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Recent tension–torsion test results on nano-grained NiTi shape memory alloy micro-tubes are reported in this paper. It is discovered that: (1) during uniaxial tensile loading, the stress-induced transformation in the micro-tubes is realized by the initiation and growth of a macroscopic spiral martensite band with a quite sharp austenite–martensite...
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Context 1
... effect of applied shear stress on the tensile response as well as on the changes in the morphology of the tube surface; (2) to obtain the initial transformation stress locus (see Section 4.3) under the biaxial stress state. The tensile stress-tensile strain curves at different given shear stresses (constant values of applied torque) are shown in Fig. 7. It is seen that, at relatively low levels of shear stress, the responses of the tube in the axial direction are unstable as demonstrated by the small stress peak (load drop) and the subsequent stress plateau in the tensile stress-tensile strain curves. The tube surface morphology observation indicates that the transformation is still ...
Context 2
... in Figs. 3 and 4 (uniaxial tension). No variation in band orientation with the applied shear stress is observed at these relative low shear stresses. With further increase in the shear stress level, the above localized deformation mode becomes less and less dominant and finally the stress-strain curves become monotonically hardened as shown in Fig. 7. The transformation strain distribution over the tube tends to be more homogeneous and band morphology with a diffusive A-M interface can be observed on the tube surface. Eventually, no band can be observed on the tube surface and deformation becomes homogeneous along the axial ...
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Citations
... Stress-induced martensitic transformation in pseudoelastic NiTi appears (typically, in tensiondominated loadings) as localized instabilities in the form of martensite bands, and subsequently progresses via propagation of the instabilities in the form of patterned interfaces (macroscopic transformation fronts) that separate the domains of low-strained austenite and high-strained martensite (e.g., Shaw and Kyriakides, 1997b,a;Sun and Li, 2002;Reedlunn et al., 2014). Due to the high strain incompatibilities that exist within the transformation front and the ensuing large local stresses, it can be reasonably inferred that propagating instabilities can vitally influence both the functional fatigue and structural fatigue of the material. ...
... It has been the prevailing view that NiTi undergoes a homogeneous transformation under shear and shear-dominated loading. This was first demonstrated by Sun and Li (2002) who investigated the transformation morphology of NiTi tubes under combined non-proportional tension-torsion by using optical microscope. The authors have also found that the transformation mode gradually changes from localization to homogeneous as the loading alters from tension-dominated to shear-dominated. ...
... Of exceptional interest was the observation of transformation localization in the form of longitudinal (almost parallel to the tube axis) transformation bands under torsion and torsiondominated loading. Interestingly, despite the localized nature of the transformation, the global shear response remained monotonic and smooth, without any indication of localization effects, thus posing an apparent contradiction to the common consensus (note that the experimental setup employed by Reedlunn et al. (2020a) differs to some extent from that of Sun and Li (2002)). In addition to this, the observed pattern exhibited unique characteristics that further distinguish it from its rival pattern under tension, including a mild strain variation (2.8%-3.5%) between the low-strained and highly-strained domains and no propagation of the bands. ...
... It has been the prevailing view that NiTi undergoes a homogeneous transformation under shear and shear-dominated loading. This was first demonstrated by Sun and Li (2002) who investigated the transformation morphology of NiTi tubes under combined non-proportional tension-torsion by using optical microscope. The authors have also found that the transformation mode gradually changes from localization to homogeneous as the loading alters from tension-dominated to shear-dominated. ...
... The Lüders-type deformation behaviour of NiTi has been observed to occur only in tension, and not in compression [11,12], shear [11,13], or biaxial tension [14]. Some studies reported the observation of strain localisation in simple shear [15] and torsion of thin wall tubes [16] and wires [17], but in all these cases the stress response increases monotonically with strain without the signature upper-lower yielding and the stress plateau of the Lüders deformation. ...
... Many studies have demonstrated the absence of Lüder-type deformation in such samples. Reedlunn et al.[16] conducted a complete tension-compression-torsion test of thin wall tube samples, following an earlier work by Sun and Li[13]. Both works demonstrate clearly the formation of a Lüders band under tension and the disappearance of the Lüders band when shear deformation becomes dominant.This also suggests that absence of Lüders-type deformation in shear cannot be argued for the lack of sufficient sample gauge length.Another interesting case is stress-induced martensitic transformation of NiTi in bending mode[12,50,51]. ...
