The use of the Hoffman yield criterion in finite element analysis of anisotropic composites

A return mapping algorithm has been developed for the Hoffman yield function of anisotropic plasticity. The accuracy of the algorithm has been assessed by means of iso-error maps for trial stress increments in the deviatoric and volumetric plane. A tangent operator that is consistent with the developed integration algorithm has been formulated. The Hoffman model has been applied to a plate structure and to two shell structures.     

Calcium channel types with distinct presynaptic localization couple differentially to transmitter release in single calyx-type synapses

We studied how Ca2+ influx through different subtypes of Ca2+ channels couples to release at a calyx-type terminal in the rat medial nucleus of the trapezoid body by simultaneously measuring the presynaptic Ca2+ influx evoked by a single action potential and the EPSC. Application of subtype-specific toxins showed that Ca2+ channels of the P/Q-, N-, and R-type controlled glutamate release at a single terminal. The Ca2+ influx through the P/Q-type channels triggered release more effectively than Ca2+ influx through N- or R-type channels. We investigated mechanisms that contributed to these differences in effectiveness. Electrophysiological experiments suggested that individual release sites were controlled by all three subtypes of Ca2+ channels. Immunocytochemical staining indicated, however, that a substantial fraction of N- and R-type channels was located distant from release sites. Although these distant channels contributed to the Ca2+ influx into the terminal, they may not contribute to release. Taken together, the results suggest that the Ca2+ influx into the calyx via N- and R-type channels triggers release less effectively than that via P/Q-type because a substantial fraction of the N- and R-type channels in the calyx is localized distant from release sites.     

The higher injury risk of abnormal kidneys in blunt renal trauma

In an effort to better understand some of the ethical and legal dilemmas that health professionals face in their daily activities, this study identified and analyzed the opinions and expectations of medical and nursing students at the University of S?o Paulo, Brazil, regarding the autonomy of adolescents in receiving health care services. Over the 1995-1996 period, interviews were done with students in the first and last years of their programs in the School of Medicine and the School of Nursing of the University of S?o Paulo. The results from the first-year students were compared with those of the final-year students. The interview questions dealt with such issues as providing medical care to adolescents when a parent or guardian was not present, adolescents making their own decisions on health care services, the confidentiality of information that adolescents provide, the circumstances in which that information could be given to parents or guardians, and situations where there were conflicts between the decisions of parents and adolescents. Despite variations among the opinions and expectations of the students, the overall results suggest that most of the medical and nursing students do not accept the concept of "health adulthood" or autonomy for adolescents and that the students instead choose to rely on current legal definitions of adulthood.     

Viscoplasticity for instabilities due to strain softening and strain-rate softening

Three viscoplastic approaches are examined in this paper. First, the overstress viscoplastic models (i.e. the Perzyna model and the Duvaut–Lions model) are outlined. Next, a consistency viscoplastic approach is presented. In the consistency model a rate-dependent yield surface is employed while the standard Kuhn–Tucker conditions for loading and unloading remain valid. For this reason, the yield surface can expand and shrink not only by softening or hardening effects, but also by softening/hardening rate effects. A full algorithmic treatment is presented for each of the three models including the derivation of a consistent tangential stiffness matrix. Based on a limited numerical experience it seems that the consistency model shows a faster global convergence than the overstress approaches. For softening problems all three approaches have a regularising effect in the sense that the initial-value problem remains well-posed. The width of the shear band is determined by the material parameters and, if present, by the size of an imperfection. A relation between the length scales of the three models is given. Furthermore, it is shown that the consistency model can properly simulate the so-called S-type instabilities, which are associated with the occurrence of travelling Portevin-Le Chatelier bands. © 1997 John Wiley & Sons, Ltd.     

