2.2. Formulations of fracture conditions
Formation of fracture origin by cementite cracking (Stage I)
It is known that a cracking of brittle phase such as cementite works as a trigger of cleavage fracture initiation.
We employ the probability of cementite cracking formulated in the author’s recent study (Shibanuma et al.,
2013) as the fracture condition of Stage I. They formulated it as a function of cementite particle thickness and
macroscopic stress and strain, based on an observation result and an estimation of internal stress of the cementite
particle by FEA. A brief overview of the study is shown as follows.
It can be assumed that a cementite cracking occurs when t
he internal stress of cementite particle reaches to the
fracture stress. Thus they formulated the probability of cementite cracking as a function of the size and the estimated
value of the internal stress of cementite particle.
The estimation formula of internal stress
of the cementite particle from macroscopic stress and strain was
developed as a function of the maximum principal stress and the equivalent plastic strain , based on the numerical results of FEA, as