13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- founded on [1]. For turbine runners, this strain would lead to component failure due to the component tolerances. In this case a strain level in the ligament of the components of 0.2 % is discussed. Figure 8. With a decreasing ligament strain, the net section stress is reduced, which results in a reduced crack tip loading for constant crack length. As it is shown in Figure 8, the limit in initial crack size therefor increases. It is clear, that with growing of alimit, the wall thickness must have a minimum larger than alimit. For the creep-brittle 1Cr-steel 28CrMoNiV4-9 a ligament strain of 1 % could not be attained even for crack sizes of 0.3 mm. Instead of modelling initial crack sizes below 0.3 mm a ligament strain of 0.8 % was accepted, too. Figure 9. Considering the relation between the reference strain and the creep rupture strain (see Figure 5) it correlates well with the observed dependence of the initial crack size and the creep rupture strain, i.e. above 20 % of creep rupture strain alimit and r remain constant. This analysis was conducted defining crack initiation at a crack growth of 0.1 mm. In general a crack propagation of 0.5 mm is assumed. So the above applied strain based failure criterion is met 0 5 10 15 20 25 30 0 1 2 3 4 5 6 7 8 9 10 0,2 % Ligament strain 30CrMoNiV4-11 / AMA [T = 550 °C] 28CrMoNiV4-9 / AGB [T = 550 °C] X12CrMoWVNbN-10-1-1 / 1A [T = 600 °C] Limit of initial crack size agrenz / mm 1% Ligament strain Creep rupture strain Au / % 0,5 % Ligament strain liga = 0,8% 0 5 10 15 20 25 30 0 1 2 3 4 5 6 7 8 9 10 a = 0,5 mm a = 0,1 mm lig = 1% 28CrMoNiV4-9 / AGB [T = 550 °C] 30CrMoNiV4-11 / AMA [T = 550 °C] X12CrMoWVNbN-10-1-1 / 1A [T = 600 °C] Limit of initial crack size alimit / mm 1% Ligament strain Creep rupture strain Au after 100.000 h / % 0,5 % Ligament strain liga = 0,8%
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