Today, specific material data, mainly from static material tests, are not sufficient to design parts of a product to assess lifetime behaviour, especially when local complex loading occurs like low cycle fatigue (LCF) and thermo mechanical fatigue (TMF). In this work, specimen tests are carried out under LCF and TMF loading with different materials, where mechanical and thermal properties are considered. The investigated materials are an aluminium alloy, cast iron, copper alloy and nickel. The TMF tests are performed by using a TMF test rig, designed by the Chair of Mechanical Engineering (AMB). The results of these tests allow a description of the material behaviour depending on the mechanical and thermal boundary conditions. Commercial Software for Finite Element Analysis (FEA) enables a fit with the test results to calculate stress and strain values for different loading conditions. These results are the input data for the final lifetime calculation. A number of approaches are given to determine the lifetime for the investigated materials. Finally, a special modified energy method -"unified energy approach" -is presented and compared with conventional approaches.Key

Cyclic loading of metallic engineering components at constant elevated or fluctuating temperature causes a complex evolution of damage which can hardly be described. At the same time the thermal expansion is restricted in some regions, due to the complex geometry of the components. Therefore, mechanical stresses occur and cyclic plastic deformation leads to thermo-mechanical fatigue (TMF) of the material. A careful analysis and comparison of the experimental results, based on a systematical variation of the relevant influence factors, allow to develop empirical models for computing the fatigue life of thermo-mechanical loaded components made of Ni alloys (IN718, Ni200/201). Different lifetime calculation methods were executed and analyzed to fulfil the requirements for the fatigue analysis of components made of these alloys. In addition, the model parameters can be optimized using these results. Based on the parameters, which define the usability of the investigated lifetime models, e.g. temperature, stress, plastic strain, the limiting combination of the parameters can be described. This graphical representation of the “border line” or limiting combination can also be used for any other materials. The results show that the main lifetime calculation models can only applied in a limited range. This limited range is mainly defined by temperature, strain constraining and their combination.

KurzfassungZyklische Beanspruchung bei konstant steigender oder variabler Temperatur führt bei metallischen Bauteilen zu einer komplexen Schädigungsentwicklung, die nur schwer mit einfachen Mitteln zu beschreiben ist. Während des Betriebes tritt in vielen Bauteilen thermomechanische Ermüdung (TMF) auf, z.B. in Kokillen in der Stahlproduktion, Turbinenscheiben, Zylinderköpfen. Daher ist es wichtig, die thermomechanischen Ermüdungseigenschaften dieser Materialien zu untersuchen und zu charakterisieren. TMF-Versuchsserien wurden durchgeführt, um eine Basis für Schädigungsparameter und energiebasierende Ansätze zu finden. In diesem Beitrag werden die TMF-Ergebnisse von galvanisch beschichtetem Nickel, kaltgewalztem wärmebehandeltem Nickel (Ni200/201) und einer geschmiedeten Nickelbasislegierung für Turbinenscheiben (IN718) dargestellt und verglichen.

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