Local Mechanical Response of Superelastic NiTi Shape-Memory Alloy Under Uniaxial Loading

Abstract

In this paper, we focus on the local mechanical response of superelastic NiTi SMA at different temperatures under uniaxial loading. In situ DIC is applied to measure the local strain of the specimen. Based on the experimental results, two types of mechanical response, which are characterized with localized phase transformation and homogenous phase transformation, are identified, respectively. Motivated by residual strain accumulation phenomenon of the superelastic mechanical response, we conduct controlled experiments, and infer that for a given material point, all (or most) of the irreversibility is accumulated when the transformation front is traversing the material point. A robust constitutive model is established to explain the experimental phenomena and we successfully simulate the evolution of local strain that agrees closely with the experimental results.

A digital image correlation study of a NiTi alloy subjected to monotonic uniaxial and cyclic loading-unloading in tension

“…Since then, researchers have routinely applied DIC to characterise strain fields during the mechanical testing of thermo-mechanically processed NiTi alloys [3,[9][10][11][12][13][14]. For example, DIC investigations revealed the development of a region of strain concentration upon deviation from linearity towards the end of the elastic region; suggesting the initiation of phase transformation just prior to the onset of the macroscopic stress plateau [1].…”

“…For example, DIC investigations revealed the development of a region of strain concentration upon deviation from linearity towards the end of the elastic region; suggesting the initiation of phase transformation just prior to the onset of the macroscopic stress plateau [1]. Thereafter, further localisation in the strain fields results in Lüders/transformation band(s) that propagate along the gauge length with higher macroscopic tensile strains [1,3,11,12,[14][15][16]. The bands are typically inclined at ~60° with respect to the loading direction [12,[17][18][19][20][21] and divide the gauge length into domains with varying local axial strain extrema comprising fully martensitic and as-yet untransformed B2 regions [11,12,14,15].…”

“…While a single band propagates along the gauge length during monotonic uniaxial tension, multiple criss-crossing bands develop during both cyclic loading-unloading in tension and fatigue tests [1,9,11,13,15,19]. However, the mechanism behind the forward propagation of the mobile front of single bands along the gauge length and the switching from single to multiple criss-crossing bands is unclear.…”

Applying Full-Field Measurement Techniques for the Thermomechanical Characterization of Shape Memory Alloys: A Review and Classification

“…Example 4: Comparing at Different Temperatures the Strain Fields Measured with DIC in a Superelastic NiTi Thin Sheet [39] In experimental mechanics, DIC is mainly used to observe 2D displacement and strain fields on the outer surface of flat specimens, so there are many such examples in the SMA literature. Ref.…”

A comparative study of a NiTi alloy subjected to uniaxial monotonic and cyclic loading-unloading in tension using digital image correlation: The grain size effect

“…Superelastic NiTi shape memory alloys are well-known to exhibit a reversible, stress-induced transformation between the cubic B2 (austenite) and the monoclinic B19′ (martensite) phase when repeatedly loaded and unloaded within the stress plateau region of their macroscopic stress-strain curves [1][2][3][4][5][6]. One method widely combined with mechanical testing to study such reversible transformation behaviour in NiTi alloys [1,3,[7][8][9][10][11][12] is digital image correlation (DIC); an in-situ optical correlation technique that allows calculating the surface strain by tracking the displacement of random speckle patterns created along the sample gauge length. Thus, DIC is capable of characterising heterogeneous deformation [13][14][15].…”