Rate-dependent Multiaxial Transformation Ratchetting And Fatigue Failure Behaviors Of Superelastic NiTi Alloy Micro-tubes

Super-elastic Ni Ti alloy has been widely used in aerospace,biomedical,civil engineering,automotive engineering and other fields.The super-elastic Ni Ti alloy-based devices are often subjected to cyclic loads under different frequencies in service.The repeated martensitic transformation leads to a degeneration of super-elasticity,i.e.,the functional fatigue.At the same time,the progressive damage is induced inside the material by the repeated martensitic transformation and accumulated transformation-induced plasticity,which finally leads to the fatigue fracture of tested samples,i.e.,the structural fatigue.Therefore,the researches on the cyclic martensitic transformation and fatigue failure of super-elastic Ni Ti alloy have attracted widespread attention and become a focus in the field of smart materials.At present,most of experimental studies just focused on the rate-dependent cyclic deformation behavior of super-elastic Ni Ti alloy under uniaxial loadings.However,the studies on the rate-dependent transformation ratchetting of the alloy under multiaxial cyclic loadings,especially for the non-proportional multiaxial ones,are relatively insufficient.In addition,most of existing macroscopic thermo-mechanically coupled cyclic constitutive models for super-elastic Ni Ti alloy neglected the anisotropy of transformation ratchetting and martensitic reorientation under non-proportional multiaxial loadings,and thus the transformation ratchetting at different loading rates and with different loading paths cannot be described effectively.Moreover,the fatigue failure behaviors of super-elastic Ni Ti alloy under uniaxial tension,pure torsion and multiaxial loadings remain to be unclear,and the fatigue life prediction model that reflecting the rate-dependent and path-dependent fatigue lives should be developed.Therefore,it is necessary to conduct the fatigue tests of super-elastic Ni Ti alloy at different loading rates and with different loading paths,so as to investigate the dependences of transformation ratchetting and fatigue failure behaviors on the loading rate and loading path,and establish the thermo-mechanically coupled three-dimensional cyclic constitutive model and fatigue life prediction model to describe the rate-dependent multiaxial transformation ratchetting and fatigue failure behaviors,respectively.For this purpose,the transformation ratchetting and fatigue failure behaviors of super-elastic Ni Ti alloy micro-tubes at different loading rates and with various loading paths were systematically investigated in this thesis and the main contents are listed as follows:(1)Stress-controlled fatigue tests of super-elastic Ni Ti alloy micro-tubes under uniaxial tension,pure torsion,proportional and non-proportional tension-torsion cyclic loading conditions at different loading rates were carried out.The evolution features of transformation ratchetting in different loading cases were summarized,the influence of loading rate on the transformation ratchetting was discussed,and the tension-torsion anisotropic and rate-dependent transformation ratchetting of super-elastic Ni Ti alloy were revealed.At the same time,the dependence of transformation ratchetting on the loading order of tension-torsion under the non-proportional multiaxial cyclic loading conditions was discussed.(2)In the framework of small deformation,the total strain was decomposed into six parts:elastic strain,thermal expansion strain,martensitic transformation strain,martensitic reorientation strain,transformation-induced plastic strain and reorientation-induced plastic strain.The expressions of driving forces and thermal balance equation were derived through the first and second thermodynamics laws.The anisotropic correction factor based on the second invariant of deviatoric stress J₂ was introduced into the equations of transformation hardening and plastic slip resistances.As a result,a thermo-mechanically coupled cyclic plastic constitutive model was established to reasonably describe the rate-dependent multiaxial transformation ratchetting of super-elastic Ni Ti alloy.(3)From a series of stress-controlled fatigue tests and by analyzing the fatigue life and damage accumulation of super-elastic Ni Ti alloy,the dependence of fatigue life on the loading rate and loading order was revealed,and finally the primary factor affecting the damage accumulation and fatigue life was summarized.Further,the total damage was divided into the transformation-induced damage and plasticity-induced damage,respectively,and the damage evolution equation and fatigue failure model were established,which can reflect the influences of loading path and loading rate on the damage and fatigue life.The rate-dependent damage evolution rule of super-elastic Ni Ti alloy under uniaxial tension,pure torsion and multiaxial loading conditions with different loading paths were predicted reasonably,and all the predicted fatigue lives are located within in the twice error band,further,most of them are located within in the 1.5 times error band.