The application of principles of soil mechanics to the modelling of pastes

This paper presents a simple model of the stress-strain behaviour of pastes made from non-damaging particles. It is based on an earlier model by Chandler and Macphee (Cem Concr Res 33:265–270, 2003) and Sands and Chandler (accepted for publication in Géotechnique) which is de-dimensionalised to reduce the number of model parameters and facilitate their establishment. The model is first tested against the results of experimental work on sands by Oka et al. (Géotechnique 49(5):661–680, 1999) and very successful simulations of torsion tests on hollow cylinders are obtained by finding the most suitable values of two adjustable parameters. The model is also checked against mechanical tests performed on a number of simple pastes. An experimental procedure used for the tests on various compositions of pastes is presented, together with the results of the tests and simulations using the model. Good agreement is achieved with the model, showing that some recent developments in soil models might provide a guide to developing and improving models for dense granular pastes operating in the frictional regime.     

Critical state behaviour of granular materials from isotropic and rebounded paths: DEM simulations

This paper presents the results of numerical simulations using the three-dimensional discrete element method (DEM) on the critical state behaviour of isotropically compressed and rebounded assemblies of granular materials. Drained and undrained (constant volume) numerical simulations were carried out. From these numerical simulations of drained and undrained tests, it has been shown that the steady state is same as the critical state. Critical state for both isotropically compressed and rebounded assemblies form unique curved line that can be approximated by a bilinear line as proposed by Been et al. [Géotechnique 41(3): 365–381, 1991]. Further more, evolution of the internal variables such as average coordination number and induced anisotropy coefficients during shear deformation has been studied.     

Influence diagnostics in a general class of beta regression models

In this article, we consider the issue of assessing influence of observations in the general class of beta regression models introduced by Simas et al. (Comput. Stat. Data Anal. 54:348–366, 2010), which is very useful in situations in which the response is restricted to the standard unit interval (0,1). Our results generalize those in Espinheira et al. (Comput. Stat. Data Anal. 52:4417–4431, 2008a; J. Appl. Stat. 35:407–419, 2008b), which only apply to linear beta regression models. We define some residuals, and a Portmanteau test for serial correlation. Further, some influence methods, such as the global, local, and total local influence of an individual and generalized leverage, are discussed. Moreover, we also derive the normal curvatures of local influence under various perturbation schemes. Finally, simulation results and an application to real data show the usefulness of our results.     

Failure mechanisms in granular media: a discrete element analysis

This paper attempts to numerically validate the concept of diffuse failure using a discrete element method. First, the theoretical background is reviewed, and it is shown how the kinetic energy of a system, initially at rest after a loading history, is likely to abruptly increase under the effect of disturbances. The vanishing of the second-order work thus constitutes a basic ingredient, related to both the pioneering work of Hill (J Mech Phys Solids (6):236–249, 1958) and the notion of bifurcation applied to geomechanics (Vardoulakis and Sulem in Bifurcation analysis in geomechanics, Chapman & Hall Publisher, London, 1995). Discrete numerical simulations were performed on homogeneous three-dimensional specimens, and the three basic conditions that must be satisfied in order to observe a failure mechanism are numerically checked. Finally, this work illustrates the phenomena that are likely to affect in situ slopes, for instance, when the loading (due to weather conditions or human activities) meets the three basic conditions for a failure mechanism to develop.     

About Two-Component Physics of HTSC

We present a first-principles, microscopic calculation of the ground state of cuprates indicating the presence of two groups of charge carriers in these compounds associated with free and localized states. The localized component arises due to a charge density wave instability in the free component. The instability occurs in underdoped cuprates where the attractive Coulomb interaction between dopant impurities and charge carriers becomes strongly over-screened at low carrier densities and divides charge carriers into two orthogonal states. Within this new two-component state a novel quasi-particle entity, a microscopic Coulomb clump (CC), emerges. Our results are completely consistent with the analysis of the Hall effect and the ARPES spectra made earlier by Gorkov and Teitelbaum (GT) (Phys. Rev. Lett., 97:247003, 2006, and J. Phys.: Conf. Ser., 108:012009, 2008) that includes also available neutron scattering, NMR and μSR data. The density of localized component is temperature dependent, which is due to thermal activation of bound holes from Coulomb clumps. The clumps also induce nanoscale superstructures observed in Scanning Tunneling Microscope (STM) experiments (Pan, et al. Nature, 413:282–285, 2001; Dubi, et al. Nature, 449:876–879, 2007; Gomes, et al. Nature, 447:569, 2007; Lee, et al. Nature, 442:546, 2006; McElroy, et al. Science, 309:1048, 2005; Zhu, et al. Phys. Rev. Lett., 97:177001, 2006) and are responsible for the pseudogap and Nernst effect in HTSC. Following GT we stress the importance of these findings for the pseudogap physics. We present a first-principles, microscopic calculation of the ground state of cuprates indicating the presence of two groups of charge carriers in these compounds associated with free and localized states. The localized component arises due to a charge density wave instability in the free component. The instability occurs in underdoped cuprates where the attractive Coulomb interaction between dopant impurities and charge carriers becomes strongly over-screened at low carrier densities and divides charge carriers into two orthogonal states. Within this new two-component state a novel quasi-particle entity, a microscopic Coulomb clump (CC), emerges.     

