Computational and experimental analyses of martensitic transformation propagation in shape memory alloys

Abstract

This study investigates the creation of heterogeneous martensitic transformation field in shape memory alloy structures with geometrical nonuniformity through experiments and finite element computational analysis.Geometrically graded superelastic NiTi thin plates with series and parallel design configurations with respect to the loading direction have been created. The nonuniform transformation evolutions within these structures are presented by thermal images captured by an infrared camera during tensile experimentation. It is found that the transformation propagation is along the loading direction in a sample with the series design configuration unlike that in a sample with the parallel design configuration. Also, the geometrically graded structures exhibit stress gradient over stress-induced transformation. The transformation propagation and the deformation behaviour of these structures are presented by numerical modelling which is in good agreement with experimental data.

Experimental and numerical data for transformation propagation in NiTi shape memory structures

“…It is expected that the transformation propagates along the loading direction from the narrow end to the wide end as illustrated in Ref. [1]. Fig.…”

“…1(b) of Ref. [1]. The material exhibited perfect pseudoelastic deformation behaviour at the room temperature with the forward transformation stress of ∼330 MPa and the transformation strain of ∼0.06.…”

“…2(b) of Ref. [1].

Video 1The martensitic transformation propagation within a NiTi structure with uniform geometry during tensile loading.

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“…1(b) of Ref. [1]). In the following subsections, the local stress and strain fields of different types of geometrically graded NiTi structures are presented.…”

A mechanical criterion for Lüders-type deformation of polycrystalline NiTi

“…The Lüders deformation of NiTi has also been studied by various means in addition to mechanical testing. Video cameras [1,[18][19][20], infrared cameras [1,[21][22][23][24] and digital image correlation (DIC) technique [12,15,23,[25][26][27][28] have been used to observe the macroscopic behaviour of Lüders bands in situ during mechanical testing. Synchrotron X-ray diffraction [29][30][31][32], neutron diffraction [33] and electron backscatter diffraction (EBSD) [34] techniques have also been used to elucidate the phase transformation and microscopic metallurgical processes of the Lüders-type deformation.…”