Unexpected liquefaction under isotropic consolidation of idealized granular materials

The drained isotropic compression behaviour of very loose and fully saturated monodisperse glass beads in triaxial compression is investigated in this paper. Short cylindrical samples were prepared by moist-tamping technique and isotropically compressed in a classical axisymmetric triaxial machine. Very loose glass bead samples exhibit numerous local collapses with sudden volumetric compaction and axial contraction of very large amplitude and experience ultimately global collapse with spontaneous liquefaction under undetermined external isotropic stress. Excess pore pressure instantaneously generated at the beginning of the collapse phenomenon, and rapidly dissipated with the usual drainage system, shows a global complex system with local dynamic instability. The dynamic time evolution of the excess pore pressure \(\varDelta U\) consists in a first and fast transient phase I at constant volume and constant axial strain with large spikes \(\varDelta U^{peak}\), followed by an intermediate second phase II of large increase of volumetric compaction and axial contraction at stabilizing \(\varDelta U_{stable}\) or constant effective stress \(\sigma ^{\prime }\) and finally a third and longest phase III of excess pore water pressure dissipation at nearly constant axial strain toward the initial back-pressure. For local collapses, the second phase is totally missing. Upon ignoring the local collapses, very loose idealized granular materials have a unique and global isotropic compressibility behaviour, independently of the void ratio at the end of the fabrication stage; and liquefaction-free for dense state below a threshold void ratio at fabrication \(e_{30}^{liq}\), representing the transition from global instability with total failure to local instability with partial collapse. This paper provides the first reported spontaneous liquefaction instability under isotropic consolidation. It gives the necessarily conditions for an isotropic liquefaction and emphases some usually hidden or partially developed mechanisms underlying the diffuse instability phenomenon and adds a new intriguing layer to the complex behaviour of idealized granular materials in classical drained triaxial isotropic compression.     

Structural,dielectric, piezoelectric,ferroelectric and electro-caloric properties of Ba1−xCaxTi0.975(Nb0.5Yb0.5)0.025O3 lead-free ceramics

Polycrystalline samples of Ba_1?xCa_xTi_0.975(Nb_0.5Yb_0.5)_0.025O₃ (where x = 0.15, 0.2 and 0.3, abbreviated as BCTYN) were prepared by the conventional solid state reaction method. The effect of calcium (Ca) substitution in BaTi_0.975(Nb_0.5Yb_0.5)_0.025O₃ (abbreviated as BTYN25) on the structural, dielectric, piezoelectric and ferroelectric properties and electro-caloric effects (ECE) was investigated. X-ray diffraction (XRD) results at room temperature showed that the BCTYN samples in the composition x < 0.3 exhibited a pure tetragonal perovskite structure. Dielectric measurements showed a classical ferroelectric behavior for all samples. With the increase of the Ca content, the Curie temperature (T_C) was still maintained with a small shift towards low temperature. The evolution of the Raman spectra was studied as a function of compositions and temperatures. The Raman bands confirmed the structure and the phase transition of the BCTYN ceramics. By adding Ca, the piezoelectric properties and the remanent polarization (P_r) are relatively maintained for the compositions x = 0.15 and x = 0.2. A piezoelectric coefficient of d₃₃ = 130 pC/N and a planar electromechanical coupling factor of k_p = 28% were obtained for these compositions. Two different methods were used to calculate the electro-caloric coefficients of the BCTYN ceramics. The incorporation of Ca was found to enhance the electro-caloric strength (ξ = ΔT/ΔE) within a broad temperature range with a best value of ξ = 0.2?Kmm/kV for x = 0.2.     

The preparation and thermodynamic properties of Fe(II)---Fe(III) hydroxide-carbonate (green rust 1); Pourbaix diagram of iron in carbonate-containing aqueous media

