13th International Conference on Fracture June 16–21, 2013, Beijing, China Certainly, if the sample surface contains some scratches, it will further increase the occurrence possibility of crack initiation. Generally, the relationship between the “crack initiation area” size and fatigue strength or fatigue limit (σ-1) proposed by Murakami can also be used for Mg alloys, which can be expressed as [13]: 1 1/6 in (HV+120) ( A ) σ β − = (1) Where HV is the Vicks Hardness of Mg matrix, β is a coefficient and Ain is the crack initiation area. b) a) c) d) Fig. 3. Secondary electron images showing the overall fracture surfaces of samples: a) S1 (95MPa for 5E8 cyc), b) S2 (110MPa for 5.2E8 cyc), c) S3 (105MPa for 5.1E8 cyc) and d) S4 (100MPa for 5.3E8 cyc) From Fig. 3, it can be seen that the crack initiation area for all failed samples is very similar. Then, the fatigue strength of alloys should be more dependent on the hardness values. Based on the results in Table 1, the hardness of the alloy with Y content of 1wt% is relatively higher. According to Equation (1), the calculated fatigue strength corresponding to 109 cycles should be higher than that of others, which is consistent with the measured results (Fig. 2). -4-
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