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Structural Engineering and Mechanics
  Volume 22, Number 5, March30 2006 , pages 563-574

Unique local deformations of the superelastic SMA rods during stress-relaxation tests
Muhammad Ashiqur Rahman and Mujibur Rahman Khan

    This paper studies mechanical behavior of the superelastic shape memory alloy (SMA) rods in terms of local deformations and time via tensile loading-unloading cycles for both ends fixed end constraints. Besides the unique stress induced martensitic transformation (SIMT), SMA? time dependent behavior when it is in mixed-phase condition upon loading and unloading, also need careful attention with a view of investigating the local deformation of the structural elements made of the same material. With this perspective, the so-called stress-relaxation tests have been performed to demonstrate and investigate the local strains-total strains relationships with time, particularly, during the forward SIMT. Some remarkable phenomena have been observed pertaining to SIMT, which are absent in traditional materials and those unique phenomena have been explained qualitatively. For example, at the stopped loading conditions the two ends (fixed end and moving end of the tensile testing machine) were in fixed positions. So that there was no axial overall deformation of the specimen but some notable increase in the axial local deformation was shown by the extensometer placed at the middle of the SMA specimen. It should be noted that this peculiar behavior termed as ?nertia driven SIMT?occurs only when the loading was stopped at mixed phase condition. Besides this relaxation test for the SMA specimens, the same is performed for the mild steel (MS) specimens under similar test conditions. The MS specimens, however, show no unusual increase of local strains during the stress relaxation tests.
Key Words
    SMA (Shape Memory Alloy); SIMT (Stress Induced
Martensitic Transformation); stress relaxation
; local strain; overall strain; inertia driven SIMT
Department of Mechanical Engineering, Bangladesh University of Engineering & Technology, Dhaka 1000, Bangladesh

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