Особенности контактного разрушения льда

The formation of an intermediate layer under hydrostatic compression at a shear appearing due to the action of converging and diverging fronts of stress momentums (pulses) is considered. Continuous monitoring of deformational changes in the structure of ice was carried out using acoustic methods. The features of contact ice breaking in the diverging fronts of stress pulses are considered by the example of the slow impact of a rigid spherical indenter on an ice plate simulating half-space. Using the piezoelectric accelerometer, an oscillogram of the impact was recorded and a generalized dependence of the reduced stress on the reduced instantaneous velocity of the impact (semi-cubic parabola) was obtained. It is established that under conditions of the experiment (smooth convex indenter surface and icy half-space) a thin intermediate layer is formed, the properties of which determine the physical similarity in the family of curves «instantaneous force-instantaneous velocity». A rheological model with due regard for the change in the microstructure of ice during the impact is proposed. Quantitative determinations of the deformation changes in structure of solid ice samples were performed under intensive plastic deformation in a matrix with a profile similar to the Laval nozzle. The deformations created by the piston caused forced vibrations in the ice. The working surface of the piston in the form of an ellipsoid together with the smooth walls of the matrix and the reverse cone created conditions for parametric resonance and the formation of fronts of highfrequency stress pulses. Under influence of these pulses, zones with a superplastic fine-crystalline structure of ice (cumulative effect) were formed in ice. In the outlet cylindrical channel, a flow around an obstacle of the ice with the structure of an intermediate layer (dynamic viscosity 20 MPa s) and the distribution of velocities of motion over the channel cross section were studied. The obtained results can be used to simulate the processes of contact destruction of deep rocks by a support or an ice-resistant platform loaded with an ice field.Проанализированы закономерности эволюции структуры пресноводного льда в условиях гидростатического сжатия при сдвиге под действием сходящихся и расходящихся фронтов импульсов напряжений. Исследовано обтекание льдом препятствия, распределение скоростей в цилиндрическом канале, формирование промежуточного слоя вблизи поверхности шарового индентора при медленном ударе. Количественно определены деформационные изменения структуры льда непосредственно в процессе контактного разрушения

Физическое моделирование режимов движения ледников

Modes of «dry friction» at glacier-bedrock interface (ice sliding and flow) were simulated under uniform compression with different combinations of mechanical and thermodynamic factors. Effect of ice structure in the intermediate layer was considered in terms of strength of ice adhesion to complex-shaped substrate for typical cases: at frictional contact of ice frozen to the walls of the cylindrical matrix; when ice was pressing-through a confusor (with contraction ratio = 30); extruding the ice in a plastic state through a pipe. For these tests, а collapsible matrix was used. It consists of three sections: the feed cylinder, the convergent channel (confuser) and the forming pipe. Changes of ice during severe plastic deformation were monitored by acoustic emission in the range from 10 Hz to 25 kHz. Relationship between the size of moving structural elements, their natural resonant frequency, density and acoustic capacitance was applied. A theoretical model was verified. Correlation of amplitude-frequency spectra of acoustic emission at the frictional contact with the acoustic spectrum of natural oscillations of the glaciers from distant sources was confirmed. The results can be applied to remote sensing studies of ice movement modes at the glacier bedrock.Воспроизведены основные режимы движения ледников по сухому ложу – скольжение и течение, включая образование промежуточного слоя. Выявлен эффект формирования спиралевидной структуры льда в условиях высокого гидростатического давления. Теоретическая модель отражает суть физических процессов интенсивной пластической деформации в пресноводном льду, к которой относится и рекристаллизация. Определены структура, физико-механические, прочностные и акустические характеристики льда в промежуточном слое, рассмотрено также их влияние на режим скольжения. Разработанный акустико-механический метод может быть применён для дистанционного изучения режимов движения ледников по ложу

Волновые структуры в ледяном поле и их влияние на прочность солёного льда

The heterogeneity of the strength of the ice field of the dynamic type of formation with an area of 800 m2 was investigated in time and space. It is shown that stationary periodic wave structures are formed in a closed volume of an ice field lying on the surface of a liquid in a rectangular basin. In a case of absence of any external influences, the dominant source of elastic waves in the ice is the coherent radiation of them on freezing of water, i.e. the ice field itself. Another wave structures, i.e. standing waves, form secondary ice textures in the ice field with diminished strength. Local hardness of ice was chosen as the criterion of strength. The recurrence of local hardness values as a function of coordinates of the measurement points in the longitudinal and transverse profiles of the ice field was determined. The hardness values vary from 40 to 60% with an axial force measurement error of 5%. The experimental relations are approximated by periodic curves, in which the maxima and minima of local hardness correlate with the nodes and antinodes of standing waves, respectively. The decrease in local ice hardness in the secondary textures is explained by high-frequency dynamic metamorphism. The wavelengths corresponding to bending-gravitational and longitudinal waves are identified, with the interference of which stationary periodic wave structures are formed. A similar regularity of changes in local hardness was revealed also in a river ice. The results obtained allow us to consider nonlinear wave phenomena as one of the factors controlling the spatial-temporal variability of the ice strength characteristics.Установлено, что в ледяном поле динамического типа формирования под действием стоячих волн образуются стационарные периодические волновые структуры. Определены масштабы таких структур. Изменение локальной твёрдости объясняется высокочастотным динамическим метаморфизмом льда в зонах пучностей стоячих волн. Показано, что источником упругих колебаний может быть когерентное излучение упругих волн при замораживании воды. Формирование аналогичных волновых структур в природном льде подтверждено полевыми измерениями твёрдости в речном ледяном покрове. Нелинейные волновые явления рассматриваются как один из факторов пространственно-временнóй изменчивости прочностных характеристик льда

