Strain (ε) is defined as the amount of deformation per unit length of an object when a load is applied. Strain is calculated by dividing the total deformation of the original length by the original length: ε = ΔL / L where ε is strain, ΔL is change in length, L is initial length. The magnitude of measured strain is generally very small and, as such, is expressed as micro-strain (μ-strain), which is ε x 10-6.
Strain gages convert mechanical deformation into an electrical signal. If a wire is held under tension, it gets slightly longer, its cross-sectional area reduces and its resistance increases. The rate of change in resistance to the change in strain is called the gage factor (GF) and is given by: GF = (ΔR / R) / ε where ΔR is change in resistance, R is initial resistance, and ε is strain.
Strain gages can be made from a variety of materials. Each material has its advantages and disadvantages in terms of performance and application.