Cosmic string Y-junctions: a comparison between field theoretic and Nambu-Goto dynamics

We explore the formation of cosmic string Y-junctions when strings of two different types collide, which has recently become important since string theory can yield cosmic strings of distinct types. Using a model containing two types of local U(1) string and stable composites, we simulate the collision of two straight strings and investigate whether the dynamics matches that previously obtained using the Nambu-Goto action, which is not strictly valid close to the junction. We find that the Nambu-Goto action performs only moderately well at predicting when the collision results in the formation of a pair of Y-junctions (with a composite string connecting them). However, we find that when they do form, the late time dynamics matches those of the Nambu-Goto approximation very closely. We also see little radiative emission from the Y-junction system, which suggests that radiative decay due to bridge formation does not appear to be a means via which a cosmological network of such string would rapidly lose energy.Comment: 17 pages, 17 figures; typo correctio

Detecting and distinguishing topological defects in future data from the CMBPol satellite

The proposed CMBPol mission will be able to detect the imprint of topological defects on the CMB provided the contribution is sufficiently strong. We quantify the detection threshold for cosmic strings and for textures, and analyze the satellite's ability to distinguish between these different types of defects. We also assess the level of danger of misidentification of a defect signature as from the wrong defect type or as an effect of primordial gravitational waves. A 0.002 fractional contribution of cosmic strings to the CMB temperature spectrum at multipole ten, and similarly a 0.001 fractional contribution of textures, can be detected and correctly identified at the 3 level. We also confirm that a tensor contribution of r=0.0018 can be detected at over 3, in agreement with the CMBPol mission concept study. These results are supported by a model selection analysis

CMB power spectra from cosmic strings: predictions for the Planck satellite and beyond

We present a significant improvement over our previous calculations of the cosmic string contribution to cosmic microwave background (CMB) power spectra, with particular focus on sub-WMAP angular scales. These smaller scales are relevant for the now-operational Planck satellite and additional sub-orbital CMB projects that have even finer resolutions. We employ larger Abelian Higgs string simulations than before and we additionally model and extrapolate the statistical measures from our simulations to smaller length scales. We then use an efficient means of including the extrapolations into our Einstein-Boltzmann calculations in order to yield accurate results over the multipole range 2 < l 3000 in the case of the temperature power spectrum, which then allows cautious extrapolation to even smaller scales. We find that a string contribution to the temperature power spectrum making up 10% of power at l=10 would be larger than the Silk-damped primary adiabatic contribution for l > 3500. Astrophysical contributions such as the Sunyaev-Zeldovich effect also become important at these scales and will reduce the sensitivity to strings, but these are potentially distinguishable by their frequency-dependence.Comment: 18 pages, 16 figure

Processing and properties of bone-analogue biodegradable and bioinert polymeric composites

This paper summarizes the processing and properties of bone-analogue composites aimed to be used in temporary or permanent orthopaedic applications. The studied matrices were two biodegradable starch based blends (with ethylene-vinyl alcohol copolymer or with cellulose acetate) and three high density polyethylene (HDPE) grades. Composites of these materials with hydroxyapatite (HA—the main inorganic constituent of the human bone) were produced by extrusion compounding and subsequently injection moulded. A non-conventional injection moulding technique known as shear controlled orientation in injection moulding (SCORIM) was used deliberately to induce a strong anisotropic character to the processed composites. For the case of HDPE based composites, an alternative reinforcement system based on carbon fibres (C fibres) was also studied. For that, a special moulding technique that combines, in a single equipment, a compounding with an injection unit was used. Composites featuring a sandwich like structure were also produced by mono-sandwich injection moulding. These composites combine a HDPE/HA outer layer and HDPE/C fibre reinforced core. The aim is to produce composites with a mechanical behaviour matching that of human cortical bone and simultaneously a strong bioactive (bone-bonding) character. For all the cases, the mechanical performance of the produced composites was assessed and the structure developed investigated and related to the processing conditions. It was possible to produce, both biodegradable and bioinert matrix composites, with properties that might allow for their application the orthopaedic orthopaedic field.Subprograma Ciência e Tecnologia do 2.º Quadro Comunitário de Apoio, Ministério da Ciência e Tecnologia (Portugal)

Interactions of Cosmic Superstrings

We develop methods by which cosmic superstring interactions can be studied in detail. These include the reconnection probability and emission of radiation such as gravitons or small string loops. Loop corrections to these are discussed, as well as relationships to $(p,q)$-strings. These tools should allow a phenomenological study of string models in anticipation of upcoming experiments sensitive to cosmic string radiation.Comment: 22 pages, 6 figures; v2: updated reference

Reinforcement and structure development in injection molding of bone-analogue composites

Composites of high density polyethylene (HDPE) with hydroxyapatite (HA—the main inorganic constituent of human bone) were produced by extrusion compounding and subsequent injection molding. Shear controlled orientation in injection molding (SCORIM) was used deliberately to induce a strong anisotropic character in the composite materials. Bi-composite moldings featuring a sandwich like morphology were also produced by mono-sandwich injection molding. These composites combine a HDPE/HA outer layer and HDPE/carbon fiber reinforced core. For all the cases, the mechanical performance of the produced composites was assessed and the structure developed investigated and related to the processing conditions

Deformation modes and anisotropy in magnesium alloy AZ31

A strongly textured sheet of magnesium alloy AZ31 has been subjected to tensile testing at temperatures between ambient and 300&deg;C. Structures have been examined by optical and transmission electron microscopy and also by atomic force microscopy to quantify surface displacements seen at grain boundaries. Plastic anisotropy varies strongly with test temperature as was observed previously by Agnew and Duygulu. The present findings do not support the view that crystallographic &lt;c + a&gt; becomes a major contributor to deformation at higher temperatures. Rather, the material behaviour reflects an increasing contribution from grain boundary sliding despite the relatively high strain rate (I 0-3 s-1) used in the mechanical tests.<br /

Structure development and control of injection-molded hydroxylapatite-reinforced starch/EVOH composites

This article reports on the development of novel composites of starch-based polymers reinforced with hydroxylapatite (HA). Two different grades of blends of starch and ethylene vinyl alcohol copolymers were reinforced with up to 30% (wt) of sintered and nonsintered HA. The initial compounding stage was carried out either in a rotating drum or by twin-screw extrusion (TSE). Compounds were injection molded using both conventional molding and a shear-controlled orientation in injection molding (Scorim) technique. It was possible to obtain composites with very good mechanical properties, approaching those of cortical bone. The best results were achieved for 30% sintered HA composites processed by TSE and Scorim, due to the in situ formation of HA fibers, and the development of molecular orientation and more compact microstructures in the moldings. This was confirmed by SEM analysis of the fracture surfaces, x-ray diffraction, and Raman spectroscopy. The biodegradability of the composites was also assessed and was found to be faster than that of the matrixPRAXIS XXI Programm