13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- This value was chosen on the one hand to avoid the need of modelling crack propagation in the FE-calculations and on the other to be able to detect crack initiation within the experiments. In Figure 6 the calculation procedure is schematically shown. The optimisation of the results with respect to the initial crack length and the accumulated strain within the ligament are done iteratively. The concept for describing crack initiation was first validated by applying on already run experiments. In Figure 7 are the results shown for the analysed 10Cr and 1Cr-steels. The results of the FE-calculations are in good agreement with the defined reference strain for all materials and different ductility. The failure criterion now can be applied to find out, if there is a limit crack size below a fracture mechanics assessment is not necessary. The limit of initial crack size alimit will be determined in correlation to the creep rupture strain corresponding to loading times of 100.000 h and more, which cannot be done experimentally. The creep rupture strain for the 10Cr-steel is assumed to be constant between 100.000 h and 200.000 h. For the 1Cr-steels the low creep rupture strain at 100.000 h is extrapolated as constant up to 200.000 h. a) X12CrMoWVNbN10-1-1 b) 30CrMoNiV4-11 c) 28CrMoNiV4-9 Figure 7. Validation of the strain based failure criterion for 10Cr and 1Cr-steels The limit of initial crack size alimit sought, is defined dependent on the accumulated inelastic strain within the ligament of the specimen. It is that size of surface type crack for the DENT specimen, when simultaneously the failure criterion in the crack-tip region is met and the selected creep strain is reached. To quantify the influence of specimen geometry and size, these have been varied. The results of the calculations are shown in Figure 8. It can be seen, that there is small scatter of the data with respect to geometry and size of the specimens at a constant creep rupture strain. In general it can be stated, that with increasing size (especially breadth) the limit of initial crack size increases, too. An increasing ductility also leads to a higher limit of initial crack size. Also different levels of ligament strain have been analysed. This is derived for the operation of different components. A strain level of 1 % within the net section stress area is valid for thick-walled components and is 100 1000 10000 100000 0,1 1 10 100 c1,qmin (Cocks&Ashby method applied), t(a= 0,1 mm) X12CrMoWVNbN10-1-1 / 1A, T = 600 °C Creep strain c / % Time t / h r (mean curve) Ds20 Ds60 Cs25 100 1000 10000 100000 0,1 1 10 100 r (mean curve) D30 D60 D15 D9 Creep strain c / % Time t / h c1,qmin (Cocks&Ashby method applied), t(a= 0,1 mm) 30CrMoNiV4-11 / AMA, T = 550 °C 100 1000 10000 100000 0,1 1 10 100 r (mean curve) D9 Creep strain c / % Time t / h c1,qmin (Cocks&Ashby method applied), t(a= 0,1 mm) 28CrMoNiV4-9 / AGB, T = 550 °C
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