Mechanical Behavior Of Shape Memory Alloys Research Articles

Shock-induced transformation of nitinol shape memory alloy: Effect of stress state on transformation

Due to its numerous practical applications and intriguing phase transformation behavior, shape memory alloys (SMAs) have garnered significant research and development interests. In the past, most studies on the mechanical behavior of SMAs have been conducted under uniaxial stress loadings. Limited research on SMAs under shock loading has not provided conclusive results regarding their transformation behavior and transformation stress under such loading. Additionally, there is a lack of comprehensive understanding regarding the effects of different stress states on transformation behavior. The main objectives of this study are to address these issues. To achieve these objectives, a series of shock wave experiments were designed and conducted. Additionally, quasi-static and dynamic uniaxial stress experiments were carried out to establish a baseline for comparison. The results revealed that the transformation stress under dynamic uniaxial strain shock loading was approximately 1.92 GPa in contrast to 0.5 GPa (quasi-static) to 0.8 GPa (dynamic) observed in uniaxial stress loading. The transformation behavior exhibited noticeable rate sensitivity for both types of loading. There appeared to be a critical strain rate above which the austenite phase was driven to a metastable state. This estimated critical axial strain rate along the loading direction was approximately 2 × 103/s–4 × 103/s for uniaxial stress loading and approximately 2 × 106/s for uniaxial strain loading. The apparent high transformation stress for uniaxial strain loading can likely be attributed to a combination of high-pressure confinement and high strain rate. However, determining their relative contributions remains an open issue.

Probabilistic seismic evaluation of SMA‐based self‐centering braced structures considering uncertainty of regional temperature

AbstractSmart materials such as shape memory alloys (SMAs) have unique material properties that can potentially mitigate structural damage against earthquake. However, the mechanical behavior of SMAs is sensitive to temperature variation. The traditional deterministic analysis, which does not take into account the influence of temperature, may lead to inaccurate and sometimes unsafe predictions of actual behavior of SMA‐based structures. This research aims to develop a modified framework based on the performance‐based earthquake engineering methodology, taking account of the uncertainty of regional temperature distribution for temperature‐dependent (TD) structures, where the SMA‐based one is considered as a representative case. As the main part of the methodology, regional seismic fragility is generated by combing temperature distribution properties and joint seismic fragility. The proposed methodology is applied to a six‐story self‐centering concentrically‐braced frame (SC‐CBF) with SMA‐based SC braces located in different regions in China. A simplified thermo‐mechanical model capturing the TD characteristics and possible failure of SMA cables is utilized. Deterministic nonlinear static and dynamic analysis is first conducted to understand the structural response. Then, the joint seismic fragility analysis of the SC‐CBF under different intensity levels of ground motion and temperatures is conducted via the incremental dynamic analysis method, by which the seismic performance of the structure can be fully captured. Finally, the proposed methodology is utilized to comprehensively evaluate the regional seismic fragility and collapse risk of the SC‐CBF in different regions. The results indicate that the ignorance of the temperature effect could underestimate the risk of collapse.

A computational study of shape memory effect and pseudoelasticity of NiTi alloy under uniaxial tension during complete and partial phase transformation

The demand of shape memory materials (SMMs) especially, shape memory alloys (SMAs) is increasing day by day in engineered products and commercial applications. Therefore, it becomes necessary to understand the various determinants that influence the behaviour of SMAs. The mechanical behaviour of SMA is highly non-linear in nature and totally dependent on working temperature, material constituents, heat treatment method, and mechanical loading. SMAs are used in a wide range of applications due to their ability to recover their predominated shape when heated, and to the hysteresis, they show during a loading-unloading cycle before recovering their predominated state. These two properties, called shape memory effect/thermal memory effect (SME/TME) and pseudoelasticity/superelasticity (PE/SE), make these alloys interesting for many industrial applications, such as the aerospace, transportation, and medical industries. The objective of this paper is the computational study of SE/PE and SME/TME of NiTi alloy (Nickel-Titanium alloy) i.e. a type of SMA under uniaxial tension during complete and partial phase transformation by using Finite Element Method (FEM) simulations. The computational study of NiTi alloy cylinder model by a rectangle in a 2D axisymmetric geometry with the composition of 50% nickel and 50% titanium in atomic scale, which is also known as nitinol 50 is done using COMSOL Multiphysics. Three studies are performed by using COMSOL Multiphysics: a parametric sweep study shows the PE/SE effect at different fixed temperatures, a prescribed displacement sweep study shows the PE/SE effect is a partial unloading-partial loading loop, and the SME study is portrayed after increasing the temperature. The results confirmed that the proposed computational study represent an effective approach for the simulation of the SMA behaviour of most of the significant properties of NiTi alloy.

A Macromechanical Constitutive Model of Shape Memory Alloys Under Uniaxial Cyclic Loading

AbstractIn this paper, the thermomechanical behavior of shape memory alloys (SMAs) subjected to uniaxial cyclic loading is investigated. To obtain experimental data, the strain-controlled cyclic loading-unloading tests are conducted at various strain-rates and temperatures. Dislocations slip and deformation twins are considered to be the main reason that causes the unique cyclic mechanical behavior of SMAs. A new variable of shape memory residual factor was introduced, which will tend to zero with the increasing of the number of cycles. Exponential form equations are established to describe the evolution of shape memory residual factor, elastic modulus and critical stress, in which the influence of strain-rate, number of cycles and temperature are taken into account. The relationship between critical stresses and temperature is modified by considering the cycling effect. A macromechanical constitutive model was constructed to predict the cyclic mechanical behavior at constant temperature. Based on the material parameters obtained from test results, the hysteretic behavior of SMAs subjected to isothermal uniaxial cyclic loading is simulated. It is shown that the numerical results of the modified model match well with the test results.

A phenomenological model for a Cu-Al-Be shape memory alloy pseudoelasticity under complex loading

For the recent years, some multiaxial loadings are performed on Shape Memory Alloys (SMA) in the range of pseudoelasticity (stress induced martensitic transformation). For instance the tension-torsion tests on Cu-Zn-Al tubes of Rogueda et al. [1], the triaxial proportional loadings of Gall et al. [2], the tension (compression) torsion tests on Ti-Ni tubes of Raniecki et al. [3] and the recent biaxial compression loadings on Cu-Al-Be of Bouvet et al. [4]. These recent experimental results show that the mechanical behavior of SMA is affected by the multiaxiality of the stress tensor. Indeed, some comparisons between simulations with classical models and experimental results under nonproportional loadings permit to conclude that more work must be done to increase the accuracy of models. The aim of this paper is to present a new phenomenological modeling based on the thermodynamics framework of irreversible processes. This model uses two transformation surfaces, the first one controls the forward transformation (austenite phase transforms to martensite phase) and the second one the reverse transformation (martensite phase transforms to austenite phase). This model is shown to give good results for the prediction of complex multiaxial nonproportional tests.