13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- at earlier loading times, i.e. it is more conservative, if a crack growth of 0.5 mm is tolerable. Otherwise, for a fixed loading time and net section stress, a larger initial crack size is tolerable if at crack initiation a defined ligament strain is reached. This was investigated for a fixed loading time of 100.000 h, shown in Figure 9. It can clearly be seen, that the initial crack sizes increase if crack initiation occurs for a crack growth of 0.5 mm at the same ligament strain. Since no crack propagation was modelled in the numerical analysis, the crack growth was approximated using: k m C* dt da (4) With the C*-parameter, see [14], [22]. The parameters C and m are dependent on the material and are listed in Table 1. Table 1. Crack propagation parameters X12CrMoWVNbN10-1-1 T = 600 °C 30CrMoNiV4-11 T = 550 °C m 0,25 0,25 k 0,18 1,05 Within the small crack propagation of 0.4 mm a constant crack growth rate can be assumed. The total time span of 100.000 h is then the sum of the time to reach the strain based failure criterion with a crack propagation of 0.1 mm and the crack growth to a = 0.5 mm. 4. Summary Modern steels for high temperature operation, e.g. in power plants, are showing high deformability. Classical methods, determining failure formulated on the stress field, are highly conservative. Improving the assessment aiming at a better material usage requires a concept which includes the actual deformability. Additionally, for high ductile materials, crack initiation does not readily start before a certain strain within the net section stress region is accumulated, defining monitoring measures or the replacement of the component. To be able to determine this crack size dependent on the material´s ductility a more accurate failure criterion is needed. A strain based concept has been introduced and applied on three 10Cr- and 1Cr-steels with different ductility. It correlates a characteristic strain within the crack tip region of the specimen and a material dependent reference strain. Determining the stress and strain field within the crack tip at a certain loading time requires Finite-Element modelling and calculations to crack initiation. With a crack initiation criterion with a crack growth of 0.5 mm modelling of the crack propagation is essential. To avoid this effort, crack initiation was attained for a smaller crack growth of 0.1 mm. The concept was validated on fracture mechanics specimens with loading times up to 20.000 h. It could be shown that the initial crack sizes of interest, i.e. such sizes which does not show crack initiation before a certain strain is reached within the ligament section, is dependent on the materials ductility. For a ligament strain of 1 % the limit of initial crack size is up to 1 mm for creep rupture strains of 20 % and more. Reducing the ligament strain is leading to an increase of the limit of initial crack size. 5. Discussion and conclusions Within the creep regime, local failure can be described through the available deformability of the material. With regard to crack initiation it would lead to a less conservative concept, than using stress based formulations, since the deformability does markedly have impact. The formulation of failure set up could be validated on fracture mechanics experiments, providing a limit of initial crack size for practical operation times. This limit characterises crack sizes for which crack initia-
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