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In order to investigate the propagation of cleavage cracks across grainboundaries, a theoretical model of a polycrystal, which consists of prismatic grains, hasbeen developed. Using computer simulation a brittle cleavage crack is allowed to nucleateand propagate across the model. The results indicate that about 32% of the overallfracture surface consists of accommodation, either brittle or ductile, at the grainboundaries. This prediction has been examined by carrying out fracture experiments onspecimens containing columnar ferrite grains formed within C-Mn steel weld metal. Theresults demonstrate that there is indeed a substantial amount of grain boundary failure,which in this case is ductile.