PSI - Issue 5

Maria Paarmann et al. / Procedia Structural Integrity 5 (2017) 869–874 M. Paarmann et al./ Structural Integrity Procedia 00 (2017) 000 – 000

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3. Numerical crack growth simulations on power plant components

On the basis of the experiments, power plant components were simulated numerically. Therefore, uncracked models were built in ABAQUS to import them in FRANC3D. In this program quasi-static crack growth simulations were executed starting at a user defined initial crack front. The examined components are a ball-shaped part, a turbine bypass valve and a boiler circulation pump (Fig. 2). Within the scope of this paper, the results for the ball-shaped part are exemplarily presented. There are two relevant positions for crack growth initiation (Fig. 2a). Both were identified from XFEM-simulations on the one hand under constant inner pressure and on the other hand under transient thermal loading (Schulz et al. , 2014). Crack initiation position F results from the inner pressure, while position E follows from transient thermal loading. The simulated scenarios (sc.) contain an emergency shutdown (from 545°C to 50°C) within three minutes as a worst case situation (scenario 1.1). Moreover, the influence of the temperature gradient was investigated by decreasing the temperature from 545°C to 50°C within thirty minutes (scenario 1.2) and decreasing the temperature from 545°C to 300°C also within three minutes (scenario 1.3). The inner pressure of 26.6 MPa was simulated as steady state scenario 2. Superimposed thermal and mechanical loading was also investigated (see (Paarmann and Sander, 2016)).

Initiation position F

a)

b)

c)

Initiation position E

Fig. 2. (a) Model and crack initiation positions in the ball-shaped part; (b) model of the simulated turbine bypass valve; (c) modelled sector of the boiler circulation pump

All simulations started with a semi-circular crack, which had an initial crack depth a 0 = 5 mm. The cyclic stress intensity factors (SIF) were calculated for all scenarios with a stress ratio of R = 0. The investigations on both crack initiation positions lead to similar results. In Fig. 3 the results of pure thermal loadings normalized on scenario 2 are shown for crack initiation position F. It can be recognized that pure mechanical loading (scenario 2) leads to significant smaller stress intensity than pure thermal loading within a short shutdown duration (scenario 1.1 and 1.3). The slight differences between scenario 2 and 1.2 also underline the large influence of the temperature gradient on crack growth. While the SIF of scenario 2 (see (Paarmann and Sander, 2016)) rises, the SIF of thermal influenced scenarios decreases over the progressing crack growth. It originates from a drop of the stresses over the wall thickness due to a decreasing temperature. A comparison of scenario 2 and 1.2 also shows that beginning at a crack depth of about 27 mm the temperature leads at the deepest point of the crack front to smaller SIF than under pure mechanical loading.