The plastic strain effect on cleavage microcracks propagation

The effect of plastic strain by tension on microcracks propagation is investigated in probabilistic and deterministic statements on the basis of the experimental results for the critical brittle fracture stress and the calculations of brittle fracture probability with local approach model. Investigation is performed for two materials: 2Cr-Ni-Mo-V steel used for WWER-1000 RPV in the thermally-embrittled state and low-alloyed low-strength steel of St.3 grade taken as model material ruptured by cleavage up to plastic strain >50 %. Experimental study includes uniaxial tension of smooth round bars over wide temperature range, mainly over brittle fracture range, and SEM examination of the fracture surfaces. Brittle fracture probability is calculated with the Prometey model. It has been found that the plastic strain effect on the microcrack propagation probability is caused by two reasons: (1) the critical brittle fracture stress increase due to formation of new barriers for microcrack under plastic deformation, and (2) the working volume decrease due to the neck formation in tensile round bar. A procedure has been proposed to separate the above different contributions of plastic deformation in the brittle fracture probability.