Failure of columnar saline ice under biaxial compression: Failure envelopes and the brittle‐to‐ductile transition

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Abstract
Experiments were performed on the compressive failure of columnar saline ice loaded biaxially at temperatures from −40°C to −50°C and at strain rates from 10−6 s−1 to 10−1 s−1. The major stress (σ11) was applied across the columns, and the minor stress was applied proportionally either across the columns (σ22) or along the columns (σ33). The results show that the macroscopic behavior changes from brittle to ductile upon reducing the strain rate to a critical level and that the transition strain rate first increases and then decreases with increasing across‐column confining stress. The brittle strength increases sharply under low degrees of across‐column confinement and then decreases upon increasing σ22 further. This behavior is manifested in a truncated, Coulombic‐type of failure envelope where failure on the rising branch is by splitting (zero confinement) and by shear faulting (moderate confinement) within the loading plane and on the descending branch by spalling out of the loading plane. The ductile strength increases monotonically with increasing confining stress and with both decreasing temperature and increasing strain rate. The ductile failure envelope is semielliptical in shape and can be described by Hills criterion for the yielding of plastically orthotropic materials. Correspondingly, the components of the inelastic “strain vector” in the X1‐X2 plane are reasonably normal to the failure envelope. Neither the brittle nor the ductile strength is affected by a confining stress along the columns (σ33). Brittle behavior is explained in terms of the frictional crack‐sliding wing crack mechanism, and ductile behavior is related to both basal (under low confinement) 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 sliding.