Papers by Keyword: Instability

Abstract: Compression tests of Mg-4Al-3Ca-1.5Zn-1Nd-0.2Mn Magnesium alloy as-extruded had been performed in the compression temperature range from 200°C to 350°C and the strain rate range from 0.001 s¹ to 1 s¹ and the flow stress data obtained from the tests were used to develop the power dissipation map, instability map and processing map. The optimum parameters for hot working of the alloy had been determined. According to the processing maps, the most optimal temperature range is 280°C to 350°C and most optimal strain rate range is 0.001 S^-1 to 1 S^-1.

Abstract: Modelling of large-scale deformation patterning in geomaterials is important for predicting instabilities and failures in the Earths crust. Shear band formation and the evolution of the bands is a predominant mechanism of deformation patterning. Independent rotations of separate grains/particles can affect the pattern formation by adding the effect of rotational degrees of freedom to the mechanism of instability. To model this mechanism we use a special experimental technique based on digital image correlation in order to recover both displacement and independent rotation fields in 2D physical models of granular material. In the physical model the particles are represented by smooth steel monodispersed disks with speckles painted on them to enable the rotation reconstruction. During the loading the deformation pattern undergoes stages of shear band formation followed by its dissolution due to re-compaction and particle rearrangement with the subsequent formation of multiple shear bands merging into a single one and the final dissolution. Also, patterns of rotations are observed at an intermediate scale between the scale of the particles and the scale of the shear band.

Abstract: The Hall-Heroult process for the production of aluminium is based on the electrochemical reduction of alumina (Al₂O₃) dissolved in a cryolite (Na₃AlF₆) based electrolyte. Instability in cell voltage is referred to as noise. Normal voltage noise is inevitable due to bubble evolution and it has little effect on performance parameters such as, current efficiency and power consumption. Metal rolling noise (wavy noise) is caused by the disturbances in cell magnetic field and it affects the cell current efficiency adversely. Investigating the causes of the cell instability in the aluminium smelting cells can lead to better cell performance. Understanding the variation in cell voltage is critical for cells, because magnitude of voltage determines the energy consumption pattern in the process and hence, any saving on voltage can save energy. Voltage affects the current efficiency of the cell and an optimum cell voltage leads to higher current efficiency without compromising on energy consumption. Magnetic, current distribution, heat loss and voltage at zero current measurements along with online current and voltage signal can help to identify the problems and their combined effects on the performance of the cells. In order to estimate the loss in current efficiency of the aluminum electrolysis cells due to metal instabilities, measurements were performed and data analyzed. The present paper analyses the effect of voltage fluctuations (noise) during metal instability along with cause of instability and its effect on current efficiency of the cell. Measurements carried out to estimate the deviations from the normal cell operations are also discussed.

Abstract: The instability of seepage and stress field coupling in unsaturated soil caused by wet process, resulted stability change in unsaturated soil.Thus,it is important to analysis the conditions of the coupling stability not in theory but in engineering. A coupled seepage and stress field model was derived from the transport laws in continuum mechanics and Darcy's law, integrated with fluid motion in porous media. Then, a linear instability analysis was performed to investigate the general conditions of which kind would trigger the non-stability of the coupling field. Finally, some variables, which have a significant effect on the onset of the instability of the unsaturated system subjected to the instability of the coupled model, were proposed in some simplified conditions.

Abstract: In condition of extraction roadway with existing bolt support at Zhao Lou coal mine as the object, the substantial function and evolution mechanism of anchor rod and anchorage structure are studied by using a variety of means; perfected suggestions about roof separation indicator are put forward; the separation phenomenon and the bolt support structure interaction state and the correlation of roadway stability are researched; method of determination of the critical value of roof separation are put forward and the criterion of the bolting structure failure and surrounding rock losing stability of roadway with the bed-separation volume as the main indicator are provides the basis for.

