Shape Memory Elements Research Articles

Shape Memory Elements in Bending: Influence of the Shape of Their Cross-Section

Shape Memory Elements in Bending: Influence of the Shape of Their Cross-Section

TiAlV/Al2O3/TiNi shape memory alloy smart composite biomaterials for orthopedic surgery

Abstract The structure and properties of TiNi/Al 2 O 3 /Ti-alloy composite implant materials consisting of a Ti-alloy body and active TiNi shape memory elements is discussed The structure of composites and ceramic/metal interfaces were evaluated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), accompanied with energy and wave dispersive X-ray microanalysis (EDX, WDX). Mechanical testing of composite materials was performed using an Inova tensile/compression test machine. Bone/implant interaction and design parameters are also discussed. The structure of the TiNi alloy is modified in the casting process. Similar effects to those observed in the annealing process occurred in TiNi materials during casting and cooling of composites. The Ti-alloy matrix is composed of a mixture of α and β phases. The interaction between the ceramic layer and the metallic components depends on the cooling rate in the casting process. Different reaction products were identified on the Al 2 O 3 /metal interface with dependence on the applied cooling rate of the composite. The fatigue life of the investigated composites is greater than 10 7 cycles, and mechanical properties are sufficient to carry loads expected in human medicine. A newly developed smart implant enables quasielastic fixation with an active role of bone tissue in load carrying process, which has a positive impact on treatment.

The electrical transport properties of shape memory alloys

Abstract The effectiveness of shape memory alloys as active elements in thermal actuators is proved by the huge amount of applications developed in the last few decades. In recent years, interest has kept growing in the sensing capabilities that they show when they are heated by an electrical current. The electrical resistance measured thereby gives an easily available feedback of the actuator displacement, provided that a suitable relation is found between the two quantities. It becomes, therefore, compulsory to investigate several alloys and different testing conditions to best fit applications with a suitable material. In this paper, a recently developed apparatus is presented. It allows driving the shape memory element with a finely controlled current waveform while the electrical resistance and the displacement are precisely measured. Besides simulating real operation conditions, this apparatus does not suffer from the high temperature limitations that affect currently used thermostatic-bath-based equipment, it will allow investigating the martensitic transformation and its fundamental properties in the newly developed high-temperature alloys (e.g. NiTiHf). The results of testing, under several constant stress states, two specific materials are also discussed: equiatomic NiTi wires, which show poor sensing features and melt spun TiNiCu ribbons, with remarkably promising sensing/actuating characteristics.

Shape memory micro-actuation for a gastro-intestinal intervention system

This paper describes the design of a prototype gastro-intestinal intervention system based on an inchworm-type of mobile robot. This type of device is a kind of vehicle for inspection through the colon, something that is currently impossible due to the large number of turns in the intestinal system. Eventually, tools for intervention can be added in the future. The overall system is about 95 mm long and has a diameter of 15 mm. The robot consists of three main modules: an extension and contraction module, a two degree-of-freedom bending module and two locking modules. All these modules are to be actuated by shape memory elements. The main part of the paper describes a modular actuator for realising bending motion. The design can be compared to a single vertebra, where stacking several elements can form a spinal column. It will be shown that this design greatly facilitates the control. The electromechanical interconnection of the different parts was an integral part of the design as well as techniques for selectively addressing the different actuators. In order to increase the performance of the proposed design, the last part of the paper discusses some production aspects of the shape memory elements.

Simulation of Cyclic Displacement by Counterpoised Shape Memory Elements

The most common shape memory actuation procedure involves a shape memory element that works against a conventional elastic biasing element. Actuation is provided by the movement of the contact interface separating the two elements. The interface then occupies one location at high temperature and another location at low temperature. Certain applications call for switching between two separate interface locations, both of which are stable at a common ambient temperature. A shape memory element working against a conventional elastic biasing element cannot generally give such capability. However, two shape memory elements working against each other does give this capability, provided that the ambient temperature supports stress-free martensite, and provided as well that the contact interface thermally isolates the elements during short duration heating spikes which trigger such a switch. There is an obvious operational benchmark involving one element with fully oriented martensite and the other element with random martensite. The benchmark operation exchanges these states between the two elements, in which case the device stroke follows immediately from the material’s transformation strain. This benchmark behavior is not achievable due to elastic strain effects. Here we analyze the achievable behavior of such a device using a recent model for shape memory behavior. This model tracks the state of the shape memory material in terms of austenite and two variant martensite. This enables the determination of the device’s shakedown behavior in terms of repeatable per-stroke strain delivery, and also gives the temperature excursions necessary for the associated full displacement.

Shape memory materials and hybrid composites for smart systems. Part II shape-memory hybrid composites

By hybridizing or incorporating shape-memory materials with other functional materials or structural materials, smart composites can be fabricated which may utilize the unique functions or properties of the individual bulk materials to achieve multiple responses and optimal properties, or, to tune their properties to adapt to environmental changes. A variety of shape-memory hybrid composites have been designed and manufactured, with shape-memory elements being either the matrix or the reinforcement. The hybrid composites provide tremendous potential for creating new paradigms for material–structural interactions and demonstrate varying success in many engineering applications. This review, from the standpoint of materials science, will give a state-of-the-art survey on the various shape-memory hybrid smart composites developed during the last decade. Emphasis is placed on the design, fabrication, characterization and performance of fibre-reinforced, particle-reinforced and multi-layered thin-film shape-memory composites.

