Local Deformation Characteristics of the Shape Memory Alloy Rods during Forward and Reverse Stress Induced Martensitic Transformations

This study deals with the local strain–overall strain relationship of the superelsatic shape memory alloy (SMA) rods observed for consecutive tensile loading– unloading cycles. The local strains are measured by an extensometer and also by strain gages. On the other hand, the total strains of the specimens are measured from displacements of the loading machine's fixture. Test results show that during the stress induced martensite transformation (SIMT), the local strains of the mid portion are significantly different from those of the overall strains of the specimen. This phenomenon is so distinct that it appears the start and finish points of the forward SIMT and as well as the reverse SIMT can be identified by simply plotting midpoint local strain versus the over-all strain of the specimens. This unique local deformation pattern is explained elaborately in terms of the critical stress barrier. The critical stress necessary to initiate SIMT is found to be higher for the mid portion of the specimen than it is for any other portion of the specimen. It is shown from this study that local stress concentration is mainly responsible for the above mentioned unique phenomena that are independent of the specimen size and gage length. The local strain, however, should be measured by highly sensitive strain gages to observe the above mentioned phenomena.