Chemistry Reference
In-Depth Information
als, nitinol has a better memory and can recover from more severe
deformations.
In addition to memory, SMAs have another, related property that
enables them to snap back from loading strains. Elasticity is a property
of a material in which it returns to its original shape aft er being bent
or dented by a pressure, as from carrying a load, for instance. A rubber
ball is elastic—a fi nger poking the surface will create a dent, but the ball
regains its round shape when the fi nger is removed. Many materials are
elastic, at least to a certain extent, though all such materials have a limit
beyond which they cannot recover, in which case the material breaks or
is permanently deformed.
Stress created by a load will cause an SMA to deform, but if the
temperature is high, it will also cause the material to transition into the
soft , deformable phase. Th is process is similar to the transition that oc-
curs from high to low temperatures, as discussed in the sidebar on page
120, except in this case the material started out in the high-temperature
phase and it is the stress that causes the transition—the temperature
does not need to change. When the load is removed, the SMA automati-
cally regains its original shape, unless the resulting strain was too much.
Th e advantage in using an SMA over other materials is that SMAs can
recover from greater strains. Nitinol can bend about 15-20 times more
than steel before breaking.
(A) Wire made of a special kind of smart material has been fashioned
into a certain shape. (B) The wire gets bent out of shape. (C) Heat is
applied, elevating the material's temperature. The wire automatically
returns its original shape. (D) The shape is maintained after the
material cools.
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