On Using a Dual Bound Approach to Characterize the Yield Behaviour of Porous Ductile Materials Containing Void Clusters

Two constitutive models for porous ductile materials are employed together to predict the yield behaviour of ductile materials containing void clusters. In this dual bound approach, the upper and lower bound constitutive models of Gurson (1977) and Sun and Wang (1989) are each evaluated in order to obtain upper and lower estimates for the material behaviour. By combining these two solutions, a predictive band can be created to capture the experimental variation in the yielding behaviour. Although these constitutive models have been derived with the assumption of a periodic void distribution, real materials contain void clusters that can significantly alter the onset of yielding and fracture. Therefore it is of great interest to determine if using dual constitutive models can produce an acceptable first-order approximation of the yielding behaviour in these materials. In the present work, the upper and lower bound yield loci are superimposed over numerical data available in the literature for the yielding of materials containing void clusters. It is shown that the dual bound approach is able to capture the material behaviour over a wide range of practically encountered stress triaxialities.     

A damage model for crack prediction in brittle and quasi-brittle materials solved by the FFT method

In this paper, we present a damage model and its numerical solution by means of Fast Fourier Transforms (FFT). The FFT-based formulation initially proposed for linear and non-linear composite homogenization (Moulinec and Suquet in CR Acad Sci Paris Ser II 318:1417–1423 1994; Comput Methods Appl Mech Eng 157:69–94 1998) was adapted to evaluate damage growth in brittle materials. A non-local damage model based on the maximal principal stress criterion was proposed for brittle materials. This non-local model was then connected to the Griffith criterion with the aim of predicting crack growth. By using the proposed model, we carried out several numerical simulations on different specimens in order to assess the fracture process in brittle materials. From these studies, we can conclude that the present FFT-based analysis is capable of dealing with crack initiation and crack growth in brittle materials with high accuracy and efficiency.     

Rashba Conduction Band Spin-Splitting for Asymmetric Quantum Well Potentials

Asymmetric quantum well potentials are expected to produce a conduction band spin-splitting which contributes to Dyakonov–Perel (Sov. Phys. Solid State 13:3023, 1971) spin relaxation. Much experimental work has focused on the effect of an electric field on spin dynamics (Karimov et al., in Phys. Rev. Lett. 91:246601, 2003) and little on asymmetry from alloy engineering. By combining time-resolved Kerr rotation measurements with transient spin grating measurements in GaAs/AlGaAs quantum wells we have compared the conduction band spin-splitting resulting from asymmetric alloy engineering with that from applied electric field. The latter is easily measurable, whilst the former is no greater than that in symmetric wells. These results are consistent with an envelope function approximation model that considers the potential profile in both the conduction and the valence bands (Winkler, in Springer Tracts in Modern Physics, vol. 191, 2003).     

Another generalization of the geometric distribution

A new generalization of the geometric distribution with parameters α>0 and 0<θ<1 is obtained in this paper. This can be done either by using the Marshall and Olkin (Biometrika 84(3), 641–652, 1997) scheme and adding a parameter to the geometric distribution or by starting with the generalized exponential distribution in Marshall and Olkin (Biometrika 84(3), 641–652, 1997) and discretizing this continuous distribution. The particular case α=1 led us to the geometric distribution. After reviewing some of its properties, we investigated the question of parameter estimation. The new distribution is unimodal with a failure rate that is monotonically increasing or decreasing, depending on the value of the parameter α. Expected frequencies were calculated for two overdispersed and infradispersed examples, and the distribution was found to provide a very satisfactory fit.     

Effect of a temperature change from 20 to 50°C on the basic creep of HPC and HPFRC

This research was performed in order to study the basic creep of High Performance Concretes (HPC) under uniaxial compression at 20 and 50°C. The aim of this work is to contribute to a better understanding of the basic creep phenomena of HPC at moderate temperature and to provide experimental data which will be used in Thermo-Hydro-Mechanical models such as those necessary for the National project CEOS.FR (Sellier, Thermo hydro mechanical numerical modelling, invited paper at the CEOS International workshop on Control of cracking in R.C. structures: a major step towards serviceability, 2009). The article also presents the fitting of a model considering the effect of temperature via an Arrhenius law affecting its viscous modules (Sellier and Buffo-Lacarriere, Eur J Environ Civ Eng 10:1161–1182, 2009). The concretes are those envisioned for future storage structures of Intermediate Level Long-Life Nuclear Wastes. The research programme has been established with four HPC, two non fibrous and two fibrous; the kinetics and amplitude of basic creep under uniaxial compression are measured during several months at 50°C and compared to those obtained at 20°C for the same materials (Camps, PhD thesis, 2008). Experimental results show that the average creep at 50°C is about twice the creep at 20°C. Besides, results show that this amplification depends on the binder type; the sensitivity to the temperature rise is greater for blended cement based concretes than for OPC based ones. The creep increase due the temperature rise is higher for the HPC under study than for ordinary concretes inventoried in a literature survey. The creep amplitude of HPC seems correlated to their amount of secondary C–S–H. At last, the fitting of the model parameters on the experimental results shows that the values of activation energy are quite close to those obtained by other authors on ordinary concretes (Bazant et al., J Eng Mech ASCE 130(6): 691–699, 2004).     

