PSI - Issue 13

Kai Suzuki et al. / Procedia Structural Integrity 13 (2018) 1065–1070 Kai Suzuki et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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3.2. Scatter in fatigue crack growth rate The fatigue cracks on the surface of the specimen were observed by the replica method. Based on the observations, the fatigue crack lengths of the HEA and LEA were measured, as shown in Fig. 4. Here, the stress amplitudes for the HEA and LEA were selected to be 270 and LEA 220 MPa, respectively, which were the same as the ratio of stress amplitude to tensile strength, σ a / σ B . The fatigue lives of the HEA at 270 MPa and LEA at 220 MPa were 1.1×10 6 and 5.0×10 5 cycles, respectively. The data in Fig. 4 include those of the main crack and subcracks. The main crack was defined as the one that directly caused the failure.

Fig. 4. Fatigue crack length plotted against the number of cycles of crack propagation. (a) HEA and (b) LEA.

Figure 5 shows the fatigue crack growth rates plotted against the crack length. Figure 5(a) and (b) shows the crack growth rates of the respective alloys. Here, the cracks denoted as crack 1 are the main cracks of the two alloys. For a clear comparison, all of the data in the two alloys were plotted in the same figure (Fig. 5(c)). Comparing the crack growth rates of the HEA with those of the LEA, we can see that most of the crack growth rate data showed similar values when plotted against the crack length. An important difference was observed in the data points surrounded by the green and blue circles in Fig. 5(c). That is, the growth of small cracks with a length of <500 μm in the HEA temporarily stopped, as indicated by the green circles, whereas the small cracks of the LEA propagated continuously. Furthermore, in the LEA, as the crack length increased, the scatter in crack growth rates decreased. In contrast, in the HEA, the scatter in the crack growth rates remained even at a relatively long crack length because of the temporal deceleration of the crack growth, as indicated by the blue circles. As a result, the scatter in crack growth rates of the HEA was larger than that of the LEA at the same ratio of stress amplitude to tensile strength.

Fig. 5. Fatigue crack growth rates plotted against fatigue crack length. (a) HEA, (b) LEA, and (c) HEA and LEA.