Generation of Recovery Stresses : Thermodynamic Modelling and Experimental Verification

Shape memory elements generate significant recovery stresses when the shape recovery associated with the reverse martensitic transformation is impeded during heating. This process of stress generation is influenced by many parameters. A fundamental model has been developed to describe this generation of recovery stresses. The model is based on a generalised thermodynamic analysis of shape memory behaviour. The mathematical approach and the assumptions in this model are selected in such a way that the calculations yield close approximations of the real behaviour and that the final mathematical equations are relatively simple. The paper presents also experimental measurements of recovery stress generation for complex restraining conditions. Comparison of the experimental results with the calculated results confirm that this model can be used to predict quantitatively the generation of recovery stresses.

CYCLING EFFECTS, FATIGUE AND DEGRADATION OF SHAPE MEMORY ALLOYS

The stability and lifetime of a shape memory device is characterised by the changes of its transformation temperatures, its cold shape, its hot shape, its global two way shape memory effect, its recovery stress. This global behaviour is influenced by a complex combination of internal and external parameters. Internal parameters are : the alloy system, the alloy composition, the type of transformation and the lattice structure including defects. External parameters are : the thermomechanical treatments, the way of training, the applied stress, the imposed shape memory strain, the amplitude of temperature cycling, the absolute temperature of the environment. The possible physical mechanisms which are at the origin of the limited lifetime of any shape memory element and which are, to a greater or lesser extent, controlled by the above mentioned internal and external parameters, are : the stabilisation of specific martensite variants, the creation of lattice defects during transformation-cycling (transfornation plasticity or defects that could triger unwanted variants), changes in the order of the lattice. changes in defect density and/or defect configuration as a result of ageing. This paper will present an overview of reported and new observations related to those aspects of shape memory alloys. The specific functional properties of shape memory alloys, necessitate an extension of the usual definition of fatigue. Three different types of fatigue have to be considered in the case of shape memory alloys. 1. Failure by fracture due to stress or strain cycling at constant temperature. Three different situations are possible : the material is martensitic during cycling; the material remains beta during cycling: the martensite is stress induced during cycling above the Ar temperature. This type of fatigue has so far received the most attention in the literature / 1-20/. 2. Changes in physical, mechanical and functional properties such as the transformation temperatures, the transformation hysteresis, two way memory effect, ... due to pure thermal cycling through the transformation /21-28/. 3. The degradation of the shape memory effect due to stress, strain or temperature cycling in or through the transformation region /30-37/. The three main parameters to be considered in the study of the global lifetime of a shape memory alloy are thus temperature, stress and the macroscopic shape strain during deformation. It might be Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1991429 C4-190 JOURNAL DE PHYSIQUE IV important for the latter to distinguish between the transformation induced plasticity. this is the shape strain under load during the forward transformation and the spontaneous shape strain due to the one or two way memory effect. The origin of the global lifetime or fatigue behaviour of shape memory alloys is due to accumulation of defects and structural changes like the change in order of the beta phase or martensite or the formation of other types of martensite. All types of changes will have an influence on the transformation temperatures, the transformation hysteresis, the reproducibility of the one and two way memory effect, and the amount of cycles before fracture. Generally, the main concern in the lifetime of a shape memory alloy is the stability of the required shapes at different temperatures or the stability of recovery stress or work output. Therefore information on the temperature and stress profile including the amount of cycles during the required lifetime is a prerequisite to make the proper choices in alloy composition, thermomechanical treatment and design. This paper will present some important results related to classic fatigue, thermal cycling and degradation of the shape memory effect.

CHARACTERISATION OF THE MICROSTRUCTURAL STATE OF A CuZnAl INDUSTRIAL SHAPE MEMORY ALLOY

The thermomecanical treatments used for the industrial production have a great effect on the transformation temperatures of the CuZnAl shape memory elements. We characterize the microstructural state of samples submitted to several kinds of thermomecanical treatments. We used the transmission electron microscopy and X-ray diffraction. A particular effort was made on the observation of the alloy long range order state. The observations show different default structures and different long range ordered domain sizes according to the kind of treatment. The order state is characterized by a size and a number of long range ordered domains 82 and L21, but it seems to be necessary to estimate the short range order degree.

Applications of titanium-nickel shape memory alloys

The shape memory alloy based on the TiNi phase has given rise to a wide range of commercially viable products. Two applications are described which illustrate the different ways in which shape memory can be used in an industrial product. In one of these, the change in shape is prevented so that the shape memory tubular element can be used as a mechanical coupling to join two pipes together securely. Such couplings have replaced welding in hydraulic lines in aircraft and ships and in the repair of undersea pipe lines. The other application demonstrates the use of a tubular shape memory element in torsion to operate a boom latch and release mechanism for the deployment of the booms on a space satellite. It is shown that a shape memory alloy can satisfy a tight and exacting specification in operating as an actuator.

Reply to ’’Comments on ’Efficiency of the solid-state engine made with Nitinol memory material’ ’’

In reply to the comments of Mohamed and Banks, we outline the conditions under which a Ni-Ti shape-memory element operates in a solid-state engine. The expression for the energy output of this element, which was used in the original paper, is derived here, and further aspects of the efficiency formula for this engine are explained.