Fatigue resistance of branching phase-transformation fronts in pseudoelastic NiTi polycrystalline strips

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

“…4a, the transformation bands are illustrated on the axial strain contours. At 0.0190 macroscopic strain, the upper and lower mobile bands are inclined at angle η of ~60° to the sample axial direction, which is in accordance with previous reports in the literature [10,17,18,[47][48][49][50]. A local kink of ~2° is highlighted at the intersection of the upper mobile front and the sample edge.…”

“…-a new band with an opposite inclination angle compared to a pre-existing band) at the mobile front of the preexisting strained transformation band. This corresponds to macroscopic stress relaxation and subsequent stress plateau as well as a straightening of the sample edge [16][17][18]22]. It was observed that retained transformation bands of higher local strains accumulate through the gauge length upon unloading, and the macroscopic stress-strain curve changes from plateau type to curvilinear elastic with successive cycles.…”

“…During uniaxial monotonic tension, DIC strain measurements indicated the initiation of phase transformation towards the end of the elastic region [1,16]. The typically inclined transformation band(s) sub-divide the gauge length into domains with either local strain maxima (B19′) or minima (B2); while the band(s) accommodate continuous transition of strains between the B2 and B19′ domains [1,4,10,11,16,17]. Local kinks occur at the intersections of the mobile band(s) and the sample edges and diminish along with the widening of the mobile band fronts upon further straining [16,18].…”

“…In the case of cyclic loading-unloading with small strain increments per cycle, DIC and high resolution DIC observed that the early development of single inclined band(s) gives way to multiple criss-crossing transformation bands [7,16,17,[19][20][21]. Zhang and He [17] pointed out that the band patterns depend on the sample geometry and structure. The observed widening of a single inclined band front during monotonic tension and the formation of multiple criss-crossing patterns during cyclic loading-unloading were studied in terms of shear strain [18,22].…”

Full-field microscale strain measurements of a nitinol medical device using digital image correlation

“…Indeed, DIC has been used previously for validating finite element (FE) biomechanics models of femoral [25], hemipelvic [26], craniofacial [27], and dental [28] implants as well as replica [29] and cadaveric [30,31] femurs. Digital image correlation has also been used to investigate the mechanics of polycrystalline nickel titanium (nitinol) starting in the mid 2000s [32,33], with interest rapidly increasing since that time (e.g., [34][35][36][37][38][39][40][41][42][43][44][45][46]; Figure 1). However, direct strain measurement on finished nitinol cardiovascular devices using either strain gauges or full-field DIC has been impractical to date due to limitations of scale, with typical strut cross-sectional dimensions measuring only a few hundred microns as illustrated in Figure 2.…”