MODELING CLEAVAGE FRACTURE OF BAINITIC STEELS

The origin of brittle fracture of polycrystalline metals failing by cl eavage is most frequently associated to slip-induced cracking of some non-metallic brittle particle or inclusion (a carbide in ferritic stee ls). When the size of the particles is smaller than the grain size of the metallic matrix, the nucleating event of a macroscopic failure res ults from the successive occurrence of three simple events: slip-induc ed cleavage of a particle, transmission of the microcrack to the neigh bouring grain across the particle/matrix interface and propagation of the grain-size microcrack to the neighbouring grains across the grain boundary. On the basis of this scheme, a statistical ''weakest link'' fracture model has been developed which takes into account the presenc e of two independent distributions of structural elements (isolated pa rticles and matrix grains) with two barriers for cleavage propagation (the particle/matrix interfaces and the grain boundaries), characteriz ed by a crack arrest capability well over the crack propagation resist ance of the cleavage planes of the crystalline lattices of the two pha ses. An application of the model to the prediction of the fracture str ess and the critical stress intensity factor as a function of the temp erature of a bainitic steel is presented.