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External Stresses (external + stress)
Selected AbstractsDerivation of Recovery Kinetics From Stress Relaxation Tests,ADVANCED ENGINEERING MATERIALS, Issue 3 2010Sheila Bhaumik The recovery behavior of a commercial aluminum alloy 3103 was investigated by the means of two alternative experimental methods: stress relaxation (SR) and double tension tests (DT). In case of SR, the stress,time evolution after deformation was recorded, and for DT the yield stress after several recovery times were measured. The DT tests were further sub-divided into tests with and without external load during recovery. The results revealed that the recovery kinetics is clearly accelerated by the external stress during the SR. However, the difference between the DT and SR stresses is much larger. It is caused by continued dislocation glide after the deformation, which causes continued plastic elongation of the specimens. This is demonstrated quantitatively by appropriate evaluation models for both experiments. In contrast to DT, the SR evaluation accounts for the elastic SR due to plastic elongation, but the recovery parameters are the same ones as for DT. This makes it possible to replace DT by SR experiments, which are materially less laborious. [source] Quantifying effects of particulate properties on powder flow properties using a ring shear testerJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 9 2008Hao Hou Abstract Effects of particle size, morphology, particle density, and surface silicification, on powder flow properties were investigated using a ring shear tester. Flow properties were quantified by flow function (FF), that is, unconfined yield strength, fc, as a function of major principal stress. A total of 11 powders from three series of microcrystalline cellulose (MCC): Avicel (regular MCC, elongated particles), Prosolv (silicified MCC, elongated particles), and Celphere (spherical MCC), were studied. Particle size distribution in each type of MCC was systematically different. Within each series, smaller particles always led to poorer powder flow properties. The slope of FF line was correlated to degree of powder consolidation by external stress. A key mechanism of the detrimental effect of particle size reduction on flow properties was the larger powder specific surface area. Flow properties of Celphere were significantly better than Avicel of comparable particles size, suggesting spherical morphology promoted better powder flow properties. Flow properties of powders different in densities but similar in particle size, shape, and surface properties were similar. When corrected for density effect, higher particle density corresponded to better flow behavior. Surface silicification significantly improved flow properties of finer MCC, but did not improve those of coarser. © 2008 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 97:4030,4039, 2008 [source] Constrained Sintering of Low-Temperature Co-Fired CeramicsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2006Aravind Mohanram This paper discusses the effect of uniaxial compressive stress and pressureless constraint on the microstructure, density, and shrinkage anisotropy during the sintering of two commercial low-temperature co-fired ceramic (LTCC) systems, i.e., Heraeus CT2000 (CT) and DuPont 951Tape (DU). Under uniaxial compression, the ratio of axial to transverse shrinkage of DU is significantly higher than that of CT. A simple linear viscous theory was used to estimate the change in the strain rates produced by the external stress and the stress required to produce zero shrinkage. The theory was found to overestimate the measured stress-induced strain rates. The uniaxial compressive stress required for zero overall shrinkage was estimated to be ,60 kPa for DU and 80 kPa for CT. The estimate for the DU materials was in good agreement with the experimental data, but there was significant deviation for the CT material. Higher viscosity and higher constraining stresses led to lower densities in pressure-less constrained CT specimens compared with DU. [source] The fractal nature of elastic aftereffect in metallic materialsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 3 2007V. Novikov Abstract Experimental investigations of the phenomenon of the elastic aftereffect of a number of metallic materials have been carried out. It was found that the time-dependent deformation may be described by a power function of time with fractional index. A mathematical model that describes the elastic aftereffect on a basis of fractal ideas about the process of deformation is constructed. A concept of a fractal set of relaxation periods is offered as an explanation of the elastic stability of the crystals, , or of their instability, i.e., emergence of residual deformation under the influence of external stress on the crystal system. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Plasticity and Grain Boundary Diffusion at Small Grain Sizes,ADVANCED ENGINEERING MATERIALS, Issue 8 2010Gerhard Wilde Bulk nanostructured,or ultrafine-grained materials are often fabricated by severe plastic deformation to break down the grain size by dislocation accumulation. Underlying the often spectacular property enhancement that forms the basis for a wide range of potential applications is a modification of the volume fraction of the grain boundaries. Yet, along with the property enhancements, several important questions arise concerning the accommodation of external stresses if dislocation-based processes are not longer dominant at small grain sizes. One question concerns so-called "non-equilibrium" grain boundaries that have been postulated to form during severe deformation and that might be of importance not only for the property enhancement known already today, but also for spectacular applications in the context of, e.g., gas permeation or fast matter transport for self-repairing structures. This contribution addresses the underlying issues by combining quantitative microstructure analysis at high resolution with grain boundary diffusion measurements. [source] Self-Assembled Organic,Inorganic Hybrid Elastic Crystal via Biomimetic MineralizationADVANCED MATERIALS, Issue 33 2010Halei Zhai Organic,inorganic hybrid rhombs with regular shape and a lamellar superstructure are biomimetically fabricated by the cooperative self-assembly of bovine serum albumin (BSA), sodium bis-2-ethylhexyl sulfosuccinate (AOT), and calcium phosphate (see figure). Although crystalline calcium phosphate is the main component, the hybrid crystals are elastic and they can be flexible under external stresses. [source] Trehalose extends longevity in the nematode Caenorhabditis elegansAGING CELL, Issue 4 2010Yoko Honda Summary Trehalose is a disaccharide of glucose found in diverse organisms and is suggested to act as a stress protectant against heat, cold, desiccation, anoxia, and oxidation. Here, we demonstrate that treatment of Caenorhabditis elegans with trehalose starting from the young-adult stage extended the mean life span by over 