PSI - Issue 46

J. Bialowas et al. / Procedia Structural Integrity 46 (2023) 49–55 J. Bialowas et al. / Structural Integrity Procedia 00 (2021) 000–000

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(2010), crankshafts by Choi and Pan (2009) and wheelset axles by Zerbst et al. (2013). The latter will be used as an example of application in this article. Wheelset axles are among the most safety-critical components of the rolling stock. Therefore, manufacturers and operators seek for possibilities to increase the reliability and duration of inspection intervals of these components. A promising approach to extend inspection intervals is to use mechanical surface treatments like the deep rolling process, see Nalla et al. (2003), Regazzi et al. (2020). Fig. 1 shows a schematic of the deep rolling process for cylindrical geometries, e.g. wheelset axles. Thereby, a machine, similar to a lathe, presses a work roller with a distinct force onto the surface of an axle. The axle rotates around its axis while the work roller moves in axial direction creating a spiral processing path on the surface of the axle. The distance in axial direction of the tool covered in one turn of the axle is called feed, depicted with the variable f in Fig. 1. The contact pressure introduces plastic strains in the surface and near-surface area of the component. This plastic deformation remains and induces residual stresses. The remaining residual stresses in axial direction vary in depth starting with compressive stresses in the vicinity of the surface, change to tensile at a certain depth where they reach a maximum, and finally fall towards zero with further increasing depth (cf. Fig. 3 (a) for a residual stress profile of a deep rolled wheelset axle). Regazzi et al. (2017) describes the stress in axial direction (σ zz ) as the key stress component of wheelset axles because the predominant load of axles is rotating bending. Despite the fact that this process is already frequently employed, the knowledge about actual advantages of this manufacturing step and detailed information on the residual stress state are limited. Maierhofer et al. (2014) suggested an analytical approach towards optimization of the process, where a process model was used for the calculation of residual stresses based on the Hertzian theory. This model gives a rough estimate of the depth of resulting compressive stresses. The sole information about the magnitude of stress on the surface and the penetration depth are not always sufficient for further calculations. The process parameters as well as the geometry of the components and used tools have a significant influence on the strain and stress fields. For this reason, a method is needed that provides additional insight into the strain and stress distribution. Non-destructive experimental approaches as X-ray diffraction can provide meaningful results for residual stresses for the surface and near surface area but deteriorate in accuracy with measuring depth. Destructive testing as for example the hole drilling method exhibit similar problems, especially when the plastic deformations are close to the yield point, as pointed out by Schajer and Whitehead (2018). Although the measuring depth is higher (up to 2 mm), it is not sufficient for the measurement of residual stresses of deep rolled axles. In order to find the ideal settings of the deep rolling parameters computational methods can overcome some of the difficulties of experimental methods. Multiple authors already performed simulations of the deep rolling process with the finite element method. There are three central features of such simulations: (i) the material model, (ii) the choice of the simplified model geometry and the corresponding boundary conditions, and (iii) the evaluated region of interest: (i) The material model must account for the cyclic nature of this process. The work roller not only rolls over the surface in tangential direction but also in axial direction, rolling over a distinct point on the surface multiple times.

Fig. 1 Simplified representation of the deep rolling process with the process-determining parameters: load ( L ), feed ( f ), edge radius of the work roller ( R WR ), work roller diameter ( D WR ) and axle diameter ( D axle )