Consitutive Model Study And Its Finite Element Application Of Superelastic NiTi Alloy

NiTi shape memory alloys have many engineering applications including in bio-medical devices and implants, aerospace and spaceflight structures, micro-electro-mechanical systems and the field of tribology due to its unique super-elasticity, shape memory effect and good biological compatibility. These structure components are often subjected to complex loading including high temperature, high stress and thermo-mechanical cyclic loading due to their special engineering application field. The plastic deformation occurs in the super-elastic NiTi alloy at high temperature or with high stress, which will partly degrade the super-elastic effect. Also, the stress-induced martensitic transformation and its reverse transformation occur in super-elastic NiTi alloy under the cyclic tension-unloading conditions. With the increasing number of cycles, the material parameters and phase transformation behaviors will change, which will sharply degrade the super-elastic effect of the alloy. However, the referable results are mainly obtained from the strain controlled tension-unloading or cyclic loading and hardly involve plastic deformation at high temperature or with high stress and cyclic deformation under stress-controlled cyclic loading. In order to improve the design of shape memory alloy components and to assess their reliability, it is necessary to carry out the experimental observations about the deformation behaviors of the super-elastic NiTi alloy under the stress-controlled cyclic loading. Moreover, it is important to establish a constitutive model to describe the plasticity and transformation ratcheting behavior of the NiTi alloy.The following conclusions on the super-elasitc NiTi alloy were obtained in this paper:1. Detailed experimental observations were carried out for super-elastic NiTi alloy under the stress-controlled cyclic loading conditions with four different loading waveforms. It is shown from experimental results that super-elastic NiTi alloy presents apparent transformation ratcheting, such as peak strain and valley strain increase with the increasing number of cycles; the transformation stresses and dissipation energy decrease apparently during the stress-controlled cycling; the valley strain increases more quickly than the peak strain with the increasing number of cyclies; the transformation ratcheting and its evolution depend greatly upon the applied stress level and loading chart. Some significant conclusions are obtained, which are helpful for establishing cyclic constitutive model describing the transformation ratcheting of the super-elastic NiTi alloy.2. In the framework of general plasticity, based on the existing experimental results, a super-elastic constitutive model considering plasticity is proposed. Further, implicit stress intergration and new expressions of consistent tangent moduli are derived for the transformation and plasticity, and the proposed model was implemented into the finite element software ABAQUS by user-subroutine UMAT. Comparing with the simulations by the proposed model and from the structure samples verifies the reasonability of implementation.3. A new constitutive model describing the transformation ratcheting of super-elastic NiTi alloy is proposed. The capability of the proposed model to predict the transformation ratchetting of the NiTi shape memory alloy was verified by comparing the simulated results with the corresponding experimental ones. Further, the proposed model was also successfully implemented into the finite element software ABAQUS.4. Based on the dimensional analysis and finite element calculation, the process of indentation was disussed with help of the implemented model considering plasticity, and unusual scaling relationships between hardness, critical loading and material propoties were obtained.