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WCES-2010

Educational stand using shape memory alloys to enhance teaching of smart materials

Daniel Amarieia *, Doina Frunzaverdeb, Ion Velaa, Gilbert Rainer Gillicha

a Center of Advanced Research, Design and Technology, Eftimie Murgu University, P-ta Traian Vuia 1-4, Resita 320085, RomaniabEftimie Murgu University, P-ta Traian Vuia 1-4, Resita 320085, Romania

Received November 10, 2009; revised December 11, 2009; accepted January 21, 2010

Abstract

The goal of this paper is to present a simple but effective stand used for demonstration of Shape Memory Alloys (SMA) behaviour. The hands-on device is developed to offer experiences, previously unavailable, in order to introduce SMAs to anyone,but especially to undergraduate mechanical or materials engineering students.The experimental set-up was designed in order to test SMA active elements in the Device Designing and Robotics Laboratory of Center of Advanced Research, Design & Technology - CARDT from Eftimie Murgu University Resita,. The device features an SMA wire weight-lifter and an actuated spring to demonstrate the force magnitude exerted by the active element under the actionof temperature. The set-up is used as a teaching tool and as an outreach demonstration, being an effective tackle in assisting students to learn about SMAs.Many other experiments and demonstrations, such as identifying thermal transfer functions, predicting and observing the displacement and understanding the phase transition can be derived from this setup. It is found that these demonstrations help tobridge the gap between theory and intuition during a student’s learning process. These demonstrations can motivate students to pursue further studies in smart materials and structures. © 2010 Elsevier Ltd. All rights reserved.

Keywords:Shape memory alloys; smart materials; experiment; teaching tool.

1. Introduction

Smart materials and structures, cited as one of the "key technologies for the 21st Century," is an emerging and important class of materials that gains little exposure to undergraduate engineering students in current engineering curricula. Shape Memory Alloy (SMA) is an important class of smart materials that has been actively researched for their mechanical actuation and control of dynamic systems.

In recent years innovative implementations of SMA include actuation of dynamic systems, vibration control of structures or bio-medical devices. In the close future, there will be a growing need for technologies using the unique

* Daniel Amariei. Tel.: +40-745-862-830; fax: +40-255-207-501 E-mail address: d.amariei@uem.ro

1877-0428 © 2010 Published by Elsevier Ltd.doi:10.1016/j.sbspro.2010.03.829

Procedia Social and Behavioral Sciences 2 (2010) 5104–5108

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characteristics of SMA. To respond to this future need, there is an immediate need to integrate smart materials and structures into mechanical and materials engineering curricula.

It has been shown that a significant portion of students are visual, sensing, and active learners who are at a disadvantage when taking traditional engineering lecture courses that do not allow them to experience the technology and concepts being taught in class. It is necessary for them to touch, feel, and see examples before they can fully understand and process the course concepts.

To assist in the teaching of smart materials and to expose SMA to a wider based student body, a series of demonstrations and experiments have been developed directly in the Device Designing and Robotics Laboratory of Center of Advanced Research, Design & Technology - CARDT from Eftimie Murgu University of Resita.

To further enhance undergraduate learning in the area of smart materials, the development of these demonstrations was assigned as a senior capstone design project allowing the students in mechanical, materials engineering and electrical engineering to gain hands-on experience in designing an intelligent system.

The most innovative component of these remote demonstrations/experiments is the utilization of different kinds of smart materials, i.e., materials that are considered to be "responsive." The produced response is often the conversion of one form of energy to another in useful quantities. For example, a SMA returns to a trained shape from a deformed state when heated above its transformation temperature. While there are many kinds of smart materials, the demonstrations and experiments described in this paper focus on the use of SMAs.

2. Shape Memory Alloys

Shape Memory Alloy (SMA) materials are metallic alloys that have the special property of being able to return to a pre-determined, or "trained," shape from a deformed state when the material is heated above its transformation temperature. A number of alloy types are known to exhibit the Shape Memory Effect (SME), or the ability to revert to a trained shape when heated, including gold-cadmium and nickel-titanium, or Nitinol. The SME follows from the changes in atomic crystal structure that the alloy forms at different temperatures.

For example, above its transformation temperature, Nitinol exists with a strong cubic packed Austenite structure. However, at lower temperatures a Martensite structure is formed in which any four contiguous atoms form a weak structure resembling a parallelogram in two dimensions. These martensite structures form a row of parallelograms "tilting" uniformly in a single direction. The next row of martensite structures tilts in the opposite direction, as pictured in Figure 1, creating the so-called twinning effect, where the rows appear to be symmetrical, or twinned, through a plane of symmetry.

