EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Current and future developments in lightweight design of cyclically loaded components require the use of high-strength materials. Additionally, longer and longer operating times are demanded, hence a safe service life is of outstanding importance. It is widely known that certain alloys can still fail after exceeding 1e7 cycles and a change in the failure mechanism from surface induced to inclusion induced crack initiation may occur. Investigations in the very high cycle fatigue (VHCF) regime are therefore essential for a reliable design and dimensioning of structural components. Since fatigue tests up to 1e9 cycles are not economically feasible with conventional servohydraulic or electromechanical systems, there is a need for innovative testing techniques.

Two VHCF testing techniques developed at the Chair of Mechanical Engineering at the Montanuniversitaet Leoben are presented. The first technique allows the testing of cylindrical bar specimens under tension-compression stress. Due to the high test frequency of approx. 1000 Hz, 1e9 cycles can be tested within 12 days. The second high-frequency fatigue testing methodology enables the testing of thin-walled, component-like, disc-shaped structures. Experiments with component-like structures are important for the validation of lifetime assessment models. They contribute to the improvement of assessment concepts based on conventional specimens.

The focus of the presented investigations is on the characterisation of the fatigue strength of a high-strength precipitation hardening stainless steel (X5CrNiCuNb16-4). Woehler tests up to 1e9 cycles reveal the material behaviour in the VHCF regime. Fractures over 1e7 cycles are observed and the fracture surfaces are examined in detail. The fatigue strength of the cylindrical bar specimens is compared with that of the component-like structures. The developed methodologies extend the spectrum of fatigue testing techniques and new insights into the assessment of real components at very high cycles can be obtained.