2.1. Microscopic process of cleava
ge fracture initiation
We assume that the microscopic process of
cleavage fracture initiation in ferrite-cementite steels is composed of
three stages; (I) formation of fracture origin by cementite cracking, (II) propagation of the crack in cementite into
the ferrite matrix and formation of a cleavage crack, and (III) propagation of the cleavage crack across ferrite grain
boundary (see Fig. 1). It is assumed that a macroscopic brittle f
racture occurs if a series of the fracture conditions of
the above stages is satisfied. In the following section, the fracture conditions of respective stages are formulated.