Abstract: Depending on the type of systems, the resonance states are either short and without inducing structural changes, or lead to forced variations of the elasticity and damping parameters with the additional energy absorption. For the material physical systems, that simulate the dynamic behavior of equipments, industrial plants and constructions, the operation in resonance mode is unstable being characterized by pulse variations in the band-pass. In this case, the necessary energy input, from outside, when the operation state is maintained within the band-pass, inevitably leads to the modification of the elastic and damping parameters and to structural degradation processes appearance. Thus the resonance band, Δω, considered as a significant parameter of the excitation, as well as the specific relaxation duration considered as an intrinsic system parameter are in a functional correlation of the dynamic system. When the dissipation increases Δc > 0, the relaxation duration drops with Δt=m/Δc, in such a way that the relation Δω Δt = 1 is valid, similar to the Heisenberg indeterminacy formula for subatomic particles. It can be observed that for Δt tending to 0 and for very high values of Δc the resonance state can be achieved for any value of the excitation pulsation as a discrete variable parameter by the modification of the viscous component. In the ideal case, the resonance interval is reduced to a single value that corresponds to its own pulsation. The systems dynamic behavior generated by periodic functions resulting in either forced vibrations or wave propagation processes in viscoelastic media is characterized by doubtful operation and physical instability within resonance.

Abstract: There is a wide application for the string-net structure in the areas of architecture and spaceflight. In the area of spaceflight, the string-net structure is mainly used in the deployable antenna. The three-dimensional string-net structure is a kind of typical string-net structures applied on the deployable antenna. Here the string-net structure is analyzed. At first, composition of the string-net structure is presented. Then the dynamical relaxation method is used to obtain the solution for movement of the structure after its instability. At last, using numerical simulation, tensions in the strings are computed. It is proved finally that the three-dimensional string-net structure still possesses the capability of keeping its shape and stiffness after its instability.

Abstract: The conservation of historical timber structures requires both the knowledge of structural behavior of complete building and behavior of any single element in structure. Because of a poor communication between the institutions involved in the conservation and restoration of buildings and institutions that urgently require renovation of structures, renovation of historic buildings is often performed inappropriately due to engineering and architectural approach. Buildings are frequently subjected to inappropriate interventions, replacements of structural elements, or even worse, they are demolished. First case study in this paper is Cathedral from Gospic city which was subjected to inappropriate reconstruction in 1990s. Second case study is small traditionally constructed timber chapel in village Lijevi Stefanki which represents a valuable monument of rural cultural heritage.

Abstract: Every compressor has a stall line. In the vicinity of the stall line, the flow field is inherently unsteady due to the interactions between adjacent rows of blades, formation of small stall cells, flow separation and the viscous effects including shock-boundary layer interactions. These factors may aggravate to a state of local breakdown of flow or a total breakdown of flow or with a disastrous flow reversal. This paper starts with an overview on the previous researches about compressor stall and surge. Subsequently, it describes the effects of these instabilities in overall engine performance and design with/ without any control. The main objectives of this paper are to review the phenomenon of instability and methodology to suppress the rotating stall and surge by enlarging the stable operating range of compressor with the application of various control systems. This paper surveys research developments in this field and also tries to find an improved solution to increase the engine performance by applying various surge control strategies. Finally, the paper focuses on some recommendations towards a better design of compressor especially for aircraft power plant.

Abstract: The function of three fields coupling of water-heat-mechanics is derived and established. The finite difference software FLAC3D is applied to simulate the affection of three fields coupling of water-heat-mechanics of the slope in the saturated state. The mechanism of instability and failure of the slope is revealed from two aspects like freezing depth and frost heaving deformation in the condition extreme snow hazard. The results show that freezing only occurs on the slope surface in the short time which makes no affection on the deep part; with the time lapsed the temperature in each depth decreases gradually and additionally with the increasing of the depth the effect of the surface temperature slows down. In the shallow part the temperature declines with the surface temperature declining and slows down with the increasing of depth. In the process of freezing the displacements occur on the surface to the direction of free boundary. In addition, the frost displacements increase with the time lapsing.