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Asymmetric Tension/Compression Response of NiTi and its Effect on Bending of Tubes

Nathan Bechle (University of Texas at Austin), Stelios Kyriakides (University of Texas at Austin)

Instability in Solids and Structures

Mon 9:00 - 10:30

Barus-Holley 190

Nearly equiatomic NiTi can undergo reversible solid-to-solid stress induced phase transformation at room level temperatures. In tension the transformation leads to localized deformation of several percent that tends to spread at nearly constant stress and deformation is recovered on unloading––pseudoelastic behavior. By contrast, under compression the transformation strains are smaller, the stress is higher, the response is monotonic, and the deformation is homogeneous. This study aims to understand how this complex asymmetric material behavior affects structures. To this end NiTi tubes subjected to pure bending under rotation control at constant temperature. It is found that phase transformations lead to moment plateaus during both loading and unloading. On loading the nucleation of martensite leads to the coexistence of high curvature for the transformed sections and low curvature for the untransformed sections. The high curvature spreads gradually while the moment remains nearly constant. A lower moment plateau is traced on unloading with similar localized bending. This coexistence of homogeneous and inhomogeneous deformation is highly temperature dependent and further complicated by the latent heat of transformation.