The influence of bi-layer metal-matrix composite coating on the strength of the coated material

Deformation and fracture in aluminum with a bi-layer composite coating are studied numerically. Dynamic boundary-value problems in the plane-stress formulation are solved by the finite element method, using ABAQUS/Explicit. Isotropic elastoplastic and elastic-brittle constitutive models are used to simulate the mechanical response of the aluminum matrix and carbide ceramic particles, respectively. Microstructure of the composite coatings takes into account the complex shape of particles explicitly. To investigate the crack initiation and propagation in ceramic particles, a Huber type fracture criterion was chosen that takes into account the type of local stress state: bulk tension or compression. The influence of the arrangement of the coating layers on the fracture of ceramic particles and on the macroscopic strength of the coated materials is studied. Plastic strain localization, crack patterns and residual stress formation are numerically investigated during cooling followed by tension of the coated materia

Computational analysis of the influence of residual stress on the strength of composites with different aluminum matrices and carbide particles

A numerical study of the mechanical behavior of aluminum matrix–carbide particle composites subjected to combined thermomechanical loading is carried out. The composite structure, corresponding to that observed experimentally, is explicitly taken into account in the calculations. The mechanical response of the aluminum matrix and carbide particles is described using the isotropic elastic–plastic and elastic–brittle models. A fracture criterion of the maximum equivalent stress acting in the local regions of volumetric tension is used to study the crack initiation and propagation in the particles. The dynamic plane stress boundary value problems of cooling and tension of the composites are solved by the finite element method ABAQUS/Explicit. The influence of the coolinginduced residual stress and thermomechanical properties of the matrix and particle materials on the strength of the composites is investigated. A positive or negative effect of the residual stress is found to depend on the ratio between the particle strength and the matrix yield stress. Compressive residual stress formed in the particle after the cooling increases the strength of composites with hard matrices and low-strength particles. A decrease in the matrix–particle interfacial curvature results in a change in the fracture mechanism from in-particle cracking to debonding, which increases the composite strength. Composite elongation upon the fracture onset decreases with the volume fraction of the particles

Computational analysis of the influence of thermal residual stresses on the strength of metal-matrix composites

Deformation and fracture in metal-matrix composite materials is investigated during cooling followed by tension or compression. Microstructure of the composite comprising aluminum matrix and single boron carbide particle of irregular experimentally observed shape – is taken into account explicitly in calculations. Constitutive models describe isotropic elastoplastic and elastic or elastic-brittle behavior of the aluminum matrix and ceramic particles, respectively. Huber type fracture criterion takes into consideration the crack origination and growth in material local regions experiencing bulk tension. Developed numerical method used to simulate three-dimensional geometry of ceramic particles assumes the invariance of the mechanical fragmentation of rocks and brittle materials. Three-dimensional and plane-stress boundary-value problems in the dynamic formulation are solved numerically by the finite-element software package ABAQUS. Deformation of the composite subjected to tension from the initial undeformed state is compared to that produced by tension being a successor of the composite cooling from the recrystallization to room temperatures. Residual thermal stresses are found to increase the strength of the composite and change the fracture patterns from in-particle cracking to debonding

On the Capacity of Hybrid Wireless Networks with Opportunistic Routing

This paper studies the capacity of hybrid wireless networks with opportunistic routing (OR). We first extend the opportunistic routing algorithm to exploit high-speed data transmissions in infrastructure network through base stations. We then develop linear programming models to calculate the end-to-end throughput bounds from multiple source nodes to single as well as multiple destination nodes. The developed models are applied to study several hybrid wireless network examples. Through case studies, we investigate several factors that have significant impacts on the hybrid wireless network capacity under opportunistic routing, such as node transmission range, density and distribution pattern of base stations (BTs), and number of wireless channels on wireless nodes and base stations. Our numerical results demonstrate that opportunistic routing could achieve much higher throughput on both ad hoc and hybrid networks than traditional unicast routing (UR). Moreover, opportunistic routing can efficiently utilize base stations and achieve significantly higher throughput gains in hybrid wireless networks than in pure ad hoc networks especially with multiple-channel base stations

DTN Support for News Dissemination in an Urban Area

Part 3: DTN and Sensor NetworksInternational audienceWe are studying the practicality of news dissemination over a Delay Tolerant Network (DTN) in an urban area. The target application is the distribution of the electronic version of a newspaper in a large city. Therefore, although strict time constraints do not apply, spreading the information should be achieved within a reasonable delay. We consider that mobile users subscribe to their content of interest and expect to receive it within their journey from their home to their office. We provide two contributions. Firstly, we consider a simple DTN environment when content is distributed solely through inter-contact of mobile nodes. We derive analytical expressions for the packet delay in such environments and suggest how to improve effectively the expected message delay in the case of an area with low or high density of mobile nodes. Secondly, if the delay is found to be excessive, we suggest the deployment of some data kiosks in the environment to better support the dissemination of content. Data kiosks are simple devices that receive content directly from the source, usually using wired or cellular networks. We investigate both an upper bound and a lower bound of the number of data kiosks to distribute the content over a geographical area within an expected delay objective. We also show the important property that those bounds scale linearly with the contact rates between a mobile node and a data kiosk. The analytical results are validated through simulation using a number of mobility models

Hydrogen dynamics in heavy alkali metal hydrides obtained through inelastic neutron scattering

Inelastic neutron scattering spectra from polycrystalline NaH, KH, RbH and CsH, measured at low temperature in the energy transfer range 3 meV < E < 500 meV, are reported. From the medium-energy regions, coinciding with the optical phonon bands, accurate hydrogen-projected densities of phonon states are extracted and compared to ab initio lattice dynamics results. The overall agreement is very good. Further lattice dynamics calculations, based on a pairwise Born–Mayer semi-empirical potential scheme, were also performed, providing only limited and qualitative agreement with the experimental data. In conclusion, incoherent inelastic neutron spectroscopy proves to be a stringent validation tool for lattice dynamics simulations of H-containing materials.Fil: Auffermann, G.. Max Planck Institut fur Chemische Physik fester Stoffe; AlemaniaFil: Barrera, Gustavo Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de la Patagonia "San Juan Bosco". Facultad de Ciencias Naturales - Sede Comodoro. Departamento de Química; ArgentinaFil: Colognesi, D.. Istituto di Fisica Applicata ‘Nello Carrara’. Consiglio Nazionale delle Ricerche; ItaliaFil: Corradi, G.. Istituto di Fisica Applicata ‘Nello Carrara’. Consiglio Nazionale delle Ricerche; ItaliaFil: Ramirez Cuesta, A. J.. Rutherford Appleton Laboratory; Reino UnidoFil: Zoppi, M.. Istituto di Fisica Applicata ‘Nello Carrara’. Consiglio Nazionale delle Ricerche; Itali