Deep rolling is a frequently used post-treatment process to increase the service life of railway axles. Through the process application, the surface roughness is reduced, the surface is work-hardened and compressive residual stresses are introduced in the near surface region in a controlled manner. These three effects have a positive impact on service life, and in the investigated case of railway axles, the introduced residual stress state constitutes the most significant potential. The compressive residual stresses introduced near the surface, which are responsible for the positive lifetime effect, are inevitably balanced by compensating tensile residual stresses below the surface. Therefore, it is generally challenging to estimate the actual potential of the process application properly. Classical design methods consider only the condition at the surface and are thus not suitable for a surface layer-based analysis. In the present publication, the concept of local fatigue strength is applied for the fatigue strength assessment of deep rolled railway axles. The locally permissible fatigue strength at each surface-layer depth of the railway axle is determined as a function of local properties and compared with the locally occurring load. This allows a proper fatigue design in depth and, at the same time, the estimation of the critical point at which the potential of crack initiation and thus failure occurs. Based on the methodology presented, it is possible to determine the fatigue benefit achieved by deep rolling railway axles considering the influence of various deep rolling parameters, and to further systematically optimise the significant process parameters. An increase in fatigue strength of 4.7 % can be proven by deep rolling the railway axle with reference parameters compared with the not deep rolled state. By applying the proposed optimisation, fatigue strength can be further increased up to 10.4 %.

To optimise the benefits of the deep-rolling process in the service life context of treated components, the process application must be investigated. In addition to the reduction in surface roughness and near-surface material strengthening, compressive residual stresses are introduced, which are primarily responsible for the increase in service life for components, especially in the case of high-strength steel materials. A numerical parameter sensitivity analysis is performed in order to investigate the introduced residual stresses in detail. For this purpose, a validated deep-rolling simulation model is used, which replicates the deep rolling of a railway axle made of the high-strength steel material 34CrNiMo6. The model is based on an elastic-plastic Chaboche material model parameterised on uniaxial tensile and LCF test results and validated with residual stress measurements. Using this model as a basis, the effect of the main process parameters deep-rolling force, feed rate, friction coefficient, number of overruns, tool geometry, and shaft geometry on the resulting residual stress state are investigated. The results reveal that the deep-rolling force has the most significant influence on the introduced residual stress state and should therefore be highlighted. In the case of applying a deep-rolling force of more than 10 kN, maximum compressive residual stresses of around − 1000 MPa are introduced, and a strong saturating behaviour is shown. Maximum compensating tensile residual stresses of + 100 MPa occur below the surface. The main influence of the deep-rolling force is the effective depth achieved, which is determined by the depth of the zero crossing. This varies from 1 mm with an applied force of 2 kN to more than 3.5 mm with 20 kN. Furthermore, the results are analysed to conclude suggestions for the process’s applicability, and a proposal for an optimised deep-rolling treatment is presented. There multiple deep rolling with decreased deep-rolling forces is used to achieve a comparably optimised residual stress state. In summary, with the presented results, a contribution to a deeper understanding of the deep-rolling process can be achieved; the influence of the most important process parameters on the residual stress in-depth profiles is established; an optimisation proposal is presented; and correlations are found. Thus, the base work for further fatigue strength assessments and the optimisation of the deep-rolling process regarding the increase of service is laid.

Cord-rubber materials are used in crucial components of rail vehicles, such as air spring bellows in secondary suspension. A detailed understanding of the material behaviour of these components is thus of the utmost importance at an early stage of development. In general, a minor knowledge of the fatigue behaviour is currently all that is available today, while empirical methods involving designer experiences are in most cases essential requirements for this work. The design can be carried out more efficiently based on a representative cord-rubber composite specimen than on an entire air spring bellow. In this paper, the design of such representative specimens is shown taking different geometries and test conditions into consideration. It is found, that a flat specimen design is suitable for analysing the base material under different loading scenarios. The design and optimisation of the specimen geometry was carried out using finite element analysis, which was validated by means of optical strain measurement. The test procedure for the specimen was designed to provide a sound transferability to experimental testing of the components. A fracture pattern study was carried out using radiography and micro computed tomography. The results show, that the dominant damage mechanism is the separation of the layers from each other, denoted as delamination. In conclusion, the developed specimen is well suited for further investigations of the composite material. Furthermore, it will significantly accelerate the development of new air springs and new layups in particular. Future work will focus on a systematic investigation of the fatigue behaviour of the cord-rubber composite air-spring bellows based on the fatigue data of the representative specimens designed in this work.