On Comparison of Experiment and Theory for Ultrasonic Attenuation in Polycrystalline Niobium

In this communication comparison of experimental attenuation results in polycrystalline niobium (Zeng et al., J. Nondestruct. Eval. 29:93–103, 2010) with scattering-induced attenuation models is reexamined. Reasonable agreement is found between those results and the standard Stanke and Kino model (in J. Acoust. Soc. Am. 75:665–681, 1984) contradicting the conclusions of Zeng et al.     

Numerical study of the effect of nonlinear control on the behavior of a liquid drop in elongational flow with vorticity

The problem of controlling a liquid drop suspended in an arbitrary two-dimensional elongational flow with vorticity is revisited. Bentley and Leal (J Fluid Mech 137:219–240, 1986) kept the drop centroid at the stagnation point using a linear proportional control strategy in a four-roll-mill apparatus that projects the drop’s motion onto the stable flow direction of the stagnation point. A nonlinear strategy based on the Poincaré–Bendixson theory to ensure a periodic motion of the drop centroid inside a prescribed area around the stagnation point is proposed and studied. In addition, a detailed numerical study is presented to illustrate the effect of the control on the drop motion. The present strategy is effective, allowing for deformation and changes in the drop orientation by less than 1% for extreme flow conditions that cannot be achieved by a four-roll-mill setup.     

A time-discrete model for dynamic fracture based on crack regularization

We propose a discrete time model for dynamic fracture based on crack regularization. The advantages of our approach are threefold: first, our regularization of the crack set has been rigorously shown to converge to the correct sharp-interface energy Ambrosio and Tortorelli (Comm. Pure Appl. Math., 43(8): 999–1036 (1990); Boll. Un. Mat. Ital. B (7), 6(1):105–123, 1992); second, our condition for crack growth, based on Griffith’s criterion, matches that of quasi-static settings Bourdin (Interfaces Free Bound 9(3): 411–430, 2007) where Griffith originally stated his criterion; third, solutions to our model converge, as the time-step tends to zero, to solutions of the correct continuous time model Larsen (Math Models Methods Appl Sci 20:1021–1048, 2010). Furthermore, in implementing this model, we naturally recover several features, such as the elastic wave speed as an upper bound on crack speed, and crack branching for sufficiently rapid boundary displacements. We conclude by comparing our approach to so-called “phase-field” ones. In particular, we explain why phase-field approaches are good for approximating free boundaries, but not the free discontinuity sets that model fracture.     

Subsampling weakly dependent time series and application to extremes

This paper provides extensions of the work on subsampling by Bertail et al. in J. Econ. 120:295–326 (2004) for strongly mixing case to weakly dependent case by application of the results of Doukhan and Louhichi in Stoch. Proc. Appl. 84:313–342 (1999). We investigate properties of smooth and rough subsampling estimators for sampling distributions of converging and extreme statistics when the underlying time series is η- or λ-weakly dependent.     

Three-dimensional chemo-thermomechanically coupled simulation of curing adhesives including viscoplasticity and chemical shrinkage

Based on the one-dimensional material model developed by Liebl et al. (Arch Appl Mech, 2011) a three-dimensional viscoelastic-viscoplastic material model for small deformations of curing adhesives on the basis of continuum mechanics is proposed in this contribution. The model describes the most relevant phenomena which occur during curing processes in the automotive industry and includes the effects of temperature and degree of cure on the mechanical properties of the material. Thermal expansion as well as chemical shrinkage are also contained. The yield stress for the viscoplastic part of the model goes back to the work of Schlimmer and Mahnken (Int J Numer Meth Eng 63:1461–1477, 2005), but is formulated in reference to the degree of cure and the temperature. Therefore this model considers chemo-thermomechanical coupling and extends the plasticity approach of Schlimmer and Mahnken, which is devised for cured adhesives, to the whole curing range, from the uncured to the fully cured adhesive. A peculiar focus is hereby laid on epoxy resins used in the automotive industry as structural adhesives.     

