Energy Release Rates for the Peeling of Flexible Membranes and the Analysis of Blister Tests

Solutions are given for the debonding of a strip by extending the usual peel test analysis. Exact solutions are obtained for concentrated loads and for uniform pressure. Consideration is also given to energy based solutions and to the effect of residual strains. The axisymmetric case is then analysed in detail and the exact series solution compared with the constant stress assumption. It is observed that only slight errors arise in using the latter and a general scheme is developed for deriving expressions for G based on this assumptions. Solutions for concentrated forces, island blisters and constrained blisters are derived. The effects of large displacements and of plastic yielding are also described. It is observed that solutions for G in the form of energy divided by area created are very useful in that the parameters relating this ratio to G are reasonably insensitive, thus rendering experiments simple. This revised version was published online in August 2006 with corrections to the Cover Date.     

Application of terahertz spectroscopy for remote express analysis of gases

An overview of the main methods of terahertz spectroscopy used for the detection of gases and gas mixtures is presented. Special attention is given to the methods and equipment used for remote express sensing of gases in the atmosphere to ensure a quick and high-precision identification of gases which is a relevant task important both from fundamental and applied points of view. The developed to date methods are analyzed and their parameters, which are largely determined by the available equipment, are discussed. The techniques and equipment used for generation and detection of terahertz radiation are considered including new trends in terahertz technology, emergence of new types of radiation sources, and upgrading of existing ones. Data on the use of the terahertz spectroscopy for the detection of gases in the laboratory and in open air are summarized. Key challenges and prospects for further application of the terahertz spectroscopy are outlined.     

Parametric analyses of diffusion of activated sources in disposal forms

The primary objective of a low-level radioactive waste disposal facility is to isolate low-level radioactive waste from the public and the environment until the radionuclides in the waste have decayed to levels at which the hazard is negligible. Two fundamental concerns must be addressed when attempting to isolate low-level waste in a disposal facility on land. The first concern is isolating the waste from water, or hydrologic isolation. The second is preventing movement of the radionuclides out of the disposal facility, or radionuclide migration. Particularly, we have investigated here the latter scenario. Empirical curves describing the progression of the altered zone are obtained and they are used to define the correspondent altered thickness in the numerical calculations for a cement sample. Subsequent numerical analyses of diffusion of activated sources in waste disposals are considered for cemented containers. The adopted DAMAGE code takes into account a mass conservation equation and the linear momentum balance equation for the multiphase material. The mathematical model is based on the theory developed by Bazant for concretes and geomaterials; the fluid phases are considered as a unique mixture interacting with a solid phase. Short- and long-term diffusion analyses are performed for different characteristics of the grout and the results are presented in terms of radionuclides concentration. Indications on the minimum grout thicknesses able to resist to radioactive fluxes up to 1000 years are given.     

Applicability of the method of fundamental solutions

The condition number of a matrix is commonly used for investigating the stability of solutions to linear algebraic systems. Recent meshless techniques for solving partial differential equations have been known to give rise to ill-conditioned matrices, yet are still able to produce results that are close to machine accuracy. In this work, we consider the method of fundamental solutions (MFS), which is known to solve, with extremely high accuracy, certain partial differential equations, namely those for which a fundamental solution is known. To investigate the applicability of the MFS, either when the boundary is not analytic or when the boundary data are not harmonic, we examine the relationship between its accuracy and the effective condition number.Three numerical examples are presented in which various boundary value problems for the Laplace equation are solved. We show that the effective condition number, which estimates system stability with the right-hand side vector taken into account, is roughly inversely proportional to the maximum error in the numerical approximation. Unlike the proven theories in literature, we focus on cases when the boundary and the data are not analytic. The effective condition number numerically provides an estimate of the quality of the MFS solution without any knowledge of the exact solution and allows the user to decide whether the MFS is, in fact, an appropriate method for a given problem, or what is the appropriate formulation of the given problem.     

Buckling strength prediction of CFRP cylindrical panels using finite element method

An important factor in that carbon fiber reinforced plastic (CFRP) materials are not widely used in civil and offshore application, is the lack of generally applicable design criteria for composite structures. This is particularly true in the case of buckling strength prediction of composite shell structures, where many problems are left to the designer and high safety factors are frequently used, that reduce the overall structural efficiency. The parameters investigated for design to account for the buckling response include fiber orientations and radius to thickness ratio. In so far as design for buckling is concerned, it is well known that a key step is to investigate the influence of the initial geometric imperfection. Both eigenvalue and incremental analyses are performed, the latter including the effect of the initial geometric imperfection shape and amplitude. The results are used to estimate the imperfection sensitivity for such CFRP curved panels. As a result, ‘knockdown’ factors are estimated for use in design.     

