PSEUDOMOMENTUM AND MATERIAL FORCES IN NONLINEAR ELASTICITY - VARIATIONAL FORMULATIONS AND APPLICATION TO BRITTLE-FRACTURE

This work examines critically the various formulations of the balance of linear momentum in nonlinear inhomogeneous elasticity. The corresponding variational formulations are presented. From the point of view of the theory of elastic inhomogeneities, the most interesting formulations are those which, being either completely material or mixed-Eulerian, exhibit explicitly the inhomogeneities in the form of material forces. They correspond to the balance of pseudomomentum, a material convector which is seldom used but which we show to play a fundamental role in the Hamiltonian canonical formulation of nonlinear elasticity. The flux associated with pseudomomentum is none other than the Eshelby material tensor. Applying this formulation to the case of an elastic body containing a crack of finite extent, the notion of suction force acting at the tip of the crack follows while a fracture criterion a la Griffith can be deduced from a variational inequality. Possible extensions to higher-grade elastic materials and inelastic materials are indicated as well as the role played by pseudomomentum in the quantization of elastic vibrations.