Abstract: Since the superposition principle applies only to linear operations, dividing strain into volumetric strain and deviatoric strain does not necessarily allow strain energy—a quadratic function of strain—to be decomposed into volumetric energy and distortional energy. This study demonstrates that in linear isotropic materials, spherical stress does not induce deviatoric strain, and deviatoric stress does not induce volumetric strain. Consequently, the strain energy can be decomposed into volumetric energy and distortional energy. However, for anisotropic materials, because part of the volumetric strain is generated by deviatoric stress, half the product of spherical stress and volumetric strain does not represent volumetric energy. Therefore, strain energy in anisotropic materials cannot be decomposed into volumetric energy and distortional energy.