Effect of Compressive and Tensile Pre-strains on Durability of Nitinol

Abstract

Fatigue testing of tension-tension and diamond specimens of nitinol exhibits improved fatigue life when subjected to tensile pre-strains due to induced plasticity. The improvement is associated to the presence of compressive residual stresses surrounding the inclusions, which act as a barrier to crack initiation. The study here extends this work to demonstrate the impact of type of residual stresses (i.e. compressive vs. tensile) combined with different modes of deformation on the durability. Diamond specimens, crimped and stretched to 9% pre-strain to induce tensile and compressive residual stresses, are tested at a particular mean strain (i.e. 3.50%) at different strain amplitudes and up to 10 million cycles. In addition, FEA analysis showing the impact of superelasticity on generating residual stresses around the inclusions is presented. Pre-straining is more prominent in nitinol when compared with conventional alloys where the total stored energy is lower due to the absence of superelasticity.