178,594 research outputs found
Exact solution for free vibration of thick rectangular plates made of porous materials
No full textFree vibration analysis for thick rectangular porous plate saturated by inviscid fluid is presented. Material properties of the plate are assumed to vary through the thickness according to a simple cosine law in term of a coefficient related to plate’s porosity. Based on Reddy’s third-order shear deformation plate theory and considering the effect of fluid in pore network of the porous medium, the equations of motion are obtained. The plate is assumed to be simply supported on two opposite edges and the rest of edges are free, clamped or simply supported. For the sake of analytical solution, a unique approach is employed to decouple the equations of system. Exact frequencies rising from the solution are obtained for a rectangular porous plate made of Berea sandstone. The roles played by mechanical constraints on edges, fluid, geometrical dimensions of plate as well as the effect of the coefficient of plate porosity are investigated. It is found that porosity function affects significantly on the fundamental frequency of the system. Also, the effect of fluid on the dynamic response of plate is studied in detail
On the effect of coupled solid-fluid deformation on natural frequencies of fluid saturated porous plates
No full textThis paper is concerned with the effect of deformation coupling between solid and fluid on the free vibration characteristics of isotropic rigid porous rectangular plates under undrained condition. Mindlin plate theory is employed to model the moderately thick porous plate. The problem addressed is formulated by using Hamilton's principle, which leads to a set of partial differential equations dealing with frequency response of the plate. The governing equations of motion are solved analytically for Levy-type porous plates by introducing some auxiliary functions. The fluid viscosity is assumed to be very low so that it cannot cause energy dissipation in system. Numerical results are obtained in non-dimensional form. The accuracy of the solution is confirmed by making some comparisons of the obtained frequencies with those available in literature. It is found that the effect of coupled solid-fluid deformation cannot be neglected when the Biot-Willis constants are considerable compared to mechanical properties of solid skeleton of the plate. It is also observed that the plate's effective stiffness increases as fluid is trapped in pore network of the porous medium
Application of Carrera Unified Formulation to study the effect of porosity on natural frequencies of thick porous–cellular plates
No full textCarrera Unified Formulation (CUF) is used for free vibration analysis of Levy-type thick rectangular porous–cellular plates. The variation of porosity through the thickness causes mechanical properties to change along the thickness. Properties are modeled by a simple cosine rule. The equations of motion are solved analytically for Levy-type rectangular porous–cellular plates using state space method. The correctness of this approach is confirmed through comparison studies with published results based on highly accurate approaches. Different orders of model are used to enhance the accuracy of the method in order to obtain the exact frequencies of thick and very thick porous–cellular plates. The effects of the coefficient of plate porosity and the thickness–length ratio as well as the aspect ratio, on the frequencies are investigated for Levy-type boundary conditions. It is found that natural frequencies of the porous–cellular thick plates decrease as the coefficient of plate porosity increases in all studied boundary conditions
An analytical study on the free vibration of moderately thick fluid-infiltrated porous annular sector plates
No full textAn exact analytical approach based on Mindlin plate theory is considered for free vibration analysis of fluid-saturated porous annular sector plates. The interconnected network of pores is saturated by inviscid fluid and the fluid is trapped in the network. The plate’s radial edges are considered to be simply supported and four auxiliary functions are used to evaluate the natural frequencies of porous plates under undrained condition. The mechanical properties of the material are considered to vary through the thickness by expressing shear modulus and density in terms of a simple cosine rule in case of plates with pores free of fluid. The present method is validated by comparing it with the results of other accurate solutions found in the literature. The influence of the coefficient of plate porosity, geometrical parameters as well as the effect of fluid on natural frequency response of porous annular sector plates under various boundary conditions are comprehensively investigated. It is found that the presence of fluid in the interconnected network of pores causes the fundamental natural frequency to increase. The method proposed in this paper may provide useful information for the future assessments of the dynamic response of porous structures when fluid–solid interaction effects are fully taken into account
Buckling response of moderately thick fluid-infiltrated porous annular sector plates
No full textIn the present article, the buckling of a fluid-infiltrated porous plate is investigated using Mindlin plate theory and an analytical procedure. A cosine rule for the pore distribution across the plate thickness is assumed with a coefficient defining porosity level. The governing stability equations are rewritten in terms of four auxiliary functions and decoupled with the aid of some mathematical manipulations. The decoupled partial differential equations are solved analytically by assuming simply supported radial edges for the plate. The critical buckling loads are calculated by considering fluid-saturated and fluid free conditions for the interconnected network of pores for different sector angles, thickness–radius ratios, coefficients of plate porosity, aspect ratios, and boundary conditions. It is found that the pore fluid compressibility affects the buckling load significantly
