The ductile-to-brittle transition and ductile failure envelopes of orthotropic ice under biaxial compression

Experiments were performed on orthotropic columnar fresh-water ice of ≈ 6mm column diamter proportionally loaded across the columns under biaxial compression at −10°C under various combinations of strain rate and stress ratio, R = σ₂₂/σ₁₁. The ductile-to-brittle transition strain rate increased from about 8 × 10⁻⁵ s⁻¹ under no confinement to about 3 × 10⁻⁴s⁻¹ under intermediate confinement ($\text{R ⋍ 0.3}$), and then decreased to about 5 × 10⁻⁵s⁻¹ under full confinement (R = 1). The ductile failure envelopes are semi-elliptical in shape and conform fairly well to those created with the use of Hill's [9] criterion for the yielding of plastically orthotropic materials. The transition strain rate is rationalized in terms of the effects of the confining stress on the ductile and the brittle failure stresses, and is modelled in terms of the suppression of creep deformation at the tips of wing cracks.