Fatigue tests as basis for permissible rail stresses
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Project data
| Titel | Title Experimentelle Ermüdungsuntersuchungen an Schienen als Grundlage zur Ermittlung erhöhter zulässiger Schienenspannungen | Experimental fatigue tests on rails as a basis for determining increased permissible rail stresses Förderer | Funding DB Netz AG Zeitraum | Period 01/2019 – 06/2021 Leiter | Project manager Prof. Dr.-Ing. Steffen Marx Bearbeiter | Contributor Chongjie Kang, M. Sc. |
Report from year book 2020
Permissible rail tensile stresses
Residual stress measurement with sectioning method
In modern railway track systems, the rails are continuously welded. The coupling effects between bridges and rails introduce track/bridge interactions when continuously welded rails are installed on bridges. The longitudinal deformations due to vertical traffic loads, temperature effects, creep and shrinkage result in additional rail stresses, relative displacements between rails and superstructure as well as additional forces in bridge supports.
The current verification value of additional rail tensile stress (92 MPa) due to track/bridge interaction in Guidelines 804 and DIN EN 1991-2 is based on investigations in 1978. This permissible stress criterion has to be re-checked for today’s track systems. With this purpose, investigations of rail structural fatigue resistance and rail residual stress are carried out on un-operated rails of 60E2 profiles, which were purposely corroded; the steel grade is R260.
The rail fatigue resistance is determined by flexural fatigue tests on whole rail specimens, whereby the influences of the rail shape, surface properties, and the manufacturing process (especially for residual stresses) can be directly included. The tests are carried out as four-point-bending tests with a span width of L = 1.90 m and a load distance of 150 mm. The tests are carried out based on the staircase method to determine the technical fatigue resistance up to N = 5 · 106 and N = 5 · 107 load cycles according to DIN EN 14730-1.
Residual stresses are additionally algebraically superimposed with other stresses in the rail. The verification of additional tensile rail stress is based on the rail fatigue resistance in the rail foot according to the current ‘Oberbauberechnung’. The residual stress is taken into account in the Smith-diagram as part of the minimum stress level; it can be determined using the following two methods: 1. the sectioning method and 2. the X-ray diffraction method
Based on the comparisons of historical results and new test results, it can be concluded that the permissible additional rail tensile stress in the guideline and codes can be assessed as conservative.