5.Thermal or magnetic(Shape memory alloys) actuators
Actuators which are actuated by thermal or magnetic energy.these types of actuators mainly uses shape memory alloys .
A shape-memory alloy (SMA, smart metal, memory metal, memory alloy, muscle wire, smart alloy). these are economical ,light in weight ,possesses max power density .
SMA is the combination of different -2 type of alloy like zink,copper,gold and iron.
SMA is an alloy which remember its original shape.when the alloy (shape-memory alloy) is heated then it change form its deform position to pre deform position.
there are mainly two types of SMA
copper aluminium nickel alloy –
these types of SMA alloy commercially available and much cheaper than the nickel titanium alloy .
nickel titanium alloy
these are most important type of alloys and these are costly than other SMA alloys.but they are much applicable then copper aluminium nickel alloy due to their stability, practicability and superior thermo-mechanic performance.
SMA can exist in two different phase
we can find out the phase position with the help of temperature and stress
in this phase the temperature of the alloy is lower and it can be converted in any shape in this phase.when the temperature of the alloy is increase then it converted into martensite to austenite.
when the temperature of the alloy is higher , then they are in ausenite phase .
Here E(T) represents martensite fraction
T represents the temperature
M(f) is the temperature at which the transition to Martensite is finished during cooling
A(s) is the temperature at which the transformation from martensite to austenite starts .
A(f) is the temperature at which the transformation from martensite to austenite finishes.
if we use the alloy again and again in this case their is a shift of temperature and properties of the material.
due to repeat-ability (repeat use of alloy material ) ,micro-structure and the properties of the alloys material changes .this change effect is known as functional fatigue .
Transition temperature it is the temperature at which the material changes its own phase from martensite to austenite
One way and Two way shape memory alloy Effects
ONE WAY SHAPE MEMORY ALLOY EFFECT
when the material at low temperature [lower than the A(s)] then it can be shaped in any shape until heated above transition temperature.Upon heating, the shape changes to its original. When the metal cools again it will remain in the hot shape, until deformed again.in this case cooling from high temperature does not cause the macroscopic shape change .
TWO WAY SHAPE MEMORY ALLOY EFFECT
when the material does not remember shapes at different-2 temperature means at low temperature it gain a shape but as the temperature increases higher than the transition temperature then it imparts different shape.it means it forget it’s shape when the temperature is change form low to high .during transformation, material changes its micro-structure due to this reason material forgets its earlier shape. in this shape effect we can not predicts the exact shape of the material .above a certain temperature the material loses the two way memory effect this is called “amnesia“
Application of SMA