Fracture Process of a Control-Rolled Steel in the Ductile to Brittle Transition Temperature Region

The mechanism of ductile to brittle transition has been investigated with the V-notched Charpy test of a control-rolled low alloy steel.(1) The absorbed energy is found to be in a unique correlation with the lateral expansion ratio of specimens irrespective of anisotropy of specimens, testing temperatures and scattering of energies. It means that the absorbed energy is determined by the amount of plastic deformation by bending, and that the change in the yield stress and the anisotropy of the specimen play a role through the critical amount of deformation.(2) The fractographic observation indicates that a stretch zone is formed at the notch root, and that multiple activations of crack sources take place. Characteristics of the fracture surface features are common to different orientations of specimens.(3) The dependence of the transition curve on the texture is strong on (110) rather than on (100).(4) The above results imply that the transition behavior is closely associated with the plastic deformation prior to the onset of brittle fracture. It is suggested that the stress controlled mechanism for one fracture source activation is not appropriate, but that some intrinsic change in the matrix during the course of plastic deformation is essential for the transition behavior.