PSI - Issue 7

Hiroshige Masuo et al. / Procedia Structural Integrity 7 (2017) 19–26 Hiroshige Masuo et Al./ Structural Integrity Procedia 00 (2017) 000–000

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materials, we need to consider the modification of defect size as Figs. 14 and 15.

(a) (b) Fig.15 Estimation of √ area eff for DMLS without HIP

(a)

(b) Fig.14 Estimation of √ area eff for EBM without HIP

V 0 ≒ 65 mm 3

S 0 ≒ 28 mm 2

7

7

EBM without HIP DMLS without HIP

EBM Surface polish without HIP DMLS Surface polish without HIP

6

6

5

5

4

4

2 Reduced variate yi (%) 3

1 Reduced variate yi (%) 2 3

1

0

0

-1

-1

-2

-2

0

100

200

300

400

500

0

100

200

300

400

500

√area max （ μ m ）

√area max （ μ m ）

Fig. 17 Statistics of extremes analysis for the defects observed at fracture origin. √ area max was calculated based on the rule of Figs. 9, 10 and12. V 0 = 65mm 3 : Control volume of surface annular zone of specimen under stress higher than 90% of nominal stress.

Fig. 16 Statistics of extremes analysis for the largest defects observed on specimen section. √ area max was calculated based on the rule of Figs. 12, 14 and 15.

Figure 16 shows the statistics of extreme analysis of the largest defects which appeared on the sections cut from a specimen with the inspection area S 0 = 28mm 2 . Figure 17 shows the statistics of extreme analysis of the defects which were observed at fatigue fracture origins. In this analysis, the modification of √area was applied based on the rule of Fig. 12. Figures 16 and 17 show that Material DMLS is graded higher than Material EBM within the current processing conditions and particle sizes and can be used as the measure for quality control of AM materials. Inserting the approximate values of fatigue limit for as-built specimens of Figs. 3 and 4 into Eq. (1), we can estimate the equivalent √ area for surface roughness. The estimated values of √ area exceed 1000 µ m for all the cases of EBM and DMLS. Since Eq. (2) is valid for √ area <1000 µ m, it can be regarded that the surface roughness produced by AM is much larger and more detrimental than other defects. Therefore, it will be of practical importance to eliminate the effect of surface roughness by polishing or other technique such as shot peening at least at critical locations. From the above discussion, it is necessary for the safe fatigue design of AM components to consider the method of the statistics of extremes analysis based on Figs. 12, 16 and 17. Considering the volume and number of productions of the components in question, the effective largest defect √ area effmax contained in large or many components can be predicted. The lower bound of the fatigue limit σ wl based on √ area effmax can be determined by the following equation.

σ wl = 1.43( HV +120)/( √ area effmax ) 1/6

(3)