There is no classification of river and lake ice that simultaneously takes into account the history of ice formation, the structure of the ice cover, and the texture of the various ice types. Such fundamental information is a prerequisite to the determination and discussion of their physical and mechanical properties.The first part of this classification is a brief description of the formation and the physical properties of the three important ice layers making up the ice cover. The second aspect of the classification deals with the texture of various ice types.

The electrical conductivity of 3Y-TZP ceramics containing SiO2 and AI2O3 has been investigated by complex impedance spectroscopy between 500 and 1270 K. At low temperatures, the total electrical conductivity is suppressed by the grain boundary glass films. The equilibrium thickness of intergranular films is 1-2 nm, as derived using the "brick-layer" model and measured by HRTEM. A change in the slope of the conductivity Arrhenius plots occurs at the characteristic temperature Tb at which the macroscopic grain boundary resistivity has the same value as the resistivity of the grains. The temperature dependence of the conductivity is discussed in terms of a series combination of RC elements.

ABSTRACT. This study gives the results and interpretation of tests made in a cold room on ice pla tes held vertically in a press and indented at various speeds with a rectangular indentor. These plates, 81 cm square, were held and frozen solid to a steel frame to represent semi-infinite ice sheets. The tests were carried out at -lO o e with indentation speeds varying from 6 X 10-5 cm/s to 3 cm/so I t was found that the nominal pressure of indentation varies as a function of loading rates in a manner similar to that of the strength of 52 ice in uniaxial compression. The pressure increases rapidly with loading rates up to its maximum value at a strain-rate of 5 X 10-4 5-1 • From there on, the pressure decreases. In the ductile range and up to the point of maximum pressure, a theoretical model was proposed where the theory of Hill (1950) of indentation of an ideal plastic body can be applied to predict an indentation pressure close to three times that of the uniaxial crushing strength.The tests did not show any so-called "ratio effect" caused by the ratio of the indentor width to the plane thickness. This effect could be explained by the influence of th e geometry of the indentor on the strain-rate. 'une fa~on semblable a celle de la resistance ultime de la glace 52 en compression uniaxiale. La press ion croit rapidement avec les taux de chargement jusqu' a un maximum correspondant a un taux de deformation de 5 X 10-4 S-I. A partir d e ce point, la pression diminue. Dans le domaine ductile e t jusqu'a ce maximum, un modele theorique es t propose ou la theorie de Hill (1950) d e l'indentation d' un plastique ideal peut s'appliquer pour predire une pression d'indentation egale a pres d e trois fois la resistance en compression unia xi ale de la glace.Les essais n'ont pas montre d'effet geometrique cause par le rapport d e la largeur de l'indenteur a l'epaisseur de la glace. Cet effet peut etre explique indirectement par l'influence d e la geometrie d e l'indenteur sur le taux de deformation. Die Versuche zeigten keinen sog. "Verhaltnis-Effekt", hervorgerufen durch das Verhaltnis zwischen Breite des Pragers und Plattendicke. Dieser Effekt konnte mit dem Einfluss d er Form des Pragers auf die V erformungsra te erklart werden. ZUSAMMENFASSUNG. M echanismus und Theorie der Priigebelastwzg von

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