Figure 1. Transformation from Austenite to Twinned Martensite Structure

The weakness of the martensite allows the material to be easily deformed by mechanical forces. When a mechanical shearing force is applied to the material at a low temperature, the crystals are re-aligned such that the rows all tilt in the same direction in a process called de-twinning (Figure 2).

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Figure 2. De-twinning of Martensite at Low Temperatures

At low temperatures, then, the material behaves like a normal, malleable metal alloy. However, the SME takes place when the alloy is heated above this transformation temperature which causes the atomic crystal structure of the atoms to change from martensite to austenite. As the atomic crystal structure changes, so, too, does the overall shape of the material, reverting to a pre-determined shape created by training the material above the transformation temperature over time. Due to the training, the overall austenite form is maintained through cooling and deformation. If any resistance is encountered during the transformation process, the material itself can exert extremely large forces to counter.

3. The Interactive SMA Demonstrations

The experimental test-bed consists of two demonstrations: a SMA Wire Weight - Lifter and a SMA Actuated Spring. User input is provided through two buttons, one for the tension and one for current. In the same frame are the

SMA wire weight-lifter on one side and the SMA actuated spring on the other side. The background information is provided in the posters behind the frame (Figure 3), aimed to introduce the audience with some basic knowledge to the properties of SMA and its transformation process. Explanations of SMA transformation changes are given in words and in diagrams to help the user better understand what makes the SMA effect possible. Additionally, test specific information will be given to outline what the demonstration presents as well as the expected outcomes of user interaction with the demonstration.

3.1. The SMA wire weight-lifter

The SMA Wire Weight-Lifter is designed to illustrate the SMA's ability to exert a large force during transformation against resistive forces. The demonstration also uses an active control system to maintain a controlled height for the weight. A 0,250 diameter Nitinol wire, twisted into a single strand, loop over one pulley mounted on a steel shaft supported by a carbon steel frame. The wire is capable of supporting a 930 gmf recovery force. One end of the strand is attached to the rigid base of the display while the other end is attached to and suspends a 100 gr. weight, from the 172 gr. weight supported. The user is able to induce Joule effect (resistive – 0,250 m wire has 20 / m resistance) heating in the SMA wires, with a recommended current intensity of 1,00 mA for this chosen diameter, causing the wire to undergo their thermal transformation from a martensitic phase to an austenite crystal structure (between 500 to 600 C) with an overall effect of a maximum 5-6 % contraction in the length of the wire. As the SMA wire contracts, the weight is lifted vertically.

3.2. The SMA actuated spring

This demonstration is a weight lifting mechanism composed by SMA Actuated Spring is designed to illustrate the Nitinol's springs (tension or compression) capacity as an actuator by lifting the weigh.

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The active component of the developed experiment is a Nitinol tension spring made from 750 μm diameter wire, the coil of the spring being 6 mm, heated by the electric current, attached to a weight of 350 g. An electrical current of 2 A activates the phase transformation between 450 and 550 C. The spring is deformed at a temperature below Mf,followed by unloading and again loading with a reaction R. Shape recovery occurs at an opposing force R during heating to a temperature above Af, so work is done. A feedback control using the measurement from a sensor is used to maintain the desired position of the weight under actuation from the Nitinol springs.

Figure 3. Posters providing background information

The other carried out tests involve a compression spring made from wire of 950 μm diameter, forming a spring coil of 9 mm. Can be activated at 55-650 C by a current of 3 A, developing a force exceeding 4 N.

In both cases, the springs are able to perform a maximum stroke of 25 mm.

Figure 4. The Experimental Stand

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4. Conclusions

This paper has presented the development, operation, and evaluation of two easy-to-use, interactive SMA wire demonstrations composed of an SMA Wire Weight-Lifter and an SMA Actuated Spring. These demonstrations are designed to assist students in learning about SMAs and their applications and to educate the general public on the topic of SMAs and smart materials in general. After the integration of the demonstrations into the "Intelligent Structural Systems" course, we hope that the SMA interactive demonstrations will be "very effective" in demonstrating the concept of SMA effect.

In addition to direct educational benefits, the indirect outcomes of these exercises include a contact to presence technology and intelligent materials, help students to correlate observed phenomena with quantified data, broadening tools that will become essential for future engineers.

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