PSI - Issue 19

Yukitaka Murakami et al. / Procedia Structural Integrity 19 (2019) 113–122 Yukitaka Murakami et al./ Structural Integrity Procedia 00 (2019) 000–000

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6. Quality control of AM materials from the viewpoint of fatigue performance Although existing standards may specify test methods and inspection methods (for example, Seifi et al (2017)), the safety standards must be made from the viewpoint of fatigue characteristics specific to AM materials. We have to pay special attention to frequent fatigue fractures from surface layer in AM materials even under tension-compression fatigue (Serrano-Munoz et al (2017), Yamashita et al (2018), Romano et al (2019)). In case of high cycle fatigue of commercial hard steels, almost all specimens fractures from internal inclusions as shown in Fig. 7. There are two reasons for frequent fracture from surface. One is the difference of the population of large defects in AMmaterials compared to the small population of large nonmetallic inclusions in commercial steels. Another is higher stress intensity factor for a surface crack even if a surface crack is smaller in size than internal cracks. The population of large defects in AM materials is much higher (Fig. 8) than the population of large nonmetallic inclusions in commercial steels. In AM materials, not only the largest defects but also second, third or even smaller defects have high probability of becoming fracture origins if they exist near surface.

Fig. 7 High cycle fatigue fracture origin at nonmetallic inclusion in bearing steel (Murakami (2002)).

Fig. 8 High population of large defects in AM material. Surface defects become fatigue fracture origins more frequently than internal defects.

Figure 9 schematically compares the crack growth behavior of a surface crack and an internal crack. Even if the surface crack is smaller than internal crack, the increase rate of stress intensity factor with crack growth is much higher than that for internal crack (Romano et al (2019)). Thus, the growth rate of surface crack overtakes that of internal crack (Murakami (2019)). Moreover, surface roughness is another serious challenge. As fatigue data shows, surface roughness is more detrimental than defects. Therefore, currently the microstructure of AM materials (phases, grain size, build direction, etc.) is not the first priority in presence of as-built surface roughness and defects.