Nonlinear interaction model of subsonic jet noise

Noise generation in a subsonic round jet is studied by a simplified model, in which nonlinear interactions of spatially evolving instability modes lead to the radiation of sound. The spatial mode evolution is computed using linear parabolized stability equations. Nonlinear interactions are found on a mode-by-mode basis and the sound radiation characteristics are determined by solution of the Lilley-Goldstein equation. Since mode interactions are computed explicitly, it is possible to find their relative importance for sound radiation. The method is applied to a single stream jet for which experimental data are available. The model gives Strouhal numbers of 0.45 for the most amplified waves in the jet and 0.19 for the dominant sound radiation. While in near field axisymmetric and the first azimuthal modes are both important, far-field sound is predominantly axisymmetric. These results are in close correspondence with experiment, suggesting that the simplified model is capturing at least some of the important mechanisms of subsonic jet noise.     

Optimization of Short Load Forecasting in Electricity Market of Iran Using Artificial Neural Networks

Accurate short-term load forecasting (STLF) is one of the essential requirements for power systems. In this paper, two different seasonal artificial neural networks (ANNs) are designed and compared in terms of model complexity, robustness, and forecasting accuracy. Furthermore, the performance of ANN partitioning is evaluated. The first model is a daily forecasting model which is used for forecasting hourly load of the next day. The second model is composed of 24 sub-networks which are used for forecasting hourly load of the next day. In fact, the second model is partitioning of the first model. Time, temperature, and historical loads are taken as inputs for ANN models. The neural network models are based on feed-forward back propagation which are trained and tested using data from electricity market of Iran during 2003 to 2005. Results show a good correlation between actual data and ANN outcomes. Moreover, it is shown that the first designed model consisting of single ANN is more appropriate than the second model consisting of 24 distinct ANNs. Finally ANN results are compared to conventional regression models. It is observed that in most cases ANN models are superior to regression models in terms of mean absolute percentage error (MAPE).     

Constitutive equations for use in prediction of flow stress during extrusion of aluminium alloys

Abstract

Constitutive equations are reviewed and discussed in the context of extrusion. It is concluded that mainly because extrusion converts directly from cast to finished product a steady state equation is adequate for use in extrusion problems and that the Zener-Hollomon type of formulation is both convenient and defined for a number of aluminium alloys. Experimental work is presented to show that if the data are correctly interpreted by suitably allowing for temperature rise, then testing in either torsion or compression yields identical equations. The physical interpretation of the various constants is discussed. Evidence is presented that differing homogenisation treatments produce differing equations and the effects of heating and cooling rates are considered. The implications are discussed in some detail. Constitutive equations are quantified for 26 aluminium alloys.     

Determination of multiple solutions of load flow equations

This paper is concerned with the problem of finding all the real solutions (all components of the solution vector must be real values) of load flow equations. Solutions in which some of the components are complex values are of no interest as they have no physical significance as a load flow solution. This problem is significant not only because of its theoretical challenge but also, its relationship with several system behavior related issues. Approaches suggested so far for solving this problem are rather ad hoc, computationally demanding and have been demonstrated only on very small systems. Further, it has been subsequently shown by others that many of these methods are not capable of finding all solutions. In this work a new approach is proposed which is more systematic and seems to have the potential to handle even large problems. We show that for any system it is possible to find the multiple load flow solutions (MLFS) corresponding to a given operating point extremely easily, starting from a set of points that are referred to as zero load solutions (ZLS) in this paper. It is shown that the complete set of ZLS is unique for a system and MLFS for any other operating point can be obtained starting from these ZLS using only the Newton’s load flow method. The set of procedures for implementing the proposed scheme are illustrated and their features are highlighted by considering several sample systems.     

