ENGINEERING ASPECTS OF SHAPE MEMORY ALLOYS PDF
of civil engineering structures, considering both passive and semi-active devices. • It discusses some aspects related to the processing of the shape memory. Engineering Aspects of Shape Memory Alloys provides an understanding of shape memory by defining terms, properties, and applications. It includes tutorials. Shape Memory Alloys (SMAs) have been on the forefront of research for the . engineering effects, and applications of shape memory alloys, including the.
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Download Citation on ResearchGate | On Jan 1, , T.W. Duerig and others published Engineering Aspects of Shape Memory Alloys. Darel E. Hodgson, Shape Memory Applications, Inc., Ming H. Wu, Memry Corporation, c memory. Although a relatively wide variety of alloys are known to exhibit the shape memory effect Proceedings of Engineering Aspects of. Shape. shape memory and superelastic properties. Shape loys or alloy compositions, but to a family of alloys .. Duering and G.R. Zadno, Engineering Aspects of.
When there is a limitation of shape recovery, these alloys promote high restitution forces. Because of these properties, there is a great technological interest in the use of SMA for different applications.
Although a relatively wide variety of alloys present the shape memory effect, only those that can recover from a large amount of strain or generate an expressive restitution force are of commercial interest. SMA based on Ni-Ti are the alloys most frequently used in commercial applications because they combine good mechanical properties with shape memory.
The remarkable properties of SMA have been known since the 's. In , Chang and Read noted the reversibility of the Au-Cd alloy not only by metallographic observations, but also by the observation of changes in resistivity. Nevertheless, it was only in the 's that SMA attracted some technological interest. In , Buehler and co-workers, of the U.
Naval Ordnance Laboratory, discovered the shape memory effect in an equiatomic Ni-Ti alloy which began to be known as Nitinol, as a reference to the initials of the laboratory. Raychem developed the first industrial application of SMA for the aeronautic industry during the 's.
In , Andreasen, of Iowa University, made the first implant of a superelastic orthodontic device 1,2. Today, these applications are being developed in different fields of science and engineering. Basically, SMA present two well-defined crystallographic phases, i. Martensite is a phase that, in the absence of stress, is stable only at low temperatures; in addition, it can be induced by either stress or temperature.
Depending on the type of transformation experienced by these alloys, the crystal structure of martensite can be either monoclinic or orthorhombic 4,5. When martensite is induced by temperature, it is called twinned martensite. The twinned martensite has 24 variants, i. On the other hand, when martensite is induced by stress, these 24 variants of twinned martensite become only one variant.
As a consequence, there is a crystallographic orientation, aligned with the stress direction, which is called detwinned martensite. The austenite phase is stable only at high temperatures, having a single variant with a body-centered cubic crystal structure.
Martensitic transformation explains the shape recovery in SMA. This transformation occurs within a range of temperatures which varies according to the chemical content of each alloy 7. In general, four characteristic transformation temperatures can be defined: MS and MF, which are the temperatures at which the formation of martensite starts and ends, respectively, and AS and AF, which are the temperatures at which the formation of austenite starts and ends, respectively. Recent studies have shown that, depending on specific conditions, some SMA can present another crystallographic phase known as R-phase.
The crystal structure of the R-phase is rhombohedric 4,5. Because of their remarkable properties, SMA can be used in a large number of non-medical applications SMA can solve problems in the aerospace industry, especially those related to vibration control of slender structures and solar panels, and non-explosive release devices 11, Micromanipulators and robotic actuators have been employed in order to mimic the smooth movement of human muscles 13, SMA are commonly used as external actuators or as SMA fibers embedded in a composite matrix so that they can alter the mechanical properties of slender structures for the control of buckling and vibration Biomedical applications of SMA have been extremely successful because of the functional properties of these alloys, increasing both the possibility and the performance of minimally invasive surgeries 2,16, The biocompatibility of these alloys is one of the important points related to their biomedical applications as orthopedic implants 18 , cardiovascular devices 2 , and surgical instruments 16 , as well as orthodontic devices and endodontic files This article presents a brief discussion of the thermomechanical behavior of SMA, and a description of their main applications in the biomedical field as cardiovascular and orthopedic devices and as surgical instruments.
