PSI - Issue 13

Taketo Kaida et al. / Procedia Structural Integrity 13 (2018) 1076–1081 Taketo Kaida et al. / Structural Integrity Procedia 00 (2018) 000–000

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Fig. 1. (a) Specimen geometry of the fatigue test. (b) An overview of the fatigue-fractured specimen used for the fractographic characterizations.

3. Results and discussion Figure 2 exhibits the fracture surface at various ∆ K values, with Figure 2(a) showing the formation of the ridgelines parallel to the crack propagation direction at ∆ K = 8.1 MPa √m . With increasing ∆ K to 22.9 MPa √m , the ridgelines were bent and merged, and a Chevron pattern appeared partially, as shown in Figure 2(b). The Chevron pattern became more distinct, when ∆ K was further increased to 43.0 MPa √m (Figure 2(c)). The formation of Chevron pattern is attributed to the activation of the secondary slip system underneath the fracture surface (Suzuki et al., (1987)). Since the contribution of the secondary slip increases as ∆ K increases, the Chevron patterns were observed to be clearer at high ∆ K areas. Furthermore, at ∆ K = 43.0 MPa √m , there were vertical lines intersecting the Chevron patterns. Most importantly, no striation was observed on the fracture surface.

Fig. 2. Fatigue fracture surfaces at (a) ∆ K = 8.1, (b) ∆ K = 22.9, and (c) ∆ K = 43.0 MPa √m . Figure 3 shows the results of the EBSD measurements underneath the fracture surface. Figures 3 (a-c) show the grain reference orientation deviation (GROD) maps at the crack lengths corresponding to respective ∆ K in Figs. 2 (a) to (c). The GROD map shows the crystallographic grain orientation angle from the average orientation in a target grain. Since the specimen used in this study was single crystal, the average orientation is determined across the specimen. The GROD map qualitatively corresponds to plastic strain distribution (Nishikawa et al., (2011)). At the ∆ K of 8.1 MPa √m , the plastic zone was developed widely, and the plastic strain was large, compared with that of the higher ∆ K regions. In terms of the mechanical severity, the higher plastic strain at the relatively low ∆ K is unreasonable. In the GROD map, at low ∆ K area of 8 MPa √m , the observation region is close to the notch tip (420 μm). Therefore, the large plastic deformation is the result of the fatigue crack initiation process. At 22.9 MPa √m , the plastic zone was observed to be within the range of 30 μm fro the fracture surface. At 43.0 MPa √m , the plastic zone size increased to the range of ~65 μm. Furthermore, a separate plastic zone appears at 43.0 MPa √m , as indicated by red arrows. The interval between the two consecutive plastic zones was in the range of 10-20 μm. At 1.9 mm, the crack length was higher than 1.0 mm, satisfying the condition where ∆ K th tends to be crack-length-independent, i.e. the crack after the crack-initiation-affected region is a mechanically long crack (Murakami and Shimizu, (1988)). As a result, Paris's law is applicable for estimating the crack propagation rate, d a /d N, which was determined to be 10 μm/cycle at 43.0 MPa √m . Since the plastic zone intervals at high ∆ K area were also 10 to 20 μm, the separate plastic zones correspond to plastic strain evolution, caused by the fatigue loading cycles.