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2010
Conference Paper
Titel
Contribution of ultrasonic rail stress analysis to track maintenance
Abstract
The continuously welded rail track has some advantages and is a request for the traffic with elevated and high speed. Since there is a significant limitation of the thermal expansion of the rails, the stress state of the rails is of concern. A major condition concerns the temperature of the rails being welded. The so called rail neutral temperature is regarded as a quality measure for a quasi stress free state of the track. In case the temperature regulation during welding has been fulfilled, the thermal expansion of the track causes stresses within acceptable limits and are not causing a buckling or cracking problem. In the other case, the zero stress line is shifted to higher tensile or to higher compressive values and the resulting stress state may cause a problem. Unfortunately, there is no reliable and applicable technique which enables the measurement of the rail neutral temperature. Furthermore, there is also no technique which reliably measures the thermally induced longitudinal stress in a rail. In all cases, the total stress state of the rail in the track influences the measuring quantities. The total stress state of a rail in the track is composed by the stress caused during the manufacturing process of the indivi-dual rail, by the stress introduced during the bedding of the track and finally by the stress caused by the traffic. The thermally induces stress comes additionally to all that. Mainly caused by the roller straightening process, there is a tensile stress state in the head and in the foot area of the new rail which is compensated by compressive stresses in the web. The stress component along the length is significantly larger in value than the other two stress components. Hence, the stress state can be regarded as one dimensional with the dominant component along the length. Different non destructive approaches are developed and results are achieved which are in satisfying agreements with the results of destructive techniques. But in case of the rails of the track the situation is different. There is a three dimensional stress state in the head of used rails. There are compressive stresses along the length, along the width as well as along the height in the upper part of the rail head until about 12 till 20 mm of distance from the running surface. And it has been found, that the stress state in the web and foot is not significantly changed by the influence of the traffic. The non-destructive techniques, successfully applied on new rails, need to be adapted to the more complex stress situation in used rails. A new concept of the evaluation and the analysis of the stress state of rails in the track has to be developed, based on those results which can reliably achieved by the application of non-destructive techniques. One possibility is using the achievements of the ultrasonic techniques and is taking advantage of the magnetic techniques to characterize the state of structure as e.g. the local changes of structure and plastic deformation in order to enable a better correction of the structure influence on the ultrasonic data for stress analysis. Even in well conditioned beds, substructures as culverts and underpasses have to be regarded with particular attention. On the one hand side, the homogeneity of the roadbed is interrupted and longitudinal (along rail length) gradients of the stress state of the rails have to be taken into account. It is obvious and confirmed by experiences that the rails in the entrance and exit areas of tunnels and bridges as well as the rails in the vicinity of switches face extreme conditions and stress loads. With regard to the track maintenance strategy the models describing the roadbed and soil conditions on the one hand and the traffic and thermally induced stress conditions on the other have to be modified by terms taking the total stress states and their local changes into account. Movable testing systems and also permanently installed sensors to enable periodical and continuous measurements have to be adapted to deliver the needed information.
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