Formation of new materials based on cast aluminum alloy

We show the dynamic formation of composite materials using the polycrystalline cast alloy Al + 12% Si as the example. The volume fraction of the synthesized framework (“influence zones”) of the composite material has been estimated to be at a level of 11%. Upon the introduction of lead a material practically insoluble under the experimental conditions was synthesized in the framework. Upon the introduction into the Al + 12% Si cast alloy of silicon carbide particles, we obtained a material with a specific corrosion exceeding by a factor of 5.6 the given index of the source matrix material.[1–7]     

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.     

Deformed metals and alloys with a structural scale from 5 nm to 100 nm

The processing, structure and properties of deformed metals and alloys with a structural scale from the micrometer to the nanometer dimensions has been the subject of a recent viewpoint set [1]. The present paper will focus on deformed metals and alloys with a structural scale from 5 nm to 100 nm, concentrating on materials processed by high pressure torsion (HPT), surface mechanical attrition treatment (SMAT) and sliding. A detailed microstructural characterization has been followed by an analysis of the relationship between structural features and processing parameters. In this analysis, some general approaches have been applied for example scaling of the evolution of the boundary spacing. This analysis is the basis for a brief discussion of the relationship between the microstructural parameters and the strength.     

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.     

Elastic property prediction by finite element analysis with random distribution of materials for heterogeneous solids

In the present paper, finite element method is employed to predict the effective material properties of heterogeneous materials via random distributions of the constituent materials. With the random distributing strategy, massive parametric analysis via finite element becomes feasible for multi-phase heterogeneous solids. Using a two-phase bi-continuous material as an example, the effects of the specimen size with respect to the characteristic size of the micro-structural size and the element density on the predicted effective properties are considered. The numerical predictions of the effective properties are checked by two analytical bounds which were proposed by Hashin and Shtrikemn (1963) through the principle of variation and the matrix-fiber model. Some discussions on the finite element prediction are also made to clarify the status of the present work in the composite mechanics research.     

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.     

Computation of transient hygroscopic stresses in unidirectional laminated composite plates with cyclic and asymmetrical environmental conditions

External variations in temperature and moisture are of primary importance for the long term behaviour of structures made of polymer matrix composites, because they induce residual stresses within laminated composite plates. It was shown (Hahn et al., 1978; Benkeddad et al., 1995, 1996; Tounsi et al., 2000, 2002, 2004; Adda-Bedia et al., 2001; Tounsi and Adda-Bedia, 2003a, b) that the heterogeneity and the anisotropy of such plates, have an influence on the distribution of transient hygroscopic stresses through the thickness of composite plates. The aim of the present paper is to present a simplified approach for the calculation of transient hygroscopic stresses within unidirectional laminates in the case where these latter are exposed to the cyclic and unsymmetric environmental conditions. Several examples are presented to assess such stresses and to demonstrate the efficiency of the used method. These stresses have to be taken into consideration for the design of composite structures submitted to a moist environment.     

A variational void coalescence model for ductile metals

We present a variational void coalescence model that includes all the essential ingredients of failure in ductile porous metals. The model is an extension of the variational void growth model by Weinberg et al. (Comput Mech 37:142–152, 2006). The extended model contains all the deformation phases in ductile porous materials, i.e. elastic deformation, plastic deformation including deviatoric and volumetric (void growth) plasticity followed by damage initiation and evolution due to void coalescence. Parametric studies have been performed to assess the model’s dependence on the different input parameters. The model is then validated against uniaxial loading experiments for different materials. We finally show the model’s ability to predict the damage mechanisms and fracture surface profile of a notched round bar under tension as observed in experiments.     