30% without any side effects. Surprisingly, trehalose treatment starting even from the old-adult stage shortly thereafter retarded the age-associated decline in survivorship and extended the remaining life span by 60%. Demographic analyses of age-specific mortality rates revealed that trehalose extended the life span by lowering age-independent vulnerability. Moreover, trehalose increased the reproductive span and retarded the age-associated decrease in pharyngeal-pumping rate and the accumulation of lipofuscin autofluorescence. Trehalose also enhanced thermotolerance and reduced polyglutamine aggregation. These results suggest that trehalose suppressed aging by counteracting internal or external stresses that disrupt protein homeostasis. On the other hand, the life span-extending effect of trehalose was abolished in long-lived insulin/IGF-1-like receptor (daf-2) mutants. RNA interference-mediated inactivation of the trehalose-biosynthesis genes trehalose-6-phosphate synthase-1 (tps-1) and tps-2, which are known to be up-regulated in daf-2 mutants, decreased the daf-2 life span. These findings indicate that a reduction in insulin/IGF-1-like signaling extends life span, at least in part, through the aging-suppressor function of trehalose. Trehalose may be a lead compound for potential nutraceutical intervention of the aging process. [source] Mechanics of column beds: II.AICHE JOURNAL, Issue 3 2003Modeling of coupled stress-strain-flow behavior The rheological behavior of a bed of packing material during its consolidation was investigated using an elastic-plastic model, the Frictional Material Model. This model takes into account the behavior of the packing material under compression stress, its consolidation, its internal friction, its friction against the column wall, and the dependence of the bed's permeability on the local void fraction. The complexity of the problem arises from the nonlinear behavior of the relationships between the critical parameters controlling the column bed behavior and the external stresses applied to this bed. Solutions of the model were calculated for combinations of axial compression and seepage stresses, the latter corresponding to the flow of the mobile-phase stream under typical conditions used in HPLC. The results demonstrate the importance of the internal angle of friction of the packing material used on the degree of radial and axial heterogeneity of the beds of chromatographic columns packed with this material. [source] Strain-Controlled Tensile Deformation Behavior and Relaxation Properties of Isotactic Poly(1-butene) and Its Ethylene CopolymersMACROMOLECULAR SYMPOSIA, Issue 1 2004Mahmoud Al-Hussein Abstract The tensile deformation behaviour of poly(1-butene) and two of its ethylene copoloymers was studied at room temperature. This was done by investigating true stress-strain curves at constant strain rates, elastic recovery and stress relaxation properties and in-situ WAXS patterns during the deformation process. As for a series of semicrystalline polymers in previous studies, a strain-controlled deformation behaviour was found. The differential compliance, the recovery properties and the stress relaxation curves changed simultaneously at well-defined points. The strains at which these points occurred along the true stress-strain remained constant for the different samples despite their different percentage crystallinities. The well-defined way in which the different samples respond to external stresses complies with the granular substructure of the crystalline lamellae in a semicrystalline polymer. [source] Mutations in two global regulators lower individual mortality in Escherichia coliMOLECULAR MICROBIOLOGY, Issue 1 2008Fanette Fontaine Summary There has been considerable investigation into the survival of bacterial cells under stress conditions, but little is known about the causes of mortality in the absence of exogenous stress. That there is a basal frequency of cell death in such populations may reflect that it is either impossible to avoid all lethal events, or alternatively, that it is too costly. Here, through a genetic screen in the model organism Escherichia coli, we identify two mutants with lower frequencies of mortality: rssB and fliA. Intriguingly, these two genes both affect the levels of different sigma factors within the cell. The rssB mutant displays enhanced resistance to multiple external stresses, possibly indicating that the cell gains its increased vitality through elevated resistance to spontaneous, endogenous stresses. The loss of fliA does not result in elevated stress resistance; rather, its survival is apparently due to a decreased physical stress linked to the insertion of the flagellum through the membrane and energy saved through the loss of the motor proteins. The identification of these two mutants implies that reducing mortality is not impossible; rather, due to its cost, it is subject to trade-offs with other traits that contribute to the competitive success of the organism. [source] Gliding dislocations in Bi2Te3 materialsPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 1 2009N. Peranio Abstract In Bi2Te3, dislocations were found with an uniquely high mobility at room temperature. The gliding dislocations were analysed and their effect on the thermoelectric properties is discussed. The glide of dislocations was induced by heating with a focused electron beam at 120 keV, external stresses were not applied. The dislocations were bowed out in the glide direction and were only pinned at the surface of the samples. Stereomicroscopy combined with image simulations yielded basal plane dislocations with a density of 109 cm,2 and Burgers vectors of type ,110,. Video sequences showing the glide of single dislocations and groups of dislocations were recorded. Isolated dislocations showed a high mobility in ±,110, direction at a velocity of 10,100 nm s,1. Dislocation dipoles were pinned and did not glide. Dislocations equidistantly arranged within the same glide plane showed a collective movement. Dislocations piled up in different glide planes were fixed and acted as barriers for gliding dislocations. The motion of dislocations was attributed to residual shear stresses of about 10 MPa, and their glide directions depended on the sign of the Burgers vector. Attractive and repulsive forces of dislocations directly visualise the forces due to the elastic strain fields of other dislocations. The relevance of phonon scattering on dislocations in Bi2Te3, particularly due to their high mobility and density, was confirmed by two inspections: (i) Dislocations decrease the lattice thermal conductivity due to phonon scattering on the elastic strain field. The phonon mean free path was estimated to about 800 µm at 3 K and agreed with published data. (ii) The dislocation resonance theory of Granato and Lücke predicts an interaction between phonons and dislocations acting as oscillating strings. The attenuation of ultrasound was estimated and was compared with published data. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | |||||||||||||||||||