DEPENDENCE OF CRACK FORMATION ON CRYSTALLOGRAPHIC ORIENTATION FOR ICE

Observations are reported of crack propagation in columnar- grain, polycrystalline ice subjected to constant compressive load applied perpendicular to the long axis of the columns. About three-quarters of the cracks observed were transcrystalline and the remainder occurred at grain boundaries. The plane of the cracks tended to be parallel to the direction of the applied load. Transcrystalline cracks tended to propagate either parallel or perpendicular to the basal plane. At least two-thirds of the grain boundary cracks were associated with boundaries for which the slip plane of one or both of the adjacent grains was close to parallel or perpendicular to the boundary. It is shown that the observations are consistent with the hypothesis that a minimum number of independent slip systems are required for a grain to conform to an arbitrary deformation under constraints imposed by neighboring grains.L'auteur expose ses observations au sujet de la propagation des dislocations dans la glace polycristalline \ue0 structure colonnaire, soumise \ue0 une charge constante de compression appliqu\ue9e perpendiculairement au grand axe des colonnes. Les trois quarts environ des dislocations observ\ue9es traversaient les cristaux et les autres se produisaient aux joints des grains. Le plan des dislocations avait tendance \ue0 se placer parall\ue8lement \ue0 la direction d' application de la charge. Les dislocations traversant les cristaux tendaient \ue0 se propager soit parall\ue8lement, soit perpendiculairement au plan de base du cristal. Les deux tiers au moins des dislocations des joints de grains impliquaient des joints o\uf9 le plan de glissement de l'un ou des deux grains adjacents \ue9tait presque parall\ue8le ou perpendiculaire au joint. L'auteur montre que ces observations concordent avec l'hypoth\ue8se exigeant la formation d'un nombre minimal de syst\ue8mes de glissement ind\ue9 pendants pour qu'un grain ob\ue9isse \ue0 une d\ue9formation arbitraire provoqu\ue9e par les contraintes dues aux joints voisins.Peer reviewed: NoNRC publication: Ye