PSI - Issue 19

Motoki Nakane et al. / Procedia Structural Integrity 19 (2019) 284–293 Author name / Structural Integrity Procedia 00 (2019) 000–000

289

6

3.2. Comparison of fatigue strength between grinding and lathe machining finished materials Fig. 6 describes comparison between fatigue data of SUS 316L obtained in this study and fatigue data of emery polishing and lathe machining finished SUS316LTP bar specimen by Nakane et al. (2019). In this figure, fatigue strength of SUS316L emery polishing specimen is almost equal to that of emery polishing bar of SUS316LTP. This is attributed to the fact that ultimate tensile strength of 556 MPa and proof strength of 238 MPa for SUS316LTP are almost equal to those of SUS316L subjected in this study. Fig. 6 also reveals that high cycle regime fatigue strength of SUS316 LTP lathe machining finish specimen with Rz =137  m is almost same as those of emery polishing specimens of both SUS316 LTP and SUS316L. In addition, Fig. 6 indicates fatigue strength of grinding finished specimen is much lower than that of lathe machining finished specimen even though maximum roughness height Rz of grinding finished specimen is less than 1/10 of lathe finished specimen. Now, the surface finish effect factor K sf is introduced by the following equation (Fukuta et al., 2014, Nakane et al., 2019), where K sf is defined by ratio of fatigue strength of emery finished specimen  ta-emery over surface finished specimen  ta-Rz at same fatigue life. K sf =  ta-emery /  ta-Rz (1) All fatigue date was fitted by Langer type equations as  ta = A ･ N f – B + C (where A, B and C are constants), and K sf for each surface finish conditions were derived as shown in Fig. 7 by using fatigue strength at 10 8 cycles of each fitted Langer type equation. From Fig. 7, it can be stated that K sf for Type 2 grinding conditions is obviously higher than other grinding conditions and Type 2 grinding condition decreases fatigue strength of materials.

10

10

○ Emery polishing ◆ Grinding (Type 2)

○ Emery polishing ■ Grinding (Type 1) ◆ Grinding (Type 2) ● Grinding (Type 3)

● Emery polishing bar(SUS316LTP)(Nakane, 2019) ▲ Lathe machining bar(SUS316LTP, R z =137μm) (Nakane, 2019)

0.1 Total strain amplitude ε ta (%) 1

0.1 Total strain amplitude ε ta (%) 1

0.01

0.01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

1.E+08

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

1.E+08

Number of cycle to failure (cycles)

Number of cycle to failure (cycles)

Fig. 5 S-N curves of SUS316L.

Fig. 6 Comparison of S-N curves.

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

○ Emery polishing ■ Grinding (Type 1) ◆ Grinding (Type 2) ● Grinding (Type 3) ▲ Lathe machining bar (SUS316LTP, R z =137μm) (Nakane, 2019)

K sf

0.1

1

10

100

1000

Maximum roughness height Rz (μm)

Fig. 7 Surface finish effect factor K sf for each grinding conditions.