Determination of absolute emission cross sections for electron-impact-induced line radiation in the vacuum ultraviolet

A method and an experimental setup have been developed for measuring absolute photoemission cross sections for electron-impact-induced line radiation in the vacuum ultraviolet (VUV). Unparalleled low uncertainties for the cross sections were achieved mainly from the use of the Berlin electron storage ring as a primary standard source in the VUV for the determination of the responsivity of the spectrometer-detector system used and from the use of a spinning rotor gauge as a secondary standard for the determination of the target gas density. As the first result we present a photoemission cross section for the Ar II 3s3p(6)(2)S(?)-3s(2)3p(5)(2)p?(3/2) transition at 91.98 nm for 2-keV electron-impact energy of 1.167 × 10(-18) cm(2) with a relative uncertainty of 4.4% (√3σ value). This low uncertainty demonstrates the suitability of the setup for further cross-section measurements.     

Structural relationships between measures based on heart beat intervals: potential for improved risk assessment

Decreased left ventricular ejection fraction is the most commonly used risk factor for identification of patients at high-risk for lethal ventricular arrhythmic events. Twenty-four-hour electrocardiographic (ECG) approaches to risk stratification include: counts of ventricular premature contractions (VPCs), measures of heart rate variability (HRV), and heart rate turbulence (HRT) which has two components, turbulence onset and turbulence slope (TS). Refinement of these ECG risk stratifiers could enhance their clinical utility. We explored the structural relationships between heart rate (HR) and HRV and HRT measures. Our goal was to separate out the component of these measures due to the underlying average heart rate (HR), thus potentially reducing the variability of the measures and increasing their power to stratify risk. We proposed re-scaling tachograms of heart-beat intervals so that the re-scaled tachogram has a HR of 75 (or equivalently an average interval of 800 ms) and calculating HRV and HRT from the rescaled time series. We also explored the relationship between the number of VPCs and HRT. We showed that TS is structurally related to the number of VPCs (and hence to the length of the ECG recording). We proposed an adjusted TS that is independent of the number of VPCs. We also addressed the ability of shorter ECG recording to estimate HRV and HRT measures. We evaluated standard and rescaled HRV and HRT measures using qualifying ambulatory ECG recordings from 744 patients in the Cardiac Arrhythmia Suppression Trial. We found that measures based on the rescaled tachogram had reduced variance (20% to 40%). Correlations between measures were also substantially reduced. We also found substantial circadian effects on some, but not all HRV indices, not explained by the circadian pattern in HR and possibly pointing to additional measures for risk prediction. In conclusion, we found that adjusting for HR and the number of VPCs in heart-beat related ambulatory ECG measures has the potential to significantly improve the power of these measures to risk stratify cardiac patients.     

First-Principles Study of Elastic Constants and Interlayer Interactions of Complex Hydrated Oxides: Case Study of Tobermorite and Jennite

It is a common perception that layered materials are soft in the interlayer direction. Herein, we present results of first-principles calculations of the structure and elastic constants of a class for hydrated oxides, tobermorite, and jennite, which illustrate that this is not the case, if (1) the interlayer distance is such that coulombic interlayer interactions become comparable to the iono-covalent intralayer interactions and (2) the existence of interlayer ions and water molecules do not shield the coulombic interlayer interactions. In this case, the mechanically softest directions are two inclined regions that form a hinge mechanism. The investigated class of materials and results are relevant to chemically complex hydrated oxides such as layered calcium–silicate–hydrates (C–S–H), the binding phase of all concrete materials, and the principle source of their strength and stiffness. In addition, the first-principles results may serve as a benchmark for validating empirical force fields required for the analysis of complex calcio–silicate oxides.     

Multisurface Chemoplasticity. I: Material Model for Shotcrete

Employing a thermodynamic framework, chemomechanical couplings for shotcrete are treated in this paper. A new material model based on multisurface chemoplasticity is presented. It accounts for hydration kinetics and chemomechanical couplings related to the strength growth and the evolution of stiffness properties as well as for autogeneous shrinkage in early-age shotcrete. The underlying intrinsic material functions, which are independent of field and boundary conditions, are determined from standard material tests. As for the numerical treatment of the constitutive equations of the material model, an extended form of the return mapping algorithm is proposed. The constitutive equations are applied to a two-surface chemoplastic model, consisting of a Drucker-Prager loading surface and a tension cut-off. In a companion paper, the proposed material model is employed for 2D structural analyses of tunnels driven according to the New Austrian Tunneling Method.     

