Investigation of local strains of the superelastic shape memory alloy rods for tensile loading-unloading cycles

Abstract

This dissertation studies mechanical behaviours of the superelastic shape memory alloy (SMA) rods in terms of local deformations and time via tensile loading-unloading cycles for different end constraints. Most of the applications and researches on this unique functional material have .been mainly centered to its property of superelasticity , that is the ability to recover the original shape from the deformed state when the mechanical load, which causes the deformation, is withdrawn. Besides this unique property, the material's (superelastic SMA) behaviours when it is in mixed-phase condition upon loading and unloading, also need careful attention with a view of investigating on the local deformation of the structural elements made of the same materials. With this perspective, the so called stress-relaxation tests have been performed to demonstrate and investigate the unique local strains- total strains relationships particularly during the forward and reverse stress induced martensitic transformations (SIMT). Several experiments have been carried out to investigate the relation of stress and strain with time. Changing behaviours ofthe local strains during mixed phase conditions of the super elastic SMA rods have also been studied under dead weight for some specific period of time. With this view, creep tests at room temperature have been performed and for comparison similar test results are obtained for the traditional material like mild steel. . Some remarkable phenomena have been observed pertaining to SIMT, which are absent in traditional materials like mild steel and stainless steel and those unique phenomena have been explained qualitatively.