Virtual Extensometer Analysis of Martensite Band Nucleation, Growth, and Strain Softening in Pseudoelastic NiTi Subjected to Different Load Cases

Abstract

Pseudoelastic NiTi shape memory alloys exhibit different stress–strain curves and modes of deformation in tension vs. compression. We have recently shown that under a combination of compression and shear, heterogeneous deformation can occur. In the present study, we use digital image correlation to systematically analyze how characteristic features of the nominally uniaxial engineering stress–strain curves (particularly the martensite nucleation peak and the plateau length) are affected by extensometer parameters in tension, compression, and the novel load case of shear-compression. By post-experimental analysis of full surface strain field data, the effect of the placement of various virtual extensometers at different locations (with respect to the nucleation site of martensite bands or inhomogeneously deforming regions) and with different gauge lengths is documented. By positioning an extensometer directly on the region corresponding to the nucleating martensite band, we, for the first time, directly record the strain-softening nature of the material—a specific softening behavior that is, for instance, important for the modeling community. Our results show that the stress–strain curves, which are often used as a basis for constitutive modeling, are affected considerably by the choice of extensometer, particularly under tensile loading, that leads to a distinct mode of localized deformation/transformation. Under compression-shear loading, inhomogeneous deformation (without lateral growth of martensite bands) is observed. The effects of extensometer gauge length are thus less pronounced than in tension, yet systematic—they are rationalized by considering the relative impact of differently deforming regions.

A mechanical criterion for Lüders-type deformation of polycrystalline NiTi

“…With these thoughts, we hypothesize that elastic contraction of the sample outside the band is the source of this "additional" elongation required for the formation of the Lüders band. This is consistent with the observed stress drop [1] and elastic strain drop [25] observed at the initiation of a Lüders band. In addition, it is also known that elastic deformation occurs by the same mechanism of lattice waves of atoms, i.e., also at the speed of sound in the solid, thus has the intrinsic ability to match in speed with the martensitic transformation.…”

“…The initiation of the band is associated with a stress drop, as commonly observed. Associated with the stress drop is an elastic strain reduction of the austenite part outside the newly formed Lüders band, as reported by Elibol and Wagner [25] and later by Shariat et al [26]. During the propagation of the Lüders band, the strain inside the band remains relatively constant at a high value corresponding to the end of the global stress plateau whereas the strain outside the band also remains constant at a low value corresponding to the beginning of the stress plateau [4,5,23].…”

“…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.…”

Eş Kanallı Açısal Pres (EKAP) Yöntemi ile Şekillendirilmiş Ti-6Al-4V Alaşımının Mekanik Özellikleri ve Makroskobik Deformasyon Davranışı Arasındaki İlişki

“…DIC fotoğraflarının proses edilmesinde ve analizinde (lokal deformasyon ve yüzey gerinim alanları bağlamında) DIC yöntemine entegre ARAMIS yazılım paketi kullanılmıştır (GOM 2010). DIC yöntemi ile, çekme deneyleri esnasında ölçülen ve kaydedilen gerinim alanlarının daha sonra sisteme entegre bu yazılım vasıtası ile işlenmesinde sanal ekstansometre analizi uygulanarak mühendislik gerinimleri yüksek hassasiyetle hesaplanmıştır (sanal ekstansometre yöntemine dair detaylı bilgi için bkz Elibol and Wagner 2018)…”