H. O. K. Kirchner

On the fracture toughness of snow

Abstract The release of a dry-snow slab avalanche involves brittle fracture. It is therefore essentially a non-linear fracture mechanics problem. Traditional snow-stability evaluation has mainly focused on snow strength measurements. Fracture toughness describes how well a material can withstand failure. The fracture toughness of snow is therefore a key parameter to assess fracture propagation propensity, and hence snows lope stability. Fracture toughness in tension KIc and shear KIIc was determined with notched cantilever-beam experiments in a cold laboratory. Measurements were performed at different temperatures and with different snow types of density ρ = 100–300 kgm–3, corresponding to typical dry-snow slab properties. The fracture toughness in tension KIc was found to be larger (by about a factor of 1.4) than in shear KIIc. Typical values of the fracture toughness were 500–1000 Pam1/2 for the snow types tested. This suggests that snow is one of the most brittle materials known to man. A power-law relation of toughness KIc on relative density was found with an exponent of about 2. The fracture toughness in tension KIc decreased with increasing temperature following an Arrhenius relation below about –8°C with an apparent activation energy of about 0.16 eV. Above –6°C the fracture toughness increased with increasing temperature towards the melting point, i.e. the Arrhenius relation broke down. The key property in dry-snow slab avalanche release, the critical crack size under shear at failure, was estimated to be about 1 m.

Fracture toughness of snow in shear and tension

Abstract Snow slab avalanche release involves brittle fracture. The fracture criterion in mixed mode for dry snow ( ρ =170 kgxa0m −3 ) was determined by cantilever beam experiments: ( K Ic 2 + K IIc 2 ) 1/2 =430±90 Paxa0m 1/2 . Fracture toughness in shear, K IIc , is about the same as in tension, K Ic .

Snow as a foam of ice: Plasticity, fracture and the brittle-to-ductile transition

Abstract At strain rates lower than 10−4 s−1. snow deforms plastically and fractures in a ductile manner; at higher strain rates it is brittle. The brittle-to-ductile transition has an activation energy of 0.6 ± 0.1 eV. Plasticity preceding fracture is characterized by an activation energy of 0.6 ± 0.05 eV for temperatures below –6°C. and about 2.7 ± 0.4eV above. The basic deformation mechanism of snow. an ice foam, is power-law creep of ice. As in silicon, the activation energy of the brittle-to-ductile transition is the lowest of the activation energies of all deformation processes available, but in ice these are the same, 0.6eV, for dislocation glide diffusion and sublimation.

Fracture toughness of snow in tension

Abstract Notched cantilever beams of snow were broken under their own weight to determine the fracture toughness of snow with densities between 100kgm−3 and 540 kgm−3. It varies with the relative density according to K lc = 7.84 (ρsnow/ρice)2.3 kPam½. Snow in tension is one of the most brittle materials known to man.

Plastic homology of bcc metals

Abstract The temperature dependence of the yield stress T c of four bcc transition metals α-Fe, Nb, Mo and Ta, and an alkali metal K as well, follows a scaling relation of T c/T p against kT/b 3(TpKT)½, where T p is the Peierls stress, b the length of the Burgers vector and K T the line tension factor for a screw dislocation calculated for elastic anisotropy. The same relation seems to apply for the other three bcc transition metals V, Cr and W, suggesting its general applicability at least for low strain rates (about 10−4 s−1). The result implies that the plasticity of bcc metals is characterized by only one non-elastic parameter, the Peierls stress T p.

Local disordering and reordering phenomena induced by mobile dislocations in short-range-ordered solid solutions

Abstract Dislocation motion in short-range-ordered solid solutions under stress are investigated in situ by transmission electron microscopy. For the first time, local reordering created by the back slip of mobile dislocations is proven to exist. Dislocation movement within pile-ups produces local disordering by forward slip during loading, and local reordering by back slip during unloading. Within a pile-up, the driving force for this mechanism is the diffuse antiphase-boundary energy created by the movement of the leading dislocation.

Yield strength and brittle-to-ductile transition of boron-nitride and gallium-nitride

Boron-nitride and gallium-nitride with cubic (zincblende) structure are industrial materials with great potential. The only mechanical parameters that have been measured are the Vickers and Knoop hardnesses of cubic boron-nitride (c-BN), 49 GPa and 43 GPa, respectively. They are larger than the values for SiC. No hardness values of c-GaN have been published. This paper predicts the temperature dependence of the critical shear stress {tau}{sub c} and the brittle-to-ductile transition (BDT)-temperature of c-BN and c-GaN by use of the universal relation which was recently found for tetrahedrally bonded crystals.

Dangerous defect distributions in snow

Abstract Because of an ever-present random distribution of flaws, the yield stress of snow is not a unique function of density but follows a statistical Weibull distribution. For snow of density 140 kgm−3, 50% of the specimen breaks at 6300 Pa, and the Weibull exponent is m = 2.1, very low compared with metals or even ceramics. This indicates that snow is inhomogeneous and full of flaws.