CLEAVAGE FRACTURE BEHAVIOR IN NOTCHED SPECIMENS OF 16MNR STEEL

The cleavage initiation ofnotched specimens of a low alloy hot rolled steel (16\,MnR) with carbides andinclusions at -196^\circC and -130^\circC is studied in this paper.The results of tensile tests,four point bending tests of single and double notched specimens, microscopicobservations and finite element method calculations show that two types ofcleavage initiation sites in notched specimens can be identified, oneisrelated to the inclusions ahead of the notch root (IC type) and theothersrelated to the inclusions located ahead of string cracks far from the notch root(SIC type). The types of initiation sites are influenced strongly bytemperature, changing from IC type at -196^\circC to SIC type at-130^\circC. In both IC and SIC initiation mechanisms, the cracknucleation is induced by inclusions and the final fracture is controlled bythe propagation of a ferrite grain-sized crack into the matrix grain. The cleavagefracture of IC initiation type in notched specimens can be described by adual-criterion model, i.e. a critical plastic strain \varepsilon_\rm p \ge \varepsilon_\rm pc for initiating a crack nucleus and a critical tensilestress \sigma_yy \ge \sigma_f for its propagation. While for SIC initiationtype, the dual-criterion model is to be modified with the expression of\varepsilon_\rm p + \varepsilon _\rm ps \ge\varepsilon_\rm pc and \sigma _yy +\sigma_yy\rm s \ge \sigma_f.