Fatigue crack growth due to overloads in plain concrete using scaling laws

Scaling laws are represented in power law form and can be utilized to extract the characteristic properties of a new phenomenon with the help of self-similar solutions. In this work, an attempt has been made to propose a scaling law analytically, for plain concrete when subjected to variable amplitude loading. Due to the application of overload on concrete structures, acceleration in the crack growth process takes place. A closed form expression has been developed to capture the acceleration in crack growth rate in conjunction with the principles of dimensional analysis and self-similarity. The proposed model accounts for parameters such as, the tensile strength, fracture toughness, overload effect and the structural size. Knowing the governed and the governing parameters of the physical problem and by using the concepts of self-similarity, a relationship is obtained between the different parameters involved. The predicted results are compared with experimental crack growth data for variable amplitude loading and are found to capture the overload effect with sufficient accuracy. Through a sensitivity analysis, fracture toughness is found to be the most dominant parameter in accelerating the crack length due to application of overload.

Vapor-liquid-solid growth of Si nanowires: A kinetic analysis

A steady state kinetic model has been developed for the vapor-liquid-solid growth of Si whiskers or nanowires from liquid catalyst droplets. The steady state is defined as one in which the net injection rate of Si into the droplet is equal to the ejection rate due to wire growth. Expressions that represent specific mechanisms of injection and ejection of Si atoms from the liquid catalyst droplet have been used and their relative importance has been discussed. The analysis shows that evaporation and reverse reaction rates need to be invoked, apart from just surface cracking of the precursor, in order to make the growth rate radius dependent. When these pathways can be neglected, the growth rate become radius independent and can be used to determine the activation energies for the rate limiting step of heterogeneous precursor decomposition. The ejection rates depend on the mechanism of wire growth at the liquid-solid interface or the liquid-solid-vapor triple phase boundary. It is shown that when wire growth is by nucleation and motion of ledges, a radius dependence of growth rate does not just come from the Gibbs-Thompson effect on supersaturation in the liquid, but also from the dependence of the actual area or length available for nucleation. Growth rates have been calculated using the framework of equations developed and compared with experimental results. The agreement in trends is found to be excellent. The same framework of equations has also been used to account for the diverse pressure and temperature dependence of growth rates reported in the literature. © 2012 American Institute of Physics.

Analysis of fatigue crack growth in RC beams

In this work, a fatigue crack propagation model developed using dimensional analysis for plain concrete is used in conjunction with the steel closing force to predict the crack growth behavior of reinforced concrete beams. A numerical procedure is followed using the proposed model to compute the fatigue life of RC beams and the dissipated energy in the steel reinforcement due to shake down behavior. Through a sensitivity study, it is found that the structural size is the most sensitive parameter on which the crack growth rate is dependent. Furthermore, the moment carrying capacity of an RC beam is computed as function of crack size by considering the effect of bond slip.

Effect of Ti concentration on the growth of Nb3Sn between solid Nb(Ti) and liquid Sn

The change in the growth rate of the Nb3Sn product phase because of Ti addition is studied for solid Nb(Ti)-liquid Sn interactions. The growth rate increased from no Ti to 1 at.% and 2 at.% of Ti in Nb, and the activation energy decreased from 221 kJ/mol to 146 kJ/mol. Based on the estimated values, the role of grain boundary and lattice diffusion is discussed in light of the possibility of increased grain boundary area and point defects such as antisites and vacancies.

An enthalpy-based model of dendritic growth in a convecting binary alloy melt

Purpose-In the present work, a numerical method, based on the well established enthalpy technique, is developed to simulate the growth of binary alloy equiaxed dendrites in presence of melt convection. The paper aims to discuss these issues. Design/methodology/approach-The principle of volume-averaging is used to formulate the governing equations (mass, momentum, energy and species conservation) which are solved using a coupled explicit-implicit method. The velocity and pressure fields are obtained using a fully implicit finite volume approach whereas the energy and species conservation equations are solved explicitly to obtain the enthalpy and solute concentration fields. As a model problem, simulation of the growth of a single crystal in a two-dimensional cavity filled with an undercooled melt is performed. Findings-Comparison of the simulation results with available solutions obtained using level set method and the phase field method shows good agreement. The effects of melt flow on dendrite growth rate and solute distribution along the solid-liquid interface are studied. A faster growth rate of the upstream dendrite arm in case of binary alloys is observed, which can be attributed to the enhanced heat transfer due to convection as well as lower solute pile-up at the solid-liquid interface. Subsequently, the influence of thermal and solutal Peclet number and undercooling on the dendrite tip velocity is investigated. Originality/value-As the present enthalpy based microscopic solidification model with melt convection is based on a framework similar to popularly used enthalpy models at the macroscopic scale, it lays the foundation to develop effective multiscale solidification.

