FAILURE OF COLUMNAR SALINE ICE UNDER BIAXIAL COMPRESSION - FAILURE ENVELOPES AND THE BRITTLE-TO-DUCTILE TRANSITION

Experiments were performed on the compressive failure of columnar sali ne ice loaded biaxially at temperatures from 40 degrees C to -5 degree s C and at strain rates from 10(-6) s(-1) to 10(-1) s(-1). The major s tress (sigma(11)) was applied across the columns, and the minor stress was applied proportionally either across the columns (sigma(22)) Or a long the columns (sigma(33)). The results show that the macroscopic be havior changes from brittle to ductile upon reducing the strain rate t o a critical level and that the transition strain rate first increases and then decreases with increasing across-column confining stress. Th e brittle strength increases sharply under low degrees of across-colum n confinement and then decreases upon increasing sigma(22) further. Th is behavior is' manifested in a truncated, Coulombic-type of failure e nvelope where failure on the rising branch is by splitting (zero confi nement) and by shear faulting (moderate confinement) within the loadin g plane and an the descending branch by spalling out of the loading pl ane. The ductile strength increases monotonically with increasing conf ining stress and with both decreasing temperature and increasing strai n rate. The ductile failure envelope is semielliptical in shape and ca n be described by Hill's criterion: for the yielding of plastically or thotropic materials. Correspondingly, the components of the inelastic ''strain vector'' in the X(1)-X(2) plane are reasonably normal to the failure envelope. Neither the brittle nor the ductile strength is affe cted by a confining stress along the columns (sigma(33)). Brittle beha vior is explained in terms of the frictional crack-sliding wing crack mechanism, and ductile behavior is related to both basal (under low co nfinement) and nonbasal (under high confinement) dislocation processes . The brittle-to-ductile transition is attributed to the relaxation of crack tip tensile stresses and is modeled by incorporating the crack size and the resistance to creep, crack propagation, and frictional sl iding.