Deep rolling is a powerful tool to increase the service life or reduce the weight of railway axles. Three fatigue-resistant increasing effects are achieved in one treatment: lower surface roughness, strain hardening and compressive residual stresses near the surface. In this work, all measurable changes introduced by the deep rolling process are investigated. A partly deep-rolled railway axle made of high strength steel material 34CrNiMo6 is investigated experimentally. Microstructure analyses, hardness-, roughness-, FWHM- and residual stress measurements are performed. By the microstructure analyses a very local grain distortion, in the range < 5 µm, is proven in the deep rolled section. Stable hardness values, but increased strain hardening is detected by means of FWHM and the surface roughness is significantly reduced by the process application. Residual stresses were measured using the XRD and HD methods. Similar surface values are proven, but the determined depth profiles deviate. Residual stress measurements have generally limitations when measuring in depth, but especially their distribution is significant for increasing the durability of steel materials. Therefore, a numerical deep rolling simulation model is additionally built. Based on uniaxial tensile and cyclic test results, examined on specimen machined from the edge layer of the railway axle, an elastic–plastic Chaboche material model is parameterised. The material model is added to the simulation model and so the introduced residual stresses can be simulated. The comparison of the simulated residual stress in-depth profile, considering the electrochemical removal, shows good agreement to the measurement results. The so validated simulation model is able to determine the prevailing residual stress state near the surface after deep rolling the railway axle. Maximum compressive residual stresses up to about -1,000 MPa near the surface are achieved. The change from the induced compressive to the compensating tensile residual stress range occurs at a depth of 3.5 mm and maximum tensile residual stresses of + 100 MPa at a depth of 4 mm are introduced. In summary, the presented experimental and numerical results demonstrate the modifications induced by the deep rolling process application on a railway axle and lay the foundation for a further optimisation of the deep rolling process.

Rail vehicle air spring bellows consisting of cord rubber composites are subject to a wide variety of stresses. It is therefore crucial to understand the fatigue behavior of such components, as the development process involves the use of empirical methods, which are both time‐consuming and costly. This paper outlines a methodology for assessing the fatigue strength of air spring bellows through representative specimen testing. Ten component tests and a validated numerical model of the air spring bellows are presented. The primary loading scenario investigated in this study is pure pulsating tensile stress with a strain ratio R in the range of 0–0.25. This scenario is considered to be highly damaging, leading to delamination. Comprehensive damage analysis of the specimen is performed using microcomputed tomography. To ensure the transferability of the results, a failure criterion is defined as 20% elongation of the specimen, based on the defined 10% internal pressure drop for the bellow. After statistical evaluation, the transferability of the specimen tests to the component is validated using a local fatigue approach. The correlation coefficient R² is 0.84, indicating a sound agreement between model and tests with 7 points out of 10 within a scatter factor of 2.

Cord-rubber composites are subjected to a wide range of loads in various applications. However, their fatigue behavior remains relatively under-researched. To address this gap, a set of representative specimens was developed, and a validated numerical model was employed to assess fatigue-relevant parameters. In this study, we present the results from two series of tests with different strain ratios (R values). One series was subjected to a pure pulsating tensile strain (R ~0), while the second series experienced an increased mean strain with an R ratio between 0.2 and 0.3. A direct comparison of the two series demonstrated that a higher strain ratio results in a longer service life. This is reflected in an increase in the slope (k) from 13 to 23, as well as an increase in the ultimate fiber strain from 8% to 11% at N_d = 50,000 load cycles for a survival probability of 50%. Both series indicate a comparable scatter in the test results. This comparative analysis shows that the strain ratio significantly impacts the fatigue behavior of cord-rubber composite materials based on cyclic tests under different loading conditions. The findings of this study demonstrate the necessity of considering different load situations when evaluating or designing components.