The .16 Mnr Steel, Ductile-brittle Transition Cleavage Fracture,

In low temperature cleavage physical model, it is of paramount importance to ascertain the nature of the critical event in the formation of a cleavage crack.. Three point bending (3PB) tests of precracked specimens and notched specimens were carried out for coarse and fine grain 16MnR steel at ductile-brittle transition temperature. Based on observations of fracture surfaces and detailed observation of configuration changes at two kind of specimens by metallographic cross sections in specimens unloaded at various applied loads, sophisticated FEM calculations of distributions of stress, strain and triaxiality ,the fracture behavior at crack tip is analyzed. The results revealed that before critical load is reached ,the crack tip is blunted only and its vicinity three criteria for cleavage fracture are satisfied in different regions separated from each other. Hence, the cleavage fracture can not be triggered. For notched specimen, the critical event of cleavage fracture determined by ferrite grain size is the propagation of a grain-sized crack into a neighboring grain. For precraked specimen , the elevation of normal stress ahead of the ductile crack is related to the stress intensification and the strain hardening. The mechanism of ductile-brittle transition was considered as the result of the transition of stress as determined by the second phase particle to that as determined by the ferrite grain. At the same time, the precraked specimen presented the characteristics of cleavage in a notched specimen.Based on ductile damage mechanics of Gurson-Tvergaard-Needlemen model in software Abaqus, the reasons for difference between elastic-plastic behavior and involving ductile damage behavior in steel 16MnR was discussed in detail. It is compared the relationship between void fraction due to growth and void fraction due to nucleation at different applied load.