Micro-macro Constitutive Model For NiTi Shape Memory Alloys And Its Applications

NiTi shape memory alloys (SMAs) have been receiving increasing attention andapplications in the past two decades because of their unique shape memory effect(SME), pseudoelasticity (PE) and good biocompatibility. During employment SMAsinevitably undergo complicated multiaxial loading, which was rarely conducted in theprevious research. It was found that the pseudoelastic property under tension ismarkedly different from that under compression, which can be attributed to theformation the different martensitic variants. The martensitic transformation undermultiaxial loading is more complex. Therefore, it is necessary to investigate theproperties of SMAs under multiaxial loading, and establish the constitutive model forthe thermodynamics behavior based on the characteristics of martensitic variants. It cannot only enrich the research of constitutive relations, but also be of significance forapplications of SMAs.In this paper, the constitutive behavior of a NiTi SMA is investigated, the mainresearch work and results are listed as follows:①The PE of polycrystalline NiTi SMA subjected to strainig by various biaxialtension/compression-torsion paths is investigated experimentally. It shows that thepseudoelastic properties of the material under different biaxial loading paths aremarkedly different. The experimental results may procide a foundation for modeling theconstitutive behavior under multiaxial loading.②A micro-macro constitutive model is proposed base on the work by Gall et al.,taking into account different elastic properties of austenite and martensite. The volumefractions of the24martensite variants are taken as the internal variables, an interactionmatrix is introduced to describe the interaction between the martensitic variants, basedon which, the constitutive model for a singlecrystal is obtained. And the constitutivemodel for a polycrystalline material can then be obtained by assembling the singlecrystals with different orientations with finite element method (FEM). The typicalthermodynamics characteristics of the NiTi SMA are described with the constitutivemodel, such as the tension-compression yield asymmetry, strain rate effect, effects oftemperature and multiaxial pseudoelasticity. The simulated results show that thepseudoelasticity is closely related to the volume fractions of the martensitic variants, thestrain rate effect is mainly induced by the latent heat of phase transformation, the forward and reserved transformation critical stresses increase obviously with theincrease of temperature.③The fraction of twinned-martensite is introduced, incorporating the correctedstress-temperature phase diagram, and assuming that the mechanism of reorientation ofmartensitic is same as phase transformation, a constitutive model is obtained, which candescribe SME phenomenon. In addition, according to the experimental results oftwo-way SME, the amplitude of transformation strain is corrected to describe thetwo-way SME at different stress levels. The constitutive model considering the twinmartensite can describe the SME of the NiTi SMA under uniaxial and multiaxial loading,it proves that assuming the mechanism of the martensitic reorientation is the same asthat of the phase transformation.④The initial investigation of an intravascular stent is conducted with the proposedthe constitutive model, the Mises equivalent stress and the distribution of martensite inthe process of compression/self-expansion, as well as the relationships between theradial force and the stent diameter are obtained. The results are significant for the designand the optimization of the intravascular stent.