Seismic stability of a long unsupported circular tunnel

The stability of a long unsupported circular tunnel (opening) in a cohesive frictional soil has been assessed with the inclusion of pseudo-static horizontal earthquake body forces. The analysis has been performed under plane strain conditions by using upper bound finite element limit analysis in combination with a linear optimization procedure. The results have been presented in the form of a non-dimensional stability number (gamma H-max/c); where H = tunnel cover, c refers to soil cohesion and gamma(max) is the maximum unit weight of soil mass which the tunnel can support without collapse. The results have been obtained for various values of H/D (D = diameter of the tunnel), internal friction angle (phi) of soil, and the horizontal earthquake acceleration coefficient (alpha(h)). The computations reveal that the values of the stability numbers (i) decrease quite significantly with an increase in alpha(h), and (ii) become continuously higher for greater values of H/D and phi. As expected, the failure zones around the periphery of the tunnel becomes always asymmetrical with an inclusion of horizontal seismic body forces. (c) 2012 Elsevier Ltd. All rights reserved.

A finite population model of mlecular evolution: theory and computation

This article is concerned with the evolution of haploid organisms that reproduce asexually. In a seminal piece of work, Eigen and coauthors proposed the quasispecies model in an attempt to understand such an evolutionary process. Their work has impacted antiviral treatment and vaccine design strategies. Yet, predictions of the quasispecies model are at best viewed as a guideline, primarily because it assumes an infinite population size, whereas realistic population sizes can be quite small. In this paper we consider a population genetics-based model aimed at understanding the evolution of such organisms with finite population sizes and present a rigorous study of the convergence and computational issues that arise therein. Our first result is structural and shows that, at any time during the evolution, as the population size tends to infinity, the distribution of genomes predicted by our model converges to that predicted by the quasispecies model. This justifies the continued use of the quasispecies model to derive guidelines for intervention. While the stationary state in the quasispecies model is readily obtained, due to the explosion of the state space in our model, exact computations are prohibitive. Our second set of results are computational in nature and address this issue. We derive conditions on the parameters of evolution under which our stochastic model mixes rapidly. Further, for a class of widely used fitness landscapes we give a fast deterministic algorithm which computes the stationary distribution of our model. These computational tools are expected to serve as a framework for the modeling of strategies for the deployment of mutagenic drugs.

Texture evolution in high strain rate deformed Cu-10Zn alloy

High strain rate deformation behavior of Cu-10Zn alloy was studied. A weak texture with fine grain size was observed at high strain rate. The weak texture has been attributed to activity of higher number of slip systems under dynamic loading conditions. Twinning has minimal role on texture. (C) 2012 Elsevier B.V. All rights reserved.

Fracture Analysis of High strength and Ultra high strength Concrete beams by using Finite Element Method

This paper presents the details of nonlinear finite element analysis (FEA) of three point bending specimens made up of high strength concrete (HSC, HSC1) and ultra high strength concrete (UHSC). Brief details about characterization and experimentation of HSC, HSC1 and UHSC have been provided. Cracking strength criterion has been used for simulation of crack propagation by conducting nonlinear FEA. The description about FEA using crack strength criterion has been outlined. Bi-linear tension softening relation has been used for modeling the cohesive stresses ahead of the crack tip. Numerical studies have been carried out on fracture analysis of three point bending specimens. It is observed from the studies that the computed values from FEA are in very good agreement with the corresponding experimental values. The computed values of stress vs crack width will be useful for evaluation of fracture energy, crack tip opening displacement and fracture toughness. Further, these values can also be used for crack growth study, remaining life assessment and residual strength evaluation of concrete structural components.

The influence of temperature and strain rate on the deformation response and microstructural evolution during hot compression of a titanium alloy Ti-6Al-4V-0.1B

Hot deformation behavior of a hypoeutectic Ti-6Al-4V-0.1B alloy in (alpha + beta) phase field is investigated in the present study with special reference to flow response, kinetics and microstructural evolution. For a comparison, the base alloy Ti-6Al-4V was also studied under identical conditions. Dynamic recovery of alpha phase occurs at low temperatures while softening due to globularization and/or dynamic recrystallization dominates at high temperatures irrespective of boron addition. Microstructural features for both the alloys display bending and kinking of alpha lamellae for near alpha test temperatures. Unlike Ti-6Al-4V, no sign of instability formation was observed in Ti-6Al-4V-0.1B for any deformation condition except for cavitation around TiB particles, due to deformation incompatibility and strain accumulation at the particle-matrix interface. The absence of macroscopic instabilities and early initiation of softening mechanisms as a result of boron addition has been attributed to microstructural features (e.g. refined prior beta grain and alpha colony size, absence of grain boundary alpha layer, presence of TiB particles at prior beta boundaries, etc.) of the respective alloys prior to deformation. (C) 2012 Elsevier B.V. All rights reserved.

