A stenotic vessel can be opened using net-Shape tubes called “ stents” leading to the restoration of the bloodstream. Compared to the commonly used stainless steel stent, self-expandable stents have some advantages. They do not suffer from the risks of damage to the vascular tissue due to the balloon expansion. Moreover, overexpansion for compensating the elastic recoil is not needed, and there is no constant force applied on the artery until the occlusion of the device by the artery stops. However, the stent cannot restore the original dimensions of the vessel in the case of calcified plaques. Self-expandable stents can be utilized for the treatment of atherosclerotic lesions in the carotid, coronary, and peripheral arteries. Shape Memory Alloys (SMAs), mainly NiTi (nitinol), are employed for self-expandable vascular stent applications. Nitinol is widely applied for medical devices and implants due to its excellent fatigue performance, mechanical properties, and biocompatibility, which make this alloy suitable for long-term installations. Other materials used for self-expandable cardiovascular stents are Shape Memory polymers (SMPs). Shape Memory effect is triggered by the hydration of polymers or temperature change preventing the collapse of small blood vessels. This review has focused on the mechanisms and properties of SMAs and SMPs as promising materials for stent application.

To assess the thermo-mechanical behavior of Shape Memory Alloys and analyzing these special materials, a simple constitutive integrated model, named micro-plane, is proposed. The model deals with shear and normal on plane stress/strain components and also on plane shear orientation as well. The proposed simple model is capable of predicting three-dimensional behavior as the superposition of on plane elastic and inelastic deformations. In the case of static constraint, two on plane stress/strain components and corresponding orientations could be obtained by transferring the stress/strain tensor. Then to calculate the on plane deformations, a plane constitutive law is needed to assess unknown strains/stresses. To represent the capability of this model, the predicted different test data across time and temperature domains are compared with the experimental results. In these test results the Shape Memory Alloys behavior as: super elasticity under various temperatures, loading rate effects, asymmetry in tension and pressure, various loops of loading and unloading, hydrostatic pressure effects, different proportional tension-shear biaxial loading and unloading, and also deviation from normality due to non-proportional tension-shear biaxial loading and unloading, are investigated and presented. The interesting well accuracy of results proves the strength and capability of the proposed model.

Today, there is an increasing interest in use of seismic isolation to protect the structures against earthquakes. The most common type of seismic isolation is called base isolation, in which the isolation layer is installed under foundations to separate the structures from the ground and reduce the earthquake forces. However, the use of this type of seismic isolation faces some difficulties such as construction in congested urban areas or construction of near sea structures. Furthermore, for seismic retrofitting of existing buildings, the installation of the isolation under foundations is difficult or even impossible; so, it needs to be located on the middle floors of the buildings. The method of isolation design is called floor or middle story isolation. Despite the advantages of seismic isolations, they have some limitations such as instability in large deformations, residual displacement, and the need for replacement after severe earthquakes. The use of Shape Memory Alloys (SMA) regarding their unique properties is considered as an appropriate solution to overcome the above problems. These smart materials show high strength and strain capacity, high recentering ability, and high resistance to corrosion and to fatigue. The purpose of this study is to investigate the effect of combination of middle story isolation utilized by Natural Rubber Bearing (NRB) and iron-based Shape Memory Alloys in steel structures and to compare the performance of such structures with and without the presence of the Shape Memory alloy in the middle-story isolation system. Then, a three-story steel structure has been modeled and evaluated. For this purpose, a structure with floor isolation and iron-based Shape Memory alloy was modeled in OpenSees computer program. The structures were then subjected to seismic loading. The results were presented in the form of story drift, floor acceleration, floor shear forces, and base shear. The outcome of this research showed that the use of these Alloys in the middle-story isolation reduced the overall base shear and floor shear forces. The overall story drift, floor acceleration and displacement are reduced; with the exception at the isolation level. Thus, utilizing the natural rubber bearing isolator along with the iron-based Shape Memory alloy can be considered as a desirable system for the seismic protective design of buildings.

In the last two decades, there has been an increasing interest in structural engineering control methods. Shape Memory Alloys and seismic isolation systems are examples of passive control systems that use of any one alone, effectively improve the seismic performance of the structure. Characteristics such as large strain range without any residual deformation, high damping capacity, excellent re-centering, high resistance to fatigue and corrosion and durability have made Shape Memory alloy an effective damping device or part of base isolators. A unique characteristic of Shape Memory Alloys is in recovering residual deformations even after strong ground excitations. Seismic isolation is a device to lessen earthquake damage prospects. In the latest research studies, Shape Memory alloy is utilized in combination with seismic isolation system and their results indicate the effectiveness of the application of them to control the response of the structures. This paper reviews the findings of research studies on base isolation system implemented in the building and/or bridge structures by including the unique behavior of Shape Memory Alloys. This study includes the primary information about the characteristic of the isolation system as well as the Shape Memory material. The efficiency and feasibility of the two mechanisms are also presented by few cases in point.

In this paper, the superelastic response of porous Shape Memory Alloys (SMAs) containing spherical pore Shape with pore volume fraction between 5% and 40% has been considered. Using digital images processing, the distribution of pores in 2D images of porous NiTi SMA has been extracted. In this method, the 3D distribution of pores has been appraised with the Monte Carlo method and 3D porous SMA models have been established. To investigate the superelastic behavior of Shape Memory Alloys, the Lagoudas’ s phenomenological model was used, in which a phase transformation function was used. To homogenize the porous SMAs, the Young’ s modulus and the phase transformation function have been assumed to be a function of the pore volume fraction. Based on the proposed constitutive model a numerical procedure was proposed and executed by the commercial finite element code ABAQUS with developing a user material subroutine. The numerical results show that the Young’ s modulus and the phase transformation function are the approximately linear function of the pore volume fraction; furthermore, these results demonstrate the accuracy of the proposed homogenization method to predict the superelastic behavior of porous SMAs.

The phase transformation phenomenon due to the crystallographic change of Shape Memory Alloys subjected to mechanical or thermal loading is very complicated. Regarding the thermo-mechanical coupling effects in Shape Memory Alloys, in case of high loading rates, heat generation/absorption during the forward/reverse transformation, will lead in temperature dependent variation and consequently affects its mechanical behavior. In this paper, a numerical algorithm based on the finite element method is proposed to investigate complex mechanical, thermal, and coupled behavior of Shape Memory Alloys, including both exclusive behaviours of these Alloys, that are superelasticity and Shape Memory effect. Several key examples are simulated and discussed to assess the efficiency and accuracy of proposed algorithm.

Shape Memory Alloys (SMA) are widely used as new materials with higher potential in implants, self expanding NiTi stents, etc. for medical applications and in actuators, micro actuators, components for isolation of vibrations, etc. for non-medical applications. In this study, Shape Memory effect, superelasticity, and excellent damping capability, three special characteristics of this material, are discussed and an extended one-dimensional constitutive model for prediction of superelastic behavior is proposed. The model is based on strain as control variable (strain driven). Stress induced austenit-martensite evolutionary equations are first proposed and time discrete model is then obtained through backward Euler scheme. The solution of the time-discrete model is approached by a modified return map algorithm for phase transition. Then, tangent modulus is used for the quadratic convergence of the Newton method ill the phase transition conditions. Muller model helps us to predict material behavior in the pseudo-elastic hysteresis. In the next step, extended solution algorithm is proposed and different experimental test results are discussed. Finally, the proposed model results are compared with the result of some experimental studies, which shows good agreements.