PSI - Issue 7

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

14 4

(a)

(b)

(c)

Fig. 5 Defects at fatigue fracture origin of Material A

800

○ ： Material B （ dierction - L ） ● ： Material B （ dierction - T ）

700

600

500

b2

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

a2

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. 6 S-N data of Material B

(a) (b) Fig. 7 Defects at fatigue fracture origin of Material B Figure 6 shows S-N data for Material B. Figure 7 shows the defects observed at fracture origin for Material B. These defects have various kinds and irregular shapes. Figure 8 shows a defect which was observed at fracture origin in contact with specimen surface in Material A. It was very common at fracture origin to observe defects inclined to specimen surface. Compared to rolled steels, the direction of defects in AM materials is random and not aligned in the identical direction. The varieties of defects are originated from lack of fusion or various qualities of powder. Since it is very difficult to identify the presence, shape and size of defects in advance of fatigue test, it is extremely difficult to predict the fatigue limit of individual specimens prior to fatigue test. Nevertheless, in order to quantify the fatigue limit, we need to pay attention to the mechanics aspect of defects. The representative dimension of a defect can be expressed with √ area in terms of fracture mechanics concept (Murakami, Y. 2002). From the viewpoint of the statistics of extremes, defects larger than the defect observed at the fracture origin should possibly exist in the specimen. Then, why did the defects at fracture origin appear mostly