Micromechanics-based viscoelastic damage model for particle-reinforced polymeric composites

The objective of this study is to develop a micromechanics-based viscoelastic damage model that can predict the overall viscoelastic behavior of particle-reinforced polymeric composites undergoing damage. The emphasis here is that the present model successfully combines a rate-dependent viscoelastic constitutive model and a damage model. The Laplace transform based on the Boltzmann superposition principle and the ensemble-volume averaged method suggested by Ju and Chen (Acta Mech 103:103–121, 1994a; Acta Mech 103:123–144, 1994b) are extended toward effective viscoelastic properties. Further, the probability of the distribution function of Weibull (J Appl Mech 18:293–297, 1951) is adopted to describe a damage model that is dependent on damage parameters. A series of numerical simulations including parametric studies, and experimental comparisons are carried out to give insight into the potential capacity of the present micromechanics-based viscoelastic damage framework.     

Study of powder flow patterns in a Couette cell with axial flow using tracers and solid fraction measurements

We present different aspects of dense granular flows in a Couette geometry using a variety of particulate materials with shape and size distributions. Tracer studies point to an apparent coupling of particle size with flow and stress field gradients. While there is a clear industrial motivation to use “real” materials as a means to expand basic physical and engineering research in granular dynamics, the current study suggests additional academic motivations. Indeed, particles with distributed characteristics uncover rich interactions between flow and stress fields that might otherwise go un-noticed with model materials such as spherical glass beads. Distribution of size and shape play a strong role in how stress is transmitted in granular media (Kheiripour Langroudi et al. in Powder Technol 203:23–32, 2010) and how particle pattern arrangements evolve. Direct solid fraction measurements, using a capacitance probe, show that dense particle flows exhibit significant variations in solid fraction in both sheared and stagnant layers. Furthermore, these measurements also show different dependence of the solid fraction on shearing rate: solid fraction decreases in sheared layers and increases in stagnant layers as the shear rate is increased. From these results the thickness of the shear band could be estimated and was found to vary as a function of particle shape and the roughness of the container walls. The main result is that shear stress (or torque) (see also Kheiripour Langroudi et al. in Powder Technol 197:91–101, 2010) and solid fraction profiles depend on particle shape and whether or not an extra degree of freedom in their movement is provided so that the system can dilate under various shear states in the Couette cell. This extra degree of freedom is assured in the present experimental work by allowing a slight axial outflow from the Couette device while the driven shear fields are in the radial and tangential directions.     

A comment to the paper by Waltman et al., Scientometrics, 87, 467–481, 2011

In reaction to a previous critique (Opthof and Leydesdorff, J Informetr 4(3):423–430, 2010), the Center for Science and Technology Studies (CWTS) in Leiden proposed to change their old “crown” indicator in citation analysis into a new one. Waltman (Scientometrics 87:467–481, 2011a) argue that this change does not affect rankings at various aggregated levels. However, CWTS data is not publicly available for testing and criticism. Therefore, we comment by using previously published data of Van Raan (Scientometrics 67(3):491–502, 2006) to address the pivotal issue of how the results of citation analysis correlate with the results of peer review. A quality parameter based on peer review was neither significantly correlated with the two parameters developed by the CWTS in the past citations per paper/mean journal citation score (CPP/JCSm) or CPP/FCSm (citations per paper/mean field citation score) nor with the more recently proposed h-index (Hirsch, Proc Natl Acad Sci USA 102(46):16569–16572, 2005). Given the high correlations between the old and new “crown” indicators, one can expect that the lack of correlation with the peer-review based quality indicator applies equally to the newly developed ones.     

Asymptotic behavior of robust estimators in partially linear models with missing responses: the effect of estimating the missing probability on the simplified marginal estimators

In this paper, we consider a semiparametric partially linear regression model where missing data occur in the response. We derive the asymptotic behavior of the robust estimators for the regression parameter and of the weighted simplified location estimator introduced in Bianco et al. (Comput. Stat. Data Anal. 54:546–564, 2010a). For the latter, consistency results and the asymptotic distribution are derived when the missing probability is known and also when it is estimated.     