Distributed Adaptive Algorithms for Optimal Opportunistic Medium Access

We examine threshold-based transmission strategies for distributed opportunistic medium access in a scenario with fairly general probabilistic interference conditions. Specifically, collisions between concurrent transmissions are governed by arbitrary probabilities, allowing for a form of channel capture and covering binary interference constraints as an important special case. We address the problem of setting the threshold values so as to optimize the aggregate throughput utility of the various users, and particularly focus on a weighted logarithmic throughput utility function (Proportional Fairness). We provide an adaptive algorithm for finding the optimal threshold values in a distributed fashion, and rigorously establish the convergence of the proposed algorithm under mild statistical assumptions. Moreover, we discuss how the algorithm may be adapted to achieve packet-level stability with only limited exchange of queue length information among the various users. We also conduct extensive numerical experiments to corroborate the theoretical convergence results.     

The use of service limits for efficient operation of multistationsingle-medium communication systems

Time limits are the major mechanisms used for controlling a large variety of multistation single-medium computer-communication systems like the FDDI network and the IEEE 802.4 Token Bus. The proper use of these mechanisms is still not understood and rules for efficient system operation are not available. The authors' objective is the derivation of such rules. They use a cyclic polling model with different service limits (k-limited service) at the different queues, thus emulating time limits. They are interested in determining these k-limit values so as to minimize the mean waiting cost of messages in the system. A simple approximative approach is proposed for two major problems: one in which a limit is set on the token rotation time and one in which no limits are imposed. The approach is tested for a variety of cases and is shown to be very effective     

Correlated Shadow-Fading in Wireless Networks and its Effect on Call Dropping

We discuss a statistical model to generate correlated shadow-fading patterns for wireless systems in the absence of detailed propagation and landscape information. The currently available autocorrelation models result in anomalous effects that depend on traffic density and mobility, as they propose independent random processes for each mobile. Our approach involves generating a pre-computed fading map with the right marginal distributions and spatial correlations, which avoids inconsistencies such as providing widely differing values for mobiles close to each other. The correlations are introduced via a Gaussian random field, which has a covariance structure that depends on a set of parameters which can be computed from local measurements. The model is efficiently implemented using standard linear-algebra methods. We conclude by describing a simulation experiment to study the effect of correlations on call dropping. The experiment reveals a strong relationship between call dropping and the correlation length of the fading pattern, and indicates circumstances under which dropping may be relatively high.     

Generalized processor sharing with light-tailed and heavy-tailed input

We consider a queue fed by a mixture of light-tailed and heavy-tailed traffic. The two traffic flows are served in accordance with the generalized processor sharing (GPS) discipline. GPS-based scheduling algorithms, such as weighted fair queueing, have emerged as an important mechanism for achieving service differentiation in integrated networks. We derive the asymptotic workload behavior of the light-tailed traffic flow under the assumption that its GPS weight is larger than its traffic intensity. The GPS mechanism ensures that the workload is bounded above by that in an isolated system with the light-tailed flow served in isolation at a constant rate equal to its GPS weight. We show that the workload distribution is in fact asymptotically equivalent to that in the isolated system, multiplied with a certain pre-factor, which accounts for the interaction with the heavy-tailed flow. Specifically, the pre-factor represents the probability that the heavy-tailed flow is backlogged long enough for the light-tailed flow to reach overflow. The results provide crucial qualitative insight in the typical overflow scenario.     

Stability of Parallel Queueing Systems with Coupled Service Rates

This paper considers a parallel system of queues fed by independent arrival streams, where the service rate of each queue depends on the number of customers in all of the queues. Necessary and sufficient conditions for the stability of the system are derived, based on stochastic monotonicity and marginal drift properties of multiclass birth and death processes. These conditions yield a sharp characterization of stability for systems where the service rate of each queue is decreasing in the number of customers in other queues, and has uniform limits as the queue lengths tend to infinity. The results are illustrated with applications where the stability region may be nonconvex.     

A solution technique for placing digital integrated circuitry on printed circuit boards

A mathematical formulation of the problem is presented using the concepts and terminology of graph theory. This formulation includes both stages of the placement problem wherein the vertices of the circuit graph are partitioned into several subsets (representing integrated circuit packages) which are, in turn, assigned to geometric positions corresponding to the printed circuit board specifications. Utilizing the manhattan metric, a heuristic procedure is described which provides minimum length placements on two-sided printed circuit boards consistent with the requirements of PC board designers. Computational results from problems arising in actual design efforts are discussed.