PSI - Issue 7

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Yoichi Yamashita et al. / Procedia Structural Integrity 7 (2017) 11–18 Yoichi Yamashita et Al./ Structural Integrity Procedia 00 (2017) 000–000

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3. Results and Discussion When we discuss the fatigue properties of materials made by AM, we should first pay attention to the ideal or upper bound of fatigue strength which we can expect for the case without influence of defects. It is well known as shown in Fig. 3 that there is a very good correlation between fatigue limit and Vickers hardness HV up to HV =~400. For HV >400, fatigue limit drops drastically due to presence of small defects (Murakami, Y. 2002). There is the robust empirical formula (Eq. (1)) between fatigue limit σ w and HV for HV <400 (Nishijima, S. 1980, and Murakami, Y. 2002). σ w,ideal = 1.6 HV ± 0.1 HV (1) Where, the units are σ w,ideal in MPa and HV in kgf/mm 2 . Since the Vickers hardness of the material investigated in this study ranges from HV =465 to 474, the ideal fatigue strength can be estimated to be around σ w,ideal = 744-758 MPa.

(a) N: normalized, Q.T: quenched and tempered,

(b) Zero mean stress. [Garwood, M.F. et al.1951]

R=-1 zero mean stress specimens. [Nishijima, Y. et al. 1980]

Fig. 3 Relationship between hardness and the fatigue limit (Murakami, Y. (2002)).

Figure 4 shows S-N data for Material A. There is no apparent difference in fatigue strength between T- and L- directions. All specimens fractured from defect and fatigue failure results show a large scatter due to scatter of the defects at fracture origin. The locations of defects of fracture origins are mostly in contact with specimen surface. The specimens which ran out for N =10 7 cycles were tested again at higher stress to identify the fatal inclusion which lead specimen to failure. The second or third test was carried out at stress 40MPa higher than the previous test to avoid the coaxing effect. Figures 5(a) and (c) are typical defects at fracture origins in Material A. Figure 5 (b) is identified as a pore.

800

□ ： Material A （ dierction - L ） ■ ： Material A （ dierction - T ）

700

600

500

B3

100 Stress Amp ｌ itude σ a [MPa] 200 300 400

A2

C2

B2

C1

B1

A1

Runout

0

1.E+05 10 5

1.E+06 10 6

1.E+07 10 7

1.E+08 10 8

1.E+04 10 4

Cycles to Failure N f [cycles]

Fig. 4 S-N data of Material A