Optimal pairing computation over families of pairing-friendly elliptic curves

Vercauteren introduced the concept of optimal pairing, which by definition can be computed by using at most (log₂ r)/φ(k) + log₂ k basic Miller iterations, where r is the order of the groups involved and k is the embedding degree Vercauteren (IEEE Trans Inf Theory 56(1):455–461, 2010). Freeman et al. summarized and proposed all of the new constructions of pairing-friendly elliptic curves that currently exist Freeman et al. (J Cryptol 23(2):224–280, 2010). In this paper, we give an optimal pairing for each family of pairing-friendly curves in Freeman et al. (J Cryptol 23(2):224–280, 2010) by taking the Ate or R-ate pairing approach.     

Crack tip fields in a viscoplastic solid: monotonic and cyclic loading

The asymptotic stress and strain field near the tip of a plane strain Mode I stationary crack in a viscoplastic material are investigated in this work, using a unified viscoplastic model based on Chaboche (Int J Plast 5(3):247–302, 1989). Asymptotic analysis shows that the near tip stress field is governed by the Hutchinson–Rice–Rosengren (HRR) field (Hutchinson in J Mech Phys Solids 16(1):13–31, 1968; Rice and Rosengren in J Mech Phys Solids 16(1):1–12, 1968) with a time dependent amplitude that depends on the loading history. Finite element analysis is carried out for a single edge crack specimen subjected to a constant applied load and a simple class of cyclic loading history. The focus is on small scale creep where the region of inelasticity is small in comparison with typical specimen dimensions. For the case of constant load, the amplitude of the HRR field is found to vanish at long times and the elastic K field dominates. For the case of cyclic loading, we study the effect of stress ratio on inelastic strain and find that the strain accumulated per cycle decreases with stress ratio.     

Understanding the role of open peer review and dynamic academic articles

We welcome the commentary by L. Egghe (Scientometrics, this issue) stimulating discussion on our recent article “Natural selection of academic papers” (NSAP) (Scientometrics, 85(2):553–559, 2010) that focuses on an important modern issue at the heart of the scientific enterprise—the open and continuous evaluation and evolution of research. We are also grateful to the editor of Scientometrics for giving us the opportunity to respond to some of the arguments by L. Egghe that we believe are inaccurate or require further comment.     

High density polyethylene (HDPE): Experiments and modeling

The uniaxial tension (loading and unloading), creep and relaxation experiments on high density polyethylene (HDPE) have been carried out at room temperature. The stress–strain behavior of HDPE under different strain rates, creep (relaxation) behavior at different stress (strain) levels have been investigated. These experimental results are used to compare the simulation results of a unified state variable theory, viscoplasticity theory based on overstress (VBO) and a macro-mechanical constitutive model for elasto-viscoplastic deformation of polymeric materials developed by Boyce et al. (Polymer 41:2183–2201, 2000). It is observed that elasto-viscoplasticity model by Boyce et al. (Polymer 41:2183–2201, 2000) is not good enough to simulate stress–strain, creep and relaxation behaviors of HDPE. However, the aforementioned behaviors can be modeled quantitatively by using VBO model.     

Extended rotation-free shell triangles with transverse shear deformation effects

The paper extends recent work of the authors to include transverse shear effects on rotation-free triangular element for plates (O?ate and Zárate in Int J Numer Methods Eng 83(2):196–227, 2010). Two new shell triangular elements are presented, the EBST+ and the EBST+1. Transverse shear deformation effects are important for thick shells, as well when the shell is laminated or formed by composite material. The ingredients for the element formulation are: a Hu-Washizu type mixed functional and linear interpolation for the displacement field. In both elements presented a finite volume approach is used for computing the bending moments and the curvatures over a patch of elements. The nodal translational degrees of freedom of the original enhanced basic shell triangle (EBST) are extended with the two shear deformation angles via two different approaches. The first one uses a linear interpolation of the rotation angles inside the element (EBST+) and the second one assumes a constant field for the rotation angles (EBST+1). For the thin shell case the shear angles vanish and the new elements reproduce the good behaviour of the original thin EBST element. As a consequence the elements can reproduce the solutions for thick to thin shells situations without exhibiting shear locking. The numerical solution for the thick shell case can be found iteratively starting from the deflection values for the Kirchhoff theory using the original thin EBST element. Examples of the good performance of the new rotation-free shell triangles are given.