Browsing by Subject "shape memory alloy"

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Access status: Open Access ,
A finite-strain model for a superelastic NiTi shape memory alloy
(Wydawnictwa AGH, 2022) Jiang, Dongjie; Xiao, Yao
A finite-strain constitutive model of a superelastic NiTi shape memory alloy is proposed in this paper. Via backward Euler implicit integration scheme and the incorporation of material softening, the model is implemented into finite element code to reproduce a Lüders like deformation of a superelastic NiTi. The simulation results are in agreement with the experimental results, indicating that the constitutive model can reasonably predict the mechanical behavior of a superelastic NiTi. A parametric study further verifies that the magnitude of softening modulus has a significant effect on the stress-strain response and Lüders-like deformation of a superelastic NiTi.
Access status: Open Access ,
Modeling of temperature-dependent cyclic performance of superelastic NiTi shape memory alloy
(Wydawnictwa AGH, 2022) Xiao, Yao; Ju, Shuangyan; Lin, Jianping
In this paper, a three-dimensional micromechanical-based constitutive model is proposed to describe the temperature-dependent performance of a cyclic deformed superelastic NiTi shape memory alloy. The dominant texture of the specimen is prescribed as <111> direction along the longitudinal direction. Apart from martensitic transformation, various mechanisms regarding superelastic degradation are taken into consideration. In order to be extended from the single-crystal scale to the polycrystalline version, the constitutive model is implemented into finite element software. It is verified that the measured cyclic response of a superelastic NiTi is well reproduced by the presented approach. Furthermore, the predicting capability of the proposed model is verified by simulating the mechanical behavior of NiTi tube subjected to cyclic bending.
Access status: Open Access ,
Testing of a spring with controllable stiffness
(Wydawnictwa AGH, 2006) Rączka, Waldemar Stanisław
The paper shows the structure of a mechanical spring with controllable stiffness and outlines the temperature control systems solutions. This spring was developed in the Department of Process Control, AGH University of Science and Technology (AGH-UST). Its main components are bars made from shape memory alloy (SMA). The unique properties of SMA are outlined. Of particular importance are parameters that affect the spring special behaviour. The spring was tested in an experimental set-up and the compiled test results are provided. Chosen results was shown for different excitations and working temperatures. Obtained results show that the spring built using SMA has many interesting features. It is clear that the spring changes its stiffness during changing working temperature. Unfortunately spring stiffness has nonlinear behaviour so that in the paper was proposed a nonlinear mathematical model of the spring. Model was developed having in mind its usage. It was built for designing a controller spring stiffness.
Access status: Open Access ,
Thermomechanical modeling on the crack initiation of NiTi shape memory alloy
(Wydawnictwa AGH, 2022) Jiang, Dongjie; Xiao, Yao
The fracture of shape memory alloys is distinct from that of conventional metals, owing to the coexistence and interaction of multiple special features such as martensitic transformation, dislocation-induced plasticity, thermomechanical coupling and others. In this paper, the impact of thermomechanical behavior upon the crack initiation of a NiTi shape memory alloy under Mode I loading is investigated numerically and verified experimentally. A constitutive model incorporating phase transformation, plasticity and thermomechanical coupling is established. Via backward Euler integration and finite-element implementation, the longitudinal strain, martensite volume fraction and temperature field in the vicinity of the crack tip are furnished. The effects of grain size and loading rate on J-integral are revealed. The grain size dependence of crack initiation is non-monotonic. For the samples with grain sizes of 1500 nm, 18 nm and 10 nm, the shielding effect takes place in front of the crack. Additionally, the anti-shielding effect is detected for samples with grain sizes of 80 nm and 42 nm. The parametric study shows that loading rate imposes limited influence on J-integral, which is attributed to a small scale transformation. The decrement of yield stress and the increment of transformation hardening modulus can alleviate the anti-shielding effect and arouse the shielding effect upon crack initiation. The presented results shed light on the design and fabrication of high toughness phase transformable materials.
Access status: Open Access ,
TiNi shape memory alloy tension at various temperatures - infrared imaging of shape memory effect and pseudoelasticity
(2011) Pieczyska, Elżbieta; Tobushi, Hisaaki
The mechanical characteristics and the infrared imaging of stress-induced martensite transformation developing in TiNi shape memory alloy (SMA) subjected to tension in various conditions with respect to the SMA austenite finish parameter have been presented. Based on the mechanical curves and their related temperature changes it was found that onset of the martensitic transformation appears at the end of the elastic part of the stress-strain curve, since the temperature starts to increase before the knee in the curve. The uniform temperature distribution observed on the specimen surface at this stage confirms that the initial martensite transformation is macroscopically homogeneous. For the shape memory effect behavior the uniform temperature distribution on the specimen surface was observed during the complete process of the SMA loading which means that the transformation process is macroscopically homogeneous. For the shape memory alloy pseudoelasticity behavior bands of higher temperature have been recorded during the specimen loading and bands of lower temperature during its unloading, manifesting localized Lü ,ders-like deformation, caused by the transformation process. As the transformation advances, the higher temperature changes have been recorded. Taking advantages from the infrared technique a new results concerning nucleation and development of the martensite forward and reverse transformation have been obtained. Finally, some examples of the SMA new applications due to the SMA shape memory effect have been presented and discussed.