Steady state crack propagation in solids is analyzed as a thermally activated process. The fracture mechanics concept of a crack driving force is formally introduced to molecular rate theory. This representation of crack propagation appears to be, in many aspects, similar to that of the motion of a dislocation under a shear stress across thermal obstacles. The basic thermodynamic relations are derived for steady state crack propagation using assumptions similar to those well accepted in theories of deformation based on thermally activated dislocation motion.

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Slow crack propagation data in PMMA previously published by Atkins et al. are reanalyzed in terms of a thermodynamic model in which the activation area is inversely proportional to the crack extension force. Important thermodynamic activation parameters are derived from the data. A correlation between the activation area and the crack extension force is observed in many brittle materials; it is compared to Li's general correlation for activation areas in creep.

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