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

17 7

Where, the units are σ wl : MPa, HV :kgf/mm

2 , √ area

effmax : µ m.

8.0

7.0

6.0

Material A

0.0 Reduced variate ｙ j （ % ） 1.0 2.0 3.0 4.0 5.0

Material B

-1.0

V 0 = 235.6mm 3

-2.0

0

100

200

300

400

500

√area max （μ m ）

Figure 12 Statistics of extremes of the defects at fracture origin of Material A and B in terms of √ area effmax . ( V 0 : Specimen volume)

Regarding the fatigue limit of Ni-based superalloy 718, a supplementary description may be necessary in terms of VHCF. There are several literature which report fatigue failure at cycles longer than 10 7 cycles from subsurface grain showing a facet at fracture origin. Thus, the possibility of “no fatigue limit” has arose for this alloy. Similar results were reported on Ti-6Al-4V. However, this phenomenon can be understood as the failure from a large grain as the extreme value of grain size distribution which is preferentially oriented to applied stress and is eventually regarded equivalent to a defect with delayed crack initiation in interior of specimen. Actually, the application of the √ area parameter model to these large grains with facet works quite well. 4. Conclusions The fatigue properties of a Ni-based Superalloy 718 manufactured by AM were studied and the perspective for fatigue design was discussed from the viewpoint of the effect of defects. 1. The Ni-based Superalloy 718 manufactured by AM contained various irregularly shaped defects which were observed by microstructural investigation on the as-built material and also at fatigue fracture origins. 2. Defects were mostly gas porosity and those made by lack of fusion. 3. There is no apparent difference in fatigue strength between T- and L- directions both for Material A and B. 4. Particular effect of surface on defects present near surface must be carefully considered. Since the orientations of defects in AM materials are random, a defect in contact with specimen surface has higher influence in terms of the effective defect size √ area eff than the real size of the defect from the viewpoint of fracture mechanics. 5. The statistics of extremes analysis based on the fracture mechanics evaluation of defects is useful for the quality control of AM. 6. Considering the volume and number of productions of components, the lower bound of the fatigue limit σ wl based on √ area effmax can be predicted by the √area parameter model. References Beretta, S., Roman, S., 2017.A comparison of fatigue strength sensitivity to defects for materials manufactured by AM or traditional processes, Int. J. Fatigue, 94, 178–191. Günther,J., Krewerth, D., Lippmann, T., Leuders, S., Tröster, T., Weidner, A., Biermann, H.,Niendorf, T.,2017. Fatigue life of additively manufactured Ti–6Al–4V in the very high cycle fatigue regime, International Journal of Fatigue 94, 236–245., and References included in this paper.