The “Chunnel” Fire.?II: Analysis of Concrete Damage

In Part I of this study, a material model for the in-situ behavior of rapidly heated concrete was developed that accounts explicitly for the dehydration of concrete and its cross-effects with deformation (chemomechanical couplings) and temperature (chemothermal couplings). In this part of the study, the model is used in finite-element analysis of the tunnel rings of the Channel Tunnel (the “Chunnel”) exposed to fire. An analysis of the finite-element results—i.e., the profiles of temperature, dehydration, stresses, and plastic strains—clearly shows that the thermal spalling that occured during the Chunnel fire is initiated by an in-plane biaxial compressive stress clog closed to the heated surface. The compressive stresses are caused by restrained thermal dilatation and are bounded by chemoplastic softening due to dehydration. They provoke permanent radial deformation, which can be attributed to spalling. The role of thermal damage and thermal decohesion is discussed by comparing elastic, chemoelastic, and chemoplastic stress developments during the 10 h fire exposure. It is found that the salient feature to capture the initiation of thermal spalling at a structural level is the chemoplastic softening behavior at a constitutive material level. It is also shown that a reinforcement on the cold-side, as well as steel fiber reinforcement of concrete, in tunnel rings may significantly increase the risk of thermal spalling.     

An inverse problem analysis for the determination of probalistic parameters of concrete behavior modeled by a statistical approach

When the Monte Carlo method is used to simulate the heterogeneous behavior of concrete in the framework of a finite element probabilistic analysis, N samples of the vector of random variables (tensile strength, Young's modulus, etc.) are generated from a specific probability density function. If the uncertainties of these material parameters are assumed to vary spatially following a normal distribution, the N samples corresponding to a simulation are function of the mean and the standard deviation that define the Gauss density function. The problem is that these statistical moments are not known,a priori, for the characteristic volume of the finite elements for which the problem has been discretized. In this paper an algorithm is proposed to evaluate the parameters characterizing the statistical distribution (e. g. for a normal distribution: the mean and the standard deviation) for a given response of the structure (for instance, a mean load-displacement curve) following an inverse analysis procedure. A very simple mechanical system is used to verify the feasibility of the procedure. By way of example, it is shown that this kind of inverse problem for the identification of statistical parameters is suitable for concrete.

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Résumé Si la méthode de Monte-Carlo est utilisée pour simuler l'hétérogénéité du béton dans le cadre d'une analyse probabiliste par éléments finis, N échantillons du vecteur des variables aléatoires (résistance à la traction, module d'Young, etc.) sont générés à partir d'une fonction de densité de probabilité donnée. Si la dispersion de ce matériau peut être représentée par une distribution normale, les N échantillons correspondant à une simulation, sont fonction de la moyenne et de l'écart type qui définissent la fonction de densité de Gauss. Le problème c'est que ces moments statistiques ne sont pas connus,a priori, pour le volume caractéristique des éléments finis selon lesquels le problème a été discrétisé. Dans cet article, on propose un algorithme pour l'évaluation des paramètres qui caractérisent la distribution statistique (comme exemple, pour une distribution normale: la moyenne et l'écart type) correspondant à la réponse donnée d'une structure (par exemple: une courbe force-déplacement) suivant un processus d'analyse inverse. Un système mécanique très simple est utilisé pour vérifier la faisabilité du processus. Il est montré, à partir d'un exemple, que ce type d'analyse inverse peut être utilisé pour le béton.

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Editorial note COPPE/UFRJ and LCPC are RILEM Titular Members.     

Modeling of Early-Age Creep of Shotcrete. II: Application to Tunneling

This paper presents the application of the material model presented in the companion paper to the structural analysis of tunnels driven according to the new Austrian tunneling method (NATM). On the one hand, two-dimensional simulations of the excavation of a tunnel driven according to the NATM are presented. They show the significance of shotcrete creep for the soil-shotcrete compound structure. These simulations constitute the main part of the paper. On the other hand, as an outlook, a hybrid method is addressed that combines, (in the framework of nonlinear finite-element analyses) the aforementioned material law with three-dimensional in situ displacement measurements performed continuously during the construction process of NATM tunnels. This method enables continuous safety assessment of tunnels and, hence, contributes essentially to online monitoring of tunnels.