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Probabilistic life-time modelling at high temperature fatigue

Probabilistic fatigue life calculation of Ni-base alloys under high-temperature fatigue loadings.

Typical inherent scatter in fatigue life of a nickel base superalloyTypical inherent scatter in fatigue life of a nickel base superalloy

Further development and improvement of high efficient gas turbine and combined cycle power stations is - using traditional test planning and data evaluation methods - very time-consuming and cost-intensive.

Consequently, mathematical modeling as well as numerical simulation gain in importance, as they offer a quick approach to reliable component design.
Design of metallic high-temperature components using probabilistic methods is up to now a rarely used procedure, even though the relevant phenomena are known since decades. Using innovative probabilistic methods, insecurities in experimentally determined material properties arising from a limited number of tests can be minimized and clearly separated from inherent scatter of experimental caused by microstructure.
Ongoing research in close collaboration of FZ Jülich, Siemens Energy and ICS (Institute of Computational Science) Lugano addresses the development of a probabilistic model for Low-Cycle-Fatigue (LCF) life prediction including the influence of superimposed creep induced damage of a cast Ni-base alloy.

At FZ Jülich, IEK-2 microstructure analyses at turbine blades and material specimens as well as an extensive set of high-temperature fatigue tests to determine the LCF lifetime behavior are performed. LCF - tests with variation of specimen geometry are performed to investigate the influence of surface-volume ratio on LCF life as an experimental baseline for validation of the probabilistic life prediction model developed by Siemens and ICS. LCF tests with previous creep loading will be performed to experimentally determine the influence of creep induced volume damage on LCF life. Furthermore, inherent scatter of LCF behavior caused by the influence of grain size and grain orientation relative to the load direction on crack initiation and propagation will be investigated by optical and scanning electron microscopy.

Typical stress-strain hysteresis during a LCF- testTypical stress-strain hysteresis during a LCF- test