Rate-dependent transformation ratcheting-fatigue interaction of super-elastic NiTi alloy under uniaxial and torsional loadings: Experimental observation

Abstract To investigate the interaction between the rate-dependent transformation ratcheting and fatigue failure of super-elastic NiTi shape memory alloy (SMA) thin-walled tubes, uniaxial and torsional stress-controlled fatigue tests were conducted at various loading rates. The experimental results show that the uniaxial transformation ratcheting is more obvious than the torsional one and the torsional fatigue lives are significantly longer than the uniaxial ones, which imply that a more obvious transformation ratcheting leads to a shorter fatigue life. The rate-dependent transformation ratcheting and fatigue failure of super-elastic NiTi SMA can be attributed to the significant thermo-mechanical coupling effect.