Investigations on different fatigue design concepts using the example of a welded crossbeam connection from the underframe of a steel railcar body

For thin-walled steel structures, welded joints often turn out to be the weak spots under cyclic loading. For the dimensioning of welded safety components in vehicle construction, strength concepts are necessary that are well-defined and well-reviewed, in terms of their predictive accuracy. Within the scope of the research results presented here, the applicabilities of the nominal, structural and notch stress approaches have been examined on the basis of different arc welded and cyclic loaded steel structures, taken from the railway sector. In detail, this is a crossbeam connection from the underframe of a railcar body. Different sheet thicknesses in the range 2.7 mm–4 mm in combination with a misalignment lead to an increase of load in the region of the weld seams. Several different components and specimens with critical regions of failure have been tested under cyclic loading with constant amplitude. With the help of strain gauges, the (technical) crack initiation has been determined. A short review of the possibility of obtaining information about the crack initiation by ultrasonic-burst-phase-thermography is also given. The specimens were the basis for the application and evaluation of the different concepts for the assessment of fatigue life. The numerical determination of the nominal, structural and notch stresses has been performed with finite-element models. For a local approach, according to the notch stress analysis, a submodelling technique has been used. The FE-models have been compared with the experimental data by means of optical 3D deformation analysis and strain measurements with strain gauges. Existing weld seam and specimen tolerances have been included through the use of parametric models. Finally the experimental and computational results have allowed the derivation of structural and notch S– N curves for the crack initiation and the rupture of the specimen.