When a body is deformed by an applied stress, the strain is the ratio of the dimensional change to the original or unstrained dimension. The strain may be a ratio of lengths, areas, or volumes. Linear strain is the ratio of the change in length of an object, such as a bar, to its original length while being stretched or compressed. Shearing strain describes the change in shape of an object whose opposite faces are pushed in different directions.
Hooke's law for elastic materials states that strain is proportional to stress up to the material's elastic limit.
A strain gauge is a device used to measure mechanical strain such as that caused by the movement of a bridge or glacier. Electrical strain gauges used the phenomenon (first studied by Lord Kelvin in 1856) that if a wire is strained, its electrical resistance is changed. An electrical strain gauge usually consists of a grid of wire filaments printed onto a metal foil. The gauge is cemented onto the structural surface whose strain is to be measured. As the strain in the structure changes, the strain in the gauge changes too, thus producing changes in resistance that can be measured.