Study Of The Dynamic Mechanical Properties Of Shape Memory Alloy

Shape memory alloy(SMA)is a kind of material, in which phase transformation will take place at certain temperature and stress. Due to their particular properties, such as shape memory effect, pseudoelastisity and ferroelasticity, shape memory alloys have been receiving increasing attention in recent years, and are extensively used in many fields, such as industry, aviation, national defense, instruments and medical devices, etc.The compressive response of a NiTi shape memory alloy is investigated at various initial temperatures, over a wide range of strain rates, using a material test system and a split Hopkinson pressure bar technique. It has been found out that the mechanical behavior of NiTi alloy with martensitic state shows strain rate sensitivity. The phase and dislocation yield stresses grow with the increase of the strain rate. The work hardening coefficients on the stages of the phase deformation and the dislocation plastic flow do not change with an increase of the strain rate. The slope of the first apparently elastic part of a stress-strain curve has a little increase for strain rates in the range 5×10^-4～2.3×10³s^-1, while the slope of the second apparently elastic stage of the deformation monotonously grows with an increase of the strain rate. The stress-induced martensite transition stress and the yield stress of the parent austenite decrease with the increasing initial temperature. The role of reverse stress-induced martensite transformation through unloading curves is studied.The 1D strain shock compression experiments of NiTi alloy are performed using the single stage gas gun equipment, and the spall strength is obtained. Two-wave structure is not observed in the experimental profiles of pressure wave.We study the microstructure of the NiTi shape memory alloy by SEM. During the compression of specimen of the NiTi alloy, it's fracture character is along the 45°direction of the loading. The microstructure of the spall target, shows a mixed mode failure. Indeed, ductile void growth was noted as well as inter- and transgranular failure.As a smart material, the study of constitutive relation is critical for its application and development. A constitutive model that includes the effects of elasticity, crystal reorientation (twinning or detwinning), phase transformations, and plasticity is presented. The model is applied to a NiTi alloy, which is known to exhibit the shape memory effect. The inelastic strains for phase transformation and reorientation are based on the material's phase diagrams. The model captures the effects of martensitic transformation and reorientation; both of the phenomena are coupled with the alloy's phase diagrams, which are defined in terms of temperature, hydrostatic pressure, and shear stress. Computational simulations are presented that demonstrate the ability of the model to capture the relevant physical response of the material. The model replicates the experimental data for the broad range of strain rates and temperatures.