... As z continuously increases from 0 to 1, Eq. (37) leads to a unique solution which is given Figure 9, the existence of a stress gradient under torsional loading will make the phase transformation takes place step by step from the surface to the center at a smaller scale, which will behaves more stable than under tension loading. This agrees well with the experimental observations reported in Sun & Li (2002). Therefore, if the multiaxial loading can introduce higher stress gradients inside the material, the transformation instability will be reduced. ...
Transformation-induced plasticity (TRIP) in pseudoelastic polycrystalline NiTi shape memory alloys (SMAs) is investigated through tensile tests combined in-situ infrared and digital image correlation (DIC) observations, and a new TRIP concentration phenomenon is revealed. To this end, an instability evolution model for solid–solid phase transformation is established to explain the multi-scale mechanisms of TRIP in SMAs. It is shown that, during the stress-induced transformation, the local stress at the grain scale evolves discontinuously inside austenite-martensite mixtures, presenting the maximum magnitude at the austenite side of the transformation interfaces. This local stress field triggers TRIP although the macroscopic stress still evolves “smoothly” with a relatively low magnitude. As the temperature increases, a further plasticity in austenite is required to lower the discontinuity at the austenite-martensite interfaces. Due to the accumulation of the heat generated in transformed regions, the temperature at the moving transformation interface continuously increases till reaching the maximum at the end of the transformation. Therefore, TRIP is concentrated at the final transformation band fronts, manifesting as concentration peaks in the residual strain map. The concentration of TRIP also shows a strong rate/frequency dependence: when the loading rate increases, TRIP accumulates to a higher average density throughout the transformation region while the concentration is significantly relieved.
... The retained dislocations can be treated as the preexisting dislocations for the next loading cycle. According to the phase field simulations, heterogeneous martensite nucleation prefers to occur around stress-concentration sites caused by dislocations [52]. Moreover, the retained dislocations facilitate the martensite nucleation by increasing the internal strain energy thus reducing the critical stress [34]. ...
Deformation and phase transformation of a cold-rolled, textured, shape memory alloy sheet Ni50.6Ti49.4 are investigated via real-time, in situ synchrotron X-ray diffraction and digital imaging correlation. The specimens are subjected to tensile loading along two orthogonal directions. Bulk stress–strain curves (macroscale), strain fields (mesoscale) and X-ray diffraction patterns (microscale) are obtained simultaneously. Martensite nucleation is highly correlated with the preexisting defects. Different from the widely observed Lüders bands within defect-free NiTi, uniform martensite phase transformation occurs in the cold-rolled alloy as a result of the relatively widely distributed preexisting defects, giving rise to progressive growth of martensite, relatively uniform strain fields, the sigmoidal stress–strain curves and increased strain hardening rate. Different favorably oriented martensite variants are induced due to the initial texture, and the martensite phase transformation is more active for loading along the less favorable orientation, giving rise to the anisotropy in the phase transformation.
... Where the ambient temperature is is the surface temperature in and h is the heat exchange coefficient of the ambient medium. It is known that the temperature field in the domain Ф is heterogeneous during the transformation of martensite due to the existence of subdomains and interfaces [21]- [22]- [23]. ...
The shape memory alloys belong to the smart materials thanks to their thermomechanical proprieties' reply to thermal or to mechanical loading. These materials can change shape, stiffness, displacement, natural frequency, and many mechanical characteristics in response to stress or to heat such as conduction, convection or radiation. However, heating by convection or conduction are the most useful and studied methods unlike radiation. Therefore, this paper aims to study the radiation effect on the shape memory alloy behavior
... In contrast, under compression the transformation leads to a monotonic hysteresis with higher stress, lower strain, and essentially homogeneous deformation (e.g., Jacobus et al., 1996;Orgeas and Favier, 1998;Bechle and Kyriakides, 2014;Reedlunn et al., 2014;Elibol et al., 2015). More complex manifestations of the tension/compression asymmetry became apparent in biaxial experiments on pseudoelastic NiTi tubes under combined axial force and torsional loadings (e.g., Helm and Haupt, 2001;Sun and Li, 2002;Yu et al., 2015;Reedlunn et al., 2020) and axial force-internal pressure (e.g., Kyriakides, 2016a, 2016b). ...
... In the vicinity of equibiaxial tension, the material reverted to hardening and nearly homogeneous deformation. Biaxial experiments have also revealed anisotropy, apparently as a result of processing induced texture (e.g., Sun and Li, 2002;Bechle and Kyriakides, 2016a;Reedlunn et al., 2020). Thus, the loci of the transformation stresses of the axial force -internal pressure results trace an elongated trajectory along the equibiaxial direction with modest anisotropy. ...
