PSI - Issue 19

Shota Hasunuma et al. / Procedia Structural Integrity 19 (2019) 194–203 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

202

9

8.5  m, 1500  m a 0 2 c 0 750  m, 1500  m

No.1-AM :

2 c 0

a 0

4.0  m, 70  m No.4-AM : 35  m, 70  m

10 1 10 4 Crack depth a ,  m 10 2 10 3

10 3 10 4 Surface crack length 2 c ,  m 10 1 10 2 10 3

10 4

10 3

10 4

Number of cycles N , cycle

Number of cycles N , cycle

(a) Surface crack length

(b)Crack depth

10 0

10 -2 Aspect ratio a / c 10 -1

10 3

10 4

(c)Aspect ratio Fig.13 Results of crack growth prediction. Number of cycles N , cycle

The low cycle fatigue life was predicted using the above model. We predicted the fatigue life of the No.1-AM and No.4-AM specimen under  = 1 % condition. We used crack growth characteristics of stainless steel described in JSME S NA1-2004: in Kozokenzensei hyouka handbook henshuiinkai (2005).  = 602 MPa was used according to Fig. 7. Dowling’s simple formulation was used for the calculation o f  J ; Dowling(1977). The Newman – Raju solution was used for the geometric factor to consider the effect of the aspect ratio; Newman and Raju(1984). In the cases of No.1-AM specimen, the initial surface crack length, 2 c 0 and the initial crack depth a 0 was determined to be 1500 and 17  m, respectively, according to Fig. 4. 2 c 0 and a 0 of No.4-AM specimen was determined to be 70 and 8  m, respectively, according to Fig. 5. Fig. 13 shows the predicted results. Fig. 13 (a), (b) and (c) show variation of surface crack length, crack depth and aspect ratio of crack, a / c , respectively. Symbols and solid lines indicate experimental and prediction results, respectively. This method predicts unsafe fatigue life, because the predicted crack depth is less than the real crack depth; i.e., the actual depth cannot be measured by surface observation because the deepest part of the scratch is closed. The prediction method is thus modified so that the initial crack depth is determined from the initial surface crack length under the assumption of a semicircular crack shape; i.e., a 0 is 750 and 35  m for No.1-AM and No.4 AM, respectively. Predictions made using this method are shown in Fig. 13 by dashed lines, and the fatigue life of the prediction becomes shorter than the experimental results. The above results indicate that the method can