At a point in a stressed body, the plane state of strain is fully characterized by three quantities (εx, εy, γxy), which can be transformed to find normal and shear strain in any orientation, the principal strains, and the maximum shear strain via strain-transformation equations or Mohr's circle of strain; because strain gauges measure only normal strain, γxy must be inferred indirectly from three normal-strain measurements taken in different directions (a strain rosette) using the strain-transformation equation. Separately, the theory establishes that the elastic constants of an isotropic material — modulus of elasticity E, shear modulus G, and Poisson's ratio μ — are interdependent (G = E / [2(1+μ)]), and defines the bulk modulus K = E / [3(1−2μ)] relating hydrostatic pressure to dilation (fractional volume change).
At a point in a stressed body, the plane state of strain is fully characterized by three quantities (εx, εy, γxy), which can be transformed to find normal and shear strain in any orientation, the pri…