Fatigue Crack Growth in Cold-Rolled and Annealed Polycrystalline Superelastic NiTi Alloys

Abstract

The fatigue crack growth in cold-rolled (CR) and annealed (AN) polycrystalline superelastic NiTi shape memory alloys under cyclic stressing are investigated. The fatigue crack growth morphologies of compact tension specimens are recorded. Experimental results show that the main cracks of the AN specimens grow along the original crack plane, while the crack paths of the CR specimens orient at 18 $$^{o}$$ angle to the original horizontal crack plane. The latter is due to the strong anisotropy of elastic modulus and hardness after the cold rolling. Besides, the fatigue crack growth rates of the AN specimens in the regime of $$\Delta K=4~\hbox {MPa}\sqrt{\mathrm{m}}\sim 9~\hbox {MPa}\sqrt{\mathrm{m}}$$ are lower than those of the CR ones. The AN specimens have higher fatigue crack growth resistance than the CR specimens due to the stronger shielding mechanism caused by plastic deformation and reduced anisotropy by annealing.