Theoretical-experimental study of shock wave-assisted metal forming process using a diaphragmless shock tube

The use of high-velocity sheet-forming techniques where the strain rates are in excess of 10(2)/s can help us solve many problems that are difficult to overcome with traditional metal-forming techniques. In this investigation, thin metallic plates/foils were subjected to shock wave loading in the newly developed diaphragmless shock tube. The conventional shock tube used in the aerodynamic applications uses a metal diaphragm for generating shock waves. This method of operation has its own disadvantages including the problems associated with repeatable and reliable generation of shock waves. Moreover, in industrial scenario, changing metal diaphragms after every shot is not desirable. Hence, a diaphragmless shock tube is calibrated and used in this study. Shock Mach numbers up to 3 can be generated with a high degree of repeatability (+/- 4 per cent) for the pressure jumps across the primary shock wave. The shock Mach number scatter is within +/- 1.5 per cent. Copper, brass, and aluminium plates of diameter 60 mm and thickness varying from 0.1 to 1 mm are used. The plate peak over-pressures ranging from 1 to 10 bar are used. The midpoint deflection, circumferential, radial, and thickness strains are measured and using these, the Von Mises strain is also calculated. The experimental results are compared with the numerical values obtained using finite element analysis. The experimental results match well with the numerical values. The plastic hinge effect was also observed in the finite element simulations. Analysis of the failed specimens shows that aluminium plates had mode I failure, whereas copper plates had mode II failure.

The Role of Strain Rate Response on Tribological Behavior of Metals

In an effort to study the role of strain rate response on the tribological behavior of metals, room temperature experiments were conducted by sliding commercially pure titanium and a-iron pins against an H-11 die steel flats of various surface textures. The steel flat surface textures were specifically prepared to allow for imposing varying amounts of strain rates at the contacting interface during sliding motion. In the experiments, it was observed that titanium (a harder material than iron) formed a transfer layer on H-11 steel surface textures that produced higher strain rates. In contrast, the titanium pins abraded the steel surfaces that produced lower strain rates. The iron pins were found to abrade the H-11 steel surface regardless of the surface texture characteristics. This unique tribological behavior of titanium is likely due to the fact that titanium undergoes adiabatic shear banding at high strain rates, which creates pathways for lower resistance shear planes. These shear planes lead to fracture and transfer layer formation on the surface of the steel flat, which ultimately promotes a higher strain rate of deformation at the asperity level. Iron does not undergo adiabatic shear banding and thus more naturally abrades the surfaces. Overall, the results clear indicated that a materials strain rate response can be an important factor in controlling the tribological behavior of a plastically deforming material at the asperity level. DOI: 10.1115/1.4007675]

Evolution of Texture and Microstructure in Commercially Pure Titanium with Change in Strain Path During Rolling

The evolution of microstructure and texture in commercially pure titanium has been studied as a function of strain path during rolling using experimental techniques and viscoplastic self-consistent simulations. Four different strain paths, namely unidirectional rolling, two-step cross rolling, multistep cross rolling, and reverse rolling, have been employed to decipher the effect of strain path change on the evolution of deformation texture and microstructure. The cross-rolled samples show higher hardness with lower microstrain and intragranular misorientation compared to the unidirectional rolled sample as determined from X-ray diffraction and electron backscatter diffraction, respectively. The higher hardness of the cross-rolled samples is attributed to orientation hardening due to the near basal texture. Viscoplastic self-consistent simulations are able to successfully predict the texture evolution of the differently rolled samples. Simulation results indicate the higher contribution of basal slip in the formation of near basal texture and as well as lower intragranular misorientation in the cross-rolled samples.

Non-Stationary Semi-Markov Decision Processes on a Finite Horizon

We introduce and study a class of non-stationary semi-Markov decision processes on a finite horizon. By constructing an equivalent Markov decision process, we establish the existence of a piecewise open loop relaxed control which is optimal for the finite horizon problem.

Operator-splitting finite element algorithms for computations of high-dimensional parabolic problems

An operator-splitting finite element method for solving high-dimensional parabolic equations is presented. The stability and the error estimates are derived for the proposed numerical scheme. Furthermore, two variants of fully-practical operator-splitting finite element algorithms based on the quadrature points and the nodal points, respectively, are presented. Both the quadrature and the nodal point based operator-splitting algorithms are validated using a three-dimensional (3D) test problem. The numerical results obtained with the full 3D computations and the operator-split 2D + 1D computations are found to be in a good agreement with the analytical solution. Further, the optimal order of convergence is obtained in both variants of the operator-splitting algorithms. (C) 2012 Elsevier Inc. All rights reserved.

