On surface energy and the continuum thermodynamics of brittle fracture

When a separating body is viewed as a nonequilibrium thermodynamic process, the full thermodynamic nature of the surface energy induced by crack propagation becomes apparent. Within such a general framework it is no longer possible to view the surface energy as a material constant. For the self-propagating crack the induced surface energy is shown to depend explicitly on the square of the crack propagation speed. It is also shown that a separating body produces entropy even though the mechanical response of the solid may be elastic. The introduction of surface quantities such as surface energy into the continuum description of the fracture process forces a major departure from the mechanics appropriate to the nonseparating body. Local equations of balance are no longer obtainable as derived consequences of postulated global balance equations. They must instead be imposed as separate additional postulates.