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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1. Introduction

In future, renewable energies shall substitute fossil power stations. This will lead to a much more flexible operation of conventional power plants, which in turn leads to enhanced cyclic stresses. Therefore, fatigue and fatigue crack growth become much more, and the influence of creep fatigue becomes less important. These new load cases were not considered when the power plants were designed. To minimize risks, the influence of these loads on the power plant components has to be evaluated. For investigating fatigue crack growth the knowledge of the load in the component and the strength of the material are very important. Thus, crack growth experiments were carried out to get a reliable database to describe the fatigue crack growth at elevated temperatures. This is important, because the power plant components operate at elevated temperature and the temperature has an enormous influence on the crack growth rate, which is shown e.g by Petit, Henaff and Sarrazin Baudoux (1999), Chen, Kawagoishi and Nisitani (2000) and UEMATSU et al. (2008). The load in the power plant components is determined with numerical linear-elastic simulations. The simulations are carried out exemplarily at three relevant components. For one component the results are shown in detail. The numeric simulations deliver e.g. the stress intensity factor (SIF) solution for diverse crack positions and load cases. Finally as a result, residual lifetimes can be predicted starting from an initial crack for the selected components. To simulate crack growth in power plant components, it is necessary to know and to describe their temperature dependent crack growth behavior. Therefore, fatigue crack growth tests were carried out with C(T)-specimens, which have been machined in different orientations from a decommissioned high pressure bypass valve made of X20CrMoV12-1. Thus, various crack orientations can be evaluated separately. Beside the orientation, the test frequency, the normalized K -gradient ( Test Method for Measurement of Fatigue Crack Growth Rates , 2015), R -ratio and the temperature were varied. The main influencing factors are temperature and R -ratio. The other parameters have no or just a minor influence on the crack growth rate. For all investigated temperatures, the crack growth tests have shown that the R -ratio has almost no influence in the PARIS-regime. In the threshold range higher R -ratios lead to lower threshold values. This expected trend is observed for all investigated temperatures. However, the influence of the R -ratio decreases for increasing temperatures. Fig. 1 exemplarily shows mean curves of the crack growth data for R = 0.1 and R = 0.5 for the investigated temperatures. Both R -ratios show that the crack growth rates in the PARIS-regime increase for increasing temperatures. Further, it is noticeable that the threshold values in Fig. 1a of the elevated temperatures are significantly lower than the threshold value of the tests at room temperature. But, for higher temperatures the trend of decreasing threshold values is not existent. 2. Experiments for describing material behavior under elevated temperatures

a)

b)

R = 0.1

R = 0.5

Fig. 1. (a) Mean curves of the crack growth tests for R = 0.1, and (b) for R = 0.5 (Mutschler and Sander (2016))