Fatigue Crack Propagation Behavior in Commercial Purity Ti Severely Deformed by Accumulative Roll Bonding Process

Fatigue properties of commercial purity titanium sheets severely deformed by the accumulative roll bonding (ARB) process were investigated. The ARB process was carried out up to 6 cycles (equivalent strain, e eq. = 4.8). The sheets ARB processed by 2-, 4- and 6-cycle consist of fine equiaxed grains and elongated lamellar grains. In the sheet ARB processed by 6-cycle, the mean size of fine equiaxed grain was 89 nm, and the mean thickness of the lamellar grains were 67 nm. The tensile strength increased with increasing the number of the ARB cycle. Fatigue crack growth tests were performed to clarify the fatigue properties such as the crack growth rate and threshold stress intensity factor range for crack growth (ΔK th ). The ΔK th of the ARB processed specimens were smaller than that of the starting sheet. The ΔK th decreased with increasing the number of the ARB cycle until 4-cycle. However, the ΔK th of 6-cycle specimen was larger than that of the 4-cycle specimen. Fracture surface of the 6-cycle specimen was different from that of the 2- and 4-cycle specimens. Fatigue crack propagation behavior changes between 4- and 6-cycle specimens. On the other hand, the crack growth rate decreases with increasing the number of the ARB cycle.