Seasonal variability of rainfall recorded in growth bands of the Giant African Land Snail Lissachatina fulica (Bowdich) from India

Seasonal rainfall patterns in Bangalore, India, have been reconstructed using stable isotopic ratios in the growth bands of Giant African Land Snail shells. The present study was conducted at Bangalore, India which receives rain during the summer months. The oxygen isotopic record in the rainwater samples collected during different months covering the period of the summer monsoon of the year 2008 is compared with the isotopic ratio in the gastropod growth bands deposited simultaneously. The chronology of the shell growth band is independently established assuming the growth rate observed in a chamber experiment maintaining similar relative humidity and temperature conditions. A consistent pattern observed in the isotopic ratio in the gastropod growth bands and rainwater is demonstrated and provides a novel approach for precipitation reconstruction at seasonal and weekly time scales. This approach of using isotopic ratios in the gastropod growth bands for rainfall can serve as a substitute for filling gaps in rainfall data and for cases where no rain records are available. In addition, they can be used to determine the frequencies and magnitudes of dry spells from the past records. (C) 2013 Elsevier B.V. All rights reserved.

Delay optimal scheduling of a discrete-time batch service queue for point-to-point channel code rate selection

We consider the problem of characterizing the minimum average delay, or equivalently the minimum average queue length, of message symbols randomly arriving to the transmitter queue of a point-to-point link which dynamically selects a (n, k) block code from a given collection. The system is modeled by a discrete time queue with an IID batch arrival process and batch service. We obtain a lower bound on the minimum average queue length, which is the optimal value for a linear program, using only the mean (λ) and variance (σ2) of the batch arrivals. For a finite collection of (n, k) codes the minimum achievable average queue length is shown to be Θ(1/ε) as ε ↓ 0 where ε is the difference between the maximum code rate and λ. We obtain a sufficient condition for code rate selection policies to achieve this optimal growth rate. A simple family of policies that use only one block code each as well as two other heuristic policies are shown to be weakly optimal in the sense of achieving the 1/ε growth rate. An appropriate selection from the family of policies that use only one block code each is also shown to achieve the optimal coefficient σ2/2 of the 1/ε growth rate. We compare the performance of the heuristic policies with the minimum achievable average queue length and the lower bound numerically. For a countable collection of (n, k) codes, the optimal average queue length is shown to be Ω(1/ε). We illustrate the selectivity among policies of the growth rate optimality criterion for both finite and countable collections of (n, k) block codes.

Effect of shrinkage induced flow on binary alloy dendrite growth: An equivalent undercooling model

This paper presents a theoretical model for studying the effects of shrinkage induced flow on the growth rate of binary alloy dendrites. An equivalent undercooling of the melt is defined in terms of ratio of the phase densities to represent the change in dendrite growth rate due to variation in solutal and thermal transport resulting from shrinkage induced flow. Subsequently, results for dendrite growth rate predicted by the equivalent undercooling model is compared with the corresponding predictions obtained using an enthalpy based numerical method for dendrite growth with shrinkage. The agreement is found to be good. Published by Elsevier Ltd.

Phase evolutions, growth kinetics and diffusion parameters in the Co-Ni-Ta system

Diffusion couple experiments are conducted to study phase evolutions in the Co-rich part of the Co-Ni-Ta phase diagram. This helps to examine the available phase diagram and propose a correction on the stability of the Co2Ta phase based on the compositional measurements and X-ray analysis. The growth rate of this phase decreases with an increase in Ni content. The same is reflected on the estimated integrated interdiffusion coefficients of the components in this phase. The possible reasons for this change are discussed based on the discussions of defects, crystal structure and the driving forces for diffusion. Diffusion rate of Co in the Co2Ta phase at the Co-rich composition is higher because of more number of Co-Co bonds present compared to that of Ta-Ta bonds and the presence of Co antisites for the deviation from the stoichiometry. The decrease in the diffusion coefficients because of Ni addition indicates that Ni preferably replaces Co antisites to decrease the diffusion rate. (C) 2014 Elsevier B.V. All rights reserved.

Gold Catalyzed Plasma Assisted Growth of Germanium Nanoneedles

A systematic study of Gold catalyzed growth of Ge nanoneedles by PECVD at low temperatures (<400 degrees C) is presented. Morphology, growth rate and aspect ratio of the needles are studied as a function of power, gas flow rate and chamber pressure. Nanoneedles were grown at pre-defined positions with catalytic particles obtained by e-Beam Lithography and lift off. This opens up the possibility of using Ge Nano needles in photovoltaic, nanoelectronics and nanosensor device applications.

