Alberto Coda

SMA Numerical Modeling Versus Experimental Results: Parameter Identification and Model Prediction Capabilities

In this work, we briefly review the one-dimensional version of a well-known phenomenological shape memory alloy (SMA) constitutive model able to represent the main macroscopic SMA macroscopic behaviors (i.e., superelasticity and shape-memory effect). We then show how to identify the needed parameters from experimental results and, in particular, from strain-temperature tests. We finally use the obtained material parameters to test the prediction properties of the model, comparing numerical results with some experiments (different from those used for the identification), and we discuss model capabilities and further required enhancements.

Characterization of Inclusions in VIM/VAR NiTi Alloys

Inclusions content is important for the mechanical behavior and performances of NiTi-based products particularly in fatigue-rated devices. Higher inclusions content has been correlated to reductions in transformation temperatures and strain recovery under mechanical or thermo-mechanical cycling. Moreover, most fatigue fractures show inclusions at the initiation site. However, there is a general lack of information on the nature and characteristics of such inclusions, especially those typically recognized as intermetallics oxides. In this study, the common scanning electron microscopy technique has been used to investigate the chemistry and morphology of inclusions in commercial standard VIM/VAR binary NiTi alloys. The defined experimental procedure, results, and their significance will be presented and discussed.

Inclusions Size-based Fatigue Life Prediction Model of NiTi Alloy for Biomedical Applications

Current standards consider the size and distribution of inclusions in semi-finished material, but do not place requirements on final biomedical devices made of NiTi shape memory alloys. In this paper, we analyze this by comparing the fatigue performances of NiTi superelastic wires obtained by different processes through a simple bilinear model of fatigue response in terms of strain life. The fracture surfaces of failed wires are analyzed through SEM microscopy and data regarding the presence of particles, and their morphology is recorded and analyzed using Type-I extreme value distribution. The results show a strong correlation between the fatigue limit of wires (in terms of strain) and the predicted extreme values of inclusions at fracture origin. Then, following the concept of treating the inclusions as ‘small cracks,’ a simple relationship between fatigue limit strain range and inclusion size is proposed based on ΔKth data from the literature. The model is compared with the fatigue data obtained from the tested wires.

Investigation on the Hysteretic Behavior of NiTi Shape Memory Wires Actuated Under Quasi-Equilibrium and Dynamic Conditions

The functional characterization of SMAs for actuation is typically performed by measuring the specimen deformation under constant load during a controlled thermal cycling across transformation temperatures. Under dynamic actuation, transformation temperatures different from those measured in quasi-equilibrium conditions have been observed. The aim of this work is to better investigate and understand these phenomena. Direct and indirect heating of shape memory wires under several loading conditions are examined in detail. According to the experimental results, the hypothesis is to consider the observed differences as an effect of the thermal cycling rate on the internal friction. However, the presented data seem do not fully confirm this idea. Further experiments will be carried out in order to directly measure the internal friction of the material under the same working conditions.

Smartflex NiTi Wires for Shape Memory Actuators

Shape Memory Alloys (SMAs) are active metallic materials classified nowadays as “smart” or “intelligent” materials. One of the main areas of interest is that of actuators. The use of Shape Memory Alloys in actuators offers the opportunity to develop robust, simple and lightweight elements that can represent an alternative to electro-magnetic actuators commonly used in several fields of industrial applications, such as automotive, appliances, etc. The obvious simplicity of mechanical design and minimum number of moved parts is amazing for an actuator. NiTi SMAs demonstrated to have the best combination of properties. Due to its relatively high recovery stress and strain, actuators providing significant force and stroke can be designed. There are perhaps thousands of applications of NiTi-based actuators mentioned in literature and in patents. Successful applications will build on SMA strengths whilst taking into account its weaknesses. SAES Getters S.p.A., thanks to its vertically integrated process and to the scientific and quality approach, developed a NiTi-based wires family which can represent a very good solution for shape memory actuators. The mechanically stabilized SAES Smartflex NiTi actuators show a very sophisticated profile of properties. In this paper the mechanical, thermal and electrical response of these shape memory wires, at diameters ranging from 25 to 500 mm, under different working conditions, simulating the actual operating condition in real actuators, will be examined in depth and discussed, in order to direct the design of the actuator so that the functional properties of the material can be completely exploited. The thermomechanical properties have been investigated and measured by several methods. The most common and useful tests for these commercially available wires will be also described.

Recent Advances in Materials Development for Emerging SMA Applications

Shape Memory Alloys (SMAs) are active metallic materials classified as “smart” or “intelligent” materials along with piezoelectric ceramic and polymers, electro-active plastics, electro-rheological and magneto-rheological fluids and others. SMAs show a multitude of different and dependent properties interesting for technological applications. These properties depend on the peculiar deformation mechanisms, accounting for the so-called shape memory effect. SMAs are nowadays used in quite different fields, like thermo-mechanical devices, anti-loosening systems, biomedical applications, mechanical damping systems, in some cases employed for large scale civil engineering structures. These multifunctional materials can be naturally considered as sensor-actuator elements demonstrating large possibilities for applications in high-tech smart systems. The use of SMAs in actuators offers an excellent technological opportunity to develop reliable, robust, simple and lightweight elements within structures or as stand-alone components that can represent an alternative to electro-magnetic actuators commonly used in several fields of industrial applications, such as automotive, appliances, consumer electronics and aerospace. NiTi-based SMAs demonstrated to have the best combination of properties, especially in terms of the amount of work output per material volume and the large amount of recoverable stress and strain. However, there are several limiting factors to a widespread diffusion of SMAs to technological fields. For instance, SMAs display a critical dependence of the shape-memory related properties, like transition temperatures, on their actual composition. For this reason, a great care in the production steps, mainly based on casting processes, is required. Another critical aspect, that is to be considered when dealing with SMAs, is the strong influence of their thermo-mechanical history on their properties. This may disclose interesting perspectives of application to smart devices in which different aspects of the shape memory phenomenology, like one and two way shape memory effect, pseudoelasticity, damping capacity, etc., are used. Last, but not least, one of the most debated aspects around NiTi alloys is microcleanliness. This concept is becoming increasingly important as the industrial market moves to smaller, lower profile devices with thinner structures.In this work a general overview about the peculiar behavior of NiTi alloys along with their main issues, the shape memory components under development, and the main efforts and directions for materials improvement will be presented and discussed. A bird’s-eye view on the future opportunities of NiTi-based shape memory actuators for industrial applications will also be given.Copyright