13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- Figure 12. Microstructure dependent growth rate of crack #1 Figure 13. Micro-hardness indentations along section of crack face (dotted red line) Even though the correlation between the distribution of the phases in the specimen and the acceleration and deceleration of the crack extension process is quite good, it has to be kept in mind that extension of an edge crack is a three-dimensional process, and that conclusions based only on surface observation may be misleading. Therefore a micro-specimen was prepared with an artificial edge notch, and the crack extension from the notch was monitored using the experimental set-up with two travelling microscopes as shown in Figure 9. The initial scans were made in the unloaded state and are given in Fig. 14 a) for the side view (vertical direction) of the micro-specimen and in Fig. 14 b) for the front view (horizontal direction). The fatigue crack initiated at the notch as expected and propagated in both directions. After a total of 45.000 load cycles, which represents approx. 96 % of the lifetime, the fatigue crack has reached a length of 1693.75 µm in the vertical direction (see Fig. 14 c)) and 1002.62 µm in the horizontal direction (see Fig. 14 d)). It seems that
RkJQdWJsaXNoZXIy MjM0NDE=