Regularities in formation of stressed state in roof rocks of mined-out space during development stage

Formation of the stress-and-strain state of the rock mass in the roof of mined coal seam depends on the development of the mined-out space. It is believed that the coal seam is located deep enough and it can be assumed that the effect of the daylight surface on its condition can be neglected. In this case, the solution is based on the analytical approach using methods of the complex variable theory and it is reduced to the construction of a single permission analytical function. The paper reviews the evolution of the deformation processes in development of the mined-out space in presence of a hard-to-collapse elastic roof, which is capable of sinking smoothly over time, without sudden caving on the landings on the floor. A particular attention is paid to the phase when the roof and the floor touch each other, i.e. the roof caving, starting from the first touching and up to its complete caving. In this case, two sections of the hanging roof are formed, that are gradually reducing in length as the dimensions of the mined-out space increase. The area of roof caving is progressively increasing, and the vertical compressive stresses at the boundary are gradually rising, tending to reach the initial vertical pressure at the depth of the formation before the start of its mining. Tension zones relative to the horizontal and vertical stresses are identified, that are attributed to the areas of roof hang-up, which may determine the location of zones with higher methane and formation water permeability, both in the rocks between the seams and in the coal seam.

Evaluation of remodeling and geometry on the biomechanical properties of nacreous bivalve shells

Mollusks have developed a broad diversity of shelled structures to protect against challenges imposed by biological interactions(e.g., predation) and constraints (e.g., $$pCO_2$$ p C O 2 -induced ocean acidification and wave-forces). Although the study of shell biomechanical properties with nacreous microstructure has provided understanding about the role of shell integrity and functionality on mollusk performance and survival, there are no studies, to our knowledge, that delve into the variability of these properties during the mollusk ontogeny, between both shells of bivalves or across the shell length. In this study, using as a model the intertidal mussel Perumytilus purpuratus to obtain, for the first time, the mechanical properties of its shells with nacreous microstructure; we perform uniaxial compression tests oriented in three orthogonal axes corresponding to the orthotropic directions of the shell material behavior (thickness, longitudinal, and transversal). Thus, we evaluated whether the shell material’s stress and strain strength and elastic modulus showed differences in mechanical behavior in mussels of different sizes, between valves, and across the shell length. Our results showed that the biomechanical properties of the material building the P. purpuratus shells are symmetrical in both valves and homogeneous across the shell length. However, uniaxial compression tests performed across the shell thickness showed that biomechanical performance depends on the shell size (aging); and that mechanical properties such as the elastic modulus, maximum stress, and strain become degraded during ontogeny. SEM observations evidenced that compression induced a tortuous fracture with a delamination effect on the aragonite mineralogical structure of the shell. Findings suggest that P. purpuratus may become vulnerable to durophagous predators and wave forces in older stages, with implications in mussel beds ecology and biodiversity of intertidal habitats.

DC characterization of advanced fine-filamentary MgB2 superconducting wires.

MgB2 wires with 114, 192 and 342 filaments of size 14-19 µm manufactured by HyperTech Research, Inc. have been subjected to low temperature DC measurements. R(T), I-V characteristics, critical currents and stress and strain tolerances of these wires differing by filament architecture and filament size sheathed by resistive CuNi alloys were measured and compared with the literature data. It was found that these fine-filamentary wires have high engineering current densities not reduced by twisting up to 10 mm, sufficient strain tolerances and therefore are promising for future applications where minimised AC losses are required due resistive sheaths, thin MgB2 filaments and short twist pitches.

Magic and Witchcraft Implications for Democratization and Poverty-Alleviating Aid in Africa

