has proven suitable for every head check situation encountered on German Railways since March 2005 (see figure 4).

Application The process-oriented software – conceived to meet the specific requirements of the grinding activity – permits a continuous overall view of the rail surface condition. This offers the possibility to control the grinding process in function of the depth of defect and position on the transverse profile. The strong point of the software is the simultaneous recording, filtering and analysis of the eddy current signals of all eight probes in real time. The strongest signal per side is selected, converted and indicated on the screen. Data concerning the depth of defect, position and count of residual head checking is continuously available. Thus there is no loss of time spent analyzing previously saved data. This aspect is crucial as it avoids retarding the grinding operation due to measuring activity. Options for the monitoring settings allow refinement of the analysis during grinding.

Figure 4 : Example of possible probe configuration and their range of measure

The scan data page represents for the operator the most important part of the software. The configuration is aligned with the train on the track. Both rails are indicated with their own field of results. Between them is the symbolic image of the train with the measurement trolley at its extremity. As the train advances, the train image scrolls over the virtual grinding section and updates the results. Kilometer points can be set which complete the view with information to locate the affected sections of rail. The status of probes and eddy current devices are indicated continuously as well as the direction of train movement (see figure 5). The correlation between measured voltage and depth of crack is defined by an internal calibration curve developed on artificial as well as on real cracking [1]. Being mounted on a rail grinder, the system is designed for self validation, as it is not able to clearly identify cracks with distances between each other smaller then 2mm. When falling short of this threshold, the signal form changes and this can lead to false depth indication as the standard calibration curve is based on single cracks. A statistical approach is being considered to offer a remedy allowing an assessment of crack fields rather than of single cracks. It is based on practical field measurements. The more data acquired (including metal removal details), the more precise the curve becomes. The uncertainty of the angle of penetration, which is still not measurable with non destructive methods, is also being taken into account. For the moment, the system functions with the standard calibration curve for DB manual trolleys and HC inspection trains [2].

Figure 5 : Scan Data page

Left rail

Right rail

Symbolic train

By means of the EC-Probes page, the status can be controlled in more detail. The distance at the rail surface, the measured depth and count of each of the eight probe signals are indicated for every meter run, whereby the depth of defect is given for the crack of greatest depth. The correct functioning of the guidance device as well as the current mechanical configuration can be checked (see figure 6). The positions relative to the mechanical configuration of the probes in the guidance device are chosen in a table of probes. This information is permanently available wherever indication of the position is needed in the software. An integrated software tool guides the operator through the calibration process of newly installed probes and saves automatically the so acquired electrical properties. Information relative to the track section being ground can be entered at any moment during measurement. Saved files can be loaded and overviewed for printing or completion with supplementary information. The user language as well as the documentation language, colors, day or night mode and other settings are defined on the parameter page, which contains all options.

Figure 6: Probe status

Documentation Results can be plotted as diagrams and retained for each grinding pass in the form of A4 color printing. In addition to the grinding results, the documentation comprises all information relative to the section being ground in order to offer the user a complete and easy-to-handle document. It is possible for users to choose various options for printing to ensure consistent documentation (see figure 7). Conclusions and further activities Speno International has developed a easy-to-use system that provides real-time knowledge of head check presence. It is foreseen that the system will be accepted shortly by German Railways. The equipment has proven reliable in daily use. The core part of the software consists of filtering and analyzing eddy current signals in order to present to the machine operator only the information of interest. Filters for identifying head checks from other marks such as welding seams are well established and tested. As the system is able to measure other types of rolling contact fatigue defects it will be necessary to develop corresponding calibration curves and of refining filters. The HC Grinding Scanner completes the existing systems on board grinding trains for measuring the rail transverse and longitudinal profiles. Thus all grinding criteria can be measured and documented before and after grinding – an important step toward controlled metal removal when maintaining rail. Acknowledgements The development success of the above described system is mainly due to the excellent cooperation of the department of Non Destructive Testing of German Railways (Kirchmöser, R. Krull) and the Federal Institute for Materials Research and Testing (Berlin, Germany, R. Pohl, M. Thomas and R. Casperson).

Figure 7 : Print example of initial measure

References [1] R. Krull, M. Thomas, R. Pohl, S. Rühe. “Eddy-current Detection of Head Checks on the Gauge

Corners of Rails: Recent Results”, Conference on Railway Engineering, London (2003) [2] R. Krull, H. Hintze, M. Thomas, T. Heckel. “Nondestructive testing of Rails today and in the Future”,

ZEVrail Glasers Annalen 127, pp. 286-296, (2003) [3] R. Pohl, R. Krull, S. Rühe. “Einsatz der Wirbelstromprüfung zur Detektion von Head Checks an

Fahrkanten von Schienen”, ZfP-Zeitung 81, pp. 38-40, (2002)