13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- measurements of crack length and the crack tip deformation behavior. a) loading stage installed in SEM; b) specimen installed in loading stage. Figure 5. In-situ SEM fatigue testing setup The specimen configuration and preparation are almost the same as that in optical microscope experiment. In this experiment, the final polishing is done by a vibration polishing machine with 1~3 um polishing suspension. The specimen is undergoing cyclic loading and observed in-situ under SEM. In the following comparison for aluminum and steel, the stress ratio is 0.1. The measurements are usually taken after 50 to 100 cycles to ensure the crack growth is stable. During the testing, one loading cycle is divided into many steps, similar with the testing under optical microscopy. The applied loading increases/decreases at a very slow rate. CTOD is measured directly from the processed images. The CTOD is defined as the distance of crack surface at the place of crack tip in last cycle. With the same reference, the crack growth rate at each cycle could be measured, as shown in Figure 6. Figure 6. Images of crack tip position and CTOD under maximum loadings Using the measuring method, two sets of experiments on steel 4043 and aluminum 7075-T6 have been performed. The results are shown in Figure 7. It shows that CTOD reduces to zero when SIF is less than 5 MPa·m0.5 for Al 7075-T6, while for steel 4340, CTOD is not zero under the minimum loading. The crack tip details are shown in Figure 8. It shows the crack closure in Al 7075-T6 clearly but the crack remains open for steel 4340. (a) Al 7075-T6 (b) Steel 4340 Figure 7. CTOD during unloading process on Al 7075-T6 and steel 4340 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 0 5 10 15 20 25 30 CTOD (um) K (MPa·m0.5) cycle 1 cycle 2 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0 2 4 6 8 10 12 CTOD (um) K (MPa·m0.5) Cycle 1 Cycle 2
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