PSI - Issue 13

Hiroaki Ito et al. / Procedia Structural Integrity 13 (2018) 1105–1110 Author name / Structural Integrity Procedia 00 (2018) 000–000

1108

4

3. Fatigue experiment 3.1. Tested Steels

The model proposed above is validated by comparing with experimental results using three different steels (B, C, and D) with various grain sizes and volume fraction of pearlite phases. Optical micrographs of the steels are shown in Fig. 3. The distribution of the ferrite grain diameter and pearlite band thickness of each steel are shown in Fig. 4. Material properties of them are shown in Table 1, where d � is the mean area diameter of ferrite grain, r � is the carbon concentration, σ � is the yield stress, σ � is the ultimate tensile strength, τ � � is the friction stress of ferrite grain, τ � � is the friction stress of pearlite particle and R � is the reduction rate in area under tensile test. Table 1 Monotonic tensile properties and friction strength to move dislocations of tested steels Steel d � [ μ m] r � [%] σ � [MPa] σ � [MPa] τ � � [MPa] τ � � [MPa] R � B 56.0 0.18 216 430 55 77 0.72 C 45.8 0.09 215 375 51 72 0.73 D 24.5 0.09 260 395 51 72 0.79

100 μm

100 μm

100 μm

(a) Steel B

(b) Steel C

(c) Steel D

Fig. 3 Optical micrographs (polished and 2% nital etched) and volume fractions of pearlite phases of the steels

0,0 0,2 0,4 0,6 0,8 1,0

0,0 0,2 0,4 0,6 0,8 1,0

0 20 40 60 80 100 120 B C D Pearlite band thickness [μm]

0 20 40 60 80 100 120 140 B C D Ferrite grain diameter [μm]

Area fraction

Equivalent area fraction

Fig. 4 Distribution of the ferrite grain diameter and pearlite band thickness of the steels. (a)Ferrite grain diameter and (b) Pearlite thickness 3.2. Condition of the experiments Fig. 5 shows the condition of the fatigue test. Test specimens with a notch (Fig. 5(a)) were fatigued under axial tension/compression by an Instron 8871 test machine, as shown in Fig. 5(b). The stress ratio was set to R=-1 with five load amplitude levels. The S-N curves of these three steels obtained from the experiments were obtained.