Most materials expand when heated. As long as the temperature change isn't too large, each dimension of an object expands according to the equation:

ΔL = L_oαΔT

or L = L_o (1+αΔT)

where L_o is the original length.

A bimetallic strip is composed of two different materials that have different expansion coefficients. Let's say these materials are brass and steel, which have coefficients of linear expansion of

Brass: 19 x 10^-6 /°C

Steel: 11 x 10^-6 /°C

Say you have a strip of each material, and the strips have equal lengths at room temperature. The strips are bonded together into one bimetalic strip. When this is heated the brass expands more than the steel, and the strip curves with the brass on the outside of the curve. If the strip was cooled from room temperature instead it would curve the other way, with the steel on the outside.

Bimetallic strips are used as switches in thermostats.

An object of dimensions L_o, W_o and H_o has a volume:

V_o = L_o W_o H_o

When the temperature changes the volume changes to:

V = L_o W_o H_o [1 + αΔT]³

V = V_o [1 + 3αΔT + 3(αΔT)² + (αΔT)³]

αΔT is much less than 1, so (αΔT)² and (αΔT)³ are negligible. So:

V = V_o [1 + 3αΔT]

V = V_o [1 + βΔT]

where β, the coefficient of volume expansion, is three times larger than α the coefficient of linear expansion.

If an object is heated or cooled and it is not free to expand or contract (it's tied down at both ends, in other words), the thermal stresses can be large enough to damage the object, or to damage whatever the object is constrained by. This is why bridges have expansion joints (check this out where the BU bridge meets Comm. Ave.). Even sidewalks are built accounting for thermal expansion.

Materials that are subjected to a great deal of thermal stress can age prematurely. Airplanes are good examples - over the life of a plane the metal is subjected to thousands of hot/cold cycles that can weaken the airplane's structure.

Another common example of thermal stress occurs with water, which expands by about 10% when it freezes. If the water is trapped in an enclosed space when it freezes the ice can exert significant pressure on the container.