ABSTRACT & RÉSUMÉ & ZUSAMMENFASSUNG: The belief in occult forces is still deeply rooted in many African societies, regardless of education, religion, and social class of the people concerned. According to many Africans its incidence is even increasing due to social stress and strain caused (among others) by the process of modernization. Most often magic and witchcraft accusations work to the disadvantage of the poor and deprived, but under particular circumstances they become a means of the poor in the struggle against oppression by establishing "cults of counterviolence." Magic and witchcraft beliefs have increasingly been instrumentalized for political purposes. Apparently they can be used to support any kind of political system, whether despotic or democratic. The belief in occult forces has serious implications for development cooperation. Development projects, which constitute arenas of strategic groups in their struggle for power and control over project resources, are likely to add further social stress to an already endangered precarious balance of power, causing witchcraft accusations to flourish. In addition, witchcraft accusations may serve as indicators of hidden social conflicts which are difficult to detect by other methods. ----------------------------------------------------------------------------------------------------------------------------------------------------------------- RÉSUMÉ : [ La magie et la sorcellerie : conséquences pour la démocratisation et l'aide à la réduction de la pauvreté en Afrique ] - La croyance en forces occultes est encore profondément enracinée dans des nombreuses sociétés africaines, indépendamment de l'éducation, de la religion et de la classe sociale des personnes concernées. Selon des nombreux Africains, son incidence augmente encore en raison du stress social et de la tension causée (entre autres) par le processus de modernisation. Le plus souvent, les accusations de magie et de sorcellerie font mal aux pauvres et aux personnes défavorisées, mais dans des circonstances particulières, elles deviennent un moyen pour les pauvres dans la lutte contre l'oppression en établissant des « cultes de contre-violence ». Les croyances magiques et sorcelleries ont de plus en plus été instrumentées à des fins politiques. Apparemment, ils peuvent être utilisés pour soutenir tout type de système politique, qu'il soit despotique ou démocratique. La croyance en forces occultes a de sérieuses implications pour la coopération au développement. Les projets de développement, qui constituent des arènes de groupes stratégiques dans leur lutte pour le pouvoir et le contrôle sur les ressources du projet, sont susceptibles d'ajouter un stress social supplémentaire à un équilibre de pouvoir précaire déjà menacé, ce qui entraînera des accusations de sorcellerie. En outre, les accusations de sorcellerie peuvent servir d'indicateur de conflits sociaux cachés qui sont difficiles à détecter par d'autres méthodes. ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------- ZUSAMMENFASSUNG: [Magie und Hexerei: Implikationen für Demokratisierung und armuts-lindernde Entwicklungshilfe in Afrika] - Der Glaube an okkulte Mächte ist immer noch fest verankert in vielen afrikanischen Gesellschaften, unabhängig von Ausbildung, Religion und sozialer Klasse der betroffenen Menschen. Viele Afrikaner glauben sogar, dass Hexerei weiter zunimmt durch die wachsenden gesellschaftlichen Gegensätze im Rahmen der Modernisierung. In der Regel wirken Hexenanschuldigungen zum Nachteil der Armen und Entrechteten. Aber unter bestimmten Umständen können sie durch die Etablierung von ‚Kulten der Gegengewalt‘ auch zum Mittel der Armen in ihrem Kampf gegen Unterdrückung werden. Der Glaube an Magie und Hexerei wird in Afrika zunehmend für politische Zwecke instrumentalisiert. Augenscheinlich kann er zur Unterstützung jeglicher politischer Systeme, ob despotisch oder demokratisch, genutzt werden. Aus dem Glauben an okkulte Mächte ergeben sich gravierende Implikationen für die Entwicklungszusammenarbeit. Entwicklungsprojekt, die auf lokaler Ebene Schauplatz strategischer Gruppen in ihrer Auseinandersetzung um Macht und Kontrolle über Projektmittel bilden, sind dazu angetan, weiteren sozialen Stress zum ohnehin schon prekären Machtgleichgewicht hinzuzufügen, und heizen Hexenanschuldigungen damit an. Davon abgesehen, können Hexenanschuldigungen als Indikator für versteckte soziale Konflikte dienen, die durch andere Methoden der Entwicklungszusammenarbeit kaum aufzudecken sind.

Study on the surface quality of 60vol% SiCp/Al2024 composites with large-grained

In this paper, the finite element cutting simulation model with irregular distribution of multiple particles is established, the stress and strain distribution of SiC particles in the process of machining, as well as the material removal mechanism are analyzed. The effects of cutting velocity and feed per tooth on the surface quality of the material are also analyzed. The effect of feed per tooth on subsurface damage is revealed. The results show that in the micro-milling of SiCp/Al2024 composites, the particle removal form is mainly crushing and extraction. The surface defects of the workpiece mainly include pits, scratches, cracks, and extrusion damage. When the cutting velocity increases, the surface defects gradually change to crack, which can improve the surface quality of the workpiece. Increasing the feed per tooth will increase the surface defects of the workpiece and lead to poor surface quality. When the feed per tooth increased from 0.428 µm to 0.714 µm, the subsurface damage thickness increased from 35.2 µm to 47.3 µm.

Meso- and Macro-Mechanical Analysis of the Frost-Heaving Effect of Void Water on Asphalt Pavement

In cold regions, many types of structural damages are caused by the frost heaving of asphalt pavements. Hence, it is important to quantitatively determine the frost-heaving effect of asphalt pavement using a mechanical method to control frost-heaving damage. In this study, first, the internal voids of the asphalt mixture were regarded as a single void, and the water phase transition generating the freezing water in the voids was simulated using a simplified hollow sphere model to create a uniform internal pressure. Second, the prediction equation of the equivalent linear expansion coefficient was proposed by taking the phase transition of water in the saturated asphalt mixture voids into account. A step function was used during the phase transition of water to determine the sudden change in the equivalent linear expansion coefficient, heat capacity, density, and thermal conductivity. Finally, the typical cooling conditions were simulated with the water phase transition and the nonwater phase transition. The experimental results showed that the proposed model could accurately simulate the effect of frost heaving. Higher stress and strain were generated on the surface and in the interior of the pavement, and the positions of maximum stress and strain occurred on the pavement surface under the frost-heaving conditions. The compressive strength of the asphalt mixture in a uniaxial compression test is about 4.5–6 MPa with a single freeze–thaw cycle. Furthermore, when frost heaving occurs on the asphalt pavement between 5.8 and 6.5 MPa, the numerical simulation method can be used to calculate the internal stress of the structure, which found that the compressive stress under the frost-heaving condition was the same magnitude as the compressive strength under the freeze–thaw testing condition.