01/12/2019
Finite Elemente illustration

Computed damage variable demonstrates the locations (red) where the nucleation of the macroscopic cracks is expected during the service.

Source: BAM, Division Experimental and Model Based Mechanical Behaviour of Materials

Performance of hot metallic components subjected to cyclic loadings is of primary concern in the design and reliability analysis of engines, turbo-machines, power plants, etc. One crucial example is the use of gas turbines as jet engines in the aircrafts. The critical components of a gas turbine operate under severe conditions including high temperatures, oxidation and mechanical loadings due to the centrifugal force and vibrations. Consequently, the increased risk of failure attributed to the thermo-mechanical fatigue (TMF) significantly limits the time in service of the jet engine. This is a vital issue for the safety of flights, which therefore requires the development of feasible computational techniques to predict the fatigue lifetimes.

The paper reviews three classes of models and computational approaches for assessment of service lives of hot metallic components. In combination with the conventional finite element software, these techniques can be efficiently involved in engineering practice. The idea behind the so-called incremental lifetime models is that a continuous damage evolution is assumed during each loading cycle. This can be readily incorporated in the finite element framework as an additional ordinary differential equation. The lifetime consumption of an arbitrary complex loading path, including intricate stress states, multiaxial loadings etc., can be therefore easy evaluated. Another class of lifetime modeling relies on certain parameters of stress-strain-hystereses as the inputs for an empirical lifetime rule. Despite the link to the deformation history is missing, these parametric approaches are beneficial in view of ease of use. The performance of these methods is verified by LCF and TMF tests on the austenitic cast iron Ni-Resist D-5S which is used for exhaust gas turbocharger hot parts (see figure).

Finally, the modern computational approaches to accelerate the cyclic simulations based on history variables are discussed. The goal is to approximate the structural response during the fatigue history based on the integration of a very small number of loading cycles, which is performed at selected stages of the degradation process. A benchmark example demonstrated that an unsaturated viscoplastic response and stress redistribution can be accurately approximated while keeping the computational efforts drastically low.

Computational Methods for Lifetime Prediction of Metallic Components under High-Temperature Fatigue
Vitaliy Kindrachuk, Bernard Fedelich, Birgit Rehmer, Frauke Peter
published in Metals, 2019, Volume 9(4), pp. 390
BAM, Division Experimental and Model Based Mechanical Behaviour of Materials