Carbonate-containing green rust 1, GR1(CO₃²⁻), is prepared by oxidation of Fe(OH)₂ in aqueous solution. Ferrous hydroxide is precipitated from NaOH and FeSO₄·7H₂O solutions and carbonate ions are added as a Na₂CO₃ solution. For sufficiently large concentrations of sodium carbonate, SO₄²⁻ ions do not play any role during the oxidation process and, at the end of the first stage of reaction, Fe(OH)₂ oxidizes into GR1(CO₃²⁻). In the second stage of reaction, GR1(CO₃²⁻) oxidizes into α-FeOOH goethite except when the transformation of ferrous hydroxide is partial, which leads to the formation of magnetite. From the X-ray diffraction analysis of GR1(CO₃²⁻), lattice parameters of its hexagonal cell are found to be a = 3.160 ± 0.005 Å and C = 22.45 ± 0.05 Å. From the Mössbauer analysis of the stoichiometric GR1(CO₃²⁻), which leads to a Fe²⁺:Fe³⁺ ratio of 2:1, the chemical formula is established to be: [Fe₄^(II)Fe₂^(III)(OH)₁₂][CO₃·2H₂O]. The 78 K Mössbauer spectrum of the compound can be fitted with three quadrupole doublets, two Fe²⁺ doublets d₁ and D₂ corresponding to isomer shifts (IS) of 1.27 and 1.28 mm s⁻¹ and quadrupole splittings (QS) of 2.93 and 2.67 mm s⁻¹, respectively, and one Fe³⁺ doublet D₃ with an IS of 0.47 mm s⁻¹ and QS of 0.43 mm s⁻¹. These three doublets were already used to fit the Mössbauer spectrum of chloride-containing GR1(Cl⁻) [see J.M.R. Génin et al., Mat. Sci. Forum8, 477 (1986) and J.M.R. Génin et al., Hyp. Int. 29, 1355 (1986)]and therefore are characteristic of GR1 compounds. From the recording of electrode potential E and the pH of the suspension versus time during the oxidation, the standard free enthalpy of formation of stoichiometric GR1(CO₃²⁻) is estimated to be ΔG °_f = − 966.250 cal mol⁻¹. Knowing the chemical formula and ΔG °_f of GR1(CO₃²⁻) the Pourbaix diagram of iron in carbonate-containing aqueous solutions is drawn.     

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.     

The preparation and thermodynamic properties of Fe(II)Fe(III) hydroxide-carbonate (green rust 1); Pourbaix diagram of iron in carbonate-containing aqueous media

Carbonate-containing green rust 1, GR1(CO₃²⁻), is prepared by oxidation of Fe(OH)₂ in aqueous solution. Ferrous hydroxide is precipitated from NaOH and FeSO₄·7H₂O solutions and carbonate ions are added as a Na₂CO₃ solution. For sufficiently large concentrations of sodium carbonate, SO₄²⁻ ions do not play any role during the oxidation process and, at the end of the first stage of reaction, Fe(OH)₂ oxidizes into GR1(CO₃²⁻). In the second stage of reaction, GR1(CO₃²⁻) oxidizes into α-FeOOH goethite except when the transformation of ferrous hydroxide is partial, which leads to the formation of magnetite. From the X-ray diffraction analysis of GR1(CO₃²⁻), lattice parameters of its hexagonal cell are found to be . From the Mössbauer analysis of the stoichiometric GR1(CO₃²⁻), which leads to a Fe²⁺:Fe³⁺ ratio of 2:1, the chemical formula is established to be: [Fe₄^(II)Fe₂^(III)(OH)₁₂][CO₃·2H₂O]. The 78 K Mössbauer spectrum of the compound can be fitted with three quadrupole doublets, two Fe²⁺ doublets d₁ and D₂ corresponding to isomer shifts (IS) of 1.27 and 1.28 mm s⁻¹ and quadrupole splittings (QS) of 2.93 and 2.67 mm s⁻¹, respectively, and one Fe³⁺ doublet D₃ with an IS of 0.47 mm s⁻¹ and QS of 0.43 mm s⁻¹. These three doublets were already used to fit the Mössbauer spectrum of chloride-containing GR1(Cl⁻) [see J.M.R. Génin et al., Mat. Sci. Forum8, 477 (1986) and J.M.R. Génin et al., Hyp. Int. 29, 1355 (1986)]and therefore are characteristic of GR1 compounds. From the recording of electrode potential E and the pH of the suspension versus time during the oxidation, the standard free enthalpy of formation of stoichiometric GR1(CO₃²⁻) is estimated to be ΔG °_f = − 966.250 calmol⁻¹. Knowing the chemical formula and ΔG °_f of GR1(CO₃²⁻) the Pourbaix diagram of iron in carbonate-containing aqueous solutions is drawn.     

An investigation into the electrostatic force representation for electrically actuated microelectromechanical systems

The ever-increasing demand for microelectromechanical systems (MEMS) in modern electronics has reinforced the need for extremely accurate analytical and reduced-order models to aid in the design of MEMS devices. Many MEMS designs consist of cantilever beams with a tip mass attached at the free end to act as a courter electrode for electrical actuation. One critical modeling aspect of electrically actuated MEMS is the electrostatic force that drives these systems. The two most used representations in the literature approximate the electrostatic force between two electrodes as a point force. In this work, the effects of the representation of the electrostatic force for electrically actuated microelectromechanical systems are investigated. The system under investigation is composed of a beam with an electrode attached to its end. The distributed force, rigid body, and point mass electrostatic force representations are modeled, studied, and their output results are compared qualitatively. Static and frequency analyses are carried out to investigate the influences of the electrostatic force representation on the static pull-in, fundamental natural frequency, and mode shape of the system. A nonlinear distributed-parameter model is then developed in order to determine and characterize the response of electrically actuated systems when considering various representation of the electrostatic forces. The results show that the size of the electrode may strongly affect the natural frequencies and static pull-in when the point mass, rigid body, and plate representations are considered. From nonlinear analysis, it is also proven that the representation may affect the hardening behavior of the system and its dynamic pull-in. This modeling and analysis give guidelines about the usefulness of the electrostatic force representations and possible erroneous assumptions that can be made which may result in inaccurate design and optimal performance detection for electrostatically actuated systems.

