Change of constitutive relations due to interaction between strain-gradient effect and material gradation

For classical elasticity, the constitutive equations (Hooke's law) have the same functional form for both homogeneous and nonhomogeneous materials. However, for straingradient elasticity, such is not the case. This paper shows that for strain-gradient elasticity with volumetric and surface energy (Casal's continuum), extra terms appear in the constitutive equations which are associated with the interaction between the material gradation and the nonlocal effect of strain gradient. The corresponding governing partial differential equations are derived and their solutions are discussed.