PERCOLATION OF CLEAVAGE THROUGH A FIELD OF GRAINS AND THE CLEAVAGE FRACTURE RESISTANCE OF POLYCRYSTALS: EXPERIMENTS AND MODELS

摘要

To ascertain the role of grain boundaries in the cleavage fracture resistance of polycrystals in the lower shelf region in steel, experiments were carried out at -30C on the modes of advance of cleavage cracks through a field of randomly oriented grains of 4-6mm size in a Fe-2%Si alloy, permitting detailed observation of the chronology of percolation of the crack front. Utilizing both previously developed models of fracture resistance of individual grain boundaries and new considerations of bridging shears along grain boundaries connecting adjacent primary cleavage facets in adjoining grains, together with standard weight function approaches of determining local stress intensities along piece-wise straight current fracture fronts it was possible to model in considerable detail both the chronology of percolation of cleavage through grains as well as to model the overall cleavage fracture resistance of polycrystals. The experiments and associated analysis have established that grain boundaries play a dual role in governing the cleavage fracture resistance of polycrystals: first through their random misorientation of principal cleavage planes in grains away from the median fracture plane resulting in a 31% increase in fracture resistance, and second, through the required Bridging-shear processes along grain boundaries connecting primary cleavage facets.