A strain gage can measure strain in only one direction—the axis along which the strain gage is mounted. To effectively measure the three independent components of plane strain (extensional strain along X and Y axis, as well as shear strain), three independent strain measurements are needed. Strain gage rosettes are used to perform such measurements.
A strain gage rosette is an arrangement of two or three closely positioned strain gages, separately oriented to measure the strains along different directions of the underlying surface of the object being measured.
Strain-gage manufacturers offer three basic types of strain gage rosettes.