| Due to excellent properties of super-elasticity,damping resistance and shape memory effect,super-elastic NiTi alloy can be used as shock absorbers in high-rise buildings,vascular scaffolds in biomedicine,and fasteners in aerospace.In actual service conditions,super-elastic NiTi alloy is often subjected to mechanical loadings at different temperatures and loading histories,leading to a super-elastic degradation of the material,that is,the transformation ratchetting behavior.The transformation ratchetting behavior can lead to functional and structural fatigue failures of the material.However,most of the existing studies focus on the transformation ratchetting behavior of super-elastic NiTi alloy at a constant temperature,and there are few studies considering the influences of temperature and thermo-mechanical loading history on the transformation ratchetting behavior.Therefore,it is necessary to conduct experimental and constitutive model studies of superelastic NiTi alloy at different temperatures and loading histories.The results will be helpful for the structural design and reliability evaluation of NiTi-based devices.The main contents of this work are summarized as follows:(1)The transformation ratchetting behavior of super-elastic NiTi alloy at different temperatures was investigated.The effects of temperature and thermo-mechanical loading history on the transformation ratchetting behavior were discussed.The experimental results show that the transformation ratchetting behavior becomes more obvious with increasing the temperature.In the cooling condition,the transformation ratchetting behavior of the material will be promoted by the thermal-mechanical cyclic loading history,and it becomes more obvious with the higher the temperature before temperature change.Compared with the untrained material,the transformation ratchetting strain of the material decreases,and its evolution is more stable.In the heating condition,compared with the untrained material,the dissipation energy density of the material increases,and the transformation ratchetting behavior is more significant,indicating the promotion effect of thermo-mechanical training on the transformation ratchetting behavior.However,the fatigue life of the material after the training reduces,indicating that the thermo-mechanical cyclic loading can reduce the fatigue life of the material.(2)Based on the thermodynamic framework and experimental results,a cyclic constitutive model for temperature-dependent transformation ratchetting behavior was established.The transformation strain and residual strain were considered as internal variables,their driving stress expressions and rate forms were deduced from the free energy expression.The thermal expansion strain and the temperature-dependent equations of material parameters were introduced to consider the temperature dependence in the cyclic constitutive model.The evolution of the yield surface was proposed to simulate super-elastic degradation by considering the different evolution rates between the forward and reverse transformation stresses.An incremental form of piecewise hardening energy was proposed to capture the hardening behavior by considering the evolution of the hardening modulus during the martensitic transformation.The evolution equation of residual strain was proposed to simulate residual strain accumulation during the cyclic loading by considering the decay of accumulation rate.Finally,the temperature-dependent transformation ratchetting cyclic constitutive model was established.(3)Based on the established transformation ratchetting constitutive model,the consistent plastic multiplier and tangent stiffness matrix were derived.The user-defined material subroutine UMAT was compiled,and the constitutive model was implemented into the finite element software ABAQUS.The finite element models of a single element and simple structure were established to simulate the effects of temperature and thermomechanical loading history on the transformation ratchetting,and the mechanical response of the structure.The comparison between simulated and experimental results verifies the rationality of finite element implementation. |
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