# Browsing by Subject "NiTi"

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Item Constitutive modeling of pseudoelastic NiTi and its application to structural problems(2017-08) Jiang, Dongjie; Kyriakides, S.; Landis, Chad M.; Kovar, Desiderio; Liechti, Kenneth M; Ravi-Chandar, KrishnaswamyShow more Nearly equiatomic NiTi exhibits the unique characteristic of pseudoelasticity, i.e. it can be deformed to strain levels of several percent that is fully recoverable on unloading. Under tension, the phase transformation results in strain of nearly 7% and inhomogeneous deformation while the stress remains essentially constant. By contrast, under compression, transformation occurs at a much higher stress, the induced strain is nearly one-half and deformation is essentially homogeneous. These material nonlinearities interact with structural instabilities to produce hitherto unknown intriguing structural responses. A constitutive modeling framework based on J₂-type kinematic hardening is developed to address the tension/compression asymmetry of pseudoelastic NiTi, including inhomogeneous versus homogeneous deformations. Its performance is then evaluated in the numerical studies of four nonlinear NiTi structures that were previously investigated experimentally. The first problem considered is the buckling and recovery of an axially compressed NiTi tube. The modeling efforts reproduce the major events observed, including onset of axisymmetric wrinkling, collapse by progressive development of buckling lobes with three circumferential waves, erasure of the lobes and recovery of deformation upon unloading. The study shows that the tension/compression asymmetry plays the key role in bringing the calculated response close to the experimental one and addition of plastic deformation further improves the results. The second problem involves a NiTi strip under tension. The constitutive model properly accounting for softening in the tensile response reproduces the closed hysteresis with two stress plateaus with correct levels and extents, as well as localization of deformation into inclined bands that propagate in the specimen. In the third problem, the constitutive model is used to analyze the tension test of a NiTi tube. The simulation reproduces a closed hysteretic response with two stress plateaus close to the measured one. The tubular geometry of the specimen imposes helical and multi-pronged localization patterns. The last problem studied is the bending of a NiTi tube. The simulation reproduces the major features of the experimental results: the hysteretic moment-end rotation response with two plateaus; localization of curvatures; progressive development of diamond patterns of higher strain on the tensioned side and the nearly homogeneous deformation on the compressed side.Show more Item Evolution of localization in NiTi shape memory alloys and its effect on structures(2016-05) Bechle, Nathan Joseph; Kyriakides, S.; Landis, Chad M; Liechti, Kenneth M; Ravi-Chandar, Krishnaswa; Kovar, DesiderioShow more Nearly equiatomic NiTi can be strained to several percent and fully recover upon unloading (pseudoelastic behavior). This property is derived from solid-state transformations between the austenitic (A) and martensitic (M) phases, which can be induced by either changes in temperature or stress. In concert with prior results in tension, stress-induced phase transformation leads to localized deformation associated with the nucleation and propagation of the M-phase during loading and the A-phase during unloading. By contrast, it is demonstrated that under compression, transformation stresses are higher, strains are smaller, and the deformation is essentially homogeneous. This tension-compression asymmetry and unstable material behavior have an effect on the response and stability of NiTi structures. This is demonstrated with pure bending of tubes, and axial compression of cylindrical shells. Pure bending results in localization that leads to the coexistence of two curvature regimes. In the axial compression of the shell, transformation induces buckling and collapse, both of which are recoverable upon unloading. A requirement for the analysis and design of such structures is constitutive models that capture the material instability and asymmetry. Furthermore, the extensions of these material nonlinearities to the multiaxial setting must be addressed. To this end, results from a series of experiments on pseudoelastic NiTi tubes loaded under combined axial load and internal pressure are presented in which radial stress paths in the axial-hoop stress space were traced. Stereo digital image correlation was used to monitor the evolution of transformation-induced deformation. Results spanning axial-to-hoop stress ratios from -1.0 to uniaxial tension revealed that, but for a narrow region near equibiaxial tension, transformation leads to localized helical deformation bands with helix angles that vary with the stress ratio, while the stresses remain nearly constant. In the vicinity of equibiaxial tension, the material exhibits hardening and homogeneous deformation. Loci of the transformation stresses, while exhibiting minor anisotropy, traced an elongated non-Mises trajectory in the axial-hoop stress space. By contrast, the transformation strains exhibit significant anisotropy between axial and hoop dominant stress paths. Moreover, strains around the equibiaxial stress state, where material hardening and homogeneous deformation was observed, are significantly smaller than in the rest of the stress space. The strain anisotropy has a corresponding reflection on the energy dissipated during transformation with axial dominant stress paths dissipating significantly more energy than hoop dominant ones, with less dissipation observed in the neighborhood of equibiaxial stress.Show more Item Localization instabilities in pseudoelastic NiTi tubes under multiaxial stress states(2022-07-01) Kazinakis, Karlos Thomas Leonidas; Kyriakides, S.; Landis, Chad M.; Kovar, Desiderio; Liechti, Kenneth M.; Ravi-Chandar, KrishnaswamyShow more Nearly equiatomic NiTi has a unique property called pseudoelasticity in that strain of several percent is recoverable at room temperatures. This characteristic is attainable due to solid-state transformations between the austenite and martensite phases. It is well established that the transformation in tension is associated with localization during the loading/unloading stress plateaus of its hysteresis. By contrast, the transformation in compression is essentially homogeneous and occurs at much higher stresses and lower strains. Recently conducted biaxial experiments on NiTi tubes revealed, in addition to tension/compression asymmetry, an inherent anisotropic behavior. The interaction of these material nonlinearities with geometric instabilities results in challenging structural problems and the need for adept constitutive models is vital. Hence, a J₂-type kinematic hardening model was developed by our group, which incorporates asymmetry and is now extended to include anisotropy. Two numerical studies of NiTi tubular structures incorporate this framework within a finite element analysis aiming to reproduce the experimental responses and the transformation-induced strain patterns. The first problem investigates the buckling and collapse of a thin-walled NiTi tube under pure bending. The analysis captures the moment-end rotation response and the distinct diamond patterns that develop demonstrating how their interaction with ovalization leads to buckling and collapse. Parametric sensitivity studies illustrate the roles of the diameter-to-thickness ratio, geometric imperfections, and some key aspects of the model to the stability of the structure. The second problem examines thin-walled NiTi tubes under combined axial force and internal pressure. The simulations reproduce well the stress-average strain responses and the transformation stress loci, while for hoop dominant stress paths the extents of the transformation strains are somewhat overpredicted. The evolution of localization in the form of high or low strain helical bands, the variation of helix angles with 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 weak deformation patterns. The special case of equibiaxial tension is studied further as it highlights the effects of asymmetry and anisotropy in the constitutive model on structural behaviorsShow more