13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- There are two heats shown, which differ only with respect to the initial heat treatment. Due to the loading times up to 100.000 h the material properties are not supposed to differ. As shown, the creep rupture strain is about 20%-25% and can be approximated by a constant. The creep rupture strain of the two 1Cr-steels is shown in Figure 3. It can clearly be seen, that the rupture strain is decreasing with increasing loading time. Both steels differ in the initial heat treatment. The creep rupture strain itself, however, is not an appropriate measured variable, due to the influence of the reduction of area gained in creep tests on materials with high ductility. Concerning thick-walled components, no transverse strain is observed. To meet this problem a reference strain was introduced, which does not include the influence of the reduction of area on the creep rupture strain value, see Figure 4. Two different formulations have been analysed. One is based on the volume of the creep test specimen. This method was preferred for the implementation of the failure criterion. The second method is based on the strains at the onset of the tertiary creep stage. 30CrMoNiV4-11 28CrMoNiV4-9 Figure 3. Creep rupture strains of 1Cr-steels at T = 550 °C 1 (d d ) 2 d 2 u1 u2 0 r,1 (2) r,2 2/3 min u 2/3 t t (3) with: tu – time to failure, 2/3 – Strain at onset of tertiary creep t2/3 – Timepoint at onset of tertiary creep Figure 4. Definition of a reference strain The reference strain according to Eq. (2) is used as material parameter within the failure criterion, which relates a characteristic strain within the specimens to the reference strain. The reference strain is referred as r. The reference strain can be plotted over the creep rupture strain, see Figure 5. This yields the correlation of the creep rupture strain and the calculated initial limit crack length alimit. 10 100 1000 10000 100000 0 5 10 15 20 25 30 35 40 45 50 30CrMoNiV4-11 / AMA 28CrMoNiV4-9 / AGB T = 550°C Creep rupture strain Au / % Time to failure tF / h du du1 du2
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