expansion bellows

From Wikipedia
Expansion Bellows (No listing)

Metal Bellows
Metal bellows are elastic vessels that can be compressed when pressure is applied to the outside of the vessel, or extended under vacuum. When the pressure or vacuum is released, the bellows will return to its original shape (provided the material has not been stressed past its yield strength).
Bellows technology of the twentieth and twenty-first century is centered on metal bellows. These high-technology products bear little resemblance to the original leather bellows used traditionally in fireplaces and forges.
There are two main types of metal bellows - formed and welded. Formed bellows are produced by a variety of processes, including cold forming (rolling), electroforming and hydroforming. They are also called convoluted bellows or sylphons.

Welded bellows (also called edge-welded, or diaphragm bellows) are manufactured by welding a number of individually formed diaphragms to each other. The comparison between the two bellows types generally centers around cost and performance. Hydroformed bellows generally have a high tooling cost, but, when mass-produced, may have a lower piece price. However, hydroformed bellows have lower performance characteristics due to relatively thick walls and high stiffness. Welded metal bellows are produced with a lower initial tooling cost and maintain higher performance characteristics. The drawback of welded bellows is the reduced metal strength at weld joints, caused by the high temperature of welding. Electroformed bellows can be produced with modest tooling costs and with thin walls (25 micrometres or less), providing them with high sensitivity and precision in many exacting applications.[1]


Expansion Joint
An expansion joint is an assembly designed to safely absorb the heat-induced expansion and contraction of various construction materials, to absorb vibration, or to allow movement due to ground settlement or earthquakes. They are commonly found between sections of sidewalks, bridges, railway tracks, piping systems, and other structures.

My comment:
If you are working with piping I would only use the term "Expansion Joint."