Low-cycle fatigue behavior of commercially pure titanium

Low-cycle fatigue tests have been carried out on annealed commercially pure titanium under strain-controlled conditions. The relation between a plastic strain range and a number of cycles to failure obeyed Manson-Coffin's rule. The initiation of fatigue cracks was observed successively with the aid of a replication technique and microstructural deformation was measured on the surface of a specimen. Results show that fatigue crack initiation is intergranular at strain ranges larger than 1%. On the contrary, slip band cracks appear in lower strain ranges. At specified grain boundaries, microstructural deformation concentrates at or near grain boundaries and irreversible steps between grains form gradually and this leads to intergranular cracking. The above fatigue behavior of pure titanium is mainly due to fewer slip systems (only three) than other metals with bcc or fee crystal structures.