Model of quasi-ductile deformations that bridges the scales

This study proposes a model of the deformation of materials susceptible to microcracking that is based on the thermodynamics of irreversible processes and fracture mechanics. The study considers the process on the atomic-, micro-, meso- and macroscales that are connected by the process of homogenization. The limitations of the process are carefully spelled out.     

Shattering impact fragmentation of slender nanoprojectiles

Molecular dynamics simulations of the rigid-anvil collision test are performed to achieve the complete pulverization of slender nanoprojectiles. The simulation setup mimics the traditional Taylor test (the flat-ended nanoscale bars collide with a rough rigid wall) at striking velocities that reach an awesome range from 20 km/s to 120 km/s. The objective is to investigate, so called, shattering fragmentation, defined by the complete disintegration (pulverization) of the slender monocrystalline nanoprojectile into the cloud of monatomic debris (m_max?=?m_max1?≡?1). The critical impact energy associated with this transition from the stochastic to the deterministic fragment distribution is investigated at two widely different initial temperatures of the slender nanoprojectile while scaling its size in a self-similar manner by varying their widths (diameters) at a fixed aspect ratio. For all but the smallest nanoprojectiles, the minimum achievable m_max???m_max1 is discussed based on the physically-limiting striking velocity range.

     

Quantitative elektrolytische erzeugung von permanganationen

In order to realize coulometric permanganatometric determinations theoretical and experimental investigations have been carried out. By the study of anodic potentials on platinum and of electrolytic oxydation processes the possibility of the quantitative formation of permanganate ions from manganous sulfate in aqueous sulfuric acid solutions could be demonstrated. Under definite conditions of anodic potential, concentration of anolyte, and relation between manganous sulfate and sulfuric acid, by different current densities, a quantitative generation of permanganate ions can be attained, in a sure and simple manner, and can also be utilized for different secondary oxidation processes.Thus the fundamental condition has been created for the realization and the development of the coulometric permanganatometry, and thereby the possibility of the coulometric generation of permanganate and standardization of volumetric solutions based on permanganate has also been created, as well as for the further utilization of the coulomb as a standard in analytical chemistry.     

Brittle to Quasi-Brittle Transition

Abstract. The present study focuses on the kinetic and non-deterministic aspects of the brittle to quasi-brittle transition. A solid is approximated by a lattice formed by the interacting continuum particles and the evolution of damage is estimated using particle dynamics. The onset of transition is measured by the rate of the change of correlation length. The proposed method is illustrated on the examples of creep rupture, strain localization and dynamic expansion of a circular void in a brittle plate.Sommario. Viene posta l'attenzione sugli aspetti cinetici e non deterministici della transizione dal comportamento fragile a quello quasi-fragile. Un solido viene approssimato da un reticolo formato da particelle interagenti e l'evoluzione del danno viene stimata tramite la dinamica delle particelle. L'inizio della transizione viene misurato tramite la variazione della lunghezza di correlazione. Il metodo proposto viene illustrato su esempi di rottura per creep, localizzazione della deformazione e l'espansione di un foro circolare in una piastra fragile.     

On strain-rate sensitivity and size effect of brittle solids: transition from cooperative phenomena to microcrack nucleation

An idealized brittle microscale system is subjected to dynamic uniaxial tension in the medium-to-high strain-rate range $(\dot \varepsilon \in \;[100\;s^{-1},\;1 \times 10^{7} \;s^{-1}])$ to investigate its mechanical response under constrained spatial and temporal scales. The setup of dynamic simulations is designed to ensure practically identical in-plane stress conditions on a system of continuum particles forming a two-dimensional, geometrically and structurally disordered, lattice. The rate sensitivity of size effects is observed as well as the ordering effect of kinetic energy. A simple phenomenological expression is developed to account for the tensile strength sensitivity of the small-sized brittle systems to the strain-rate and extrinsic size effects, which may serve as a guideline for formulation of constitutive relations in the MEMS design. The representative sample is defined as a square lattice size for which the tensile strength becomes rate-insensitive and an expression is proposed to model its evolution between two asymptotes corresponding to the limiting loading rates. The dynamics of damage accumulation is analyzed as a function of sample size and loading rate.     

Stationary polarons in discrete molecular chains

Properties of the large acoustic polarons in discrete molecular chains have been investigated within the adiabatic approximation. It turns out that practically all the polaron features are determined by the single parameter‐coupling constant which represents the ratio between the small polaron binding energy and the electron bandwidth. Three different types of stationary solutions were found corresponding to weak, intermediate, and strong coupling limits, respectively. In the weak coupling regime, that is, for the values of coupling constant exceeding the limit of the applicability of continuum approximation but lower than the critical one ( $g_C$ ), we observe symmetric bond‐centered solution corresponding to the polaron positioned in the middle between the adjacent lattice sites. When coupling constant overgrows, this critical value transition toward the site‐centered state occurs. It takes place continuously through the intermediate asymmetric state whose position gradually approaches lattice site as coupling constant increases. One of the main consequences of the lattice discreteness is the emergence of the periodic potential, Peierls‐Nabarro potential relief, through which polarons have to pass to transfer along the chain. The conditions for the polaron propagation are formulated in terms of the threshold velocity. © 2012 Wiley Periodicals, Inc.     

Brittle and quasi-ductile damage at large strain rates

The present study suggests that molecular and particle dynamics simulations can provide the basis for estimates of qualitative and quantitative aspects of damage in materials with inferior cohesive strength subjected to high strain rate loading. Two examples demonstrate that the traditional continuum models are of limited value in modeling of the class of considered problems.     

Multi-wavelength laser sensor for intruder detection and discrimination

An intruder detection and discrimination sensor with improved optical design is developed using lasers of different wavelengths to demonstrate the concept of discrimination over a distance of 6 m. A distinctive feature of optics is used to provide additional transverse laser beam scanning. The sample objects used to demonstrate the concept of discrimination over a distance of 6 m are leaf, bark, black fabric, PVC, wood and camouflage material. A camouflage material is chosen to illustrate the discrimination capability of the sensor. The sensor utilizes a five-wavelength laser combination module, which sequentially emits identically-polarized laser light beams along one optical path. A cylindrical quasi-optical cavity with improved optical design generates multiple laser light beams for each laser. The intensities of the reflected light beams from the different spots are detected using a high speed area scan image sensor. Object discrimination and detection is based on analyzing the Gaussian profile of reflected light at the different wavelengths. The discrimination between selected objects is accomplished by calculating four different slopes from the objects' reflectance spectra at the wavelengths 473 nm, 532 nm, 635 nm, 670 nm and 785 nm. Furthermore, the camouflage material, which has complex patterns within a single sample, is also detected and discriminated over a 6 m range by scanning the laser beam spots along the transverse direction.     

Coulometrische permanganatometrische bestimmung von oxalat-, ferro- und arsenitionen

Methods based upon the quantitative electrolytic generation of permanganate in aqueous sulfuric acid solutions of manganous sulfate have been developed for the coulometric permanganatometric determination of oxalate, ferrous, and arsenious ions; the end-point being ascertained by means of colour changes of indicators. In this way coulometric permanganatometry has been established and the possibility of its large practical utilisation has been shown.Thus, by the creation of the coulometric permanganatometry, the coulomb, replacing the mass of the standard, has penetrated into still another part of analytical chemistry for measurement and standardization.