Creep buckling and rupture of columns with arbitrary symmetric cross section

A practical method of analysis of simply supported columns subjected to a steady-state creep is developed. The method can be applied to the columns of an arbitrary symmetric cross section manufactured from materials that follow Norton's law. Two modes of failure are considered, i.e. creep buckling and the failure associated with allowable stresses. Numerical examples presented for actual materials illustrate the effects of various parameters on the creep buckling.     

Parametric instability of thick,orthotropic, circular cylindrical shells

Summary The dynamic instability of simply supported, finite-length, circular cylindrical shells subjected to parametric excitation by axial loading, is investigated analytically. The shell is taken to be orthotropic, due to closely spaced longitudinal and/or circumferential stiffeners or to many layers of fiber-reinforced composite material either oriented at angles of 0° and 90° (cross-ply) or at + and – (angle-ply) with respect to the shell axis. The theory used is a general first-order shear deformable shell theory introduced by Hsu, Reddy, and Bert; it can be considered to be the thick-shell version of the popular Sanders-Koiter thin-shell theory. By means of tracers, this theory can be reduced to thick-shell versions of the theories of Love (and Loo) and of Donnell (and Morley). Quantitative results are presented to show the effects of shell geometry, materials, and fiber orientation on the stability boundaries.With 10 FiguresA considerably abbreviated version of this paper was presented at Euromech Colloquium 219 on Refined Dynamical theories of Beams, Plates and Shells and Their Applications, Universität-Gesamthochschule Kassel, Federal Republic of Germany, September 23–26, 1986.     

Technology challenges for virtual overlay networks

An emerging generation of mission-critical networked applications is placing demands on the Internet protocol suite that go well beyond the properties they were designed to guarantee. Although the “next generation internet” (NGI) is intended to respond to the need, when we review such applications in light of the expected functionality of the NGI, it becomes apparent that the NGI will be faster but not more robust. We propose a new kind of virtual overlay network (VON) that overcomes this deficiency and can be constructed using only simple extensions of existing network technology. In this paper, we use the restructured electric power grid to illustrate the issues, and elaborate on the technical implications of our proposal     

An Investigation of Numerical Modeling of Transient Heat Conduction in a One-Dimensional Functionally Graded Material

This article presents a closed form analytical solution for one-dimensional transient heat conduction in a material where the thermal conductivity varies linearly through the thickness but the thermal diffusivity is held constant. This solution is used to validate the results from finite-difference and finite-element approximations that account for this variation at the element level. This was motivated by a suggested limitation on the minimum time step used in the commercial finite-element software code ABAQUS for quadratic elements. Good agreement was found between the analytical and numerical approximations, indicating that conventional numerical techniques may be sufficiently robust to analyze heat conduction problems in functionally graded materials without the use of special elements. The minimum time step constraint was found to be unnecessary for a convective boundary condition for the one-dimensional elements and property variation used in this study.     

dcOvercoming Communications Challenges in Software for Monitoring and Controlling Power Systems

The restructuring of the electric power grid has created new control and monitoring requirements for which classical technologies may be inadequate. The most obvious way of building such systems, using TCP connections to link monitoring systems with data sources, gives poor scalability and exhibits instability precisely when information is most urgently required. Astrolabe, Bimodal Multicast, and Gravitational Gossip, technologies of our own design, seek to overcome these problems using what are called "epidemic" communication protocols. This paper evaluates a hypothetical power monitoring scenario involving the New York State grid, and concludes that the technology is well matched to the need.     

The next-generation Internet: unsafe at any speed?

Speed alone will not make future Internet applications secure. I propose a new networking isolation capability, termed a virtual overlay network (VON). Such software-based virtual networks, layered on top of physical networks, may provide the isolation that critical applications need. Although a VON offers a response to the reliability and security needs of critical applications, it would be prohibitively costly to implement using contemporary technologies. Extending an existing router feature and coupling it with well-understood group communication techniques, however, could support VONs at low cost, with good scalability     

Comparative Study of Microwave Sintering of Zinc Oxide at 2.45, 30, and 83 GHz

Temperature gradients that develop in ceramic materials during microwave heating are known to be strongly dependent on the applied microwave frequency. To gain a better understanding of this dependence, identical samples of ZnO powder compacts were microwave heated at three distinct widely separated frequencies of 2.45, 30, and 83 GHz and the core and surface temperatures were simultaneously monitored. At 2.45 GHz, the approximately uniform "volumetric" heating tends to raise the temperature of the sample as a whole, but the interior becomes hotter than the exterior because of heat loss from the surface. At 30 and 83 GHz, this interior to exterior temperature difference was found to be reversed, especially for high heating rates. This reversal resulted from increased energy deposition close to the sample's surface associated with reduced skin depth. A model for solving Maxwell's equations was incorporated into a newly developed two-dimensional (2-D) heat transport simulation code. The numerical simulations are in agreement with the experimental results. Simultaneous application of two or more widely separated frequencies is expected to allow electronic tailoring of the temperature profile during sintering.     

An approach to optimization of shape memory alloy hybrid composite plates subjected to low-velocity impact

The paper presents an approach to the problem of optimum design of composite plates subjected to low velocity impact. The deflections and stresses are reduced by employing prestrained shape memory alloy (SMA) fibers which are in the martensitic phase when embedded within the plate. At an elevated temperature, the SMA fibers transform into the austenitic phase and tend to contract. However, due to a constraint, the contraction is either completely prevented or reduced resulting in significant tensile recovery stresses. This tension reduces deformations and stresses in the plate subjected to low-velocity impact.The solution in the paper addresses an impact of cross-ply plates with SMA fibers embedded within the layers oriented in both directions. An approach to optimization considered in the paper involves variations of the volume fractions of SMA fibers in each direction subject to a constraint on the total volume of the shape memory alloy. It is shown that an application of SMA fibers can significantly reduce deflections and stresses. A further benefit can be achieved by an optimization of a distribution of volume fractions of SMA fibers between the layers.     

Compressive Response of Composites Under Combined Fire and Compression Loading

The thermal buckling of an axially restrained composite column that is exposed to a heat flux due to fire is studied by both analytical and experimental means. The column is exposed to fire from one-side and the resulting heat damage, the charred layer formation and non-uniform transient temperature distribution are calculated by the thermal model developed by Gibson et al. (Revue de l’Institute Francais du Petrole 50:69–74, 1995). For the thermal buckling analysis, the mechanical properties of the fire-damaged (charred) region are considered negligible; the degradation of the elastic properties with temperature (especially near the glass transition temperature of the matrix) in the undamaged layer, is accounted for by using experimental data for the elastic moduli. Due to the non-uniform stiffness and the effect of the ensuing thermal moment, the structure behaves like an imperfect column, and responds by bending rather than buckling in the classical Euler (bifurcation) sense. Another important effect of the non-uniform temperature is that the neutral axis moves away from the centroid of the cross section, resulting in another moment due to eccentric loading, which would tend to bend the structure away from the fire. In order to verify the mechanical response, the compressive buckling behavior of the same material subjected to simultaneous high intensity surface heating and axial compressive loading were investigated experimentally. Fire exposure was simulated by subjecting the surface of rectangular specimens to radiant heating in a cone calorimeter. Heat flux levels of 25 kW/m², 50 kW/m² and 75 kW/m² were studied. All specimens exhibited buckling and subsequent catastrophic failure, even at compressive stresses as low as 3.5 MPa under a surface heat flux of 25 kW/m². Details of the experimental procedure, including modifications made to a cone calorimeter to allow simultaneous mechanical loading are presented.