Исследования ледников на основе акустических измерений

The possibility of glacier ice flow studies using the method of acoustic emission (AE) in frequency range from 15 Hz to 20 kHz has been considered. A portable acoustic line system has been developed and a number of methodological issues (mounting of acoustic sensors into glacial ice, their location, reliability of acoustic coupling, etc.) have been solved. Acoustic studies of glacial ice have been performed; rock fall effect, ice cracking and ice movement on bedrock have been simulated. Correspondences of AE parameters to specific sources have been identified. The results of acoustic studies on Aldegondabreen (Spitsbergen), Central Tuyuksu and Molodezhny glaciers (northern Tien Shan) have been summarized. The dependence of the adhesive strength of ice with smooth substrate (serpentenite) on the shear rate has been considered; the effect of tor-shaped obstacle on shear force has been estimated. It is shown that the acoustic effects at cohesive ice failure on obstacles are similar to the observed natural acoustic vibrations generated in glaciers from distant sources. The results might be applied in development of the mobile ice lab and system for remote acoustic monitoring the processes in the bottom layers of glaciers.Рассмотрены возможности исследования движения льда в леднике и его анализа с помощью метода акустической эмиссии в диапазоне частот 15–20 кГц. Создана портативная измерительная акустическая линия и решён ряд методических вопросов. Выполнены акустические исследования ледникового льда, имитированы камнепад, трещинообразование в леднике и его движение по ложу. Установлены соответствия параметров акустической эмиссии определённому источнику. Обобщены результаты акустических исследований на ледниках Альдегонда (Шпицберген), Центральный Туюксу и Молодёжный (Северный Тянь-Шань). Рассмотрена зависимость адгезионной прочности соединения льда с гладкой подложкой (серпентинит) от скорости сдвига, оценено влияние препятствия в виде тора на силу сдвига. В собственных акустических спектрах исследованных ледников обнаружены акустические эффекты, характерные для разрушения льда на препятствиях. Полученные результаты можно использовать для создания мобильной ледниковой лаборатории и системы дистанционного акустического мониторинга процессов в придонных слоях ледника

Двойная периодичность механических свойств тонкого ледяного поля, сформированного в условиях бокового стеснения

Experimental data and results of theoretical modeling of the bending of a viscoelastic floating ice plate formed under constrained deformation are analyzed. When a thin plate of ice is frozen on the water surface under conditions of constrained deformation, which may be caused, for example, by the rigid walls of the pool, periodic changes in physical properties occur in it, in particular, periodic penetration resistance. Experimental results confirming this fact were obtained during tests of a thin ice cover at the Krylov State Research Center (Saint-Petersburg, Russia). A characteristic feature of the test results is that their spatial distributions can be represented with sufficient accuracy as an overlap of two periodic functions with significantly different periods: long-wave and short-wave components. In this paper, a detailed analysis of experimental data is given, which makes it possible to isolate these components. Furthermore, the theoretical model that explains the physical causes for double periodicity is proposed. The model assumes viscoelastic quasi-static deformation of the ice plate caused by small fluctuations of the water level in the basin and random disturbances of its surface. An analytical solution for the model case of cylindrical bending is derived. The solution is presented in the form of an expansion in terms of eigenfunctions of differential operators generated by the boundary value problem under study. It has been established that when a thin plate of ice freezes under conditions of constrained deformation, there are at least two reasons for the appearance of a periodic structure: a general loss of stability as an elastic structure and a local loss of stability by a viscoelastic-plastic mechanism. The results obtained can be used in the development of the theory of ice compression, in assessing the causes of variation in the local strength of ice fields and the possibility of their artificial destruction.Обсуждаются двойная периодичность (cжатие в плоскости ледяного поля и реологический характер локализации и накопления изгибных деформаций в нём), методы её экспериментальной идентификации, математическое моделирование и причины появления. Установлены две причины возникновения периодичности: общая потеря устойчивости упругой конструкции и локальная неустойчивость по вязкоупругому механизму. Первая зависит от геометрии пластины в целом и условий её закрепления, вторая - от толщины, усилия обжатия и физико-механических свойств льда

Measurement of the e+e−→K+K−π+π−e^+e^- \to K^+K^-\pi^+\pi^- cross section with the CMD-3 detector at the VEPP-2000 collider

The process $e^+e^- \to K^+K^-\pi^+\pi^-$ has been studied in the center-of-mass energy range from 1500 to 2000\,MeV using a data sample of 23 pb$^{-1}$ collected with the CMD-3 detector at the VEPP-2000 $e^+e^-$ collider. Using about 24000 selected events, the $e^+e^- \to K^+K^-\pi^+\pi^-$ cross section has been measured with a systematic uncertainty decreasing from 11.7\% at 1500-1600\,MeV to 6.1\% above 1800\,MeV. A preliminary study of $K^+K^-\pi^+\pi^-$ production dynamics has been performed

Study of the process e+e−→ppˉe^+e^-\to p\bar{p} in the c.m. energy range from threshold to 2 GeV with the CMD-3 detector

Using a data sample of 6.8 pb$^{-1}$ collected with the CMD-3 detector at the VEPP-2000 $e^+e^-$ collider we select about 2700 events of the $e^+e^- \to p\bar{p}$ process and measure its cross section at 12 energy ponts with about 6\% systematic uncertainty. From the angular distribution of produced nucleons we obtain the ratio $|G_{E}/G_{M}| = 1.49 \pm 0.23 \pm 0.30$