Application of response surfaces for reliability analysis of marine structures

Marine structures subjected to multiple environmental loads (i.e. waves, current, wind) are considered. These loads are characterized by a set of corresponding parameters. The structural fatigue damage and long-term response are expressed in terms of these environmental parameters based on application of polynomial response surfaces. For both types of analysis, an integration across the range of variation for all the environmental parameters is required. The location of the intervals which give rise to the dominant contribution for these integrals depends on the relative magnitude of the coefficients defining the polynomials. The required degree of numerical subdivision in order to obtain accurate results is also of interest. These issues are studied on a non-dimensional form. The loss of accuracy which results when applying response surfaces of too low order is also investigated. Response surfaces with cut-off limits at specific lower-bound values for the environmental parameters are further investigated.

Having obtained general expressions on non-dimensional form, examples which correspond to specific response quantities for marine structures are considered. Typical values for the polynomial coefficients, and for the statistical distributions representing the environmental parameters, are applied. Convergence studies are subsequently performed for the particular example response quantities in order to make comparison with the general formulation. For the extreme response, the application of ‘extreme contours’ obtained from the statistical distributions of the environmental parameters is explored.     

Compression and shear of a layer of granular material

A classical problem in metal plasticity is the compression of a block of material between rigid platens. The corresponding problem for a layer of granular material that conforms to the Coulomb-Mohr yield condition and the double-shearing theory for the velocity field has also been solved. A layer of granular material between rough rigid plates that is subjected to both compression and shearing forces is considered. Analytical solutions are obtained for the stress and velocity fields in the layer. The known solutions for steady simple shear and pure compression are recovered as special cases. Yield loads are determined for combined compression and shear in the case of Coulomb friction boundary conditions. Numerical results which describe the stress and velocity fields in terms of the normal and shear forces on the layer at yield are presented for the case in which the surfaces of the platens are perfectly rough. Post-yield behaviour is briefly considered.     

Analysis of cracked disk specimens

A Fredholm integral equation of the plane problem for an arbitrarily loaded disk with an arbitrarily located inner crack is obtained and its solution in the form of double series in power of two parameters is provided. A general formula to define stress intensity factors of central crack is given by which the stress intensity factors for some types of loading are derived and analysed. It is shown that disks loaded by centrifugal forces due to uniform rotation and compressed in the plane of the crack are particularly suitable in fracture toughness testing of brittle materials. It is found that for disks stretched by two forces applied at some distance from the crack the stress intensity factor remains constant in a large range of crack lengths. Therefore they are recommended to be used in subcritical crack growth rate testing. Suitable grips are developed and appropriate correction for their effect is made in the stress intensity factor formula. Recommendations for a choice of optimal dimension ratios for a disk specimen are given.     

Compression and shear of a layer of granular material

A classical problem in metal plasticity is the compression of a block of material between rigid platens. The corresponding problem for a layer of granular material that conforms to the Coulomb-Mohr yield condition and the double-shearing theory for the velocity field has also been solved. A layer of granular material between rough rigid plates that is subjected to both compression and shearing forces is considered. Analytical solutions are obtained for the stress and velocity fields in the layer. The known solutions for steady simple shear and pure compression are recovered as special cases. Yield loads are determined for combined compression and shear in the case of Coulomb friction boundary conditions. Numerical results which describe the stress and velocity fields in terms of the normal and shear forces on the layer at yield are presented for the case in which the surfaces of the platens are perfectly rough. Post-yield behaviour is briefly considered.     

Probability of liquefaction spread

In order to predict the potential of liquefaction spread for sandy soil surface ground, a methodology is presented which evaluates probability of fraction of liquefaction spread for a zone under study, conditional on earthquake hazard. The SPT values are considered variables which are spatially correlated. The method which is used on this study is a Monte Carlo simulation based on a disjoint set of sampling density functions to make easy the sampling of random variables which are, otherwise, to be sampled from a joint probability density function of general distribution.The ground is modelled to consist of three dimensional discrete semi-zones having discrete elements in the vertical direction.A liquefaction potential index that is widely used in the design practice in Japan to account for the distribution of the safety factor in the soil-column direction, is employed as the performance function for the probability of liquefaction spread, and the numerical results are presented as a fragility curve of liquefaction spread.     

