Univ. Prof. Dr. Ing.

Bergs, Thomas

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  • Publication
    Numerical Modeling of the Redistribution of Residual Stresses in Deep Rolled Cross Bores in Shafts from GJS700-2
    ( 2024)
    Uhlmann, Lars
    ;
    Reissner, Felix-Christian
    ;
    Rathnakar, Shashaank Nambla
    ;
    Herrig, Tim
    ;
    Baumgartner, Jörg orcid-logo
    ;
    Bergs, Thomas
    Lightweight design efforts are generally limited by highly stressed areas. In the case of shafts with cross bore the cross bore forms a notch. Due to geometry and position, those notches lead under cyclic torsional loading to stress peaks in the component, which appear as highly stressed areas. In order to counteract tensile stress peaks, compressive residual stresses may be induced into the surface layer by means of surface treatments such as deep rolling. The induction of compressive residual stresses may delay crack initiation and growth. When deep rolled components are subsequently subjected to cyclic loading, the induced residual stresses are redistributed until a stable residual stress state is established, which is decisive in the assessment of the fatigue strength. The influence of deep rolling on the surface properties of cross bores in shafts made of GJS700 and the redistribution behavior of the induced residual stresses under subsequent cyclic torsional loading is mostly unknown. The objective of this work was therefore to identify the cause-effect relationships between the deep rolling parameters (pressure, overlap) as well as the cyclic loading and the resulting surface properties. Therefore, experimental investigations of deep rolling and of the subsequent cyclic torsional loading were carried out. Subsequently, the process sequence was modeled numerically consisting of a deep rolling and a torsion model. The experimental tests were used to validate the models. Finally, the cause-effect relationships between the deep rolling parameters on the residual stresses and the redistribution due to cyclic torsional loading were investigated.
  • Publication
    Improved lifetime estimation of shot-peened shaft bores using a numerical approach
    ( 2024)
    Reissner, Felix-Christian
    ;
    Uhlmann, Lars
    ;
    Baumgartner, Jörg orcid-logo
    ;
    Herrig, Tim
    ;
    Bergs, Thomas
    Shot peening is commonly used to improve the fatigue strength of mechanical components. The peening process involves the use of high-energy mechanical impacts to create compressive residual stresses and a material hardening on the surface of the component, which can significantly increase its resistance to fatigue. Accurate lifetime prediction is important for optimizing the design of shot-peened components and ensuring their reliability and safety. Due to nonlinear material behavior and the simulation of contact, estimating the lifetime of shot-peened shaft bores under cyclic loading conditions remains a challenge. In this study, the lifetime estimation of shot-peened shaft bores using a combination of experimental testing and finite element analysis is investigated. A series of experiments was conducted on shot-peened shaft bores made of EN-GJS-700 and 34CrNiMo6, using different peening parameters such as intensity and coverage. The specimens with shot-peened shaft bores were subjected to cyclic loading in a fatigue testing machine and the lifetime was experimentally identified. The results were used to develop a methodology based on finite element analysis, which considers the effect of the shot peening parameters on the residual stress distribution, hardening and the resulting fatigue life. The results of the proposed methodology were validated against the experimental data and showed good agreement with the experimental results. The derived methodology can be used to estimate the lifetime of shot-peened shaft bores under different loading and peening conditions and provides a useful tool for optimizing the design of shot-peened components in fatigue engineering applications.