A DISCRETE ELEMENT FORMALISM FOR MODELLING WEAR PARTICLE FORMATION IN CONTACT BETWEEN SLIDING METALS

The paper describes an advanced discrete-element based mechanical model, which allows modelling contact interaction of ductile materials with taking into account fracture and surface adhesion by the cold welding mechanism. The model describes these competitive processes from a unified standpoint and uses plastic work of deformation as a criterion of both local fracture and chemical bonding of surfaces in contact spots. Using this model, we carried out a preliminary study of the formation of wear particles and wedges during the friction of rough metal surfaces and the influence of the type of forming third body (interfacial) elements on the dynamics of the friction coefficient. The qualitative difference of friction dynamics in the areas of the contact zone characterized by different degrees of mechanical confinement is shown

Finishing the euchromatic sequence of the human genome

The sequence of the human genome encodes the genetic instructions for human physiology, as well as rich information about human evolution. In 2001, the International Human Genome Sequencing Consortium reported a draft sequence of the euchromatic portion of the human genome. Since then, the international collaboration has worked to convert this draft into a genome sequence with high accuracy and nearly complete coverage. Here, we report the result of this finishing process. The current genome sequence (Build 35) contains 2.85 billion nucleotides interrupted by only 341 gaps. It covers ∼99% of the euchromatic genome and is accurate to an error rate of ∼1 event per 100,000 bases. Many of the remaining euchromatic gaps are associated with segmental duplications and will require focused work with new methods. The near-complete sequence, the first for a vertebrate, greatly improves the precision of biological analyses of the human genome including studies of gene number, birth and death. Notably, the human enome seems to encode only 20,000-25,000 protein-coding genes. The genome sequence reported here should serve as a firm foundation for biomedical research in the decades ahead

Detection of Subsea Permafrost Using Shallow Georesistivity at Three Arctic Sites

This study focuses on a coupled understanding of coastal erosion and submarine permafrost dynamics at 3 arctic sites, where long-term observations of coastal change, historical subsea permafrost core data from the 1980s and recently obtained geophysical data of subsea permafrost are available. In this context, observations of changes in coastal geomorphology are coupled with geophysical observations along a historic submarine sub-bottom profiles. The rate of permafrost degradation in the littoral zone is controlled by a number of factors. The sea bottom water temperature and salinity control warming and salt penetration into the sediment column. Sediment deposition, re-suspension and transport by wave action and entrainment in ice are important in determining initial rates of degradation in the water depths where wave cycles reach the sea bed. Where the water depth is less than the sea ice thickness, bottom-fast ice (BFI) forms and affects the sea bed. The duration of inundation controls the length of time over which these factors influence the sea bed and increases with seaward distance from the coastline. Thus, the rate of coastal retreat affects the inclination of the IBPF table within the sediment. If erosion is rapid, and permafrost degradation rates in the littoral zone are negligible, the permafrost table lies close to the sea bed (upper figure A). If other factors are similar, a low coastal retreat rate leads to a steeper inclination of the IBPF table (middle figure B). Observations show inclinations between 0.14 and 2° below horizontal [Overduin et al., 2007] that vary between and within sites, suggesting that the relative importance of factors affecting IBPF table inclination varies spatially between sites and over time at each site (e.g. lower figure C). Here we present observations of the geoelectrical resistivity of the seabed, and compare interpreted IBPF positions from this observations with observations from the literature. We include near shore (< 10 m water depth) IBPF table inclinations determined by probing, drilling and temperature and their dependence on coastline position change rates

Kinetic approach to the development of computational dynamic models for brittle solids

The paper presents an approach to developing the mathematical formalism of the discrete element method to numerically study the inelastic behavior and fracture of brittle materials under dynamic loading. The approach adopts the basic principles of the kinetic theory of strength which postulate the finite time of nucleation of discontinuities and relaxation of local stresses in the material. A general methodology is proposed for constructing dynamic (kinetic) models of the mechanical behavior of a discrete element based on quasi-static models and using three dynamic material parameters (time parameters). The physical meaning of these parameters is discussed, and a method is proposed for estimating the magnitude of the parameters for a considered material using standard experimental data on its mechanical characteristics. The approach is verified by a dynamic formulation of two-parameter models of inelasticity and strength of brittle materials within the method of simply deformable discrete elements. The proposed way to the dynamic generalization of conventional quasi-static mechanical models is applicable to various Lagrangian numerical methods and makes it possible to numerically study the dynamic behavior features and to predict the mechanical characteristics of brittle materials at different strain rates (up to strain rates 103 s−1) and different types of stress state