The well-known tension/compression asymmetry exhibited by nearly equiatomic NiTi has been previously modeled using a hardening potential for compression and a partially softening one for tension to represent pseudoelastic phase transformations (Jiang et al., 2016b). The present study first extends this constitutive model to include anisotropy revealed in the combined axial-internal pressure experiments on NiTi tubes of Bechle and Kyriakides [2016a]. The model is then calibrated anew, implemented in a finite element analysis of tubes and used to simulate the entire range of biaxial experiments performed. Overall, the simulations reproduce well the stress-average strain hystereses and the transformation stress loci, while for hoop dominant stress paths the extents of the transformation strains are somewhat over-predicted. The evolution of localization in the form of high or low strain helical bands, the variation of helix angles with respect to the stress ratio, and the dissipated energy compare favorably. The hardening response and essentially homogeneous deformation exhibited in the neighborhood of the equibiaxial stress state is reproduced, but with reduced hardening and mild inhomogeneity. Despite some minor differences, the results demonstrate the overall success of the analysis in reproducing the phenomena-rich behavior exhibited by tubular NiTi structures under biaxial loadings.
... Vibrations and ultrasound methods are often used to study the viscous properties of polymers [11]. However, uniaxial tension remains one of the main methods for characterizing the mechanical response of biological tissues, superelastic materials, and shape memory alloys. ...
Samples of skin, tendons, muscles, and knitwear composed of NiTi wire are studied by uniaxial cyclic tension and stretching to rupture. The metal knitted mesh behaves similar to a superelastic material when stretched, similar to soft biological tissues. The superelasticity effect was found in NiTi wire, but not in the mesh composed of it. A softening effect similar to biological tissues is observed during the cyclic stretching of the mesh. The mechanical behavior of the NiTi mesh is similar to the biomechanical behavior of biological tissues. The discovered superelastic effects allow developing criteria for the selection and evaluation of mesh materials composed of titanium nickelide for soft tissue reconstructive surgery.
... In NiTi tubes, the instability occurs via nucleation of a helical martensite band that evolves into a cylindrical domain possessing ring-shaped or prong-like transformation fronts (e.g. Li and Sun, 2002;Sun and Li, 2002;Feng and Sun, 2006;Bechle and Kyriakides, 2014;Bechle and Kyriakides, 2016;Reedlunn et al., 2014Reedlunn et al., , 2020a. ...
... Moreover, the transformation stress in uniaxial compression is higher compared to that in uniaxial tension, while the transformation strain in compression is smaller, and those of shear lie in between (e.g. Orgéas and Favier, 1998;Sun and Li, 2002;Grabe and Bruhns, 2009;Mao et al., 2010;Bechle and Kyriakides, 2014;Reedlunn et al., 2014Reedlunn et al., , 2020a. Concluding, the experimental observations indicate a significant tension-compression asymmetry (and also tension-shear asymmetry) of NiTi in terms of the mechanical response and in terms of homogeneity of transformation. ...
... For instance, diamond-shaped patterns have been observed in NiTi tubes under bending (Bechle and Kyriakides, 2014;Reedlunn et al., 2014). The effect of initial torsion on the transformation pattern in NiTi tubes under combined tension-torsion has been investigated by Sun and Li (2002). It has been reported that, as the level of the initial torsion increases, the transformation pattern alters from a propagating instability to a homogeneous transformation. ...
This paper is concerned with modelling of propagating instabilities and transformation patterns in NiTi tubes subjected to combined tension-torsion loading. A recently developed gradient-enhanced finite-strain model of pseudoelasticity is employed for this purpose, and respective finite-element computations are carried out. It is shown that the model is capable of representing a number of experimentally observed effects. The major effect, which has not been successfully modelled to date, is that the transformation is inhomogeneous under tension-dominated loading and alters towards a homogeneous transformation as the level of torsion is increased. To capture this effect, the model must deliver a non-monotonic (up-down-up) stress-strain response in tension and a monotonic one in torsion, and this can be achieved if the model includes three features: tension-compression asymmetry, transverse isotropy of the transformation strain, and deformation-dependent hardening/softening response. A detailed study is also carried out regarding the transformation yield locus. The results reveal an ambiguity in determination of the yield locus for tension-dominated loading and hence an ambiguity in determination of the tension-compression asymmetry. This aspect seems to have been overlooked in the literature despite its impact on correct interpretation of experimental results.
