Local and global strains and strain ratios in shape memory alloys using digital imagecorrelation

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

“…5a, 6a, 7a, and 8a. Choosing three material [21], and Bewerse et al [22], the sharp transition between high-strain and low-strain regions is attributed to transformation fronts associated with the local martensite transformation, and thus regions undergoing different magnitudes of local strain (large vs. small) are corresponding to regions with different phases (martensite vs. austenite). As is demonstrated in Fig.…”

“…5d, several declining transformation boundaries, which are also referred to as Lüders deformation bands (LDBs), pass through the width of the sample. In light of the in situ EBSD studies of Mao et al [21,22], it is pointed out that LDB does not purely keep to the principal of the maximum shear stress, but is possibly a result of the interaction between the mechanics and the martensite crystallography. In the present experiments, the declining angle of the LDB ranges from 57°to 62°, which agrees with the detailed studies indicating that shear angle of the LDB varies from 48°to 61° [23][24][25].…”

Stress induced martensite at the crack tip in NiTi alloys during fatigue loading

“…Even though these numerical methods represent useful design tools to simulate the macroscopic response of simple or complex SMA based systems, special care should be taken when they are used to study the local effects in the proximity of high stress concentration regions. In particular, it was demonstrated that the high values of local stresses arising in the crack tip region of NiTi alloys cause stress-induced phase transition mechanisms [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30], which significantly affect the crack tip stress distribution and, consequently, the crack evolution under both static and fatigue loading conditions. Despite the increasing number of research activities on fracture and fatigue of NiTi alloys in recent years, much effort should be devoted for an effective understanding of the role of the phase transformations in the crack formation and propagation mechanisms and in the stress state generated at the crack tip.…”

“…Within this context, the development and application of full field techniques to analyze the local transformation mechanisms near geometrical discontinuities and, in particular, in the crack tip region, represent a highly challenging scientific goal. For this purpose, synchrotron X-ray micro-diffraction (XRD) [13][14][15], infrared thermographic (IR) [16] and Digital Image Correlation (DIC) [18,19] techniques were recently applied, to better understand the mechanisms of phase transformation in the notch and/or crack tip proximity. In particular, a pseudoelastic NiTi alloy for medical applications were analyzed in [13] by using miniature compact-tension (CT) specimens, which were directly obtained from thin-walled tubes, similar to those used for manufacturing self-expanding stents.…”

“…Strain and texture evolution near the crack tip of a martensitic NiTi alloy were analyzed in [14], by synchrotron X-ray experiments, after fatigue crack propagation in a compact-tension (CT) specimen; it was found that texture evolution is mainly due to detwinning, the main deformation mechanism in martensitic NiTi alloys. Both martensitic and austenitic alloys were analyzed in [15], by using miniaturized CT specimens [19] after fatigue crack propagation, which revealed the presence of detwinned martensite at the crack tip of both martensitic and austenitic specimens. An austenitic NiTi alloy was analyzed in [17] by DIC analysis of thin edge cracked specimen, which allowed direct measurement of the crack tip strain field related to stress-induced transformation.…”