A Constitutive Description For Cyclic Pseudoelasticity Of Niti Shape Memory Alloy

NiTi shape memory alloys (SMAs) have been receiving increasing attention andachieving increasing application in many fields, such as medicine, automobile, aviationand aerospace, due to their excellent pseudoelastic property. In practical application,NiTi SMAs will inevitably be subjected to multiaxial cyclic loading，which mayseverely affect the constitutive behavior, the damage and degradation, the invalidationas well as the life of the materials. Thus a realistic constitutive description for thepseudoelastic behavior of such materials under non-proportional cyclic loading is oftheoretical and application significance.In this thesis, based on the analysis of the experimental results of NiTi SMAssubjected to cyclic and biaxial loading, and making use of the constitutive descriptionfor SMAs involving coupled transformation and plastic deformation, the correspondingnumerical algorithm and the program are developed for the analysis of the response ofthe pseudoelastic behavior of SMAs subjected to cyclic and biaxial loading. Theresponses of a NiTi SMA subjected to cyclic tension, biaxial proportional andnonproportional tensile-shear straining are computed and compared with theexperimental results, respectively. The satisfactory agreement between the computedand the experimental results demonstrates the validity of the proposed approach. Themain progress achieved in this thesis is listed as follows:1.The constitutive characteristics of NiTi SMAs subjected to cyclic tension areanalyzed, including transformation stress, recoverable strain, residual strain, plasticstrain, dissipated work density and recoverable energy density, etc. It showed thatplastic strain increases with the increase of the maximum applied strain, and the plasticstrain distinctly affects the residual strain, transformation stress, maximum stress, andthe dissipated work density in the subsequent cyclic process. With the increase ofnumber of cycles, the recoverable strain, the transformation stress and the dissipatedwork density will tend to their saturated states.2.Combined with the experimental results, a reasonable explanation is suggested atthe micro level for reduction of the forward/inverse transformation platforms and thedeviation of the starting points in the stress-strain curve with the increase of the numberof cycles.3.The typical responses and the characteristics of NiTi SMAs subjected proportional and non-proportional straining by different paths in the biaxial tensile andshear strain plane are numerically analyzed, respectively, the effects of maximumequivalent strain is also investigated. Significant differences are found though in all thecases involved the NiTi SMAs exhibit distinct pseudoelasticity.The computed results are compared with the experimental results, and thesatisfactory agreement demonstrates the validity for the proposed approach to beapplied in the analysis of the pseudoelastic behavior of the NiTi SMAs subjected tocyclic and biaxial loading.