On the capacity of quantized gaussian MAC channels with finite input alphabet

In this paper, we investigate the achievable rate region of Gaussian multiple access channels (MAC) with finite input alphabet and quantized output. With finite input alphabet and an unquantized receiver, the two-user Gaussian MAC rate region was studied. In most high throughput communication systems based on digital signal processing, the analog received signal is quantized using a low precision quantizer. In this paper, we first derive the expressions for the achievable rate region of a two-user Gaussian MAC with finite input alphabet and quantized output. We show that, with finite input alphabet, the achievable rate region with the commonly used uniform receiver quantizer has a significant loss in the rate region compared. It is observed that this degradation is due to the fact that the received analog signal is densely distributed around the origin, and is therefore not efficiently quantized with a uniform quantizer which has equally spaced quantization intervals. It is also observed that the density of the received analog signal around the origin increases with increasing number of users. Hence, the loss in the achievable rate region due to uniform receiver quantization is expected to increase with increasing number of users. We, therefore, propose a novel non-uniform quantizer with finely spaced quantization intervals near the origin. For a two-user Gaussian MAC with a given finite input alphabet and low precision receiver quantization, we show that the proposed non-uniform quantizer has a significantly larger rate region compared to what is achieved with a uniform quantizer.

A finite-rate-of-innovation signal sampling perspective of the source-filter model of speech production

We consider the speech production mechanism and the asso- ciated linear source-filter model. For voiced speech sounds in particular, the source/glottal excitation is modeled as a stream of impulses and the filter as a cascade of second-order resonators. We show that the process of sampling speech signals can be modeled as filtering a stream of Dirac impulses (a model for the excitation) with a kernel function (the vocal tract response),and then sampling uniformly. We show that the problem of esti- mating the excitation is equivalent to the problem of recovering a stream of Dirac impulses from samples of a filtered version. We present associated algorithms based on the annihilating filter and also make a comparison with the classical linear prediction technique, which is well known in speech analysis. Results on synthesized as well as natural speech data are presented.

Mechanism for the Compressive Strain Induced Oscillations in the Conductance of Carbon Nanotubes

We report on the monotonic increase and the oscillation of electrical conductance in multiwalled carbon nanotubes with compressive strain. Combined experimental and theoretical analyses confirm that the conductance variation with strain is because of the transition from sp(2) to configurations that are promoted by the interaction of walls in the nanotubes. The intrawall interaction is the reason for the monotonic increase in the conduction, while the oscillations are attributable to interwall interactions. This explains the observed electromechanical oscillation in multiwalled nanotubes and its absence in single-walled nanotubes, thereby resolving a long-standing debate on the interpretation of these results. Moreover, the current carrying capability of nanotubes can be enhanced significantly by controlling applied strains. DOI: 10.1103/PhysRevLett.110.095504

Strain localization effect on system reliability based design of bridge abutments under earthquake loading

The component and system reliability based design of bridge abutments under earthquake loading is presented in the paper. Planar failure surface has been used in conjunction with pseudo-dynamic approach to compute seismic active earth pressures on an abutment. The pseudo-dynamic method, considers the effect of phase difference in shear waves, soil amplification along with the horizontal seismic accelerations, strain localization in backfill soil and associated post-peak reduction in the shear resistance from peak to residual values along a previously formed failure plane. Four modes of stability viz. sliding, overturning, eccentricity and bearing capacity of the foundation soil are considered in the analysis. The series system reliability is computed with an assumption of independent failure modes. The lower and upper bounds of system reliability are also computed by taking into account the correlations between four failure modes, which is evaluated using the direction cosines of the tangent planes at the most probable points of failure.

Influence of strain localization on reliability based design of bridge abutments using pseudo-dynamic method

The paper focuses on reliability based design of bridge abutments when subjected to earthquake loading. Planar failure surface has been used in conjunction with pseudo-dynamic approach to compute the seismic active earth pressures on the bridge abutment. The proposed pseudo dynamic method, considers the effects of strain localization in the backfill soil and associated post-peak reduction in the shear resistance from peak to residual values along a previously formed failure plane, phase difference in shear waves and soil amplification along with the horizontal seismic accelerations. Four modes of stability viz. sliding, overturning, eccentricity and bearing capacity of the foundation soil are considered for the reliability analysis. The influence of various design parameters on the seismic reliability indices against four modes of failure is presented, following the suggestions of Japan Road Association, Caltrans Bridge Design Specifications and U.S Department of the Army.