PSI - Issue 13

Kohei Kishida et al. / Procedia Structural Integrity 13 (2018) 1032–1036 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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Fig. 1. Specimen geometries for (a) tensile and (b) fatigue tests (unit: mm). (c) Notch geometry in the fatigue test specimen (unit: μ m). The stress – strain curve at 433 K also showed distinct serrations, but two factors causing the serrations at 433 K can be considered: (1) the occurrence of DSA with increased carbon diffusivity, (2) carbide formation during deformation, i.e., dynamic precipitation (Chan et al., 1997). The present data are insufficient to determine which of these factors caused the serrations. Therefore, this study further investigates this issue using fatigue and hardness test results.

Fig. 2. Nominal stress – nominal strain curves at 293 K and 433 K in the Fe-0.016C-1.0Si alloy.

Fig. 3. Stress amplitude – fatigue life diagrams. The results of the Fe 0.017 alloy were obtained from the previous study (Li et al., 2018) .

3.2. Fatigue properties and micro fatigue crack propagation behavior Figure 3 shows the diagrams of stress amplitude versus number of cycles to failure at 293 K and 433 K. The Fe- 0.016C-1Si specimen tested at 200 MPa and 293 K did not show failure even at 2.4 × 10 7 cycles, but the fatigue crack was observed to propagate for the number of cycles in the range from 2.1 × 10 7 to 2.4 × 10 7 cycles as shown in Fig. 4. Although non-propagation of the fatigue crack was not confirmed, we determined the fatigue limit of the Fe- 0.016C-1Si alloy to be 200 MPa where fatigue failure did not occur for more than 10 7 cycles. The Fe-0.017 C alloy with the same notch geometry showed the fatigue limit of 185 MPa at 293 K — i.e., the addition of Si increased the fatigue limit at 293 K, perhaps owing to the solution strengthening. The Fe-0.016C-1Si specimen tested at 160 MPa and 433 K did not show failure until 2.0 × 10 7 cycles; however, similar to the case at 293 K, the crack did not stop propagating even when the number of cycles reaches 2.4 × 10 7 .