Daniel Iliescu

On the initiation of shear faults during brittle compressive failure: A new mechanism

Brittle materials loaded under compression generally fail by shear faulting. This paper addresses the initiation of the fault. It presents direct observational evidence from ice, which is used as a model material for rock, and shows that wing cracking and “splay cracking” are important processes in the localization of deformation, both prior to and during fault initiation. Wing cracks develop at the tips of sliding intergranular cracks and tend to align with the maximum principal stress. Splay cracks emanate from one side of the sliding parent crack. The theme of the paper is that the splay cracks play the dominant role in triggering the fault. The central idea is that the slender columns between the splay cracks are more likely to buckle and fail than are the columns between adjacent wing cracks because they do not have two fixed ends; instead, the end stemming from the inclined parent crack is free. A moment is then applied by frictional sliding of the parent inclined crack, and this causes the fixed-free columns to break at a much lower stress than the fixed-fixed columns. Columns created near a free surface are more likely to fail than those created elsewhere, and this explains the observation that shear localization tends to initiate near free surfaces. A first-order calculation shows that the failure stress of the splay-created columns is of the same order of magnitude as the terminal failure stress.

Microstructural characterization of ice cores

Abstract In this paper, we outline the use of Raman spectroscopy coupled with scanning confocal optical microscopy for determining the microstructural location of impurities in ice-core specimens. We also demonstrate how the orientations of grains and the misorientations across grain boundaries can be determined to high precision for ice polycrystals using either selected area channeling patterns or electron backscatter patterns in a scanning electron microscope.

Preliminary microstructural and microchemical observations on pond and river accretion ice

Abstract Scanning electron microscopy and X-ray microanalysis were employed to investigate the morphology of impurities present in pond and river freshwater ice. Several structural features, 0.4 to roughly 1.2 mm in diameter, with distinct morphological characteristics were identified in the pond ice. The features reside both in the interior of the grains as well as at grain boundaries. In river ice, the impurity aggregates were significantly smaller than in the pond ice (the majority with sizes ⪝20 μm) with no particular shape and were concentrated in the grain boundaries.

Containerless Consolidation of Mg Powders Using ECAE

In this article, we present results of consolidation of Mg powders using equal channel angular extrusion. A novel feature of the processing is that we were able to compact the powders without either prior cold compaction or canning of the powders. Increased hydrostatic pressure was used to aid the consolidation using the simple expedient of a metal plug in front of the powder. The porosity, density, and tensile properties of the resulting compacts are described.

Brittle compressive failure of ice: monotonic versus cyclic loading

Abstract Biaxial compression experiments (at −10 °C at strain rates between 3−510−3 s−1 have confirmed that the terminal strength of fresh-water S2 ice brought to failure under moderate confinement ((σ22/σ11)≅0.07) in about 10 rapid compressive cycles of increasingly larger loads is greater by a factor of 1.5 than the strength of the same material loaded monotonically. The strengthening effect becomes discernible when the ice is brought to failure in approximately 4–5 cycles, and then saturates after roughly 10 cycles even though inelastic strain at failure continues to increase. Damage accumulates more slowly under cyclic loading, but eventually leads to the development of the same kind of macroscopic shear fault that accompanies terminal failure under monotonic loading, provided that the loading and unloading strain rates are relatively high ( 3−510 −3 s −1 ) and equal. Upon lowering the unloading rate by one order of magnitude, ductile behavior is induced and shear faulting is suppressed. The behavior is explained in terms of the relaxation of internal stress.

On the effects of temperature on the strength of H2SO4-doped ice single crystals

Previously, it was shown that the reduction in peak stress of single crystal ice at -20°C due to H 2 SO 4 , Δσ, could be described by Δσ = KC 1/2 , where C is the sulfuric acid concentration and k is a constant. Here we show that the strength reduction due to H 2 SO 4 is more general and that the reduction is greater as the temperature decreases. Δσ was again found to be proportional to C 1/2 at -10°C, but K at -10°C was significantly less than at -20°C.

Effects of impurities and their redistribution during recrystallization of ice crystals

In order to examine the effects of solutes on recrystallization and subsequent grain growth in ice, both doped and undoped ice single crystals were extruded through a 120° equal-channel angular extrusion jig, in order to impart a large shear strain (∼1.15). Upon subsequent annealing at -3°C, the original single crystals recrystallized, in most cases to a new single crystal with a different orientation. Increasing the solute concentration (for H 2 SO 4 to ∼200-300 ppb, and for NaCI, KCI and MgSO 4 to >5 ppm) was found to significantly retard the growth and possibly, for H 2 SO 4 -doped ice, the nucleation of new grains in the strained ice single crystals. This is indicative of how soluble impurities can retard grain growth in ice cores. It was also found that the migrating grain boundaries surrounding the newly formed grains contained large concentrations of impurities, often observed as filaments. These could have formed by the grain boundaries sweeping up impurities from the lattice into the boundary or by their diffusion to the boundary - mechanisms whereby impurities could be concentrated into the grain boundaries in ice cores - although the latter mechanism seems unlikely since it would require very high diffusion rates.

Brittle compressive failure of ice : proportional straining vs proportional loading

Proportional straining experiments have been performed on columnar-grained S2 freshwater ice biaxially compressed across the columns at -10°C at a strain rate of (4.5 ± 1.5) x 10 -3 s -1 . The results are compared with those obtained earlier (Iliescu and Schulson, 2004) from the same kind of material deformed to terminal failure under the same conditions, but through proportional loading. The exercise shows that the biaxial strength is practically independent of the path taken, at least under low confinement where Coulombic shear faulting limits terminal failure. First-year sea ice is expected to exhibit the same behavior.