     

Large electrocaloric effect in lead-free Ba1-xCaxTi1-yZryO3 ceramics under strong electric field at room-temperature

In this study, lead-free Ba_1-xCa_xZr_yTi_1-yO₃ (BCTZ(x, y)) ceramics were prepared by means of the classic solid-state reaction method. The morphotropic phase transition (MPB) from the orthorhombic to the tetragonal phase (O-T) was identified in this composition. Besides, the identification of those two structures at room temperature (RT) was made possible thanks to an X-ray diffraction (XRD) study. In order to determine the phase transitions dielectric measurements were conducted. Based on Maxwell equation, the electrocaloric (EC) effect in the studied ceramics was performed via the indirect method. The compositions gave maximum EC temperature changes (ΔT) at above their T_C on application of a 3?kV/mm electric field. These temperature changes are ΔT?=?0.565?K at T_EC=?392?K, ΔT?=?0.548?K at T_EC=?365?K and ΔT?=?0.235?K at T_EC=?307?K for BCZT(10%,5%), BCZT(13%,10%) and BCZT(20%,15%), respectively. At RT, these compositions provided a very interesting EC coefficient (ξ?=?ΔT/ΔE) compared to the pure BaTiO₃ (BT). These values, lying between 0.105 Kmm/kV and 0.188Kmm/kV for ΔE?=?3?kV/mm, are also greater than those related to some lead-based ferroelectric.     

An integrated production and maintenance planning model with time windows and shortage cost

In this paper, we tackle the problem of integrating production and maintenance. The production problem addresses the issue of determining the production lot sizes of various items. Preventive maintenance is carried out in time windows to restore the production line to an ‘as-good-as-new’ status, and when a production line fails, a minimal repair is carried out to restore it to an ‘as-bad-as-old’ status. The resulting problem is modelled as a linear mixed-integer program. It takes into account demand shortage and the reliability of the production line. Computational experiments are carried out to show the effectiveness of the integrated model compared with a separate classical model for different instances, and the results obtained are analysed in detail.     

Structural and dielectric properties of Na1?xBaxNb1?x(Sn0.5Ti0.5)xO3 ceramics

Lead-free (1 − x)NaNbO₃/xBa(Ti_0.5Sn_0.5)O₃ (x = 0.1, 0.125, 0.15, 0.175, 0.2, and 0.3) ceramics were elaborated by the conventional ceramic technique. Sintering has been made at 1523 K for 2 h. The crystal structure was investigated by X-ray diffraction with CuKα radiation at room temperature. As a function of composition, these compounds crystallize with tetragonal or cubic symmetry. Dielectric measurements show that the materials have a classical ferroelectric behavior for compositions in the range 0.10 ≤ x ≤ 0.15 and relaxor one for compositions in the range 0.15 < x ≤ 0.30. Temperatures T _C or T _m decrease as x content increases. The ferroelectric behavior has been confirmed by hysteresis characterization. For x = 0.1, a piezoelectric coefficient d ₃₁ of 42.146 pC N⁻¹ was obtained at room temperature. The evolution of the Raman spectra was studied as a function of temperature for x = 0.1.     

Urinary calculi in children: contribution of anamnesis, biological exploration and physical analysis of calculi to the etiological diagnosis

BACKGROUND: It is always of importance to define the cause of urinary calculi disease in children to prevent recurrence and possible impairing of renal function. Nevertheless, etiology is not always easy to prove and must be deduced from both clinical and biological arguments. PATIENTS AND METHODS: The aim of this prospective study including 39 Tunisian children with urinary stones was to identify etiology and stone risk factors and detail the part of clinical and biological data and results of physical analysis of stones in determining the cause of the stone. RESULTS: In 31 cases among 39, clinical and biological data were not sufficient to identify clearly the stone etiology. When considering the structure and stone composition, the cause of the stone could be determined in 97.4% of the cases. An inherited disease was found responsible for the stone in 11 children, urinary tract infection in 13 cases, idiopathic hypercalciuria in nine cases and a nutritional deficiency disease in seven cases. In one case, polycystic kidney disease with metabolic risk factors could explain the stone process. No precise etiology was found in one case. Among infection stones, struvite stones could be related to urea-splitting bacteria while other calculi, containing whitlockite and protein matrix could be related to other micro-organisms. Earlier severe chronic diarrhoea episodes were noted in six among seven children presenting stones with a nucleus mainly composed of ammonium urate. CONCLUSION: Clinical data, biological data from both urine and blood of the patients and also the structure and composition of the stones are needed to identify the cause of urinary calculi. Such a procedure could provide the stone etiology in most cases.