Mechanical Properties And Phase Transition Waves Of NITI Alloy Under Combined Stresses

NiTi shape memory alloy is a typical phase transition material, phase transition can greatly affect the mechanical response and wave propagation properties of material and structure. In this paper, the quasi-static and dynamic phase transition behaviors of pseudoelastic NiTi alloy under combined stress are investigated, and the theoretical and experimental research of combined phase transition waves in thin-walled NiTi tubes are proposed.Phase transition behavior of NiTi shape memory alloy was studied systematically through quasi-static experiments under different loading paths (uniaxial compression and tension, pure torsion and proportional tension/compression-torsion) and different temperatures range from28℃to150℃. Experimental results show apparent pseudoelastic effect and tension-compression asymmetry. The initial phase transition stress increased with the rise of temperature; meanwhile the pseudoelastic effect and tension-compression asymmetry were gradually weakened. NiTi alloy show elastic-plastic behavior when the temperature reached a certain threshold, and the tension-compression asymmetry disappeared. Based on the data of critical phase transition points measured from the experiments, three phase transition criterions were discussed on σ-τ stress plane, and their applicability were compared respectively. Finally preliminarily discussed the calculated method of shear stress and shear strain during the torsion of NiTi circular bar, and revised the experiment data of combined loading.The quasi-static and dynamic phase transition behaviors of pseudoelastic NiTi alloy under compression-shear loading were studied through a new combined compression-shear technique based on split Hopkinson pressure bar. The data processing methods were studied and verified by quasi-static experiment of NiTi alloy. A series of experiments of NiTi alloy were performed at different impact velocities and different loading angles, and the dynamic equivalent pressure curves were given. The initial phase transition points and the phase transition surfaces predicted by a dynamic phase transition criterion were given. The results show that the material was sensitive to the strain rate, and the stress of initial phase transition increased with the strain rate during390-860s-1. By a high resolution camera and high-speed photography following the quasi-static and dynamic experimental process respectively, there was no relative slip between the specimen and the beveled ends from the comparison of the photographs of the experiment, which confirmed the validity of the experiment method.The incremental constitutive relation and governing equations with combined stresses for phase transition wave propagation in a thin-walled tube were established based on the phase transition criterion considering both the hydrostatic pressure and the deviatoric stress. It was found that the centers of the initial and subsequent phase transition ellipses were shifted along the σ-axis in the σ-τ plane due to the tension-compression asymmetry induced by the hydrostatic pressure. The wave solution offered the "fast" and "slow" phase transition waves under combined longitudinal and torsional stresses in the phase transition region. The results show some new stress paths and wave structures in a thin-walled tube with phase transition, differing from those of conventional elastic-plastic materials.The split combined compression torsion Hopkinson bar was designed and built based on SHPB, and experiment technique of pre-torqued tubes under longitudinal impact was achieved. The combined stress wave of strainless steel tube was measured, and the quick wave and slow wave of the strain signal was quantitative analysed by elastic-viscoplastic models, which show the experiment technique was feasible. The combined stress and strain signals of pre-torqued tube under impact were calculated by the material parameters of NiTi. The final stress rate in and out the initial phase transition surface were achieved during the combined loading of NiTi tube, and the property of combined phase transition was analysed. The separate of quick phase transition waves and slow phase transition waves was not clear at present experiment, but has a good agreement with the theoretical model. As the NiTi tube have several problems such as:the initial phase transition stress is too high, the crack strain is low, and the tension-compression asymmetry. To obtain desired signals of combined phase transition waves, it must be improved the compression-torsion loading into tension-torsion loading or decreased the initial phase transition stress through heat treatment.