The Middle to Later Stone Age transition at Panga ya Saidi, in the tropical coastal forest of eastern Africa
No full textThe Middle to Later Stone Age transition is a critical period of human behavioral change that has been variously argued to pertain to the emergence of modern cognition, substantial population growth, and major dispersals of Homo sapiens within and beyond Africa. However, there is little consensus about when the transition occurred, the geographic patterning of its emergence, or even how it is manifested in the stone tool technology that is used to define it. Here, we examine a long sequence of lithic technological change at the cave site of Panga ya Saidi, Kenya, that spans the Middle and Later Stone Age and includes human occupations in each of the last five Marine Isotope Stages. In addition to the stone artifact technology, Panga ya Saidi preserves osseous and shell artifacts, enabling broader considerations of the covariation between different spheres of material culture. Several environmental proxies contextualize the artifactual record of human behavior at Panga ya Saidi. We compare technological change between the Middle and Later Stone Age with on-site paleoenvironmental manifestations of wider climatic fluctuations in the Late Pleistocene. The principal distinguishing feature of Middle from Later Stone Age technology at Panga ya Saidi is the preference for fine-grained stone, coupled with the creation of small flakes (miniaturization). Our review of the Middle to Later Stone Age transition elsewhere in eastern Africa and across the continent suggests that this broader distinction between the two periods is in fact widespread. We suggest that the Later Stone Age represents new short use-life and multicomponent ways of using stone tools, in which edge sharpness was prioritized over durability.1. Introduction 1.1. Identifying the MSA-LSA technological transition in eastern Africa 1.2. Site and environment 2. Materials and methods 3. Results 3.1. Local stone sources 3.2. Lithic materials, frequency, and size 3.3. Core reduction 3.4. Flakes 3.5. Retouched flakes 4. Discussion 4.1. The Panga ya Saidi lithic sequence in context 4.2. Overview of the Panga ya Saidi sequence 4.3. The MSA-LSA lithic transition in eastern Africa and beyond 5. Conclusion
Appropriate Similarity Measures for Author Cocitation Analysis
No full textWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
On natural frequencies of Levy-type thick porous-cellular plates surrounded by piezoelectric layers
No full textIn this paper, an analytical solution for free vibration of rectangular porous-cellular plates enclosed by piezoelectric layers is presented by using third-order shear deformation plate theory. Using Hamilton’s principle and Maxwell equation, the governing equations of the system are obtained for both closed and open circuit conditions. Due to the coordinate dependency of mechanical properties of porous materials, the governing equations of motion are highly coupled. By using four auxiliary functions, these equations convert into two independent partial differential equations. The decoupled equations are solved analytically by employing Levy-type boundary conditions for the plate. Finally, after validation of the obtained results, the effects of various parameters such as porosity and geometrical dimensions on the natural frequencies of plate are investigated for different electrical and mechanical boundary conditions. It is found that the natural frequencies of the plate decrease as the coefficient of plate porosity increases. Also, the piezoelectric layers cause the natural frequency of the plate to increase in various vibrating modes
An investigation over the effect of piezoelectricity and porosity distribution on natural frequencies of porous smart plates
No full textThe eigenvibration characteristics of a smart plate with piezoelectric layers and porous-cellular core are investigated in the present article. The core plate is assumed to be composed of materials that contain pores and the porosities may be distributed according to different mathematical rules. Variational principle is applied in order to derive the continuous system equations on the basis of Mindlin plate theory. A highly efficient analytical modeling for eigenfrequency analysis of the smart plate is presented under the assumption that both Skempton’s pore pressure coefficient and normal elongation through the thickness are negligible. Unlike numerical methods that require huge computational cost, this approach enables us to find the system’s response for rectangular plates with arbitrary dimensions. To examine the validity of the present framework, multiple comparison studies are made between the extracted results and those available in the literature. It is shown that the type of porosity distribution influences strongly on the way that frequency changes. Furthermore, it is found out that it is necessary to consider electrical effects for plates with open circuit condition unlike the other electrical condition
"Closing the R&D Gap, Evaluating the Sources of R&D Spending"
No full textBoth spending and tax policies have been implemented in the United States with the goal of stimulating private sector research and development (R&D). Karier questions whether current R&D policy, especially the research and experimentation tax credit, can contribute to closing the gap between nondefense expenditures on R&D in the United States and such expenditures in other countries, such as Japan and Germany. He also explores possible changes to our current R&D policy to make it more effective.
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