Thermomechanical behavior SMA present typical thermomechanical behaviors, like pseudoelasticity and shape memory effects one-way and two-way. This section presents a short discussion of these behaviors, explaining the macroscopic phenomenological aspects related to each one Pseudoelasticity Pseudoelasticity occurs whenever an SMA sample is at a temperature above AF the temperature above which only the austenitic phase is stable for a stress-free specimen.
Thus, one can consider an SMA sample subjected to a mechanical loading at a constant temperature above AF. The stress-strain curve s-e in Figure 1 , left side, illustrates the macroscopic behavior of SMA, showing the pseudoelastic phenomenon.
At this point, the crystal structure of the sample is totally composed of detwinned martensite.
For higher stress values, SMA presents a linear response. From point D on, the sample presents an elastic discharge. When the loading-unloading process is finished, SMA have no residual strain. However, since the path of the forward martensitic transformation does not coincide with the reverse transformation path, there is a hysteresis loop associated with energy dissipation. Another way to observe the pseudoelastic effect is indicated on the right side of Figure 1.
At this temperature, there is only one phase, i. At a constant temperature, a mechanical loading is applied promoting the appearance of the detwinned martensite,.
Applications of Shape Memory Alloys for Neurology and Neuromuscular Rehabilitation
Figure 1. See text for explanation of process.
Figure 2 , left side, shows the stress-strain curve of an SMA sample at a low temperature less than MF, the temperature below which only the martensitic phase is stable where the shape memory effect can be noted. When the sample is subjected to a mechanical loading, the stress reaches a critical value, point A, when the transformation of the twinned martensite into the detwinned martensite begins, ending at point B.
When the loading-unloading process is finished, the SMA sample presents a residual strain point C. This residual strain can be recovered by sample heating, which induces the reverse phase transformation. This is the shape memory effect, also known as one-way shape memory effect. This phenomenon can be understood from a motion of the hysteresis loop shown on the stress-strain curve in Figure 1.
Since the temperature goes down, the hysteresis loop moves down as well. The right side of Figure 2 presents an alternative way to observe the shape memory effect. At this temperature, the sample has only the austenitic phase. When the temperature of the SMA sample decreases and crosses the line related to MS, the phase transformation begins to take place and the twinned martensite replaces the austenite. This transformation is concluded when the sample temperature is below MF,.
When this load vanishes the sample presents a residual strain,. Figure 2. Shape memory effect. For abbreviations, see legend to Figure 1. The primary characteristic of the two-way effect is associated with the presence of a specific phase in a specific setting. In this way, the sample has a shape in the austenitic state and another in the martensitic state.
The change of temperature produces a change in sample shape without any mechanical loading. In order to obtain the two-way effect, it is necessary that the SMA sample be trained. Typically, there are two training procedures 23 : shape memory effect cycling cycles of shape memory effect and the training through the appearance of the detwinned martensite, the stress-induced martensite training.
Both induce considerable plastic strains.
Engineering Aspects of Shape Memory Alloys
Figure 3 shows a schematic presentation of the two-way effect. Another cooling returns the sample to its low temperature shape,.
It should be pointed out that, in contrast to the one-way shape memory effect, it is not necessary to apply mechanical loading in order to alter the sample's shape at low temperature. Figure 3. Two-way shape memory effect.
This is a crucial factor for the use of SMA devices in the human body A biocompatible material does not produce allergic reactions inside the host, and also does not release ions into the bloodstream. The period during which a biomaterial remains inside the human body is an important aspect to be considered concerning its use. Generally, the biocompatibility of a material is strongly related to allergic reactions between the material surface and the inflammatory response of the host.
Several aspects can contribute to these reactions such as patient's characteristics health, age, immunological state, and so on , and material characteristics rugosity and porosity of the surface and individual toxic effects of the elements present in the material Several investigations have been conducted in order to establish the biocompatibility of Ni-Ti-based alloys, and to exclude intrinsic hazards involved in their applications 24, The analysis of aspects related to the biocompatibility of these alloys is performed by assessing each of their elements, nickel and titanium, separately.