Estimation of Particle Size Distribution Based on Observed Weights of Groups of Particles

Particulate matter is collected on some sampling medium. The particles of interest are collected by some means, and weighed as a group. Weights are observed for a number of replicate samples. These observed weights may include background contribution from particles existing on the sampling medium prior to its use. Given these data, plus similar date to establish the background, the problem is to estimate the average number of particles per sample, and their weight distribution. The estimation is accomplished by equating sample moments to population moments. The first four moments of the population are found for an arbitrary weight distribution function possessing finite first four moments. Specific estimates are found in the event the weight distribution is exponential in form, and approximate sampling variances of these estimates are derived. A numerical example is included.     

Particle toughening of polymers by plastic void growth

An analysis is given for the toughening of particle filled polymers assuming that plastic void growth around debonded, or cavitated particles is the dominant energy absorbing mechanism. The controlling parameter is the debonding (or cavitation) surface energy which triggers the growth of a plastic void around the particle which in turn enhances the toughness. Literature data are examined for particles ranging from 0.01 to 25 μm in radius in thermoset resins and it is found that the surface work for very small particles is the surface work of adhesion while for larger sizes it is some fraction of the matrix toughness. This larger debonding energy is found to be proportional to the particle radius. Large toughness increases are predicted, and observed, in the nano-range, i.e. 0.01 μm which are shown to require good particle dispersion and high matrix ductility. More modest increases are predicted at the micron scale but these are more robust.     

Uncertainty Estimation of Some Sorption Isotherms Used for the Moisture Conditioning of Grains

Sorption isotherms relate the equilibrium condition between the moisture content of a solid material and the relative humidity of ambient where such material is placed. Isotherms are useful in many fields where the water vapor of ambient affects the properties of materials. In particular, the information given by the sorption isotherms is useful in the moisture conditioning of solid materials, which are used for the calibration of moisture meters for grains, cereals and wood, among others. There are many isotherm models (almost one for each material). However, most of them do not estimate the uncertainty or, in some cases, the estimation is incomplete. On the other hand, the most known method for uncertainty evaluation is given by the Guide to the Expression of Uncertainty in Measurement (GUM). However, this guide has some restrictions to be satisfactorily used; for example, the model of measurand must be linear and have a known probability distribution function for the model inputs and similar uncertainty values. So often, the sorption isotherm models used for grains are highly nonlinear. Therefore, the GUM could not provide reliable results. To overcome this, an alternative method is the use of Monte Carlo simulation method, which is suggested for nonlinear models in the GUM supplement. In this paper, the uncertainty estimation was done with the GUM and Monte Carlo methods applied to some sorption isotherms, which are used for grains and cereals. The results with both methods showed some discrepancies, which are due mainly to the nonlinearity of models.     

Postgrouped sampling method of estimation

A postgrouped sampling is considered for estimating the (finite or finite), population mean. Double sampling and an empirical-weighted estimator is used. Unbiasedness, variance and efficiency are considered. Its properties are discussed allowing the simple random sampling with replacement (SRSWR) design in the first phase, and in each stratum for the second phase. It is shown that for a fixed sample size in each postgroup, the variance of the proposed estimator with less prior information is asymptotically equivalent to the usual stratified estimator for fixed allocation. Some examples are provided for natural populations., The method is also extended to simple random sampling without replacement (SRSWOR) design in the first phase, and in each stratum for the second phase. Unbiased variance estimation is provided for both types of sampling designs.     

Development and evaluation of a high-frequency ultrasound-based system for in vivo strain imaging of the skin

The elastic properties of skin are of great interest in dermatology because they are affected by many pathological conditions. In this paper, a technique for in vivo mechanical strain imaging of the skin based on high-frequency ultrasound (HFUS) is presented. Elastic skin properties are assessed applying suction to the skin surface with a stepwise increased vacuum and estimating the resulting displacements in a spatially resolved manner. Acquired radio frequency (RF) echo signals and their envelope are analyzed for this purpose. A computer-controlled vacuum system with a digital pressure control loop was developed for precise and reproducible deformation. In a first processing step, the skin surface is segmented. Local axial strains inside the skin are estimated from axial displacements, which are estimated from consecutive echo signal frames analyzing the phase of the complex cross correlation function of analytical echo signals. Furthermore, speckle tracking is applied to estimate axial and lateral displacements and to quantify axial and lateral strains. The correlation coefficient of windowed echo signals compensated for displacements are used as a measure to validate the estimated strains, which is essential to accomplish reliable in vivo measurements. Phantom experiments were performed to validate the proposed technique. Results of in vivo measurements are presented, showing the potential for mechanical strain imaging in the skin in vivo.