Experimental study on the very high cycle fatigue properties of 6065A aluminum alloy for high speed train

Fatigue tests in the very high cycle (106~1010) fatigue regime of 6065A aluminum alloy with cylinder and plate specimens were conducted by employing symmetric tension and compression loading with a frequency of 20 kHz, and the fatigue fractures were observed by scanning electron microscopy (SEM). The results show that the S-N curve of 6065A alloy specimens displays the characteristics of stepwise decrease and is different from that of the infinite life characteristics in traditional fatigue. The fatigue fracture will still occur when the fatigue life is more than 107 cycles. The fatigue crack expands in shear manner along with 45° specimen axial direction. The stepwise decrease characteristics of cylinder specimen S-N curve correspond to the double-initiation mechanism of fatigue crack. In the low or high cycle regimes of lower fatigue life, the fatigue crack initiates in the local stress or strain concentration on specimen surface. In the very high cycle regime of higher fatigue life, the fatigue crack initiates from the subsurface or internal defect of specimen. The fatigue properties of the plate specimens are markedly lower than those of cylindrical specimens. The fatigue crack displays the characteristics of multi-source nucleation, and surface crack initiation is dominant.