Thermally activated crack propagation takes place by consecutive bond breaking and occasional mending steps. This process can be represented by a differential equation that describes the crack-size distribution as a function of time for single-step processes. Because of the essential physical similarity of crack propagation with diffusion, the functional solution of the fracture equation can be developed advantageously by utilizing the mathematical theory of diffusion. The results describe the crack-size distribution as a function of the surface free energy (bond energy), temperature, as well as the loading and geometrical boundary conditions of the test specimen.