Dual-crack failure behaviors of milled Ti-6Al-4V alloy in conformal contact fretting fatigue

Conformal contact between two cylindrical surfaces is widely presented in practical engineering of hole connections, the resulted fretting due to small oscillatory movement between the connected parts could cause significant performance degradation of the components. To promote effective fatigue control method for crucial components, the fatigue failure mechanism of conformal contact parts needs to be taken more investigation. In this paper, a series of conformal contact fretting fatigue (CCFF) tests subjected to Ti-6Al-4V alloy were conducted and two types of cracks with different morphologies were reported and inspected detailly. The first type crack initiated on the hole surface and propagated in the direction with an angle around 42° to the contact surface, further to change its direction roughly to 70° regards to the loading direction, while the second type crack initiated at the newly fretting boundary and propagated in a direction approximately perpendicular to the loading direction. The results showed that the dual-crack failure was attributed to the strength and stiffness loss/degradation of the specimen caused by the early propagation of the first type fretting crack. The second fretting crack was also potential to initiate at a newly formed fretting contact region and mainly presented ductile fracture with an approximately tripled growth rate of the crack. Moreover, the cracking pattern was a mixture of intergranular and transgranular in the crack initiation regime while the transgranular cracking dominated the short crack propagation regime. Besides, the misorientation angle and the kernel average misorientation (KAM) of the grains in the crack formation zone (CFZ) increased by 45.2% and 3 times, respectively, while these values in the adjacent region of the crack tip zone (CTZ) were increased by 22.2% approximately. Significant geometrically necessary dislocation (GND) density was presented at both the grain boundaries and inside the grains in the CFZ, while the GNDs tended to accumulate inside the whole grain and at the grain boundaries for the regions that adjacent to and far away from the crack path, respectively.