PSI - Issue 57

Malik Spahic et al. / Procedia Structural Integrity 57 (2024) 833–847 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Figure 1: During start-up, the rotor surface is at a higher temperature than the rotor center causing thermal stress

This cyclic change from compressive stress to tensile stress and vice versa, creates stress-strain cycles causing fatigue damage. At higher temperatures, creep comes into play as well. Creep will cause a stress relaxation of the thermal stresses, not only during the start-up, but also in nominal condition when residual stress is present from the start-up due to local yielding at stress concentrations. An example of a complete cycle at the first stage groove of a steam turbine rotor including creep stress relaxation is shown in Figure 2. It starts off with a compressive stress which turns into a residual tensile stress in nominal operation. Creep stress relaxation then causes it to reduce, at shutdown a tensile stress is again introduced until a new cycle begins at the next start.

Figure 2: Stabilized stress-strain cycle at the HP groove including creep stress relaxation

In different damage assessment procedures (R5 [2], ASME [3]), creep relaxation is accounted for by enhancing the fatigue strain range. On top of that, a ductility exhaustion or time fraction based damage assessment is included to account for the creep damage. Many of these assessments are based on linear-elastic stress simulations, although non linear material models have been developed as well, enabling to combine both plasticity and creep models [4]. Some use methods based on neural networks to decrease the computational time [5,6]. The damage assessment can be based on isothermal LCF testing or service-like testing in which both temperature and strain is controlled throughout the cycle. An example of tests on 1%Cr material performed by ENGIE Laborelec is shown on Figure 3.

Figure 3: Stress-strain cycle of TMF service-like tests