SHAPE MEMORY ALLOYS

Presented by

Afsal .f

S7 Mechanical

Shape memory alloys (SMA's) are metals, which exhibit two very unique properties, pseudo-elasticity, and the shape memory effect.

The most effective and widely used alloys include Ni Ti (Nickel Titanium), Cu Zn Al and Cu Al Ni

The two unique properties described above are made possible through a solid state phase change, that is a molecular rearrangement, which occurs in the shape memory alloy solid state phase change is similar in that a molecular rearrangement is occurring, but the molecules remain closely packed so that the substance remain solid

Introduction

General principles

Shape memory metal alloy can exist in two different temperature dependent crystal structures (phases) called martensite (lower temperature ) and austenite ( higher temperature or parent phase ).

General principles

Martensite is relatively soft and easily deformed phase exists at lower temperature

Molecular structure is twinned Upon deformation this phase takes on

second form Austenite Austenite, the stronger phase occurs at

higher temperature Shape of Austenite is cubic

Shape Memory Effect

It is observed when the temperature of a piece of SMA is cooled to below the temperature Mf

At this stage the alloy is completely composed of Martensite which can be easily deformed

After distorting the SMA the original shape can be recovered by heating the wire above the temperature Af

Shape Memory Effect

The Shape memory effect is currently being implemented in:

Coffepots The space shuttle Thermostats Vascular Stents Hydraulic Fittings (for Airplanes)

Pseudo-Elasticity

Pseudo-elasticity occurs in shape memory alloys when the alloy is completely composed of Austenite (temperature is greater than Af). Unlike the shape memory effect, pseudo-elasticity occurs without a change in temperature. The load on the shape memory alloy is increased until

the Austenite becomes transformed into Martensite simply due to the loading;

Pseudo-Elasticity

Figure shows the process of pseudo elasticity

Examples of applications in which pseudo-elasticity is used are:

Eyeglass Frames Medical Tools Cellular Phone Antennae

Applications Aircraft Maneuverability 

The wire on the bottom of the wing is shortened through the shape memory effect, while the top wire is stretched bending the edge downwards, the opposite occurs when the wing must be bent upwards. The shape memory effect is induced in the wires simply by heating them with an electric current

Hinge less shape memory alloy Flap 

Medical Applications

1. Stents

2. Vena-cava Filters

3. Dental and Orthodontic Applications

Bone Plates

Bone plates are surgical tools, which are used to assist in the healing of broken and fractured bones.

The breaks are first set and then held in place using bone plates in situations where casts cannot be applied to the injured area. Bone plates are often applied to fractures occurring to facial areas such the nose, jaw or eye sockets.

Repairs like this fall into an area of medicine known as osteosynthesis.

Fracture repair

Plate and screw

Reconstruction of fractured face using bone plates

Shape memory alloys mimic human muscles and tendons very well. SMA's are strong and compact so that large groups of them can be used for robotic applications,

Robotic muscle

Working of SMA in a Robotic muscle

MILITORY APPLICATION OF SMA

For the military, one of the key features of SMA’s is that they can produce particularly strong forces without creating a detectable signature - such as a wake of the type produced by a propeller

SMA actuators can give an underwater vehicle characteristic more like those of a fish or other underwater species

Research is focusing on developing underwater vessels that "mimic" the movement of fish

Artist’ s impression of an underwater vehicle using SMA’s.

Advantages

Bio-compatibility Diverse Fields of Application Good Mechanical Properties (strong, corrosion resistant)

Disadvantages

These alloys are still relatively expensive to manufacture and machine compared to other materials such as steel and aluminum. Most SMA's have poor fatigue properties

Some applications of SMA

 Aircraft flap/slat adjusters  Arterial clips  Automotive thermo  Cold start vehicle actuators  Contraceptive devices  Electrical circuit breakers  Fibre-optic coupling

Filter struts Fire dampers Fire sprinklers Gas dischargeIntraocular lens mount Kettle switches Keyhole instruments Key-hole surgery instruments

Conclusion Future applications include engines in cars and airplanes

and electrical generators utilizing the mechanical energy resulting from the shape transformations.

Nitinol with its shape memory property is also envisioned for use as car frames.

possible automotive applications using SMA springs include engine cooling, carburetor and engine lubrication controls.