Fermi Glass Versus High-Temperature Superconductivity Behavior

Very recently BaSrNiO₄ was reported to be a Fermi glass (Schilling et al. in J. Phys., Condens. Matter 21:015701, 2009). Its structure is essentially the one of K₂NiF₄ as is that of La₂CuO₄, in which the occurrence of high-temperature superconductivity (HTS) upon hole doping was first reported (Bednorz and Müller in Z. Phys. B 64:189, 1986; Adv. Chem. 100:757, 1988). The carriers in both have mainly e_g character, and move in a stochastic potential as documented by a number of experiments. The difference of the two behaviors is mainly ascribed to the formation of intersite bipolarons (Kabanov and Mihailovic in J. Supercond. 13:950, 2000) , which is estimated to be up to two orders of magnitude larger in La₂CuO₄ than in BaSrNiO₄. From this it follows that for HTS to occur, a large bipolaron formation energy in layered structures is required.     

The MARE Project

The international project “Microcalorimeter Arrays for a Rhenium Experiment” (MARE) aims at a direct and calorimetric measurement of the electron antineutrino mass with sub-electronvolt sensitivity. MARE is divided in two phases. The first phase consists of two independent experiments using the presently available detector technology to reach a sensitivity of the order of 1 eV and to improve the understanding of the systematic uncertainties peculiar of this technique. In parallel to these experiments, a wide R&D program will single out the appropriate detector configuration, the read-out scheme and the large array technology for the second phase of MARE. In the second phase, the selected techniques will be applied to the realization of large arrays with as many as 10000 detectors each. At least five arrays will be then deployed to collect the statistics required to probe the antineutrino mass with a sensitivity of at least 0.2 eV, comparable to the one expected for the Katrin experiment (KATRIN Design Report, [2004]). On behalf of the MARE collaboration.     

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.     

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.     

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.     

Stability analysis of undrained adiabatic shearing of a rock layer with Cosserat microstructure

Stability of undrained shearing in a classical Cauchy continuum has been first analyzed by Rice (J Geophys Res 80(11):1531–1536, 1975) who showed that instability occurs when the underlying drained deformation becomes unstable (i.e. in the softening regime of the corresponding drained stress-strain curve). However Vardoulakis (Int J Numer Anal Methods Geomech 9:339–414, 1985; Int J Numer Anal Methods Geomech 10:177–190, 1986) has shown that Rice’s linear stability analysis, if performed at the state of maximum deviator, leads to a sharp transition from infinitely stable to infinitely unstable behaviour, which indicates that the solution of the considered initial-value problem does not exist and consequently that the corresponding problem is mathematically ill-posed. Vardoulakis (Géotechnique 46(3):441–456, 1996; Géotechnique 46(3):457–472, 1996) proposed a regularization of the ill-posed problem in the softening regime by resorting to a second grade extension of plasticity theory. In this paper, the kinetics of a granular material is described by a Cosserat continuum as first suggested by Mühlhaus and Vardoulakis (Géotechnique 37:271–283, 1987) and we incorporate the effect of shear heating due to the dissipation of the frictional work. The undrained adiabatic limit is applicable as soon as the slip event is sufficiently rapid and the shear zone broad enough to effectively preclude heat or fluid transfer as it is the case during an earthquake or a landslide. It is shown that shear heating has a destabilizing effect and that instability can occur in the hardening regime if the amount of dilatant strengthening is not sufficient as compared to the effect of thermal pressurization of the pore fluid. It is shown that the linear stability analysis with macro and micro inertia terms leads to the selection of a preferred wave length of the instability mode corresponding to the instability mode with fastest (but finite) growth coefficient.     

Growth Dynamics and Faceting of 3He Crystals

³He crystals start to show facets on their surface only at about 100 mK, well below the roughening transition temperature. To find out the reason for this discrepancy, we have performed the first quantitative investigation on the growth dynamics of the faceted and rough surfaces of ³He crystals in the temperature range of 60–110 mK. We have applied an original method to obtain the variation of the overpressure on the crystal surface by measuring its curvature and height locally using a Fabry–Pérot interferometer. The growth of the rough surface was found to be limited by the transport of the latent heat which elaborates in the liquid, in accordance with theoretical predictions (Puech L., et al. in J. Low Temp. Phys. 62:315, 1986; Graner F., et al. in J. Low Temp. Phys. 75:69, 1989 and 80:113, 1990) and previous measurements near the minimum of the melting curve (Graner F., et al. in J. Low Temp. Phys. 75:69, 1989 and 80:113, 1990). The mobility of an elementary step on a facet was shown to be limited by the latent heat transport as well. The values obtained for the step free energy are by two orders of magnitude smaller than at ultra low temperatures, which we show to be the result of quantum oscillations of the solid-liquid interface, which quickly become damped when temperature decreases below 100 mK.