How does a strain gauge work?

A strain gauge measures strain (deformation) based on the principle that electrical resistance changes when a conductor is stretched or compressed. The gauge consists of a thin metallic foil pattern on a backing, bonded to the test surface. When the surface deforms, the gauge deforms with it, changing resistance: Delta_R/R = GF x epsilon, where GF is gauge factor (typically 2 for metal, 100+ for semiconductor) and epsilon is strain. Strain gauges are used in load cells, pressure sensors, and structural monitoring.