Issue 35

Takamasa Abe et alii, Frattura ed Integrità Strutturale, 35 (2016) 196-205; DOI: 10.3221/IGF-ESIS.35.23 203 Figure 16 : Beach mark on fracture surface of Fa=6kN. Figure 17 : 3-dimentional observation of fatigue crack propagation at the root region ( F a =5 kN, N =10 7 cycles). Relation between crack initiation origin and the shape of the unwelded portion The above observations confirmed that multiple crack origins occur in the loaded test piece. In this subsection, we consider the location of these crack origins with regard to the shape of the unwelded portion. As a quantitative evaluation index we define the root gap as the distance from 10μm opposite the crack propagation direction in the unwelded portion. The relationship between root gap and position across the sample width is plotted in Fig. 18. Typical observations of the unwelded portion are presented in Fig.19. The root gap in the unwelded portion is typically 0.02 - 0.03mm (see circles labeled (b) and (d) in Fig. 18), but multiple narrow gaps in the unwelded portion are also observed (enclosed by circles (a ) and (c) in Fig. 18). This unwelded sharp portion may be the origin of a fatigue crack. Figure 18 : Measurement of the root gap. 7.29 × 10 5 3.05 × 10 5 5.65 × 10 5 Origin(a) Origin(b) 5mm Polish distance from the edge of specimen [mm] ( F a =9 kN, N / N f =50%) 0 10 20 30 40 50 0 Root gap [mm] 0.01 0.02 0.03 0.04 (c) (d) Polish distance from the edge of specimen [mm] ( F a =6 kN, N / N f =50%) 0 10 20 30 40 50 0 Root gap [mm] 0.01 0.02 0.03 0.04 (b) (a) -0.2 0.2 0 10 30 50 0 0.5 1.5 40 1.0 Loading direction 20