Optimal control analysis of the dynamic growth behavior of microorganisms

Understanding the growth behavior of microorganisms using modeling and optimization techniques is an active area of research in the fields of biochemical engineering and systems biology. In this paper, we propose a general modeling framework, based on Monad model, to model the growth of microorganisms. Utilizing the general framework, we formulate an optimal control problem with the objective of maximizing a long-term cellular goal and solve it analytically under various constraints for the growth of microorganisms in a two substrate batch environment. We investigate the relation between long term and short term cellular goals and show that the objective of maximizing cellular concentration at a fixed final time is equivalent to maximization of instantaneous growth rate. We then establish the mathematical connection between the generalized framework and optimal and cybernetic modeling frameworks and derive generalized governing dynamic equations for optimal and cybernetic models. We finally illustrate the influence of various constraints in the cybernetic modeling framework on the optimal growth behavior of microorganisms by solving several dynamic optimization problems using genetic algorithms. (C) 2014 Published by Elsevier Inc.

Effect of Nb orientation and deformation on the growth of Nb3Sn intermetallic superconductor by bronze technique

The growth of Nb3Sn by bronze technique on single crystals and deformed Nb is studied. The grain boundary diffusion-controlled growth rate is found to be higher for Nb-(0 1 3) than Nb-(0 1 1) because of smaller grain size of Nb3Sn. The difference in grain size is explained with the help of surface energies leading to different nucleation barrier. Significantly finer grains and higher growth rate of the product phase is found for rolled Nb because of available defects acting as potential nucleation sites.

Divergence in Patterns of Leaf Growth Polarity Is Associated with the Expression Divergence of miR396

Lateral appendages often show allometric growth with a specific growth polarity along the proximo-distal axis. Studies on leaf growth in model plants have identified a basipetal growth direction with the highest growth rate at the proximal end and progressively lower rates toward the distal end. Although the molecular mechanisms governing such a growth pattern have been studied recently, variation in leaf growth polarity and, therefore, its evolutionary origin remain unknown. By surveying 75 eudicot species, here we report that leaf growth polarity is divergent. Leaf growth in the proximo-distal axis is polar, with more growth arising from either the proximal or the distal end; dispersed with no apparent polarity; or bidirectional, with more growth contributed by the central region and less growth at either end. We further demonstrate that the expression gradient of the miR396-GROWTH-REGULATING FACTOR module strongly correlates with the polarity of leaf growth. Altering the endogenous pattern of miR396 expression in transgenic Arabidopsis thaliana leaves only partially modified the spatial pattern of cell expansion, suggesting that the diverse growth polarities might have evolved via concerted changes in multiple gene regulatory networks.

Effect of Ni content on the diffusion-controlled growth of the product phases in the Cu(Ni)-Sn system

A strong influence of Ni content on the diffusion-controlled growth of the (Cu,Ni)(3)Sn and (Cu,Ni)(6)Sn-5 phases by coupling different Cu(Ni) alloys with Sn in the solid state is reported. The continuous increase in the thickness ratio of (Cu,Ni)(6)Sn-5 to (Cu,Ni)(3)Sn with the Ni content is explained by combined kinetic and thermodynamic arguments as follows: (i) The integrated interdiffusion coefficient does not change for the (Cu,Ni)(3)Sn phase up to 2.5 at.% Ni and decreases drastically for 5 at.% Ni. On the other hand, there is a continuous increase in the integrated interdiffusion coefficient for (Cu,Ni)(6)Sn-5 as a function of increasing Ni content. (ii) With the increase in Ni content, driving forces for the diffusion of components increase for both components in both phases but at different rates. However, the magnitude of these changes alone is not large enough to explain the high difference in the observed growth rate of the product phases because of Ni addition. (iv) Kirkendall marker experiments indicate that the Cu6Sn5 phase grows by diffusion of both Cu and Sn in the binary case. However, when Ni is added, the growth is by diffusion of Sn only. (v) Also, the observed grain refinement in the Cu6Sn5 phase with the addition of Ni suggests that the grain boundary diffusion of Sn may have an important role in the observed changes in the growth rate.

Effect of Ga content in Cu(Ga) on the growth of V3Ga following bronze technique

Diffusion controlled growth rate of V3Ga in the Cu(Ga)/V system changes dramatically because of a small change in Ga content in Cu(Ga). One atomic percent increase from 15 to 16 leads to more than double the product phase layer thickness and a decrease in activation energy from 255 to 142 kJ/mol. Kirkendall marker experiment indicates that V3Ga grows because of diffusion of Ga. Role of different factors influencing the diffusion rate of Ga and high growth rate of V3Ga are discussed. (C) 2015 Elsevier Ltd. All rights reserved.