PSI - Issue 19

Masanori Nakatani et al. / Procedia Structural Integrity 19 (2019) 294–301 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

300

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Fig. 7. Relationship between  K th ( R = - 1) and defect size √ area in Ti-6Al-4V alloy fabricated by AM (Masuo (2018)).

Table 3. Estimated defect size and equivalent crack size.

Surface roughness √ calculated using Eq. (3) (  m) S z (  m) R Sm (  m)

Equivalent crack size √ R (Eq. (4) or (5)) (  m)

Estimated  K th (Eq. (6)) (MPam 1/2 )

HV (kgf/mm 2 )

AM process

 w (MPa)

HIP

No

374 349 380 342

140 195 155 210

16505

5.2 7.3 5.1 6.9

EBM

646

696

264

Yes

1655 9635

No

DMLS

260

535

203

Yes

970

Here, the units are  w : MPa, HV : kgf/mm 2 , √ area :  m. In the any case, the calculated √ area is much larger than upper limit which √ area parameter model can apply. This result indicates that √ area parameter model cannot deal with the surface roughness of as-built specimen. Therefore, the surface roughness of as-built specimen should be considered as long crack. Murakami (2002) proposed an evaluation method of equivalent defect size, √ R , for periodic surface notches as artificial surface roughness with a depth of a (the vertical distance of root-to-peak) and a pitch of 2 b (the horizontal distance of peak-to-peak) as followed. √ R 2 = 2.97 ( 2 ) − 3.51 ( 2 ) 2 − 9.74 ( 2 ) 3 , for 2 ≤ 0.195 (4) √ R 2 ≈ 0.38, for 2 ≥ 0.195 (5) Here, maximum value of maximum height S a and mean width of profile elements R Sm were substituted for a depth of a and a pitch of 2 b respectively. The estimated √ R are also summarized in Table 3.  K th was calculated using Eq. (6). Δ th = 0.65 × 2σ √ √ R (6) The calculated  K th was lower than the  K th value for a forged Ti-6Al-4V alloy; about 10 MPam 1/2 . The  K th value of AM Ti-6Al-4V alloy has a tendency to be low compared with that of conventional Ti-6Al-4V alloy (Liu (2019)). In addition, it is reported that  K th value for R = 0.1 ranges 2 to 5 MPam 1/2 in AM Ti-6Al-4V alloy. Thus, the