Shape memory alloys

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WELCOME

Shape Memory Alloy

SHAPE MEMORY ALLOYS(The Metals That Came from Outer Space)

GUIDED BY : PRESENTED BY: MANJU GEORGE ELDHO PETER

Dept. of Civil Engg. S7 CE ROLL

NO:21 MBITS

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INTRODUCTIONShape Memory Alloys are materials that “remember”

their original shape.

If deformed, they recover their original shape upon heating.

They can take large stresses without undergoing permanent deformation.

They can be formed into various shapes like bars, wires, plates and rings thus serving various functions.

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HISTORY1938: Arne Olande observed shape

and recovery ability of Au-Cd alloy.

1938: Greninger and Mooradian observed the formation and disappearance of

martensitic phase by varying the temperature of a Cu-Zn alloy.

1962-63: Ni-Ti alloys were first developed by the United States NavalOrdnance Laboratory.

Nitinol – Nickel Titanium Naval Ordnance Laboratories. Shape Memory Alloy

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PROPERTIESSMA’s exhibit 2 important properties.1. Shape Memory Effect2. Pseudo-Elasticity

Shape Memory Effect

Austenite and Martensite – Two phases exhibited by SMA’s in solid state.

SMA moulded into parent shape in austenite phase.

Shape Memory Alloy

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What is shape memory effect (SME)?

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If deformed in martensite phase, the parent shape is regained upon heating.

As and Af are the temperatures at which transformation from martensite to austenite starts and finishes.

Transition dependant on temperature and stress.

Repeated use of shape memory effect leads to functional fatigue.

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Pseudo-Elasticity

Occurs without temperature change.

This property allows the SMA’s to bear large amounts of stress without undergoing permanent deformation.

Applications: Reading glasses.

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Temperature of SMA is maintained above transition temperature.

Load is increased until austenite transforms to martensite.

When loading is decreased, martensite transforms back of austenite.

SMA goes back to original shape as temperature is still above transition temperature.

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WORKING OF SMA’SA molecular

rearrangement in the SMA’s austenite and martensite phase is responsible for its unique properties.

Martensite is relatively soft and occurs at lower temperatures.

Austenite occurs at higher temperatures.

The shape of austenite structure is cubic.Shape Memory Alloy

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The un-deformed Martensite phase is the same size and shape as the cubic Austenite phase on a macroscopic scale.

No change in size or shape is visible in shape memory alloys until the Martensite is deformed.

To fix the parent shape, the metal must be held in position and heated to about 500°C.

The high temperature causes the atoms to arrange themselves into the most compact and regular pattern possible resulting in a rigid cubic arrangement (austenite phase).Shape Memory Alloy

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APPLICATIONS1. Reinforcement

SMA’s are particularly beneficial for construction in seismic regions.

If SMA is used as reinforcement, it will yield when subjected to high seismic loads but will not retain significant permanent deformations.

The seismic performance of reinforced concrete and steel columns with SMA bars was investigated by Wang (2004).

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2. Bolted Joints Beam-column and column-foundation joints

are the weakest links in a structure during an earthquake.

SMA materials can be effectively employed in such joints to reduce their vulnerability by dissipating greater energy through large plastic deformation and then recovering it on removal of load.

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3. Prestressing

The benefits of employing SMAs in prestressing include:

i. Active control on the amount of prestressing ii. No involvement of jacking or strand-cuttingiii. No elastic shortening, friction, and anchorage

losses over time

SMA strands are used in pre tensioning and post tensioning.

SMA’s in prestressing have the potential for creating smart structure.

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4. Restrainers

One of the main problems of bridges during earthquakes is their unseating because of excessive relative hinge opening and displacement

. Limitations of existing unseating-prevention devices include small elastic strain range,

limited ductility, and no recentering capability.

These limitations can be overcome by introducing SMA restrainers as they have larger elastic strain range and can be brought back to its original position even after deformation

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CONCLUSIONSMA’s have the potential to be used

effectively in seismic regions.

The high cost of SMAs is a major limiting factor for its wider use in the construction industry.

Their capability to allow the development of smart structures with active control of strength and stiffness and ability of self-healing and self-repairing opens the door for exciting opportunities, making them the construction material of the future.

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REFERENCE1. K Otsuka, C M Wayman, Shape Memory Materials,

Cambridge University press, 1999.2. Kauffman, George and Isaac Mayo, (1996) The Story of

Nitinol: The Serendipitous Discovery of the Memory Metal and Its Applications, The Chemical Educator, VOL. 2, pp1430-4171.

3. Peter Filip and Karel Mazanec, (1995)  Influence of work hardening and heat treatment on the substructure and deformation behaviour of TiNi shape memory alloys, Scripta Metallurgica et Materiala, Volume 32, Issue 9, 1 May 1995, pp 1375-1380.

4. Rogers, Craig., (1995), Intelligent Materials, Scientific American Sept. 1995:, pp154-157.

5. Lin, Richard., Shape Memory Alloys And Their Applications, (Created: January 21, 1996. Last modified: February 22, 2008), <www.stanford.edu>, (July 17, 2011). Shape Memory Alloy

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Thank You