Influence of the cyclic thermal and mechanical loading history on crack closure, fatigue crack growth and damage interaction of cast nickel-base alloys

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Components of gas turbines are exposed to high thermal and mechanical loads resulting from start-up and shut-down cycles. Thus, thermomechanical fatigue (TMF) of the used nickel-base alloys is a major criterion for the assessment of the strength of the components. Due to thermal transients and structural mechanics, varying loading histories are present in critical areas of the components during service, where the phase angle between thermal and mechanical load varies locally (ouf-of-phase, in-phase, phase-shift). Meaningful fatigue life predictions can, however, only be obtained if the lifetime limiting mechanisms of crack growth are understood. In this regard, crack closure and the interaction of different damage mechanisms have a significant, but today not quantifiable influence, since the material properties are temperature-dependent, the material behaves time-dependent and, hence, the phase angle plays an important role. In this project, on the one hand, crack growth, crack closure and the damage interaction are investigated experimentally. On the other hand, numerical investigations with the finite-element method are used to identify correlations between crack closure, crack growth and material properties. On the basis of the experimental and numerical results, a mechanism-based model is developed which can reproduce the influence of the temperature-dependent mechanical properties in combination with the phase angle and the time-dependency on the TMF crack growth of the polycrystalline cast nickel-base alloy IN100.