The rate of subsea permafrost degradation in the 25 years following coastal erosion at Muostakh Island, Laptev Sea

Submarine permafrost is usually created by the inundation of terrestrial permafrost by seawater. The inundation of permafrost follows coastal erosion or relative sea level rise. Low modern sea level rise rates in Siberia mean that coastal erosion is the main mechanism of formation of submarine permafrost. Coastal sections composed of fine-grained sediments that have high ground-ice contents, such as the long Yedoma coastline of eastern Siberia, are especially vulnerable to mechanical and thermal erosion processes [Romanovskii et al., 2004]. When such frozen soft sediments thaw and/or erode, then the state of the permafrost is determined by the transition from sub-aerial to submarine conditions, and the processes that accompany this transition. These include removal of the upper horizons of material, sediment dynamics along the beach and shore face profile, saltwater diffusion, changing thermal regime and sea ice dynamics. For example, driven by the influence of warm and salty seawater, permafrost begins to thaw once inundated due to thermal and chemical degradation. As a result, submarine permafrost degradation may be rapid near the coast, and shows a spatially variable dependence on this set of processes [Overduin et al., 2007]. Our objectives are to investigate changes in coastal geomorphology in combination with geophysical investigations of submarine permafrost distribution in the near-shore zone (< 10 m water depth), in order to infer which processes dominate permafrost degradation in this highly dynamic coastal setting

Analysis of the Quasi-Static and Dynamic Fracture of the Silica Refractory Using the Mesoscale Discrete Element Modelling

Computer modelling is a key tool in the optimisation and development of ceramic refractories utilised as insulation in high-temperature industrial furnaces and reactors. The paper is devoted to the mesoscale computer modelling of silica refractories using the method of homogeneously deformable discrete elements. Approaches to determine the local mechanical properties of the constituents from the global experimental failure parameters and respective crack trajectories are considered. Simulations of the uniaxial compressive and tensile failure in a wide range of quasi-static and dynamic loading rates (102 s&minus;1) are performed. The upper limit of the dynamic loading rates corresponds to the most severe loading rates during the scrap loading on the refractory lining. The dependence of the strength, fracture energy, and brittleness at failure on the loading rate is analysed. The model illustrates that an increase in the loading rate is accompanied by a significant change in the mechanical response of the refractory, including a decrease in the brittleness at failure, a more dispersed failure process, and a higher fraction of the large grain failure. The variation of the grain&ndash;matrix interface&rsquo;s strength has a higher impact on the static compressive than on the static tensile properties of the material, while the material&rsquo;s dynamic tensile properties are more sensitive to the interface strength than the dynamic compressive properties

Acoustic emission characterization of sliding wear under condition of direct and inverse transformations in low-temperature degradation aged Y-TZP and Y-TZP-AL2O3

Abstract In this research, results of the investigation of the sliding friction and wear of yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) and Y-TZP-Al2O3 samples preliminarily subjected to low-temperature degradation are reported. The investigation was carried out using a pin-on-disk tribometer with simultaneous recording of acoustic emission (AE) and vibration acceleration. The sliding wear process was found to be determined by dynamic direct and inverse Y-TZP transformations detected by monoclinic and tetragonal X-ray diffraction peak ratios. The AE signals generated under direct and inverse transformations can be used to characterize wear and friction mechanisms as well as direct and inversed sliding-induced phase transformations. The AE signal energy grows with the friction coefficient and the inverse transformation degree. Reduction of the AE signal energy indicates establishing the mild wear stage caused by effective stress-induced direct martensitic transformation. The AE signal median frequency increases in the case of lower friction. Numerical studies of wear subsurface fracture under conditions of stress-induced martensitic transformation were used to elucidate the role played by the phase transformation in Y-TZP and Y-TZP-Al2O3. Martensitic transformation in Y-TZP was described with use of the non-associated dilatant plasticity model. Simulation results particularly show that increase in the value of dilatancy coefficient from 0 to 0.2 is accompanied by 25%−30% reduce in characteristic length and penetration depth of sliding-induced subsurface cracks. As shown the AE may be an effective tool for in-situ monitoring the subsurface wear of materials experiencing both direct and inverse transformations