REVERSIBLE CREEP OF BUILDING MATERIALS UNDER CONSTANT AND REPEATED  LOADING

M. Lordkipanidze,  L.Minkin,  N.Chakhvashvili, N.Bochorishvili 
(Georgia)

The results obtained for various materials (concrete, tufa, gypsum, silicate glass, acrylic plastic glass, rubber, naphthalene monocrystal)  in liquid surface-active media were analogous and showed that the limit deformation of reversible creep is achieved under the cyclic loading many times quicker than under constant the loading. The only exception is steel in the gaseous hydrogen medium. This suggests an idea that it is the phase state of a surface-active medium that plays the key role. The nature of such an accelerated development of pre-critical cracks under the action of a liquid surface-active medium can be described as follows. When a specimen experiences tension in a medium under constant tensile stress, we observe pre-critical (according to Griffits) cracks (micro crevices),  the emergence of which is caused by the combined action of mechanical stress and a surface-active medium, which, on the one hand, promotes the rupture of bonds in a solid body, and, on the other hand, provides the stabilization of embryo cracks, which, according to thermodynamics, should close. The kinetics of the growth of such micro cracks is determined by the velocity with which a surface-active medium arrives at the crack vertex through fine channels.

We should emphasize one more important peculiarity of the reversible creep phenomenon of solid bodies in surface-active media – this is the complete reversibility of the phenomenon independently of the number of cycles of placing the specimen in an active medium which is subsequently removed. The specimen’s properties remain constant: in the absence of the medium, the elastic deformation value does not change and in the presence of the medium the reversible creep parameters also remain constant.  This shows the reversibility of formation  processes of pre-critical cracks in both thermodynamic and dynamic senses.

The phenomenon of reversible creep of solid bodies under the action of surface-active media serve as a unique tool of the investigation of the formation and growth of pre-critical cracks in stressed materials.

Further investigation of this phenomenon can facilitate the understanding of the nature and mechanism of catastrophic failures of stressed structures.





THERMAL EXPANSION OF POLYCRYSTALLINE SI1-XGEX(X≤0.02) ALLOYS IN THE 20-800°C TEMPERATURE INTERVAL

I.Tabatadze, D.Mkheidze, V.Kuchukhidze, M.Kadaria, G.Aroshvili
(Georgia)

Study of polycrystalline Si-Ge alloys thermal expansion by means of dilatometric method in the wide temperature range (20-800°C) is conducted. Experiments are performed on dilatometer with quartz gauges, which is equipped with displacement capacitor sensor, featuring information exchange digital port. A special program is utilized for reading data from the sensors and processing algorithms via RS232 interface.

Experimentally is demonstrated non-monotonic changes of thermal expansion linear coefficient in a wide temperature range. A comparative analysis of temperature dependence of the thermal expansion coefficient for SiGe alloys was conducted.  It is show  that an increase in the concentration of the Ge results into a decrease of critical temperature of non-monotonic decline of linear thermal expansion by 20-30°C. All samples are characterized by clearly manifested non-monotonic temperature changes of thermal expansion, when the temperature changes at the rate of 3-5°C/min.

The experimental results referred above suggest that the non-monotonic thermal expansion of SiGe polycrystalline alloys in the range of 200-500°C could be caused by configuration and concentration transformations in structural defects.