3.3 Very high-cycle fatigue
The S-N data associated with stress amplitudes σa and
fatigue life Nf of LZ91 alloy under asymmetric axial
loading with a stress ratio of R = -1 are shown in Figure 5 .
The S-N data of some other Mg-Li alloys and magnesium alloys were also
included for comparison [19, 31, 32]. “Run-out specimens”
represents that the fatigue life of the specimen reached a limit of
1×107 (for LAZ832-0.5Y alloy) and
1×107 (for the rest three alloys). The nonlinear
fitting curves of these fatigue data were obtained by Basquin’s
equation, which were plotted by dash lines.
It can be seen that the S-N curve of LZ91 alloy exhibits a continuous
decreasing trend, even in the range of
107-109 cycles. By comparing the S-N
curve of LZ91 alloy and LAZ832-0.5Y alloy, it can be found that the
curve of LZ91 alloy decreases much more slowly. Meanwhile, the fatigue
performance of the LZ91 alloy is much better than that of the
LAZ832-0.5Y alloy when the fatigue life exceeds 105cycles. As compared to conventional magnesium alloys [19, 32], whose
fatigue tests were also conducted at R=-1 and a loading frequency of
~20 kHz, the fatigue resistance of the LZ91 alloy is
obviously lower, about 20-35 MPa. However, as the fatigue life
approaching 109 cycles, the fatigue strength gap
between them is narrowing, less than 20 MPa at 109cycles. Due to the significant difference in mechanical properties
between LZ91 and these two alloys, the lag of LZ91 in fatigue strength
seems inevitable. After that, in order to compare the fatigue
performance of these alloys, we calculated their fatigue ratios (the
ratio of fatigue strength to the ultimate tensile strength), and the
results are displayed in Table 4 . Clearly, the fatigue ratio of
the LZ91 alloy, about 0.46, is significantly higher than that of the
other three alloys (0.29, 0.27, 0.30 for LAZ832-0.5Y alloy, ZK60, AZ31,
respectively), which indicates that the LZ91 alloy exhibits relatively
good fatigue performance.