 In the case of viscous fluids, plastic and elastic solids, the pressure force is generalized to the stress tensor, and changes in volume are generalized to the volume multiplied by the strain tensor .These then form a conjugate pair.If formula_25 is the "ij" component of the stress tensor, and formula_26 is the "ij" component of the strain tensor, then the mechanical work done as the result of a stress-induced infinitesimal strain formula_27 is