8. Evaluation of Fatigue Strength of High-Speed Vehicle Body Members

      Fatigue damage to steel Shinkansen body structures is caused by the action of inside/outside pressure differences when the airtight structures pass each other in tunnels at high speeds. Modern high-speed vehicles have an aluminum alloy body structure to reduce weight, but evaluation criteria for fatigue strength, including welded joints, are yet to be clearly defined.
     The Railway Technical Research Institute (RTRI) has created a fatigue design diagram highlighting the frequency of stress occurrence by performing a fatigue test using a test model simulation (Fig. 1). The simulation selected a number of typical welded joints where heavy load is assumed in commercial vehicles running at 270 km/h as follows: the weld zones of hollow extruded shapes, the reinforced weld zone between the roof carline and cant rail, and the weld zone between the side pole and side beam. The range and frequency of stress occurrence during the period of commercial service were also estimated by examining the results of two types of test: body structure air-tight load tests (to find the relationship between inside and outside pressure differences) and generated stress and actual running tests (to find the relationship between inside and outside pressure differences and their occurrence). The life of vehicle was then evaluated based on the degree of cumulative damage determined by the newly developed fatigue design diagram. The evaluation results showed that the fatigue level poses no problem for practical use under current usage conditions. The degree of fatigue damage is expected to extend close to the allowable limit if commercial speed is further increased; this fatigue design diagram can be used to evaluate such a case. Such diagrams will also be created for other joints to define evaluation criteria for body structures.

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