Nickel, although necessary to life, is a highly poisonous element 2. Studies have shown that persons having systematic contact with nickel present problems such as pneumonia, chronic sinusitis and rhinitis, nostril and lung cancer, as well as dermatitis caused by physical contact.
Unlike nickel, titanium and its compounds are highly biocompatible; moreover, due to their mechanical properties, they are usually employed in orthodontic and orthopedic implants 2. The oxidation reaction of titanium produces an innocuous layer of TiO2 which surrounds the sample. This layer is responsible for the high resistance to corrosion of titanium alloys, and the fact that they are harmless to the human body. Inquiries concerning the biocompatibility of Ni-Ti alloys began shortly after their discovery in This method allows predicting the extent of crack tip transformation region as well as the resulting stress distribution.
Furthermore, based on this model fracture control parameters for SMAs have been proposed 39 , i.
However, despite the large number of research reports on fracture of NiTi SMAs, much effort should be paid for an effective understanding of the role of phase transformations on the crack formation and propagation mechanisms. Within this context, systematic experiments and theoretical studies were carried out in this investigation with the aim of capturing the actual stress-strain distribution at the crack tip.
The effects of temperature on fracture properties of SMAs, within the pseudoelastic regime of the alloys, were also analyzed. In particular, temperature controlled fracture tests were carried out, by using single edge crack specimens made of a commercial pseudoelastic NiTi alloy.
The DIC method was applied to capture displacement and strain distribution in the crack tip region. Furthermore, the experimental results were critically analyzed by using a recent analytical model 37 and it was demonstrated that this model is able to correctly capture the effects of temperature on the crack tip stress distribution.
As a consequence, the model can be actually used to define an effective fracture toughness parameter for SMAs by taking into account the real thermo-mechanical loading conditions, including both mechanical load and temperature, and the resulting stress-induced transformation phenomena.
The rolling direction is parallel to the tensile axis. Almost straight crack paths normal to the load direction, initiating from EDM notches, were obtained in all specimens as illustrated in the optical micrographs of Fig.
Isothermal displacement controlled 0. The tests were executed at different temperatures within the pseudoelastic regime of the alloy, i. Two specimens for each testing temperature were analyzed, as illustrated in Fig.
Mechanical tests were carried out by an electro-dynamic testing machine Instron E equipped with and a special system for open-air temperature control. In this system heat is provided by a Peltier cell, which is directly applied on one side of the specimen. A K-type thermocouple and an electronic control driver unit is used for feed forward temperature control, with an accuracy of 0.
Direct measurements and control of the specimen temperature allows to avoid possible temperature variations arising during tensile test, due to the latent heat associated with stress-induced transformations. It includes tutorials, overviews, and specific design examples—all written with the intention of minimizing the science and maximizing the engineering aspects.
Although the individual chapters have been written by many different authors, each one of the best in their fields, the overall tone and intent of the book is not that of a proceedings, but that of a textbook. The book consists of five parts. Part I deals with the mechanism of shape memory and the alloys that exhibit the effect. It also defines many essential terms that will be used in later parts. Part II deals primarily with constrained recovery, but to some extent with free recovery. There is an introductory paper which defines terms and principles, then several specific examples of products based on constrained recovery.
Finally, Part V deals with superelasticity, with an introductory paper and then several specific examples of product engineering. We are always looking for ways to improve customer experience on Elsevier. We would like to ask you for a moment of your time to fill in a short questionnaire, at the end of your visit. If you decide to participate, a new browser tab will open so you can complete the survey after you have completed your visit to this website.
Thanks in advance for your time.Google Scholar  Moser, K. Generally, the biocompatibility of a material is strongly related to allergic reactions between the material surface and the inflammatory response of the host. Although a relatively wide variety of alloys present the shape memory effect, only those that can recover from a large amount of strain or generate an expressive restitution force are of commercial interest.
Accepted December 4, Naval Ordnance Laboratory, discovered the shape memory effect in an equiatomic Ni-Ti alloy which began to be known as Nitinol, as a reference to the initials of the laboratory.
The Simon filter Figure 4 represents a new generation of devices that are used for blood vessel interruption in order to prevent pulmonary embolism.